geneva cam mechanism

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PROJECT REPORT

On

Project title: Geneva Cam Mechanism

5

th

Semester ofDIPLOMA ENGINEERING

IN

MECHANICAL ENGINEERING

B.S. PATEL POLYTECHNIC, KHERVA

Submitted By Guide By

Ankit Mevada (126440319162) Mr. H. S. PatelHarsh Patel (126440319161) Mechanical Engineering Department

B.S.Patel Polytechnic, Kherva


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Certificate

This is to certify that Mr. Mevada Ankit K. having Enrolment No:126440319162 have

completed Part-I IDP Project work having titleGeneva Cam Mechanism.

He has undergone the process of shodh yatra, literature survey and problem Definition. He is

supposed to carry out the residue IDP Part-II work on same problem during Semester-VI for the

final fulfilment of the IDP work which is Prerequisite to complete Diploma Engineering.

Institute Guide Head of Department

Mr. H.S. Patel Mr. K.P. Patel

Mechanical Engineering Mechanical Engineering

Department Department

B.S.Patel Polytechnic, Kherva B.S.Patel Polytechnic, Kherva


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Certificate

This is to certify that Mr. Patel Harsh R. having Enrolment No.: 126440319161 has completed

Part-I IDP Project work having title Geneva Cam Mechanism.

He has undergone the process of shodh yatra, literature survey and problem Definition. He is

supposed to carry out the residue IDP Part-II work on same problem during Semester-VI for the

final fulfilment of the IDP work which is Prerequisite to complete Diploma Engineering.

Institute Guide Head of Department

Mr. H.S. Patel Mr. K.P. Patel

Mechanical Engineering Mechanical Engineering Department Department

B.S.Patel Polytechnic, Kherva B.S.Patel Polytechnic, Kherva


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INDUSTRY DEFINED PROBLEM/PROJECT (IDP) STATEMENT FORM

STUDENT PARTICULARS-1

FIRST NAME ANKITKUMAR

LAST NAME KIRITBHAI MEVADA

MOBILE NO. +91 8735933694 +91 9825723235

EMAIL [email protected]

COLLEGE

NAME B.S.PATEL POLYTECNIC, GANPAT UNIVERSITY.

ADDRESS

33/ Alok Tenament, Part-2, Maheshwari Nagar road,

Vastral, Ahmedabad.

BRANCH MECHANICAL ENGINEERING

SEMESTER 5th

SEM. YEAR 2014-2015

TEAM NAME

SIGNATURE

OF STUDENT

mailto:[email protected]:[email protected]:[email protected]


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INDUSTRY PARTICULARS

---INDUSTRY COORDINATOR---

NAME KIRITBHAI B. MEVADA

CONTACT

ADDRESS

33/ Alok Tenament, Part-2, Maheshwari Nagar road,

Vastral, Ahmedabad.

MOBILE NO. +919825723235

EMAIL -

-----INDUSTRY----

NAME LAXMI PHARMA MACHINES

ADDRESS

75, Bileshwar ind. Estate, Opp. GVMM, Nr. Ring Road,

Odhav-Kathwada Road, Odhav, Ahmedabad-382415.

CONTACT

NO. +91 79 22900488 MOBILE +91982572325

NAME OF INDUSTRIAL ESTATE BILESHWAR INDUSTRIAL ESTATE

COMPANY

LOGO


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INDEX

SR. NO. TOPICS NAME

1. Introduction Of Project & background of the invention

2. Project Descriptions

3. Implementation Procedure, Study And Data Collection

4. Description Of Project

5. Manufacturing Aspects/Process Planning

6. Kinematics Of Jeneva

7. Advantages And Limitations

8. Cost Estimation

9. Conclusions

10. Expected Out Come/Future Modifications

11. References


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Introduction

The Geneva drive or Maltese cross is a gear mechanism that translates a

continuous rotation into an intermittent rotary motion. The rotating drive wheel has

a pin that reaches into a slot of the driven wheel advancing it by one step. The

drive wheel also has a raised circular blocking disc that locks the driven wheel in

position between steps.

BACKGROUND OF THE INVENTION

Geneva Mechanisms are widely used in motion picture film projectors to

intermittently advance film through a film gate having a projection aperture. The

film is moved or advanced by a Geneva Mechanism (also known as a “Maltese

Cross”) until an image frame is in alignment with the projection aperture. The film

is then held stationary for a discrete time period during which light is passed

through the aperture, film frame, projection lens, and onto a screen. Thisintermittent frame-by-frame motion of the film is enabled by the Geneva

Mechanism, which comprises one portion, the driver, which rotates continuously,

and which causes intermittent rotation of a second portion, the star wheel. In a

motion picture projector the star wheel shares its central shaft with a sprocket, the

teeth of which are engaged with perforations in the film. Therefore, when the

driver moves the star wheel, both the star wheel and the film experience a resulting

intermittent motion. Other mechanisms, including servo motors and a Mitchell

Movement, have been used to drive film through a gate in an intermittent manner, but the Geneva Mechanism has proven itself over the past century to be

particularly well suited to accurately drive a load (the film) in an intermittent

fashion when the time allowed for motion of the load is minimal. For example,

motion picture film is typically projected at a rate of 24 frames per second, such

that a new film frame is positioned in the projection aperture every {fraction

http://en.wikipedia.org/wiki/Gearhttp://en.wikipedia.org/wiki/Rotationhttp://en.wikipedia.org/wiki/Rotationhttp://en.wikipedia.org/wiki/Gear


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(1/24)} second, or approximately 42 ms. The typical projector Geneva Mechanism

moves a film frame into the projection aperture with an indexing time of one-

fourth of the frame period, or approximately 10.5ms.

During essentially all of this indexing time, a shutter must block the light

incident to the film to prevent the appearance of “travel ghost” (smear of the image

caused by film motion). The projectable frame time, which would appear to be

approximately three-fourths of the total frame period, is further reduced to only

approximately one-half of the total frame period because the typical motion picture

projector employs a two-bladed shutter, which causes two blanking periods per

frame of the film, in order to raise the apparent frame rate to 48 frames per second,

and thereby greatly reduce the apparent flicker perceived by the human eye.

Furthermore, it is necessary for these two shutter intervals to be nearly equal in

duration in order to limit perceived flicker. Therefore, since one blanking periodmust be approximately one-fourth of the frame period in order to blank the

projected image as the film moves, the other blanking period must be of essentially

the same duration.

A basic Geneva Mechanism, as is commonly employed in motion picture

projectors, is described in U.S. Pat. No. 1,774,789. In general, a Geneva

Mechanism is used to convert uniform rotary motion to incremental rotary motion.

Typically, such a mechanism includes a star wheel having a plurality of radially

extending straight slots spaced equally around the periphery of the star. Interposed between these slots are concave cam guide surfaces, which, like the slots, are

uniformly dimensioned and arranged. A driver component, comprised of a

restraining cam, a drive arm extending from the base of the cam, and a drive pin

near the far end of the drive arm, is employed for indexing the star wheel. The

restraining cam has a side cam surface, which is convex and configured to interact

with the concave cam guide surfaces of the star wheel. The close contact of this

convex cam surface to the concave cam guide surfaces restrains the star wheel

from experiencing rotary motion except during the periods in which the star wheelis driven by the drive pin. The star wheel is thus restrained intermittently, and in a

manner such that the straight slots sequentially receive the drive pin.

The number of slots radially disposed around a Geneva Mechanism's star

wheel is variable, and may be any whole number greater than 2. As the number of

straight slots is changed, specific features of the mechanism such as component


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sizes, the speed and duration of the intermittent motion, and the forces or loads

applied to the drive pin and star wheel, and to the load (film) all vary as well. For

example, the Geneva Mechanisms used in motion picture projectors (which vary

some in detail depending on the projector) typically use a star wheel with four

equally spaced straight-sided slots. Such star wheels with four straight slots areengaged with the driver pin for 90° of a 360° revolution of the driver, producing

the intermittent motion. Thus, in the case of film projected at 24 frames per second,

the star wheel and film experience movement during an indexing time of only

approximately 10.5 ms of the approximately 42 ms available time per frame. By

comparison, a star wheel with three straight slots will also experience intermittent

motion once per revolution of the cam, but the engagement of the star wheel with

the driver pin occurs over only 60° of the 360° revolution of the driver. If used in a

motion picture projector at 24 frames per second, a star wheel with three slotswould utilize an indexing time of approximately 7 ms per frame to move a film

frame into the projecting aperture. While a three slot star wheel would thus

decrease the time required to move the film and thereby increase the available

projection time, the acceleration forces applied to the drive pin, slots, and the load

(the film and film perforations) are greatly increased over those of a four-slot

mechanism, making the three-slot mechanism undesirable for use in a projector.

On the other hand, a star wheel with five straight slots will have any one slot

engaged with the driver pin over 118° of the 360° revolution of the driver, for an

indexing time of approximately 14 ms versus the approximately 42 ms available

time per frame. This is not desirable in a projector, as the light efficiency to the

screen would be significantly reduced when compared with those of a four-slot

mechanism, and a more powerful lamp would be needed to obtain the same screen

luminance. However, the acceleration forces on the drive pin, star wheel, and load

(film perforations) would be reduced relative to a three or four straight slot star

wheel.

Accordingly, it would be very advantageous to find a geometry for a Geneva

Mechanism in `6 incurring a substantial increase in the acceleration forces applied

to the drive pin, star wheel slots, and the load (the film and film perforations). U.S.

Pat. No. 1,801,969 proposes to solve this problem by altering the slots of the star

wheel to have curved surfaces. Thus, as the drive pin moves in engagement with

these curved slots, the acceleration and velocity experienced by both the star wheel


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and the load (film) are altered, and the indexing time is greatly reduced when

compared with that of a star wheel with an identical number of straight slots

operating at the same frame rate. However, the designs for the curved star wheel

slots described in U.S. Pat. No. 1,801,969 fail to adequately provide for the forces

applied both to the drive pin and to the star wheel slot surfaces with which the pinis in contact. As a result, a drive pin and/or slot surfaces manufactured per U.S.

Pat. No. 1,801,969 would quickly experience overloading failure during operation.

Furthermore, the curved star wheel slot geometry as described in U.S. Pat. No.

1,801,969 also fails to provide sufficient slot width at the mouth of the slot for the

drive pin to enter and exit the slot without very heavy interference.

It should therefore readily be appreciated that there currently exists a need

for an improved Geneva Mechanism, which includes a driver with a cam and a star

wheel; in which a load can be advanced intermittently with a reduced indexingtime required to experience the intermittent motion, while the load forces applied

to the star wheel, drive pin, and the load are all controlled, without sacrificing any

of the advantages normally attendant to the use of a Geneva Mechanism. It should

also be apparent, that other improved configurations for Geneva Mechanisms, in

which the indexing time required to experience the intermittent motion can be

reduced while the forces on the load driven by the star wheel and on the drive pin

are also reduced, would likewise be advantageous. Likewise, it should be apparent

that yet other improved configurations for Geneva Mechanisms, in which theindexing time is effectively unchanged, but where the load forces applied to the

star wheel, drive pin, and load are substantially reduced, would also have value.

Finally, it should be readily apparent that such improved Geneva Mechanisms

could be employed in devices generally, and in motion picture projectors

particularly, with an advantageous affect.


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SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide for a drive mechanism

such as a Geneva Mechanism for intermittently moving a load, such as film, in amotion picture projector; where the time for intermittent motion is reduced, the

load on the mechanism is reduced, the load on the object being moved is reduced,

or some combination thereof.

The present invention relates to a Geneva Mechanism for intermittently driving a

load. The mechanism comprises: a rotating driver having a drive pin; and a star

wheel operationally associated with a load which is to be intermittently driven,

with the star wheel comprising a plurality of radially extending slots, and each of

the radially extending slots being curved along a substantial portion of its length.The drive pin of the rotating driver is engageable with one of the radial extending

slots to drive the star wheel in an incremental angular manner, such that a drive pin

load on the drive pin and forces applied to the load operationally associated with

the star wheel remain in control, and the star wheel reaches a peak acceleration and

deceleration in a continuous and controlled manner.

The present invention further relates to a Geneva Mechanism for intermittently

driving a load which comprises: a rotating driver having a shaped drive pin; and a

star wheel operationally associated with a load which is to be intermittently driven,with the star wheel comprising a plurality of radially extending slots which are

located periodically around a circumference of the star wheel. The shaped drive pin

has a controlled shaped surface and is engageable with one of the radially

extending slots to drive the star wheel in an incremental angular manner while

minimizing load forces on the drive pin, the star wheel and the load to be

intermittently driven.

The present invention also relates to a drive mechanism for driving a load which

comprises a first rotary member which provides an essentially uniform rotarymotion, with the first rotary member comprising a drive pin; and a second rotary

member operationally associated with a load which is to be driven. The second

rotary member comprises a plurality of radially extending slots which are curved

along a substantial portion of a length of the slots; with each of the slots being

engageable with the drive pin of the first rotary member during a rotation of the


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first rotary member, to provide an incremental angular rotation to the second rotary

member while a drive pin load on the drive pin and forces applied to the load

which is to be intermittently driven remain in control.

The present invention further relates to a projection system for motion picture film

which comprises a film gate having a projection aperture, such that film frames of

the film move past the projection aperture; a light source; illumination optics for

directing light from the light source into the projection aperture of the film gate; a

projection lens for projecting an image of each of the film frames onto a screen; a

Geneva Mechanism for moving the film progressively past the projection aperture,

with the Geneva Mechanism comprising a rotating driver having a drive pin, and a

star wheel operationally associated with the film, with the star wheel comprising a

plurality of radially extending slots which cooperate with the drive pin of the

rotating driver to cause an intermittent rotation of the star wheel upon a rotation ofthe rotating driver and thereby cause and an intermittent motion of the film; and a

shutter mechanism which blocks light incident onto the film frames of the film in

synchronization with the intermittent motion of the film. One of the drive pin or

the radially extending slots being shaped in a controlled manner to permit the star

wheel to reach a peak acceleration and deceleration in a continuous manner.

The present invention further relates to a projection system for film which

comprises a film gate having a projection aperture, such that film frames of the

film move past the projection aperture; and a drive mechanism for moving the film progressively past the projection aperture. The drive mechanism comprises a first

rotary member having a drive pin and a second rotary member operationally

associated with film. The second rotary member comprises a plurality of slots

which cooperate with the drive pin of the first rotary member to cause an

intermittent rotation of the second rotary member and thereby cause an intermittent

motion of the film. One of the drive pin or the slots being shaped in a controlled

manner to permit the second rotary member to reach a peak acceleration and a peak

deceleration in a continuous manner.


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Background

The name derives from the device's earliest application in mechanical

watches, Geneva, Switzerland being an important centre of watch making. The

Geneva drive is also commonly called a Maltese cross mechanism due to the visual

resemblance when the driven wheel has four spokes. Since they can be made small

and are able to withstand substantial mechanical stress, these mechanisms are

frequently used in watches.

In the most common arrangement, the driven wheel has four slots and thus

advances by one step of 90 degrees for each rotation of the drive wheel. If the

driven wheel has n slots, it advances by 360°/n per full rotation of the drive wheel

.

Because the mechanism needs to be well lubricated, it is often enclosed in an oil

capsule. The Geneva converts continuous rotary motion into intermittent rotary

motion. When one of the slots in the driven wheel is obstructed, the device, then

called a Geneva Stop, can be used to limit the number of turns of the input wheel.

In this form, the ×Geneva was used to limit the wind-up in clockwork springs.

http://en.wikipedia.org/wiki/Clockhttp://en.wikipedia.org/wiki/Clockhttp://en.wikipedia.org/wiki/Genevahttp://en.wikipedia.org/wiki/Switzerlandhttp://en.wikipedia.org/wiki/Maltese_crosshttp://en.wikipedia.org/wiki/Degree_%28angle%29http://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://en.wikipedia.org/wiki/Degree_%28angle%29http://en.wikipedia.org/wiki/Maltese_crosshttp://en.wikipedia.org/wiki/Switzerlandhttp://en.wikipedia.org/wiki/Genevahttp://en.wikipedia.org/wiki/Clockhttp://en.wikipedia.org/wiki/Clockhttp://en.wikipedia.org/wiki/Clock


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Abstract

Converting constant rotary motion into intermittent rotary motion gives rise to a

range of useful applications in silicon micromachining. This paper discusses the

design and fabrication of one such mechanism called the Geneva Wheel

mechanism. The standard SUMMiT process has been made use of in developing

this. All the related mathematics of the Geneva wheel was developed and the

system was analysed.

The Geneva Stop Mechanism (MME), designed by EDIBON, is a mechanism thattransforms continuous circular motion into intermittent motion. It is a positive

drive in which the driven wheel is positively moved or locked. It is made of

aluminum and consists of a rotating drive wheel and a driven wheel with six slots.

The drive wheel has a pin that reaches into a slot of the driven wheel, advancing it

by one step. The drive wheel also has a raised half-moon locking disc that locks

the driven wheel in position between steps.The drive wheel and the driven wheel

are mounted in two graduated discs, where the angle of both wheels can be

measured.


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Design approach

Many factors contribute to a successful Geneva mechanism design, such as

materials used, surface finish, tolerances, loads, stress levels, lubricant, etc.

Unsuccessful experimental applications of this mechanism usually result in two

modes of failure: pin wear and wheel breakage. Of these two modes, wear is the

hardest to control.

The present design approach will be to reduce wear by altering the geometry of the

Geneva wheel to reduce the contact stress while maintaining acceptable stress

levels in other regions of the wheel. R. C. Johnson3 showed that an optimum

wheel diameter exists for minimum wear stress.

In this paper, consideration is given to two additional dimensions (pin diameter

and tip thickness) on the wear stress and certain internal beam stresses. This

paper will begin by defining the wheel geometry and then developing the

relationships between this geometry

and the wheel inertia, the maximum pin load, the contact stress, and the internal

wheel stresses. These performance parameters will be normalized to the

corresponding param-

eters of a set of predefined “standard” Genevas for convenience in interpreting

results. For the “standard” set chosen, curves will show the stress and load

parameters as

a function of inertia and speed. The normalized curves GENEVA WHEEL.

Figure l(a) Geometry of the Geneva mechanism; (b) beam section geometry. will

show the effect of geometrical differences between any Geneva wheel and the“standard” Geneva.

Graphical curves for 4-, 5-, 6-, and 8-slot Genevas are shown although the concept

can be extended to Genevas with any number of slots. The complexity and

voluminous nature of 2 the calculations prohibit any complete closed form solution

of the problem, and therefore it was necessary to use a digital computer (IBM


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7094) for most of the results. For this reason, no detailed derivations will be given,

and the emphasis will be on the results obtained and how they can be used in the

analysis and synthesis of Geneva mechanism.

3


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4


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Geneva Wheel Mechanism

The basic structure of a four slot Geneva wheel is shown in Fig.1. The

system consists of a constantly rotating disk coupled with a slotted disk, which

gives rise to the desired discrete motion. A rotation of 2p radians of the former

causes 2p/N radians of rotation of the latter, where N is the number of slots

available on the slotted disk. Thus, one complete rotation of the slotted wheel

requires N complete rotations of the other disk, thereby also increasing he otal time

period. The conversion mechanism of this disk system is as follows.

Referring to Fig.1, pinwheel W rotates constantly about axis A and as shown

below, has a pin ‘a’ attached to it. This pin ‘a’ engages into the slots ‘s’ of the

Geneva Wheel G (a basic 4-slot Geneva mechanism is shown here) and rotates it

as long as it is engaged with the slot. While the wheel W rotates continuously, the

Geneva wheel G has a discrete rotation about axis ‘b’. Wheel G has a rotation

time period of t when it is moving along with disk W and n idlingtime period,

when the pin ‘a’ is not inside one of the slots ‘s’and is moving freely. The three

quarter wheel ‘L’ is placed inorder to prevent any unintentional rotation of wheelG hile itis idling. For a four slot Geneva mechanism, the rotation timeperiod. By

varying the number of slots on G, one can varythe time period and the angular

displacement of the same. Ifthis system is now coupled with some optical system

like amicromirror (through a rack and pinion kind of arrangement),then it can be

used to deflect light rays in different directions(by discretely positioning the

moving mirror by using thediscrete angular positions of the Geneva wheel) thereby

givingrise to an optical switching technique.


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In the following sections, four slot and six slotGeneva wheels have been analysed

and a design layout hasbeen provided. Along the same lines, multiple slot wheels

canbe designed. The basic criterion that has to be maintained indesigning any

number of slotted Geneva wheel is that, the pinhas to enter and leave the slots

radially. This will again bediscussed in detail in the following sections.


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Internal Geneva Wheel

When the dwell period must be less than 180°, other intermittent drive mechanisms

must be used. The internal Geneva wheel is one way of obtaining this form of

motion. The main

advantage of the internal Geneva wheel, other than its smooth operation, is its

sharply defined dwell period. A disadvantage is the relatively large size of the

driven member, which

increases the inertial forces resisting acceleration/deceleration. For proper

operation, the

drive pin (crankpin) must enter and leave the slot tangentially. Structurally, the

internal Geneva wheel differs from the external Geneva wheel in that the distance

of the crank center from the wheel center is less than the wheel radius. However,

this leads to significant differences in the mechanics of the system. The dwell

period of all internal Geneva wheels is less than 180°, leaving more time for the

star wheel to reach maximum velocity, lowering the acceleration. The highest

value of acceleration occurs when the crankpin enters or leaves the slot, however

the acceleration curve does not reach a peak within the range of motion of the

driven wheel. The geometrical maximum would occur in the continuation of the

curve, but this continuation has no significance since the driven member will have

entered the dwell phase associated with the high angular displacement of the

driving member. This geometrical maximum falls into the region representing the

motion of the external Geneva wheel.

The design of the internal Geneva mechanism is very similar to that of the external

mechanism. The maximum angular velocity occurs when the crank angle is zero

with respectto the centerline c. The maximum angular acceleration occurs when the crank

enters the slot.

Figure 3 shows a plot of the angular acceleration of the wheel with respect to the

crank angle for 3, 4, 5, 6, 8, 10, and 12 slotted Geneva wheels with unit crank link

length (r1=1). It can be seen that there exits a non-zero angular acceleration


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component as the crankpin makes contact with the Geneva slot. In fact this is the

maximum angular acceleration of the system during the non-dwell phase. Once

again this leads to a singularity, hence an infinite jerk upon contact.

CNC Cut Geneva Drive

A Geneva Drive is defined by Wikipedia as “a gear mechanism that

translates a continuous rotation into an intermittent rotary

motion.” Although replaced by servo drives in many cases, these were once used

in movie projectors and to power rotary tables in industrial assembly lines. This iswhere I first saw this type of drive, and after redoing practically the entirety of

these machines, the venerable Geneva mechanism was still at the heart of it,

working like a champ after 20+ years. Like most cam-type devices, they may not

afford nearly the adjustability or the “easy” electrical programming of a servo of

PLC controlled device, but they will run for literally decades on end with little

maintenance.

So after that introduction, I recently machined one out of MDF on my router. I

plan on making one that works a bit better, and is possibly motor-driven, but check

out the video below for my prototype:

This is a crude model a this point, but I’ve included the G-code and DXFs for the

mechanism at the end of the article. Everything was cut with a 5/16 inch bit that

I’ve been using as a flycutter on my router, providing a relatively high pocketing

speed. Unfortunately, it doesn’t provide for cutting small holes. This could be

worked around by changing the tool, but it’s a prototype, so the middle hole was

crudely marked with the cutter, then drilled with my manual mill. The cutinaccuracy is, I would assume, partly to blame for the reverse not working as it

should in the video.

The design of this was done on Draftsight (see my review of this AutoCAD clone),

with few problems. I’ve never designed one of these before, and there are some

equations to use if you want to do it that way. Instead, I did it visually, drawing

http://en.wikipedia.org/wiki/Geneva_drivehttp://www.jcopro.net/2012/03/01/dramatically-improve-your-zen-toolworks-pocketing-speed-with-a-dremel-115-flycutter/http://www.jcopro.net/2012/03/01/dramatically-improve-your-zen-toolworks-pocketing-speed-with-a-dremel-115-flycutter/http://www.jcopro.net/2010/12/29/draftsight-review-like-autocad-but-free/http://www.jcopro.net/2010/12/29/draftsight-review-like-autocad-but-free/http://www.jcopro.net/2012/03/01/dramatically-improve-your-zen-toolworks-pocketing-speed-with-a-dremel-115-flycutter/http://en.wikipedia.org/wiki/Geneva_drive


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the basic circles first, making everything fit, and rotating each piece around the

central axis. This is probably easiest to do with a four-stroke device like I made,

but I’m sure other numbers could be done this way.

After I cut the two rotating elements out, I placed them both on a spare piece ofMDF, spaced out by the circular cavity, and drilled pilot holes for the axes. I then

put a few washers under the driven element and bolted them both down lightly. It

works OK, but I plan to redo this design so that the drive gear, the driven “cross”,

and the base can be cut out at one time with a CNC router. I’m sure I’ll have to do

some finishing work, especially if I end up putting a motor on it (maybe recycled

from the “rotary engine” model project). Be sure to check back on this project, or

just do yourself a favor and subscribe to the RSS feed!

The Geometry of the Geneva Mechanism

In the four slot Geneva stop, both wheels are the same size.

In Geneva drives with different numbers of slots a little geometry soon reveals the

relative sizes of the wheel.

http://www.jcopro.net/2011/12/08/another-radial-engine-model-attempt/http://feeds.feedburner.com/Jcoprohttp://feeds.feedburner.com/Jcoprohttp://www.jcopro.net/2011/12/08/another-radial-engine-model-attempt/


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For example, in the six-slot design, left, the slots are sixty degrees apart. The drive

pin needs to to enter the top of the slot at an angle of ninety degrees. With the

angles fixed, working out the dimensions is straight forward. In the six slot

example, construct a right angled triangle with one the the angles being sixty

degrees. The radii (radiuses?) of the two wheels are the lengths to the two shortestsides of the triangle and the wheels are separated by the length of the longest side.

The Geneva drive is named after the city of its invention where it was used in the

construction of clocks. Originally the Geneva mechanism was used as a way of

preventing springs from being wound too tight. One of the slots would be blanked

off so the winder could only be turned a fixed number of turns. This mechanism is

known as a Geneva stop or Geneva stop works.

In this case the spring would be connected to the smaller wheel, the slotted wheelis there to limit the number of turns. After five turns, the pin hits the blanked out

slot, arrowed, and the spring is prevented from being over wound.

The Geneva drive is used to provide intermittent motion, the drive wheel turns

continuously, the pin on the drive wheel then turns the cross shaped piece quarter

of a turn for each revolution of the drive wheel. The crescent shaped cut out in the

drive wheel lets the points of the cross pass, the rest of the circle locks the slotted

wheel into place while it is stationary. Drive motion can be changed by changing

the number of slots in the slotted wheel. The Geneva drive mechanism is used

commonly in film projectors to move the film on one frame at a time then hold it

stationary as the bright projector light is shone through it.

Specialized Mechanisms GENEVA WHEEL

MechanismThe Geneva drive or Maltese cross is a gear mechanism that translates a

continuous rotation into an intermittent rotary motion. The rotating drive wheel

has a pin that reaches into a slot of the driven wheel advancing it by one step. The


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drive wheel also has a raised circular blocking disc that locks the driven wheel in

position between steps.

The name derives from the device’s earliest application in mechanical watches,

Switzerland and Geneva being an important center of watchmaking.The Geneva

drive is also commonly called a Maltese cross mechanism due to the visualresemblance when the driven wheel has four spokes. They are used in watches and

for the main reason of being made small and can withstand mechanical stress.

In the most common arrangement, the driven wheel has four slots and thus

advances for each rotation of the drive wheel by one step of 90°. If the driven

wheel has n slots, it advances by 360°/n per full rotation of the drive wheel.

Analysis of the output motion of the cam

As discussed above, if the input member roller-crank completes one revolution, the

roller must traverse the open cam problem twice. Consequently, the output motion

of the cam while the

Roller retraces its path is governed by the problem that prescribes the motion of the

cam in the rest part of the cycle. It is now clear that we can specify the motion of

the cam in only a half the cycle and the remaining half is determined automatically,

thus limiting control over the complete motion. However, by studying the motion

in the second half we can design the cam problem to have better control over its

motion.


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Types Of Geneva Drive

As we know that GENEVA Mechanisms are very important in Automated machine

tools, today we look towards 20 different types of Geneva mechanisms. Geneva

mechanisms are nothing but the Indexing mechanisms like Dividing head. We can

adjust the Geneva mechanism so as to rotate or transfer work around the working

stations within some fixed Intervals

1) Locking-arm Geneva drive: The driving follower on the rotatinginput crank of this Geneva enters a slot and rapidly indexes the output. In this

version, the roller of the locking-arm (shown leaving the slot) enters the slot to

prevent the Geneva from shifting when it is not indexing.


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2) Planetary gear Geneva drive: The output link remains stationarywhile the input gear drives the planet gear with single tooth on the locking disk.

The disk is part of the planet gear, and it meshes with the ring-gear Geneva to

index the output link one position.

7

3) Four bar Geneva drive: A four-bar Geneva produces a long-dwell

motion from an oscillating output. The rotation of the input wheel causes a driving

roller to reciprocate in and out of the slot of the output link. The two disk surfaces

keep the output in the position shown during the dwell period.


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4) Twin Geneva drive: The driven member of the first Geneva acts as thedriver for the second Geneva. This produces a wide variety of output motions

including very long dwells between rapid indexes.

8

5) Groove cam Geneva drive: When a Geneva is driven by a rollerrotating at a constant speed, it tends to have very high acceleration and

deceleration characteristics. In this modifi

cation, the input link, which contains the driving roller, can move radially while

being rotated by the groove cam. Thus, as the driving roller enters the Geneva slot,it moves radially inward. This action reduces the Geneva acceleration force.


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6) Locking slide Geneva drive: One pin locks and unlocks the Geneva;

the second pin rotates the Geneva during the unlocked phase. In the positionshown, the drive pin is about to enter the slot to index the

Geneva. Simultaneously, the locking pin is just clearing the slot.

7) Rapid transfer Geneva drive: The coupler point at the extensionof the connecting link of the four-bar mechanism describes a curve with twoapproximately straight lines, 90° apart. This provides a favorable entry

situation because there is no motion in the Geneva while the driving pin moves

deeply into the slot. Then there is an extremely rapid index. A locking cam, which


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prevents the Geneva from shifting when it is not indexing, is connected to the input

shaftthrough gears.

8) Dual-track Geneva drive: The key consideration in the design ofGeneva’s is to have the input roller enter and leave the Geneva slots tangentially

(as the crank rapidly indexes the output). This is accomplished in the novel

mechanism shown with two tracks. The roller enters one track, indexes the

Geneva 90° (in a four-stage Geneva), and then automatically follows the exit slot

to leave the Geneva. The associated linkage mechanism locks the Geneva when it

is not indexing. In the positionshown, the locking roller is just about to exitfrom the Geneva.


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11) Internal groove Geneva drive: This arrangement permits the rollerto exit and enter the driving slots tangentially. In the position shown, the driving

roller has just completed indexing the Geneva, and it is about to coast for90° as

it goes around the curve. (During this time, a separate locking device might be

necessary to prevent an external torque from reversing theGeneva.)


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12) Progressive oscillating drive: A crank attached to the planet gearcan make point P describe the double loop curve illustrated. The slotted output

crank oscillates briefly at the vertical positions.

13) Sinusoidal reciprocator drive: This reciprocator transforms rotarymotion into a reciprocating motion in which the oscillating output member is in the

same plane as the input shaft. The output member has two arms with rollers which

contact the surface of the truncated sphere. The rotation of the sphere causes theoutput to oscillate.

14) Controlled output escapement: The output in this simplemechanism is prevented from turning in either direction unless it is actuated by the


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input motion. In operation, the drive lever indexes the output disk by bearing on

the pin. The escapement is cammed out of the way during indexing because the

slot in the input disk is positioned to permit the escapement tip to enter it. But as

the lever leaves the pin, the input disk forces the escapement tip out of its slot and

into the notch. That locks the output in both directions.

15) Parallel guidance drive: The input crank contains two planet gears.The center sun gear is fixed. By making the three gears equal indiameter and

having gear 2 serve as an idler, any member fixed to gear 3 will remain parallel to

its previous positions throughout the rotation of the input ring crank.


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16) Rotating-cam reciprocator drive: The high-volume 2500-ton press is designed to shape such parts as connecting rods, tractor track links, and

wheel hubs. A simple automatic-feed mechanism makes it possible to produce

2400 forgings per hour.


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17) An external Geneva drive. The driver grooves lock the driven wheel pins during dwell. During movement, the driver pin mates with the driven-wheel

slot.

18) An internal Geneva drive. The driver and driven wheel rotate in

same direction. The duration of dwell is more than 180º of driver rotation.


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19) A spherical Geneva drive. The driver and driven wheel are on perpendicular shafts. The duration of dwell is exactly 180° of driver rotation.

Design

Design of the Geneva wheel has been done using the 4-level polysilicon surface

micromachining technology by Sandia National Laboratories. All the four levels of

polysilicon are required for designing this mechanism. The SUMMiT process [2,3]

and the layout design have been discussed in detail in this section.

3.1 The Summits Process:-

The Sandia Ultra-planar, Multi-level MEMS Technology (Summit) process[2,3,4]

is a standard process developed by the Sandia National Laboratories. A cross-

section of the main layers in the process is shown in Fig.2. In this process four

layers of polysilicon alternated by sacrificial silicon dioxide layers are laid down.

The first level is a silicon dioxide and nitride stack layer. The oxide layer in this


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level is used as an insulating layer. he nitride layer acts as an etch stop and

protects this oxide layer when the sacrificial oxide etch is carried out. he four

polysilicon layers function as the structural layers used for developing various

micromachined structures. The oxide layers alternating between the polysilicon

layers are used to physically isolate the polysilicon layers. Once the wholestructure has been developed, these oxide layers are etched away and polysilicon

structures are released. The thickness of the various layers is given in Fig.2.

Fig.2. Layers of the SUMMiT process Courtesy: Sandia National Laboratories.

The two things that are unique to his process and which make a variety of designs

possible at the micro level are the conformal SACOX2 layer and planarized

SACOX3 layers.

Wheel Design:

Two types of slot designs for the Geneva wheel were considered. The designs were

laid out using the Cadence software for MEMS layouts. Since the technology file

available with the software allowed for only three levels f polysilicon, the

structural poly0 level was not laid down. For any N slotted wheel, the angle by

which the slotted wheel rotates for a given rotation of the constantly rotating wheel

is 2p/N. The slots are thus placed at 2p/N radians intervals. An important

requirement is that during every rotation, the pin should enter and leave the slots in

such a way that the tangent to the constantly rotating wheel at the pin passes

through the center of the slotted wheel. This means that if ‘r’ is the radius of the

constantly moving disk,then the distance ‘D’ between the centers of the two disks

has to be:

D = r/sin (p/N)

and the radius of rotation ‘R’ of the Geneva wheel is given by:R = r/tan( p/N)

The minimum length of the slot through which the pin on disk W moves should

be:

S = D-[(D-R)+(D-r)]


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= R+r-D

Applying these relations to the wheel shown in Fig.1 we get: Aa = ab = Ab/ 2

Fig.3. Layout of a four slot Geneva wheel using Cadence

The layout of a four slotted wheel in Cadence is shown in Fig. 3 with projections

on the constantly rotating wheel to so that it can be moved with a probe. Theconstantly rotating disk can be rotated using a Sandia microengine[5,6,7,8] driven

by comb-drives. The gears of the microengine can be made to mesh with the gears

of the constantly rotating wheel that can be provided on it. To avoid unintentional

rotation of the Geneva wheel, a truncated wheel (with a chopped arc angle of 4p/N

radians) is placed on the constantly moving disk, which stops any rotation of the

Geneva wheel when the pin is moving freely and is not engaged with any of the

slots.

The design should therefore, have the truncated disk and the Geneva wheel on the

same polysilicon layer and the constantly moving disk in another layer, which

would mesh with the microengine. The Geneva wheel and the chopped disk are

made on the Poly2 layer and the constantly moving disk lies below in the Poly1

layer. The engaging pin on this disk is placed on Poly2 layer. The pins holding the


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Geneva wheel and the other disks are then placed on Poly3 layer, which gets

contacted to the Poly0 layer and allows rotation of

the disks after release.

A gear can be fabricated on poly1 layer concentric with the Geneva wheel, whichcan then be meshed with a rack to convert the intermittent rotation of the disk into

discrete linear motion. This can then be applied to micromirrors and other systems

requiring such motions.


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Uses and applications

One application of the ×Geneva drive is in movie projectors: the film does not run

continuously through the projector. Instead, the film is advanced frame by frame,

each frame standing still in front of the lens for 1/24 of a second (and being

exposed twice in that time, resulting in a frequency of 48 Hz). This intermittent

motion is achieved using a Geneva drive. (Modern film projectors may also use an

electronically controlled indexing mechanism or stepper motor, which allows for

fast-forwarding the film.) The first uses of the ×Geneva drive in film projectors go back to 1896 to the projectors of Oskar Messter and ×Max Gliewe and the

Teatrograph of Robert William Paul. Previous projectors, including Thomas

Armat's projector, marketed by Edison as the Vita scope, had used a "beater

mechanism", invented by Georges Demenÿ in 1893, to achieve intermittent film

transport.

Geneva wheels having the form of the driven wheel were also used in mechanical

watches, but not in a drive, rather to limit the tension of the spring, such that it

would operate only in the range where its elastic force is nearly linear. If one of theslots of the driven wheel is occluded, the number of rotations the drive wheel can

make is limited. In watches, the "drive" wheel is the one that winds up the spring,

and the Geneva wheel with four or five spokes and one closed slot prevents over

winding (and also complete unwinding) of the spring. This so-called Geneva stop

or "Geneva stop work" was the invention of 17th or 18th century watchmakers.

http://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://en.wikipedia.org/wiki/Movie_projectorhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Stepper_motorhttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://en.wikipedia.org/wiki/Oskar_Messterhttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Max%20Gliewehttp://en.wikipedia.org/wiki/Robert_William_Paulhttp://en.wikipedia.org/wiki/Thomas_Armathttp://en.wikipedia.org/wiki/Thomas_Armathttp://en.wikipedia.org/wiki/Thomas_Edisonhttp://en.wikipedia.org/wiki/Georges_Demen%C3%BFhttp://en.wikipedia.org/wiki/Watchhttp://en.wikipedia.org/wiki/Spring_%28device%29http://en.wikipedia.org/wiki/Elastic_forcehttp://en.wikipedia.org/wiki/Elastic_forcehttp://en.wikipedia.org/wiki/Spring_%28device%29http://en.wikipedia.org/wiki/Watchhttp://en.wikipedia.org/wiki/Georges_Demen%C3%BFhttp://en.wikipedia.org/wiki/Thomas_Edisonhttp://en.wikipedia.org/wiki/Thomas_Armathttp://en.wikipedia.org/wiki/Thomas_Armathttp://en.wikipedia.org/wiki/Thomas_Armathttp://en.wikipedia.org/wiki/Robert_William_Paulhttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Max%20Gliewehttp://en.wikipedia.org/wiki/Oskar_Messterhttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://en.wikipedia.org/wiki/Stepper_motorhttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Movie_projectorhttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Geneva


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Other applications of the ×Geneva drive include the pen change mechanism in

plotters, automated sampling devices, indexing tables in assembly lines, tool

changers for CNC machines, banknote counting and so on. The Iron Ring Clock

uses a Geneva mechanism to provide intermittent motion to one of its rings. A

Geneva drive was used to change filters in the Dawn mission framing Camera usedto image the asteroid 4 Vesta in 2011. It was selected to ensure that should the

mechanism fail at least one filter would be usable.

Internal Geneva drive

Internal Geneva drive.

Animation showing an internal Geneva drives in operation.

An internal ×Geneva drive is a variant on the design. The axis of the drive wheel of

the internal drive can have a bearing only on one side. The angle by which the

drive wheel has to rotate to effect one step rotation of the driven wheel is alwayssmaller than 180° in an external ×Geneva drive and always greater than 180° in an

internal one, where the switch time is therefore greater than the time the driven

wheel stands still.

The external form is the more common, as it can be built smaller and can withstand

higher mechanical stresses.

http://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://en.wikipedia.org/wiki/Plotterhttp://en.wikipedia.org/wiki/Indexing_%28motion%29http://en.wikipedia.org/wiki/CNChttp://en.wikipedia.org/wiki/Banknotehttp://en.wikipedia.org/wiki/Iron_Ring_Clockhttp://en.wikipedia.org/wiki/Dawn_%28spacecraft%29http://en.wikipedia.org/wiki/4_Vestahttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://en.wikipedia.org/wiki/Stress_%28mechanics%29http://en.wikipedia.org/wiki/Stress_%28mechanics%29http://en.wikipedia.org/wiki/Stress_%28mechanics%29http://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Genevahttp://en.wikipedia.org/wiki/4_Vestahttp://en.wikipedia.org/wiki/Dawn_%28spacecraft%29http://en.wikipedia.org/wiki/Iron_Ring_Clockhttp://en.wikipedia.org/wiki/Banknotehttp://en.wikipedia.org/wiki/CNChttp://en.wikipedia.org/wiki/Indexing_%28motion%29http://en.wikipedia.org/wiki/Plotterhttp://search.conduit.com/Results.aspx?ctid=CT3327494&searchsource=55&UM=2&q=Geneva


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Assembly Drawings


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Kinematics

Motion curves for one turn of the drive wheel, from top to bottom: angular position

θ, angular velocity ω, angular acceleration α and angular jerk ja.

The figure shows the motions curves for an external four-slot Geneva drive, in

arbitrary units. There is a discontinuity in the acceleration when the drive pin

enters and leaves the slot. This generates an "infinite" peak of jerk (Dirac peak),

and therefore vibrations.

http://en.wikipedia.org/wiki/Jerk_%28physics%29http://en.wikipedia.org/wiki/Dirac_distributionhttp://en.wikipedia.org/wiki/Dirac_distributionhttp://en.wikipedia.org/wiki/Jerk_%28physics%29


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Geneva mechanism

Geneva mechanism, also called Geneva Stop, one of the most commonly used

devices for producing intermittent rotary motion, characterized by alternate periods

of motion and rest with no reversal in direction. It is also used for indexing (i.e.,

rotating a shaft through a prescribed angle).

In the Figure the driver A carries a pin or roller R that fits in the four radial slots in

the follower B. Between the slots there are four concave surfaces that fit the

surface S on the driver and serve to keep the follower from rotating when they are

fully engaged. In the position shown, the pin is entering one of the slots, and, on

further rotation of the driver, it will move into the slot and rotate the follower

through 90°. After the pin leaves the slot, the driver will rotate through 270° while

the follower dwells— i.e., stands still. The lowest practical number of slots in a

Geneva mechanism is 3; more than 18 are seldom used. If one of the slot positions

is uncut, the number of turns that the driver can make is limited. It is said that theGeneva mechanism was invented by a Swiss watchmaker to prevent the over

winding of watch springs. For this reason it is sometimes called a Geneva stop.

Early motion-picture projectors used Geneva mechanisms to give the film a quick

advance while the shutter was closed, followed by a dwell period with the shutter

open.

http://www.britannica.com/EBchecked/topic/537596/shafthttp://www.britannica.com/EBchecked/media/61209/http://www.britannica.com/EBchecked/topic/566289/stillhttp://www.britannica.com/EBchecked/topic/636716/watchhttp://www.britannica.com/EBchecked/topic/229000/Genevahttp://www.britannica.com/EBchecked/topic/229000/Genevahttp://www.britannica.com/EBchecked/topic/636716/watchhttp://www.britannica.com/EBchecked/topic/566289/stillhttp://www.britannica.com/EBchecked/media/61209/http://www.britannica.com/EBchecked/topic/537596/shaft


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APPLICATION OF GENEVA MECHANISMS IN INDUSTRY

Geneva mechanisms are generally used in induction unit. In industry we use

this cam in pharma machines. This cam has great precision work.

There are three basic types of Geneva motion uses in our industry as shown in thefigures below.

External the most common type.

Internal

Spherical

This is rarely used.


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Because the driven wheel in a Geneva motion is always under full control ofthe driver there is no problem with overrunning. Impact is still a problem unlessthe slots of the driven wheel are accurately made and the driving pin enters theseslots at the proper angle. For best results the pin should be shaped so that the pin

picks up the driven member as slowly as possible. Impact can also be reduced byleaving the top and bottom of the slot open. The fingers that form the slot will thenhave some resiliency. However strength is of primary importance and the slotmust be bridged by a web.

External Geneva and Internal Geneva have been used for both light and heavyduties. They are frequently used as inputs to high speed devices e.g. high speedmechanical counters use a Geneva between the first and second wheels. Mutilated

pinions, which connect succeeding stages, could not absorb the shocks transmittedfrom the first to the second wheel.

When input and output shafts must be perpendicular few intermittent mechanismsare as suitable as the spherical Geneva, but this type is bulky and not practical forsignificant power levels. Molded or cast spherical Geneva’s are adequate for light

duty applications.

Typical Geneva with special characteristics are those driven by 4 bar linkages forimproved acceleration characteristics, Geneva’s with variable dwells , Geneva’sused as planets in planetary chains and those combined with cycloid cranks.


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Mutilated Gears

Gears can be used in several ways to produce intermittent motion. A typical unitis the "mutilated gear" shown in the figure below. In this case some of the teeth

have been removed from the driver and a partial holding surface has been added toeach gear to prevent slight rotation of the drum gear during the dwell period.

Mutilated gears can be run without holding rings but it is not desirable no matterhow slow the motion, the teeth of the driver will sooner or later top the teeth of theoutput gear. Since the teeth will meet near a centerline, even small input torquecan produce large toggle forces that can damage the teeth.

Mutilated gears of the type shown in the figure below are subject to large impact

loads and accelerations if the driving speed is high. The shape of the first teeththat will mesh is sometimes modified to reduce impact but only a slight advantageis gained. Attempts have been made to slack mount the first teeth but only a slightadvantage is gained. Geneva or star wheels are usually preferred for high speedsand for high power applications.

A desirable feature with mutilated gears is their indexing accuracy and in additionto the inherent accuracy of the gears, the output is always under control on the

input. Mutilated gears as shown below are used in almost all counters, they areinexpensive reasonably precise and efficient. They stand up well under the type ofloading found in instruments. The mutilated pinions as shown below are virtuallyidentical to the gear above except that the locking ring on the output gear has beeneliminated. Every other tooth on the input end of the pinion has been cut away sothat the remaining teeth can hold the pinion during dwell periods. In counters thedriver has only two teeth, but it can have any even number of teeth.


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Cycloid Gears

With cycloid intermittent gearing the input and output remains in constantmesh. Cycloid gearing provides considerable latitude in selection of operatingcharacteristics- decelerations, dwell periods, ratio of input to output motions

etc. A basic cycloid mechanism is shown in the figure below. In this type thedrive pin or roller must on the pitch circle of the planet gear if the output crank isto stop, otherwise, the output will either slow and not stop or actually reverse themotion.

There are many other variations of this type of mechanism including hyper-cyclonical, epicyclical & peri-cycloidal arrangements. These devices are very


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versatile and can be used with Geneva’s for additional output variations.

The arrangement as shown below can be classed with the hyper-cycloidalgear arrangements since the driver moves around the inside of a ring gear. In thiscase however the driver is constrained from rotating by a fixed pin. The inputshaft turns the eccentric, which is mounted with a sliding fit within the internalgear and is concentric with it. The amount the ring gear, is indexed by the internalgear is determined by relative size diameter of the eccentric and location of the

pin. In this case the ring rotates 36 degrees for every 360 degrees of input motion,remaining at rest for the remaining 324 degrees of rotation. Accelerations are lowand the two gears are always in mesh. Since the inner gear is really only link of afour bar mechanism, the sliding pin can be replaced by a link loosely pinned to the

gear and to the frame. This arrangement is reliable, inexpensive, quiet andcompact. A company "Ikongear" manufactures a gear reduction design similar, in

principle, to this mechanism. Below.

Star Wheels

A different type of intermittent motion mechanism is the star wheel. In thearrangement shown in the figure below pins are used as teeth on the driver, butinvolutes teeth can be used instead. This is another versatile mechanism. It

provides considerable freedom in choosing operating parameters. The output wheel


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for example can be made to rotate more than one revolution. This is not possiblewith pure simple Geneva’s. Star wheel devices can rotate at different amounts ateach index point. Accelerations and decelerations can be controlled more readilythan in a mutilated gear pair." By careful shaping of the teeth. Internal pairs arealso possible.

Cams

Various types of cams can be used to produce intermittent indexing rotations. As

an example the scroll-shaped disc cam showed below indexes a wheel 180o when

the solenoid pulls the levers down and a further 180o when the solenoid is releases

the levers.


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A face cam as shown below is also often used for indexing. The reciprocatingdrive arm moves a pin or roller back and forth in the zig -zag groove in the face ofthe wheel. This simple arrangement is used in moderate speed counters. As withmany inexpensive cam drives efficiency is not high. Impact is light particularly ifthe drive arm is itself driven by a properly shaped cam. In this form of drive thereis little danger of over travel.

A cylindrical cam as shown below can serve as the driver in another type ofindexing drive. a typical commercial unit can handle moderate to high loads atspeeds of 1000 steps /minute.

The cam system shown below is used often for Sequenced grabs. Every time thecentre shaft is lowered down the ring in which internal slots are machined iscaused to rotate a fixed angle. This is used for sequenced grabs in which each


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alternate ring position conforms to a grab open position and the other positionsconform to a grab locked closed position.

The mechanism shown below is a diagrammatic representation of a ball pointretracting mechanism. In practice this mechanism is a cylindrical mechanismarranged such that at each press of the end projection the pen is sequentiallyextended and retracted. The diagram below show the cylinder flattened out toillustrate the action.

One significant advantage of cam drives over most other intermittent motion drivesis that the cams can be shaped to control such dynamic factors as impact,acceleration and dwell periods. However since small changes in cam contour canresult in significant changes in performance each design must be tailored to the

particular application.


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Advantages

• Available in a wide variety of sizes.

• Maintains good control of its load at all times.

• Have little wear leading to a very long life span.

Disadvantages

• Limited number of dwells from 3 to 18 per rotation.

• Has a greater instantaneous change of acceleration than a cam mechanism.

• Very difficult to change timing once design is chosen.


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Acknowledgement

We wish to thank Prof. K.S.J. Pister, Department of Electrical Engineering,

University of California, Berkeley for this invaluable guidance during the course ofdesigning this system. We also thank Elliot Hui and Karen Cheng, University of

California, Berkeley for giving us great technical assistance during the design of

this system.


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References

[1] I. I. Artobolevsky, Mechanisms in modern engineering design, Vol. III, MIR

Publications Moscow, 1979.

[2] Sniegowski J. F., “Chemical Mechanical Polishing: Inhancing the

manufacturability of MEMS”.

[3] Sniegowski J. F. and Rodgers M. S., “Manufacturing Microsystems-on-a-chip

with a 5-Level micromachining technology”.

geneva cam mechanism

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