me 222: kinematics of machines and...

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Suril V. Shah

ME 222: Kinematics of Machines and Mechanisms

[L9] Practical Considerations

1/24/19 1IIT Jodhpur

Suril V. Shah

Some practical considerations

Pin Joints versus Sliders and Half JointsCantilever or Straddle Mount?Short LinksBearing RatioCommercial SlidesLinkages versus CamsSelection of Motors and Drivers

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Suril V. Shah

Practical considerationsMany factors need to be considered to create good-quality designs

Not all of them are contained within the applicable theories

A great deal of art based on experience is involved in design as well.


Suril V. Shah

At joint two materials rub together

Such a rubbing interface is called a bearing.

Proper material and good lubricationare the key to long life of joint.

Choice of joint type affects ability to provide good clean lubrication

Pin Joints versus Sliders and Half Joints


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Simple revolute joint is the clear winner for several reasons

Easy and inexpensive to design and to build a good quality pin joint

In its pure form—also called sleeve or journal bearing

Geometry of pin-in-hole traps a lubricant film within its interface

Promotes a condition called hydrodynamic lubrication in which the parts are separated by a thin film of lubricant

Replacement lubricant can be done through radial holes

Seals can easily be provided at the ends of the hole

Revolute (Pin) Joints


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Available in large variety of sizes for revolute joints

Relatively inexpensive

Available pre-lubricated and with end seals.

Good ability to trap lubricant within the roll cage

Relatively high rolling speed of the balls or rollers

Promotes long life

Ball and roller bearing


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This has a spherical, sleeve-type bearing

Self-aligns to an out of parallel shaft

Its body threads onto the link, allowing links to be made from round stock with threaded ends

Allow adjustment of linklength.

Spherical Rod End Bearings


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Pillow blocks and flange-mount bearings

Fitted with rolling or sleeve-type journal bearings.

Pillow block: mounting to a surface parallel to the pin axis

Flange mount: fasten to surfaces perpendicular to the pin axis.

Revolute joints pivoted to ground


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Require a carefully machined and straight slot or rod

Bearings often must be custom made

Lubrication is difficult to maintain as not geometrically captured

Lubrication is resupplied either by running the joint in an oil bath or by periodic manual regreasing.

An open slot tends to accumulate airborne dirt

This can act as a grinding compound when trapped in the lubricant and accelerate wear.

Prismatic (Slider) Joints


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Suffer even more acutely from the slider’s lubrication problems

Two oppositely curved surfaces in line contact, tend to squeeze any lubricant out of the joint.

Joint needs to be run in an oil bath for long life.

Requires housing the assembly in an expensive oil-tight box with seals

Higher (Half) Joints


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These joints are used extensively in machinery with great success.

Some common examples can be found in an automobile. The windshield wiper mechanism is a pin-jointed linkage. The pistons in the engine cylinders sliders and bathed in oil. The valves are operated by cam-follower and drowned in oil.

You probably change your engine oil fairly frequently. When was the last time you lubricated your windshield wiper? Has this linkage (not the motor) ever failed?

An example


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A cantilevered joint has the pin (journal) supported only.

It is necessary when a crank must pass over the coupler.

Cantilever beam is weaker than a straddle-mounted beam.

Straddle mounting can avoid applying a bending moment to the links by keeping the forces in the same plane.

Straddle-mounted pin is in double shear a cantilevered pin is in single shear.

It is good practice to use straddle-mounted joints wherever possible.

Cantilever or Straddle Mount?


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If crank is short it is not possible to provide suitably sized pins or bearings at each of its pivots.

The solution is to design the link as an eccentric crank

The fixed pivot is placed a distance e from the center of this circle equal to the required crank length.

Lubricating large-diameter journal can be difficult.

Short Links


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Need for straight-line motion requires extensive use of linear sliding joints.

There is a very basic geometrical relationship called bearing ratio, which if ignored leads to several problems.

The bearing ratio (BR) is defined as the effective length of the slider over the effective diameter of the bearing:

BR = L / D.

Bearing ratio


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Effective length is the distance over which the moving slider contacts the stationary guide

Effective diameter is the largest distance across the stationary guides in a plane perpendicular to motion

A common example of a device with a poor bearing ratio is a drawer in an inexpensive piece of furniture

You have probably experienced the sticking and jamming that occurs with such a drawer

For smooth operation this ratio should be greater than 1.5, and never less than 1


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Pin-jointed linkage has all the advantages of revolute joints

Cam-follower has all the problems associated with the half joint.

Pin-jointed linkage with good bearings is a potentially superior design, and should be the first explored

Linkages have relatively large size compared to the output displacement in comparison to Cam.

Linkages are relatively difficult to synthesize, and cams are relatively easy to design.

Dwells are easy to get with cams, and difficult with linkages.

Linkages versus Cams


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Linkages can survive hostile environments, with poor lubrication, whereas cams cannot

Linkages have better high-speed dynamic behavior than cams.

Linkages are less sensitive to manufacturing errors, and can handle very high loads.

Cams can match specified motions better.

So the answer is far from clear-cut. It is another design trade-off situation.

Linkages versus Cams: Comparison


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Need of driver to provide the input motion and energy.

Continuous rotary input: Motor is the logical choice.

Motors come in a wide variety of types. Most common energy source for a motor is electricityAir and hydraulic motors

Motors and drivers


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Motors provide continuous rotary output.

Stalled momentarily against a load

It can not tolerate a full-current, zero-velocity stall for more than a few minutes without overheating.

Electric motors are classified by Electrical configurationFunction or application Rated power

Electric motors


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Electrical configuration


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Functional classificationsGear motorsServomotorsStepping motors

Rated powerSubfractional (> 1/20 HP)Fractional (1/20 - 1 HP)Integral (> 1 HP)


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Have electrical configurations: permanent magnet, shunt-wound, series-wound, and compound-wound

Rotating armature coils connected to the stationary field coils—in parallel (shunt), in series, or in combined series-parallel (compound)

Permanent magnets replace the field coils in a PM motor.

Each configuration provides different torque-speed characteristics.

Torque-speed curve describes how motor will respond to an applied load

DC motors


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Torque varies greatly with speed

power = torque x angular velocity.

Torque is maximum at stall (zero velocity)

This is an advantage when starting heavy loads (unlike IC engine)

Permanent Magnet DC Motors


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Flatter slope around the rated torque

Less speed-sensitive to load variation

It stalls very quickly

Typically used on fans and blowers.

Shunt-wound DC Motors


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More speed-sensitive (large slope)

Starting torque as high as 800% of full-load

No theoretical maximum no-load speed

Run away if the load is removed.(Maximum speed 30,000 rpm.)

Gives a “soft-start”

Sewing machines and portable grinders

Series-wound DC Motors


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Combination of shunt-wound and series-wound

Speed sensitivity is greater than a shunt-wound but less than a series-wound

It will not run away when unloaded.

Has high starting torque and soft-start capability

Good choice for cranes and hoists that experience high inertial loads and can suddenly lose the load due to cable failure

Compound-wound DC Motors


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Controller increases and decreases the current against changing load to maintain speed.

Speed-controlled (typically PM) DC motors will run from an AC source.

Cost of such motor is high.

Speed control DC motors


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Least expensive way to get rotary motion.

Synchronous motors “lock on” to the AC line frequency and run exactly at synchronous speed.

Nonsynchronous motors have a small amount of slip and lag the line frequency by about 3 to 10%.

The synchronous motor speed ns = 120f/p.

AC motors


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Shaded pole and permanent split capacitor: starting torque lower than their full-load torque.

Split-phase and capacitor-start motors: boost the start torque

Single-phase motors


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Differ mainly in their starting torque and speed sensitivity near full-load point.

Three-phase motors

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Motor’s output shaft is connected by gearbox

Gearmotor with integral gearbox is available

Available in variety of output speeds and power ratings.

Gearmotors


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Servomotors are made in both AC and DC configurations

Closed-loop-controlled (sensors for position/velocity feedback)

Precise positioning of the output device

Capable of holding a fixed position against a load.

Have fast-response

Aircraft and guided missiles, numerically controlled machine, robots

Servomotors


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Brushless permanent magnet motors.

Intermittent motion devices (no continuous rotary motion)

They run open loop, without feedback

Resolution depends on the number of magnetic strips.

Relatively small and have low drive torque capacity

Have high holding torque.

Expensive and require special controllers

Stepper Motors


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Require a compressed air or hydraulic source.

Less energy efficient due conversion of the energy.

Air motors used factories where compressed air is available.

Cost of all the ancillary equipment is high.

Hydraulic motors are found cranes, aircraft, and ships.

Air and Hydraulic motors


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These are linear actuators (piston in cylinder)

Provide a limited stroke, straight-line output

Input of either compressed air or hydraulic fluid

Share the high cost, low efficiency

Farm and construction equipment

Air and Hydraulic Cylinders


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Electromechanical (AC or DC) linear actuators

Share some of the limitations of air cylinders

Energy inefficient

Very short strokes (about 2 to 3 cm) length

Develop a force that varies exponentially over the stroke

Inexpensive, reliable, and have very rapid response times.

They cannot handle much power.

Used in camera shutters, electric door or trunk locking systems in automobiles

Solenoids


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