proyecto final maquinas 2 (informe y apendice con drawing) (1)

7/31/2019 Proyecto Final Maquinas 2 (Informe y Apendice Con Drawing) (1)

1/35

Polytechnic University Puerto RicoHato Rey, PR

Department of Mechanical Engineering

Final Project:Design of a Jackscrew


2/35

Abstract

In this group project we were presented with a great job in our hands developing a

screw jack. The first part of the developing we gathered information to know the best

way to start the design with the right tools according to the requirement made by the

professor. Our Jack needed to lift 2tons and have a maximum lift of .8m. The material

use for the jack is steel 1030. We did various test to assure our jack works correctly

and with efficiency, body stresses, even the buckling consideration. An automotive jack

is a device used to raise all or part of a vehicle into the air in order to facilitate repairs.

Any time a jack is used, it's critical that the vehicle be in a stable position on a flat

surface. Be sure that the jack is pushing up against a solid frame member that will

support the weight of the vehicle, or else you will need to repair more than your tire.


3/35

Table of Contents

Abstract ...................................................................................................................... 2

Introduction ................................................................................................................ 4

Theory ........................................................................................................................ 5

Materials and strength ................................................................................. 6

Mechanical Analysis .................................................................................... 7

Tensile Strength ........................................................................................... 8

Types of Screw .......................................................................................... 10

Power Screws ............................................................................................ 10

Bending Stress........................................................................................... 12

Shear Stress .............................................................................................. 12

Tensile /Compressive stresses .................................................................. 12

Combined stresses .................................................................................... 13

B kli t 13


4/35

IntroductionA mechanical jack is a device which lifts heavy equipment. The most common form

is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be

performed. Car jacks usually use mechanical advantage to allow a human to lift a

vehicle by manual force alone. More powerful jacks use hydraulic power to provide

more lift over greater distances. Mechanical jacks are usually rated for a maximum

lifting capacity.

For this design it is required for the jack to be able to have a lifting capacity of

2Tons and a lifting clearance of .8m.
http://en.wikipedia.org/wiki/Mechanical_advantagehttp://en.wikipedia.org/wiki/Hydraulic_machineryhttp://en.wikipedia.org/wiki/Hydraulic_machineryhttp://en.wikipedia.org/wiki/Mechanical_advantage


5/35

Theory

A screw is one of the six simple machines. A simple screw is a helical inclined plane.

A screw can convert a rotational force (torque) to a linear force and vice versa. The ratio

of threading determines the mechanical advantage of the machine. More threading

increases the mechanical advantage. A rough comparison of mechanical advantage

can be done by taking the circumference of the shaft of the screw and divide by the

distance between the thread

A screw is a shaft with a helical groove or thread formed on its surface and provision

at one end to turn the screw. Its main uses are as a threaded fastener used to hold

objects together, and as a simple machine used to translate torque into linear force. It

can also be defined as an inclined plane wrapped around a shaft.
http://en.wikipedia.org/wiki/Simple_machinehttp://en.wikipedia.org/wiki/Inclined_planehttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Mechanical_advantagehttp://en.wikipedia.org/wiki/Mechanical_advantagehttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Inclined_planehttp://en.wikipedia.org/wiki/Simple_machine


6/35


7/35

The same type of screw or bolt can be made in many different grades of

material. For critical high-tensile-strength applications, low-grade bolts may fail,resulting in damage or injury. On SAE-standard bolts, a distinctive pattern of marking is

impressed on the heads to allow inspection and validation of the strength of the bolt.

However, low-cost counterfeit fasteners may be found with actual strength far less than

indicated by the markings. Such inferior fasteners are a danger to life and property

when used in aircraft, automobiles, heavy trucks, and similar critical applications.Gradings are indicated as markings, while grade 0 is the lowest, grade 10 is the

highest. Here is the sequence of bolt strength and markings, from least to most. Grade

0, 1 and 2 bolts have no markings, grade 3 has 2 radial lines, grade 5 has 3, grade 6

has 4, grade 7 has 5, grade 8 has 6, grade 9 has 7, grade 10 has 8.

In some applications joints are designed so that the screw or bolt willintentionally fail before more expensive components. In this case replacing an existing

fastener with a higher strength fastener can result in equipment damage. Thus it is

generally good practice to replace fasteners with the same grade originally installed.

Mechanical Analysis

A screw or bolt is a specialized application of the inclined plane. The inclined
http://en.wikipedia.org/wiki/Counterfeithttp://en.wikipedia.org/wiki/Counterfeit


8/35

is very similar to that performed to predict wedge behavior. Wedges are discussed in

the article on simple machines.Critical applications of screws and bolts will specify a torque that must be applied

when driving it. The main concept is to tension the bolt, and compress parts being held

together, creating a spring-like assembly. The stress thus introduced to the bolt is

called a preload. When external forces try to separate the parts, the bolt experiences no

strain unless the preload force is exceeded.As long as the preload is never exceeded, the bolt or nut will never come loose

(assuming the full strength of the bolt is used). If the full strength of the bolt is not

used (for example, a steel bolt threaded into aluminium, then a thread-locking adhesive

or insert may be used.

If the preload is exceeded during normal use, the joint will eventually fail. Thepreload is calculated as a percentage of the bolt's yield tensile strength, or the strength

of the threads it goes into, or the compressive strength of the clamped layers (plates,

washers, gaskets), whichever is least.

Tensile Strength

Screws and bolts are usually in tension when properly fitted. In most applications
http://en.wikipedia.org/wiki/Simple_machinehttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Spring_%28device%29http://en.wikipedia.org/wiki/Strain_%28materials_science%29http://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Washer_%28mechanical%29http://en.wikipedia.org/wiki/Gaskethttp://en.wikipedia.org/wiki/Gaskethttp://en.wikipedia.org/wiki/Washer_%28mechanical%29http://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Strain_%28materials_science%29http://en.wikipedia.org/wiki/Spring_%28device%29http://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Simple_machine


9/35

tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt

has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield

strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.

Tensile ultimate strength is the stress at which the bolt fails (breaks in half).

Tensile yield strength is the stress at which the bolt will receive a permanent set (an

elongation from which it will not recover when the force is removed) of 0.2 % offset

strain. When elongating a fastener prior to reaching the yield point, the fastener is saidto be operating in the elastic region; whereas elongation beyond the yield point is

referred to as operating in the plastic region, since the fastener has suffered permanent

plastic deformation.

Mild steel bolts have property class 4.6. High-strength steel bolts have property

class 8.8 or above. An M10, property class 8.8 bolt can very safely hold a static tensileload of about 15 kN.

There is no simple method to measure the tension of a bolt already in place

other than to tighten it and identify at which point the bolt starts moving. This is known

as 're-torqueing'. An electronic torque wrench is used on the bolt under test, and the

torque applied is constantly measured. When the bolt starts moving (tightening) thetorque briefly drops sharply - this drop-off point is considered the measure of tension.

Recent developments enable bolt tensions to be estimated by using ultrasonic
http://en.wikipedia.org/wiki/Yield_strength#Definitionhttp://en.wikipedia.org/wiki/Yield_strength#Definitionhttp://en.wikipedia.org/wiki/Newtonhttp://en.wikipedia.org/wiki/Newtonhttp://en.wikipedia.org/wiki/Yield_strength#Definitionhttp://en.wikipedia.org/wiki/Yield_strength#Definition


10/35


11/35

torques. Steel nuts are used for only occasional adjustment and limited duty so as to

avoid galling of like materials.

Standard pitches for metric diameters


12/35

Bending Stress

The maximum bending stress occurs at the root of the thread. It is calculated byassuming the thread is a simple cantilever beam built in at the root. The load is

assumed to act at mid point on the thread. The maximumum stress is provided by the

bending moment relationship M/I = /(y) =e/R. that is = M.y/I

The section under bending has a length = .dm.n

The width of the section at the thread root = b. The Moment of Inertia at the root I = .dm.n.b

3 /12

The distance from the centroid the the most remote fibre ..y = b/2.

The Bending Moment M = W.h/2

The maximum bending stress is therefore.

Shear Stress

Both the nut and screw threads are subject to traverse shear stress resulting from

the bending forces. For a rectangular section the maximums shear stress occurs at the

neutral axis and equals


13/35

Combined stresses

Based on maximum shear stress theory...

The shear stress caused by torque on the screw =

The value of the combined stress is therefore

This equation always applies when the screw is in tension. When a screw is in

compression and the length is greater than 8 time the root diameter then the buckling

stress has to be considered.

Buckling stress

When the screw is longer than 8 times the root diameter it must be considered a

column. Long columns with are dealt with using the Euler equation. Columns with

slenderness ratios of less than 100 are considered as short columns. The slenderness

ti i th l th (b t t ) / L t di f ti f th ti


14/35

Analysis of ResultsData and Assumption:

Coefficient of Friction:

Safety Factor:

Single thread:

Screw, Crown and Base Material:

Carbon Steel 1030 (C, 0.27% - 0.34%; Fe, 98.67% - 99.13%; Mn, 0.6% - 0.9%; P,

0.04% ; F, 0.05% )

Given Force:


15/35

Using the Bearing System table for Dimensional Series 02, the approximated valueof C is:

Dimensions of the Bearing ( ):

Calculus to find : Part 2: Buckling Analysis


16/35

Part 3: Maximum Diameter (), Minimum Diameter (), Mean Diameter ()y Pitch (

)

According with the Square Thread Table for Metric

(http://en.wikipedia.org/wiki/Square_thread_form)

For a Maximum Diameter of 44mm, the correct pitch is 7mm.

Part 4: Calculus to find the Lead:

Part 5: Thread Dimensions
http://en.wikipedia.org/wiki/Square_thread_formhttp://en.wikipedia.org/wiki/Square_thread_formhttp://en.wikipedia.org/wiki/Square_thread_form


17/35

Part 7: Calculus to find the Load-Lowering Torque

Part 8: Calculus to confirm the Self-Locking between the power screw and

the crown

According with our design, the power screw has Self-Locking.

Part 8: Efficiency:


18/35

Part 11: The bearing stress B is, with one thread ( ) carrying 0.38F Part 12: The thread-root bending stress b with one thread ( ) carrying0.38F

Part 13: Tri-Axial Analysis and Distortion Energy Theory (von Mises stress)

( ) ( ) ( )

[ ]


19/35

Part 14: Maximum Shear

The Maximum Shear is lower than the Allow Shear, therefore our design accomplish

with the basic fundamentals of mechanical to avoid mechanical failure due to the

chosen material and apply forces


20/35

Mohr Circle Diagram for three-dimensional stress


21/35

Conclusion

By studying the conditions under which the jack will be subjected to we were able to

identify the principle dimensions of the power screw. For a major diameter of 44 mm we

find that the screw will withstand the loads associated with that of a power screw.

Moreover, with a applied load of 2 tons and the allow force equation we find the minor

diameter for the our power screw, the value of this diameter is 35.12 mm. Using the

Square Thread Table for Metric, the maximum diameter is 44 mm and the pitch is 7

mm.

After calculating the dimensions of the power screw and the axial bearing we

calculate the Load-Raising Torque and Load-Lowering Torque, our results is 278.93

Nm and 233.48 Nm, respectively. Assuming a friction coefficient of 0.2 and lead of

7mm our power screw has self-locking. The efficiency of this power screw is 7.83%.

According with assigned load of 2 tons and our minor diameter of 37mm the power

screw jack wont buckle assuming a safety factor of 3. After calculating the Shear and

Axial stress we find the 3 axial stresses and 3 shear stresses for the three-dimensional


22/35

References

Screw Jacks . (n.d.). Engineering ToolBox. Retrieved October 17, 2010, from

http://www.engineeringtoolbox.com/screw-jack-d_1308.html

Budynas, R., & Nisbett, J. K. (2006). Shigley's Mechanical Engineering Design (Mcgraw-Hill

Series in Mechanical Engineering) (8 ed.). New York: McGraw-Hill

Science/Engineering/Math.

Jackscrew - Wikipedia, the free encyclopedia. (n.d.). Wikipedia, the free encyclopedia. Retrieved

October 17, 2010, from http://en.wikipedia.org/wiki/Jackscrew


23/35

Appendix


24/35

All units are in milimeter and de gree

CC

Group #3 - Jac k screw:Assembly Detail

SECTION C-C

SCALE 1 : 3

SCALE 1 : 30

D

DETAIL DSCALE 1 : 1

70

30

45

6215

44

20R

138

28

201

12

20


25/35

Fina l Projec t: Jac k Sc rew

Axia l BearingSHEET 1 OF 1SCALE: 1:2 WEIGHT:

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

ENG A PPR.

CHEC KED

DRAWN

Group # 3

All units are in m ilimeter a nd de gree

ITEM NO. PART NUMBER QTY.1 inner housing 1

2 ba ll b earing 243 outer housing 1

3

1

19

118

110

2

62


26/35

Group # 3


All units are in milimet er and de gree

CHEC KED

ba ll bea ringSHEET 1 OF 1SCALE: 4:1 WEIGHT:

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

ENG A PPR.

DRAWN

12


27/35

Group # 3



ENG A PPR.

SCALE: 1:16

CHEC KED

SHEET 1 OF 1baseWEIGHT:

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

DRAWN

819.793

C

C

SECTION C-CSCALE 1 : 4

110

670

30 50.706

120.00

38.119

120.00

120.00

105.750

R

853.822

25

15


28/35

5

396

800

1716.822

2

Group # 3


1

3

4


120.00

439.822

R69

ITEM NO. PART NUMBER QTY.1 power sc rew 1

2 lever 13 Axia l Bearing 1

4 base 15 tube 1

ENG A PPR.

CHEC KED

SCALE: 1:25Emsablaje2SHEET 1 OF 1WEIGHT:

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

DRAWN


29/35

CC

DETAIL DSCALE 1 : 1

R

15

28

20

30

70

138

45

20

62

441

12

20

SECTION C-C

SCALE 1 : 3

SCALE 1 : 30

D

Group #3 - Jac k screw:Assembly Detail


30/35

Group # 3


All units are in m ilimeter a nd de gree

ENG A PPR.

CHEC KED

inner housingSHEET 1 OF 1SCALE: 2:3 WEIGHT:

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

DRAWN

12

110 R1

70

4

4


31/35

Group # 3



ITEM NO. PART NUMBER QTY.

1 power sc rew 1

2 lever 13 Axia l Bearing 1

4 base 15 tube 1

CHEC KED

Jack ScrewSHEET 1 OF 1SCALE: 1.20

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

ENG A PPR.

DRAWN

5

3

2

1

4


32/35

SECTION A-A

B

38

30

138

DETAIL BSCALE 3 : 2

3.500

3.500

7


Group # 3

All units are in milimete r and d egree

CHEC KED

lever SHEET 1 OF 1SCALE: 1:2 WEIGHT:

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

ENG A PPR.

DRAWN

15

A

A

20R

126R

62

62


33/35


34/35

SECTION A-ASCALE 1 : 10

B

44

DETAIL BSCALE 3 : 2

3.50

3.50

7100

100 R20


Group # 3


CHEC KED

power sc rewSHEET 1 OF 1SCALE: 1:10 WEIGHT:

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

ENG A PPR.

DRAWN

850

15

A

A


35/35

R2

R2


Group # 3

All units are in milimete r and d egree

CHEC KED

tube SHEET 1 OF 1SCALE: 1 : 4 WEIGHT:

REVDWG. NO.

A

SIZE

TITLE:

NAME DATE

CO MM ENTS:

Q.A.

MFG A PPR.

ENG A PPR.

DRAWN

15 20

400

30

proyecto final maquinas 2 (informe y apendice con drawing) (1)

Documents