iNIGER DELTA UNIVERSITY

WILBERFORCE ISLAND

BAYELSA STATE

DEPARTMENT OF MECHANICAL ENGINEERING

FACULTY OF ENGINEERING

A PROJECT ON

SCISSOR LIFT DESIGN FOR USE IN THE AUTOMOTIVE INDUSTRY

PRESENTED

BY

MACAULAY OLETU STANLEY

UG/07/0898

IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THEAWARD OF A BACHELOR OF ENGINEERING (B.Engr.) DEGREE.

SUPERVISOR: Engr. Dr. E.M. ADIGIO

AUGUST 2012

ii

CERTIFICATION

This is to certify that this project work was carried out and successfully completed by

MACAULAY OLETU STANLEY of the department of mechanical engineering, Niger

Delta University, Wilberforce Island, Amassoma, Bayelsa state, during the 2011/2012

Academic session.

Sign. . . . . . . . . . . . . . . . . . . . . . . . . .25/08/2012 . . . . .Engr. Dr. E.M. ADIGIO Dateproject supervisor

Sign . . . . . . . . . . . . . . . . . . . . . . . . . . 25/08/2012. . . .Engr. Dr. E.M. ADIGIO DateHead of Department

Sign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Prof. C. J. IGBEKA DateDean, Faculty of Engineering

Sign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Name of External Examiner Date

iii

DEDICATION

This project work is dedicated to all Nigerian Student who are working endlessly to

make the Nation Great, and also not forgetting all Nigeria youths who are willing to be

educated but does not have the opportunity to go to school. This work is also dedicated

to my parents, my uncles whose financial help, prayers, love and care, has been a source

of encouragement all this years. And above all to God almighty.

iv

ACKNOWLEDGEMENT

I am indeed grateful to God Almighty for his grace and enduring mercies, guidance,

protection, favour throughout this programme.

I also express my gratitude to my project supervisor, Dr. E. M. Adigio for his support

and word of encouragement to me in course of my research.

I also wish to express my gratitude to all the lecturers of the department of mechanical

engineering who took me one course or the other in course of my training, Dr. E.M.

Adigio, Prof. L.A. Salami, Prof. O.O. Oyeameobi, Dr. E.E. Jumbo, Dr. Charles, Dr. Okah-

Avae, Engr. N.A. Yousuo, Engr. M.G. Anomeidei, Engr. A.R.S. Dienagha, Mr. P.Y.

Olisa, Mr. Eremasi, Mr.Amakiri, Mr. ThankGod, Ms. Amos, and many others too

numerous to mention who have imparted me in one way or the other, may God

almighty bless you all.

I also acknowledge my parent Mr. and Mrs. MACAULAY OKOLOSI for their prayer

and support.

Also to all my friends,; Ikue Hope, Eviano Felix, Afriki Preye, Uwazor Moses, Akpekpe

Jirius, William Kenneth, Pena Blaise, my roommate; Michael Obaro, and to all my

entire course mate and others too numerous to mention.

vCONTENTSCERTIFICATION ........................................................................................................................................ iDEDICATION ........................................................................................................................................... iiiACKNOWLEDGEMENT......................................................................................................................... ivCHAPTER 1 ................................................................................................................................................11.0 INTRODUCTION ..........................................................................................................................11.1 PROBLEM STATEMENT..............................................................................................................2

1.1.1 PROPOSED SOLUTION ..........................................................................................................21.2 CHALLENGES/DEFICIENCY FACED WITH THE USE OF VARIOUS LIFTINGPLATFORM ................................................................................................................................................3

1.2.1 HYDRAULIC LIFTING PLATFORM:....................................................................................31.2 2 ELECTRIC LIFTING PLATFORM..........................................................................................51.2.3 PNEUMATIC SYSTEM/AIRBAG ..........................................................................................61.2.4 SCISSOR LIFT................................................................................................................................. 6

1.3 AIMS/OBJECTIVES ......................................................................................................................7CHAPTER TWO.........................................................................................................................................82.0 LITERATURE REVIEW.......................................................................................................................82.1 LIMITING DEFLECTION IN SCISSOR LIFT ............................................................................11

2.3 SUMMARY OF DEFLECTION ........................................................................................................12CHAPTER THREE ...................................................................................................................................143.0 METHODOLOGY, DESIGN ANALYSIS .......................................................................................143.1 CONCEPT SELECTION....................................................................................................................143.1.1 LIFT FRAME ................................................................................................................................143.1.2 SCREW SELECTION ..................................................................................................................143.1.3 BEARING SELECTION..............................................................................................................143.1.4 LOCKING WHEEL .....................................................................................................................153.1.5 SYSTEM ANALYSIS ...................................................................................................................153.1.6 FRAME .........................................................................................................................................15

3.2 SCISSOR LIFT FRAME EQUATION...............................................................................................173.2.1 EQUATION OF THE SCISSOR LIFT FRAME ........................................................................18

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3.2.2 EQUATION FOR THE POWER SCREW DESIGN ................................................................243.2.3 NUT DESIGN ..............................................................................................................................273.2.4 BEARING SELECTION..............................................................................................................283.2.5 SCREW SHAFT STRESS.............................................................................................................283.2.5.1 CRITICAL LOCATIONS.....................................................................................................283.2.5.2 FATIQUE FAILURE OF THE SHAFT...............................................................................29

3.3 WHEELS..............................................................................................................................................29CHAPTER FOUR .....................................................................................................................................314.0 INPUT SPECIFICATIONS ................................................................................................................314.1 STRENGTH AND RIGIDITY OF THE SYSTEM ...........................................................................364.2 RESULT AND DISCUSSION ...........................................................................................................37CHAPTER 5 ..............................................................................................................................................395.0 CONCLUSIONS AND RECOMMENDATION.............................................................................39REFERENCES ...........................................................................................................................................41APPENDIX....................................................................................................................................................43DRAWINGS ..................................................................................................................................................43

1CHAPTER 1

1.0 INTRODUCTION

According to Tredgold (1827), engineering is the act of directing the great source of

power in nature for the use and convenience of man. Engineers, like artist start with

blank sheet of paper on which ideas develop and take conceptualized shapes. In the

ancient time, man applied engineering knowledge to reduce difficult and complex task

to easy and simplified task. Before the invention of weight lifting device such as screw

jack, hydraulic jack, crane, etc., the early man apply a crude way of lifting objects to

great heights through the use of ropes and rollers, which was mostly applied in the

construction industry, where, it was used to raise mortar (cement, sand & water).

After the industrial revolution, with the advent of automobile, the automotive industry

was also faced with the challenge of load lifting, because of the bulkiness of some

automotive parts. The automotive industry deals with various components made of

metal, rubber, ceramics, etc., assembled mechanically to move people and goods from

one place to the other. Because of the interface between the automobile and human

lives, there is need for standardization of its component parts to improve its

performance and efficiency and to reduce failure. For this reason care has to be taken

during production and assembly of its component parts. Many tools and equipment

used in the automotive industry are designed to help the personnel working in a

production facility. Other tools are produced to help the operators of the machine. Such

2tools include the lifting device, generally called jack. This report presents the study of a

scissors lift for the automotive industry.

1.1 PROBLEM STATEMENT

Automotive parts are mostly made of metal, which is a major reason for its large

weight, and as such requires devices of lifting and displacement of same. In an

automobile production and assembly facility, components have to be raised to certain

heights which could be more convenient to the personnel working on it. When such

device is not available workers are often forced to bend from the waist to access the

components which can lead to strains and major discomforts or even serious injuries

that could affect productivity

Consequently, an assembly table that will be adjustable will be required for use in the

automotive industry to improve the efficiency of personnel working in a production or

assembly facility. In order to do this, a mechanism is recommended to be incorporated

into a table platform where the height is adjustable.

1.1.1 PROPOSED SOLUTION

In recent years, various platforms or devices with various means of application have

been produced for use in the automotive industry. The automotive industry have also

experience the influx of various lifting platform, some of which are;

3? Electrically operated lifting device which is operated by the turning effect of electricmotor to drive the gear which will eventually turn a screw shaft to raise or lower

load.

? Hydraulic operated lifting platform which utilize the pressure power developedfrom hydraulic oil to raise or lower a load.

? Pneumatic lifting device which make use of air to create pressure or vacuum to raiseor lower load.

? Recent research also shows the use of air bag for raising or lowering load. (MichaelAdel, PE (2008): Understanding Scissors Lift Deflection).

All this lifting devices have contributed greatly to the advancement recently being

experienced in the automotive industry, but most of them are still faced with various

challenges. This report presents a scissors mechanism with a table platform that will be

horizontal at every level. The proposed mechanism is a double scissors for stability.

1.2 CHALLENGES/DEFICIENCY FACED WITH THE USE OFVARIOUS LIFTING PLATFORM

1.2.1 HYDRAULIC LIFTING PLATFORM:

The hydraulic lift makes use of fluid pressure to produce smooth movement during

lifting. It has some benefits when compared to other lifting device; firstly, its

dependency on power supply is eliminated. Secondly, it allows smooth movement

without jerking due to steady increase in fluid pressure, majority of lift platform in

market make use of hydraulic. Above all it has a high capacity in terms of load lifting.

4In conclusion, hydraulic lifts are heavier because of the amount of fluid in circulation in

the system, in extreme cold conditions, the fluid can falter or get frozen which might

leads to leakage in hydraulic lines or pipes. The challenges of this system are;

a) It is not economical to the common technician or artesian.

b) It requires trained personnel to operate it.

c) Since it make use of oil, it require a temperature range for it proper storage.

d) It is very difficult to move from place to place due to its complex design.

e) Studies have shown that hydraulic lift that operates on two cylinders at most time

experience delay in one of the cylinder to actuate due to poor cross feeding between

cylinders.

f) Sometime debris from improperly preserved oil block oil tubes and at times

disrupts proper functioning of the system.

g) There is always problem of valve failure.

h) Hydraulic system requires too many accessories to function efficiently.

i) There is risk of slipping while working with hydraulic system, due to leakage that

might emanate from the system.

j) Hydraulic system is not flexible for usage because its component parts are not fully

attach as a whole.

k) There is frequent problem of seal leakage.

l) Aging problem of oil leads to failure in valves and shorter life of pumps.

m) There is problem of accumulation of debris in oil tank.

51.2 2 ELECTRIC LIFTING PLATFORM

These lift devices make use of electromagnetic power to raise or lower through the use

of electric motor. The device could be very expensive and there is high probability of

jerking during startup of the device through the torque created by the electric motor.

The device could be put to stand still during electricity/power outage, and there is

potential of electrocution when electric cables are exposed. The challenges of this

system are;

a) Due to frequent raising and lowering of the lift, there is possibility of snapping in

the electric cable which could lead to exposure of the cable and could lead to

electrocution.

b) It requires other accessories to be operated.

c) Electric lift cannot be used where the electric power is fluctuation.

d) It requires trained personnel to operate it successfully.

e) It requires regular maintenance.

f) The electrical control unit must not be exposed to water or higher temperature.

g) Electrically operated solenoid valve could easily get damage during operation

with irregular voltage supply.

h) Over heating in electrical coil could damage the system.

i) Fuses easily blow-out when they are used as safety device.

j) Dirt in electrical system could also lead to malfunctioning of the system.

k) It is expensive to acquire.

61.2.3 PNEUMATIC SYSTEM/AIRBAG

This device also operates like the hydraulic device, but it acquires its driving

force/pressure from the air. Testimonies from operators of this device show that the

failure rate of the device is very high due to frequent air leakage during operation as a

result of failure in valves. The challenges of this system are;

a) There is high risk of air leakage.

b) Pneumatic systems are frequent with valve leakage.

c) The air bag is not flexible during usage.

1.2.4 SCISSOR LIFTA scissor lift is a device used for lifting purposes, its objectives is to make the table

adjustable to desirable height.

A scissor lift provide the most economic dependable and versatile methods of lifting

loads, it has few moving parts, which may only require lubrication. This lift table raises

the load smoothly to any desired height. The scissor lift can be used in combination

with any of the previously mentioned application, i.e. hydraulic, pneumatic or

electrical. In order to reduce the inadequacies of the devices mentioned above, a scissors

mechanism is proposed. This mechanism is incorporated with a power screw and the

top of the scissors is attached a table platform. This device will make use of the power

generated from a power screw to raise or lower a platform manually.

71.3 AIMS/OBJECTIVES

The aim of this study is to design a scissors lifting device that can be used in the

automobile sector. The design conditions are to meet the following specifications;

a) The device is limited to an average load of 280 kg

b) The device will have a maximum lift of 150 cm

c) This objective is desirable to be achieved through the rotation of the screw to raise or

lower the scissor platform.

d) The system must be operated on a flat surface.

8CHAPTER TWO

2.0 LITERATURE REVIEW

A lifting device is a system that allows small force (effort) to overcome a large force or

load (Alexander et al, 1978, Smith, 1981, Nelkon, 1985). There are practically hundreds

of uses for lift tables in manufacturing, warehousing and distribution facilities. The

addition of this device (lift table) makes job faster, safer and easier. Some typical

applications include; machine feeding and off-loading, product assembly, inspection

quality control repair, feeding and offloading conveyor levels. The commonest method

for operating a scissors lift is the use of a power screw. According to Allens et al, (1980)

power screws are devices that provide means for obtaining large mechanical advantage.

Other researchers (Jain, 1988, Shigley and Mischke, 2001) define the power screw as a

device used in converting rotary motion into uniform longitudinal motion

The manually operated scissor lift is a device that makes use of a horizontally placed

power screw to overcome large load through less effort applied on the power screw, by

turning the power screw with the aid of a ratchet handle on one side of the device. The

device is capable of lifting an average load with little effort applied.

One of the most important factors of lift platform is its stability. Knowing that stability

is a source of concern for a lift platform, its positioning should be on a flat surface and

the load should be place or concentrated at the centre of gravity of the table. Other

constraint to be considered is the deflection of the unit. Deflection in scissor lift can be

9defined as the resulting change in elevation of all parts of a scissor lift assembly,

typically measured from the floor to the top of the platform deck, whenever load is

applied to or removed from the lift (Michael-Adel, 2008). The ANSI MH29.1 safety

requirement for industrial scissor lifts states that All industrial scissor lift will deflect

under load. The industry standard goes on to outline the maximum allowable

deflection base on platform size and number of scissor mechanism within the lift design

(WBC Standards, 1989).

Scissor lift deflection becomes more critical in material handling applications where the

lift must interface with adjoining, fixed elevations, especially when transferring rolling

load. In these cases, it is important that any difference in elevation between adjoining

surfaces during material transfer be minimize or if not totally eliminated.

Before attempting to discuss how to limit scissor lift deflection, it is important to

understand the contributing factors to a lift total deflection. An open or raised scissor

acts very much like a spring would apply a load and it compresses, remove a load and

it expands. Such component within the scissor lift has the potential to store or release

energy when loaded and unloaded (and therefore deflects). There are application-

specific characteristics that may promote deflection, understanding these root causes

helps to pin-point and apply effective measures to limit them

(www.autoquip.com2011).

Leg deflection due to bending is as a result of stress which is driven by the total weight

supported by the legs, scissor leg length, and available leg cross section. The longer the

10

scissors leg length, the more difficult it is to control the bending under load

(www.autoquip.com). Other important component is the platform structure.

Platform bending will increase as the load centre of gravity move away from the centre

(unevenly distributed load) or at an edge (eccentric loading) of the platform. Also as the

scissor open during rising of the lift, the rollers could roll back towards the platform

hinges and create an increasingly unsupported overhung portion of the platform

assembly. Increase platform strength via increase support structure material height

does improve resistance to deflection, but also contributes to an increase collapsed

height of the lift (www.autoquip.com).

The lift base frame is usually placed on the floor and may not experience deflection. For

cases where the scissor lift is mounted to an elevated or portable frame, the potential for

deflection increases. To effectively resist deflection, the base frame must be rigidly

supported from beneath to support the point loading created by the two scissor leg

hinges(www.autoquip.com ).

Scissors lifts are assembled with pins at all the hinge points and each pin has a running

clearance the outer diameter of the pin and the inner diameter of its clearance hole or

bushing. The more scissor pairs or pantographs, that are stack on top of each other, the

more pinned connections there are to accommodate movement, or deflection, when

compressing these running clearances under load.

11

Load placement also plays a large role in scissor lift deflection. Off-centre load causes

the scissor lift to deflect differently than with centre or evenly distributed loads. End

load (in line with the scissor) are usually share well between the two scissor leg pairs.

Side load (perpendicular to the scissor) however are not share as well between the

scissor legs pairs, and must kept within acceptable design limits to prevent leg

twist(unequal scissor leg deflection)which in addition to platform movement due to

deflection, often result in poor roller tracking, unequal axle pin wear, and misalignment

of cylinder mounts (www.autoquip.com).

As mention above, degree of deflection is directly related to change in system pressure

and change in component stress as a result of loading and unloading. Scissor lift

typically experiences their highest system pressure and highest stress (and therefore the

highest potential for deflection) with the first 20% of the total available vertical travel

from the fully lowered position (www.autoquip.com).

2.1 LIMITING DEFLECTION IN SCISSOR LIFT

Selecting a lift with design capacity greater than required for the application; most

scissor lift design for duty at higher capacities will experience less stress in all structural

components as well as lower system pressures, at lower, working capacities. Reduced

stresses and pressure always result in reduced deflection.

Avoid transfer of load within the first 20% of lift travel: To minimize stress and

deflection, at transfer elevations, it is critical to design the conveyor or transfer system

12

to ensure that these elevations are above the scissor lifts critical zone of the first 20%

of the lift available travel.

Transfer load over fixed end of the lift platform: First if possible, load should not be

transfer over the sides of a raised scissor lift is much more difficult to control deflection

when the load is not shared equally between the two scissor legs pairs. Load transfer

should be made over the hinge or fixed end of the lift platform to avoid placing

concentrated load on the less supported, over hung end of the platform, provided the

platform is equipped with trapped roller or is otherwise capable of withstanding this

edge loading without risk of platform tipping up or losing contact with the rollers.

Ensure that the based frame is lagged down and fully supported:

First, base frame should be adequately attached to the surface on which they are

mounted. Base frame that are not bolted, welded or otherwise attached to withstand the

upward force created by the eccentric loading of the platform will contribute to

deflection by bending or moving while resisting such forces. Bases must also be rigidly

supported beneath the entire perimeter of the frame in order to withstand without

deflection on the four point loads imposed upon the frame from above by the four

scissor-legs. Two moving roller and two fixed hinge points.

2.3 SUMMARY OF DEFLECTION

Although, there are few literatures on the design of scissors lift, this chapter has

highlighted the features and constraints in the design and fabrication of a prototype

13

unit. ANSI MH29.1 accurately points out that it is the responsibility of the

user/purchaser to advise the manufacturer where deflection may be critical to the

application. It has been noted that there are industrial best practices which can be

applied to reduce the impact or amount of deflection being experienced.

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CHAPTER THREE

3.0 METHODOLOGY, DESIGN ANALYSIS

In this section all design concepts developed will be discussed and based on evaluation

criteria and process developed followed by a design.

3.1 CONCEPT SELECTION

The design was selected from an already made product in the market with modification

in various parts and section to further enhance the functionality of the design.

3.1.1 LIFT FRAME

A scissor lift design was chosen because of its ergonomics as compared to other heavy

lifting devices in the market. Scissor lift frame are very sturdy and strong with increase

structural integrity.

3.1.2 SCREW SELECTION

The horizontal spindle screw selection was selected from a variety of screw application,

because of its important to the scissor lift, and after much consideration a final decision

would be a square thread.

3.1.3 BEARING SELECTION

The inclusion of a bearing is to reduce the effort required to turn the screw spindle,

knowing that since the device is to be operated manually, it would not be an easy task

15

turning the spindle to either raise or lower load on the platform. The bearing will be

attached to one end of the spindle, while the other end will be attached to a handle.

3.1.4 LOCKING WHEEL

A wheel is attached to the device to enable it to be movable from one place to the other,

but a locking device would also be mounted for safety.

With much attention on the above mentioned design considerations, the optimum aim

is the manufacturability, functionality and the economic availability of the design, in

general its ergonomic advantages.

3.1.5 SYSTEM ANALYSIS

Mathematical model will be developed for all the components of the device and parts of

the design to prove its performance in real sense. The system will be divided into the

following for proper presentation of fact and figures to ease design effort:

a) Frame

b) Screw spindle

c) Wheel

3.1.6 FRAME

The frame is made up of the scissor arms which are acting as the support to the entire

structure. A table bed platform will be at the top of the scissor link arm. Also a similar

bed will be at the bottom of the frame to accommodate the scissor link mechanism

when fully collapsed.

16

Figure 3.1 shows a schematic drawing of a scissor frame with 2-tier and the forces. It has

six possible ways of application of load, but for the scope of this study, only three of

them will be discussed. The alphabet at any particular point is used to identify linear

force but with a subscript to identify where the force is acting on. At each joint there is

also six possible forces and moment, but only few will be taken into consideration

because of the symmetry of various joint and parts of the structure, i.e. reaction forces at

similar but opposite point will also be similar but might be of different direction when

all conditions are met. M will stand for the moment about any point, while will be the

weight inherent in the system (i.e. weight). Also W will be used to represent applied

load i.e. , , & in the X, Y, & Z directions.

Figure 3.1 A Schematic Drawing of a Scissors Lift Showing the Forces

17

3.2 SCISSOR LIFT FRAME EQUATION

For a careful and proper calculation of the reacting force throughout the lift, it will be

proper to begin at the top of the lift where a known magnitude of load is applied. Then

using the principle of static equilibrium, reaction forces at the first level will be gotten.

The forces at the top of the second level will also be known because it is equivalent to

the reaction forces at the first tier, because they are equal and opposite forces. Also

reaction forces at the bottom of the second tier can also be determined.

Figure 3.2 Schematic Drawing of Scissors Structure

For easier calculations in order to reduce computations in the analysis, a number of

assumptions were made. Calculations will be made without recourse to the weight or

reaction force of the power screw spindle on the entire structure. It will also be assumed

that all four joints at the bottom are pinned to the ground as in Figure 2. It will also be

assumed that all frictional forces are negligible so that the principle of conservation of

energy is applied so that power screw forces could be calculated directly. It is also

assumed that at the top, the joint at either the front or the back are attached via roller,

while the others via pins.

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3.2.1 EQUATION OF THE SCISSOR LIFT FRAME

With various possible ways of loading;

1.) Load type 1: centered load in the negative y-direction(normal ,loading)

Figure 3.3 Schematic Drawing of Scissors Structure Showing Vertical Load

Total load in the system will be the applied load plus the weight of the scissor lift.

.. Total weight of the system in the negative Y-direction will be

= + u 3.1Since it is vertical and centered load, the reaction forces on the left side of the lift are

identical to the right side. This implies that reaction forces are symmetrical about the y-

z plane.

From Figure 3.4a below;

, , & = /4 = ; = 0, F =0.The reaction forces from the first tier will be

19

2 2 = 0. 3.2Assuming that;

2 = ( ) ; = ; then = ; where + = 2 ; M=0

Figure 3.4 Schematic Drawing Showing Load Distributions

Figure 3.5 Triangles of Forces

M = F sin+ . cos+ . cos+ I( ) sin. cos + . cos = sin ( )

cd/2

F

20

cos + = sin ( ).

= tan ( ).= (Change in reaction force between the two tiers)

Let = , = , = =Since one part of the joint at the top is connected to a roller, equal zero.

3.3

Solving for , =0, + +

= ( ) = 3.4In Summary

Load type 2; moment about the z-axis

= = = = = +4 .= = = +2 tan .= +tan . = 0.

21

Note; only the top joint that is connected with pin that will support load, the roller end

will not support load in the x-direction.

= cos (Force couple). = . =0 (from fig 2b).cos + cos = 0 3.5

= . = 0

Figure 3.6 schematics of loading resulting in moment in z-direction

a b

Figure 3.7 reacting forces due to moment in z-direction

22

+ + = 0 .= =

= 0.cos + cos + sin = 0 3.6

Since = 0, = 0. i.e. + = 0= 0.

The above equation shows that the forces at the 2nd tier are identical to the load at the

1st tier. this implies that reaction forces at 2nd level is the same as the 1st level and only

the force at the 1st level need to be determined. Therefore the remaining equations are;

= 0 + + = 0From figure 2c above

= = = . = 0.+ = 0, = = 0Summarily;

= = .= = .= = .= = = 0

23

Load type 3; moment about x-axis

.

Due to symmetry in the y-z plane, , = , = , = .Also since the applied has no z-component all reaction load in the z-direction are zero.

I.e. the front half of the lift as in above in fig 3.8b.

= 0. = 0.= .= 0.

Figure 1.7 schematic of loading resultingin moment in x-direction

a b

cFigure 3.8 schematic of loading resulting in moment in x-direction and reacting forces in b and c above

24

+ = 0=

The fourth type of loading would be moment in the y-axis, but this type of loading do

not have applied load (I.e. at the top of the lift) so therefore it will not be considered

since it is unrealistic.

N.B: this type of loading is statistically indeterminate.

3.2.2 EQUATION FOR THE POWER SCREW DESIGN

In power screw application, effort is applied at the mean radius of the screw by one

revolution and the load is lifted axially by the pitch p of the thread, for a single start

thread, = tan ( ). (Jindal U.C. 2010).

NB: root diameter = , thread thickness at root t= , shearing area per thread inscrew= , number of thread in nut supporting the load n= =The choice of square thread was necessary because loss of motion could be tolerated

but cant be tolerated in trapezoidal or acme thread. (Version 2ME, IIT Kharagpur).

Summarily;= 0= == = = 0

25

The minor diameter can be obtained from the formula = (shigley, 2008).Where = axial stress in the body of the screw due to load F. also the helix angle of the

thread could be estimated assuming the load is concentrated at a point as below.

Figure 3.9a schematic of a screw

Figure 3.9b schematic of square thread

tan = ( ) (Allens et al 1980).Where =helix angle of the thread in degree, n=number of engaged threads. P=pitch of

the thread, =mean diameter of the screw spindle.

25

The minor diameter can be obtained from the formula = (shigley, 2008).Where = axial stress in the body of the screw due to load F. also the helix angle of the

thread could be estimated assuming the load is concentrated at a point as below.

Figure 3.9a schematic of a screw

Figure 3.9b schematic of square thread

tan = ( ) (Allens et al 1980).Where =helix angle of the thread in degree, n=number of engaged threads. P=pitch of

the thread, =mean diameter of the screw spindle.

25

The minor diameter can be obtained from the formula = (shigley, 2008).Where = axial stress in the body of the screw due to load F. also the helix angle of the

thread could be estimated assuming the load is concentrated at a point as below.

Figure 3.9a schematic of a screw

Figure 3.9b schematic of square thread

tan = ( ) (Allens et al 1980).Where =helix angle of the thread in degree, n=number of engaged threads. P=pitch of

the thread, =mean diameter of the screw spindle.

26

Coefficient of friction in the screw spindle thread, tan = (smith 1981).Where =coefficient of friction, =friction angle in degree.

Effort required to raise the load = tan( + ) = , where = tan

Turning moment of the screw to raise the load = tan( + )NB: if > , after removal of effort P, the load will come down without any rotationalmoment on the nut. But if < , the load will remain in position after removal of theeffort. i.e. (self locking).

The effort required to lower the load will be = tan( ).While the turning moment required to lower the load = tan( )

Shear stress develop in the screw = = ( )Shear area per thread in nut is = =

Shear stress develop in the nut = Bearing area per thread = ( ) = (2 )Bearing stress develop in screw and nut = ( )

Maximum principal stress developed in the screw = + ( ) +

27

BEARING STRESS DEVELOPED( )= (shigley 2008).

Where =no of engaged thread.The principal stresses are found as follows:

= + + 4 (Stephens 1979)

Maximum shear stress; = ( ) . ( ) . ( )

Where = 0, = , = (Shiglet & Mischke 2001)

{ = 0, = , = 0.3.2.3 NUT DESIGN

The main item in the design of the nut is the height; it depends on the bearing pressure

between thread of the screw and nut (Jain 1988).

Bearing pressure = , where n =number of thread in contact with nut.

The height of the nut h will be h= (Jain 1988).

Therefore the shear stress in the thread of the nut can be obtained by the formula

= (Jain 1988).

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Where t is the thickness or width of the thread.

3.2.4 BEARING SELECTION

The type of bearing suitable for this type of loading will be the single-row-deep-groove

roller bearing. It is selected because of its increased load capacity when force is applied

radially

Figure 3.10 ball bearings

3.2.5 SCREW SHAFT STRESS

3.2.5.1 CRITICAL LOCATIONS

There will be no normal stress due to bending at one end of the screw shaft spindle

because it is attached to a bearing (Shigley 2008).

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Axial stress is also neglected because of the attachment of bearing. So therefore, only

bending and torsion stress will be considered.

[ = , = , = , = = ]. (shigley, 2008)Where = alternating bending stress, =mid range bending stress, = alternating

shear stress, = mid range shear stress, = alternating bending stress, =bending

moment for mid range, =alternating bending torque, & =mid range torque.

Where = 1 + ( 1), = 1 + ( 1) From chart of notch-sensitivity for steel.3.2.5.2 FATIQUE FAILURE OF THE SHAFT

Since the bending and torsion will be steady due to the stress distribution along the

shaft (ST. VENANT. principle) because of equal geometry along the screw pitch, the

expression for a suitable diameter of the screw spindle to withstand fatigue failure will

be

d={ [ (4 + 3 ] + [4( ) + 3( ) ] }

3.3 WHEELS

Wheels are made up of mild steel having diameters of 150mm and shaft diameter of

25 mm. the wheels are chosen on the base on the design load criteria which can

sustain the external load and well as the equipment load during transpiration in

industrial line. The main function of using wheels for this equipment is that machine

30

can be moved from corner to the other corner of the industry premises as per the

requirement to lift the load.

31

CHAPTER FOUR

4.0 INPUT SPECIFICATIONS

Research carried out in the automobile industry reveals that one of the heaviest

components of the automobile car is the car engine which is of average weight of

272 kg. By this factor, our control weight would be 280 kg for safer design of the scissor

lift table platform for assembly purposes in the auto industry.

5cm

75cm

5cm 20cm

75cm140cmcm

Figure 4.1: schematics of scissor lift specification

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Table 4.1 Input specifications

S/N COMPONENT symbols Value Unit

1 Max allowable load W 280 kg

2 Inherent weight in the system 150 kg

3 Length of each scissor arm D 140 Cm

4 Max. lift of the platform 150 Cm

5 Table platform dimensions L (150 90) CmSCREW SPINDLE SPECIFICATION

6 Mean diameter of screw spindle 30 Mm

7 Pitch of the thread P 6 Mm

8 No of engaged thread N 1

= cos =31From fig 4.1,

75cm

5cm

20cm

5cm

140cm

Figure 4.2: force resolution in lift mechanism

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Reaction forces along the horizontal direction at the end of the first tier.

= = = = +2 tan = 280 + 1502 tan 31 = 357.82Horizontal force at the middle of the first tier = = = 715.6For load type 2, force couple = = = = 215

= 215 , = , = = .

= 0, = 0, = 0

For loading type (3).

= 0, = 0, = 0 & = 0.= =

Where b=80cm=0.8m, due to the symmetry of the system, = & = . = 2 = 2 0.8 215 = 344 .NB: The effort required to raise the load will be equivalent to the from the power screw.

Given n=1, p=6, = 0.15( & ) {Design & system mechanicalhandbook 2nd edition, Mc Graw hill 1985}

= 2 cos = 2 1.40 cos 31 215 = 516 = 51.600= = cos = 2 = 5161.4 cos 31 = 430

34

L=np=1 6 = 6, = 33 , (normal series square thread) {Design & system mechanicalhandbook 2nd edition, Mc Graw hill 1985}

= = 33 = 30 = 0.03, = = 33 6 = 27 = 0.027= (280 + 150) = 430 , = tan ( ) = tan = 3.643( ).

Axial stress = = ( ) . = 608.32 = tan = tan (0.15) = 8.53( , > )Effort required to raise the load = tan( + )= 430tan( 3.643 + 8.53)=92.75NEffort required lowering the load = tan( ) = 430 tan(8.53 3.643) =36.76N

= 430 0.032 tan 12.173 = 1.39 Turning moment of the screw to raise the load= 2 tan( + )

Turning moment of screw to lower the load

= tan( ) = 430 . tan 4.887 = 0.551 Efficiency of the screw (square thread) = = ( )= ( ) = . . = 0.295 = 29.5% ( < 50%, . ) (machine

design by Judal. U. C.)

The effort required at the handle;

35

From = , = , = .From above, = 1.39 , = 150 = 0.15 .Effort required at the handle= . . = 9.27 Shear stress developed in the screw = = ( )Shear stress developed in the screw =

= ( ) = ( ) = 1.69 = 1.69

Bearing stress developed in the screw and nut = ( )= =1.521 = 1.52

Maximum principal stress developed in the screw = + + =. + . + (1.69) = 0.76 + 1.85 = 2.16 = 2610

36

Table 4.2 Results

S/N LOAD TYPE QUANTITY SYMBOLS VALUE UNIT

1 Max. lift of scissor arm 31 Degree2 Load type 1 Vertical force at the end of first tier , , , 107.5 N

Horizontal force at the end of firsttier

, , , 357.82 NHorizontal force at middle of first tier 715.6 N

3Load type 2

Vertical force at the end of first tier , , , 215 NHorizontal force at the end of firsttier

, , , 0 NMoment in the z-direction in 1st tier 516 Nm

4Load type 3

Vertical force at the end of first tier , , , 215 NHorizontal force at the end of firsttier

, , , 0 NMoment in the x-direction in 1st tier 344 Nm

5 Effort required to raise the load 92.75 N

6 Effort required to lower the load 36.76 N7 Turning moment of screw to raise the

load1.39

8 Turning moment of screw to lowerthe load

0.55 9 Efficiency of the screw 29.5 %10 Effort at the handle of the screw 9.27 N

4.1 STRENGTH AND RIGIDITY OF THE SYSTEM

The reaction force for load type 1 & 2 are all in the x-y, and are symmetric in the x-y plane.Because of this a cross bracing is recommended between left and right side of the lift to increaserigidity of the system. Reaction force from load type 1 & 2 will completely unstressed cross

37

bracing in the left and right side of the system. It may be used for buckling purpose. While loadtype 3 have a reaction force that may stress cross bracing. In most application the scissor liftoperate on a level ground. In this application is the most important significant load, althoughload type 2 and 3 may also be present if the load at the top of the lift is not centered. While for asloping ground other loading type which is beyond my scope may be present. The type of loaddetermines the type and the amount of cross-bracing required. Load type 1 & 2 do not stresscross bracing as is not too small but load type 3 affect cross bracing. Another thing that alsoadd to the rigidity of the lift members is the property of the screw and nut, such as the efficiencyof the nut, e.g. self locking of the nut.4.2 RESULT AND DISCUSSION

Table 4.3 Cost analysis of projectS/N PARTICULARS QUANTITY COST (N)1 Pins 12 24002 Rollers 4 20003 Screw spindle 1 90004 Bolt and nuts 5 4005 Spindle nut 1 2006 Steel plates (half sheet) 1 2, 5007 Flat bars 2 1,6508 Square pipe 2 2,6509 Angle iron 1,20010 Tyres 4 2,40011 Paints 2 Litres 1,50012 cutting disc 2 50013 Electordes 70014 Blades 3 600PROCESSING15 Fabrication and welding 6,500

38

16 Transport 3,500TOTAL 35,600

39

CHAPTER FIVE

5.0 CONCLUSIONS AND RECOMMENDATION

The design of a portable work platform elevated by the turning effect of a horizontal

screw spindle was carried out successfully meeting the required design standards. The

portable work platform is operated by turning a handle attached to the horizontal

spindle. The scissor lift is only for average load, because the higher the load the higher

the effort required. The screw operated scissor lift is simple in use and does not require

much maintenance. For the present dimension we get a lift of 150cm, the scissor lift can

lift a load of 280kg. The main constraint of this device is its high initial cost, but has a

low operating cost. The shearing tool should be heat treated to have high strength. The

device affords plenty of scope for modifications for further improvements and

operational efficiency, which should make it commercially available and attractive.

Hence, it should have a wide application in engineering industries and not just

automotive industry alone. Thus, it is recommended for the engineering industry and

for commercial production.

RECOMMENDATIONS

It is recommended that the screw and thread should be lubricated frequently so as to

reduce the amount of effort required to operate the system. This also reduces the

amount of wear between the screw and the nut. It is also suggested that the spindle and

40

nut should not be exposed to moisture so that it would not be susceptible to corrosion

thereby reducing its strength and toughness.

41

REFERENCES

Alexander j., Kaman P., Jack P., and David S. (1978), Physic for Engineering

Technology. John Willey and Sons Inc USA.

Allens H. R., Alfred R. H. and Herman G. L. (1980): Theory and Problems of MachineDesign. Schaums Outline Series. McGraw Hill Book Company

ASTM F-1166, (2007) Standard Practice for Human Engineering Design for MarineSystems

Elevating work platform, retrieved online at www.Wikipedia, 21/04/2011. (Last

accessed 15 July 2012).

Institution of Civil Engineers (Great Britain) (1870), Minutes of Proceedings of theInstitution of Civil Engineers. The Institution, pp 215 note 1.http://books.google.com/books?id=EwNRAAAAYAAJ&pg=PA215. Retrieved 1 May2012

Jain P. K. (1988):Machine/Mechanical Engineering Design Khanna Publishers. 5thEdition

Jindal U.C. (2010). Machine Design

Nelkon M. (1985): Principles of Physics. Collins International Textbook Dept. 8th

Edition

Shigley E. J. and Mischke R. C. (2001); Mechanical Engineering Design. McGraw Hillcompanies. Inc. New York, 6th Edition

Shigleys Mechanical Engineering Design (2008). Richard G. B. and Nisbett J. K.Design (2008). 8th Edition

Smith K. (1981) Mechanical Engineering Principles. Vol. 1. Pitman Education Ltd.

42

Snook, Stover H., and Vincent M. Ciriello, (1991) "The Design of Manual HandlingTasks: Revised Tables of Maximum Acceptable Weights and Forces," Ergonomics,Volume 34, No. 9,

Spackman H. M. (1989); NC Technical Document 1550 (Mathematical Analysis ofScissor Lifts)Stephens R. C. (1979): Strength of Material Theory and Examples. Edward Arnoldpublishers Ltd London.

The NIOSH Work Practices Guide for Manual Lifting (1981)Determining Acceptable Weights of Lift--Effective from March 1981 to July 1993--by Henry G. Wickes, Jr., P.E., CSP and Gary S. Nelson, Ph.D., CSP, Consultants

Understanding Scissor Lift Deflection, retrieved online at www.Autoquip.com

21/04/2011 (Last accessed 15 July 2012).

WCB Standards: A324 Forklift mounted work Platforms retrieved online 21/04/2011.

(Last accessed 15 July 2012).

43

APPENDIXDRAWINGS

44

392 30

12O 12 400

225 150 400

3 4

400

400

4

4

All dimensions inmillimetres

Tolerance unlessotherwise stated???????

0.05

Drawn by S. O. Macaulay Course:B.Eng. 28/08/12Bottom bed frameDrawing No. BE100812Niger Delta University

45

5,66 O 15O 8368

128 1,79

2

A

DETAILA

B

DETAIL B

All dimensions inmillimetres

Tolerance unlessotherwise stated???????

0.05

Drawn by S. O. Macaulay Course:B.Eng. 28/08/12Screw SpindleDrawing No. BE101812Niger Delta University