DESIGN AND FABRICATION OF ROBOTIC END

EFFECTOR USING 4-BAR LINKAGE

A PROJECT REPORT

Submitted by

ARUN.S 52409114002

CHARLES GNANAKUMAR.G 52409114007

KARTHIKEYAN.G 52409114024

MADHAN.E 52409114027

in partial fulfillment for the award of the degree

of

BACHELOR OF ENGINEERING

in

MECHANICAL ENGINEERING

KINGSTON ENGINEERING COLLEGE, VELLORE

ANNA UNIVERSITY: CHENNAI 600 025

APRIL 2012

www.kingston.ac.in, info@kingston.ac.in

BONAFIDE CERTIFICATE

Certified that this project report “DESIGN AND FABRICATION OF

ROBOTIC END EFFECTOR USING 4-BAR LINKAGE” is the bonafide

work of the following students who carried out the project work under my

supervision.

1. Arun.S (52409114002)

2. Charles Gnanakumar.G (52409114007)

3. Karthikeyan.G (52409114024)

4. Madhan.E (52409114027)

SIGNATURE SIGNATURE Mr. D.SARAVANAN M.E., Mr. S.MAHENDIRAN M.E., HEAD OF DEPARTMENT SUPERVISOR Assistant Professor, Department of Mechanical Engineering Department of Mechanical Engineering Kingston Engineering College, Kingston Engineering College, Chittoor main road, Vellore - 632 059. Chittoor main road, Vellore - 632 059.

Submitted for the Anna University Practical Examination Held on ___________________ at Kingston Engineering College, Chittoor Main Road, Vellore-632 059.

Signature of Examiners: ______________________ ______________________ INTERNAL EXAMINER EXTERNAL EXAMINER

ACKNOWLEDGEMENT

At this pleasing moment of having successfully completed our project, we wish to convey

our sincere thanks and gratitude to the management of our college and our beloved Chairman

Thiru.D.M.KATHIR ANAND, M.B.A., (USA) who provided all the facilities to us.

We would like to express our sincere thanks to our principal Dr.G.BASKAR, M.E.,

Ph.D., FIE for forwarding us to do our project and offering adequate duration in completing our

project.

We are also grateful to the Head of the department Mr.D.SARAVANAN, M.E., for his

constructive suggestions and encouragement during our project.

With deep sense of gratitude, we extend our earnest and sincere thanks to our guide

Assistant Prof. Mr.S.MAHENDIRAN, M.E., Department of Mechanical Engineering for his

kind guidance and encouragement during this project.

We also express our in depth thanks to our teaching and non-teaching staffs of

Mechanical Engineering Department in KINGSTON ENGINEERING COLLEGE.

i

TABLE OF CONTENTS

CHAPTER TITLE PAGE

ABSTRACT iii LIST OF TABLES iv

LIST OF FIGURES iv

LIST OF SYMBOLS AND vi

ABBREVIATIONS

1 INTRODUCTION

1.1 TYPES OF END EFFECTORS 1 1.2 TYPES OF GRIPPER MECHANISMS 2

2 GRIPPER FORCE ANALYSIS

2.1 INTRODUCTION 7

2.2 OTHER TYPES OF GRIPPERS 8

2.2.1 Vacuum Cups 9

2.2.2 Magnetic Gripper 10

2.2.3 Adhesive Grippers 12

2.3 TOOLS AS END EFFECTORS 13

2.4 POWER AND SIGNAL TRANSMISSION 13

2.5 CONSIDERATIONS IN GRIPPER 14

SELECTION

2.6 DESIGN CALCULATION OF WORM 16

GEAR DRIVE

ii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

3 MATERIALS AND COST ESTIMATION

3.1 DESIGN OF THE PROJECT 20

3.1.1 Before Assembly 20

3.1.2 After Assembly 22

3.2 COST ESTIMATION 23

3.2.1 Material Cost 23

3.2.2 Machining Cost 23

3.2.3 Miscellaneous cost 23

3.2.4 Total Cost 23

3.3 MATERIAL INTRODUCTION 24

3.3.1 Base Plate 24

3.3.2 Gripper Plate 24

3.3.3 Gripper Link 24

3.3.1 Worm and Worm Gears 24

3.4 EVALUATION 25

4 SUMMARY AND CONCLUSIONS

4.1 WHAT HAS BEEN DONE 27

4.2 FUTURE DIRECTIONS 27

4.3 CONCLUSION 29

REFERENCES 30

iii

ABSTRACT

In the area of Robotics, the gripper plays a very important role as it is required to hold

and place the object at the desired location. The requirements of gripper in terms of load

capacity, and flexibility to adapt to the form of the object with tactile sensing capability which

suit the strength of the object are necessary. Extensive research work is under way in the design

of soft gripper or dexterous hand.

The mechanism is based on the motion characteristic of a parallelogram four-bar linkage

and the geometric relationship that chords of concentric circles at a central angle are parallel. The

gripper is simple in structure, easy to manufacture, and convenient to use. It has the capability to

grip a wide range of part sizes and can achieve high accuracy.

This report presents a design of a new type of robot end-effector with inherent

mechanical grasping capabilities. Concentrating on designing an end-effector to grasp a simple

class of objects, cylindrical, allowed a design with only one degree of actuation. The key

features of this design are high bandwidth response to forces.

Passive grasping capabilities, ease of control and ability to wrap around objects with

simple geometries providing form closure. A prototype of this mechanism was built to

evaluate these features.

iv

LIST OF TABLES

TABLE TITLE PAGE

2.5.1 Checklist of factors in the selection and design 15

of grippers

3.2.1 Material Cost 23

3.2.4 Total Cost 23

LIST OF FIGURES

FIGURE TITLE PAGE

1.2.1 Some possible linkages for robotic grippers 3

1.2.2 Gear and Rack method of actuating the gripper 4

1.2.3 Cam actuated gripper 5

1.2.4 Screw type gripper actuation 5

2.1.1 Force against part parallel to finger surfaces 7

tending to pull part out of gripper

2.2.1.1 Venturi device used to operate a suction cup. 9

2.2.2.1 Stripper device operated by air cylinders used 11

with a permanent magnetic gripper.

2.6.1 Worm 16

v

2.6.2 Worm Wheel 16

3.1 Model of our project 20

3.2 Assembled Model of our project 21

3.4.1 Worm Gear Drive 26

3.4.2 Prototype of End Effector 26

3.4.3 Gripper holding Rectangular Plate 26

3.4.4 Gripper holding Square Block 26

3.4.5 Gripper holding Cylindrical Rod 26

4.2 Dexterous Hand 28

4.3 Robotic arm with 4-bar linkage end effector 28

done by using SOLIDWORKS

vi

LIST OF SYMBOLS AND ABBREVATIONS

F -Force, N

µ -Coefficient of friction of the finger contact surface against the part surface

nf -Number of contacting fingers

Fg -Gripper force, N

w -Weight of the part or object being gripped, Kg

P -Negative pressure, lb/in2

A -Total effective area of the suction cup(s) used to create the vacuum, in2

σb -Bending Stress, N/mm2

σc -Contact Stress, N/mm2

E -Young’s Modulus, N/mm2

𝑀𝑡 -Initial Design Torque, N-mm

mx -Axial Module, mm

a -Revised Centre distance, mm

𝛾 -Lead angle

1

1. INTRODUCTION

An end effector is a device that attaches to the wrist of the robot arm and enables the

general-purpose robot to perform a specific task. It is sometimes referred to as the robot's "hand."

Most production machines require special purpose fixtures and tools designed for a particular

operation, and a robot is no. exception. The end effector is pan of that special-purpose tooling for

a, robot. Usually, end effectors must be custom engineered for the particular task which is to be

performed. This can be accomplished either by designing and fabricating-the device from

scratch, or by purchasing a commercially available device and adapting it to the application. The

company installing the robot can either do the engineering work itself or it can contract for the

services of a firm that does this kind of work.

Most robot manufacturers have special engineering groups whose function is to design

end effectors and to provide consultation services to their customers. Also, there are a growing

number of robot systems firms which perform some or all of the engineering work to install

robot systems. Their services would typically include end effector design.

1.1 TYPES OF END EFFECTORS

There are wide assortments of end effectors required to perform the variety of

different work functions. The various types can be divided into two major categories:

1. Grippers

2. Tools

Grippers are end effectors used to grasp and hold object. The objects are generally work

parts that are to be moved by the robot. These part-handling applications include machine

loading and unloading, picking parts from a conveyor, and arranging parts onto a pallet. In

addition to work parts, other objects handled by robot grippers include cartons, bottles, raw

materials, and tools. We tend to think of grippers as mechanical grasping devices, but there are

alternative ways of holding objects involving the use of magnets, suction cups, or other means.

2

Grippers can be classified as single grippers or double grippers although this

classification applies best to mechanical grippers. The single gripper is distinguished by the

fact that only one grasping device is mounted on the robot's wrist. A double gripper has two

gripping devices attached to the wrist and is used to handle two separate objects. The two

gripping devices can be actuated independently.

The double gripper is especially useful in machine loading and unloading applications.

With a double gripper, the robot can pick the part from the incoming conveyor with one of the

gripping devices and have it ready to exchange for the finished part. When the machine cycle is

completed, the robot can reach in for the finished part with the available grasping device, and

insert the raw part into the machine with the other grasping device. The amount of time that

the machine is open is minimized.

The term multiple gripper is applied in the case where two or more grasping

mechanisms are fastened to the wrist. Double grippers are a subset of multiple grippers. The

occasions when more than two grippers would be required are somewhat rare. There is also a

cost and reliability penalty which accompanies an increasing number of gripper devices on one

robot arm.

By definition, the tool-type end effector is attached to the robot’s wrist. One of the

most common applications of industrial robots is spot welding, in which the welding electrodes

constitute the end effector of the robot. Other examples of robot applications in which tools

are used as end effectors include spray painting and arc welding.

1.2 TYPES OF GRIPPER MECHANISMS

There are various ways of classifying mechanical grippers and their actuating

mechanisms. One method is according to the type of finger movement used by the gripper. In

this classification the grippers can actuate the opening and closing of the fingers by one of the

following motions:

3

1. Pivoting movement

2. Linear or translational movement

In the pivoting movement the fingers rotate about fixed pivot points on the gripper to

open and close. The motion is usually accomplished by some kind of linkage mechanism. In the

linear movement the fingers open and close by moving in parallel to each other. This is

accomplished by means of guide rails so that each finger base slides along a guide rail during

actuation. The translational finger movement might also be accomplished by means of a linkage

which would maintain the fingers in a parallel orientation to each other during actuation.

Fig.1.2.1: Some possible linkages for robotic grippers

Mechanical grippers can also be classed according to the type of kinematic device used to

actuate the finger movement. In this classification we have the following types:

1. Linkage actuation

2. Gear-and-rack actuation

3. Cam actuation

4

4. Screw actuation

5. Rope-and-pulley actuation

6. Miscellaneous

The linkage category covers a wide range of design possibilities to actuate the opening

and closing of the gripper. A few examples are illustrated in Fig.1.2.1

The design of the linkage determines how the input force F to the gripper is

converted into the gripping force F applied by the fingers. The linkage configuration also

determines other operational features such as how wide the gripper fingers will open and how

quickly the gripper will actuate.

Fig. 1.2.2: Gear and Rack method of actuating the gripper

Figure 1.2.2 illustrates one method of actuating the gripper fingers using a gear-and-rack

configuration. The rack gear would be attached to a piston or some other mechanism that would

provide a linear motion. Movement of the rack would drive two partial pinion gears, and these

would in turn open and close the fingers.

The cam actuated gripper includes a variety of possible designs, one of which is shown in

Fig.1.2.3. A cam and follower arrangement often using a spring-loaded follower can provide the

opening and closing action of the gripper.

5

Fig.1.2.3: Cam actuated gripper

For example, movement of the cam in one direction would force the gripper to open,

while movement of the cam in the opposite direction would cause the spring to force the gripper

to close. The advantage of this arrangement is that the spring action would accommodate

different sized parts. This might be desirable, for example, in a machining operation where

a single gripper is used to handle the raw work part and the finished part. The finished part

might be significantly smaller after machining.

An example of the screw-type actuation method is shown in Fig.1.2.4. The screw is

turned by a motor, usually accompanied by a speed reduction mechanism. When the screw is

rotated in one direction, this causes a threaded block to be translated in one direction. The

threaded block is, in turn, connected to the gripper fingers to cause the corresponding opening

and closing action.

Fig.1.2.4: Screw type gripper actuation

6

Rope-and-pulley mechanisms can be designed to open and close a mechanical gripper.

Because of the nature of these mechanisms, some form of tension device must be used to oppose

the motion of the rope or cord in the pulley system. For example, the pulley system might

operate in one direction to open the gripper, and the tension device would take up the slack

in the rope and close the gripper when the pulley system operates in the opposite direction.

The miscellaneous category is included in our list to allow for gripper actuating

mechanisms that do not logically fall into one of the above categories. An example might be an

expandable bladder or diaphragm that would be inflated and deflated to actuate the gripper

fingers.

7

2. GRIPPER FORCE ANALYSIS

2.1 INTRODUCTION

As indicated previously, the purpose of the gripper mechanism is to convert input power

into the required motion and force to grasp and hold an object. Let us illustrate the analysis that

might be used to determine the magnitude of the required input power in order to obtain a given

gripping force. We will assume that a friction-type grasping action is being used to hold the part.

A detailed study of mechanism analysis is beyond the scope of this text, and the reader might

refer to other books such as Beer and Johnson and Shigley and Mitchell.

Fig.2.1.1: Force against part parallel to finger surfaces tending to pull part out of

gripper

If a force of sufficient magnitude is applied against the part in a direction parallel to

the friction surfaces of the fingers as shown in Fig.2.1.1(a), the part might slip out of the gripper.

To resist this slippage, the gripper must be designed to exert a force that depends on the weight

of the part, the coefficient of friction between the part surface and the finger surface, the

acceleration (or deceleration) of the part, and the orientation between the direction of motion

during acceleration and the direction of the fingers.

Referring to Fig.2.1.1(b), the following force equations, Equations (a) and (b), can be

used to determine the required magnitude of the gripper force as a function of these factors.

Equation (a) covers the simpler case in which weight alone is the force tending to cause the part

to slip out of the gripper.

8

- - - - - - - - - - - (a)

Where µ=coefficient of friction of the finger contact surface against the part surface

nf= number of contacting fingers

Fg =gripper force

w = weight of the part or object being gripped

This equation would apply when the force of gravity is directed parallel to the contacting

surfaces. If the force tending to pull the part out of the fingers is greater than the weight of the

object, then Eq. (a) would have to be altered. For example, the force of acceleration would be a

significant factor in fast part-handling cycles. Engelberger suggests that in a high-speed handling

operation the acceleration (or deceleration) of the part could exert a force that is twice the weight

of the part. He reduces the problem to the use of a g f actor in a revised version of Eq. (a) as

follows:

- - - - - - - - - - - - - (b)

Where g =the g factor. The g f actor is supposed to take account of the combined

effect of gravity and acceleration. If the acceleration force is applied in the same direction as

the gravity force, then the g value = 3.0. If the acceleration is applied in the opposite direction,

then the g value = 1.0 (2 x the weight of the part due to acceleration minus 1 x the weight of the

part due to gravity). If the acceleration is applied in a horizontal direction, then use g = 2.0.

2.2 OTHER TYPES OF GRIPPERS

In addition to mechanical grippers there are a variety of other devices that can be

designed to lift and hold objects. Included among these other types of grippers are the following:

1. Vacuum cups

9

2. Magnetic grippers

3. Adhesive grippers

4. Hooks, scoops, and other miscellaneous devices

2.2.1 Vacuum Cups

Vacuum cups, also called suction cups, can be used as gripper devices for handling

certain types of objects. The usual requirements on the objects to be handled are that they be flat,

smooth and clean, conditions necessary to form a satisfactory vacuum between the object and the

suction cup.

The suction cups used in this type of robot gripper are typically made of elastic material

such as rubber or soft plastic. An exception would be when the object to be handled is composed

of a soft material. In this case, the suction cup would be made of a hard substance. The shape of

the vacuum cup, as shown in the figure (2.2.1.1), is usually round. Some means of removing the

air between the cup and the part surface to create the vacuum is required. The vacuum pump and

the venturi are two common ·devices used for this purpose.

Fig.2.2.1.1: Venturi device used to operate a suction cup.

The vacuum pump is a piston-operated or vane-driven device powered by an electric

motor. It is capable of creating a relatively high vacuum. The venturi is a simpler device and can

be driven by means of "shop air pressure." Its initial cost is less than that of a vacuum pump and

10

it is relatively reliable because of its simplicity. However, the overall reliability of the vacuum

system is dependent on the source of air pressure.

The lift capacity of the suction cup depends on the effective area of the cup and the

negative air pressure between the cup and the object. The relationship can be summarized

in the following equation

F = PA ------------------ (c)

Where F = the force or lift capacity, lb

P = the negative pressure, lb/in2

A = the total effective area of the suction cup(s) used to create the vacuum, in2

The effective area of the cup during operation is approximately equal to the unreformed

area determined by the diameter of the suction cup. The squashing action of the cup as it presses

against the object would tend to make the effective area slightly larger than the unreformed area.

On the other hand, if the center portion of the cup makes contact against the object

during deformation, this would reduce the effective area over which the vacuum is applied.

These two conditions tend to cancel each other out. The negative air pressure is the pressure

differential between the inside and the outside of the vacuum cup.

2.2.2 Magnetic Gripper

Magnetic grippers can be a very feasible means of handling ferrous materials. The

stainless steel plate would not be an appropriate application for a magnetic gripper because 18-8

stainless steel is not attracted by a magnet. Other steels, however, including certain types of

stainless steel, would be suitable candidates for this means of handling, especially when the

materials are handled in sheet or plate form.

In general, magnetic grippers offer the following advantages in robotic handling

applications:

11

Pickup times are very fast.

Variations in part size can be tolerated. The gripper does not have to be

designed for one particular work part.

They have the ability to handle metal parts with holes (not possible

with vacuum grippers).

They require only one surface for gripping.

Disadvantages with magnetic grippers include the residual magnetism remaining in

the work piece which may cause a problem in subsequent hand- ling, and the possible side

slippage and other errors which limit the precision of this means of handling. Another

potential disadvantage of a magnetic gripper is the problem of picking up only one

sheet from a stack. The magnetic attraction tends to penetrate beyond the top sheet in the

stack, resulting in the possibility that more than a single sheet will be lifted by the magnet.

This problem can be confronted in several ways.

Magnetic grippers can be divided into two categories, those using electromagnets and

those using permanent magnets. Electromagnetic grippers are easier to control, but require a

source of dc power and an appropriate controller unit. As with any other robotic-gripping

device, the part must be released at the end of the handling cycle. This is easier to accomplish

with an electromagnet than with a permanent magnet.

Fig.2.2.2.1: Stripper device operated by air cylinders used with a permanent magnetic

gripper.

12

When the part is to be released the controller unit reverses the polarity at a reduced

power level before switching off the electromagnet. This procedure acts to cancel the residual

magnetism in the work piece and ensures a positive release of the part.

Permanent magnets have the advantage of not requiring an external power source to

operate the magnet. However, there is a loss of control that accompanies this apparent

advantage. For example, when the part is to be released at the end of the handling cycle,

some means of separating the part from the magnet must be provided. The device which

accomplishes this is called a stripper or stripping device. Its function is to mechanically detach

the part from the magnet. One possible stripper design is illustrated in Fig.2.2.2.1.

Permanent magnets are often considered for handling tasks in hazardous environments

requiring explosion proof apparatus. The fact that no electrical circuit is needed to operate the

magnet reduces the danger of sparks which might cause ignition in such an environment.

2.2.3 Adhesive Grippers

Gripper designs in which an adhesive substance performs the grasping action can be

used to handle fabrics and other lightweight materials. The require-ments on the items to be

handled are that they must be gripped on one side only and that other forms of grasping such

as a vacuum or magnet are not appropriate.

Cine of the potential limitations of an adhesive gripper is that the adhesive substance

loses its tackiness on repeated usage.

Consequently, its reliability as a gripping device is diminished with each successive

operation cycle. To overcome this limitation, the adhesive material is loaded in the form of a

continuous ribbon into a feeding mechanism that is attached to the robot wrist. The feeding

mechanism operates in a manner similar to a typewriter ribbon mechanism.

13

2.3 TOOLS AS END EFFECTORS

In many applications, the robot is required to manipulate a tool rather than a work part. In

a limited number of these applications, the end effector is a gripper that is designed to grasp and

handle the tool. The reason for using a gripper in these applications is that there may be more

than one tool to be used by the robot in the work cycle. The use of a gripper permits the tools to

be exchanged during the cycle, and thus facilitates this multi tool handling function.

In most of the robot applications in which a tool is manipulated, the tool is attached

directly to the robot wrist. In these cases the tool is the end effector. Some examples of tools

used as end effectors in robot applications include:

Spot-welding tools Arc-welding torch Spray-painting nozzle

Rotating spindles for operations such as:

Drilling routing

Wire brushing

Grinding

Liquid cement applicators for assembly

Heating torches

Water jet cutting tool

2.4 POWER AND SIGNAL TRANSMISSION

End effectors require power to operate. They also require control signals to regulate their

operation. The principal methods of transmitting power and control signals to the end effector

are:

I. Pneumatic

II. Electric

III. Hydraulic

IV. Mechanical

14

2.5 CONSIDERATIONS IN GRIPPER SELECTION AND DESIGN

As indicated above, tools are used for spot welding, arc welding, rotating spindle

operations, and other processing applications. Certainly one of the considerations deals with

determining the grasping requirement for the gripper. Engelberger defines many of the factors

that should be considered in assessing gripping requirements. The following list is based on

Engelberger's discussion of these factors:

1. The part surface to be grasped must be reachable. For example, it must not be enclosed within

a chuck or other holding fixture.

2. The size variation of the part must be accounted for, and how this might influence the

accuracy of locating the part. For example, there might be a problem in placing a rough casting

or forging into a chuck for machining operations. ,

3. The gripper design must accommodate the change in size that occurs between pan loading and

unloading. For example, the part size is reduced in machining and forging operations.

4. Consideration must be given to the potential problem of scratching and distorting the part

during gripping, if the part is fragile or has delicate surfaces.

5. If there is a choice between two different dimensions on a part, the larger dimension should be

selected for grasping. Holding the part by its larger surface will provide better control and

stability of the part in positioning.

6. Gripper fingers can be designed to conform to the part shape by using resilient pads or self-

aligning fingers. The reason for using self-aligning fingers is to ensure that each finger makes

contact with the part in more than one place.

This provides better part control and physical stability. Use of replaceable fingers will

allow for wear and also for interchangeability for different part models

15

2.5.1 Checklist of factors in the selection and design of grippers

Factor

Consideration

Part to be handled

Weight and size Shape Changes in shape during processing Tolerances on the part size Surface condition, protection of delicate surfaces

Actuation method

Mechanical grasping Vacuum cup Magnet Other methods (adhesives, scoops, etc.)

Power and signal transmission

Pneumatic Electrical Hydraulic Mechanical

Gripper force (mechanical gripper)

Weight of the object Method of holding (physical constriction or friction) Coefficient of friction between fingers and object Speed and acceleration during motion cycle

Positioning problems Length of fingers Inherent accuracy and repeatability of robot Tolerances on the part size

Service conditions

Number of actuations during lifetime of gripper Replaceability of wear components (fingers) Maintenance and serviceability

Operating environment

Heat and temperature Humidity, moisture, dirt, chemicals

Temperature protection

Heat shields long fingers Forced cooling (compressed air, water cooling, etc.) Use of heat-resistant materials

Fabrication materials

Strength, rigidity, durability Fatigue Strength Cost and ease of fabrication Friction properties f or finger surfaces Compatibility with- operating environment

Other considerations

Use of interchangeable fingers Design standards Mounting connections and interfacing with robot Risk of product design changes and their effect on the gripper

design Lead time for design and fabrication Spare parts maintenance and service Tryout of the gripper in production

16

The important factors that determine the required grasping force are: ·

The weight of the object.

Consideration of whether the part can be grasped consistently about its center of mass.

If not, an analysis of the possible moments from off-center grasping should be

considered.

The speed and acceleration with which the robot arms moves (acceleration and

deceleration forces) and the orientational relationship between the direction of

movement and the position of the fingers on the object (whether the movement is parallel

or perpendicular to the finger surface contacting the part).

Whether physical constriction or friction is used to hold the part.

Coefficient of friction between the object and the gripper fingers.

2.6 DESIGN CALCULATION OF WORM GEAR DRIVE

Materials used

Worm and Worm Wheel - Mild Steel

Fig. 2.6.1: Worm Fig. 2.6.2: Worm Wheel

Assumed Values

Speed of worm (N1) = 100 rpm

Number of teeth on worm wheel (Z2) = 65

Number of starts on worm (Z1) = 4

k*kd = 1

Initial Centre Distance (a) = 68mm

Standard Values

17

For Mild Steel

Bending Stress (σb) = 165 N/mm2

Contact Stress (σc) = 190 N/mm2

Young’s Modulus (E) = 2.06*105 N/mm2

Transmission Ratio

I = Z2/Z1 = 65/4= 16.25

Initial Design Torque

[𝑴𝒕] = 𝑴𝒕*k*kd

Assume k*kd = 1.

𝑴𝒕 = P*60/2πN2

𝒂 = �𝒁𝟐𝒒

+ 𝟏� �� 𝟓𝟒𝟎

�𝒁𝟐𝒒 �[𝝈𝒄]�𝟐

[𝑴𝒕]𝟏𝟎

𝟑

68 = �6511

+ 1� �� 540

�6511�[170]�2

[𝑀𝑡]10

3

32.08=�[𝑀𝑡]3

𝑴𝒕= 33013.98 N-mm

𝑀𝑡 = P*60/2πN2 = 60∗𝑃2∗𝜋∗9.26

= 33013.98

P=32W = 0.032Kw

Axial Module

mx = 𝟐𝒂𝒒+𝒁𝟐

= 2∗6811+65

= 1.789 ~ 2mm

Revised Centre distance

a= 0.5 mx (𝒒 + 𝒁𝟐) = 0.5*2*(11 + 65)= 76mm

Pitch Circle Diameters

d1=q* mx = 11*2= 22mm

18

d2=𝒁𝟐* mx = 65*2= 130mm

Lead angle

𝜸= tan-1(𝒁𝟏𝒒

) = tan-1( 411

) = 19.98◦

Virtual Number of teeth

Zv = 𝒛𝒄𝒐𝒔𝟑𝜸

= 65𝑐𝑜𝑠319.98

= 78.3 ~ 79 teeth.

Pitch Line Velocity

V1 = 𝝅𝒅𝟏𝑵𝟏𝟔𝟎

= 𝜋∗𝟎.𝟎𝟐𝟐∗𝟏𝟎𝟎60

= 0.115m/s

V2 = 𝝅𝒅𝟐𝑵𝟐𝟔𝟎

= 𝜋∗𝟎.𝟏𝟑∗𝟗.𝟐𝟔60

= 0.063m/s

Sliding Velocity

Vs = 𝑽𝟏𝒄𝒐𝒔𝜸

= 0.115𝑐𝑜𝑠19.98

= 0.122m/s

From PSGDB for Zv =79, form factor, 𝒀𝒗=0.499

Check for Bending

σb = 𝟏.𝟗[𝑴𝒕]

𝒎𝒙𝟑 ∗𝒒∗𝒁𝟐∗𝒀𝒗 = 1.9∗33013.98

23 ∗11∗65∗0.499 = 21.97N/mm2

Since the value of design bending stress [σb] is greater than the value of induced bending stress

σb, (i.e.)

[σb] = 165 N/mm2 > σb = 21.97N/mm2

The design is safe and satisfactory.

Check for Wear

𝛔𝒄 = �𝟓𝟒𝟎�𝒁𝟐𝒒 �

� ��𝒁𝟐𝒒 +𝟏

𝒂�𝟑

[𝑴𝒕]𝟏𝟎

𝛔𝒄 = �𝟓𝟒𝟎�𝟔𝟓𝟏𝟏�

� ��𝟔𝟓𝟏𝟏+𝟏

𝟕𝟔�𝟑

[𝟑𝟑𝟎𝟏𝟑.𝟗𝟖]𝟏𝟎

19

𝛔𝒄 = 113.83N/mm2

Since the value of design contact stress [σc] is greater than the value of induced contact stress σc,

(i.e.)

[σc] = 190 N/mm2> σc = 113.83N/mm2

The design is safe and satisfactory.

20

3. MATERIALS AND COST ESTIMATION

3.1 DESIGN OF THE PROJECT

The Worm and Worm Wheel are made according to the dimensions and calculations

made in section 2.6.

According to that calculation base plate, Gripper plate and various links are designed

based on assumptions.

The Model of Worm, Worm Wheel, base plate, Gripper plate and various links made by

using SOLIDWORKS 2010 are shown below.

3.1.1 BEFORE ASSEMBLY

Gripper Base Plate

21

Worm Worm Gear

Gripper Link1

Link 2

Fig. 3.1.1: Model of our project

22

3.1.2 AFTER ASSEMBLY Isometric View

Top View

Fig. 3.1.2: Assembled Model of our project

23

3.2 COST ESTIMATION

3.2.1 MATERIAL COST

S.No COMPONENTS

NEEDED TYPE OF

MATERIAL QUANTITY MATERIAL PRICE IN `

(approx.) 1 Base plate Mild Steel 1 400

2 Gripper plate Mild Steel 2 600

3 Gripper Link Mild Steel 2 400

4 Links Mild Steel 4 800

5 Worm Gear Mild Steel 2 500

6 Worm Mild Steel 1 300

TOTAL 11 `3000 3.2.2 MACHINING COST

Machining Cost for all the Components: `3000

3.2.3 MISCELLANEOUS COST

Other Miscellaneous Cost: `2000 3.2.4 TOTAL COST

S.No STUDENTS NAME AMOUNT CONTRIBUTION IN `

1 Arun.S 2000

2 Charles Gnanakumar.G 2000

3 Karthikeyan.G 2000

4 Madhan.E 2000

TOTAL `8000

24

3.3 MATERIAL INTRODUCTION 3.3.1 BASE PLATE

It is the base of the End Effector. The Base Plate is made of Mild Steel. It gives rigid

structure to end effector and it holds all the components of the end effector like worm, worm

wheel, various links.

3.3.2 GRIPPER PLATE

It is used to hold various types of objects. It is used to pick an object from one place and

to place in another place. Generally it is made of lighter materials and the ends of the gripper

plate are knurled to provide a grip to hold the object.

3.3.3 GRIPPER LINK

It is the major part of the End Effector where it is used to transmit rotary motion of the

gear into oscillatory or reciprocating motion of the gripper plate. It is made up of cast iron to

form a rigid link between the gear and the gripper plate.

3.3.4 WORM AND WORM GEARS

A worm drive is a gear arrangement in which a worm (which is a gear in the form of

a screw) meshes with a worm gear (which is similar in appearance to a spur gear, and is also

called a worm wheel).

Worm wheels are first gashed to rough out the teeth and then hobbed to the final

dimensions

The terminology is often confused by imprecise use of the term worm gear to refer to the

worm, the worm gear, or the worm drive as a unit. Like other gear arrangements, a worm drive

can reduce rotational speed or allow higher torque to be transmitted.

Worm and Worm gear is used in this End Effector to transmit power to actuate the links.

Worm is meshed with Worm gear to transmit the power to the gripper links.

The dimensions and design procedure for worm and worm wheel is already discussed in

chapter 2.6.

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3.4 EVALUATION

In this section, important features of the End Effector are discussed in relation to

experience in the lab. Figure 3.4.2 shows a photograph of the actuated prototype End

Effector.

The base link is clamped rigidly to the edge of a table for these tests.

The key purpose of this project was to design an end-effector which works well on a

simple class of objects.

The prototype End Effector is shown in Figure 3.4.5 grasping a cylinder. As you can see,

the cylinder can be grasped adequately.

The prototype End Effector is shown in Figure 3.4.3 & 3.4.4 grasping a variety of

rectangular and square blocks.

To improve the workspace in constructing a new version the ratio of link would be

changed such that the distal link is proportionally a bit larger.

The positive aspects of current lengths are that the mechanism does not typically hit into

itself with the tip of the last link.

On the other hand the most common mode of failure during grasping is that the distal link

doesn't reach the cylinder.

A larger distal link increases the primary workspace for small diameter cylinders.

26

Fig.3.4.1: Worm Gear Drive Fig.3.4.2: Prototype of End Effector

Fig.3.4.3:Gripper Holding Rectangular plate Fig.3.4.4:Gripper Holding Square Block

Fig.3.4.5:Gripper Holding Cylindrical Rod

27

4. CONCLUSIONS

4.1 WHAT HAS BEEN DONE

A Mechanism was designed for grasping a specific class of objects, cylindrical, square

and Rectangular blocks.

Early in the design process it was decided that compliant characteristics were needed to

be able to collide with the environment at high speed.

Analysis was performed to determine the relevant kinematics, the workspace, the

grasping behavior, and the quality of the grasp.

A prototype was constructed to evaluate the design and analysis.

This was a first prototype of a new type of gripper which combines passive and active

grasping capabilities.

The mechanism currently works well as an End Effector which can grab objects rapidly.

Natural "grabbiness" and high bandwidth response to forces makes this device a good

candidate for a gripper that will come into contact with objects rapidly.

4.2 FUTURE DIRECTIONS

An end-effector such as the Worm actuated 4-bar linkage End Effector should be

designed to be very strong so it can not only grab but also pick up heavy objects.

This project concentrated on a mechanism to improve grasping.

The load capacity was analyzed but no attempt was made to increase it.

An improvement to the design would be to increase the strength while maintaining the

same desirable grasping characteristics. .

The possibility to improve the strength is to employ a block and tackle mechanism.

This would act similarly to another gear stage, increasing the force while decreasing the

speed.

Protecting the moving parts of the mechanism would also be a valuable improvement.

The advantages of the End Effector design would be maintained and some of the

problems resolved.

With more gripping surfaces, the device would be able to grasp objects with more

28

complex geometries. .

It would also have advantages over current "dexterous" manipulators which are designed

to hold objects only with the fingertips as shown in fig 4.2.

Another advantage to grabbing an object with two or more fingers is that they would

constrain it to a unique configuration, and thus the object would not tend to shift in the

grasp.

Fig. 4.2: Dexterous Hand

Fig. 4.3: Robotic arm with 4-bar linkage end effector done by using SOLIDWORKS

29

4.3 CONCLUSION

A reasonably brief and inexpensive analysis effort yielded a significant improvement in

the performance of the gripping end effector and the mating grip fixture for the end effector and

other payloads.

30

REFERENCES

1. F. P. Beer and E. R. Johnson, Jr., Vector Mechanics for Engineers, 3rd edition, McGraw-Hill,

New York, 1977.

2. F. Y. Chen. "Gripping Mechanisms for Industrial Robots," Mechanism and Machine Theory

17(5). 299-311 (1982).

3. J. F. Engelberger, Robotics in Practice, AMACOM (American Management Association),

NewYork, 1980, chap. 3.

4. M. P.Groover and E.W.Zimmers, Jr.. CAD/CAM : Computuer Aided Design and

Manufacturing, Prentice-Hall, Englewood Cliffs, NJ, 1984, chap. 10.

5. G.Lundstrom, B.Glemme, and B. W.Rocks, Industrial Robots-Gripper review, International

Fluidics Services Ltd., Bedford, England.

6. J. E. Shigley and L D. Mitchell, Mechanical Engineering Design, McGraw-Hill, New York,

1983.

7. L L. Toepperwein, M.T. Blackman, et al., "ICAM Robotics Application Guide," Technical

Report AFWAL-TR-80-4042, vol. II, Materials Laboratory, Air Force Wright Aeronautical

Laboratories, Ohio, April 1980.

8. J.M. Vranish, "Quick Change System for Robots," SME paper MS84-418, Conference

papers., Robotics Research-The Next Five Years and Beyond, Lehigh University, Bethlehem,

Pennsylvania, August 1984.

9. A. J.Wright, "Light Assembly Robots-An End Effector Exchange Mechanism." Mechanical

Engineering, July 1983, pp.29-35.

10. Mikell P.Groover, Mitchell Weiss, Roger N. Nagel, Nicholas G. Odrey., “INDUSTRIAL

ROBOTICS Technology, Programming and Applications”, 3rd edition, McGraw-Hill, New

York, 1986.