1. Design of a C – Clamp Asanga Ratnaweera Dept of Mechanical Engineering
ME 209 Machine Design I
Design of a C – Clamp
Asanga Ratnaweera
Dept of Mechanical Engineering

2. Introduction to Clamps
Some commonly used clamps
C-clamp
Pipe clamp
Hand screw clamp
Quick action clamp
Miter clamp
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

3. Asanga Ratnaweera, Department of Mechanical Engineering
C - Clamp
Some examples
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

4. Engineering Design Process
Functional requirement -> Design
1. Conceptualization
2. Synthesis
3. Analysis
4. Evaluation
5. Representation
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

5. Asanga Ratnaweera, Department of Mechanical Engineering
Design of a C - Clamp
Identification of component
Collar
Nut
Screw
Handle
Screw Head
Frame
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

6. Asanga Ratnaweera, Department of Mechanical Engineering
Design of a C – Clamp
Prevention of Failure
Decides the size of each component considering its safety under the maximum permissible load conditions
Other Design Requirements
Ergonomics, Standards, Operational Parameters.
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

7. Asanga Ratnaweera, Department of Mechanical Engineering
Modes of Failure
Compressive failure
Tensile failure
Shear failure
Bending failure
Torsion failure
Fatigue failure
Many more…….
Note : it is important to identify the maximum stress and its location for each element under a given load condition
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

8. Asanga Ratnaweera, Department of Mechanical Engineering
Stress analysis
Note 1 : Identification of actual load condition and the area is extremely important
Note 3 : Bending and torsional stresses are analyzed using the following equations
Note 2: Direction of the load applied will determine the type of the stress
Bending
Torsion
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

9. Asanga Ratnaweera, Department of Mechanical Engineering
Factor of safety
Also know as safety factor is a multiplier applied to the calculated maximum stress to which a component will be subjected. Typically, for components whose failure could result in substantial financial loss or serious injury or death, high safety factor is used.
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

10. Asanga Ratnaweera, Department of Mechanical Engineering
Screw Treads
The principal uses of threads are:
for fastening,
for adjusting, and
for transmitting power
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

11. Standard Thread Systems
SI (ISO) Unified or American ACME
Pipe Whitworth (BSW)
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Asanga Ratnaweera, Department of Mechanical Engineering

12. Typical Screw Designation
In ACME system
1/2” - 13 UNC - 2A
external thread
(B means internal)
Class of fit
(1 is loosest tolerance, 3 is tightest)
Thread Series
UNC (Unified Coarse)
UNF (Unified Fine)
Pitch (threads/inch)
Nominal Diameter
(also shown as decimal or screw #)
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

13. Typical Screw Designation
In Metric system a screw is designated by the nominal size (major diameter) and pitch being separated by the sign X. e.g. M8 X 1.
M1.6 X 0.35 M2 X 0.4 M2.5 X 0.45
M3 X 0.5 M3.5 X 0.6 M4 X 0.7
M5 X 0.8 M6 X 1 M8 X 1.25
M10 X 1.5 M12 X M14 X 2
M16 X 2 M20 X 2.5 M24 X 3
M30 X 3.5 M36 X 4 M42 X 4.5
M48 X 5 M56 X 5.5 M64 X 6
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

14. Asanga Ratnaweera, Department of Mechanical Engineering
Power Screws
Power Screws are linear actuators that transform rotary motion into linear motion.
Power screws are generally based on ACME , Square, and Buttress threads.  
ACME
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Asanga Ratnaweera, Department of Mechanical Engineering

15. Asanga Ratnaweera, Department of Mechanical Engineering
Power Screws
Square
Buttress
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Asanga Ratnaweera, Department of Mechanical Engineering

16. Asanga Ratnaweera, Department of Mechanical Engineering
Power Screws
Ball screws are a type of power screw. 
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

17. Asanga Ratnaweera, Department of Mechanical Engineering
Power Screws
Efficiencies of between 30% and 70% are obtained with conventional power screws.  
Ball screws have efficiencies of above 90%.
Power Screws are Used for the following three reasons
To obtain high mechanical advantage in order to move large loads with minimum effort. e.g Screw Jack.
To generate large forces e.g A compactor press.
To obtain precise axial movements e.g. A machine tool lead screw.
Familiar applications
clamps or vises, presses, and jacks.
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

18. Asanga Ratnaweera, Department of Mechanical Engineering
Screw Threads
Wrapping an inclined plane around a cylinder results in a screw threaded or power screw.
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

19. Asanga Ratnaweera, Department of Mechanical Engineering
Screw Threads
What is the thread?
The raised helical rib going around a screw
What is the tread angle (λ)?
The tread angle is the angle of the inclined plane.
The more threads you have the smaller the angle.
The less thread you have the bigger the angle.
Lead = pitch * number of starts
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

20. Asanga Ratnaweera, Department of Mechanical Engineering
Screw Threads
If one turn of a square thread is unwrapped, the following ramp can be obtained. L
dm – mean diameter dm
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

21. Forces on Screw Threads
Identical to pushing an object up along a thread
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Asanga Ratnaweera, Department of Mechanical Engineering

22. Forces on Screw Threads
Considering a small element of the nut
δP - small force on the element due to the torque,
δW - a small part of the load which the element is supported,
δN - the normal force,
δF - the friction force λ δW δN δF δP
Motion
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

23. Forces on Screw Threads
Considering the force equilibrium, when the screw is about to rotate λ δW δN δF δP
Motion
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

24. Forces on Screw Threads
However, the friction angle φ;
The torque required
Therefore, the total torque;
Rm – mean radius
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Asanga Ratnaweera, Department of Mechanical Engineering

25. Asanga Ratnaweera, Department of Mechanical Engineering
Swivel Head Rc F
Torque required to turn the collar
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

26. Asanga Ratnaweera, Department of Mechanical Engineering
Design of the Nut
Shear and bearing failure of threads
Bearing area
screw
nut
Shear area
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

27. Asanga Ratnaweera, Department of Mechanical Engineering
Drawing of Screw Threads
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

28. Asanga Ratnaweera, Department of Mechanical Engineering
Design of the frame
The frame is subjected to combined bending and direct stresses W Wx W W x
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

29. Asanga Ratnaweera, Department of Mechanical Engineering
Design of the frame
An I-section can be used for the frame P σ t 6t 9t
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

30. Asanga Ratnaweera, Department of Mechanical Engineering
Design of a C - Clamp
Major Design Steps
1. Calculate the torque required at the handle
To overcome the friction at the screw
To overcome the friction at the collar
2. Calculation of stress on the screw
Maximum torsional stress
Maximum direct stress
3. Bearing pressure on threads
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering

31. Asanga Ratnaweera, Department of Mechanical Engineering
Design of a C - Clamp
Major Design Step
4. Selection of number of starts
5. Check the shear failure of threads
6. Calculation of the stresses on the frame
Moment due to couple
Direct stresses
7 December 2018
Asanga Ratnaweera, Department of Mechanical Engineering