Question

11. Explain the Design Factor and Factor of Safety shortly. 12. Explain Clearance, Interference and Allowance. 13. Explain the meaning of Nominal size of a shaft. Why is the tolerance important in manufacturing? 14. Give the names of TORQUE TRANSMITTING ELEMENTS used in Machine Design 15. As Flexible Power Transmission Elements used Belts and Chains. What is the selection criteria of this component? 16. Give some examples for the types of friction clutches and brakes. 17. Show on a simple sketch the Basic geometry of a spur gear terminology. 18. If the N: Number of teeth/D: Pitch diameter [mm/m: Module (mm/teeth) and o: Circular pitch Imm write the relation between these components. How changes the geometry of a Cartooth sizes with the value of module? 19. Which geometric profiles are used for the teeth geometry? 20. What is the purpose of using Springs in machine design? 21. Advantages and Disadvantages of Rolling Contact Bearings Over Sliding Contact Bearings, 22. Give some examples for Permanent and Nonpermanent joints 23. Give names Types of lubrication 1 Hydrodynamic 2 Hydrostatic Elastohydrodynamic 4 Boundary Sold film 24 Wirite down the Thread Standards and terminology of screw thread

Answer 1

11. Design factor and Factor of safety

Design factor -

The design factor for a structure is the ratio of ultimate load to maximum permissible load that can be safely placed.It is generally provided in advance and is not an actual calculation and also often set by regulatory codes etc.when we are designing some component ,we should consider it from both manufacturing and handling point of view.we should not design components with complexities which is difficult to manufacture.This factors also you should consider it as a design factor.

Factor of safety -

It is the ratio of maximum stress to the working stress.$Factor of safety = \frac{Maximum stress}{ working or design stress}$

But incase of ductile materials, the yield points are clearly defined, so it depends on yield point and the relation changes to:

$Factor of safety = \frac{Yield point stress}{ working or design stress}$

But incase of brittle materials, the yield points are not clearly defined and depends on ultimate stress .So the relation changes to

$Factor of safety = \frac{Ultimate stress}{ working or design stress}$

A design engineer should consider the following factors before selecting the factor of safety:

• Different properties of materials and the possibility of changes of these properties during any operations.
• Reliabilty of applied load, whether it is gradual or impact.
• Exact mode of failure.
• Initial stresses acting.
• Extent of simplifying assumptions.

factor of safety is always greater than 1.

12. Clearance , Interference and Allowance

Clearance -

In an assembly it is the amount by which actual size of the shaft is less than the actual size of the mating hole.Its difference is always positive.

Interference -

In an assembly , it is the arithmetical difference by which the actual size of a shaft is larger than the actual finished size of the mating hole. Its difference is always negative.

Allowance-

Allowance is the difference between the basic dimensions of the mating parts.Allowance is positive, when the shaft size is less than the hole size and allowance is negative when the shaft size is greater than the hole size.   

13. Nominal size of a shaft.

Nominal size of shaft is the size of the part that is specified in drawing as a matter of convenience.

Importance of tolerance in manufacturing,

• Parts with proper tolerance will fit as desired.
• Liabilty of making the parts correctly.
• Reduced costs. - producing parts of unnecessarily tight tolerances are more expensive.
• No lag in production.

14. Names of some torque transmitting elements

• Couplings
• Shafts
• Power screws
• Gears
• Brakes and clutches.
• Belts
• Ropes
• Pulleys
• Chains and sprockets