DESIGN OF SHAFTS
1. Define shaft.
A
shaft is a rotating machine element which is used to transmit power from one
place to another. Shaft is used for the
transmission of torque and bending moment.
2. Differentiate between shaft and axle.
An
axle, through similar in shape to the shaft, is a stationary machine element
and is used for transmission of bending moment only. It simply acts as a support for some rotating
body.
3. What is spindle?
A
spindle is a short shaft that imparts motion either to a cutting tool or to a
workpiece.
4. What are the
materials used for shafts.
For
ordinary shafts – mild steel
For high strength shafts – alloy
steel such as Nickel, Ni-Cr steels (or) Cr – V steels.
5. What are the
types of shafts and their importance?
1. Transmission
shafts – These shafts transmit power between the source and the machines
absorbing power. These shafts carry
machine parts such as pulleys, gears etc.
they are subjected to bending in addition to twisting.
2. Machine
shafts – these shafts form an integrated part of the machine itself. The crankshaft is an example of machine
shaft.
6. What are
various types of stresses induced in the shafts. .
1. Shear
stresses due to transmission of torque.
2. Bending
stresses.
3. Stresses due
to combined torsional and bending loads.
7. What are standard sizes of transmission
shafts?
1. 25mm to 60mm
with 5mm steps.
2. 60mm to 110mm
with 10mm steps.
3. 110mm to
140mm with 15mm steps.
4. 140mm to
500mm with 20mm steps.
Standard length
– 5m, 6m and 7m.
8. On what basis the shafts are designed.
1. Based on
rigidity and stiffness 2. Based on
strength 3. Based on critical speed.
9. Differentiate the hollow shaft and solid
shaft.
The
hollow shafts are used in marine work.
These shafts are stronger per kg of material and they may be forged on a
mandrel, thus making the material more homogenous than a solid shaft.
10. Give
examples for shafts subjected to axial load in addition to torsion and bending
loads.
a. propeller
shafts of ships b.
shafts for driving worm gears
c. main shaft of
Kaplan turbines.
11. What are the desirable properties for the
materials for shafts and axles?
a. sufficient
high strength
b. a low
sensitivity to stress concentration
c. ability to
withstand heat and case hardening treatment.
d. good machinability
12. How the
shafts are designed when it is subjected to twisting moment only?
When
the shaft is subjected to torque only, then it is designed based on torsion
equation.
13. Why rotating shaft are generally made with
circular cross section?
Stress
distribution pattern will be uniform throughout the circular cross section.
14. Define
Torsional stiffness of shaft.
It
is defined as the resisting strength of a shaft to torsional load.
Mathematically
it can be calculated by the formula.
15. If the shaft
is subjected to torsion and bending moment, the shaft diameter can be determined based on the two theories namely
_________
Ans:
Guest’s theory and Rankine’s theory.
16. What are the
ways of improving lateral rigidity of shafts?
1. maintaining
proper bearing clearances
2. correct gear
teeth alignment.
17. Define
critical speed of a shaft.
Rotating
shaft tends to vibrate violently in transverse direction at certain speeds
known as critical (or) whirling speed.
When the natural frequency of vibration is equal to the speed of the
shaft, resonance will occur. Such a
value of natural frequency is called critical or whirling speed.
18. State any
two reasons for preferring hollow shaft over solid shaft.
1.
for some weight of shaft, hollow shaft can transmit 1.5 times the torque
transmitted by solid shaft.
2. for a particular power
transmission hollow shaft requires minimum weight.
19. What is
column factor?
If
a long shaft subjected to axial load (compressive load) in addition to torsion
and bending, a factor must be introduced to take the column effect into
account.
