DME II SHORT ANSWER QUESTIONS
MODULE 1
1. What is critical speed of a shaft?
Critical or whirling or whipping speed is the speed at which a rotating
shaft tends to vibrate violently in the transverse direction if the shaft rotates
in the horizontal direction. In other words, the whirling or critical speed is
the speed at which resonance occurs.
2. What are the causes of failure in shafts?
The following stresses are the causes of failure in shafts:
a) Shear stresses due to the transmission of torque (i.e. due to torsional load).
b) Bending stresses (tensile or compressive) due to the forces acting upon
machine elements like gears, pulleys etc. as well as due to the weight of
the shaft itself.
c) Stresses due to combined torsional and bending loads.
3. Differentiate between torsional rigidity and lateral rigidity of shaft.
Torsional Rigidity is the resistance to the angular twist of the shaft along its
rotational axis. GJ will represent the torsional rigidity, which is the torque
transmitted by the shaft for a unit angular twist in the unit length of the shaft.
Lateral Rigidity is the resistance to the deflection of the shaft along the lateral
axis. It is also known as flexural Rigidity (EI).
4. Compare the strength and stiffness of a hollow shaft of same outside
diameter as that of a solid shaft.
For solid circular shaft, For hollow circular shaft,
𝜏 T
=
𝑟 J
𝜋𝐷4 𝜋
J= J = 32 (𝐷 4 − 𝑑 4 )
32
16T 16T𝐷 16 T
𝜏= 𝜏= =
𝜋𝐷3 𝜋(𝐷4 −𝑑 4 ) 𝜋𝐷3 (1−𝐾 4 )
, 𝐷
Assume d = , K = 0.5
2
16 𝑇 16 𝑇
𝜏= = 1.066
𝜋𝐷3 (1 − 0.54 ) 𝜋𝐷3
It may be seen that 𝜏𝑚𝑎𝑥 in the case of hollow shaft is 6.6 % larger than in the
case of a solid shaft having the same outside diameter. Therefore, hollow
shafts are stronger than the solid shaft.
5. Enumerate the various types of flat belt drives.
1. Open belt drive. The open belt drive, as shown in Fig. 18.4, is used with
shafts arranged parallel and rotating in the same direction.
2. Crossed or twist belt drive. The crossed or twist belt drive, as shown in
Fig. 18.5, is used with shafts arranged parallel and rotating in the opposite
directions.
, 6. Explain about the different materials used for flat belts.
Leather belts: The most important material for flat belt is leather. The best
leather belts are made from Top grade steer hides. The belts are specified
according to the number of layers.
Rubber belts: consisting of layers of fabric impregnated with a rubber
composition and having a thin layer of rubber on the faces, are very flexible
but are quickly destroyed if contact with oil or grease.
Balata belts: are similar to rubber belts except that balata gum is used in place
of rubber. It is about 25% stronger than rubber belt. These belts are acid proof
and water proof and it is not affected by animal oils or alkalies.
Cotton or fabric belts: are made from woven fabric or cotton in which a
number of layers, depending upon the thickness desired are put and stitched
together. These are treated with linsoil to make it water proof.
7. Enumerate the advantages and disadvantages of a flat belt drive.
Advantage of the flat belt drive
The flat belts are very simple in design and cheaper.
Smooth running and quiet running.
Flat belts can be used for long distances, even up to 15 m.
Precise alignments of shafts and pulleys are not as critical with flat belts.
The periodic adjustment of the belt tension and belt replacement, when it is
worn out, is easier in the case of flat belts.
Flat belts are very flexible and, therefore, they have the ability to bend and
twist on pulleys and follow very tortuous paths.
Flexible and longer belts can absorb shock and torsional vibration.
They are durable.
Disadvantage of the flat belt drive
High chance of slip off.
Compared to v-belts, flat belts require significantly greater of pretension to
transmit a certain torque without slip.
Flat belts are not advisable for high outputs at high speeds. The flat belt
cannot transmit high power as timing belt does.
They are bulky, for a given transmission, flat belt drive requires a wide
pulley.
MODULE 1
1. What is critical speed of a shaft?
Critical or whirling or whipping speed is the speed at which a rotating
shaft tends to vibrate violently in the transverse direction if the shaft rotates
in the horizontal direction. In other words, the whirling or critical speed is
the speed at which resonance occurs.
2. What are the causes of failure in shafts?
The following stresses are the causes of failure in shafts:
a) Shear stresses due to the transmission of torque (i.e. due to torsional load).
b) Bending stresses (tensile or compressive) due to the forces acting upon
machine elements like gears, pulleys etc. as well as due to the weight of
the shaft itself.
c) Stresses due to combined torsional and bending loads.
3. Differentiate between torsional rigidity and lateral rigidity of shaft.
Torsional Rigidity is the resistance to the angular twist of the shaft along its
rotational axis. GJ will represent the torsional rigidity, which is the torque
transmitted by the shaft for a unit angular twist in the unit length of the shaft.
Lateral Rigidity is the resistance to the deflection of the shaft along the lateral
axis. It is also known as flexural Rigidity (EI).
4. Compare the strength and stiffness of a hollow shaft of same outside
diameter as that of a solid shaft.
For solid circular shaft, For hollow circular shaft,
𝜏 T
=
𝑟 J
𝜋𝐷4 𝜋
J= J = 32 (𝐷 4 − 𝑑 4 )
32
16T 16T𝐷 16 T
𝜏= 𝜏= =
𝜋𝐷3 𝜋(𝐷4 −𝑑 4 ) 𝜋𝐷3 (1−𝐾 4 )
, 𝐷
Assume d = , K = 0.5
2
16 𝑇 16 𝑇
𝜏= = 1.066
𝜋𝐷3 (1 − 0.54 ) 𝜋𝐷3
It may be seen that 𝜏𝑚𝑎𝑥 in the case of hollow shaft is 6.6 % larger than in the
case of a solid shaft having the same outside diameter. Therefore, hollow
shafts are stronger than the solid shaft.
5. Enumerate the various types of flat belt drives.
1. Open belt drive. The open belt drive, as shown in Fig. 18.4, is used with
shafts arranged parallel and rotating in the same direction.
2. Crossed or twist belt drive. The crossed or twist belt drive, as shown in
Fig. 18.5, is used with shafts arranged parallel and rotating in the opposite
directions.
, 6. Explain about the different materials used for flat belts.
Leather belts: The most important material for flat belt is leather. The best
leather belts are made from Top grade steer hides. The belts are specified
according to the number of layers.
Rubber belts: consisting of layers of fabric impregnated with a rubber
composition and having a thin layer of rubber on the faces, are very flexible
but are quickly destroyed if contact with oil or grease.
Balata belts: are similar to rubber belts except that balata gum is used in place
of rubber. It is about 25% stronger than rubber belt. These belts are acid proof
and water proof and it is not affected by animal oils or alkalies.
Cotton or fabric belts: are made from woven fabric or cotton in which a
number of layers, depending upon the thickness desired are put and stitched
together. These are treated with linsoil to make it water proof.
7. Enumerate the advantages and disadvantages of a flat belt drive.
Advantage of the flat belt drive
The flat belts are very simple in design and cheaper.
Smooth running and quiet running.
Flat belts can be used for long distances, even up to 15 m.
Precise alignments of shafts and pulleys are not as critical with flat belts.
The periodic adjustment of the belt tension and belt replacement, when it is
worn out, is easier in the case of flat belts.
Flat belts are very flexible and, therefore, they have the ability to bend and
twist on pulleys and follow very tortuous paths.
Flexible and longer belts can absorb shock and torsional vibration.
They are durable.
Disadvantage of the flat belt drive
High chance of slip off.
Compared to v-belts, flat belts require significantly greater of pretension to
transmit a certain torque without slip.
Flat belts are not advisable for high outputs at high speeds. The flat belt
cannot transmit high power as timing belt does.
They are bulky, for a given transmission, flat belt drive requires a wide
pulley.
