Machine Kinematics Multiple Choice Questions on “Contact Ratio”.
1. The tooth profile most commonly used in gear drives for power transmission is
a) A cycloid
b) An involute
c) An ellipse
d) A parabola
Answer: b
Clarification: It is due to easy manufacturing.
2. There are six gears A, B, C, D, E, F in a compound train. The numbers of teeth in the gears are 20, 60, 30, 80, 25 and 75 respectively. The ratio of the angular speeds of the driven (F) to the driver (A) of the drive is
a) 1/24
b) 1/8
c) 4/15
d) 12
Answer: a
Clarification: The ratio of angular speeds of F to A = TA.TC.TE/TB.TD.TF
= 20 x 30 x 25/60 x 80 x 75
= 1/24.
3. A fixed gear having 100 teeth meshes with another gear having 25 teeth, the centre lines of both the gears being joined by an arm so as to form an epicyclic gear train. The number of rotations made by the smaller gear for one rotation of the arm is
a) 3
b) 4
c) 5
d) 6
Answer: c
Clarification: Revolution of 25 teeth gear = 1 + T100/T25
= 1 + 100/25 =5.
4. Speed reduction in a gear box is achieved using a worm and worm wheel. The worm wheel has 30 teeth and a pitch diameter of 210 mm. If the pressure angle of the worm is 20o, what is the axial pitch of the worm?
a) 7 mm
b) 22 mm
c) 14 mm
d) 63 mm
Answer: b
Clarification: m = 210/30 = 7
P = πm = 22/7 x 7
= 22 mm
Axial pitch = circular pitch of the worm wheel = πm.
5. A speed reducer unit consists of a double-threaded worm of pitch = 11 mm and a worm wheel of pitch diameter = 84 mm. The ratio of the output torque to the input to rque is
a) 7·6
b) 12
c) 24
d) 42
Answer: a
Clarification: Output torque/Input torque = pitch diameter of worm wheel/ pitch of worm
= 84/11
= 7.6.
6. A pair of gears forms a rolling pair.
a) True
b) False
Answer: b
Clarification: In rolling pair one link rolls over another fixed link.
7. Spiral bevel gears designed to be used with an offset in their shafts are called ‘hypoid gears’
a) True
b) False
Answer: a
Clarification: The pitch surfaces of such gears are hyperboloids of revolution.
8. Gears with involute tooth profile transmit constant velocity ratios between shafts connected by them.
a) True
b) False
Answer: a
Clarification: For involute gears, the common normal at the point of contact between pairs of teeth always passes through the pictch point.
9. In the case of spur gears, the mating teeth execute pure rolling motion with respect to each other from the commencement of engagement to its termination.
a) True
b) False
Answer: a
Clarification: The involute profiles of the mating teeth are conjugate profiles which obey the law of gearing.
10. A pair of helical gears has fewer teeth in contact as compared to an equivalent pair of spur gears.
a) True
b) False
Answer: b
Clarification: In spur gears, the contact between meshing teeth occurs along the entire face width of the tooth, resulting in a sudden application of the load which, in turn, results in impact conditions and generates noise.