Machine Kinematics Assessment Questions and Answers on “Standard Spur Gears – 2”.
1. A spur gear of 40 teeth is machined in a gear hobbing machine using a double start hob cutter. The speed ratio between the hob and the blank is
a) 1:20
b) 1:40
c) 40:1
d) 20:1
Answer: d
Clarification: None.
2. A helical gear has the active face width equal to b pitch p and helix angle α. What should be the minimum value of b in order that contact is maintained across the entire active face of the gear?
a) p cosα
b) p secα
c) p tanα
d) p cotα
Answer: d
Clarification: b > p/tanα
b > p cotα.
3. Match the type of gears with their most appropriate description.
Type of gear Description
P. Helical 1. Axes non parallel and intersecting
Q. Spiral 2. Axes parallel and teeth are inclined to the axis
R. Hypoid 3. Axes parallel and teeth are parallel to the axis
S. Rack and pinion 4. Axes are perpendicular and intersecting,and teeth are inclined to the axis
5. Axes are perpendicular and used for large speed reduction
6. Axes parallel and one of the gears has infinite radius
a) P-2, Q- 4, R- 1, S- 6
b) P-1, Q- 4, R- 5, S- 6
c) P-2, Q- 6, R- 4, S- 2
d) P-6, Q- 3, R- 1, S- 5
Answer: a
Clarification: Helical – Axes parallel and teeth are inclined to the axis
Spiral – Axes are perpendicular and intersecting,and teeth are inclined to the axis
Hypoid – Axes non parallel and intersecting
Rack and pinion – Axes parallel and one of the gears has infinite radius.
4. Tooth interference in an external in volute spur gear pair can be reduced by
a) Decreasing center distance between gear pair
b) Decreasing module
c) Decreasing pressure angle
d) Increasing number of gear teeth
Answer: d
Clarification: There are several ways to avoid interfering:
i. Increase number of gear teeth
ii. Modified involutes
iii. Modified addendum
iv. Increased centre distance .
5. Interference in a pair of gears is avoided, if the addendum circles of both the gears intersect common tangent to the base circles within the points of tangency.
a) True
b) False
Answer: a
Clarification: None.
6. Twenty degree full depth involute profiled 19-tooth pinion and 37-tooth gear are in mesh. If the module is 5 mm, the centre distance between the gear pair will be
a) 140 mm
b) 150 mm
c) 280 mm
d) 300 mm
Answer: a
Clarification: Centre distance = D1 + D2/2 = mT1 + mT2/2 = 5(19 + 37)/2
= 140 mm.
7. If the drive efficiency is 80%, then torque required on the input shaft to create 1000 N output thrust is
a) 20 Nm
b) 25 Nm
c) 32 Nm
d) 50 Nm
Answer: b
Clarification: Module m = 2,
D/T = 2
∴ D = 80 × 2 = 160 mm
2F = 1000, or F = 500 N
Let T1 be the torque applied by motor.
TT2 be the torque applied by gear.
∴ Power transmission = 80%
Now, T1ω1= 2T2 x ω2/0.8
T1 = 25 N-m.
8. If the pressure angle of the rack is 20°, then force acting along the line of action between the rack and the gear teeth is
a) 250 N
b) 342 N
c) 532 N
d) 600 N
Answer: c
Clarification: Pcos φ = F
∴ Force acting along the line of action,
P = F/cos φ
= 500/cos20°
= 532 N.
9. Two mating spur gears have 40 and 120 teeth respectively. The pinion rotates at 1200 rpm and transmits a torque of 20 Nm. The torque transmitted by the gear is
a) 6.6 Nm
b) 20 Nm
c) 40 Nm
d) 60Nm
Answer: d
Clarification: We know, N1/N2 = T1/T2
N1 = speed of pinion
N2 = speed of gear wheel
T1 = number of teeth of gear
T2 = number of teeth of pinion
1200/N2 = 120/40
N2 = 400 r.p.m.
Since power transmitted by both gear will be equal
T1ω1= T2 x ω2
torque transmitted by gear, = 60 N-m.