# Strength of Helical Gears Questions & Answers :: Gear Design

1. Lewis form factor is based on real number of teeth.

a) True
b) False

Justification: It is based on virtual umber of teeth only.

2. Beam strength indicates the maximum value of radial force that a tooth can transmit without fatigue failure.

a) True
b) False

Justification: It indicates maximum force for bending failure and not fatigue failure.

3. A pair of helical gears consist of 25 teeth pinion gear meshing with a 90 teeth gear. Calculate the ratio factor.

a) 0.74
b) 0.88
c) 1.57
d) 1.44

Justification: Ratio factor Q=2×90/90+25.

4. A pair of helical gears consist of 25 teeth pinion gear meshing with a 90 teeth gear. Calculate the material constant k If surface hardness is 260BHN.

a) 0.64N/mm²
b) 0.88N/mm²
c) 1.08N/mm²
d) 2.66N/mm²

Justification: The material constant k If surface hardness is K = 0.16 × [BHN/100]2 = 1.08N/mm2

5. A pair of helical gears consist of 25 teeth pinion gear meshing with a 90 teeth gear. Calculate the wear strength If surface hardness is 260BHN. Also face width=35mm, module=4mm and helix angle=25°.

a) 443.5N
b) 1125.6N
c) 7971.9N
d) 1014.2N

Justification: S = bQdK/cos²ω

where ω=25⁰, d = zm/cosω, Q=2 × 90/90+25,

K=0.16 × [BHN/100]² = 7971.9 N

6. A pair of helical gears consist of 25 teeth pinion gear meshing with a 90 teeth gear. Calculate the tangential force If surface hardness is 260BHN. Also face width=35mm, module=4mm and helix angle=25⁰. The velocity of operation is 3.5m/s and service factor 1.5.

a) 1136.5N
b) 3983.7
c) 2012.6N
d) 3226.5N

Justification: S = 1.5 × P/C where C=5.6/5.6+√v and S=bQdK/cos²ω where ω=25°, d=zm/cosω, Q = 2×90/90+25, K=0.16 × [BHN/100]².

7. Which of the following is herringbone gear?

a) Left one
b) Right one
c) Both
d) None

Justification: Herringbone gear is without any groove.

8. The net axial force acting on bearing is zero in case of double helical gears while none zero in case of herringbone gears.

a) True
b) False

Justification: The net axial force acting on the bearing is zero in the case of double-helical gears as well as zero in the case of herringbone gears.

9. Helix angle of herringbone and double helical gears is relatively higher.

a) True
b) False

Justification: There is no thrust force and hence higher angles are permitted.

10. A herringbone speed reducer consist of 20 teeth pinion driving a 100 teeth gear. The normal module of gear is 2mm. The face width of each half is 30mm and Lewis factor is 0.4. If permissible bending stress is 500N/mm², then calculate the beam strength.

a) 15000N
b) 12000N
c) 8000N
d) 10000N

Justification: The beam strength is given as S = mbσY = 12000 N

11. A herringbone speed reducer consist of 20 teeth pinion driving a 100 teeth gear. The normal module of gear is 2mm. The face width of each half is 30mm and Lewis factor is 0.4. Calculate the ratio factor Q.

a) 1.2
b) 1.4
c) 1.7
d) 1.4

Justification: The ratio factor Q = 2×100/100+20. = 1.7

12. A herringbone speed reducer consist of 20 teeth pinion driving a 100 teeth gear. The normal module of gear is 2mm. The face width of each half is 30mm and Lewis factor is 0.4. Calculate the material constant K if surface hardness is 400BHN.

a) 1.25 N/mm²
b) 4.05 N/mm²
c) 3.25N/mm²
d) 2.56N/mm

Justification: The material constant K if surface hardness is K=0.16x[BHN/100]² = 3.25N/mm²

13. Helical gears mounted on parallel shafts are called crossed helical gears.

a) True
b) False

Justification: Crossed helical gears are the helical gears mounted on non-parallel shafts.

14. Crossed helical gears have very low load carrying capacity.

a) True
b) False

Justification: There is point contact and hence very less area and thus wear is comparatively rapid.

15. Calculate the shaft angle for same hand of helix if helix angle of two gears are 20⁰ and 17⁰.

a) 17°
b) 20°
c) 37°
d) 3°