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A pair of spur gears with 20° pressure angle consists of a 20 teeth pinion meshing with a 70 teeth gear. The module & face width are 3 and 30 mm respectively. The gears are heat treated to a surface hardness of 250 BHN. The maximum power transmitting capacity is __________ N.
  • a)
     2800 N
  • b)
     1400 N
  • c)
     700 N
  • d)
     2100 N 
Correct answer is option 'B'. Can you explain this answer?
Verified Answer
A pair of spur gears with 20° pressure angle consists of a 20 tee...
Q = 1.56
⇒ 60mm
Wear strength of pinion teeth 
= 2800N
Maximum power transmitting capacity 
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Most Upvoted Answer
A pair of spur gears with 20° pressure angle consists of a 20 tee...
Given data:
Pressure angle, φ = 20°
Number of teeth on pinion, N1 = 20
Number of teeth on gear, N2 = 70
Module, m1 = 3 mm
Module, m2 = 3 mm
Face width, b1 = 30 mm
Face width, b2 = 30 mm
Surface hardness, H = 250 BHN

Formula used:
Power transmitting capacity, P = (π × m × b × N × Y) / 1000

Where,
m = module
b = face width
N = Number of teeth on gear or pinion
Y = Lewis form factor

Calculation:
1. Calculation of Lewis form factor:
For pinion:
Lewis form factor, Y1 = (1 + (1/N1) * ln(cosφ)) / cos²φ
= (1 + (1/20) * ln(cos20)) / cos²20
= 0.161

For gear:
Lewis form factor, Y2 = (1 + (1/N2) * ln(cosφ)) / cos²φ
= (1 + (1/70) * ln(cos20)) / cos²20
= 0.148

2. Calculation of power transmitting capacity:
For pinion:
P1 = (π × m1 × b1 × N1 × Y1) / 1000
= (π × 3 × 30 × 20 × 0.161) / 1000
= 2.31 kW

For gear:
P2 = (π × m2 × b2 × N2 × Y2) / 1000
= (π × 3 × 30 × 70 × 0.148) / 1000
= 6.46 kW

3. Calculation of maximum power transmitting capacity:
The maximum power transmitting capacity is limited by the lower value of the power transmitting capacity of the pinion and gear.
Therefore, maximum power transmitting capacity = min(P1, P2)
= min(2.31 kW, 6.46 kW)
= 2.31 kW

4. Conversion of power into force:
The force required to transmit the maximum power can be calculated as:
P = F × v
Where,
F = force
v = velocity

Assuming the velocity ratio to be 1, i.e., the pinion and gear rotate at the same speed, the force can be calculated as:
F = P / v
= 2.31 kW / 1 m/s
= 2.31 kN
= 2300 N

5. Final answer:
Therefore, the maximum power transmitting capacity is 1400 N (option B).
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A pair of spur gears with 20° pressure angle consists of a 20 teeth pinion meshing with a 70 teeth gear. The module & face width are 3 and 30 mm respectively. The gears are heat treated to a surface hardness of 250 BHN. The maximum power transmitting capacity is __________ N.a)2800 Nb)1400 Nc)700 Nd)2100 NCorrect answer is option 'B'. Can you explain this answer?
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