Page 19 - Gear Technology Solutions

accelerator abruptly. It becomes important in cases of high ratios that back
driving is possible. This means, now, the inertia of the vehicle transfers rotation
and torque via the rotating wheels to the engine. The effect is known as engine
brake. In the case of electric vehicles, the motor becomes a generator and
charges the batteries, while the vehicle coasts with a certain break load. In
case of a self-locking gearset, the break load due to friction and the high ratio
becomes higher than the coasting torque from the wheels which causes the
wheels to lock. This effect presents an unsafe driving condition besides the
high friction wear on the tires and the fact that battery charging is prevented
[4].

Figure 3: Special Methods Dimension-Sheet
part 3, back driving factor

Back driving ability is not a “yes or no” condition but a step-less transition
between “freewheeling” and “locking”. The back driving factor in part 3 of the
Dimension-Sheet is calculated by dividing the break torque by the driving
torque based on the pinion and gear tooth geometry and a coefficient of friction
m = 0.08. A back driving factor of zero (freewheeling) is impossible and would
represent a condition without any friction. The other extreme, a back driving
factor larger than 1.0, implies a condition of solid self-locking, which is also
impossible. Back driving factors of 1.0 and above suggest that the break
torque is larger than the driving torque, which cannot be achieved with realistic
coefficients of friction.

The calculation of the values in Figure 3 is based on a SRH gearset with 4
pinion teeth and a ratio of 15. The resulting back driving factor of 0.139
indicates an excellent back driving ability. Back driving factors between 0.05

4