Speed reducers are widely available in a multitude of ratios and power ratings. Under some limited conditions, speed increasers can also be used as speed reducers. Some types of reducers cannot be used as increasers. Many worm gear reducers and some cycloidal designs will not back drive as an increaser. In the more common case of the worm reducer, if the lead angle of the worm is less than 10 degrees, the gear set is self-locking. In this case the frictional forces in the gear mesh cannot be overcome by a torque applied to the worm wheel.
Spur and helical gear reducers are the best candidates for use as a speed increaser. Generally a helical or spur gear speed reducer may be used as a speed increaser within the same speed. For example, a 4:1 ratio, helical gear speed reducer rated at 5HP, 1750 RPM input can be used as a 1:4 ratio, helical gear speed increaser rated at 5 HP, 438 RPM input. In this case, the resulting gear, bearing loads and shaft speeds are the same in both applications. This case is often not very helpful.
Why Can’t the 5HP Reducer be Used as a 20HP, 1750 RPM increaser?
Gear tooth loading is affected by speed, the higher pitch line speeds in the speed increaser will de-rate the gear capacity in a non-linear relation to shaft speed. The unit has less load capacity. Even at reduced load (5HP, 1750 RPM input) depending upon the accuracy of the gearing, the dynamic load affect may be nearly equal to the load reduction.
Bearing Life Directly Impacted by Speed:
Lubrication is Dramatically Impacted by Speed:
Lip Type Oil Seals Rated Based Upon Shaft Surface Speeds:
In summary, helical and spur gear reducers generally may be used as speed increasers within the operating range for which they were designed as a reducer. Operating a gear reduce at a higher speeds as a speed increaser will lead to significantly reduced load capacity and life, even at significantly reduced loads. Catastrophic failure due to thermal runaway is possible since lubricant churning losses and seal losses are speed dependent and not reduced with load. The gear reducer has a limited ability to dissipate these increased thermal loads.