Gearboxes: Basic Features
Torque Loadablity / Lifetime
In long term operation torque load and lateral force load causes wear (abrasion) of gearbox components:
Wear of gear wheel surface (for instance pittings) as well as wear of bearings, particularly if sinter bearings are applied.
Wear and lifetime are also effected by speed.
The lifetime of the output shaft bearing depends on
- The lateral force acting on the shaft
- The distance of this force from the bearing
If a pinion is mounted, the lateral force is about
Flateral = Torque / (dpitch/2)
Majority of JE motors use sinter bearings.
For applications with high speed, high lateral force or axial force (e.g. worm or spindle mounted on gearbox shaft), ball bearings are used.
Beside long term loadability – peak torque loadability is important feature too.
It has to be considered for gear wheel teeth, but also for connections between wheels and pinions (if wheels are pressed on knurled pinion shafts).
Very high peak torque load can occur whenever the driven application should hit a mechanical end stop and if the mechanics has low flexibility (hard stop).
Precision of transmission is effected by:
1. Backlash (angular play) between meshing gear wheel teeth.
Major effect comes from the final gear stage.
The influence of other stages is reduced by the gear ratio
JE gearboxes have a backlash B of 1…3°, usually.
It can get increased after longer lifetime.
2. Distortion of the gear wheels, caused by increasing torque load.
If that value is to reduce, a stronger and bigger gearbox size must be used.
The effect of backlash and torque distortion on accuracy can be avoided, if the positioning operation is executed always from the same direction (overdrive the position and turn back).
3. Accuracy of gear wheel shape (thickness tolerance of teeth).
There are losses due to internal friction, between gear wheels and shafts and between shafts and bearings as well as from lubrication.
For operation at rated transmission torque, efficiency per gear stage is about 90% (except worm gears).
It depends on design, material, accuracy, size and lubrication.
However, if the gearbox is operated much below of its rated transmission torque (oversized), the efficiency is lower.
Noise emission is effected by accuracy of gear wheel shape, accuracy of gear wheel distance, material, speed and torque.
Main noise emission is generated by the first gear stage (motor pinion/ first gear wheel), due to its high speed.
Plastics material, for instance hytrel, is preferable for low noise.
Noise emission can be reduced by use of inclined teeth.
It is applied typically in the first stage.