In a typical geared motor system, the function of a gearbox is to take ac motor power, increase the torque, and reduce the speed. In this case, the most essential factors at a given gearbox output shaft are its speed expressed in rpm and the amount of torque it produces. However, when it comes to the efficiency of the gearbox, there are so many aspects involved, especially the gearbox loading.
In most cases, manufacturers usually publish the efficiency of the ac motor only, excluding gearbox efficiency. This is why the efficiency of the entire system, which includes both motor and gearbox, cannot be calculated easily and understood vaguely. Unsurprisingly, it’s often useless to rely just on catalog gearbox efficiency since it only provides a single efficiency rating. Total system efficiency depends on many factors, notably the gearbox loading. Unfortunately, efficiency tolerances are usually not indicated as well as the difference in efficiencies between a gearbox that runs under normal loads and a heavily loaded one.
In every ac motor application, the rule of the thumb is that the lighter the load of the gearbox and the higher the ratio, the less probable it is that a gearbox will approach its specified efficiency. This scenario usually has poor gearbox efficiency. On the other hand, with high ratios and heavy loads, the gearbox reaches its specified efficiency. So, in small gearboxes, the heat that’s being generated due to friction, does not pose a problem because power loss is relatively small.
Conversely, in bigger gearboxes, it’s a different story. Larger gearboxes usually use oil pumps and coolers to compensate for the inefficiency due to power loss caused by the friction. All of these have something to do with the quality of the gearing, the load torque, and the number of tooth engagements or the times one gear drives another.
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