Understanding Gearbox Ratings: Input Torque vs. Output Torque

Choosing industrial components requires an apples-to-apples comparison. Be aware of how torque ratings are determined —manufacturers don't all use the same rating standards.
Let’s clear that up, so you can spec confidently.

Let’s clear that up, so you can spec confidently.

Input vs. output torque

Input torque refers to the power going into the gearbox. Output torque is the usable power produced after the gears perform their function. 

How does efficiency affect the relationship?

Gearboxes are rated for two things, primarily: 

• RPM (revolutions per minute): This is typically limited by the maximum rotational speed the gearbox can handle.
• Torque capability: In right-angle gearboxes, input torque and output torque don't usually operate at a ratio of 1:1. And the type of gear impacts how much of a divide that is. For example, when the right-angle gearbox uses helical gears, the ratio changes because helical gears are only 60% efficient. So, 100 ft-lbs of input torque will only produce 60 ft-lbs of output torque. Spiral bevel gears, on the other hand, are about 97% efficient—so there is very little change between the input and output torque. 

Why are manufacturer ratings different? 

Ratings are often confusing because some brands use input torque while others—like Branham—use output torque. A "higher" rating on paper might actually offer the same capacity as a Branham gearbox.

Using our helical gear example above, Company A might rate their gearbox at 100 ft-lbs (input torque), while our comparable Branham gearbox would show a rating of 60 ft-lbs (output torque). 

Another factor often overlooked is the service level at which the maximum torque is measured.  If one gearbox calculates torque ratings at a higher maximum temperature than the exact same gearbox, the one at the higher temperature may appear to have a higher torque rating. But digging deeper, you may find that the higher temperatures of the first gearbox do not sustain constant use. In other words, one gearbox may be measured for intermittent use and another for continuous use.

How to convert between ratings for accurate comparison?

The functional style of the gears determines the gearbox’s efficiency. Once you understand the efficiency of different types of gears, then you can do the math:

• Amount of input torque (in ft lbs) X efficiency of the type of gears in your gearbox 

Which rating is most important for your application?

While output torque drives the application, input torque dictates your motor choice. The motor must provide the necessary RPM and torque without overpowering the gearbox. Before you choose, evaluate:

• Input limitations for the device
• Output required for the application

Once you understand gearbox ratings and the key selection factors involved, easy math allows you to figure out what you need and how to compare options. 

As always, Branham engineers are happy to help you choose the most effective gearbox for your application.