Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Gearbox Power Rating

Gearbox Power Rating

Gearbox Power Rating

Does the horsepower rating of a gearbox depend on the input speed from the motor? Most catalogs seem to have the ratings based on an input speed of 1750 rpm. I'm driving my gearbox with a 1000 rpm motor. My gearbox is rated for 3/4 hp and I'm driving it with a 3/4 hp motor. I'm concerned that since I'm driving the gearbox at a lower speed with the same horsepower, more torque will be developed internally. Are gearboxes truly rated for input power, or are they rated for input power at a certain speed?

RE: Gearbox Power Rating

This shows why gear boxes are usually rated for a horsepower even though the truly important limit is torque. The electric motor has basically the same limitation. If you slow the motor it's power capacity goes down also. Note that I said capacity. The actual horsepower depends on the load, not the motor. Rating gearboxes this way makes it easy to match it with a motor.

RE: Gearbox Power Rating

The problem is that I'm not slowing the motor down. Instead of using a 3/4 hp motor rated at 1750 rpm, I'm using a different 3/4 hp motor rated at 1000 rpm.

I feel like I should be using a gearbox rated for (3/4)(1750)/(1000)= 1.31 hp. I'd round this up to a gearbox rated for 1.5 hp.

Not sure why most gearbox catalogs are layed out like this, I find it confusing.

RE: Gearbox Power Rating


The rating of your gearbox probably takes the dumping of heat into account, as well as the maximum speed of your bearings. The important parameter still is torque, which you can work out easily.

Have you called the manufacturer?


RE: Gearbox Power Rating

I called the manufacturer, I talked to a couple of people who seemed to know less than I did.

It sounds like you agree with my calculation above so I will go with a 1.5 hp box. It's not a huge cost difference and I'd rather be safe than sorry.

RE: Gearbox Power Rating

I agree.

RE: Gearbox Power Rating

Commercial gearbox designs are rated based on many factors. For example, the scuffing limit of the gears is mostly a function of power transmitted across the mesh. While fatigue life of the gears and bearings is a function of load magnitude and number of load cycles. A lower shaft speed would result in fewer load cycles, but a higher shaft torque would result in greater applied load for each cycle. Heat rejection is usually only an issue with gear drives having low efficiency, marginal lubrication, high speeds, or high power.

RE: Gearbox Power Rating

Thermal horsepower and mechanical horsepower are both requirements which need to be satisfied when sizing a reducer - but you have stumbled upon the issue of sizing for the correct output torque. There are typically service factors involved in reducer sizing, and using a reducer rated at 3/4 HP with a 3/4 HP motor may cause problems in a few months.

Maybe your gearbox literature does not list it, but a great many reducer manufacturers list power ratings at different speeds, including 1000 rpm. If the input speed is too low, the reducer primary gear may not throw lubricant off of the high speed shaft properly, but I doubt that to be the case at 1000 rpm. Sounds to me like you need to 1.) buy from a manufacturer who publishes literature which includes gear box sizing information and 2.) read that literature.

RE: Gearbox Power Rating

As said already, but I'll try to summarize:
Gearbox reducer ratings are primarily based on torque at the output shaft. Not all customers prefer to calculate this, so many manufacturers provide instead a HP rating chart at various common speeds. If you do the math, you'll see they are usually about the same torque for any given ratio/configuration. These ratings should cover the limits of the gearbox fracturing the teeth (sudden) or tooth surface fatigue (gradual) failures, whichever is lower. As you have proposed to change the motor, the torque will increase and the gearbox rating will go down, leading to either a greater risk of failure when overloaded or loss of overall lifespan.

In most cases, the rating is not as much as the peak load encountered in startup or upset conditions. Therefore rating is not enough for most cases. We call this extra margin service factor. Service factor = gearbox rating / actual load. 1.5 to 2.0 is popular for many industrial applications with well-understood loading.

The thermal rating is another limit that can come into play, but it tends to follow HP and speed. Given the same HP and lower speed motor you mentioned, the box should run cooler. If you are buying a drive where thermal rating might be a factor, try to understand how much temperature rise is expected with the thermal rating. Some drives run much hotter than you would expect.

The bearing life will change also. While the bearings will rotate through fewer load cycles per minute, the load on the bearings follows torque, which is increased. Of the two opposing factors you should see a net loss of bearing life as a result of the higher load and less optimal lubrication conditions.

Depending on your application, the gearboxes you consider may have some/all/none of this in their engineering tables. (For example, for slow-speed drives thermal may never be a practical problem and therefore omitted) It would concern me greatly if your gearbox supplier is not familiar with all of this.

Up-sizing the gearbox was a wise choice. It insures your service factor is maintained.

Hope this helps.


RE: Gearbox Power Rating


If you would like to learn more about this subject I would recommend reading "ANSI/AGMA 6013-A06 Standard for Industrial Enclosed Gear
. This standard is commonly used for rating commercial gear drives, and it provides a detailed description of the effect each variable has on the rating.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close