Gearbox Power Rating 4

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.

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.

michaelb7,

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?


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JHG

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.

I agree.

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.

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.

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.

David

michaelb7-

If you would like to learn more about this subject I would recommend reading "ANSI/AGMA 6013-A06 Standard for Industrial Enclosed Gear
Drives". 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.