Gearbox Power Rating
Gearbox Power Rating
(OP)
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
RE: Gearbox Power Rating
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?
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JHG
RE: Gearbox Power Rating
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
RE: Gearbox Power Rating
RE: Gearbox Power Rating
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
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
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
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.