Question about motors and pinions

I’ve got a question regarding an offseason project.

I’ve got a 4kw BLDC motor that I am planning to use in an existing gearbox. The motor has a 10mm dia x 34mm length shaft. In the gearbox does not have any support bearing for the end of the motor shaft. The pinion would usually sit pretty close to the motor as the gearbox housing is about 6mm thick. In order to use this motor in the gearbox I have to add an adaptor plate however, because the mounting patterns are differnet. The adaptor plate is an aditional 8mm thick.

Here is the question: would pushing the pinion farther out on the motor shaft lead to excessive radial loading on the bearings and potential premature motor death?

Do you have any sort of manual for the motor in question, or a way to contact the manufacturer?

Generally, motor shafts are designed to be loaded without the need for additional support. There are limits to how much side loading it can take, but in general “typical” pinion gears would be well within the shaft side load capabilities.

If there are limits, the supplier should have put those in the manual.


The short answer is: Maybe?

It depends a lot on how the motor is constructed and what kinds of loads it’s designed to take. For example, I wouldn’t have any issues mounting a pinion gear in the manner you described on a CIM motor because I know internally it has fairly strong bearings/bushings that will be able to handle it.

If the gearbox you’re using is the type this motor is expected to be used with (IE, RC motor + RC gearbox) then moving the pinion out a bit probably wouldn’t hurt anything. If it isn’t, or if you’re not sure, then I would be cautious. Like @wgorgen mentioned, a manual or other technical information on the motor would help with this; or you could always try taking the motor apart and examining its construction (no guarantees it will work after doing this though).

Now all that said, I would suspect in most cases that excessive radial loading, rather than killing the motor, is probably more likely to bend the output shaft resulting in damage to the gears. The only motor I can think of where the motor would die first would be something like a computer fan.


4 kw seems like a lot (~5 HP) for a 10 mm shaft.
So you are proposing to move the pinion another 8 mm away from the motor, correct?
As others have written, maybe. It depends on the motor’s construction, particularly the output shaft bearing (or bushing). Research that to see what you have.

Of course, the radial loading on a pinion at the center of a planetary gearbox is minimal, while in a spur gearbox it can be fairly high (depending on motor output used) and in a worm or bevel gearbox it is extremely high. So the kind of gearbox matters too, as does the expected power transmission.

Worst case, the motor lasts only a thousand hours instead of its rated 50,000. Would that be a problem?

It’s possible to calculate the bending stress on the shaft and do repeated load calculations and such to find out the lifetime of the shaft. The bearings are less likely to have problems than the shaft, as (theoretically) the side load on the bearing should be roughly the same independent of where the gear is, depending on the loads in question. Those can be calculated too.

A 10mm shaft will do a lot of HP at high RPMs. If you have the working torque of the motor (ar at the very least, the stall torque), the size of the pinion gear, and you know the material of the shaft, I can help walk you through calculating the life of the shaft at a given RPM.

Th side loading can kill motors. I recall in 2013, some of the team members killed a CIM after putting 2 or 3 frisbees through their shooter prototype. I suspect they had almost no compliance in the system. When we took it apart, we found it was the sleeve bearing at the closed end of the motor that failed.

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