Cantilevering with Planetary Gear Motors

[Ginger Power]

Obviously cantilevering a planetary gear motor output shaft is generally frowned upon... but is it permittable if the loads are low enough? I am working with this motor, and direct driving a 10 lb load directly off the output shaft of the motor. In doing some research I've come to the obvious conclusion that I shouldn't be putting this kind of load on this kind of motor. Is 10 lbs an excessive load for this motor, and how would I go about calculating that? The packaging requirements for the application of this motor are really tight. Direct driving seems to be the cleanest way to fit the motor in the alloted space. As a side question, does anybody have suggestions for an alternate to direct driving?

[Ivan Helmrich]

The spec. sheet gives the max radial load as 3.5 kgf, or around 8 lbs. So no, you shouldn't do that. You could probably get away with it if what you are designing doesn't need to last very long though.

[AdamHeard]

Originally Posted by Ginger Power

Obviously cantilevering a planetary gear motor output shaft is generally frowned upon... but is it permittable if the loads are low enough? I am working with this motor, and direct driving a 10 lb load directly off the output shaft of the motor. In doing some research I've come to the obvious conclusion that I shouldn't be putting this kind of load on this kind of motor. Is 10 lbs an excessive load for this motor, and how would I go about calculating that? The packaging requirements for the application of this motor are really tight. Direct driving seems to be the cleanest way to fit the motor in the alloted space. As a side question, does anybody have suggestions for an alternate to direct driving?

Are you dead certain you can't extend the shaft (with something larger OD) and support the other side?

If not, and you can afford for the planetary to get longer you can make a block to hold two larger bearings with a larger shaft. The shaft on the planetary would interface with the ID of this shaft, and the face of the planetary would mount to this block.

[GeeTwo]

Or, if you don't need as much shaft as the PG output offers, and don't really need all that much transverse force (e.g. your 10 lb), you could add another (well-supported) bearing between the PG output block and the end of the shaft. The farther away the bearing is from the PG, and the closer the load is to the extra bearing, the better.

[MaGiC_PiKaChU]

We had our whole drive train in 2014 directly on 6 cantilevered versa planetaries on CIMs... sketchy but it works when there is little or no impacts

[Al Skierkiewicz]

I answer similar questions like this every year. The mechanical side load also translates to much higher electrical loads in planetary transmissions. So while you may not damage the transmission, you may cause significant damage to the motor to the point of failure.

[Richard Wallace]

I will chime in here to support Al.

I always ask my team's students to measure free (wheels up) current with the drivetrain fully assembled, and compare that to the sum of the free currents of all the drive train motors. The difference is extraneous mechanical loss.

Extraneous mechanical losses are bad for your robot in several ways. As Al pointed out, driving those losses can overheat the motor. They rob power from your wheels, causing you to lose pushing matches. And they overheat gear meshes, bearings, fit points, etc. -- which accelerates wear and can cause early, possibly catastrophic failures.

Cantilevered loading of planetary gearboxes also increases extraneous mechanical losses. This particular loss is not easily measured with the wheels up. It is similar to the loss caused by excessive chain tension, and should be relieved as Adam and others have suggested; i.e., by supporting the output shaft on its outboard end, so that side load is not transmitted to the gear meshes.

[Chris is me]

Originally Posted by MaGiC_PiKaChU

We had our whole drive train in 2014 directly on 6 cantilevered versa planetaries on CIMs... sketchy but it works when there is little or no impacts

Note for those reading along at home: You probably shouldn't do this. It definitely exceeds the load rating of VPs to do this and you're playing with fire here.

You can get away with cantilevering VPs a little more than most planetary gearboxes - the shaft is supported well by double bearings in a solid block, so it's generally okay to put a cantilevered sprocket or whatever on it for many mechanisms as long as it's super close to the output.

I don't have any experience with the linked motor / planetary so I can't offer specific advice there, nor do I really know what a "10lb load" translates to in terms of torsion or bending torque (is it a long arm? is it far from the edge of the gearbox casing?) but I would listen to Adam's suggestions for relieving the load of the mechanism from the gearbox.

[Ginger Power]

Thanks for the awesome replies everybody. It's been a huge help.