Ever since I used Cobra motors on my mini quad, I came across the issue where the motor bell develops vertical movement ( bell is can move up and down). We asked Cobra about it and here is the reply from the CEO himself.
Further Reading: Motors for Mini Quad Beginner’s Guide
In some cases this movement can cause bad vibrations that makes the quad difficult to tune, and even jello in FPV footage.
We think this should be brought to maker attention, so we asked the very man about that issue and this is his reply. Hope that helps those who have the same issue.
Reasons for Vertical Play in Brushless motors
Typically this is caused by the shaft slowly working out of the bell over time, typically due to weak glue or insufficient tolerances. If it is causing vibrations you can can attempt to solve it.
Fix Motor Vertical Play
If you have some spare copper spacers that is an easy solution, but over time it will likely continue to increase. If you want to attempt to fix the bell, turn the motor over and place it so the shaft goes into a hole in a board that is deep enough so the pressure is on the bell rather than the shaft. Tap VERY gently with a hammer and a punch on the shaft. Check it frequently until the play is gone. If you go too far it is MUCH harder to get it back out, so don’t overshoot.
To press a shaft you can also use a screw-type C-clamp. Basically turn the clamp slowly to press the shaft in to the exact tension required. It will work its way back out overtime. A permanent fix might be to remove the shaft entirely and then use green loc-tite to secure it. But… probably not worth the price of a replacement motor.
Reply from Cobra
It is actually quite normal to have a small amount of up and down play in the motor shaft. Due to variations in the grinding process when the shafts are made, there range of acceptable shaft diameters. The manufacturing spec calls for the shaft to be 3.000mm -.006/-.012. The shaft has to be a tiny bit smaller than the inside diameter of the motor bearings in order to be able to fit properly. All of the shafts do move, because they have to in order for the motor to be assembled.
Some motors that have shafts which are on the larger end of the acceptable range require a bit more force to slide into the bearings, and once these motors are assembled, it looks like the motors have no end play. For motors which have a shaft that is on the smaller end of the acceptable diameter range, they will freely float in the bearings and move up and down rather easily. This is in no way an indication of a problem with the motor.
There does need to be a small amount of end play in the shaft in order to prevent the bearings from getting into a bind when the motor comes up to operating temperature during hard racing use. The amount of this designed in end play is around 0.25 to 0.5mm, depending on part tolerance build-up.
Cobra recently initiated an additional step in the shaft grinding process to ensure that the shaft smoothness allows for a small amount of end play in the motors, which allows the motors to operate smoothly over a wide range of temperature conditions. In the end, you have absolutely nothing to worry about, and the motor will operate perfectly fine.
President & CEO
Innov8tive Designs, Inc.