Motor Maintenance

Brush and Comm. Examination

The two main components to examine for wear are the brushes and the commuator. While the brushes are easily replaceable, maintaining the comm. requires cutting it with a special tool called a comm. lathe. Your local hobby store may do this for you. If not, you will need to have a comm. lathe do this properly. If you do not have the comm. cut, the brushes and comm. will wear even faster when you reassemble the motor. This is due to the brushes not being able to properly seat on the comm., which causes intense sparking and heat.

You can use a device called a comm. stick which will remove the glaze off of a comm. if it's not badly worn. However anything more than a slight glaze really should result in a comm. cut. Even if using a comm. stick, you will need to eventually have the comm. cut as wear increases. Replacing the armature instead of having the comm. cut is an option. However most armatures cost you about the same as the motor itself, and over time the magnets on the can lose their strength. So, in the end, you would be better off to just buy a whole new motor.

Below is an example of a badly worn comm. This is what happens if you continue to replace the brushes without having the comm. cut. The comm. takes on a concave shape. This comm. could be cut and the armature restored to service, however it is worn down so far that it will not last very long due to the amount of cutting that would have to be done. The lifespan of this motor, because of the neglect, is about half to a third of what it should have been.

The comm. below is an example of a comm. that is ready for a cut. A little work on a comm. lathe would easily restore it back to good condition. This is an example of the point you do not want to go beyond. Anything more than this amount of wear will start to cause a decrease in the lifespan of the motor. You want to cut when the comm. is discolored, not when it really starts to become physically worn down.

The brushes are another item that frequently show signs of wear. A new set of brushes only costs a few dollars, so replacing them is easy on the wallet. If the brush spring is almost fully extended into the brush hood, while looking at the assembled motor, it's a good sign that the spring needs to be replaced.

If you allow the brush to wear long enough the spring will not be able to push the brush against the comm. and the motor will quit running. This is frequently the cause of motors that just seem to stop for no apparent reason. Sometimes pushing the truck with your foot may re-seat the brush, but the run will be short-lived as within the next few minutes the brush will not be able to make contact with the comm. again.

The brush on the left is a new brush, while the brush on the right is a worn brush. Its about half the size of the new brush, due to wear, and it should be replaced.

Even if your brush isn't completely worn down it may possibly need to be replaced. When brushes become burnt they become discolored on the ends. Once the brush is burnt it becomes more abrasive on the comm. and accelerates comm. wear. This brush should be replaced. Over gearing and not allowing the motor to cool properly between runs can cause a brush to burn in this manner.

One more often neglected part of the motor to examine is the brush springs. While not a crucial as the brush or the comm. the springs should also be examined. Put them side by side and make sure they are exactly alike. If you'll notice in the picture below, the bottom of the spring on the right seems to be pointed more to the left than the other spring. Heat from the motor has caused it to loose some of it's tension. It would be best, to go ahead and replace both of these springs. If you have a hard time telling whether the springs are worn or not, I would recommend replacing them when you replace the brushes.

It should be noted that some tuned motors may not be running the exact same spring on both sides of the end bell. If that's the case I would replace them both as you can't really make a comparison between the two.