Bearings in Spur Gear Assembly

One of the critical performance (and often overlooked) issues in modern RC touring cars is bearing drag.  When most RC industry companies design RC cars, they design the plastic parts without regard for plastic shrinkage.  When plastics cool after injection molding, the plastic parts tend to shrink slightly.

For example, if a company designs an 8mm hole to fit an 8mm bearing, the hole will actually be smaller than 8mm.  It won’t be a lot smaller, but it will be small enough that it will press too much on the bearing and cause the bearing to drag internally.  For example, the AE spur gear assembly was clearly designed for 8mm bearings but the holes are actually only 7.97mm.  If the bearings are simply forced into the undersized hole, the bearings will not spin as freely as they should because the plastic is actually slightly crushing the outside of the bearings.

To alleviate this problem, RC car designers could design the holes with the plastic shrinkage in mind.  For example, they could size an 8mm hole as 8.04mm, which would shrink to 8mm.  Then the bearings would slide in without any binding.  Unfortunately, this is not common practice in the RC industry.  While there are exceptions, most RC car designers simply design holes without thinking about plastic shrinkage.

As a result, all of the bearing clearances in the plastic parts are far too tight and need to be modified carefully by hand.  There are several different ways you can accomplish this.  The easiest is to simply by the correct sized reamer.  If you don’t have the correct reamer, you can use a hobby knife, a Dremel bit (spun only by hand!), or some sand paper wrapped around a tool (such as a 2.5mm or 3.0 driver).

If you use a Dremel bit, do not use a Dremel tool to spin the bit.  It will take too much material away and cause the bearing clearance to become sloppy.  Remember, you need to increase the size of the hole by only 0.03mm.  That will mean removing 0.015mm of material all the way around the inside of the bearing race in the plastic.  That is a paper thin amount of plastic.  If you use a Dremel tool, you will most likely remove far too much material and create a hole that is no longer a perfect circle.

Personally, I like to use a combination of tools.  Reamers often leave a small ridge in the bottom of the hole, because the tips of the reamers are curved or slanted to make it easier to slide the reamer into the hole.  So I often use a Dremel bit or hobby knife to remove the small ridge that the reamers can’t reach.  I am also very found of 800 grit sand paper as a final tool to sand down anything the other tools left behind.  In a pinch, you can get by with just a hobby knife, but you need to be sure to keep the hole in a perfect circle.  Don’t accidentally shave away too much material in on spot, because that can make the hole out of round.

A perfect hole is one that the bearing will slide into without significant force.  In fact, I like my bearing to be able to literally fall out on there own.  That way I know nothing is binding.  Just make sure you don’t remove too much material, because you don’t want there to be any slop at all.

Once you are confident that the bearings can easily slide in and out of the holes in the plastic, the next step is to prepare the bearings themselves.  Most bearings are packed with thick grease.  The thick grease helps keep dirt away from the balls within the bearings, but the thick grease also creates drag within the bearings.  The balls in the bearing have to literally push the thick grease out of the way when the bearings are spinning.  A better solution is to remove the thick grease and replace it with a much thinner bearing oil.

To remove the thick grease, I remove the inner bearing shield (the non-flanged shield) using a hobby knife.  I remove the inner shield because I plan to leave it off when I am done.  The spur gear assembly itself will cover that inner side of the bearings when I am done.

With the shield removed, install the bearing into an RPM bearing blaster.  Use Trinity Buggy Blast (the one with Brian Kinwald’s picture on it) to flush the thick grease out of the bearing.

Now place two or three drops of bearing oil on the bearing balls.  Gently rotate the bearing to spread the bearing oil around.  I use Trinity bearing oil, but any bearing oil will work.

Place the bearings into the spur gear assembly.  Insert the layshaft, and connect the e-clips to the layshaft as shown in the AE TC6 manual.

Mount the spur gear assembly in the car without any belts.  We are installing it without belts at this point so we can check how well it spins.  After we are happy with how well it spins, we will need to remove it from the car, install the belts, and then re-install it into the car.

Flick the spur gear with your finger.  If you did everything right, the spur gear should spin for 5-15 seconds.  If it does not spin at all or only rotates for a couple seconds, then you need to double check the work you did throughout this page.

These same concepts can be applied to the bearings on the diffs and on the CVD axles.

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