Thread: Trex250 Manual
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Old 12-10-2008, 01:48 AM   #19 (permalink)
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Join Date: May 2007


You are absolutely correct. The original twin-bearing blade grip setup has absolutely no preloading capability. It's simply two bearings with a spacer between to prevent the bearings from interfering. As you said, the only way you could preload this setup is to force the blade grip against the hub shoulder, in which case the grip wouldn't turn. I think Align did this to 'widen' the bearing area, thus adding more stability. No preloading, though, and the slop of the grip is determined by the slop in the bearing races.

Heim's "custom spacer" video explains how to solve this problem. It's much the same, but instead uses a larger spacer outside the inner spacer, which sits against the bearings' outer races. In this setup, with a slightly narrower inner spacer (he uses a crushable one), you are basically sandwiching the bearings together, slightly more with middle races, which preloads the bearings much like the hub of a bicycle wheel.

I actually did Heim's preload experiment in a very crafty way. I took the balls out of an old bearing, which left just the inner and outer race. I used this as my set of inner and outer spacers between the two bearings in the grip. I slightly ground the inner race/spacer (since it isn't crushable) using fine emery sand paper. This moved both inner races on either side a bit closer together, and the outer were separated by the spacer provided by the scrapped outer race. So, my whole stack is 3.6mm wide. Two 1.2mm bearings, separated by a 1.2mm set of spacers which were previously a bearing. I used the longer screw that was for the blade grips to secure the stack inside the grip.

Result: perfect preload. No slop in the grip, no play, and the grip spins like a top.

The new tail hub kits with the single 2mm bearing won't provide any preload, either, unfortunately. Neall
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