# Question for 254, 22, 968, 60, or 22

Your frame side rails are only 1" wide. Your pillow blocks are on either side, so your bearings are roughly 1" apart. I know it works just fine. But please explain to me why you are able to have such a minimal distance between the bearings. I’d like to know not just that it works, but why it works. Thanks so much!

I am not from any of those teams, but I will give my crack at an answer.

The dirty little secret is that the loads in an axle of a FIRST robot are large, but not really THAT large.

If the axles can be kept short(ish) the moment is just not that great. To give you a feel for things, suppose all the weight was on 1 wheel and you had a coef. of friction of 2. That makes the resultant vector on the wheel a magnitude of 2.2XWeight = 300lbs. If you have an 8" dia wheel, that could potentially put a moment of 1200in-lbs on the axle (I know that some of the load will not be at a right angle to the axle, but we are getting rough orders of magnitued here). At 1" of separation, we can support 1200in-lbs with 1200 lbs in each bearing.

Going to McMaster and picking out the first bearing I see that with a 1/2 dia (60355K15) it has a load rating of 1150 lbs – close enough for me.

The next thing is to make sure that the box section of the frame can take that kind of moment, I don’t want to go into the gory details, but basically, the story ends well for the good guys…

Joe J.