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Old Jan 09, 2013, 07:40 AM
jackerbes is offline
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USA, ME, Ellsworth
Joined May 2008
18,098 Posts
Originally Posted by DeepEastKilla View Post
New question now becomes, Whats worse? A tight motor or a lost e-clip... Yes, I managed to loose the e-clip. I took it apart and put it back together one time with much success, but wanted to tinker more so I took it apart again... Hopefully the LHS has some e-clips that will substitute the one lost on the floor somewhere.
A tight motor would be the worst situation. If the sliding resistanceof the shaft in the inner bearing races is not too much, you'll notice that when the magnet housing is placed over the hammer heads (restrain it so it seats gently, don't let the magnets slam it into place or that can damage a bearing) it will find a location where it wants to be. That is the optimum position for it to be in when the motor is running, and I call it the "sweet spot".

You can measure where the sweet spot is by using feeler gauges in the gap between the base plate and the lower edge of the magnet housing. So when you mount the magnet housing you want to have that gap right and at the same time, (for a firewall mounted tractor motor) you want the Circlip to be up against the inner race (or against the washer/spacer that is on the shaft between the Circlip and inner race). That will prevent the pulling forces of the prop from pulling the magnet housing forward and out of it's optimum location.

Are we done tinkering yet? No! Of course not. With the motor set up as described above you may be able to push the shaft to the rear and displace it from the sweet spot. To prevent that from happening you have to place washer/spacers on the shaft inside the magnet housing so as to fill that small gap between the front bearing's inner race and the cylindrical boss inside the magnet. With that gap filled, you now have a motor that has a magnet housing that will stay in the sweet spot in either a pusher or puller configuration.

The final consideration is that when the motor heats up in use all of the various metals (aluminum and steel for the most part) will expand in all directions. The shaft will get longer, bearing tube will get longer, etc., etc. Carefully established tolerances will get larger and smaller as the different metals expand at different rates. And you need to set the motor up so that the bearings do not become too tight when the motor warms up.

Now the consideration is clearance or free play. Imagine that both the bearing tube and the shaft are going to get longer as they warm. But the aluminum will gain more length than will the shaft because that is the nature of the expansion rates in the two different metals. So, with a goal of having enough free play for the shaft to not cause the ball bearings to be pulled tightly against the sides of the bearing races, you want the magnet housing to have a small amount of fore and aft sliding movement in the races when the motor is at the ambient temperature. That will allow the motor to warm up without causing any bearing binding. That clearance will be seen as small amount of travel that changes that sweet spot gap when the housing is pushed in or pulled out.

I consider a free play of 0.001-0.002" (0.025-0.050mm) to be about the right amount for these motors.

Why do I worry myself about these kinds of details? Because once you are aware of them it is nearly impossible for an old machinist to not want to know what the clearance situation is on the bearings. And once you have learned what it is, it is impossible for an old machinist to not want to make it right.

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