Delta Motor Assembly

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Rick_B

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I'm working on two Delta (A O Smith) motors - one of which will be going on a 17" drill press. They are both 3 phase - 3/4 HP and 1 HP (86-720 and 86-920 respectively). I was putting the bearings on the 3/4 HP rotor. Fan end was fine - required pressing on. Opposite end, however, the bearing pretty much just slips on the rotor. So I thought I had a problem and moved on to the 1 HP - same thing - bearing fits correctly on fan end but slips on by hand on the opposite end.

That seemed pretty coincidental - both motors with same problem on the rotor. I need to ask the question - is this looseness by design? Ii has been suggested on another forum that fit was by design to allow expansion/contraction due to operational heat. There is a spring washer between the bearing and end bell with the fingers pressing against the bearing outer race that would theoretically keep the loose bearing from turning. However, my disassembly notes indicate that the spring washer was on the same end (fan end) where the bearing was tight and this was true on both motors.

Anyone have experience wth this type of motor design?
 
I would be inclined to put some blue loctite on the rotor shaft to assure it does not spin.
The outer race to bellhousing on the back side is usually a slip fit with a spring washer and thats ok.
 
If the fan end is pressed in, there should be little lateral movement of the rotor. So long as there isn’t excessive clearance between the bearing and the shaft ( more than.003”) you will be fine with what you have.
If you have greater than .003” , I would suggest retaining compound like locktite
 
Here's a link to bearing specifications and fits published by Timken.


I'm not sure whether you have roller or ball bearings, but I'm suspecting they are ball bearings. I went through a similar situation several years ago when replacing the deep groove ball bearings in a (circa 1960) Worthington AllSpeed drive. For that particular installation the outer race was supposed to be a slip fit that would allow the outer race to make 1 revolution per hour in the casting.

Bearing construction and fits have changed over the years. Here's a link to a publication by the BAART Industrial Group concerning more modern bearing fits. In the case of machine tools, (high speed light load) they are recommending a "transition" fit. This is similar to the previous type of fit described, but the slip fit can be on either the shaft or the housing.

Here's an excerpt from the publication:
As the name implies, a transition fit is somewhere between the two described above. Either a clearance or interference fit may occur depending on the actual sizes of the bearing bore and shaft, or outer ring and housing. The resulting fit will be dependent on the tolerances of two points of contact – either the bearing bore and the shaft or the housing and bearing O.D.

It will be unusual for a shaft and housing fit to be the same. One fit requires a clearance fit, the other, generally, the rotating ring, will require an interference fit. The rotating ring requires this interference fit because when applying the load to a looser fit, there would be slippage and a loss of efficiency, and eventually, surface damage or fretting corrosion. Often, vibrating or shaker-type applications vary from the above generality. Incorrect fits can cause premature bearing failure.

Here's a link to the publication:
baartgroup.com

How to Determine Bearing Shaft and Housing Fit - Baart Group

A proper shaft and housing fit are vital for the life of your bearings. We break down the process for correctly determining the bearing fit.
baartgroup.com baartgroup.com

Personally, I would measure the shaft in several locations. If it's the same the entire length I would make the assumption the slip fit was intended. I would also look up the bearing specifications as far as internal race diameter is concerned. In recent years I've found several offshore bearings that do not meet standards.
 
I can't think of any good reason why the bearing would be a loose fit so I assembled the motor using a shaft retainer compound. I also decided to put the spring washer where I found it - fan side end bell. Motor started up and ran fine. It will be vertical in the drill press with the fan end down so the rotor will be pressing down against the spring washer.

Rick
 
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