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Paul Alciatore
06-14-2010, 03:49 PM
I have had my Unimat DB-200 for over 40 years now and I find that the ball bearings for the pulleys are becoming more and more problematic. At least, I do not recall having this difficulty when it was new.

First, let me describe the way the pulleys are mounted. Here is a sketch (not to scale).

http://img.photobucket.com/albums/v55/EPAIII/PulleyBearings.jpg

As you can see, there are two standard, non sealed, radial ball bearings with a 1mm thick spacer between them. They measure 19mm OD, 6 mm ID and 6mm thick, nominal measurements. They are marked, “HJ 626.635 FAG GERMANY” They and the pulley itself are secured on the shaft with a simple washer and nut. The OD of the BBs is clamped in the bracket via on the OD only via the slit and SHCS. There is no radial restraint other than friction. So, the clamping force is directed around the OD and tends to squeeze the bearings.

What is happening is the bearings are binding when the SHCS is tightened. It has reached the point where, when even the slightest clamping force is applied, the bearings start to drag. This happens even when that force is too small an amount to actually hold them in place. Cleaning and re-lubricating the bearings seems to have no effect.

I can not see any other explanation for this drag, except for a compression of the outer races which destroys the clearance in the bearings and causes drag.

What I would like to do is replace the bearings with better quality units that would not be subject to such compressive problems. Perhaps a tapered roller bearing in the same size? Or what? Any suggestions would be greatly appreciated.

radish1us
06-14-2010, 04:16 PM
The bearings in question are readily available, just an "off the shelf variety", sold at every corner store bearing place.

So, after 40 years and they eventually start to complain and you want something better, why bother, just replace what's there and get another 40 years out of the mighty midget.

Or, put it up on the flea-bay and fall over at the price you get, when the auction stops.

bob_s
06-14-2010, 04:18 PM
Paul:

Best advice is to replace them with identical replacements.
1. Are you going to last another 40 years?
2.Are you overloading the bearings.

Remember ball bearings are usually rated at 1000000 revolutions at a specific working load. If operated below the rated load effectively last forever. Must derate above base load rating.

After 40 years of use I figure that the balls, or the race or both have spalled.

mikey553
06-15-2010, 08:48 PM
Paul,

I would like to offer a possible explanation for your problem. Most likely the bearings are still fine. When you turn them by hand without installing in a bracket, they do not bind, do they?

If your mounting bracket over the years developed an out of round or oversized bore, it will try to distort the outer race of the bearing when clamping with a screw. I think the distorsion, not the compression, is responsible for your problem. Your bracket bore must be perfectly round and have a tight fit to the bearings. This is the only way such design may work.

You may check this out by preparing a bore with a light press fit and no split and pressing one bearing only in it. Does it bind? It should not if the bearing is still good and you did your part.

Paul Alciatore
06-15-2010, 09:25 PM
Mikey,

I appreciate the theory, but I kind of doubt it. The bearings do turn freely when not mounted, but it takes only the very slightest amount of torque on the mounting screw to cause them to drag. But, I will check the bore.

I am wondering about the design of the bearing unit. As I see it, the bolt holds the two bearings and the pulley in a sandwich. There are three washers, two thin ones on the outsides of the two bearings and one thick one (a spacer) between them. The thin, outer washers clearly bear on the center races of the bearings and not on the outer races. I need to disassemble it again and check the center spacer/washer to see what it bears on. It is large enough to come in contact with either or both of the races (inner and outer). If it is in contact with both it could be the problem. I would think it should only contact the outer races, placing a preload on the bearings. My drawing is accurate as far as it goes, but it does not show any details of the loading on the bearings. That loading should be carefully applied in this kind of bearing.

Or, perhaps I am tightening the nut too much. Perhaps it needs to be only lightly tightened and held in place with thread locker. Perhaps I have reassembled these units improperly in the past and the bearings are damaged from improper loading. I do have new bearings on order, but want to be sure I assemble them properaly this time.

Dr Stan
06-15-2010, 09:51 PM
Paul,

I agree with the other posters that all you really need to do is replace the bearings as 40 years is an excellent run for any bearing. I do however recommend that you use FAG bearings as the replacement. When I worked as a millwright we used FAG bearings in applications where every other brand did not last long. They still make excellent bearings.

Stan

mikey553
06-16-2010, 07:15 AM
Paul,

Such design is quite common for pulley installation. The nut on the shaft must be tight. It will hold both bearings and inner spacer on the shaft. Outer washers hold the lubricant and protect the bearings from dust. Radial bearings for the pulley do not need to be axially preloaded. In fact they do not take heavy preload. Make sure you torque the nut first, and the bracket screw second. And please check the bearing bore condition.

Mike

laddy
06-16-2010, 07:53 AM
I ran across a website a year or so ago that explained exactly how to swap out the bearings. The guy sold replacement Unimat belts. Can't find the site in my favorites at the moment. Fred