Presto 8N bering

[audiocarver]

Hello
I have been having a problem with "wow" on my Presto 8N. At first I thought it was the idler wheels being out of round but it seems to be the spindle bering. I have about 0.015 inch runout on the bering and as the turntable turns it changes position and its not spinning true. I thought about machining a new bronze bering, but what about precision ball berings? Has anyone ever tried this? Thanks.

[tsullivan]

It doesn't sound like a very good plan to me. I think you would be trading wow for rumble. Making a new bronze bushing should be an easy job for any machine shop. Make sure you bring your platter into the machine shop so it can be checked for runout or a bent shaft, that could be a problem too.

Tom

[audiocarver]

After giving it a second thought I think your correct about the ball bearings creating too much rumble. Not to mention, I think the speed would be too fast to be able to adjust out with the added loss of friction.

After taking it apart it looks like what Presto has done is cut the bearing on one side and put an adjustment screw across for just enough friction drag to help maintain proper speed. Then set the bearing in place.

The turntable shaft looks good at least with a micrometer and square. I'll have them check runout.

I contacted Bearing Service and they can remanufacture the bearing, but I don't know what they will charge for it since it would be a custom one off. I would probably order two.
I am not too surprized that the bronze would wear first. I will post cost and results later. Thanks.

Andy

[cuttercollector]

There is something wrong with your reasoning or the way you are precieving the design. No system is designed to have an adjustment to slow it to the correct speed through drag and friction loss. There might be something to take up slack to provide a fit with no play, or more likely just to hold the turntable shaft captive, but not to touch anything during normal operation. A properly designed system would have the motor able to achieve and maintain it's no-load speed under load and the drive elements geared for the correct speed(s). Typically most turntables had a long shaft in a brass bushing that should have either oil or VERY light grease applied. There is a contact point of a single ball at the bottom of the bushing to take the vertical force and there should be smooth free turning action of the platter with no drag and no play or wobble when trying to rock the platter up and down from opposite sides.

[audiocarver]

It must be my misunderstanding of the design. What you describe in terms of the construction of turntable bearing is exactly what I have.

Where the misunderstanding is why they slit the bronze stock and put a screw inside the bronze part of the bearing. The machinist was even baffled by it!! If its not to add friction then it must have been used to move the bronze bearing stock apart to hold into place inside the cast assembly reducing friction on the shaft for good fit. Or maybe it was simply cut in the bronze just for lubrication retention. Or for both. Whats baffling is it was quite possible to just machine the bronze bearing stock to correct size even in the days that this recording equipment was made. A SouthBend lathe from 1920 could do it and be within one thousandth of inch.

Anyway, I took the bearing to a machinist and the spindle shaft spun true and was within one thousandth of an inch in diameter on both ends. The bronze bearing was within one thousandth of an inch inside diameter.

The problem was at the bottom end of the bearing assembly near the end cap were it is cast iron. The cast iron was worn out about five thousandth of an inch causing excessive play when moving the table up and down on opposite sides. The machinist bored out the cast iron and inserted a steel sleeve and bored out the inside of the sleeve to the size of the shaft within one thousandth of an inch. He also notched the sleeve where the end cap fits for lubrication like the original casting.

Now, there is absolutely no play in the table when moved up and down on opposite sides. And the table is much much quieter while operating.

Thanks cuttercollector, I will try to post pictures of the bearing so you can see the slot in the bronze portion and the repair sleeve that was made.

[audiocarver]

If you look inside the slit in the bronze stock you can see the threads of a screw. Unfortunatly, I can't photograph it. As you can see its not really made to be adjustable but with this screw it would be just for good fit and then forgotten about. But why Presto didn't just machine to correct size in the first place is beyond me. After all, they had to bore the cast iron part to within one or two thousandth of an inch anyway for this thing to even work.
In the other photo, you can see the sleeve that was installed at the bottom end to take up the slack from wear of cast iron. If you look close you can see the slot machined into it.

[img]http://photos.imageevent.com/audiocarver/presto8ntablebearing/websize/PICT0017.JPG

[img]http://photos.imageevent.com/audiocarver/presto8ntablebearing/websize/PICT0016.JPG
[/img]

Thanks for everyones help. My lathe is cutting great again with no "wow."

[audiocarver]

Oops. Let me try that again.

http://photos.imageevent.com/audiocarver/presto8ntablebearing/websize/PICT0017.JPG

http://photos.imageevent.com/audiocarver/presto8ntablebearing/websize/PICT0016.JPG