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View Full Version : Bridgeport - what causes this? J head.. and a bearing preload question



lakeside53
02-03-2008, 09:57 PM
My timing belt moves up when rotating one way and down when the other. When down it touches the cover and has worn a groove, and has been doing this for sometime before I bought it. I assume something isn't quite vertical and causes the belt to track , but why this changes with motor direction has me confused. The belts sheds on the bottom edge a little, is rubbing on the cover top, and that's not right. The pulley hub has a tiny amount of movement when located inside the top housing.



So.. I suspected the top pulley...

The top pulley bearings were shot. I replaced them with two Nachi 6207 C3 from H&W. The original bearings were factory ground for DB mounting and had identical length spacers. The new bearings of course have no preload and when clamped together, there's about 7-8 thou movement vertically, and a noticeable amount of movement laterally at the pulley.

I don't like that... So it opens up another can of worms...

I turned the outer spacer ring down so it was 8 thou (bearing movement) + 2 thou (a guesstimate for some preload) less than the inner ring. Bit of a pain to set up accurately in the lathe, but it came out o.k. Reassembly proved there was now no play. Powered it all up and the belt still tracks the same, but... noise (rattle) from the dog clutch is a lot less.


Preloading the bearings as I did, what should I be using as a net differential? If my 2 thou net preload is accurate, and the bearing doesn't get more than "warm" after 25 minutes at high speed, does this sound o.k.?


Others replace pulley bearings with non-DU types... do you put up with the sloppiness or do you attempt to preload them with shims or whatever?

rdfeil
02-04-2008, 04:32 AM
Hi Lakeside,

I will be no help, but I will be sympathetic and interested in any response you get. I have a 1964 Bridgeport J that I rebuilt and I have the exact same problem with the timing belt :confused: . The groove in the lower housing was there when I bought the mill so the problem was there before I did the rebuild. I have checked everything and can not find a problem. I hope you get some answers and I will read with baited breath. As for the preload on the pully bearings, I just replaced them with generic Fafner bearings and all seems fine. I didn't worry about preload because this is not a precision part of the mill (I know better is always better :p ). I hope you get some answers for both of us.

Good Luck,
Robin

aboard_epsilon
02-04-2008, 06:34 AM
my timing belt has sides on the pulley .

sort of plates bolted on with four tiny screws.

has yours ?

all the best.markj

wierdscience
02-04-2008, 08:03 AM
Is this the big pulley,which is the input to the back-gear box?As I remember it that pulley sits on a shouldered shaft with a key and nut securing it.Is there a possibility that the nut is loose?Or that the shaft is free to move up and down in the gearbox?

It seems like the pulley comes off then there is a bearing retainer held on with three or four screws and then the shaft,bearings and pinion gear all come out as a unit and everything is kept in it's place with snap rings??

lakeside53
02-05-2008, 12:49 AM
Thanks for the replies.

Everything around the timing pulley is tight and in place... The belt is new, and the bearings (top and bottom) on the shaft the timing pulley sits on are new.



Yes, the timing pulley has two plates -top and bottom. The bottom was worn so I inverted it. If I can't figure out what's causing this rubbing I might raise the bottom of the pulley slightly by turning it down on the lathe. But that's a last resort!

One of the other reasons I decided to play with the top pulley bearing preload was to see if it reduced noises coming from the dog clutch - it did... so although it's not a "precision" part, reducing any play seems to be advantageous.

macona
02-05-2008, 04:29 PM
Were the old bearing ground for a preset preload?

Spin Doctor
02-05-2008, 05:11 PM
Lets see if I can recall the parts detail of the timing/cog belt in the back section of a J head. IIRC the bearings are either fully sheiled or sealed. They are designed to be take only loads that are radial in nature appied at 90D to the centerline of the axis of rotation. They really aren't meant to set-up with a preload. That's not saying you can't get away with it but they really aren't meant for that type of use. A better choice if you could find them would be a 7207 setup in a DB or Back to Back mounting. In such a mounting the inner spacer would need to be shorter than the outer otherwise there will be no pre-load in the bearing stack. DBs require a shorter inner spacer and DFs a shortewr outside spacer. But .002", IMO that is pretty excessive. IIRC the biggest pre-load I saw when rebuilding spindles in bearings in that size range was around .0011". And that was with Class 7 bearings. Also are you sure the amount of veritcal play did not include lack of capture in the bearing bore of the outer races and spacer?

aboard_epsilon
02-05-2008, 05:22 PM
how about machining the pulleys so they have less circumference towards the middle.

not much probably 1mm tapered dish in them...on the raised castle-ations only

perhaps Bridgeport did this originally ...and yours have come to the end of their life..

or a few blobs of high modulus silicone ...in the castle-ations ...at each edge .......effect may last years ...who knows

cant think of anything else .

have you tried a new belt.

or otherwise it gets expensive...replace all bearings to do with these pulleys and the pulleys and the belt.

all the best...markj

aboard_epsilon
02-05-2008, 05:25 PM
btw ...
the way the cone pulleys are aranged on the motor shaft ...it is possible for them to move/drift down the shaft ...and press on the timing pulley.

all the best.markj

aboard_epsilon
02-05-2008, 05:44 PM
another thing

if the belt does exactly the opposite when you turn it the other way up ....
its the belt stretched on one side .

all the best.markj

lakeside53
02-05-2008, 10:41 PM
Lets see if I can recall the parts detail of the timing/cog belt in the back section of a J head. IIRC the bearings are either fully sheiled or sealed. They are designed to be take only loads that are radial in nature appied at 90D to the centerline of the axis of rotation. They really aren't meant to set-up with a preload. That's not saying you can't get away with it but they really aren't meant for that type of use. A better choice if you could find them would be a 7207 setup in a DB or Back to Back mounting. In such a mounting the inner spacer would need to be shorter than the outer otherwise there will be no pre-load in the bearing stack. DBs require a shorter inner spacer and DFs a shortewr outside spacer. But .002", IMO that is pretty excessive. IIRC the biggest pre-load I saw when rebuilding spindles in bearings in that size range was around .0011". And that was with Class 7 bearings. Also are you sure the amount of veritcal play did not include lack of capture in the bearing bore of the outer races and spacer?

No problem on the capture of the inner and outer races - there is a separate threaded "locknut" for each.

Yes, use of radial bearings with preload is a not as good as angular thrust bearings, but it does work in cases where axial thrust isn't high.

The original BP bearings are shielded M207 radial bearing that have been DB ground. High spots are marked on the inner and outer races. yes... 2 is probably excessive, so I'll likely back it down to 1 or 0.5. The problem starts with accurately measuring the "0" point"

Part of this experiment was to see if the aftermarket bearings were worth installing.. Sorry, but I don't believe so they are not unless some attempt is made to preload them.


more..

The bearings beneath the bull gear are the M208 C2 or 6208. These aren't marked DB or anything, but it's obvious they have been ground for preload. I put in a new set of Koyo 6208 Mtr grade (top quality Japanese bearings) in place of the 40 year old M208s. Hopeless... same problem -8 thou vertical play and horrible wobble on the edges of the bull gear. I put back in the original bearings... still nice and tight.... In this case there is no spacer and without machining of the housing - you're limited to adding a shim between the center races.

lakeside53
02-05-2008, 10:52 PM
how about machining the pulleys so they have less circumference towards the middle.

not much probably 1mm tapered dish in them...on the raised castle-ations only

perhaps Bridgeport did this originally ...and yours have come to the end of their life..

or a few blobs of high modulus silicone ...in the castle-ations ...at each edge .......effect may last years ...who knows

cant think of anything else .

have you tried a new belt.

or otherwise it gets expensive...replace all bearings to do with these pulleys and the pulleys and the belt.

all the best...markj


All the bearings have been replaced or in the case of the lower bull gear bearings - checked carefully (preceding post)....

So.. today I took the timing pulley, mounted in in my lathe, and skimmed the outer edge to true it. Then did the same to the sides. There was some slight wear, but it was dished to the center from wear.. and I would have thought that helped. It will be a few days before I can test the results.

I can't see any wear on the small end (steel), but...


I agree... that either something isn't perfectly in alignment or the belt is not right (but new). Even a tiny amount of difference will cause a belt to track and I doubt any BP is "perfect". Can a couple of you please shine a flashlight into the belt housing and watch the timing belt when you change direction?

lakeside53
02-06-2008, 10:18 PM
So.... my schedule changed so I got to put the BP back together.

The big timing pulley machined true on all surfaces made no difference to the belt tracking.. Neither did another belt. I'm still trying to get confirmation from others if their belts track up and down depending on direction of rotation.

I did end up taking about 10 thou off the bottom of the timing pulley. That has raised the belt so it JUST misses the housing. I'm now thinking that the problem is caused by the timing pulley being too low relative to the housing. The only way that can happen is if the lower bearing on the back gear is allowing the shaft to settle a little.... Hmmm... need to get it all apart again..

Can a couple of you please shine a flashlight into the belt housing and watch the timing belt when you change direction



BTW.. I did take another 1.5 thou off the inner spacer of the top pulley bearing set. This reduced the pre-load (estimate) to about 5 10ths. Rotated nicely and no obvious temperature build up, so it can stay there...



Andy

jacampb2
02-07-2008, 01:41 AM
I had a similar problem with the timing belt on the belt sander I modified last month. I found that no amount of adjustment would keep the thing from walking off the pulley. My pulleys had a flange cast into them to keep the belt from walking off the shaft, however I broke the flange off one pulley when I was working on it, and that was the problem pulley. In the end I machined a little collar to go outboard of the pulley and keep the belt in place, and it worked perfect.

The reason for the long winded response is this, I noticed that if I significantly reduced the tension on the timing belt, that it tracked perfectly, almost w/o regard to alignment. It had to be seriously out of alignment to cause a problem. However, whenever I put the sander under load, it would just chuck the belt off. I was unable to find a perfect compromise between tension and tracking. My suspicion is that when under too much tension, the belt stretches more on the outside then the center and makes it walk. Putting additional load on it causes even more stretch on the trailing side of the pulley and causes it to walk. Even if yours is not too tight, this last thought could explain why it changes direction of travel with motor direction change.

All of this is just conjecture. Hopefully someone will know exactly what is going on, but those were my observations.

Later,
Jason

rdfeil
02-09-2008, 04:15 PM
I am posting to bump this thread back up the list, sorry to all who have already read it:o I am hoping someone who has not read this might see it and respond. I also have the same problem with the timing belt on my Bridgport J head. I have checked everything I can check and all appears to be running true and parallel (cog pullys, shafts etc). My machine had some problems when I got it and already had a groove warn into the back gear housing. New bearings, belt etc NO help. Without the back gear engauged and the spindle runnung forward (normal milling cutter direction) the timing belt rides down the cogs and rubs slightly on the housing. Reverse the direction and it will ride up to the top pully flange. This will happen in very few rotations of the spindle. I am getting worried as the groove is fairly deep and I am concerned that it will wear through the housing. Any thoughts or things to check would be appreciated.

Thanks All,
Robin

PS: Lakeside, have you had any luck.

Mike from WI
02-10-2008, 09:19 AM
Robin and Lakeside, the timing belt tracking phenomenon that you are observing on your Bridgeportís is normal. It is caused by the belt cord member construction, this link should explain it http://www.sdp-si.com/D265/HTML/D265T010.html
As Lakeside found out most of the time the belt rubbing is caused by worn flanges. I solved the problem on mine by making a new lower flange .250 inch thick counter bored .125 inch, and do I have clearance now. At the time I thought about take a cut off the pulley but chickened out. Make or buy a couple new flanges, or flip them over as Lakeside did if they arenít worn paper thin, machine the bottom off the pulley to leave about .025 or so clearance and you should be good to go (remember a new belt is 1.125 inch wide so donít over do it).

Mike

rdfeil
02-10-2008, 07:22 PM
Mike,

Thanks for the input and link. Will make a new flange and give it a try.

Robin

lakeside53
02-11-2008, 12:14 AM
Thanks Mike.

I believe there is a little more to it in some cases as upper surface of the lower flange (after flipping) in my case was slightly below the casing that was wearing. However, my machining off the bottom of the timing pulley has belped a lot, and I'll likely take another good slice off. I'll need to check the belt position when the clutch is raised to make sure there is enough upper range left though.