Bridgeport - what causes this? J head.. and a bearing preload question

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 pulley...

The 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?

Lakeside53,
Have you though about using angular contact bearings? I read they are made preloaded. I know the class 7 bearings are a bit pricey (7XXX series). Remember there is another pulley/shaft to look at for the cause of the problem (other end of the belt). Did you replace the belt? I think we are looking at a J head pulley drive, correct?

I am going to try the double row Angular Contact type (5205) as a replacement for my surface grinder's tapered brass bearing. Although I just found this statement on this site:

http://superoll-bearing.en.alibaba....Double_Row_Angular_Contact_Ball_Bearings.html

"Detailed Product Description
The bearings accommodate radial load and axial load in both
directions, and are also able to accommodate moment load. They
are inferior to single-row and matched pair angular contact ball
bearings in terms of high-speed and high accuracy performance."

So I may try two of the single angular contact pair, if the 5205's don't work. A company I contacted wanted $5,000.00, to rebuild the severely worn spindle shaft, and no where to get new brass tapered bearings. Heck, I can buy a new 1400 lb surface grinder for a lot less than that! I buy my bearings from
http://www.bearingsdirect.com/
You can buy Japanese made and other brand bearings from them.
Joe

Yes to the new belts.... and the timing pulley at the "far" end has new bearings, top and bottom.

Bearings: yes, I considered angular contact, but this time around I just did what BP did - take radial bearings and preload them..BP had them pre-ground for DB mounting- I'm just playing with spacer depth to achieve a similar result. This isn't a "precision" part, and the bearing are only $11 each, but any play sucks so some preload is needed. What interests me is that these are the bearings you get when you buy aftermarket instead of genuine. There is no way "out of the box" that they assemble "tight". I know I'm going to have the same problem when I replace my 6208 bull gear bearings.


If I did convert to angular, I'd need shielded or sealed bearings, and just low cost Abec1. I know it will work the way I'm doing it - just not sure what preload is is correct... or even how to measure it..

Troll ebay for old stock precision bearing sets. Use "Fafnir" as one of your key words. I just got 6 SETS of Bridgeport Fafnir/Timken MM207 DU spindle bearings... BTW.. I might have the exact bearing pair of single row you need... in precision DU sets. PM me with a number.

Looks like you have the bearing thing handled. You may want to try a different timing belt, in case your new one is defective. Only since it is tracking up in one direction and the opposite when you reverse directions. I would think it would track to the same direction regardless of direction of rotation, if the plane of the pulleys were different (shafts not parallel). I had a broken flange on my timing pulley clutch sleeve (ref: 51, Lower Housing Asm), so I made one and replaced it. It stops the belt from tracking lower than the end of the spline (ref:47), which keeps the belt off the gear housing plate (ref:20, Lower Housing Asm). I can send you my old belt if you want to try a different one. It's still in one piece, just old.
I think the J and 2J heads use the same timing belts.

Ref:Parts 2J, Upper Housing Assembly; Lower Housing Assembly at:
www.icai-online.com/home.php?cat=252
Joe

PM sent

The answer is now obvious...

Why does the belt rub on some J-heads and not others? Simple... there are differences...

I have a couple of different era J-heads now - one from 1966 and the other from 1989. The 1966 rubs even though I replaced a bunch of parts; the 1989 doesn't. I tore them down and started measuring.

Timing pulley pulley:
1966 - 1.30 inches high (not including flanges)
1989 - 1.25 inches high (not including flanges)

The top of both pulleys is the exact same height from the back-gear case - the later has 50 thou taken of the bottom.

The back gear cover:
1966 - 0.450 inches high - has a raised flange around the drive hole.
1989 - 0.375 inches high - no flange.

The combined changes on the later version buy another 0.125 inches of clearance.



They didn't rub from the factory, so why now?

- Belt flapping as they age.
- Upper pulley bearings and/or case. Any slight angle (excessive drive belt tension) will push the timing belt down,
- Wear on the lower belt retainer (disk on the timing wheel) - big notches in all of mine.
- Dog clutch engagement changes over time - settles in deeper as they wear and allows the belt to go lower. With both new drive shafts ($$$) and belt retainer, the belt just clears the cover.

The fix is easy - just modify the timing pulley and cover to the later dimensions.


I made a mandrel to center the pully (just wanted to reference the center for belt face dressing). The belt retaining band screw holes were threaded deep enough.



I milled this, but it could be done with a file.

Thanks for the update and nice job of sleuthing. You have answered questions that none of the BPT experts I have spoken to have been able to.

I ended up milling the backgear cover and reducing the width of the timing pulley also. In the end, I got tired of listening to the timing belt whine and removed it completely. At the lowest pulley step and with a low VFD setting (sensorless vector type), my needs are served. The mill sounds great and if it ever become necessary, I can drop the belt back on in a half hour.

Den

I learned long ago, from repairing a woodworking bandsaw (a cheap one, the upper pulley mounting block literally crumbled, and it was replaced with a massive milled block of 6061) that the most common cause of belt shift on a pulley, particularly if the belt shifts its riding location in reverse gear, is a lack of parallellism between the two axes of rotation. If the belt starts to ride up or
down, and rides the other way in reverse, I'd suspect worn shaft bearings
allowing the shafts to come slightly out of parallel. It's the first thing I'd be
checking for.


CJ

CJ, Those of us who have tried to track down the cause of this sort of concluded, as Lakeside has established definitively, that there weren't many clearances in play even when the BPT was new. This is what left some of us scratching our heads until we bit the bullet and removed metal from our prizes

I did note the fix and if I should need it, I'll make good use of that information.

But, the lack of clearance just exacerbates the main problem, which I think is what I suggested. Adding clearance by a few mill cuts just gives you a little more range of tolerance for out-of-parallel axles/spindles/whatever you want to call them. I think that if the machine in question were to have the shafts
inspected very carefully, they'd be out of parallel by a little bit.


CJ

I'm sure the belt rides up and down from new, and all that stops it doing any damage is the pulley retainers and the clearances. At some serial number between my two.. they modified the design to allow a longer belt service life, and stop complaints.

The basic design leads to a tiny amount of "out of parallel" . The spindle pulley is a cantilever pulled by the motor drive belt, and it has to have a small clearance in the top housing to slide. Mine has machined pulleys, new shafts, bearings, (and they are ground DB bearings) balanced motor and entire top pulley bearing housing. It still rides up and down - nothing alarming or damaging.


In addition to the shafts, even the tiniest variance in the timing belt diameter over its width will make it ride up and down.

Over the last year I've peeked in maybe a dozen J heads while they are operating.. asked many on-line BP users.. and in all all of them the timing belts ride up and down depending on the direction of rotation. Sure, my sample is small in comparison to the number of J-head, but...

dennh said:

Thanks for the update and nice job of sleuthing. You have answered questions that none of the BPT experts I have spoken to have been able to.

I ended up milling the backgear cover and reducing the width of the timing pulley also. In the end, I got tired of listening to the timing belt whine and removed it completely. At the lowest pulley step and with a low VFD setting (sensorless vector type), my needs are served. The mill sounds great and if it ever become necessary, I can drop the belt back on in a half hour.

Den

Click to expand...

Yes, the belt noises are annoying. Of course, when making chips, you really don't hear them!

My next big job is to retrofit the entire top end... I currently run an Hitachi VFD on a 2hp, mostly in the 2nd belt position, and 400rpm to 4k, and it works great. I was thinking just two ratios - both driven with a decent toothed belt, but I'll likely keep the back gear. Hey, you never know when you will need to stir paint or knead bread dough.

But... it might have to wait.. today my varispeed went "clunk", and screeched.. now the brake doesn't work and... Oh well.. needed spindle bearing attention anyhow. Maybe I'll chuck the varispeed junk out and put a decent drive in that.

A bit more J Head Info

I just rebuilt the upper half of my J-Head about a month ago. The belt had been cutting into the cover for quite a while. After re-assembly, the new belt continued to rub the cover. After about 3 hrs of use, the head just locked up and wouldn't turn. I found the hub area on the underside of the timing pulley that rides on the inner bearing race had worn which allowed the timing pulley to drop down a bit. It dropped enough to gall up on the top side of the cover and would not turn. I dis-assembled and made a .050" spacer from a washer to lift the pulley up a bit. If the spacer is to thick, the step pulley will hit the timing pulley in High Range. The belt quit rubbing, the head is quiet. Life is good.