Backgear bearing replacement
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  1. #1
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    I'm about to get into my '56 3HP backgear (the gearbox on the motor), this to replace bearings - I get some nasty sounds at high speeds. If someone out there knows the bearing sizes I'd appreciate it as I'd like to order the bearings before I tear it all down for work. Also, I'm afraid of what I might fond in there - if anyone has a spare gearbox after a DC motor takeout I'd be interested in it, it couldn't be worse than the one I have on this guy.

  2. #2
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    I've got mine out with the new ones next to them. The big one at the front are dreaded
    class 7's. But I'd better go take a looksee.
    Will post info this eve. Daryl

  3. #3
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    Back again, The two big ones at the pulley
    are a matched? set. ND 20209 and 420209DTL.
    I paid $91.95 for the set, not bad. The two little ones are ND NS6203ZZ, at $4.14 ea.
    I get all mine from General Bearing in LA,
    There is a guy there, Robt. how knows EE's and Tree mills, really knows the bearings for these mach. They are a Bijur dealer too.

  4. #4
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    Thanks, Daryl. It was suggested that the little ones are the likely culprits for the noise, so I'll likely replace those first. 6203s are common, I'll get some and toss 'em in there this weekend.

  5. #5
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    Just been through my gearbox. I fitted a lip seal to run on the armiture shaft,to keep dirt out and the oil in. I used sealed bearings for the top 2 6203's. I found the same bearing as the factory for the lower pulley shaft,but without the groove for the retaining ring. They were cheaper, but needed the wxtra work of cutting in the groove, If i had it to do again, i would pay the extra and get the real thing. I also made new gears for mine...they were very noisy. By the way, i believe that if the box is noisy in high range, it can only be the large pulley bearings that are fault....The gears and the uper 6203's are not in use when you are in direct drive! Ross

    [This message has been edited by AlfaGTA (edited 01-21-2003).]

  6. #6
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    Well, I took the backgear apart tonight. Nothing too bad in there, but the rear gear on the countershaft feels a bit loose as well as the smaller gear in front of it. Adding the key to the rear gear tightens it some, so I guess that we'll see. If it still makes noise I'll look into getting a replacement for the shaft. The gears and all looked to be in good shape, so that's a relief.

    On thinking about this one issue concerns me. I was getting the loud harmonic on this guy when the motor was in direct drive, and so far as I can tell the countershaft should be completely disengaged in that mode. That must mean the the noise is in the front bearing set. They feel fine, though, so I'm kind of at a loss. The noise appeared about 2200 rpm to 2500 rpm, and 3300 to 3500 (about, there are 2 distinct bands both above 2000 and about the same width - I didn't hang around too long to get precise measurements).

    There are no gaskets on this guy, it was gasketed with some light purple sealant, probably something like 'make-a-gasket'. I was planning on putting it back together that way unless someone can show it's a real bad idea.

    [Added later]

    I looked at the front bearings, appear to be Nachi 6209, don't know the class, but they're sealed. Does this sound right?

    [This message has been edited by rke[pler (edited 01-21-2003).]

  7. #7
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    There should be four gaskets,one between the back gear housing and the motor EE-2401, one between the front cover and the main body, EE-2402. one for the little bearing cover on the front,EE-2469 and one for front cover in front of the big bearings,EE-2605. All of them are $2.00 apiece. Russ I won't let you get away with form-a-gasket. Daryl

  8. #8
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    Well, this is getting messier and messier. I've ordered all the gaskets from Monarch (as well as the gaskets * seals for the electric leadscrew reverse electric box (there were *none* on this guy and I always kind of wondered why it lost it's oil fast) so I've got some time to figure out the big bearings.

    I took the big bearings apart. Describing the assembly the outer bearing is the one with the snap ring groove, then there's a couple of rings, one about hub diameter and the outer the bearing run diameter, then the inner bearing. The snap ring restrains the bearings from moving inwards. Riding on the bearing from the inside out is the output shaft, with an integral gear and ground hub on the inside contacting the inner bearing hub. The shaft is restrained on the outside by a disc held against the outer bearing hub by the sheaves, itself pulled against the disc by 4 screws into the output shaft.

    The bearings in mine are, in fact, 6209s - standard ball bearings. Talking to Scott at Monarch he says that they should be angular contact bearings - 7209s. But with an angular contact bearing you need to displace the inner hub of the bearing relative to the outer to get preload, and on the way mine is assembled all I can do is to pull the hubs together. OK, that would be 'back-to-back', but Monarch's spec for the inner spacer says that it's thickness has to be within .0003" of the outer ring's thickness. So - where's the preload?

    [This message has been edited by rke[pler (edited 01-24-2003).]

  9. #9
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    I am going answer dumb and say its set up just like the spindle bearings in the headstock. You have a outer spacer and a inner spacer. When you lock everything down, there really isn't any preload. I hope this answer doesn't land me in the dunderhead hall
    of fame.

  10. #10
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    Russ has it partly right, so don't put yourself down fella. The bearings used in the Monarch spindle are known as "universal grind" type. That means their faces have been ground to provide a given amount of preload when the inner and outer faces of a bearing pair are in perfect alignment.This is usually accomplished by making the inner and outer spacers as close to the same size and parallelism as possible. Then clamping pressure must be applied axially to the inner and outer stackups. This is necessary to apply the preload. Give it a go and good luck.

  11. #11
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    Whoops! used the wrong salutation. I was replying to Daryl's comment about preload to your (Russ's) question about same.Now whose the dumb one?

  12. #12
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    OK, I think I have a better understanding of bearings; I was confused after pulling the bearings out of my backgear and finding standard bearings. I went out yesterday and snagged an SKF catalog, and it's got a decent section on angular contact bearings.

    I was thinking that I needed to get a pair of 7209s made for back-to-back tandem mount, one with a snap-ring groove; but EAH's comment that the Monarch bearings are 'universal grind' made me look into that. So far as I can tell it would be a lighter preload (comments?) but other than that it would be the same, mount 'em back-to-back and the takeup would hold the spacer between in position.

    I'll call a distributor with this new (to me, anyway) information and see what pops up. I'd like to replace the spacers as well if I can find a replacement near the .258 - .261" thickness.

  13. #13
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    Old thread revival! I just called Bob at General Bearings in LA. He has lost the sample of the bearing (New Departure 420209 DTL) with the snap ring groove in the OD. He used this sample for sending the grinding work out. I am sending him the one out of my gearbox and he quoted me double the price quoted in 2003 above for the set of two bearings. I think he will have a few ground at the same time so the setup charge will be shared between myself and the next customers to buy these specials.Good to know he is still there and these can be had fairly reasonably.

  14. #14
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    I bought a set from Bob Summer before last. He was very knowlegable and helpful. The grooved bearing fit perfectly.

    Greg


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