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skunkhome

Another question about BGB

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skunkhome

I did a search but was unsuccessful in finding an answer to my question.

I recently had the needle bearings seize up on my input shaft, fortunately it was while the engine was at idle so it squealed and choked down the engine so it looks like the BGB case is ok. Since the rebuilt will require parts I put a "rebuilt" BGB in the tractor and I think it has too much play between the input and output shaft. I believe the bevel gears need to be in closer contact. Is the gear engagement regulated by the shims placed between the right side plate and the BGB? If I am not mistaken there were two thin and one thicker shim that I had to apply grease to to keep them in place while installing to side plate. Is that where I need to make my adjustments?

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skunkhome

Willy, I'm not really questioning your answer because that is the answer I wanted to hear but the ball bearing seems to be sitting against a shoulder in the case. Am I wrong? If that is the case how do shims on the outside of a bearing against a rim close the gap between the gears.

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RayS

Phil, the bearing does set in a shoulder and the seal sets against the bearing. If there are no shims added I believe the gap between the side plate and the face of the seal is around .045. Shims are added to build up that space to make the bearing and shim the flush with the side plate. Otherwise the bearing and seal will move around in that dead space and affect the mesh of the gears.

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skunkhome

So I can make this adjustment without actually opening up the BGB. Just insert shims until the desired backlash is realized.

With that in minding what would be the desired backlash assuming in the keys are tight? The example I have seen on YouTube of a bad but functioning BGB looked like it turned 40-50° before taking up the slack. Mine moves through 5-6° of arc before taking up the slack. I'm thinking I should be looking for 1-2°.

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RayS

The shims will set the correct backlash if the shaft is OK where the snap ring is in front of the gear. The ones that I have rebuilt with all new parts will move about a 1/32" to 1/16" when meshed together.

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PhanDad

You can't really set the mesh. Only allow it to be worse.

A Ray stated, it's "as designed" when the bearing is against the case. The seal and stack of shims (outside of the seal), keep the bearing pressed against the case. If not enough shims are used, then the bearing could move outboard and there would be less mesh. I'm thinking 5-6 is very good.

Another thing that sometimes causes issues is the use of on incorrect spacer. There are different length spacers based on the tranny type (cross shaft is different). The spacer is designed to be clamped in compression pulling the shaft and gear against the bearing (which is held in place by the seal and shim pack).

See this post for reference:

http://www.simpletractors.com/club2/topic.asp?TOPIC_ID=142055

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skunkhome
quote:Originally posted by PhanDad

You can't really set the mesh. Only allow it to be worse.


id="quote">
id="quote">. So conceivably I could add a shim on the inside of the case between the gear and the inner race of the ball bearing.? 5-6° is about 1/4" at the outer rim of a 5" pulley.

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PhanDad

Yes, I think that would make the mesh greater. One of rfsmith1952's pics for reference:

BGB01.jpg

If you put a shim between the gear and bearing (in effect making the straight side of the driven gear longer) , that would move the driven gear toward the driver gear when everything is tightened up. I haven't rebuilt a gear box in many years. Maybe new ball bearings aren't exactly the same as the old ones - inner race face relative to the outer race face tolerance, or more tolerance in the balls to races so more lateral movement when loaded axially as in this application. But, as I stated above, I think your box is pretty tight. I wish others would comment.

BGB01.jpg.2a4f76eabf4d81c6b05368fc4ed7c1bd.jpg

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Bill725

In the design of the BGB, the backlash between the gears is set by the "original factory machining" of the housing. The backlash is determined by the machined geometry between the centerlines of the bearing bores for the input and output shafts and the dimensional relationship between the input shaft centerline and the counterbore depth for the ball bearing on the RH side. When assembled in the housing, the input shaft and drive gear are in a fixed position. The driven gear sets between the retaining ring on the inboard side and the inner race of the ball bearing on the driven shaft. The inner race of the bearing sets against the shoulder of the counterbore in the housing. The oil seal sets against the outer race of the ball bearing. As stated in an earlier post, the shims are used to fill the space between the oil seal and the RH side plate so the shaft assembly does not move. The Simplicity Large Frame Garden Tractor Service & Repair Manual states, "Stack shim pack so that half the thickness of the thicker shim protrudes the bevel gear case housing machined surface. This insures the shim pack will compress when the side plate is installed." Backlash not only sets the clearance between the gears, but it also determines where the gears contact each other. You want to make sure the gears contact each other in the center of both teeth. Placing a shim between the inner race of the ball bearing and the driven gear will reduce the clearance and change the contact location from the center of the teeth.

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PhanDad

Bill,

Much better explanation than mine in earlier posts.

Only part that I think is incorrect is "The inner race of the bearing sets against the shoulder of the counterbore in the housing". If this is true, then the inner race wouldn't be free to rotate.

I'm pretty sure the gear, inner race of the bearing, the spacer, and pulley hub are all clamped in compression between the inboard retaining ring (or shaft step in older BGBs) and the outboard nut at the pulley. The entire shaft floats axially on the tolerance of balls/races of the bearing.

I can't find a pic of a bare BGB to see how big the bore is.

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