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Davek0974
11-08-2009, 12:31 PM
Hi all,

a friend has presented me a job, its a 50mm shaft with a gear to be fitted at one end, the shaft has some cams and stuff fitted along its length. The gear was buggered and a new one has been made but he got a little over-zealous with the boring bar and ended up 0.20mm oversize, its not terrible but is a bit sloppy in fit. The gear is secured with a single fitted key and rightly so he is concerned about it working loose as the stresses vary on rotation under load.

The gear is 6" dia and 1" thick, quite chunky stuff and is under a fair bit of load. Its not a precision job, just something he is developing at home.

What options are workable for this fix? I did recommend making a new gear but costs prevent this. I also suggested boring further and sleeving but what about the keyway as the drive needs to be into the gear not the sleeve?

Next up was reducing the end of the shaft a little and fitting a thin sleeve there?

Any others or would one of these be correct here?

Thanks in advance
Dave

Timleech
11-08-2009, 12:48 PM
Hi all,

a friend has presented me a job, its a 50mm shaft with a gear to be fitted at one end, the shaft has some cams and stuff fitted along its length. The gear was buggered and a new one has been made but he got a little over-zealous with the boring bar and ended up 0.20mm oversize, its not terrible but is a bit sloppy in fit. The gear is secured with a single fitted key and rightly so he is concerned about it working loose as the stresses vary on rotation under load.

The gear is 6" dia and 1" thick, quite chunky stuff and is under a fair bit of load. Its not a precision job, just something he is developing at home.

What options are workable for this fix? I did recommend making a new gear but costs prevent this. I also suggested boring further and sleeving but what about the keyway as the drive needs to be into the gear not the sleeve?

Next up was reducing the end of the shaft a little and fitting a thin sleeve there?

Any others or would one of these be correct here?

Thanks in advance
Dave

How critical for it to run true? You could assemble with the right grade of Loctite and check for runout afterwards, it does sem to have a bit of a centring effect (make sure the key won't be holding it off centre). If it's not running true, dismantle with a bit of heat and try again, worth 2 or 3 goes I should think. If you're nervous about it holding, add a couple of grub screws as Dutch keys at 120 deg to the key.
OR, bore it out beyond the keyway, make a bush to be held with loctite & 3 dutch keys, bore to correct size & cut a new keyway.

Tim

Peter.
11-08-2009, 12:54 PM
I fixed a loose main bearing on a 125cc 2-stroke bike crankshaft once by centre-punching a pattern around the shaft and fixing with loctite bearing-fit.

wierdscience
11-08-2009, 12:56 PM
With a little lathe work and a couple of holes being drilled and tapped he could use one of these-

http://www.drillspot.com/pimages/2948/294808_300.jpg

Walter
11-08-2009, 12:58 PM
Dave,

Weld up the bore on the gear and re-bore it.
or... Weld and re-turn the end of the shaft oversize.
or... find some appropriate shim stock and fit a nice sleeve of it as is.


please excuse me for converting to inch but .2mm = .0078 inch, that's pretty small, I'd be most inclined to order some .004 (.1mm) shim stock, and carefully fit it . should work just fine.

Forrest Addy
11-08-2009, 01:12 PM
Bore the new but oversized gear to take a tapered bushing. Solves all the problems at once but it might take a bit of fiddling to match the taper. It has to fit rght

http://www.martinsprocket.com/Bushings.htm

Carld
11-08-2009, 01:17 PM
It has a keyway to index the gear. Remove the key, lightly knurl the shaft so it measures a few thousandths bigger than the bore. Then install the key, coat the shaft and gear bore with one of the high temp loose fit locktite or other brands and press the gear on. It will be a long time before you have a problem with it if ever.

barts
11-08-2009, 01:20 PM
The tapered bushing is a great solution; they're much better for reversing loads than a regular keyway as well.

Buy the bushing and slip it onto a stub shaft held in a collet, and set the compound w/ a dial indicator to match the taper.

The style of bushing w/ a flange are easier to fit if you have room as the hole location is less critical.

- Bart

Davek0974
11-08-2009, 01:21 PM
Thanks guys,

Accurate running is not as important as it coming loose while running.

I guess some 0.08mm shim will fit well, i like the loctite as well, and also the grubscrews.

Taper lock is probably a bit OTT here.

The answer lies here i think, many thanks

Dave

Davek0974
11-08-2009, 01:24 PM
Of course knurl the shaft and loctite it, that'll do nicely.

Thanks

Dave

Carld
11-08-2009, 01:31 PM
Dave, I just noticed your sig line. A lot of my friends, when watching me make "adjustments" will go :eek: and say "man, you sure wield a crafty hammer and I just grin :D .

Davek0974
11-08-2009, 01:45 PM
Dave, I just noticed your sig line. A lot of my friends, when watching me make "adjustments" will go :eek: and say "man, you sure wield a crafty hammer and I just grin :D .

:D Yep, the hammer is one of the more versatile tools, one of them and an adjustable spanner and you've got a complete Ford mechanics toolbox:D

Black_Moons
11-08-2009, 10:13 PM
Bore, make sleave, And find a rectangle key to go through the gear, sleave and shaft?

Yankee1
11-08-2009, 10:53 PM
Hi
I have used molybdenum Disulfide with a resin base to make up for a insufficient interference fit. It is sold under the name of "Magic Moly"
I used it on oil field equipment gears without any trouble afterwards.
It is sprayed on the shaft and let dry then warm up the gear and put on.
Fit the key afterwards. While the gear is warm align the keyway with the end of the key. I have built up shafts as much as .005" this way using several coats.
Yankee1

oldtiffie
11-08-2009, 11:12 PM
Hi all,

a friend has presented me a job, its a 50mm shaft with a gear to be fitted at one end, the shaft has some cams and stuff fitted along its length. The gear was buggered and a new one has been made but he got a little over-zealous with the boring bar and ended up 0.20mm oversize, its not terrible but is a bit sloppy in fit. The gear is secured with a single fitted key and rightly so he is concerned about it working loose as the stresses vary on rotation under load.

The gear is 6" dia and 1" thick, quite chunky stuff and is under a fair bit of load. Its not a precision job, just something he is developing at home.

What options are workable for this fix? I did recommend making a new gear but costs prevent this. I also suggested boring further and sleeving but what about the keyway as the drive needs to be into the gear not the sleeve?

Next up was reducing the end of the shaft a little and fitting a thin sleeve there?

Any others or would one of these be correct here?

Thanks in advance
Dave

Dave.

I guess that the gear is driven by the key but it must be held onto the shaft by something else - nut and washer etc?

The angular rotating forces are the bigger one than of the radial forces (components of the vector/force at right angles to the pressure angle at the pitch circle).

The key will take the most of the load. The shaft is for location. The shim is hard enough to take the radial load without compressing significantly.

I'd cut a 0.10mm (~0.004") shim to fit over the shaft with a gap for the key. Then just push or press it on.

Davek0974
11-09-2009, 03:50 PM
Thanks all, great assistance as always:D

Much appreciated.

Dave