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TO20 Timing Gear Backlash

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Stan/TX

02-01-2008 19:20:23




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Some of you may remember reading about the beginning of this tale - I know Jerry, George, and a certain John will.

Over the Christmas break I had 2 weeks to finish up the checking out and putting together of my TO20. All the parts were gathered, the tractor was inside so weather was not a problem. Installed new camshaft, pressed on the new crankshaft timing gear, reset the governor mechanism, put on the old camshaft timing gear because it had NO BACKLASH when meshed with the crank gear and it appeared to be in good condition. I also worked some sublte kinks out of the rocker arm shaft and assembly, adjusted the rocker arms to specs, and a bunch of other minor details. Cranked the thing up and let it run off and on for about 4-5 hours over a 3-4 day period. Even ran it up and down the road and felt really good and proud that it was finally back together and running so well.
Put it back in the garage to change the oil and filter - dropped the filter access panel and found 5 gear teeth. Obviously this meant dropping the pan, pulling the timing gear cover off and examining the gears. No shavings or messed up parts, just 5 teeth broken off the camshaft timing gear. The new gears I got are both standard. I put the new camshaft timing gear on and the backlash - the space between the 2 sets of gearteeth is 0.032" - just a LITTLE over the spec. My mechanic found out from the AGCO distributor that they no longer sell over or under crankshaft timing gears - just the standard. They only sell over and under camshaft timing gears. The best I can tell everything else is okay. The endplay on the camshaft and crank seem to be within tolerance.

Any suggestions on what size camshaft timing gear to install to get the backlash into spec - 0.001"-0.003"?

I sure have enjoyed reading all the post the last few weeks.

Thanks for any help.

Stan in warm and fiery North Central Texas.

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Stan/TX

02-02-2008 12:42:02




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 Re: TO20 Timing Gear Backlash in reply to Stan/TX, 02-01-2008 19:20:23  
Jerry,

I did not replace or do anything to the crankshaft - things appear to be okay. I did not do a detail examination or analysis of the mains and crank - I may have to before this is over.

The camshaft specs all look good - endplay and axial tolerances all good. There was no noticeable movement of any type when I installed the new camshaft. This morning I measured the distance between each set of teeth as I rotated the crankshaft 2 times. The measurments are between 0.029"-0.032" with the large percentage being 0.030". Keep in mind I am doing these measurements with a spark gap wire gauge.

I also looked at the camshaft gear with the broken teeth. The missing teeth are at the alignmentment marks (2) and right at 180 degrees around the gear (3). From the first tooth at the alignment mark the missing teeth are #1, #3, #22, #24, #25. These correspond with DC on #1 cylinder and #4 cylinder. Does that say anything that I need to know?

The other thing that is puzzling is what you bring up - just what does oversize mean. If it is diameter it would not take much to create the situation you are talking about - the camshaft gear putting pressure on the crank gear that actually tries to push the 2 shafts apart. My mechanic is still looking into that for me. By the way he indicates that we can get by with as much as 0.005" backlash between the 2 gears.

You are correct - there has to be some backlash. I noticed when I put those gears on and hand cranked the engine it was a little harder like there was some pressure between the 2 sets of gears. I think I am very lucky there was not a lot more damage. Needless to say I am not starting this thing back up until I resolve this issue.

Thanks for the input.

Stan

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Jerry/MT

02-03-2008 15:16:22




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 Re: TO20 Timing Gear Backlash in reply to Stan/TX, 02-02-2008 12:42:02  
Because the broken teeth are diametrically opposite each other it"s like the motion of the camshaft gear or the crankshaft gear or both, are eccentric along that diameter. Do you have any way of measuring the diameters of both the gears (big micrometer)? if you can you can assure your self whether the gears are made correctly or not. A lot of these replacement parts are now made where QC is something talked about but not generally practiced. How was the fit of the cam shaft at the bearing journals? Can you mic the journals in the block and the bearings on the cam.? What was the clearances on the crankshaft?
If the gears are not eccentric than there are some clearance issues or a bent camshaft or crankshaft that are causing the problem. I"d lean toward the camshaft if I had to take a guess at which one of those was bent or damamged but really that"s just a stab because it is a smaller shaft and easier to bend.

You"re going to have to do some detective work to get this figured out, Stan. You might borrow some guages and mics or rent them, etc and start making measurements. Or, pull the gears and the cam and take them to a machine shop where they can make some measurements for you.

Please keep us posted on this issue as you gather more info. I will look forward to hearing updates of your progress and will be glad to share what little I know about this issue with you.

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Don Hooks*

02-02-2008 13:06:57




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 Re: TO20 Timing Gear Backlash in reply to Stan/TX, 02-02-2008 12:42:02  
Sure sounds like something is flexing and the fact that opposite sides of the cam gear are damaged makes me think it is the crankshaft. Are you at a stage of assembly where you can take the center cap off the crank and check it for alignment - it almost seems that some slop there might be the culprit by allowing the crank to move enough to take away the backlash... Also, there is a very small chance the distributor shaft is bent and flexing the cam. I think several folks will be watching to see what you finally determine.

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Stan/TX

02-02-2008 06:50:00




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 Re: TO20 Timing Gear Backlash in reply to Stan/TX, 02-01-2008 19:20:23  
Gene,

I am in Decatur.

Stan



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Gene Stevens

02-02-2008 03:14:26




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 Re: TO20 Timing Gear Backlash in reply to Stan/TX, 02-01-2008 19:20:23  
Stan, where in NCT...I am in Wichita Falls. Gene



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Jerry/MT

02-01-2008 20:21:11




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 Re: TO20 Timing Gear Backlash in reply to Stan/TX, 02-01-2008 19:20:23  
I guess the tip might be a problem was that there was no backlash when you assembled the gears. Your suppposed to have some backlash, albiet small. But 0.032" against the spec of 0.002" is a bit much.(The shop manual is confusing on this because the spec sheet says 0.002" and the write up says "...0.002" or less." You had "less" and the teeth broke off.)

Did you replace your mains? Is the cam fully seated and the cam axial play within the specs? If the camshaft can move relative to the crankshaft (or vice versa) due to wear in their respective bearings then the backlash will increase. Measure the slop in the camshaft; it should be about 0.0014" based on the nominal camshaft and bearing journal data.

What do they mean by "oversize" and "undersize gears"? Is that a diameter or a tooth thickness. My guess is it"s diameter. If that"s true than going to a bigger diameter would reduce the backlash which is the direction you need to go.

I"m sorry I can"t be more helpful to you. I don"t have a machine design background so I"m just trying to interpret what I read in the shop manual as it applies to your problem. Hope it helps.

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