Why does this slip clutch vibrate

[BeezFun]

This slip clutch came with the pto shaft on the 5' King Kutter I bought a few years ago. There's always been more vibration in this thing than I liked, so I finally took off the slip clutch as the first step in diagnosing the problem. With the clutch off, the thing runs smooth as a sewing machine, with it on there's quite a bit of vibration.
Both the male and female sides of the clutch fit snug with their mating parts on the gearbox and pto shaft, and there's no slop in the shear bolt. I checked the run out of the outer diameter of the clutch surface, and even with the irregularities of the surface and paint, it's about 1/32". And it's such a small diameter I wouldn't think it would matter much anyway. The only other thing I can guess is that the shaft is either not perpendicular to the clutch face, or the shaft is not concentric with the female side of the clutch. The other thing I can't explain is that the vibration increases as I get the shaft more aligned, and it decreases as the angle between the gearbox and pto stub increases, which is backwards from what I'd expect.

 

[sweettractors]

Are you sure it is not slipping? Ken Sweet

 

[pmsmechanic]

Are the yokes on the pto shaft lined up?

 

[BeezFun]

Are you sure it is not slipping? Ken Sweet

Yes, I made index marks with white paint when I initially tested it and adjusted the springs.

 

[BeezFun]

Are the yokes on the pto shaft lined up?

Yes, and the symptoms I see are the opposite of what I'd expect with unsychronized yokes. Unsychronized yokes have no effect when the shaft is aligned, and get worse as the misalignment increases.

 

[pmsmechanic]

Would it be possible to run a dial gauge on the input shaft while turning the mower to see if the input shaft is aligned. Also is it possible to extend the pto shaft and attach it to the gearbox and run the mower without the slip clutch to see if the slip clutch is really the problem? I wouldn't necessarily mow grass like that. Just run it to observe if the issue still continues.

Edit to add: Is the shaft near full extension? There might be enough slop in the slip part to cause problems.

 

[BeezFun]

Are the yokes on the pto shaft lined up?

Yes, and the symptoms I see are the opposite of what I'd expect with unsychronized yokes. Unsychronized yokes have no effect when the shaft is aligned, and get worse as the misalignment increases.

 

[BeezFun]

I think I may have found a potential answer in this old post. I'll test this out once we get a day less than 95 degrees. Here's a quote from what that guy found:

"Assuming you have a slip clutch, I think there is some evidence, that the source of the vibration may the slip clutch itself. If you dont have a slip clutch, then you bent something else.

The clutch bore holes may not be concentric or possibly there is an imbalance on one part of the clutch. Most likely, after initial slippage, the small imbalances in the clutch can become stacked up, so to speak, which translates into a BIG imbalance at certain orientations of the two slip dics. (You mentioned that it got worse after you hit something, which suggests the clutch slipped a little from its orginal position. As I said in a previous post, my mower behaves exactly the same way).

If you wish to confirm that the slip clutch is the source of your vibration, then try this, since it has worked for me:

1. Mark a DOT on the center part of the slip clutch and write NUMBERS on the edge of the other half, using a bright marker or paint. Just mark the NUMBERS like the numbers on a clock face. see photo.

2. Start the tractor, engage the PTO and purposely make the clutch turn or slip a little bit. Keep repeating this by engaging/disengaging the PTO until the vibration improves. Eventually, you should find a sweet spot, meaning somewhere the vibrations should improve - maybe even similar to when it was new. Of course, the DOT/NUMBER should be changing each time you slip the clutch.

3. When you find the lowest vibration amplitude, carefully let the blades coast down freely - meaning no forward movement of the tractor or cutting of grass. When the PTO is finally at rest, observe the DOT and which NUMBER cooresponds to that DOT.

4. Make a note of which DOT-to-NUMBER gives you the lowest vibration. By the way, my style of PTO slip clutch discs are considered TIGHT when the proper bolt/spring compression dimension is achieved. I have a BUSH HOG brand mower. The KK clutch is probably similar.

In summary, I think that there is a "balance high spot" in the slip clutch assembly which becomes stacked, causing an excessive imbalance, hence the high vibrations. At some orientations, the imbalance cancels each other out. That is the condition you want.

This method should PROVE that:
-the location of the slip discs changes the vibration amplitude, getting greater or lesser as the discs turn in relation to each other and
-your slip clutch is the source of the imbalance.

When you are out mowing and the next time it vibrates badly, simply try to purposely slip that clutch making it match up as close as you can to the best number painted on your slip clutch assembly.

On the other hand if you wish to confirm that the clutch is the culprit, just remove the whole slip clutch assembly and run the mower temporarily without a clutch just to see if the vibration disappears. If the vibes improve a lot then your culprit is definitely the clutch.

Let us know if you try this technique and if it helped you or not ....

dwight"

 

[BeezFun]

Would it be possible to run a dial gauge on the input shaft while turning the mower to see if the input shaft is aligned.

That's a good idea, I will try that.

Also is it possible to extend the pto shaft and attach it to the gearbox and run the mower without the slip clutch to see if the slip clutch is really the problem? I wouldn't necessarily mow grass like that. Just run it to observe if the issue still continues.

That's what I did to determine that the slip clutch is the problem. The pto shaft is plenty long, no problem running it this way.

Edit to add: Is the shaft near full extension? There might be enough slop in the slip part to cause problems.

The shaft has 21" overlap when the clutch is in place, 15" without it. So that should be plenty whether I use the clutch or not.

 

[BeezFun]

Would it be possible to run a dial gauge on the input shaft while turning the mower to see if the input shaft is aligned.

I finally got around to putting a dial indicator on this. When I spin the blades by hand, the input shaft to the gearbox shows 0.001" runout. I thought that was pretty amazing for a middle of the road bush hog. When I put the slip clutch on the gearbox and rotate the blades by hand, the input shaft to the slip clutch shows 0.035" runout, which I think explains the problem. For kicks I checked the outer surface of the clutch disks, they had 0.020" runout. I guess I could try rotating the two halves of the slip clutch to different orientations to see if the runout is reduced, but I decided to buy a Grade 2 shear bolt and just run without a slip clutch.