Knocking Sound in PTO Shaft

[TerryR]

This evening when I shut down my rotary cutter I heard a knocking sound in the PTO shaft. It occurs once per revolution of the shaft. It's not too loud but loud enough to hear when standing next to it with the engine off and it slowly winding down. I can also feel it with my fingers on the shaft.

The shaft doesn't have a huge number of hours but is 20 years old. It probably gets less than 40 hours of service a year. It runs close to straight - less than 5 degrees of angle I'd guess.

Any ideas about what I should be looking for?

 

[boggen]

does it make sound with another implement? (to narrow it down between tractor or implement)

are your blades out of balance a good amount?

do you have vines, twin, rope, wire, etc.. tangled up underneath, or around the pto shaft itself (under the shields)? also check the stump jumper it anything tangled up around it.

is there slop in the PTO shaft, as in turn shaft at tractor side counter clock wise and it moves and finally shaft at rotary cutter turns? or vice vis clock wise?

oil in the gear box on the rotary is it to full?

is rotary cutter up in the air held up by 3pt hitch, or down on the ground and blades hitting a weed were ya sat it down?

slip clutch working correctly?

if not slip clutch, is shear bolt in good condition?

the end of pto shaft connecting to tractor. the "pin" that goes into the rear of tractor and locks pto shaft in place, is there slop in it?


---------------
ya trying to get away form actual bearings and the gear box checking.

 

[TerryR]

Thanks, Ryan,

When I heard it the cutter was sitting on the concrete floor of my shop, and the PTO was turning maybe 5 to 10 rpm. I didn't pay a lot of attention to it at the time but later got to thinking it could be something serious.

I was thinking the lower shaft turns faster than the PTO, so since it was knocking once per revolution of the shaft it had to be something turning at shaft speed and couldn't be anything below. But now I'm not sure the blade turns faster than the PTO - was I wrong about that?

There's nothing tangled in the shaft, but I didn't look below.

I need to go check it out further this morning.

 

[TerryR]

I just started it up and checked it out. First, I find that the blade turns half again as fast as the PTO, so the sound has to come from something turning at the same speed as the shaft.

I couldn't get exactly what I heard last night, but I did find that the attachment of the yoke to the PTO spline on the tractor is loose enough that I can get a little play when I shake it. I suspect that was what I was hearing as the blade spun down to a stop last night - the yoke dropping a little bit with each full revolution (don't known why it wasn't twice with each revolution). I suspect I can't get the same sound this morning is the grease on the spine was warm last night but cold this morning.

It's pin-style keeper, and that connection seems solid, so I don't think there's a real issue.

Separately, are the plastic shields on the shaft replaceable? This one has long since worn out. The yoke bearings seem sound on both ends, so I think the shaft itself is fine.

 

[PILOON]

You said , like 20 years old.
I'd then suspect a Ujoint bearing somewhere in the drive shaft is kinda dry and due to fail.
You have 2 Ujoints therefore 8 bearings and any one might be wanting grease. Probably the tiny rollers inside of a cup are nice and dry and kind of squared off.

Just a wild guess but something to consider.
Manually twisting and turning the U joints should confirm the possibility.
If you feel hesitation or slight jerking as you twist/pivot the U joint you probably have found the culprit.
If so I'd replace before a total seizure that could get nasty (and costly)

 

[DieselBound]

I'd still figure on verifying that your blade tips are hitting on something: if you've got TWO spots to hit on they that'll equate to a noise being generated per shaft rotation.

Go through your entire PTO shaft. After blowing out a U-joint on a shaft that came with a used rotary cutter that I recently bought I am chucking the entire shaft: I replaced one U-joint only to find that the other one (which didn't blow up on me) was also bad; the shaft is bent and shield/guard has seen better days (I'm also not convinced the slip clutch is good). If there's any signs of the shaft being bent then TOSS it! (likely cheapest in time and $$ to get an entire new PTO shaft)

 

[TerryR]

Thanks, guys.

I'd still figure on verifying that your blade tips are hitting on something: if you've got TWO spots to hit on they that'll equate to a noise being generated per shaft rotation.

No, the lower shaft turns one and a half revolutions for each revolution of the PTO shaft, and the sound is exactly once per revolution of the PTO shaft.

I checked the U-joints this morning and they seemed solid. But since both of you warned of that I slid back both shields and checked them again. Both are very tight - no movement and no roughness.

I'll keep watch, but I'm now pretty sure the little clunk at low speeds when not under power is the front yoke moving a bit on the spline.

 

[CCWKen]

I'd go for a U-Joint going bad or there's something under the deck that the blades are hitting.

 

[em14]

I'd go for a U-Joint going bad or there's something under the deck that the blades are hitting.

CCWKen - I have no idea how my post replaced yours! I am sorry that happened and if anyone knows what happen Please enlighten me!
Leo