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PTO Knock

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720AL

03-24-2003 17:34:26




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I have a 720LP. I connected a 7' Bush Hog rotary mower to the drawbar and started up in 4th gear. When I drive straight with the mower running, no problem. But when I turn to the left or right (especially left) a pretty loud knocking sound can be heard (and felt) and it sounds like it's comming from deep inside the bottom of the tractor. Because the pto shaft is offset to the left, i'm thinking that it's the u-joint making the noise. Except that it's a new u-joint on the pto coupler shaft.

If I turn the tractor with the pto turned off - no noise.

Does this make sense to anyone? Does anyone else have a knocking sound when pulling a large bush hog?

Thanks

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Steve

03-26-2003 09:02:25




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
Previous posts are all good things to check. I grew up on a 720 gas that did this as long as I know. About 10 years ago we had to tear it apart because the brake would not stop pto any more,the pto oil level was always to full, Dad said don,t worry about it. We tore it apart and found the brake lining was not attached to the brake plate. Replaced seals between transmission and pto. I could see no reason for that noise except possibly the end play in pto shaft itself. I removed a shim controling end play and no more noise when turning!

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720AL

03-25-2003 09:38:24




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
Thanks very much for all of the answers. I will check the mower's pto slip shaft for length and the distance from the end of the pto and the drawbar. Good ideas.

However, this did jog my old memory. Last year, I replace the pto clutch and brake linings in the tractor. A little painful, both in the work and in the wallet, but not too bad. Now I remember that the race for the pto shaft bearing on the end of the pto shaft (inside the tractor) looked really worn. It's possible that the end of the shaft is rattling around and the bearing hitting the sides of the race or (if the bearing is really worn, too) the end of the shaft could be hitting the case. (ouch!)

I'll try everything else first and if that doesn't fix it, then i'll go back into the pto housing. It will really make my wife mad because I had her hold up one end of the pto housing when I put it back together and she's going to have to help again. This is going to cost me a dinner out!

I'll post what I find for everyone as soon as I can.

Thanks very much to all.

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Steve K

03-25-2003 04:33:46




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
I have the exact same problem and my neighbor also had the same problem, but he fixed his. It was not just the ujoint causing the noise, but rather the thrust washer behind the one bevel gear in the bottom of the trans that was out. This allowed too much clearance between the 2 bevel gears causing them to hammer severely when turning only. Yes, the ujoint can cause this noise too, but don't neglect the possibility that the noise is internal. You should center your drawbar directly behind the pto and MAKE SURE your hitch pin hole is 14" behind the end of the pto shaft. If you do these 2 things and it still hammers on turns, I believe the problem is in your tractor, not the ujoint. Steve

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F-I-T

03-24-2003 19:13:23




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
720AL:

Check to see if the mower PTO shaft is too long.

If it bottoms out when you turn, it will hammer like no tomorrow.

Mark the position of the outer to inner sliding shield with a magic marker when the tractor/mower is turned to the extreme left and then right. Then disconnect and slide all the way in. It should slide on past those shortest of those two marks. If not, slide it apart and get out the hacksaw, and trim it a little at a time. You might have to trim both the shield and the inner shafts.

Another time it shows up is going through a dip, or turning while going through a dip.

Was a classic noise on old AC combines.

Hope it's simple.

Frank-in-Tallahassee
70 // 855

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Bob F.

03-25-2003 03:52:12




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 Re: Re: PTO Knock in reply to F-I-T, 03-24-2003 19:13:23  
Another possible easy fix if the PTO shaft is too long is to check the drawbar for length. It could be that it is pushed in too far and needs to be extended to the next set of holes. Out 720 LP does the same thing to a lesser degree on our rotary cutter.



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Ray

03-24-2003 18:34:09




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
The 720 pto is not in the center of the tractor,
it's offset,and when you turn left it puts it
in more of an angle to bind up or make noise.



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ChuckW

03-24-2003 18:26:35




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
I would say the U joint is causing the knock, however excessive vibration such as this telegraphing back to the PTO gear could crack, destroy the thrust washer at the PTO gear. I would recommend a 3 point mounted unit.



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scottAR

03-24-2003 18:11:05




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
Lubricate the slip joint so it can move freely. Even check to see if the shaft may be bent and not allowing it to slip. Secondly, check to make sure the u-joint yokes are lined up correctly on all the drive shafts of the machine. Your second and third yokes must be on the same plane.



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Clooney

03-24-2003 18:04:38




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
AL, if it only does it when turning it is more than likely the PTO u-joint angle causing the shaft to jump & that in turn causes the internal PTO noise. ~When power is transmitted through a single u-joint that is run at an angle, or through two u-joints that are run at different angles the shaft accelerates & decelerates for each 1/4 turn of the shaft & if there is a mass in motion on the end of that shaft [such as a brush hog] the mass velocity tends to want to stay at a constant RPM so when the shaft tries to accelerate & decelerate it forces the tractor internal PTO parts to change speed & make a knocking noise.....

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Bob Weber

03-25-2003 08:45:35




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 Re: Re: PTO Knock in reply to Clooney , 03-24-2003 18:04:38  
I had a similar situation on my B, and I modified my brush hog a bit so that I didn't have such a severe u-joint angle at the tractor PTO end when turning. I put an intermediate shaft between the hog gearbox and a support bearing I mounted near the front of the hog. Then I run a short PTO shaft (24") with a u-joint at each end between the tractor PTO and the intermediate shaft on the hog. I have my drawbar set so that the pivot point (drawbar pin) between the tractor and hog is at approximately the halfway point of the 24" shaft. Now when I turn, each of the u-joints bend at the same angle, and they are phased so that the "acceleration/deceleration" of the u-joints cancel each other out. It made a world of difference in how much smoother and quieter things run.

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Tyler, MO

03-24-2003 17:53:05




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 Re: PTO Knock in reply to 720AL, 03-24-2003 17:34:26  
Yes,
I know exactly what you're talking about. My 720 Gasoline (which is identical to your LP as far as PTO goes) makes a loud knocking noise. Mine does it especially when I turn to the right. Very rarely does it make the noise when turning to the left. My tractor has these symptoms even when using the pto for light duty i.e. fertlizer spreader, but also for the heavy duty applications like square baler, pto driven manure spreader, ect. I wish I could tell you what the problem is, but I too am dumbfounded by the noise. I just hope that it isn't causing very much damage internally. Tyler

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Ed Hadaller

03-25-2003 08:31:45




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 Re: Re: PTO Knock in reply to Tyler, MO, 03-24-2003 17:53:05  
My 630 John Deere makes that same noise. I noticed it when we were baling with it. It seems to make the noise more when making the right turns baling. Seems to me that there is some slop in the pto shaft maybe???



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