PTO Shaft Issue with Shear Bolt

[Neophyte]

I know I'm going to get alot of "I told you so's" but I wanted to share this for others that may be considering it.

My 3320 is a 32.5 HP tractor with 25 PTO HP.

My Howse 500 has a 55 HP gearbox and is recommended for a 25-45 HP tractor. Without a slip clutch, Howse recommends a Grade 5 bolt and it even says so on their website. Click on Z62, it's a Grade 5 bolt. I was never able to confirm this in their manual since I have been waiting several weeks for one and I still don't have it. Have I mentioned that Howse's customer service is terrible?

Howse Model 300, 350, 400 & 500 Parts Breakdown

So I purchased a new PTO driveshaft for my 5' rotary cutter at a local dealer. I gave them my tractor HP, PTO HP, and implement specs. They recommended a Tisco P/N AB4111WY. You can see the details by entering that P/N at this link.

Tisco Parts Catalog

It is a 4 series PTO shaft, which is rated for 35 HP & 460 Nm (torque) at 540 RPM. You can see additional details at this link.

PTO Specifications and Information

Despite the fact that Howse recommends a Grade 5 shear bolt, I was using Grade 2. However, they were popping way to easy and way too often. After I ran out of about 15 Grade 2 bolts (seriously), I decided to go with their Grade 5 recommendation. I have attached the results.

I noticed the problem when I went to collapse the PTO shaft and it would not collapse completely. I was hoping that it was a burr or dirt but I knew without looking that it was twisted. Removing the safety cover confirmed this.

I supposed it could have been worse. I could have damaged my RC gearbox or tractor PTO.

If my tractor is only 25 PTO HP, should a 35 HP PTO Shaft twist like this?

I was talking to someone about this and he said this is why you want to run at 540 RPM. He said that when running at lower RPMs, you are running at higher torques, which can damage things such as PTO shafts. Is this true?. It's a moot point though because I was running at 540 RPM the entire time.

I was reading online that typically a 5 series PTO shaft (48 HP & 620 Nm) is used for rotary cutting applications. Should I have purchased a 5 series instead of the 4 series? My dealer said that all his new Woods and Bush Hog rotary cutters are being supplied with 4 series drive shafts.

Needless to say, I just purchased a slip clutch at Agri-Supply for $63 plus S&H. I was given a price of $150 locally. This works out nicely because of the added length of the slip clutch. I will be able to un-pin the twisted shaft, cut the twisted portion out, drill it and re-pin it. The reduced length will make room for the new slip clutch and I won't need to purchase a new shaft.

Agri-Supply Slip Clutch

I still think that a shear bolt (Grade 2) is fine for field mowing. However, if you are mowing through thick brush like I am, then I think a slip clutch is a necessity.

Should I be using a Grade 5 or Grade 8 bolt on the slip clutch shaft holes?

Given the fact that you can't use a C-Ring to hold the slip clutch on to the gearbox shaft, I would think you would want a Grade 8 bolt on this side to ensure that it never shears. I think the slip clutch allows for a C-ring on the PTO shaft side. Given this, my guess would be to use a Grade 8 bolt on the gearbox side of the slip clutch and a Grade 5 bolt on the PTO shaft side.
.

 

[Farmwithjunk]

What are you mowing? I've only seen a few PTO shafts twist like that. In ALL those cases, it was from way too much HP on way too small of a piece of equipment. OR.... Hitting a sawed off telephone pole sticking out of the ground about 8". ( I won't tell you how I know about THAT one..... )

That's why I always tend to favor slip clutches. Even with them, I tend to adjust the spring "stack height" just a hair on the loose side. I want mine to slip when I hit something. (Even if that means replacing the friction disc every so often.)

I'd start off with a grade 5 bolt in the slip clutch. See if it holds. If not, then try grade 8. I'd want all the protection I could muster after seeing that shaft.

When in "operating position", was that shaft extended (retracted) to about the position you show in your pictures? Or was it extended further?

 

[Neophyte]

I was mowing field and brush. I hit a few rocks but nothing noteworthy. The brush was 1-2" thick, maybe a little more in some areas. I did hit a couple of branch clusters that may have done this. The branches were only 1-2" thick but they converged to a stump that was thicker (maybe 4-6"). The tractor bogged down a little in a few cases but it never stalled out.

On level ground, the PTO shaft was completely overlapping with only an inch or two to spare on either side. Of course, I was mowing on some pretty uneven ground where I was backing up and down bankings, which is the only time the shaft could have been this exposed.

Do you think it is a faulty PTO shaft that isn't to specs?

 

[Farmwithjunk]

Can't say for sure, but I'd venture a guess that shaft is at best MARGINAL to specs. Sometimes equipment like a bush hog will just "take a hit" that does things that can't be helped. You may hit the same rock 1000 times and that one magical moment may be the one time that breaks something. Again, that's why I like slip clutches adjusted to the least possible resistance. On mine, I adjust spring stack height 1/32nd" loose from manufacturers recomendation. It doesn't slip under normal loads, but will slip at the slightest "over-load situation". I bought an extra set of friction plates when I bought the mower, planning on them wearing out somewhat prematurely. (I do the same with bush hog blades. Buy an extra set when I buy a new mower) Spare parts are my hedge against breakage. In my years of experience, I rarely break anything if I have replacement parts on the workbench. It's always that "on back-order for 7 months" stuff I break.....

 

[greg_g]

The more a shaft is extended, the more vunerable it is to twisting - the grade of the shear bolt won't help you here. Everything I've read by manufacturers clearly indicate that a properly-sized shaft should ahve more than 6" extension (of the inside shaft) when the driven equipment is in the working position. This 6" length is usually achieved after following their "How to cut your PTO shaft". They all start out recommending you lift the implement with the TPH until the tractor PTO spline and the implement transmission input shaft are perfectly parallel with each other. That's the point at which the shaft will be compressed to it's shortest length, and represents the baseline length from which you eventually make the final cut. Cut it too short, and the inside shaft will extend too far when the implement is on the ground. At that point, you lose more than 50% of the strength of the double wall.

If/when you have a shaft of the correct working length, and a slip clutch properly adjusted to match your tractor's PTO horsepower, the grade of bolt you use to secure the slip clutch to the implement input shaft should not matter. If everything else is done right, the clutch should slip before there's enough torque applied to snap that bolt. With a properly adjusted clutch, it's NOT a shear bolt any more, it's just another fastener.

Another reason for busted bolts is excessively worn (egged-out) bolt holes, which is why I much prefer PTO driven implements with splined shafts. In the case of a round input shaft, the whole load is on the bolt. But with a splined shaft, the load is on the splines - and the bolt becomes nothing more than a fastener. But in your case, that horse is already outa the barn. Something to keep in mind though, if you're ever go shopping for a new PTO-driven implement.

And yes, when encountering a sudden heavy load, running slower than 540 PTO rpms is hard on both the tractor and the driven implement. Relative to the power curve, you're likely developing enough torque - but not yet enough horsepower to sustain it. In the case of a setup like yours, it's often the shaft that suffers.

//greg//

 

[jinman]

How are you engaging your PTO. Do you feather it up to speed or does it suddenly engage? My 45hp tractor will shear a bolt every time if I don't feather the engagement. After it's running, it works great and I only lose a grade 2 bolt when I hit something. My cutter is a Tractor Supply King Kutter with a 40hp box, so it's nothing special. However, my PTO shaft is two sizes of square tubing off an old Howse cutter instead of the solid shaft inside the outer tubing like yours.

 

[Neophyte]

Greg_g,
I followed the instructions for cutting the shaft to length. As you can see from the picture, I have much more than 6" of overlap; it's at least 24" or more of overlap when the rotary cutter is on level ground. Also, when the rotary cutter is on the ground, the shaft is almost fully collapsed with only an inch or two of inner shaft showing.

The holes are not egged out yet.


Jinman,
I always engage the PTO at idle speed and then feather it up to PTO speed. The inside shaft on my PTO shaft is not solid, it's hollow. I have the same square tube shaft that you are talking about. It was on this Howse rotary cutter prior to me replacing it with this newer one. I showed it to my dealer and he said it was rated at 40 HP. Regardless, that square PTO shaft took a major beating and never twisted. I will post a picture of it when I get a chance.

 

[greg_g]

My reply above must have garbled on the upload, and there's no way to edit in a correction anymore. What it was supposed to have said was "a properly-sized shaft should ordinarily have no more than 6" extension (of the inside shaft) when the driven implement is in the working position". This will typically be with the implement having been let all the way down by the TPH. This maximizes the amount of double-wall strength. Every unnecessary inch of extended inner shaft weakens the overall capability of the shaft.

But back when the shaft was originally cut to fit, the first measurement should have been with the implement lifted (by the TPH) so that the tractor's PTO spline and the implement transmission input shaft were on a horizontal plane. At that point, there need be no more than 1" of extension (of the inside shaft).

So. If you state there's only an inch or two when the implement is actually on the ground, your shaft may still be too long. The distance to the working position is at an angle, when you raise the implement the distance gets shorter. When the distance gets shorter, the shaft must collapse. When you run out of "collapse length", something's gotta give. As such, you may have bent it when lifting to - or past - horizontal. Whether or not that's related to the twisting I can't say. Either way, it's not good.

The bend in that shear bolt photo looks like it's moving around while rotating. That can be from it being the wrong diameter, or the wrong length. You say the holes aren't egged out, so make sure you're using a 1/2" bolt in a 1/2" hole. Too long, and it will move back and forth in the hole - bending at either end (which is what I see). Make sure that self-locking nut is snugged up all the way to the clutch housing, so that the bolt can't move back and forth in the holes.

//greg//

 

[Neophyte]

Greg_g,

In cutting the shaft, I followed the instructions as outlined in the Bush Hog Squealer manual as well as the instructions given my various members on this site.

Everything I have ever read has said there should be a minimum of 6" of overlap. This means that the inner shaft should extend at least 6" into the outer shaft, no less.

It seems to me that you would want as much overlap as possible, more inner tube within the outer tube, in order to gain the double wall stength that you are talking about. Less overlap = less double wall strength.

I had the implement spline and tractor spline on a horizontal plane when I measured for the cut (i.e., the shortest distance). With that said, I'm not sure this was the shortest distance. It almost seemed like the shortest distance was when the implement was raised beyond the horizontal plane where the implement comes closer to the tractor.

The difference between horizontal plane and implement ground level, in my case, is only an inch or two. I tested my implement through the full range of motions and it never fully collapses or binds up. That is, I can always see a little of the inner shaft so it's not too long.

The bolt is the exact size as specified by Howse and the nylock nut is tight. With the bend in it, I had a very difficult time getting it out of the hole.

I should be receiving my slip clutch this week. I will recut the shaft to accomodate the slip clutch and I will let you know how it goes.

Should my slip clutch come with a manual? Are there any good manuals online that describe the process of setting a slip clutch?

 

[greg_g]

The new slip clutch should come with an instruction sheet, but that's not a given. If not, the chart I uploaded might help. Be careful though, these spring lengths only apply to the 55hp rated clutch. Confirm the PTO horsepower rating of your tractor, consult the chart, adjust the spring length accordingly.

LIke I said before, I purposefully select implements/clutches that have splined shafts and receivers. The type that just bolts to a smooth transmission input shaft needs a lot more attention. That is; at least semi-annual disassembly cleaning, spring adjustment. Cuz if/when a smoothbore slip clutch freezes up from neglect, you've automatically defaulted to a shear bolt configuration again.

//greg//