Harden a locking pin

[ieezitin]

Hi guys..

i have a 3 point hitch bush-hog mower and the PTO angle drive is rated for a 25HP, My tractor is 30HP diesel so its a tad heavy on the torque for this mower. I dont have the original pin that connects the splined PTO drive to the knuckle of the PTO shaft as that sheared off yonks ago so i use a 7/16 grade 7 bolt, if i dont wail it or hit large branches it works fine, hitting a stump or block of concrete shears it as it should, the drive output speed for the mower is 540rpm and thats what i run it at

I get a season of use out of this set up if i dont smash it into something to large, this arrangement works fine but after a period of time it just shears on its own while im not abusing it, also the 7/16 bolt it about .050 below diameter of the hole, i compensate for the little wiggle but running up two nuts to lock it tight. I believe the bolt gets work hardened and gives up.

my question is: if I turn my own pin true to size of the hole whats the correct way to harden it for the task at hand?

Thanks

Anthony. 

[vtsteam]

It's there to protect the shaft, so a season of use seems okay to me. If you were replacing it every time you went out that would be different. I'd be wary of hardening it too much and enlarging the holes without shearing it.

If the holes are enlarged this will tend to shear the pin more quickly because it hammers.

Also it could actually be that the bolt is too hard and fatiguing. Can't remember if that grade is case hardened either. Anyway, it doesn't seem like the right pin per mfr., but if it is working and replaced once a season, and isn't enlarging the holes, seems like it's working properly as is.

Actually, forgot to ask the most basic question:

Can't you just get a proper new shear pin from the manufacturer/dealer?

[wgw]

I don't know what a bush hog mower is, but I'm guessing it may be what we call a "topper". First try to get the proper pin, if not it may be metric? 12mm is nearer than 7/16". The holes should not be worn but can wear over time. As said a years use is not too bad. Do not be tempted to fit a higher tensile pin. I used to design these things and the biggest problem was high tensile bolts instead of the proper pin. ps- try some thick grease on the pin ,see if that helps .

[ieezitin]

Hi

I know the pin is there to shear. The bolt i use works great for a season then shears, as i said in my original post i think its work hardening, if i had the correct pin it would not shear after one season it would work until i hit something hard then it would do its job, that is what i am trying to achieve. I cannot buy a pin because the holes are different, that's why i want to make one and make the largest hole a sliding fit.

My question was: whats the best way to harden the pin i am going to make.... I am going to make it out of steel. To expand a little on whats in my mind i want something around the hardness of a grade 7 bolt and its shear strength.. i am not up on tensile, proof and shear of carbon steel but i know a grade 7 is perfect, because it works now... i suspect if it fitted correctly (hole diameter) it would not fail but due to the slack fit it work hardens.

Thanks

Anthony. 

[awemawson]

So why don't you take a larger 'grade 7' and turn it down to a snug fit and use that?

[Lew_Merrick_PE]

Anthony,

A SAE J429 Grade 7 bolt has an tensile ultimate strength of 133,000 psi (minimum) and a tensile yield strength of 105,000 psi (minimum).  That is pretty much going to leave you using either tool steel or alloy steel if you want to match it.  The harden & temper process will vary depending on which type and grade of steel you wish to use.  My personal preference would be AISI S7 tool steel hardened & tempered to (approximately) Rc-20.

I can get you more information on the process should you need it.

[vtsteam]

A grade 8 is a medium carbon steel according to the chart I saw online, so that's what you'd want. Hardening is pretty straightforward heating to cherry red or non-magnetic and plunging into water or oil, depending on the type steel, it's the tempering after hardening that's important.

You shine up a portion bright, then reheat the pin gradually (in something like a toaster oven where you want it even throughout) and watch the colors of the oxides and then plunge. The temperatures here are much lower than the original hardening process, in the 400 F +- range, so a standard oven can usually reach those temps. The higher the temp, the softer the pin, but also usually the tougher.

I don't know the temper temperature (color) that would be appropriate for this particular purpose, and maybe Lew will be able to tell you that.

[Lew_Merrick_PE]

VT -- Most SAE graded bolts are made from one of the 4130 to 4340 grades of steel.  It is usually easier (and cheaper) to get a short length of tool steel than a short length of alloy steel.  At least that is my reasoning.  The tempering temperature will be determined by the (moderately) specific alloy.  Another advantage of the tool steel approach is that the temperature-transformation charts are readily available.

[petertheterrible]

Most shear pins are of a mild grade on the setups similar to what your doing for a definite reason, to break before the universal does, making the PTO shaft a propeller.  If it is the hitch point locking lug then it needs to be hard but otherwise a 4.8 bolt would do.  Look at the hole for the shear pin, as far as they go if it is oval your problem may be there.  If the shear pin brakes often, maybe your PTO clutch fitted is to tight assuming you have one fitted.  Turn the mower by hand, may it be that you have a seized bearing, if it turns to difficult it will break your pins.  Tractors only put about 70 % of their power out at the PTO, if it states that your tractor has 30 HP it will probably be the engine power and not the dyno'd power of the PTO, see your operators manual.

Reread your post, if it is at the tractors PTO side it isn't a shear pin just a hitch pin, which on most systems is spring loaded and has a taper and flat to the pin.  It should not sheer, I suggest you follow my instructions above.

[petertheterrible]

Splined shafts do not have shear pins, only pins that hold the parts together.  What would be the point of using a shear pin if it won't afford any protection to the system when it breaks?

[Manxmodder]

So why don't you take a larger 'grade 7' and turn it down to a snug fit and use that?

That sounds like a sensible solution to me.......OZ.

[vtsteam]

Splined shafts do not have shear pins, only pins that hold the parts together.  What would be the point of using a shear pin if it won't afford any protection to the system when it breaks?

Yes, forgot about that. The shear pin on my bush hog is located down at the mower head connection, The spline pin up at the PTO itself is spring loaded and built into the pto shaft universal joint housing, and crosses a recess on the side of the shaft, rather than running through a hole in it.

That isn't a shear type of arrangement. It prevents the splined shaft from uncoupling axially, the splines themselves prevent rotation so you can't possibly shear it by hitting something with the blades unless the splines ar broken off.

It is subjected to axial forces so if the mower hits something in line and somehow the 3 point fails to prevent over-extension, I guess it could break. The other possibility is there isn't enough length in the PTO shaft so it's banging when raised or lowered. In fact PTO shafts are telescoping, up to a limit to avoid putting any axial stress on that pin.

True you want to replace it with something the right size instead of a pin too small, and also if you make it yourself it should be properly strong and hardened, but a lot of the confusion here surrounds the idea that it was a shear pin being asked about, and why a shear pin would be highly hardened since the point of it is to shear. Many shear pins in fact are made of materials with a notch machined into them or have a reduced waist to encourage breakage, or made of softer materials than steel even so that they will shear.

[PekkaNF]

I have used whole loat of PTO shafts with overload clutches and some with oneway cluch action too, because our tractor had a brake integragted to PTO and some high inertia machines would kill PTO shaft when slammed.

Pekka

[vtsteam]

Yup, mine has a one way (over-running) clutch, too.

[vtsteam]

I just had to go down to the barn and check out the PTO shaft and bush hog -- hadn't used it yet this year, and I couldn't even picture the pin on the splined shaft.

Nope, what I have doesn't even remotely apply. No pin on the PTO housing knuckle. It's 3 balls that retract to lock it in place in recesses near the splines.

The bush hog has a plain shaft with a bolt shear pin. No splines. So nothing I have applies to your situation, Anthony. And I'm going to end by saying I just don't know what your situation is. You should have a pin to fit, in any case, which is what you enquired about making in the first place. But with no experience of what you have, I just don't know about the hardness issue.

[petertheterrible]

I second what vtsteam has said.
Possible remarks that might be helpful are:
1 Tractor universals are standardized and have different sized gradings in either SAE or EURO.
2 You can probably get a replacement at the local farmers supply as their is a chance that it won't be machine specific.
3 PTO shafts are sold complete or you can buy only the parts required.
4 When you buy a replacement shaft it will be longer than required in general and will need to be shortened. 
5 Fit both sides without being intact and measure the max travel by holding and gestimating the max travel required before cutting (Fully up and down position).
6 Not to be smug but it's a lawnmower and not a stumpgrinder, exercise due caution.
7 Nearly all slasher type mowers have an output speed of around 560 rpm.
8 If there isn't a problem with the gearbox or clutch, you may be overstraining the machine cutting to thick grass with to low a setting.
9 A problem rarely experienced by new types of machines is the clogging of the machine, larger machines have more wind and thrust grass more evenly out of the machine, but this can be a problem with small machines and can cause driveline problems.
10 Sharpen cutting blades.

[petertheterrible]

This might sound counterintuitive but jerking parts pf poor fit seldom cause work hardening, the bolt just gums away at the contact point.  Work hardening actually is occurs besides the contact point.  The killer actually is structural failure similar to when you bend a piece of wire until it breaks.  The steel actually softens unlike a rolling action where molecular compression occurs, hardening the metal.

With things like mowers please, please, please use KISS, as this will most likely be the most effective and cost saving principle.