Harden a locking pin

Gallery, Projects and General > How do I??

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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?

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