Hewland cotter-pin failure

I have had cotter-pin failure several times in my Mk-9 gearbox, on a FF. I spoke with Lee at WRD, who noted several others had had the same problem over the last year, and suspected cheaper material in the pins. {I got mine from Pegasus] I recall that when I got the box, it had double-nuts [I assume you torgued the first and cinched the second as a jam nut] instead of Castellated nuts, but Richard Morgan Racing changed it to the more standard setup years ago. The problem with the cotter pins began 2-3 years ago, long after the change. Does anyone know of a source for good cotter-pins, which are stronger {carbon st vs stainless} or know anything about the double-nut setup? [I can't recall which shaft had that, at the moment. The top shaft seems to be the failure area, I think.]
Thanks for your input.

That's odd. The cotter pin shouldn't even come in to play unless the bolt is loosening, which I've never had happen. Are your nuts actually loose? (that sounds bad) I wonder if the draw bolt being too tight could break cotter pins?

The cotter pin on the layshaft is really just there to hold the draw pin in place, they aren't much as true locking devices. I have never had a nut come loose enough to trap the cotter pin when I wanted to take it out again, so I don't know about failing cotter pins...

Brian

Hi Guys , I have had this happen once , it had not come out completly but 2 pieces had broken off . I only use the split pins once so I put it down to cheap chinese split pins

Where are the cotter pins breaking? Is it at the edge of the hole, where the two halves are normally bent outwards? I have seen that type of failure commonly on early (3.8L) Jaguars where the rod bolts had castle nuts. The problem, as you stated, is that the lead content is gone and the cotter pins are more brittle.

For the Jag motors I simply update the rod bots and nuts. For the Mark 9, I suggest you look at a different type of pin. I have had great success with the "hair pin" style, where the one half of the pin fits across the castle nut, the other half (big round part) simply goes around the outside of the nut ad springs back against the outside of the castle nut....EDIT: Rick calls them safety pins ...of course!

sorry for the poor explanation, do you understand what I am describing? The type of locking pin used on pip pins and the like. Because the part that fits across the castle nut is straight, there is no stress and they do not fail

Otherwise, call Taylor and get them to send along a few that they use when rebuilding Hewlands. Even if they charge a couple of bucks each, cheap insurance!

Good luck, Tom

I've always wondered why we don't just use the spring style safety pins instead of cotters on the shafts. I spend more time bending the pins back and then beating them out with a punch than I do actually changing the gears.

I've had very good lusk with the stuff from fastenal.

I bought a bag of 100 from Aircraft Spruce, good stuff. When I bought them, they were cheaper, with shipping, than buying the cheap crap locally...

but, you really need to find the cause of the breakage, because those pins should not have any load on them to speak of

We broke some years back, switched to shorter ones that we just bend out slightly instead of back on themselves.

Have never had one break since doing it this way. For an LD200 we just use 5/32 x 1" non stainless from McMaster Carr.

Never re-use obviously.

Quote:

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Originally Posted by stephen wilson

That's odd. The cotter pin shouldn't even come in to play unless the bolt is loosening, which I've never had happen.

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I agree with Stephen and Brian above.

The big question: is the cotter pin failing due to shear from the nut trying to work itself off, or due to brittle failure from being bent over.

By chance, did you recently install any new spacers in the gear stack? If not made with the correct material, the spacers may be yielding in compression, thus reducing clamp load, allowing the nut to back off and shear the cotter. If I recall correctly, this was an issue many moons ago with Staffs FF/FC trannies, which do not have provisions for cotter pins (no hole in either shaft).

Not to steal this thread, but on a related note, can someone explain to me the function of the drawbolt? Once the tranny is mated to the bell and input shaft, is the drawbolt serving any purpose?

Keeps the layshaft and input shaft together in the splined coupling that you find connecting them when you remove the side covers.

Thanks for the input, guys.
No recent shimming or changes to the gearbox. i seem to recall it happened on the layshaft nut and the pinion shaft. The breaks appear to be at the edge of the hole.
The pictures may show the problem and the solution. If it is the pinion shaft that has the problem, which I am not sure is true, then the fact that the holes are misaligned probably contributes. It is not the whole story, as they have been that way for 20 years, and I have only had the problem off and on for 2 years or so. I have the nuts which were used on it years ago, and a washer which fits, and may have been used with the nuts. I have never used a washer with the castellated nuts, so I don't see why it would be needed with the nut/jam nut setup.
Does anyone know anything about the appropriateness / reliability of the nut and jam nut setup? With or without the washer? Torque 115 for each or just the first nut?
Thanks for the advise.

IIRC, if the hole does not line up @ required torque you continue to tighten until it lines up correctly, or have a hardened washer ground to make it work. that's the way it works with an LD-200. You can also buy new nuts, sometimes that changes the stack-up.

Rick, the hole will not line up because the shaft is drilled off center. I have to shave the cotter pin to get it to go in, but I have been doing that for years, so I am not convinced that is the whole story. [It does seem worse, but I can't see how it would "get worse" so I guess I am just getting more annoyed with it.] I don't have the skills to take the box apart and replace the shaft, so I have been living with it.

As an aside there is another mistake to be made with the cotter pins and that is leaving the ends too long that you bend over after inserting it.

This last weekend at Laguna I had the gearbox jam in neutral twice during a race, the second time I coasted in so I could take the box apart with it jammed to hopefully make it easier to find. As we took off the back cover a small piece of metal went 'tink' into the drain pan. It turned out to be the 1" long leg of the too-long cotter that I had bent around the castle nut to clear the inside of the case. Over time vibration caused the metal to fatigue at the first bend and it broke off. With all the room in the back cover it decided it needed to find the one place required to jam the linkage. Twice.

I normally cut the cotter if it's too long, leaving 1/4"-3/8" out from the castle nut to bend over. I guess the last time I was in the box I didn't...

Classic 10¢ part story ruining a race.

Tom Duncan

PS: In 20 years of racing a MK9 I have never seen any sign of the castle nut turning agains the cotter pin.

[QUOTE=In 20 years of racing a MK9 I have never seen any sign of the castle nut turning agains the cotter pin.[/QUOTE]

Which makes me wonder if the cotter pins are even worth the hassle of installing. Just tighten the nuts to the desired torque and you're done (just like a Staffs).

The pin keeps the drawbolt from backing out.

Jim,

So far you haven't answered the important question.

"When you take it apart does the nut still have torque, or is it loose?"

If the nut still has torque, then definitely it is a case of poor quality pins.

OBTW, i use much shorter pins, and only spread them a bit after insertion.
That long bent over piece (already stressed from the 180 degree bend) is probably vibrating like a son of a gun.

My guess would be that the threads on the nut and/or the shaft are worn and the bugger simply loosens and works the pin. I'd bet you have no idea of the life of these parts. Replace the nut.

I keep reading this and I'm still perplexed.
For the 17 years I raced a car with a Hewland Mk series gearbox, (6, 8 and 9) I never had this issue. I never went out of my way to get anything special in cotter pins. Sometimes I used what I had available, at times were thinner than the hole in the shaft(s). I've used then "just long enough" and those that had a long section bent over. I can't even remember a time when a nut was loading the pin high enough that it would be difficult to remove (thus trying to cut the pin off).
I like the worn Nut idea.
I like puzzles and their solutions so I'm quite interested to hear what really is happening here.
Please let us know.

Well, except for the usual one. I haven't thought to check the actual torque on disassembly, but the nut is not "loose" as in no force needed to remove it loose. As for the worn threads, that may be right. I am sure the nut is old enough to vote. I am now going back to a double-nut setup, as I am sick of trying to get a pin through the mis-aligned holes. [see pics above] There is an adjacent thread on hewland torques [not mine]. That thread suggest I should be using more torque on the shaft that calls for 70 ft/lb, but if I recall, that is the lower shaft, and I think the upper shaft is mostly where this happens. I'll order a new nut, and let you know in the fall. If the double nut fails at Roebling, I'll let you know sooner!
BTW: I like the shorter pin idea.

If the nut is not loosening, then it is not the problem. It is just bad pins.

In looking at the condition of the threads on the studs and from what I can see of the threads on the shaft the box needs a complete going through. These things are not meant to last forever. Taylor sells a stud kit quite reasonably. I replace mine every three years or when the nuts show any sign of loosening. That goes for the side plates as well. Proper washers are important also. You say the hole is misaligned. The proper fix is to get the components in proper working condition (I.e. a new shaft, nut and thrust washer etc.) and assemble the box with proper torque. If you are very lucky the holes will line up - in that case go buy a lottery ticket. Odds are they will not line up. Back the nut off until the holes line up exactly - but measure how far you turn the nut as accurately as you can. Then, using the math you learned in grade school, determine how much you need to remove from the bottom of the nut so the holes will be lined up with proper torque. You probably won't hit it first time so err on the conservative side. Take off half of what you calculate. If you don't have a lathe then go to a machine shop and they will either use a lathe or a surface grinder.