Gearbox taper pin sheared

[AlanT]

As you will see above I've been struggling to understand this topic.

So I asked some "silly" questions of poor old Roger to try to clear my head.

 

Today I got the parts in my hand and all became clear.

 

All along my mental picture had the top end, the small end, of the pin sitting snugly in the fork In a taper that had been reamed at the same time as the shaft. In effect a matched pair. I thought that these were breaking because the taper wears loose.

 

So I could not see why Roger was telling us to make this end parallel! These seemed more likely to go loose.

 

I blued up the pin to check how good a contact I had between the tapered end of the pin and the blind-hole in the fork.

There wasn't any contact. The blind hole is a country mile bigger than the pin.

 

They may as well have made these pins 1/4in shorter. The small end does b'all.

All the clutch torque goes through the threaded part of the fork and pin.

 

It's AN AXIOM of fastener design and use that you don't run threads in bending.

 

The place where the threaded and unthreaded sections meet is a known as a place for a crack to start.

This is where the design concentrates the stress.

 

Roger is right IN SPADES. The small end of the pin must be made to earn its living.

 

You may say these pins will do 100K like that. They will. Mine has.

But some won't. Sooner or later more will break. Cleanliness and dry contact are needed to get a good fit and the ones that break probably didn't have this.

 

None of this would matter if it was doing something trivial on a door or boot lid.

Its where it is and what it does that makes it critical.

[Tom Fremont]

It's worth mentioning that eccentric mounting of the gearbox w/r/t the engine causes the TOB sleeve to drag on the snout and increases force required to release the clutch. Use of the dowels when assembling avoids that pitfall .

[RogerH]

Hi Al,

that empty cavity in the fork above the shaft, where the taper pin touches nothing, is where I put a bush for the parallel nose on the pin to sit in.

Relatively simple mod and the pin will never break.

 

Roger

[Lebro]

Ah--- now I also do understand !!

Thanks for explaining the explanation Alan.

 

Bob.

[AlanT]

Questions for Roger, when he is not too busy with the disaster recovery:

 

1. what diameter did you use for the small end of the pin

2. how did you hold the pin to ensure concentricity of the parallel section?

3. the best method is turning a matching female taper, which is a PITA.

 

Here are a few bits of info for anyone messing with this:

 

1. the fork was originally drilled 25/64 in, the tapping size for 7/16 UNF. This is 0.390625 in.

2. 10mm is .393701 in which is pretty close

3. I measured the taper as 2.35 dergree included angle, by a not terribly accurate method.

4. the "standard" taper, of 0.25 in per foot, is an included angle of 2.387 degree and so I believe this is what the pin actually is.

5. the small end of the pin is 7.77mm dia, as near as I can get.

 

I could make a 25/64 bush to go in the fork and take the pin.

It would have a wall-thickness of 1mm approx. This is workable, just.

 

Lets have a picture of this thing, I believe this to be an original pin:

https://flic.kr/p/ohq65b

[AlanT]

Today I drilled the cross-shaft for a roll-pin. I have lots of these and its time they found a home.

 

One of the jobs I least like doing is putting a cross hole in a shaft. I like to get them on a true diameter and this is not easy.

Here I am using a cheap Y-gauge, which makes a very sensitive turn-around test, that you are bang in the middle:

https://flic.kr/p/ohFhSu

 

Another thing is that the face of the "convenient boss" is not orthogonal to the axis of the taper-pin.

So I had to go in at an angle. It's not vital these two are exactly orthogonal but I like it that way:

https://flic.kr/p/ojtLLt

 

See also how near the square-head is to bottoming on the boss. It does not but I have touched this with a grinder so as to clean the surface for counter-boring.

 

Here is a precision ground 1/4in pin, which slides perfectly in and out of the reamed hole.

https://flic.kr/p/ohwRv5

 

But now an interesting thing happens.

 

If I apply a torque, just using hand pressure on the lever where the clutch-cylinder goes, then the 1/4in pin becomes locked.

The taper-pin is now being BENT out of line, it is not in shear BUT the cross-pin is.

 

The first time I did this I put the fork in a vice. But so little torque is required, I can do this on the kitchen table with one hand on the lever and the other testing the movement of the cross-pin.

 

So this will never break the taper-pin but it might shear the cross-pin. Note that a 3/16in looked too skinny so I have used 1/4in.

 

So now I face the acid test. Can I "Roger" the taper-pin so that the cross-pin remains UNLOCKED when I apply the torque.

Ideally we might like the cross-pin to lock up but only with quite a big torque.

 

This is not really vital, but will prove that the taper-pin is supported at the small end.

Edited July 11, 2014 by AlanT

[AlanT]

I drilled axially, up through the fork, for an 8-32 grub-screw.

 

I have the couple of reasons for doing this.

It also would assist getting out a broken pin because there is now a starter hole.

 

You don't come out in the centre of the fork and so here are some pictures, that might help someone, drill out to remove a broken pin:

 

https://flic.kr/p/o2HWU5

https://flic.kr/p/okYKbF

[RogerH]

Hi Al,

sorry for the delay - had a week-end under canvas n ext to a pub

 

Questions for Roger, when he is not too busy with the disaster recovery:

1. what diameter did you use for the small end of the pin

The nose of the taper pin was turned down to 0.25" - This gives a bush wall thickness of apprx 0.070" (0.39 - 0.25/2)

This plain shank is long enough to just go into the shaft and miss the bush.

2. how did you hold the pin to ensure concentricity of the parallel section?

I screwed the pin into a suitable nut (tight) and held the nut in a three jaw chuck. The nose can then be lined up quite well (not perfect but pretty good).

The bush is turned to match the taper pin nose.

3. the best method is turning a matching female taper, which is a PITA.

Roger

[ronhatch]

Hi all, Thanks for all the replies and info, I drilled into the top of the fork and exposed the top of the pin and I was surprised at how much "slack" room there was around the top of the pin.

So I started tapping it, no joy, and after various hammers ended up giving it some clog all to no avail,

I thought I had a brainwave, putting a masonry bit in my drill I switched to "hammer" put it on the pin head to try and persuade it but the bugger wouldn't move. so in the end I cut the fork off, cut off the protruding pin head and removed the shaft.

Even in the vice I could not tap out the pin so I drilled it out.

Following this post I will put in a roll pin and drill a starter hole for the taper pin, just in case.

Thanks for all the interesting thoughts.

Regards

Ron

[Lebro]

Does this show your mod Roger ? (not to scale)

 

 

 

Bob

Edited September 27, 2015 by Lebro

[RogerH]

Hi Bob,

you have morph'd my mod with a CND sign - well done.

Yes that is exactly what I have done.

 

Interestingly, I have just removed the taper pin from the 4A as the GB is going to be sorted.

 

I'll try and post a pic but the taper pin fits nicely into the cavity above the shaft. It was not the same diameter as the taper pin thread taping size.

The pin had done a good 100,000 miles (ten years operation) and it was cracked 360 degrees around its body but still in one piece, just.

So, a good fitting pin in a reasonable double shear will last quite some time.

 

Roger

[AlanT]

Could not bring myself to turn the pin down to 1/4in.

 

So I made a tapered bush:

https://flic.kr/p/oiJqvN

 

This has the following dimensions:

OD 9.86mm, this is a good push fit into the recess in my fork.

Height 0.153in, this sits just below the edge in the fork

 

This is also a tight fit in the tip of the pin:

https://flic.kr/p/onwcqg

 

Lined up and about to push in. You get just one go at this, so it must fit:

https://flic.kr/p/oiJqm9

 

MUST NOT push in with the taper. Will expand and jam.

Used this:

https://flic.kr/p/onTEwD

 

Seen here in-situ:

https://flic.kr/p/o4DabW

 

This must "bottom-out" in the recess AND allow the pin to mate with the shaft.

In other words the two tapers must match.

 

Here is the fit on the pin shown, by tradtional method of "bluing", in this case wth a marker pen:

https://flic.kr/p/om6VrW

 

The most difficult part of this was not being able to measure the diameter of the recess in the fork, because there is no access and its too shallow.

 

The hole is smaller than the 25/64in or 9.9mm usually used for 7/16in threads.

I believe this was drilled with 9.8mm which is the BSI alternative size.

Edited July 16, 2014 by AlanT

[RogerH]

Hi Alan,

I shyed away from a tapered bush as getting it to fit tight and stay seated would be interesting.

A parallel bush will hold the pin nose even if it migrates upwards.

 

Roger

[Tony Millward]

I agree with Rodger, a parallel pin is actually a much stronger proposition (with a cross pin for extra security).

 

Cheers

 

Tony

[AlanT]

The USA guys use parallel pins right through. They say these are "interference fit".

 

This is a long way to do a push-fit with a poor little thread to close it.

You could easily wind up with this stuck half-way and a fight to get it out.

 

I like to use a standard pin and didn't fancy another stress-riser in a thing I am hoping won't break.

[Lebro]

I am helping a fellow member to do an engine / gearbox swap in his TR3A at present. The gearbox / overdrive has been refurbished by Hardy engineering, & was all reassembled except for the clutch release parts.

Knowing about the problems with the taper pin, I was anxious to do the right things prior to re-fitting. On examination, with pin pushed as far into the shaft as possible there was only about ½mm protrusion, not enough to perform the parallel bush - parallel end to the pin trick.

I also found that the pin was not quite the same taper as the hole- it was catching at the fat end before the thin end. I mounted the pin in my lathe (using a old wheel nut to hold it -same thread 7/16" UNF) took a tiny ammount off the fat end using a fine file followed by emmery paper to polish. now fits better, & protrudes slightly more, but still not enough to machine a parallel section.

 

So we used the pin as normal without any further mods, but cross drilled, & inserted a 4mm mills pin as per photos earlier in this thread.

 

There was no evidence of the taper pin having broken before, & no cracks, but strange that it was not longer ??

 

Bob.