Cotter pins: Why? Why not?

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Brucey
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Re: Cotter pins: Why? Why not?

Postby Brucey » 17 Jun 2020, 11:20pm

Carlton green wrote: If that’s the case then I’ve missed why there’s a preferred direction for cotters to be installed.....


it could be the case that the cantilever (twisting) loads on the LH crank (with its short BB bore) are worst and that these loads when combined with the torque loads do favour one orientation over another, and that the (less harsh) loading on the RH side is less sensitive to cotter pin orientation. But I'm basically speculating here.

Any tips on filing cotters?


since you can't file them lengthwise (not without clouting the threaded part on the end, anyway) they have to be filed diagonally or sideways. Since cotter pins are impossible to hold by hand without risk of rocking, the cotter pin should be held in a bench vice when filing. Setting the flat parallel to the top of the vice jaws helps keep the angle correct. Use a long pattern file and use a good technique which keeps the file flat without rocking, if you want the surface to stay flat.

If there is lots to come off an angle grinder is a lot quicker, but keeping the surface flat is more difficult.

cheers
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Carlton green
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Re: Cotter pins: Why? Why not?

Postby Carlton green » 18 Jun 2020, 9:03am

Thank you Brucey.

The points you make with regard to filing are well made and good workshop practice. I suspect that whilst imperfectly made axles and cranks won’t help a significant part of the issue with cotter pins is the degree of fitting skill(s) that can be needed to make things work correctly and reliably. Just bolting it all together with maybe a bit of filing here and there might not always be enough to secure good results.

For the cranks to be perfectly 180 degrees apart then (assuming perfectly made axles and cranks) the taper angle on both cotter pins must be the same as each other after filing. The practice - or should that be aim - of filing parallel to the vice jaws makes sense to me, so to control the taper angle care must be taken in positioning the pin correctly (at the needed angle) in the vice’s jaws. Of course you can just do that positioning by eye and you might get good enough results but personally I’d like more control of and repeatability in production of that taper angle. Perhaps I’m going OTT but where it’s practical I like to give my self the best possible chance of something working perfectly. Is there any way of working (with hand tools) that with some accuracy controls taper angle?

Brucey
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Re: Cotter pins: Why? Why not?

Postby Brucey » 18 Jun 2020, 11:12am

you could put a small spirit level on the cotter pin once it is in the vice, to ensure that it really is flat. However this isn't really necessary; a good eye will see when the flat isn't parallel to the vice jaws with sufficient accuracy. As I noted upthread, the flats on the spindle are often not parallel to one another anyway, and you may need to allow for this if you really want the cranks to be in line.

cheers
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531colin
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Re: Cotter pins: Why? Why not?

Postby 531colin » 18 Jun 2020, 11:59am

I would just add, to make sure the cotter pins are "well in" the crank, so that the full length of the flat seats properly on the axle flat, not just hanging on by the fingernails.
On my photo of the old self-extracting cotters, you can see the marks where the axle has scuffed the cotter on its way in (and out)

ian s
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Re: Cotter pins: Why? Why not?

Postby ian s » 19 Jun 2020, 10:22pm

Much the best way of getting cotter pins in tight, or out again, is a car ball joint splitter; they work an absolute treat

Brucey
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Re: Cotter pins: Why? Why not?

Postby Brucey » 19 Jun 2020, 10:47pm

ian s wrote:Much the best way of getting cotter pins in tight, or out again, is a car ball joint splitter; they work an absolute treat


any recommendations as to which model works best?

There are posh press tools for this job e.g.

Image

but they tend to be a bit spendy. Something as good but cheaper would be welcome.

BITD VAR made a kind of 'Medieval Nutcracker' (the mere appearance of which makes grown men's eyes water) to do this job....

Image

looks just as worrying in the flesh

Image

cheers
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531colin
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Re: Cotter pins: Why? Why not?

Postby 531colin » 20 Jun 2020, 1:45pm

Now that thing could be used to dimple chainstays, with a couple of formers!

ian s
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Re: Cotter pins: Why? Why not?

Postby ian s » 26 Jun 2020, 10:14pm


iandusud
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Re: Cotter pins: Why? Why not?

Postby iandusud » 27 Jun 2020, 12:45pm

ian s wrote:Much the best way of getting cotter pins in tight, or out again, is a car ball joint splitter; they work an absolute treat


I can certainly see how that could be used to good effect to remove a cotter pin but can't see how it would do as good a job as a hammer for fitting. I'm sure a sharp blow from a hammer would exert more force.

Brucey
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Re: Cotter pins: Why? Why not?

Postby Brucey » 27 Jun 2020, 12:57pm



I've got a couple of those splitters and I have never felt that tempted to use them on cotter pins; they only exert a straight push on the part at one jaw opening distance; this is OK on car/truck ball joints (which commonly have an M10 thread on them) but on a cotter pin (M6 threaded) if the push isn't perfectly straight then the M6 thread is easily damaged.

cheers
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David Cox
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Re: Cotter pins: Why? Why not?

Postby David Cox » 27 Jun 2020, 12:57pm

Oh if only this thread had been available 45 years ago !!!

iandusud
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Re: Cotter pins: Why? Why not?

Postby iandusud » 27 Jun 2020, 2:32pm

David Cox wrote:Oh if only this thread had been available 45 years ago !!!

:lol:

ian s
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Re: Cotter pins: Why? Why not?

Postby ian s » 27 Jun 2020, 9:26pm

Brucey - you leave the nut on the thread just proud of the thread, so that the splitter does not touch the end of the pin - standard engineering practice in such situations.
Iandusud - wind as much load on the puller as you dare, then give the bit of the splitter over the end of the cotter pin a tap with the hammer, applies more load than you would with hammer alone - either fitting or dismantling - again, standard engineering practice

What also helps is to put a slight chamfer on the relevant edge of the flats on the bottom bracket spindle, so that the edge doesn't cut into the side of the cotter pin

Brucey
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Re: Cotter pins: Why? Why not?

Postby Brucey » 28 Jun 2020, 2:35am

ian s wrote:Brucey - you leave the nut on the thread just proud of the thread, so that the splitter does not touch the end of the pin - standard engineering practice in such situations...


M6 threads commonly strip before cotter pins come out if loaded in this way. The best you can do is to load both the nut and the end of the cotter pin so the load is shared between the two. If the 'push' isn't straight (as will happen with a short-lever scissor action device) the threaded part of the cotter pin tends to collapse at the base of the thread, and this will happen at a lower load than normal.

Upthread the VAR tool is pictured; this allows an almost exact parallel push onto the cotter pin, whereas the scissor action of the pictured ball joint splitter will not.

I guess overall I have a ~50% success rate removing 1/4" threaded cotter pins, dropping to about 25% if the pin has been in for decades and the joint is still tight. The success rate is lower with modern (M6 threaded) cotter pins simply because they are not as strong where the load needs to be applied. This is where 'success' is removing the pin such that it can be re-used.

No surprise there; the core of an M6 thread is about 5mm diameter, which means that the thing you are pushing on when you are trying to get it out would need to be x3.6 stronger if it were to match the strength of the cotter pin when it is being fitted, loaded in the other direction.

I can no longer buy good cotter pins so it is even more important that I find a better way of extracting the things. My next plan is to try welding onto the other end of the cotter pin, then using a slide hammer or a screw jack to remove it. This should allow the pin to come out without damage to the threaded end. Not as good as getting it out without any damage at all, but far better than the usual mess.

cheers
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iandusud
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Re: Cotter pins: Why? Why not?

Postby iandusud » 28 Jun 2020, 8:04am

ian s wrote:Iandusud - wind as much load on the puller as you dare, then give the bit of the splitter over the end of the cotter pin a tap with the hammer, applies more load than you would with hammer alone - either fitting or dismantling - again, standard engineering practice

What also helps is to put a slight chamfer on the relevant edge of the flats on the bottom bracket spindle, so that the edge doesn't cut into the side of the cotter pin


Ah, that makes sense, although I'm not sure it is necessary for fitting as I've never had problems with cotter pins coming loose that I have tapped home with a hammer (and I have fitted many. Also note I say "tapped home" no bludgeoned to death!). I do like the suggestion of a slight chamfer on the bb spindle as I have seen a lot of cotter pins damaged in this way.