Left crank coming loose

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Annoying Twit
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Re: Left crank coming loose

Postby Annoying Twit » 5 Sep 2016, 7:33pm

fastpedaller wrote:Fingers crossed (that always seems a daft expression to me, and I've just used it!) everything will now be ok.


Thanks. The new crank has gone on and fits properly at 180 degrees from the other crank and seems nice and firm. Now I see what happens next. I didn't put any threadlock on the bolt. Should I? (I have some.)

It seems that every single bike in my household (four of them) all have 170mm cranks. All I need is for one of them to have a square taper and the 'wrong' crank will be a potentially useful spare to have on hand.

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meic
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Re: Left crank coming loose

Postby meic » 5 Sep 2016, 7:40pm

I would forget about the "square aligned" crank if I was you. I bought one once, years ago falling for its low price of one Euro, it sits on the side of a shelf to remind me not to make the same mistake again.
They are nowhere near as common as the diagonally aligned ones.
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Annoying Twit
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Re: Left crank coming loose

Postby Annoying Twit » 5 Sep 2016, 8:40pm

meic wrote:I would forget about the "square aligned" crank if I was you. I bought one once, years ago falling for its low price of one Euro, it sits on the side of a shelf to remind me not to make the same mistake again.
They are nowhere near as common as the diagonally aligned ones.


Really? Two shops in Leicester had them in stock. There are 240 'square taper' cranks on ebay, but only 24 'diamond taper' ones. They seem quite common.

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meic
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Re: Left crank coming loose

Postby meic » 5 Sep 2016, 8:54pm

That is because both types are called square taper cranks as they fit onto square taper axles, not some other type of axle.
If you look at the (any) pictures you will see that most are what you are calling diagonal tapers.
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Re: Left crank coming loose

Postby Annoying Twit » 5 Sep 2016, 9:11pm

meic wrote:That is because both types are called square taper cranks as they fit onto square taper axles, not some other type of axle.
If you look at the (any) pictures you will see that most are what you are calling diagonal tapers.


Ah, I see what you mean. Oh well, I've bought a crank I may not be able to use. In the parts box it goes, perhaps never to come out again.

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meic
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Re: Left crank coming loose

Postby meic » 5 Sep 2016, 9:13pm

That makes (at least) two of us. :lol:
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mjr
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Re: Left crank coming loose

Postby mjr » 6 Sep 2016, 1:28pm

Brucey wrote:FWIW my advice is to fit a new crank without grease on the taper, and to retorque the bolt after the first 50-100 miles.

Do you advise greasing the crank bolt thread?

I ask because I don't grease the tapers but I've just been rewarded for riding through a rainstorm by the left crank coming loose on the second bike this summer :mad: :mad: :mad: (and strangely, I felt the dreaded clunk on the ride after next after the rain, not the next ride after the rain and I think it may have been the same last time) and I'm wondering if the heavy rain manages to rinse out enough grease to allow the bolt to move and start the trouble. :mad:
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Brucey
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Re: Left crank coming loose

Postby Brucey » 6 Sep 2016, 1:42pm

I normally grease the bolt thread.

Cranks shouldn't work loose in the rain, no.

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NATURAL ANKLING
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Re: Left crank coming loose

Postby NATURAL ANKLING » 6 Sep 2016, 1:42pm

Hi,
If you grease a bolt thread it is more likely to undo than if you don't.
As some 80% of torque is friction in thread.

If you have trouble with cranks coming lose it probably worth checking the bolt periodically.
I would say that if tapers are lubed you need to recheck more often as its likely that the taper has seated, bolt tension goes loose then the crank moves?

Shimano recommend lubing or not depending on the BB spindle, IIRC you only lube the taper if the BB taper is backed with a stepped boss?

Edied-
Thats a general term for all threads, if manual say grease then grease.

In the case of BB's the bolt has a large flange which gives a large area for to bite on, I wipe the thread on these bolts but don't normally lube.
They (thread) can pick up with corrosion, normally when bike is left unused outside.
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Re: Left crank coming loose

Postby Brucey » 6 Sep 2016, 1:52pm

IME if you don't grease the thread, the bolt will likely never be tight enough (at the specified torque) the joint doesn't see the intended load, and the crank can more easily work loose in service.

BTW the left crank is more likely to work loose because that joint sees both bending loads and torque loads at the same time. By contrast the RH joint more or less sees one or the other, not both at the same time.

Loosening is (IMV) mostly caused because the taper fit settles, rather than because the bolt is backing out.

If you are worried about the bolt backing out, you can use threadlock on it. However I would not advise using this until you are certain that the joint has settled. Once you use threadlock, you cannot easily check that the bolt is still snug.

cheers
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mjr
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Re: Left crank coming loose

Postby mjr » 6 Sep 2016, 3:14pm

So other than not greasing the taper, do you feel http://sheldonbrown.com/tooltips/cotter ... ml#sockets is accurate advice?

I'm struggling to understand why I'm suffering this so often now (or at least 3 bikes in 18 months feels too often to me - I don't remember suffering this much in the past, but maybe when I rode one bike all the time and was generally less careful with maintenance, the cranks corroded on hard as they were certainly a devil to remove ;-) ). I think I may go buy a new big torque wrench - I feel it's quite likely that mine has gone out of calibration and it's a cheap one that can't be reset easily.
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Re: Left crank coming loose

Postby Brucey » 6 Sep 2016, 7:26pm

there is a lot of nonsense talked about square tapers. People do 'experiments' that are poor and then draw poor conclusions from them, too. Much of SB says is correct but there is more to it than that.

BB spindles vary more than people think. They vary in angle (slightly), they vary in how big the end of the spindle is too, but more importantly they also vary in how wide the flat parts are and whether the flats are the same width or if the width varies along the length. This is really important if you are fitting new parts together that don't quite match (even if they ought to) or if you are fitting a new (slightly different) BB into a used crank.

So when JH offers a new SKF BB up to a used 1990s campag crank, and it feels like it is going to wind on for ever, actually what is happening is probably that the taper is making itself a new seat in the crank, because the SKF axle is almost certainly different from the campag one that was used previously/for which the crank was machined.

Until the axle seats fully, the loaded area is a tiny fraction of what it ought to be and the taper will be resizing itself to suit. It is very easy to test how much of the interface is really in contact, and anything that anyone says about the quality of the fit is rendered more or less meaningless if they have not done this simple check, e.g. by blueing up both parts and checking that it all gets scuffed in the same way.

Campag have always said not to grease the taper and when you fit a campag crank onto a campag spindle, the fit is bloody good; often you can push the cranks on by hand and they don't want to come off. When you wind the bolt up they hardly move. They certainly don't 'wind on forever' if you use the right parts.

Most other cranks appear not to be so well made as that; the result is that if you assemble dry and don't use much torque, the taper is not a good fit and there will usually be some settling in service subsequently.

You can circumvent much of this settling by using more torque and a little grease on the taper. This allows the axle to re-size the crank in one go, pretty much, even if it isn't a perfect fit. This is more or less what Shimano recommend, and if you want an assembly that is going to be OK most of the time and ought not come loose too much of the time (as per a bike shop's requirements) then maybe this isn't such a bad scheme. However I think you can do better than that, plus Shimano (and pretty much everyone else who says the same thing) all make the same mistake in that

a) they don't say how much grease to use and
b) they don't say what kind of grease to use
c) they don't make any discrimination between fitting new parts and used parts that either do or don't mate perfectly.

If you use lots of grease, or the wrong kind of grease, it is quite likely that you will end up with a fit that will (if you pedal hard) move around for ever, until the left crank just falls off.

With light greasing, using grease without solid lubricants, without EP additives, without the wrong soap chemistry, etc etc etc once the joint has been assembled for a few months, there is usually pretty much no evidence that it was ever greased in the first place, and there is no real need to regrease the taper when you subsequently reassemble the parts.

I've seen a few ST assemblies where there was no sign of in-service movement in the joint, but the vast majority show clear signs of fretting. Once the fretting starts, this starts cold welding aluminium to the taper faces, until the crank is quite well bonded to the spindle. When the crank is removed, the aluminium shears, leaving a little of the fretted material bonded to the steel BB spindle. You can see these marks on most disassembled ST spindles (where the joint has been good); they are usually lozenge shaped areas of light colour on the ST spindle faces. You can tell it is deposited aluminum, because it is soft enough to scratch; you can make/see the scratches under a magnifying glass.

Obviously the wrong kind of grease will inhibit/delay the fretting process and if the crank moves enough in the meantime, it'll work loose. If you repeatedly retorque a crankbolt on a ST which is greased and it is still moving, it may work itself on the taper for ever. Whilst it is moving around (i.e. not cold welded yet) it is a bad idea to use very high loads on the assembly. By contrast in a dry-fit ST joint, once it has cold-welded, retorquing the bolt ought not do any harm.

SO you can see that the right thing to do is perhaps different between whether the parts fit well to start with and whether you are happy to check and retorque the crank bolt.

[edit; I forgot to mention before; I have seen quite a few ST assemblies where the taper has been lubed and (for whatever reason) the crank has never stopped moving (perhaps only occasionally) in service, or has evidently moved for a long time before settling. In this instance the flats on the spindle generally don't show the usual fretting/galling (in which the steel becomes coated in Aluminium) at all, or show it in unusual places. What is usual in this case is that the spindle itself can suffer plucking wear; if you catch a reflection in a spindle flat, you may see a depression/worn area, roughly in the same place where cold welding ought normally occur.

I think that the (fretting/galling) mechanisms of cold welding are at work, but the resultant bond never becomes strong enough to withstand the service loads, which leads to wear instead of cold-welding. I think that the load required to cause sliding movement between the abutting faces in the ST joint (which breaks the cold welds and/or causes plucking wear in the spindle) can be lower than in a dry joint, because the contribution of friction to the joint strength is reduced by the presence of the lubricant. If a rider is strong enough, the joint can move about like that under load (when, say starting off or riding out of the saddle) for ever, which can of course eventually cause the joint to fail.

Also, I think that JH is quite wrong about the effect of flanged bolts vs bolts with washers; with the latter the usual thing is that the bolt head turns against the washer; this is very easily demonstrated by disassembling and examining a bolt/washer that has a black finish to it. If a flanged bolt is used with no washer instead, the torque required is likely to increase considerably, and will anyway vary depending on whether the flange is chrome-plated or has some other kind of finish on it, as well as if it is lubricated or not.

FWIW I suspect that if you are fitting an unknown combination of parts together, or ones where the fit is suspect, you could do worse than to

a) assemble with grease, torque to the shimano spec, then
b) disassemble, clean, check the quality of the fit, then
c) reassemble dry, torqueing to the campag spec.
d) check the bolts are snug after 50-100 miles

If in b) the fit isn't good enough, repeat a) using more torque than before.]


cheers
Last edited by Brucey on 7 Sep 2016, 12:20pm, edited 1 time in total.
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mjr
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Re: Left crank coming loose

Postby mjr » 6 Sep 2016, 8:28pm

Thanks so much for that. So just to deal with what I think is a fairly common case (and mine) of used manufacturer-chosen cranks and a new replacement Shimano cartidge bb like UN55 and IMO make this discussion too good to lose":-
Is a simple vegetable thin grease lke Pure Bike Grease a suitable grease?
Would one of the spray oils that are too thin for chains be any good at this, assuming avoiding PTFE ones like GT85?
By blueing the spindle and crank to help check the fit, do you mean blueing it, fitting it and then extracting it again to look or am I missing a trick?

New torque wrench ordered. It may be interesting to see how far out my click wrench reads...
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Re: Left crank coming loose

Postby Brucey » 6 Sep 2016, 9:32pm

mjr wrote:Thanks so much for that. So just to deal with what I think is a fairly common case (and mine) of used manufacturer-chosen cranks and a new replacement Shimano cartidge bb like UN55 and IMO make this discussion too good to lose":-
Is a simple vegetable thin grease lke Pure Bike Grease a suitable grease?
Would one of the spray oils that are too thin for chains be any good at this, assuming avoiding PTFE ones like GT85?

honestly, I don't know what to suggest for the best here; apparently simple greases/lubes often have things in them that you wouldn't expect, and no-one seems to publish all the ingredients in them. A thin lube (that washes off easily later on) may well be a good choice.

By blueing the spindle and crank to help check the fit, do you mean blueing it, fitting it and then extracting it again to look or am I missing a trick?


yes, sorry, I didn't make that clear, did I...

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NATURAL ANKLING
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Re: Left crank coming loose

Postby NATURAL ANKLING » 6 Sep 2016, 9:48pm

Hi,
Problem is that torque wrenches don't discriminate between dry and greased.
Suggest the bolt is not that tight if you use a torque wrench / difficult to operate if the part is also free to move held with other hand.

Probably a good idea to examine the BB taper for deposits and damage, I normally wipe the taper then take a look and use your fingers (Clean hands) on the taper for nasty's.
Clean out the thread in BB of mud etc, rag stuffed in should be clean after.
Clean out the socket in the crank.
Clean all of the bolt, it should run free in the mating thread with fingers, replace as necessary.
On BB bolts the shoulder sees most of the friction whilst torqueing.
Place the crank arm on the taper and hit it with the palm of hand.
This should be enough that thumping the back of crank with hand till it hurts will just budge it off again.
A minute smidgen of lube on the flats, I probably rub mine with a not so clean rag.
There is probably some residue of lube in the threaded hole.
Torque up bolt ( I normally use a longish wrench, not as longer than the crank arm) I normally use one hand on crank and one on the wrench, bringing hands together across your chest, there will be a point where the thing feels like its stopped moving creaking even as the wrench (hex key) bends and springs slightly.

To recheck the bolt after 50 miles or so apply same force, it may well move slightly or not at all.

I also pack the bolt head if no rubber seal, and rear face of crank with some butyl rubber which has aged some so its rather like plasticine.
Stops the water getting in.
Don't use a wrench longer than crank.
All for an average 11 - 12 stone man.
Keep hands free from grease / muck, I always use nitrile fabric backed gloves and keep rag for wiping hands tucked in to your front / hanging from your pocket, any other use its a rag on the bench.

DONT wipe the mess with the same rag that wipes your hands, you end up with CRAP all over the controls and bars.

Never had a crank come loose, but seen many MTB ones where the shaft protrudes through crank :?

Good idea to remove cranks periodically, though I would say that unless its stored out side / heavy off road use, they don't seize on.

From experience in motors and lathe tapers on tailstock etc, a bur or lube at all will stop the taper from seating properly, allowing it to turn which wrecks both parts, keys on tapered shafts are for location not driving force.
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