Crank bolts coming loose

[GoLefty!!]

If the tapers are not greased before torquing, the torque wrench reading is meaningless and the actual press fit is random.

 

 

Oi, greasing of square tapers should not be allowed with aluminium parts on steel axles. With a layer of grease on the axle, the ham fisted muppet using the wrench will overtighten the bolt and force the crank arm too far up the taper. This leads to excessive stress in the head of the crank arm where it will eventually crack. In engineers school we had a rule;

grease splines, not tapers. This worked well and we never had problems removing gearwheels from machinary.

 

Cottered cranks or Quartered crenks as they are known on the cape flats are absolute rubbish. I was happy to see the rise of the Cotterless crank or Quarterless crenk.

[Johan Bornman]

Engineering school does indeed tell you to keep tapers dry. No-one ever questions the prof or seem to understand why. You?ll find reference books citing both sides ? grease and dry. Very few of them explain why. The profs also don?t explain why. I think they just don?t know why or never bothered to question this themselves.  Some crumbs of wisdom are taken on face value for decades on end. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

Here is my explanation.

 

Always remember that steel and aluminium are elastic. When used in combination, the aluminium will strain more than the steel.

 

On completion of installation, a square-taper crank is in equilibrium. The forces that want to push the crank down and off the taper are in equilibrium with the bolt forcing it up the taper.

 

During installation, as the crank moves up the taper, there is considerable friction. This friction prevents all the bolt?s pressure to push the crank as far it would have if it were only to overcome pressfit forces.

After installation, when the crank is used, it starts to squirm its way up the taper away from the crank bolt. This is because the frictional forces still present in there are now overcome by the deformation of the press-fit area from your pedaling forces.

As the crank walks up the axle, the tension in the bolt decreases up to the point where it matches the press-fit forces and the crank stops moving up.

If there is too much friction during installation the initial imbalance is higher and the crank will walk up further than were there less friction to start off with.  This friction is determined by the surface roughness of the two interfaces and the presence or not of lubricant. The crank will now release tension on the bolt and the bolt will become loose enough to fall off. The crank will almost certainly now come off.

By greasing the tapers, you eliminate much of the friction and help the crank find its final resting place quicker, without affecting bolt tightness in the process.

Shimano advocates greasing (and in fact, pre-greases its axles) and Campagnolo vehemently forbids it. Neither of them gives an explanation.

As for the theory that you?ll crack the crank if you grease it too much. Well, in the Yellow Saddle Labs we have a standing experiment.

Apparatus:

Square-taper Shimano 600 crank.

Square-taper axle fixed in a pipe vice.

New crank bolt.

15mm socket with very long lever.

 

Method: Grease the taper liberally. Fit the crank and turn the bolt slowly for the crank to move up the taper. Keep on turning until the bolt breaks.

 

Result: Never once does the crank break, always the bolt.

 

Finding: A greased taper does not break a crank. Assumption: Broken cranks are caused by ungreased tapers that move up the crank by themselves, chased by someone who continuously torques the loosening bolt to spec.

 

Edit: PS -  It is very, very hard to get metal parts free of lubricant. That last molecule of grease on there doesn't come off with wiping or a single applicaton of solvent. My guess is that all tapers that have been in contact with grease (even greasy fingers) are actually greased.

Johan Bornman2008-10-21 01:56:09

[GoLefty!!]

This true, often there are dogma's that are clinged to without question.

[droo]

This true' date=' often there are dogma's that are clinged to without question.[/quote']

 

 

 

Like nasty wobbly quartered crenks

[GoLefty!!]

is ya, you know mos wut i mien

[Velouria]

 

 

Cotter cranks on the other hand are rubbish. The cotter/axle interface is too small to withstand the rigours of athletic or even sedentary pedalling and these quickly fail. Further' date=' there is no elegant way of removing them other than using a sledge hammer. The world is a better place without cottered cranks.

 

For those of you born after the goold old days, this is a cotter crank. Wave it goodbye.

 

http://www.bike-classique.com/cotter.jpg

[/quote']

 

Even as an eight year old I went through 2 crank/BB sets in a year. I hated the damn things. I suspect some of the damage caused was because of the sledge hammer approach, but that was the only way. We used to have a little tray of spare cotter pins - they got replaced quite frequently

 

[Speed Devil]

Besides the cotterpins which were inevitably stripped while tightening, the BB was a nightmare to keep clean and free from water/dust/sand etc.

I often ground bearings to a pulp in my BB/cotter crank setup.

I think sealed BB was as much of a welcome revolution as the cotterless crank.