Why would a pedal spindle work loose?

[rogerzilla]

There's no need to unscrew them regularly. Just grease them when fitting and only remove them if the pedal bearings need attention. The recommended tightening torque is very high - 35 to 55 Nm.

 

[gbb]

One thing to watch for very very carefully when removing and installing pedal axles is to inspect the threads on the axles very carefully, look for any aluminium material from the cranks and make sure they are absolutely debris free when you install the axles back in. Any debris will rip into the crank threads damaging them irreparably. This could have been a slow process of light damage each time slowly damaging the threads each time .

 

[Ajax Bay]

but loosen if the bearings seize, so that you don't break an ankle (I have seen this happen).

Presumably this is a 'fixed' issue only, Ian?

 

[Ian H]

Presumably this is a 'fixed' issue only, Ian?

Strangely no. On one occasion the rider was I think climbing & came off on unscrewing the pedal. I think this was after maintenance, the bearings tightened themselves to locked.
The other occasion was an old friend, JJ, whose bike maintenance was notorious. We had just left Barcelona airport, heading inland. We'd refitted our pedals after the flight. I assume he neglected to tighten one of his.

 

[silva]

Another thing to inspect when pedals had play / worked loose, is hairline cracks around the cranks pedal eye.
Those will grow, allowing the eye to open a little bit, and in the end cause crank failure, which can end up in a cyclist bad story.

 

[Sillyoldman]

It appears counterintuitive but they are threaded to tighten, because the mechanism of this tightening is not one of friction but of precession. The spindle effectively rolls its way tighter, rather than being screwed out of the crank arm.

This is why I can’t work out how it worked loose.

 

[Sillyoldman]

Could this be the problem, easy to get distracted and not tighten them.

No they were last removed months ago

 

[Sillyoldman]

Yes, this is where the action of friction overcomes that of precession.

Bearings are beautifully smooth so it’s not that they have seized to cause this.

 

[Sillyoldman]

Another thing to inspect when pedals had play / worked loose, is hairline cracks around the cranks pedal eye.
Those will grow, allowing the eye to open a little bit, and in the end cause crank failure, which can end up in a cyclist bad story.

Yes I will have another close look tomorrow. I rode 40 miles today having tightened the pedal yesterday. No issues all good. Will ride again tomorrow. Cheers all.