Creaky crank

[underspray]

... all three of them ...

lolz true, but a 40% saving in weight!

 

[one-eyed_jim]

I knew I shouldn't have got into a taper argument!

All I know is if you torque a bolt up to the same setting with grease it produces a higher figure on a strain gauge, this also alters with the type of grease used.

Right, more tension (strain) for the same installation torque. Agreed. If a torque figure is given, it should specify greased or dry - and that figure will vary according to the lube you use. If you torque a lubed crank to dry spec you've probably overtightened it.

Also after previous experiance as a bike shop gimp I wouldn't grease a square taper bb axle after seeing the results.
The result being that the crank is pushed too far onto the taper,just like if you over toqued it dry.

But only if you over-torque it - i.e. fit it greased to dry spec. I guess the moral of the story is that tightening torque needs to take account of lubrication. I reckon I've seen more cranks die from being ridden loose than I've seen split from over-tightening in any case.

Also after reading Jobst Brandts article he appears to neither confirm the use of grease or not.

I don't think he sees it as a big deal, but he does say that greasing can't cause crank failure in itself, and dry installation can cause galling and reaming of the bore. I think it's pretty clear from the context that he favours grease.

I can tell you that in heavy industry taper fit items would not normally be greased, the stuff I work with is usually put together dry and removed with heat.

Why is that? Why not grease them and specify a lower installation torque? Straight question.

You maybe have different experiances.

My only mechanical experience is with bikes, but I've always greased crank tapers, and never had a problem. That's why I wonder about all these crank failure horror stories. It can't just be dumb luck on my part.

 

[one-eyed_jim]

Yes, but it's the use of grease during the installation that causes the damage. It allows the crank to be over tightened which damages the taper faces.

That's not what you said before. You said:

The interface relies on friction to work.

Just to be clear, I don't think the interface relies on friction to work. I think it relies mostly on the fact that a square shaft can't rotate in a square bore. It also relies on a big bolt to hold the male and female parts in engagement.

But if damage to the taper faces is the problem, why not install with grease, but specify a lower installation torque?

 

[terryhfs]

No, it's the same thing said differently.

Grease means that the taper can be tightened more than it would without.

The interface does rely on friction in pedalling mode. A square socket and a square plug is indeed a friction fit. If either part of the interface becomes damaged the parts can move independently of each other because the torque applied during pedalling is greater than the resistance offered by the friction of the interface.

 

[one-eyed_jim]

Grease means that the taper can be tightened more than it would without.

We agree about that. The correct torque for a greased interface is lower than for a dry one. That doesn't mean that grease equals crank death. Just go easy with the spanner.

The interface does rely on friction in pedalling mode. A square socket and a square plug is indeed a friction fit.

I wonder if we're at cross purposes here. A square shaft can't rotate in a square bore whether the bore is slippery or dry - that's because of the geometry of the parts. As long as the bolt holds the parts in contact, any motion can only be because of the elasticity of the crank. You need a press fit so the crank can be loaded without developing clearance and play at the interface.

If either part of the interface becomes damaged the parts can move independently of each other because the torque applied during pedalling is greater than the resistance offered by the friction of the interface.

I think this is where we differ. If the parts were sliding across one another, I can see friction being a factor. But they're held in compressive contact by the crank bolt. When you load the crank it's the gradient in compression from the leading edge to the trailing edge of the faces that transmits torque to the axle, not friction between the faces.