Slipping rear axle
 

I have a shimano nexus 3 speed hub and it keep slipping. No matter how much I tighten the bolt, it will slip at some point. It only happens when I stand up and put a lot of force on the pedals.

Is there something I can do to stop the axle from slipping? I would love to be able to climb hills on 2nd while standing and not having to put it in 1st and slowboat it up.

What type of metal at the frame, steel or alu? Are the dropouts heavily indented?
What kind of nut? The current Shimano nut is very good.You can get much better purchase with a full sized track nut with built-in washer.
How do you tighten the nut, with a small multi-tool, adjustable wrench? I prefer a full sized ring wrench.

The frame is chromoly steel and the frame looks mostly untouched at the axle outside of skid marks from the axle slipping. I use a standard wrench and tighten it to the point the bike shop needs an extender on their wrench(which is already longer than mine) to get the wheel off.

As for the nut, it's the same one that shipped with the bike and is the standard one that ships with the inter-3.

What type of washer is between the frame & nut ? You could try a serrated washer for better grip.

Nexus three-speed? Sounds like your Y33Z20200 non-turn washer is either MIA or doesn't interface tightly with the dropout.

With an IGH like this, the gears will multiply the reverse torque on the axle and if the anti-rotation washer doesn't do it's job, you'll never be able to hold the axle merely by tightening the nuts.

The drive side dose not have that anti-rotation washer, never did. I found that odd as my inter-7 had one on both sides, but the bike shop said the inter-3 did not need it.

Sometimes oiling the threads and between nut and washer gives improved tightening.

It'd be interesting to get a picture of the dropouts and see what things might affect the slippage. A few things I look for:

a) is the axle/nut diameter appropriate for the dropout (i.e. does it bite enough metal?)
b) Do both the inside lock-nut and the outside locknut that sandwiches the dropout have sufficient serrations to grip?
c) is there some hub/axle geometry interference that prevents the outer locknut from really cinching down on the inner locknut to make the ultimately tight sandwich?
d) could some other form of chain tensioner/tug help prevent slippage?

Last resort of course is a chain tug of some sort, which only works with horizontal, back facing dropouts. But I'm wondering if any of the first 3 things listed might be the issue.