How can I prevent my shoulder bolt axle from causing my wheels to lock up?

[amfmnsam]

Coaching grade school track, I have to carry around several starting blocks. Last year, to store for the next season, I made a rude carrier from 3/4 plywood that held 5 blocks, stacked on top of each other. I put wheels on the back of the cart, using shoulder bolts, which the kind man at Home Depot helped me select. I drilled through the ends of the bolts and locked the nut on with a cotter pin, feeling clever.

In use, one of the wheels locks up, as the rotation tightens the nut on the bolt. My first fix thought is to not use a nut, take up the space with washers and let the cotter pin hold the wheel on the cart. That seems like a lot of pressure for the cotter pin.

What am I missing?

Thanks

 

[Danger]

Welcome to PF, Amfmnsam.
I have no idea what a 'shoulder bolt' is. Perhaps what we call a 'carriage bolt' or 'shank bolt'? That's a bolt with a long unthreaded section beneath the head, then a square segment, and then a rounded head with no screwdriver slots.
The obvious solution would be to use left-hand threads on the side that's giving you trouble, but if might not be easy to find them. Elsewise, you could use lock nuts or a chemical locker to prevent rotation of the nut relative to the bolt.

 

[pantaz]

In use, one of the wheels locks up, as the rotation tightens the nut on the bolt.

To properly use a shoulder bolt (aka, stripper bolt) as an axle, the hole for the threaded portion must be smaller than the shank diameter. That way, the force of tightening will be exerted only on the shoulder.

If I get a chance, I'll make a quick sketch to demonstrate.

 

[rocketboy]

If the threaded section of the bolt is long enough, could you put two nuts on it? I guess it depends how it is set up because what I'm thinking is you screw one nut on all the way such that it can't be screwed on any further so that it now serves as a barrier. On one side you have the wheel, and on the other is the frame and nut holding it. Re-reading this perhaps it makes more sense in my head than it does here haha.

Alternatively, could you drill another hole on the other side of the nut to put another cotter pin in? Now you have one on either side of the nut and it can't rotate.