Casters that retract under weight

[jmarkwolf]

I would like to deploy some caster mechanism that would cause shop stool casters to retract due to weight of the occupant.

In such a scenario the casters would retract down to the stationary feet with much less chance of the stool rolling out from under you.

With no occupant the casters need only support the weight of the stool, hence light duty casters. This would make a relatively heavy stool easy to move around when you're not sitting on it but become safe and stationary when you do apply your weight.

Any ideas?