If I understand correctly, you want to model a joint with some clearance, is that right? The trick is to measure the position and velocity of the joint, and then use a collision force law, which provides a restoring stifness (and damping) when the penetration (position) goes outside a defined range, e.g:
F = 0 if abs(x) <= x_0, k*(x-x0)+b*x_dot when abs(x) > x_0.
similar to what's in the "Backlash Actuation" subsystem of mech_gears_backlash or in the "Contact Force" subsystem of mech_bouncing_ball.
HTH,
Arnaud
