We're talking about different things. I'm not talking about the belts stretching over time (since as you say that just doesn't affect us). I'm saying when you apply a force to a belt it will stretch then spring back when you remove the force. So I agreed that when driving a ballscrew with a belt there's no problem. However I thought John S was asking about using the belt instead of a ballscrew? I.e. long open belts on either side of the gantry to push it along...