Dynamic Simulation with assemblies within other assemblies

I have a differential with on assembly of two bevel gears within the final assembly of the whole differential. I would like to create a simulation of it working but I cannot get the two bevel gears to rotate together. When I go to add a cone on cone contact jioint between them it does bnot recognmise that they are two seperate parts and wont let me individually select each of them.
4 Answers

Ive got them two gears to spin now and they rotate together properly but how can I get them to rotate at a constant velocity?