The btRigidBody is the main class for rigid body objects.
btGearConstraint(btRigidBody &rbA, btRigidBody &rbB, const btVector3 &axisInA, const btVector3 &axisInB, btScalar ratio=1.f)
Implemented by Erwin Coumans. The idea for the constraint comes from Dimitris Papavasiliou.