the motion

The driving wheels are fitted into axle boxes which slide up and down in horn blocks. These are suspended by leaf springs.

machined horn blocks and axles boxes for a model locomotive

The wheels are connected by the coupling rod which itself has pivots to allow the wheels to rise and fall independently. The rod is attached to crank pins fitted to the driving wheels. A large counterbalance weight has to be cast into the wheel to allow for the weight of the rotating rods.

coupling rods removed from a locomotive

The connecting rod is fitted to one of the crank pins and to the crosshead and converts the reciprocal motion of the piston to rotary.