Pistons attached directly to the rotor

The pistons can be attached directly to the rotor, without any connecting rod. This requires a minimum clearance and some additional sealing means between stator and rotor (grey and red respectively in the figure). It should be noted that they continue to be double-acting pistons. It reduces the complexity and makes it a low-cost option that may well be the best option at least for compressors.

Three epicyclic gear sets

An additional epyciclic gear set can be build into the device that effectively duplicates the function of the original epyciclic gear set attached to the stator. This creates a very robust configuration with an even better balance of forces at the expense of more parts, though not more moving ones. This would be the preferred configuration in high load machines.

gears 1