[en] The hysteresis clutch drive units were developed by Sperry from the original idea of the Dragon Project. They are installed in each of the five drives of the Charge Machine for protection of the mechanisms. Because of the environment in the machine, namely high purity helium, a type of clutch was required where there was no rubbing contact between the driving and driven members. The helium atmosphere also made the prediction of the load torques in the drives uncertain with time and so the pull out torque of each clutch needed to be adjustable. A further requirement was that the clutches should produce a signal when slipping. This was to be both a warning and, when used with the adjustable slip torque, a torque measuring facility. These and other requirements led to the choice of a hysteresis type clutch.