a power drive unit for cargo handling systems comprising a frame arranged to be mounted in use for raising and lowering movement relative to a supporting structure , a drive motor carried by the frame and having an output shaft , a drive roller assembly carried by said frame for engagement in use with a unit load device or the like to be moved by the pdu , a first gear train transmitting drive from said motor output shaft to a drive roller of said drive roller assembly , a rotatable lifting cam assembly carried by said frame and driven in use relative to said frame to lift and lower the frame on said mounting , a second gear train for transmitting drive from said motor output shaft to said lifting cam assembly to operate said lifting cam assembly , clutch means operable to connect and disconnect said motor output shaft to and from said second gear train , a torque limiting device limiting the torque transmitted through said clutch , when said clutch is engaged , to said lifting cam assembly , and a brake mechanism between said torque limiting device and said lifting cam assembly for braking said lifting cam assembly against rotation relative to said frame .