Abstract: A main pump (rotary vane pump 1) and a secondary pump (radial piston pump 2) are driven by way of a common shaft (3), a cam ring (30) being used to drive the secondary pump. The cam ring is coupled to the shaft (3) by way of a shear pin (33). If the auxiliary pump locks, the shear pin fractures and the cam ring (30) can rotate relative to the shaft (3). In that case, the fracture portions of the shear pin (33) are driven into their respective bores (34, 35), thereby avoiding excessive friction and overheating. Therefore, the main pump can continue to operate in spite of the failure of the auxiliary pump.