A pressurised air generating system (60) comprises a chassis (2). An air compressor assembly (3) is mounted on the chassis (2). A drive shaft (4) is drivably connected to the air compressor assembly (3). The drive shaft (4) has a drive input (5) for connection to a power take-off on a tractor vehicle. An associated mounting assembly (7) attaches to the chassis (2) for mounting the chassis (2) on the three-point linkage of the tractor vehicle with the drive input (5)positioned for connection to the power take-off (PTO) of the tractor vehicle. The drive shaft (4) has a drive input (5) at each end of the drive shaft (4). Each drive input (5) projects outwardly at a front end (9) and at a rear end (10) of the chassis (2) for connection to a PTO shaft on the tractor vehicle for mounting the system (60) either at a rear of the tractor vehicle, or at a front of the tractor vehicle.
