INJECTION DEVICE AND DRIVE COUPLING

An injection device, or a drive coupling suitable for such a device, including a housing for containing a syringe 84 with a needle 100 and a dispensing piston 92 in a bore 82, a drive element (66 fig 3) to move the piston to expel the contents of the syringe and a drive coupling to connect the drive to the piston wherein the drive coupling is compressible in length so that once the dispensing piston has made the full extent of its travel the coupling gradually compresses, preferably allowing a delatch mechanism (72 fig 3) including spring 98 to retract the syringe and needle. The drive coupling may include chamber 80 between two movable walls 86, 88, the first of which 86 is driven by the drive element and the second of which moves the dispensing piston 92 and one or more orifices 94 for bleeding flowable material from the chamber. The gradual compression may begin before the dispensing piston reaches the end of its travel. The drive coupling may alternatively include friction surfaces (fig 7), rack and pinion gears (figs 8 to 15) or relatively rotatable members including a damper spinning against air resistance (figs 16 to 21). The injection device may be an automatic syringe.