Abstract: A piston (3) for impulsively delivering mechanical energy is connected with a crank shaft (5) having a pin (5a) in co-action with a first coupling member (2a, 2b) on a rotatable member (2). The pin (5a) forms a second coupling member, which accompanies the movement of the rotatable member (2) during one half revolution thereof, whereupon an energy-absorbing means, for example, a spring (4) arranged around the piston (3) is compressed. When the rotatable member (2) has reached a limit position, the coupling engagement between the coupling members is instanteously broken, whereupon the energy stored in the spring or the like is transferred to the piston, which executes an impulsive percussion movement. The cycle is repeated when the coupling member (2a, 2b) of the rotatable member is again brought into engagement with the pin (5a).