Power wrench

#The invention refers to a power wrench with a drive portion including a hydraulic cylinder, and a driven portion, the driven portion comprising a output shaft rotatably supported in a housing and rotated at intervals by a ratchet lever.

#Hydraulic power wrenches are known that work based on the ratchet lever principle. When appropriately high hydraulic pressures are applied, such power wrenches can be used for very high torques. After a stroke of the hydraulic piston/cylinder unit, the output shaft tends to rotate backward by a small angle. This is due to the fact that the screw structure partly relaxes after each working stroke. To prevent such a relaxation, it is already known to have a blocking member engage the output shaft, the blocking member engaging an outer toothing of the output shaft, thereby preventing backward rotation. This entails the disadvantage of a safety risk caused by unreliable retaining systems. When the hydraulic unit that supplies pressure to the power screw driver is stopped, or in the event of a power breakdown or an incomplete stroke length, the apparatuses may come clear of the screw to be turned during the working process. This means a risk of accidents. Apparatuses with retaining latch systems may become twisted after the end moment has been reached and have to be detached tediously from the object to be screwed. The power wrench has to be brought up again to the maximum torque set and may lead to torque inaccuracies. After every stroke, the full clamping force is again applied to the apparatus and the screw bolt connection. This gives rise to high loads in the system and to constant bending stresses at the screw connection.

#It is an object of the invention to provide a power wrench that allows for a uniform tightening of screws without any safety risks.

#FIG. 1 is a side elevational view of a first embodiment of the power wrench.

#FIG. 2 is a front view in the direction of the arrow II in FIG. 1.

#FIG. 3 is a sectional view along line III-III in FIG. 1.

#The power wrench of the embodiment illustrated in FIGS. 1-3 has a housing 1 of a substantially L-shaped design and accommodating a drive portion 11 in one leg and a driven portion 12 in the other leg. The drive portion includes a hydraulic piston/cylinder unit (not illustrated) with a reciprocating piston rod. The piston rod drives a ratchet lever. The driven portion 12 includes a output shaft 14 supported in the housing, the output shaft being a hollow shaft (FIG. 3) with an internal hexagon profile 15. Situated between two sidewalls 10A, 10B of the housing 10 is the ratchet lever 16 coupled with an outer toothing of the output shaft 14 via a toothing 17. In a working stroke, the ratchet lever 16 takes the output shaft 14 along in one rotational direction, whereas it slides back empty during the return stroke. In this manner, the output shaft 14 is rotated at intervals.

#The friction brake 20 has a friction shoe 21 arranged radially with respect to the output shaft 14 and pressing against a friction surface 22 at the circumference of the output shaft. A tensioning device 23 has a tensioning wedge 24 provided with a threaded bore into which a tensioning screw 25 is threaded that is retained in the housing wall 10B. By tightening the tensioning screw 25, the tensioning wedge 24 is pulled outward (to the right in FIG. 3), whereby it presses against a rear wedge face of the friction shoe 21 and urges the same forward towards the friction surface 22.

#In the present instance, only a single friction shoe 21 is provided, however, a plurality of friction shoes can be provided that preferably are distributed uniformly along the circumference of the output shaft 14. The continuously operative friction brake 20 permanently applies a constant braking force on the output shaft while the output shaft rotates, so that the shaft is prevented from rotating backward.