Abstract:
The invention relates to a hydraulic power wrench comprising a ratchet lever ( 16 ) driven by a hydraulic cylinder. According to the invention, said power wrench is provided with a friction brake ( 20 ) which operates continuously between the housing ( 10 ) and the output shaft ( 14 ). In this way, reversed rotations of the output shaft following the individual working strokes are avoided.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    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. 
         [0002]    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. 
       SUMMARY OF THE INVENTION 
       [0003]    It is an object of the invention to provide a power wrench that allows for a uniform tightening of screws without any safety risks. 
         [0004]    The power wrench of the invention is defined in claim  1 . It comprises a continuously operative friction brake causing a friction force between the drive shaft and the housing. This friction force is generated permanently and is overcome by the hydraulic drive during the working stroke. The friction brake causes a uniform tightening of the screw. It allows a fitter to control several screwing operations performed with power wrenches at the same time. The power wrench is reliably positioned and is held securely in every working position, regardless of the phase of the respective stroke. No stepped latching takes place. The retaining system is active all the time. It is not necessary to switch the hydraulic system on and off. Neither do any additional bending moments act on the screw connection as is true for systems with a locking latch. The screwing tool will not jam after the last stroke. Therefore, time-consuming loosening work is eliminated. A secure removal of the apparatus is guaranteed. Since no latching and unlatching occurs, the apparatus is secured in any optional working position. 
         [0005]    The friction brake may comprise at least one friction shoe arranged substantially radially with respect to the output shaft and pressing against a circumferential friction surface. Preferably, the friction surface is provided at the output shaft or a component connected therewith. However, it is also possible to provide the friction shoe on the output shaft and to make it act on a friction surface of the housing. A plurality of circumferentially distributed friction shoes may be provided. Preferably, each friction shoe is urged against the drive shaft by means of an adjustable tensioning device. In this manner, the friction brake can be readjusted or adjusted. The tensioning device preferably comprises a tensioning wedge. 
         [0006]    In a development of the invention, it is provided for an axially operating friction brake that the friction brake has a flange of the drive shaft that presses against a friction member stationarily provided at the housing. The friction force acting on the flange brakes the output shaft. 
         [0007]    Preferably, a spring is provided that presses the output shaft towards the friction member. The friction member can be connected with a swivel ring in a manner secured against rotation, the ring pressing axially against the spring via the axial bearing. 
         [0008]    According to another aspect of the invention, the housing of a power wrench of the type mentioned above is provided with a frictionally retained rotatable index ring arranged concentrically with the drive shaft, the index ring having a centric window through which a mark on the output shaft is visible. Such an index ring may be used to indicate the rotational angle of the output shaft. The index ring is first set to the respective position of the mark by manual rotation, so as to memorize the initial position. During the screwing process, the mark will wander relative to the index ring, so that the respective screwing angle van easily be read. 
         [0009]    The following is a detailed description of embodiments of the invention made with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a side elevational view of a first embodiment of the power wrench. 
           [0011]      FIG. 2  is a front view in the direction of the arrow II in  FIG. 1 . 
           [0012]      FIG. 3  is a sectional view along line III-III in  FIG. 1 . 
           [0013]      FIG. 4  is a side elevational view of a second embodiment. 
           [0014]      FIG. 5  is a front view of the power wrench in the direction of the arrow V in  FIG. 4 . 
           [0015]      FIG. 6  is a sectional view along line VI-VI in  FIG. 4 . 
           [0016]      FIG. 7  is an enlarged illustration of a section through the friction brake and through the rotational angle display of the second embodiment. 
           [0017]      FIG. 8  is a side elevational view of a third embodiment. 
           [0018]      FIG. 9  is a sectional view along line IX-IX in  FIG. 8 . 
           [0019]      FIG. 10  is an enlarged illustration of the detail X of  FIG. 9 . 
           [0020]      FIG. 11  illustrates a section through the tensioning device of the friction brake along the line XI-XI in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    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  10 A,  10 B 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. 
         [0022]    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  10 B. 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 . 
         [0023]    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. 
         [0024]    In the embodiment illustrated in  FIGS. 5-7 , the housing  10  also includes a drive portion  11  and a driven portion  12 . The cylindrical drive portion  11  is provided with a spline  30  onto which a supporting foot (not illustrated) may be set which is placed against a stationary counter bearing to prevent a rotation of the housing  10  while screwing. 
         [0025]    Again, the housing  10  includes two parallel housing walls  10   a ,  10   b  with holes in which the output shaft  14  is supported. In this case, the output shaft  14  is solid and protrudes from one side of the housing  10 , where the output shaft  14  is provided with a plug-on square  26  on which a socket wrench may be set. A sleeve  32  sits on a spline  31  of the output shaft  14  in a manner secured against rotation, the sleeve additionally being provided with an outer toothing  33 . The toothing of a tappet member  34  meshes with this outer toothing, said tappet member being situated within the housing  10  and meshing with the ratchet lever  16 . The reciprocating ratchet lever  16  drives the sleeve  32  and the output shaft  14  via the tappet member  34 , the sleeve and the output shaft being taken along only in one direction of rotation. 
         [0026]    A slide  35 , whose front end wall  36  forms a push-button, slides in an axial recess of the output shaft  14 . A spring  37  presses the slide  35  outward. The slide has a circumferential groove  38  that, with the slide pushed in, partly receives a ball  39  movable in an axial bore of the output shaft. Thereby, an intermediate sleeve  40  provided on a shank of the output shaft  14  is uncoupled from the output shaft so that the intermediate sleeve can be pulled off when the push-button  36  is pushed in. 
         [0027]    As illustrated in  FIG. 7 , the intermediate sleeve  40 , connected with the output shaft  14  in a manner secured against rotation, has a radial flange  41  at the outer end. The same presses against a friction surface  42  of an annular friction member  43  connected with the housing  10 . The friction surface  42  may also be formed by a friction lining. Thus, the flange  41  forms a friction brake  20  together with the friction surface  42 . The brake also comprises a plate spring  49  pressing the intermediate bushing  40  against the friction surface  42 . 
         [0028]    A swivel ring  45  sits on a thread  44  of the friction member  43 , the ring being secured by means of a safety pin  46 . The swivel ring  45  embraces a needle bearing  47  axially supported at the swivel ring  45  and, on the opposite side, pressing a disk  48  against the plate spring  49 . In this manner, the swivel ring  45 , together with the plate spring  49 , is part of a tensioning device  23  for adjusting the friction force of the friction brake  20 . 
         [0029]    An index ring  50  sits on the swivel ring  45 , lockingly secured by means of an O-ring  51 . The index ring  50  has an angle scale from 0° to 360°, visible in  FIG. 4 . The index ring is arranged at the housing  10  so as to be rotatable and concentric with the output shaft. It has a centric window  53  through which a mark  52  on the output shaft can be seen. It may be rotated—with some friction—on the swivel ring to any angular position so that the position 0° can be aligned with the mark  52  on the push-button  36 . The push-button  36  is connected with the output shaft  14  in a manner secured against rotation and it thus indicates the respective rotational position of the screw as a relative position. The index ring  50  is set to zero manually in order to memorize the initial position. Thereafter, during the screwing operation, the mark  52  will wander corresponding to the progress of screwing, so that the user can read the respective rotational angle covered since the start of the screwing work or since another time during the screwing operation. The index ring, in combination with the mark  52  belonging to the output shaft, is of independent importance and is not linked to the presence of a friction brake. 
         [0030]    The embodiment illustrated in  FIGS. 8-11  includes a output shaft  14  rotatably supported between the housing walls  10   a  and  1   b  and having an outer toothing (not illustrated) engaged by a reciprocating ratchet lever  16 . The output shaft  14  has an internal hexagon profile  15 . From one end thereof (from the left in  FIGS. 9 and 10 ), a screw head is inserted, while the friction brake  20  and the index ring  50  are provided at the opposite (right) end. The friction brake  20  comprises a friction member  60  with a bushing connected with the hollow output shaft  14  in a manner secured against rotation. An annular friction disc  61  protrudes radially from the bushing, the disk having a circumferential portion  62  with two radial friction surfaces  62   a  and  62   b . These friction surfaces are engaged by friction linings  63   a ,  63   b  of the friction brake  20 . The friction lining  63   a  is mounted to an inner brake jaw  64  and the friction lining  63   b  is mounted to an outer brake jaw  65 , the brake jaws facing each other in parallel and clamp the edge  62  of the friction member  60  between them. The clamping effect is caused by two tensioning screws  66  ( FIG. 8 ) that pull the brake jaws  64  and  65  against each other and which are adjustable to set the braking force of the friction brake  20 . A retaining screw  67  is situated between the two tensioning screws  66 , which is threaded into the housing wall  10   b  and keeps the friction brake  20  on the housing  10 . 
         [0031]    In the third embodiment, the friction brake  20  is designed in the manner of a calliper brake, the two brake jaws  64 ,  65  forming a calliper straddling the edge of the friction disc  61  with the friction linings  63   a ,  63   b.