The invention relates an impulse type power wrench

including a rotation motor, and a hydraulic pulse unit intermittently coupling the motor to an output shaft for delivering repeated torque impulses to a screw joint to be tightened .

In some power wrenches of this type there is a problem to fill the pulse unit with an exactly correct amount of oil such that there will be air enough left inside the pulse unit to absorb some heat related expansion of the oil during operation of the wrench. The main reason, though, why a certain amount of air is left in the oil volume is to make the oil volume somewhat elastically compressible to thereby enhance the impulse frequency of the pulse unit. A too small percentage of air in the oil will cause a great pressure inside the pulse unit resulting in friction losses and overheating, whereas a too large percentage of air will reduce the efficiency of the pulse unit.

In some other pulse units the oil filling problem is solved by using an elastic element or accumulator to compensate for heat related expansions and to obtain a certain

elasticity of the oil volume, which means that in those cases the pulse units can be filled up completely without leaving any air in the oil volume. An impulse generator having this feature is described in US 6,110,045. However, there is still a problem in that there will inevitably be a certain oil leakage, although initially small, from the pulse unit during operation of the wrench, which means that air will penetrate into the pulse unit in a corresponding amount. The result is that that there will be an increased amount of air inside the pulse unit over time. Accordingly, the percentage of air in the oil volume will increase successively, and after some service time of the wrench the increased amount of air in the pulse unit will cause an impaired efficiency and finally a complete loss of impulse generation, so called spinning. Theoretically, it would be possible to extend the service intervals of the power wrench by substantially increasing the size of the entire oil containing volume of the pulse unit to thereby keep down the percentage of air in the oil volume caused by leakage. The result would be that the elasticity of the oil volume would be kept low during an extended power wrench operation time and that the pulse generation efficiency would be maintained at a high level for prolonged service intervals. After some time of operation, though, there will still be an undesirably high percentage of air, even in such an enlarged oil chamber. The obvious drawback of such an enlarged oil volume would be increased outer dimensions and weight of the pulse unit and, hence, the entire power wrench. That would not be acceptable .

It is an object of the invention to provide an improved power wrench including a hydraulic pulse unit with a separator for extracting air from oil to thereby keep up the pulse efficiency for substantially extended service intervals of the power wrench at maintained small

dimensions of the pulse unit. A further object of the invention is to provide a power wrench including a hydraulic pulse unit with a separator for extracting air from oil by centrifugal action, and a separate air collecting chamber for gathering separated air .

Still further objects of the invention will appear from the following specification and claims. A preferred embodiment of the invention is described below with reference to the accompanying drawings.

In the drawings

Fig. 1 shows a side view of a power wrench according to the invention.

Fig. 2 shows a longitudinal section through the pulse unit of the power wrench in Fig. 1.

Fig. 3 shows a rear end view of the separator member in Fig. 2 illustrating one embodiment of the oil-air

separator.

Fig. 4 shows an alternative embodiment of the oil-air separator .

Fig. 5 shows still another embodiment of the oil-air separator .

Fig. 6 shows, on a larger scale, a sectional view of a part of the oil-air separator with an oil filter.

Fig. 7 shows a diagram illustrating the impulse frequency of the pulse unit in relation to the amount of oil leakage. Fig. 8 shows a diagram illustrating the torque output of the pulse unit in relation to the amount of oil leakage. The power wrench shown as an example in the drawings is a pneumatically powered pistol type wrench which comprises a housing 10 with a handle 11. The latter carries at its lower end a connection piece 12 for a pressure air conduit and an exhaust air silencer 13. A throttle valve for power control is maneuvered by a trigger button 14. In the housing there is provided a non-illustrated motor, and a hydraulic pulse unit 17 with a square ended output shaft 18.

As illustrated in Fig. 2 the impulse unit comprises an inertia drive member 20 including a cylindrical element 21 with a rear end wall 22 and enclosing an oil chamber 23. The rear end wall 22 is formed with a coupling portion 24 for permanent connection to the motor and is secured to the cylindrical element 21 by a thread connection 25.

The output shaft 18 has an impulse receiving portion 29 which extends into the oil chamber 23 and is intermittently coupled to the drive member 20 via an impulse generating mechanism 30. The latter comprises a couple of piston elements 24 and rolling elements 25 rotatively locked to the output shaft and acted upon sequentially in radially inward directions by cam profiles 26 on the cylindrical element 21 so as to compress oil between them in a central high pressure chamber 34, thereby generating torque

impulses. A cam spindle 35 is secured to the rear end wall 22 and extends coaxially into the high pressure chamber 34 and is effective in returning the piston elements 24 outwardly after each impulse generation. The operation of the impulse mechanism per se is not described in any further detail since it is previously described in US Patent 6,110,045. The rear end wall 22 also comprises an air separator 37 including a substantially axially facing surface 46, and a separator element 38 which is rigidly clamped between the end wall 22 and the cylindrical element 21. The separator element 38 has a substantially axially facing surface 45 located opposite and being congruent with the surface 46 on the end wall 22. Both surfaces 45,46 have a slightly conical shape, and the separator element 38 has a central oil inlet opening 40 which communicates with the high pressure chamber 34 via a central passage 41 and two radial openings 42 located in the cam spindle 35. The openings 42 have very small flow areas and are adapted to prevent high pressure peaks from reaching the separator 37 but to let through a tiny oil flow only. Adjacent the periphery the separator element 38 is provided with an oil outlet opening 53, and close to the rotation axis A-A there is provided an air outlet opening 52. The surfaces 45,46 form between them a clearance gap 43 which serves as an oil circulation passage, and oil entering the clearance gap 43 via the inlet opening 40 will be forced outwardly by centrifugal action towards the outlet opening 52.

In the embodiment illustrated in Fig. 4 the clearance gap 43 comprise a series of substantially loop-shaped grooves 48a-e formed in the separator element 38, wherein a groove 48a communicates with the oil inlet opening 40. At their central parts all of the grooves 48a-e communicate with an air collecting chamber 50 located in the rear end wall 22 at the rotation axis A-A. The loop-shaped grooves 48 a-e are series connected to each other, wherein the downstream groove portion 48e communicate with an oil outlet opening 53 via a substantially radial oil discharge passages 51. The oil outlet opening 53 is disposed at a radial distance from the rotation axis A-A and communicates with the oil chamber 23.

The grooves 48 a-e on the surface 45 of the separator element 38 forming the clearance gap 43 are produced in any suitable way, for instance at a sintering process forming the entire separator element 38.

During operation of the impulse unit the inertia drive member 20 is rotated by the motor and when a torque load is applied on the output shaft 18 a relative rotation between the inertia drive member 20 and the output shaft 18 will take place and the rollers 25 and the piston elements 24 will be urged inwardly by the cam profiles 26 to create a high pressure in the high pressure chamber 34. Thereby, a torque impulse is accomplished in the output shaft 18. Due to an inevitable leakage there will always be some amount of air mixed into the oil volume, and if that amount becomes too large the efficiency of the impulse unit will decline. The separator 37 will see to that the amount of air is kept low as long as possible to keep up the

efficiency of the impulse unit.

At impulse generation the high pressure peaks in the chamber 34 will cause a small intermittent flow of oil to be extruded through the openings 42, and the passage 41, and reach the oil inlet opening 40 of the separator 37. Since the separator 37 is co-rotating with the inertia drive member 20 there will be a centrifugal action

influencing on the oil in the clearance gap 43. The oil will be forced outwardly through the first loop-shaped grove 48a, which means that the oil flow also is turned inwardly for a certain distance before entering the next loop-shaped groove 48 b. See the filled arrows illustrating the oil flow and the open arrows illustrating the air escape flow to the air collecting chamber 50 via air outlet openings 52a-e.

Depending on the substantial difference in density between oil and air the centrifugal action will force the oil radially outwardly and the air inwardly, which means that the air will be pressed into the central air collecting chamber 50 via the air outlet openings 52a-e, whereas the oil flow continues through the next loop-shaped groove 48b and further on through the following grooves 48c, 48d and 48e. Remaining air in the oil flow will be pressed out of the oil flow from all loop-shaped grooves 48a-e through the outlet openings 52a-e and into the air collecting chamber 50 such that when the oil leaves the separator 37 via the discharge passage 51 and the outlet opening 53 all air mixed into the oil is separated and gathered in the

collection chamber 50. By the repeated centrifugal

separations in the loops 48a-e all air mixed into the oil will be extracted, and a substantially air free oil is discharged back into the oil chamber 23. In order to enhance the separating effect the loop-shaped grooves 48a-e are asymmetric to take advance of the

retardation forces during impulse generation in the rotation direction R of the inertia drive member 21. The inertia of the oil urges the oil even stronger radially outwardly while the air is pressed inwardly to reach the air outlet openings 52a-e.

Since the entire pulse unit 17 is filled up with oil from the beginning also the air collecting chamber is filled up with oil. After some time of operation there will be gathered air in the collection chamber 50, but due to the very fine dimensions of the loop-shaped grooves 48a-e oil rather than air will be retracted by capillary action from the collection chamber 50 to return into the oil chamber 23. This means that air is gathered in the collection chamber 50, whereas oil is leaving it. Alternatively, wicks of a felt material could be arranged to suck via capillary action oil from the collection chamber 50

In the embodiment of the air separator illustrated in Fig. 5 the oil circulating loop-shaped grooves 68a-d on the separator element 38 are four in number and divided into two sections 60 and 61, whereof one section 60 comprises two loop-shaped grooves 68a, b and the other section 61 two loop-shaped grooves 68c, d. Each section has an oil inlet opening 64a, b located close to the rotation axis A-A of the inertia drive member 20 and an oil discharge passage 66a, b with an oil outlet opening 65a, b at a radial distance from the rotation axis A-A. As in the previously described embodiment each groove 68a-d has a central air outlet opening 62a-d opening into the air collection chamber 50.

During operation an oil flow is extruded from the high pressure chamber 34 into the separator 37 via the openings 41 and 42 in the valve spindle 35 and the inlet openings 64. The oil containing a certain amount of mixed in air is circulated through the loop-shaped grooves 68a-d, wherein air is pressed out through the outlet openings 62a-d and oil is leaving the separator 37 through the outlet openings 65a, b .

In Fig. 6 there is illustrated an annular filter element 70 inserted in the clearance gap 43 between the surfaces 45, 46 for separating also metallic or other undesirable particles from the oil, thereby avoiding inter alia

unnecessary mechanical wear of the impulse unit parts.

By the diagram shown in Fig. 7 there is illustrated by curve A how the pulse frequency f is kept at a lower level at an increasing amount of oil leakage L when using the air separator according to the invention compared to a faster increasing pulse frequency f at increasing amount of oil leakage L at prior art pulse units according to curve B.

By the diagram shown in Fig. 8 there is illustrated by the curve A how the torque output M of a pulse unit using an air separator according to the invention is maintained at a high level at an increasing amount of oil leakage L

compared to a faster decreasing output torque M at

increasing amount of oil leakage L at prior art pulse units according to curve B.

As illustrated by the diagrams in Figs. 7 and 8 the

employment of an air separator according to invention means an extended high performance of a pulse generator and extended service intervals of the impulse tool. This means an improved accessibility of the impulse tool as well economical advantage for the operator.