Power wrench with a movement indicator for a movable torque responsive power transmission element.

The invention relates to a power wrench having a rotation motor driving an output shaft via a power transmission which comprises a movable element that is axialIy displaceable in response to the magnitude of the transferred torque, and a movement indicator for delivering signals at displacement of the movable element.

A problem concerned with power wrenches of the above type is the difficulty to obtain a reliable indication of a set target torque level being reached, and also to obtain information of the magnitude and characteristics of the output torque. It is equally difficult to provide a simple and reliable torque responsive sensing means which is not exposed to mechanical wear and does not add to the overall dimensions of the power wrench.

In US 6,662,882 there is described a power wrench including a movement indicating device for initiating power shut-off as a torque transferring clutch releases at a set target torque level . This known power wrench is mechanically rather complicated as a torque limiting release clutch is arranged to activate a signal generating transducer via a separate activation ring element which is thrown a certain distance when hit by a clutch member at reengagement of the clutch after release. Moreover, this known device is able to indicate a clutch release movement only as the ring element is thrown past the transducer, but a successive displacement of a clutch member responsive to a changing torque output is not possible to indicate. Moreover, this known device is not only mechanically complicated but its

service life is limited as the ring element tends to break as it is exposed to impact forces when repeatedly hit by the clutch member.

In GB 2 271 522 there is described a power screw driver comprising a rotation motor, a power transmission including a reduction gearing, a torque responsive activating mechanism, and an output shaft . The torque responsive activating mechanism includes a driving part, a driven part, a cam mechanism between the driving and driven parts, and an axially displaceable sleeve engaged by the cam mechanism and having an integrated "flux shield" . The latter extends through an air gap between a stationary magnet and a likewise stationary Hall-effect sensor in the tool housing. As the torque load on the power transmission increases above a certain level the sleeve and the "flux shield" are axially displaced by the cam mechanism, whereby the magnetic influence on the Hall-effect sensor changes and a signal is generated by the latter. The signal is used for controlling the motor. The movement indicating mechanism of this known power tool is disadvantageous in that it is structurally rather complicated and requires a special and expensive power transmission design. MoreoverT the open magnetic field created by the stationary magnet is by nature rather weak at the Hall-sensor location and, hence, quite sensitive to external distortions which would affect the output signal in a negative way.

It is an object of the invention to avoid the above disadvantages by creating a power wrench with an improved contact-free movement indicating device arranged to deliver signals responsive to the axial displacement of a movable

power transmission element and, hence, responsive to the magnitude of the output torque of the power wrench.

Another object of the invention is to provide a power wrench with a contact -free movement indicating device comprising a transducer arranged to deliver a shut -off initiating signal responsive to a certain axial displacement of a power transmission element, a movement that corresponds to a certain torque output of the power wrench .

Still another object of the invention is to provide a power wrench comprising a torque responsive contact-free movement indicating device of a compact design and forming a separate transducer unit easily incorporated in most power wrench designs having a torque responsive axially discplaceable element without requiring any redesign of the power transmission.

Further objects and advantages 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, partly in section of a power wrench according to the invention.

Fig. 2 shows on a larger scale an illustration of a power transmission element and a displacement indicating device.

Fig. 3A shows a side view of a torque transferring release clutch with an axially displaceable torque transferring

element illustrated in its low torque transferring position.

Fig. 3B shows the same view as Fig. 3A but illustrating the displaceable element in a high torque transferring position.

Figs. 4A and 4B show a schematic end view a movement indicating device as being activated by a displaceable element in alternative movement directions.

Fig. 5 shows a plane view of the movement indicating device and the displaceable element shown in Figs. 4A and 4B .

The power wrench shown in Fig. 1 comprises a housing 10 with a pistol type handle 11, an electric motor 12, a power transmission 13 connecting the motor 12 to an output shaft 14, and a power control unit 16 operated by a trigger 17 on the handle 11 and connected to a non- illustrated programmable process controlling unit. The power transmission 13 comprises a torque responsive release clutch 19 of a well known type which comprises a driving clutch half 21, a driven clutch half 22, a number of torque transferring balls 23 located between the driving and driven clutch halves 21,22, and a bias spring 25 supported by an adjustable sleeve 26 and acting on the driven clutch half 22 to move the latter axially at release of the clutch 19. The driving and driven clutch halves 21,22 are provided with cam surfaces 28,29 for co-operation with the balls 23 at torque transmission from the motor 12 to the output shaft 14. The driven clutch half 22 comprises a rim portion 30 with a larger diameter than the rest of the driven clutch half 22 and forms together with the latter a movable element. The clutch half 22 with its rim portion 30 is of a magnetically conductive material and arranged to cooperate with a movement transducer in the form of a magnetically

activated Hall-type element 31. The latter is connected to a non-illustrated programmable process control unit for governing the tightening operations performed by the power wrench.

The Hall -element sensor 31 is mounted on a magnetically conductive bar or plate 35. As illustrated in Figs. 2, 4A,B and 5, the magnetically conductive plate 35 also carries a number of magnets which are arranged in two pairs 36a, 36b and 37a, 37b disposed at opposite sides of the rim portion 30 as the movable element 22 occupies an unloaded neutral position. See Fig. 5. The magnets 36a, 36b of one pair have one magnet 36a disposed with its north pole N facing the the neutral position of the rim portion 30, whereas the other magnet 36b has its south pole S also facing the neutral position of the rim portion 30. In the other pair of magnets one magnet 37a has its south pole S facing the neutral position, whereas the other magnet has its north pole N facing the neutral position. The magnets of the two pairs are arranged in such a way that the south pole S of one magnet 37a is located opposite the north pole N of a magnet 36a of the other pair, viewed in the axial direction of the clutch 19.

The arrangement of the magnets 36a, b and 37a, b results in the generation of a closed circuit of magnetic flow M _A or M _B through the metallic plate 35 and the rim portion 30, which results in the Hall -element 31 generating a signal of a certain sign and/or magnitude. For instance, when the rim portion 30 is displaced in the direction A, as illustrated in Fig. 5, a magnetic flow M _A is generated and the HaIl- element 31 generates a positive signal of a magnitude responsive to the amount of movement of the rim portion 30.

When instead the rim portion 30 is displaced in the opposite direction an oppositely directed magnetic flow M _B is generated and the Hall -element 31 generates a negative signal of a certain magnitude. Depending on the signal being positive or negative and what magnitude it has the process control unit connected to the power wrench is able to indicate whether the transferred torque is increasing and to what magnitude or whether the clutch has released at a set target torque level and is moving in its re- engagement direction.

Due to the fact that the clutch 19 is pre-tensioned to a certain degree by the bias spring 25 there will be no relative rotation between the clutch halves 21,22 until the axial force generated by the cam surfaces 28,29 and the balls 23 becomes bigger than the bias force of the bias spring 25. When however the axial force generated by the cam surfaces 28,29 and the balls 23 becomes bigger than force of the spring 25 the driven clutch half 22 will start moving axially in the direction A, which means that the rim portion 30 is displaced as well and a magnetic flow M _A is generated in the plate 35 in the direction illustrated in Fig. 4A. This magnetic flow results in a positive signal being delivered by the Hall -element 31 indicating that the clutch 19 is under an increasing torque load with the clutch halves 21,22 moving apart.

When the pre-set release level of the clutch 19 is reached and the cam surfaces 28,29 override the balls 23 the driving clutch half 22 is returned to its re-engagement position by the action of the spring 25. This means that the rim portion 30 is displaced in the opposite direction B and generates a magnetic flow M _B of an opposite direction

in the plate 35, whereby the Hall-element 31 delivers a negative signal to indicate that the clutch has released and the tightening operation is completed.

It is to be noted that the device according to the invention is able not only to indicate the actual status of the torque transferring clutch but also to enable indication of the transferred torque magnitude within a certain torque range, namely within a torque range above the torque magnitude whereat the clutch halves 21,22 start rotating relative to each other. The signal delivered by the Hall -element 31 is responsive to the distance actually displaced by the rim portion 30, and the process control unit is programmed to translate the distance related signals into transferred torque magnitudes. This makes it possible to accomplish, for instance a speed control of the power wrench during the final stage of a tightening operation and thereby obtain a more accurate result of the operation.

An important advantage gained by the movement indicating transducer according to the invention is that due to the closed magnetic flow circuit obtained by the pairs of magnets via the conductive plate with the Hall -sensor and the rim portion of the driven clutch half there is obtained a strong magnetic flow and a very distinctive activation of the Hall-sensor and, hence, a stable signal output which is not sensitive to external distortions.

A further significant feature of the power wrench according to the invention is that the movement indicating transducer is very compact. The integrated arrangement of the magnets, the conductive plate, and the Hall -sensor form in fact a

separate unit, and by being very compact it is easy to incorporate in most power wrench designs. The movement indicating transducer unit is advantageous also in that it does not require any redesign or adaptation of the power transmission, the clutch in particular, but is suitable to be incorporated in most standard type power wrenches having a mechanical release clutch, only the clutch has an axially moving clutch part that is accessible for the transducer unit, with or without at rim portion.