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The invention concerns a torque sensing unit and a power tool provided with such torque sensing unit by which an output torque responsive signal is generated by a means sensing the reaction torque transferred from a part of a reduction gearing to the tool housing.

In particular, the invention relates a torque sensing unit comprising a reaction torque transferring element connected between a ring gear of a planetary reduction gearing and the tool housing and provided with strain sensors arranged to deliver signals in response to the transferred reaction torque and, hence, the output torque of the tool.

One previously known type of torque sensing means is described in for instance US 4,620,449 and comprises an elastically deformable sleeve connecting a ring gear of the reduction gearing to the tool housing, and a strain gauge type sensor means arranged to indicate elastic torsional deformation of the sleeve responsive to the reaction torque transferred to the tool housing. This device is disadvantageous in that it requires a substantial axial space in the tool housing. It is also relatively costly to produce .

Another type of torque sensing means for a torque delivering power tool is described in US 5,172,774. This known torque sensing means comprises a wheel shaped torque transferring element which comprises a hub formed with internal splines for connection to an immobile disc mounted in the housing, and a rim portion formed with external splines for connection to a ring gear element exposed to a reaction torque. This device is advantageous in that it requires a relatively short axial space in the tool

housing, but instead it requires a large radial space and is rather complicated and expensive to manufacture.

It is a primary object of the invention to accomplish an improved torque sensing unit and a power tool provided with such torque sensing unit by which the drawbacks of the prior art devices are avoided. Accordingly, the invention intends to create a torque sensing unit for a power tool that is compact in design and simple and non-expensive to manufacture .

Further advantages and objects of the invention will appear from the following specification and claims.

A preferred embodiment of the invention is below described in detail with reference to the accompanying drawing.

In the drawing

Fig. 1 shows a longitudinal section through a power tool comprising a torque sensing unit according to the invention .

Fig. 2 shows a cross section along line II-II in Fig. 1.

Fig. 3 shows a perspective view of a torque transferring element according to the invention including a strain sensing means .

The power tool illustrated in the drawing figures is a power nutrunner for tightening screw joints and comprises a housing 10, a rotation motor 11, a reduction gearing 12, and an output shaft 13 journalled in an angle head 14. The reduction gearing 12 comprises two planetary gearings of a well known type connected in series and having a common non-rotating ring gear 16 connected to the housing 10. In order to keep down the volume of the specification the planetary gearings are not described in further detail . However, the reduction gearing 12 has a primary drive shaft formed with a sun gear 19 and connected to the motor 11.

The ring gear 16 is connected to the housing 10 via a torque sensing unit 20 arranged to transfer to the housing 10 the reaction torque exerted on the ring gear 16 during operation of the tool.

As well known in prior art, the reaction torque exerted on a ring gear of a planetary reduction gearing is proportional to the output torque delivered via the output shaft of the tool, and in order to accomplish a stationary relatively simple means to measure the output torque of the tool a torque sensing means has been employed between the ring gear and the tool housing. As described above previously known devices for this purpose have been disadvantageous by being too space demanding and/or too complicated and expensive to manufacture.

In the power tool illustrated in the drawing figures a torque sensing unit 20 including an annular torque transferring element 22 is connected between the ring gear 16 and the housing 10 and has a geometric axis coinciding with the geometric axis of the ring gear 16. This torque transferring element 22 is relatively thin, i.e. its axial length is considerably less than its radial dimensions, which means that it requires a limited axial space only of the inside area of the housing 10. The torque transferring element 22 is formed with a first pair of external and diametrically opposite engagement portions 24,25 for connection to the housing 10, and a second pair of diametrically opposite engagement portion 27,28 for connection to the ring gear 16. The first pair of engagement portions 24,25 is angularly spaced by 90 degrees relative to the second pair of engagement portions 27,28.

The first pair of engagement portions 24,25 is intended to transfer reaction forces F _R from the torque transferring element 22 to the housing 10, whereas the second pair of engagement portions 27,28 is intended to transfer torque

related tangent!ally directed forces F _τ received from the ring gear 16 to the torque transferring element 22. See Fig. 3.

The engagement portions 24,25 of the first pair are adapted to engage grooves 31,32 in the housing 10, and the engagement portions 27,28 of the second pair are adapted to cooperate with pockets 36,37 in the ring gear 16. See Fig. 2.

Between the first and second pairs of engagement portions 24,25 and 27,28, respectively, there are four identical weak zones 42 a-d which during torque transfer are exposed to bending stresses, thereby being elastically deformed. In order to measure this deformation caused by the bending stresses there are provided strain gauges 43, of which just one is illustrated in Fig. 3. These strain gauges 43 are mounted on flat surfaces 45 a-d which extend in planes A and B which are parallel to the geometric axis C of the ring gear 16. In the transverse direction these planes A and B extend along chord lines of an imaginary circle coaxial with the ring gear 16.

Suitably, each strain gauge 43 is connected in a Wheatstone Bridge circuit of an operation control or monitoring means, located onboard the power tool or at a distance from the tool as a stationary unit, whereby torque related signals are obtained for indicating the output torque of the tool . By comparing the actual signals with a predetermined set signal representing a desired final torque level it is possible to determine when the delivered output torque has reached a desired level or not and, if desired, initiate an automatic power tool shut-off.

Thanks to its compact dimensions and simple design the torque sensing unit according to the invention is not only suitable for but particularly adapted to power tool

application. The annular torque transferring element 22 may be rationally manufactured by extrusion technique where for instance a light alloy tube shaped blank with the desired cross sectional shape is extruded, and separate elements are formed by cutting that blank into "slices".