EP3938145B1

EP3938145B1 - Handheld power tool and means for absorbing torque reaction forces for such a power tool

Info

Publication number: EP3938145B1
Application number: EP20708445.0A
Authority: EP
Inventors: Jakob LINDSTRÖM; Mattias ANDRÉ
Original assignee: Atlas Copco Industrial Technique AB
Current assignee: Atlas Copco Industrial Technique AB
Priority date: 2019-03-11
Filing date: 2020-02-20
Publication date: 2023-10-11
Anticipated expiration: 2040-02-20
Also published as: WO2020182441A9; EP3938145A2; WO2020182441A2

Description

Technical field

The present invention generally relates to handheld power tools, more particularly to handheld power tools comprising means for absorbing torque reaction forces.

Technical Background

Power tools, for example fastening tools for tightening of nuts and bolts, are known to be used in various applications and industries. When using such tool for tightening,
inevitably a reaction torque caused by the turning force required to tighten the screw or bolt arises. This reaction torque must be dealt with, i.e. absorbed, by applying an equal and opposite force on the tool in order to obtain proper tightening.
In the simplest case, and given that the torque levels are not too high, the reaction torque from a power tool is commonly absorbed by the operator. I.e., to counteract the torque reaction, the operator applies an equal and opposite force on the handle of the tool.
This of course inflicts strain on the operator, strain which already at lower torque levels may pose a risk to work place safety for example if the number of tightening performed during a working day in high, and is in any case only feasible up to a certain torque level corresponding to a level which may be handled with regards to muscle strength and ergonomics of the operator.
Power tools, especially tools adapted to deliver higher torque levels, are therefore known to, in order to be able to relieve the worker from the reaction torque, be provided with so called reaction arms commonly attached to a front part of the tool and adapted to bear against a supporting object which can take up the reaction torque as seen in document WO 2016/176518 A2 . Commonly this supporting object may be a part of the work piece or an adjacent bolt. Simple reaction bars as well as advanced reactions bars suspended from the ceiling are known. However, a major disadvantage of such conventional reaction arm is the risk of pinching or even crushing injuries which may occur if a worker accidentally puts for example a hand between the reaction arm and the support where severe injuries may result. Further, such reaction arms have to be adapted to the specific application depending on the position of suitable support against which the arm may bear, and therefore are expensive and offer little or no flexibility. Further, reaction arms also require a lot of space and hence there are many applications where absorption of reaction torque is difficult due to lack of space to fit such a reaction arm.
Furthermore, there are tools known which support themselves on a washer as can be seen in document WO2016/176518 A2 .
Hence, there exists a need for improvement in the field of handling of reaction torque of power tools.

Summary of the invention

Accordingly, it would be desirable to provide a power tool comprising improved means for handling the reaction torque improving work place safety by minimizing the risk of pinching- and crushing accidents injuries. In particular, it would be desirable to provide such a power tool providing high flexibility and accessibility by not requiring adaption of the reaction means or work place. To better address one or more of these concerns a power tool as defined in the independent claim is provided. Preferred embodiments are defined in the dependent claims.
According to a first aspect of the invention a handheld power tool according to claim 1 is provided.
According to the first aspect, the power tool provides an inventive solution to the concerns described above by means of a design incorporating housing adapted to bear against the work piece surface, a nut socket and a mechanism connecting this socket and the housing. More particularly, a socket adapted to not only apply torque to the bolt but to exert a force on the bolt in an axial direction, for example in a lifting or pulling direction on the bolt, and a mechanism connecting this socket and the housing such that the axial load is provided when torque is applied and transferred to the work piece via the housing by means of a pressing of the housing against the workpiece surface surrounding the bolt, or screw, thereby in turn enabling the generation of a friction torque which in turn absorbs the reaction torque and eliminates the need for a reaction bar.
Further, the axial force provided by the mechanism is proportional to the torque applied to the bolt, or increases proportionally to the torque applied to the bolt. An important effect of this is that a constant margin against sliding may be maintained throughout the tightening process, assuming constant friction coefficients.
Hence, the invention allows the reaction torque to be absorbed by the workpiece in the area immediately around the bolt head by means of a friction torques. This makes for a very compact and versatile alternative to the known so called reaction arms, the latter since the inventive concept does not need to be adapted to the workpiece to support the reaction torque.
The invention thereby eliminates strain on the operator and in cases where a reaction bar would normally be used, the invention poses a compact and versatile option. Further, since the invention utilizes the friction between the housing and the surface, the invention also eliminates the risk of pinching- or crushing injuries which is often present when using conventional reaction bars. In other words, the inventive design hence cleverly provides not only a compact and versatile alternative to a traditional reaction bar, but also increases work place safety. Hereby, the inventive concept for example enables one hand tightening of high torque fasteners by using friction to absorb the reaction torque.
The principle of pulling on the screw also has the additional advantage of reducing the friction under the head, thus enabling a more precise estimation of the clamp load achieved as will be explained in the following section briefly explaining the operation, or functionality, of the tool according to the independent claim.
In an initial state, the housing has just contacted the work piece. Until this point, the input shaft has driven the socket via the mechanism in ordinary rotation. But, as the housing now has reached and bears against the work piece surface and can no longer follow the axial movement of the bolt and as the screw continuous to rotate the mechanism start to generate an axial force proportional to the transferred torque. The clamp force generated in the joint increases until the set torque is reached. Since the axial force generated is increasing proportionally to the torque on the bolt, a constant margin against sliding is maintained throughout the tightening process, assuming constant friction coefficients.
In general, with regards to the input shaft, the protruding portion may be a separate element attached to an input shaft axle or a protruding portion integrally formed with the shaft. Further, the tool may comprise an axial bearing arranged on the shoulder of the housing against which the protruding portion may rotatably bear.
In some embodiment, the motor is an electric motor. A tool comprising an air motor is however also conceivable within the scope of the present invention. In some embodiments, the power tool may be a battery powered tool. The weight saving and improved ergonomics due to the elimination of the need of a heavy reaction arm provided by means of the inventive (front part of the) power tool may be of particular importance to a battery powered tools which tend to be heavier to begin with due to the weight of the battery. In some embodiments, the tool may further comprise circuitry and/or be connectable to a controller (or control unit) operative to control the power tool. In one embodiment, the power tool is a tool providing a higher tightening torque, for example in the range 1500-2500 Nm. In another embodiment, the power tool is a tool providing a somewhat lower tightening torque, for example in the range 100-300 Nm.
According to the invention, the mechanism further comprises a first element engaging the drive shaft, a second element engaging the socket and a connecting arrangement providing a connection between the first and second element, wherein the connecting arrangement is arranged to provide a connection allowing a variable axial distance between the first and second element. By engaged may be understood attached to, or connected to, the input shaft and the socket respectively such that the axial- and rotational movement is constrained (?) between the first element and the shaft and between the second element and the socket respectively. The first element may for example be attached to the drive shaft by means of a press fit or similar. It follows, given the arrangement of the first and second element, that the socket in such an embodiment may be described as movably arranged with respect to the input shaft where at least a limited relative movement there between is allowed due to the nature of the connecting mechanism. Hereby, it is enabled, in some embodiments, that the socket is movably arranged between a first and a second position, wherein in the first position the front end of the socket is arranged at a distance d1 from the front end of the housing and in the second position the front end of the socket is arranged at a distance d2 from the front end of the housing, wherein d1>d2. According to the invention, the first and second element are further coaxially arranged with respect to the input shaft and wherein the connecting arrangement is further arranged to provide a connection such that first and second element are further movably arranged between a first relative angular position and a second relative angular position. For example, the first and second element may be substantially annular (or disc shaped) elements each comprising a hole through which the input shaft may extend. As the first element engages the shaft and the second element the socket, it follows that the second element and hence the socket may be rotated with respect to the input shaft, at least to a limited extent.
According to the invention, the variable axial distance between the first and second element decreases as the first and second element move from the first relative angular position to the second relative angular position. Further, in one embodiment, the second relative angular position is a position in which the mechanism exerts the axial force on the housing. I.e., there is a kinematic relationship provided between the axial and the rotational movement, or relative movement. In other words, the first and second element may be arranged in a first relative position in which no axial force is exerted and in a second relative position where the axial force is exerted on the bolt and hence acts on the housing. According to one embodiment, the first element comprises a first contact surface and the second element comprises a second contact surface, and wherein the second contact surface is an opposite surface facing the first contact surface. Hereby, the first and second contact surface may conveniently engage one another.
According to one embodiment, at least one of the first and second contact surface comprises an inclined surface portion, wherein the inclined surface portion has a first pitch. Advantageously, each of the first and second contact surfaces comprises such an inclined portion, or ramp, such that the first and second contact surface may interact in order to provide for the axial force. In such an embodiment, as the first and second element are rotated with respect to one another, the axial force generated by the mechanism is determined by this pitch. Further, the force increases proportionally to the torque applied to the bolt. Therefore, a constant margin against sliding is maintained throughout the tightening process, assuming constant friction coefficients.
According to one embodiment, the mechanism is an axial cam mechanism such that a relative rotation between the first and second element results in a change in the axial distance between the first and second element.
In the context of the present specification the term "cam mechanism" in a broad sense refers to a mechanism which transform a rotary movement into a linear movement. Further, in such an embodiment, the first and second element may also be referred to as a first and second cam ring. By cam ring should in this case be understood an element comprising a cam surface, i.e. a surface having a pitch, such that a rotation may be transformed into linear movement. In other embodiments, the mechanism may be for example a screw mechanism.
According to one embodiment, the cam mechanism further comprises at least one rolling element arranged to engage the first and the second contact surface. Such an element may be a roller or a ball.
According to one embodiment, the at least one rolling element is arranged between the first and second contact surface. Hereby, the rolling element may roll along the inclined surfaces in order to facilitate the relative rotation.
According to one embodiment, the first and the second element each comprises a portion having a substantially cylindrical shape, wherein the respective contact surface are arranged along an outer portion of a respective substantially circular cross section of the cylindrical portions of the first and second element. This facilities assembly and mounting of the elements in the housing which may be a cylindrical housing. Further, in some embodiments, the rolling elements may hence be arranged to roll between the two substantially cylindrical elements as they rotate with respect to one another.
According to one embodiment, the second element is a clutch unit comprising a contact element and a sleeve element, wherein the contact element comprises the second contact surface, wherein the sleeve element is arranged to connect the contact element and the socket and wherein the sleeve element encloses the first element and the contact element.
According to one embodiment, the mechanism comprises a plurality of rolling elements. In some embodiments, the first and second element may comprise a corresponding number of inclined contact surface portions, or ramps, such that each of the plurality of rolling elements may be arranged between a respective pair of contact surfaces.
According to one embodiment, the socket is adapted to engage under a head of the bolt. Hereby, a firm connection for applying the axial force may be established. The socket may be suitably adapted depending of the shape of the bolt head used in a specific application. In other embodiments, the socket may be adapted to engage a side of the bolt or a specific engaging structure provided on the bolt (where one example would be a hole or at least protrusion provided in the bolt in which a corresponding structure of the bolt may engage).
According to a further embodiment, the power tool further comprises an ultrasonic measurement device. Ultrasonic measurement on bolts in order to determine the clamp force achieved during bolt tightening is known. When using such methods, ultra-sonic pulses are transmitted into the bolt by means of a suitable transducer and the response time, often referred to as the time of flight, is monitored. The time of flight provides a measurement of the length of the bolt and hence, any measured increase in the time of flight corresponds to an increase of the length of the bolt, and thus, of the clamp force in the bolt. Problems associated with such ultra sound methods however include difficulties to establish a good enough contact between the ultra-sonic meter and the bolt, commonly leading to an undesired need to use special screws and/or other special arrangements on the tool. In the power tool described in the present specification however, the inventive arrangement described in the foregoing and more particularly the axial force generated thereby may cleverly be utilized to provide the additional effect of facilitating the establishment of a proper contact between the bolt head and the socket. I.e., providing the additional effect of establishment of a firm, stable contact which may be utilized to significantly improve ultrasound measurement of bolt elongation. This since, the axial force holds, or even presses the bolt head against the socket, thereby eliminating the problems relating to poor contact between the bolt and transducer described above.
According to one embodiment, the ultrasonic measurement device comprises an ultrasonic transducer and is adapted to measure time of flight of an ultrasonic wave in a bolt during tightening. Hereby, the change in length and thus the clamp load may be determined. In such an embodiment, the transducer is arranged such that contact may be established between the transducer and the bolt head, preferably in the socket. According to a second aspect of the present invention, a front part for a power tool adapted to apply torque to a bolt according to claim 11 is provided. This aspect hence relates to a separate part or accessory for a power tool and accordingly further comprises suitable means for attaching a front part housing to the housing of the tool and the input shaft to a rotating shaft of the tool. Objectives, advantages and features of the gear unit conceivable within the scope of the second aspect of the invention are readily understood by the foregoing discussion referring to the first aspect of the invention.
According to a further aspect, a screw assembly adapted for use with a power tool according to any of the exemplary embodiment listed above is provided. This since operation of the tool may be advantageously facilitated if the screw head is somewhat raised from the surface of the joint (or workpiece), hereby the socket may for example be arranged to apply a force to an underside of a bolt head and/or to the corners of an exemplary hexagonal bolt head.
Such a screw assembly may in some embodiments comprise means for facilitating the use of standards screw, such means may include a suitable washer to be arranged on (i.e. around) the screw such that access to the screw head with which the socket of the inventive power tool is adapted to engage is facilitated. More particularly, access to an underside of the screw head may be facilitated by such a washer such that the socket may engage the underside of the bolt head and/or the corners of the bolt head in order to exert an axial, pulling force on the bolt. In other embodiments, the assembly comprises a special bolt, or screw, having a shoulder arranged to provide the same functionality, i.e. to facilitate access for the socket to the underside of the bolt head and/or corners. I.e., a screw adapted to be engaged by the socket. According to a further aspect, a system comprising a power tool according to any of the embodiments described above and a screw assembly according to any of the embodiment described above is provided.
According to yet another aspect a system comprising a power tool according to any of the exemplary embodiment listed above and a control unit operative to control the power tool is provided.
Further objectives of, features of and advantages of the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

Brief description of the drawings

The invention will be described in the following illustrative and non-limiting detailed description of exemplary embodiments, with reference to the appended drawing, on which

Figure 1a is a cross sectional view of an exemplary power tool according to one embodiment in a first position.
Figure 1b is a cross sectional view of an exemplary power tool according to one embodiment in a second position.
Figure 2 is a perspective view of a component comprised by the inventive mechanism according to one embodiment.

All figures are schematic, not necessarily to scale and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.

Detailed description

Fig. 1a and b are both cross sectional views of a front part of an exemplary power tool 1 according to one embodiment, in this case a handheld bolt tightening tool 1. The tool is adapted to tighten a joint by means of applying a torque to a bolt B and comprises a rotatable input shaft 10 adapted to engage for example an output axle of a motor (not shown), a housing 11 comprising a front portion (or front end) 111 having an end surface 111a oriented substantially normal to the input shaft 10 and adapted to selectively bear against the surface S of the workpiece WP into which the bolt B is tightened. A radially protruding element 101, in the illustrated embodiment a rotationally symmetric funnel shaped collar 101 integrally formed with the input shaft 10, is arranged to rotatably bear against a shoulder 111b formed in an inner side 11' of the housing 11 such that an axial load or force may be transferred from the input shaft 10 to the housing 11 via this shoulder 111b and bearing 13 while the input shaft 10 may still rotate with respect to the housing 11. In some embodiment an axial bearing facilitating this rotation, or any other similar element such as a ball bearing or a sliding bearing, may be arranged on the shoulder 111b such that the radially protruding element 101 may bear there against. A socket 12 is adapted to engage the bolt B, for example a hexagonal bolt head of the bolt B, surface such that a force may be exerted on the bolt by the socket in an axial direction. In the illustrated embodiment, the socket 12 is adapted to engage under the bolt head, thereby establishing a firm engagement for applying a pulling force on the bolt B.
Further, the tool comprises a mechanism 20 connecting the socket 12 and the input shaft 10 and adapted to selectively exert an axial force on the bolt and housing when torque is applied to the bolt thereby pressing the end surface 111a of the housing against the work piece surface S. This axial force is proportional to the torque applied to the bolt.
The mechanism in the illustrated embodiment comprises a first element 21 engaging the drive shaft 10 and a second element 22 engaging the socket 12. A connecting arrangement 23 is arranged to provide a variable distance between the first 21 and the second 22 element, and hence in turn between the socket 12 and the input shaft 10. Since the input shaft 10 is axially constrained with respect to the housing 11, it follows that a limited relative movement is provided also between the socket and the housing.
The first and second element 21, 22 are further coaxially arranged around the input shaft 10 and the connecting arrangement is further arranged such that the first and second element 21, 22 are movably arranged between a first relative angular position and a second relative angular position. Since the first element 21 engages the input shaft, it follows that the second element 22 may be slightly turned with respect to the shaft. This movement is such that the axial distance between the elements decreases as the relative rotation occurs. Further, depending on the relative rotation, the mechanism 20 exerts or does not exert the axial force on the bolt and housing 11 as will become apparent when the operation of the mechanism is later described.
The first element 21 has an overall cylindrical shape. The second element 22 in the illustrated embodiment in turn comprises a contact element 221 and a housing element 222, where the housing element 222 engages the socket 12 and the contact element 221 similarly to the first element 21 has an overall cylindrical shape. Hence, the housing element connects the contact element 221 and the socket 12 and further encloses the first element 21 and the contact element 221 and may therefore be referred as a mechanism housing 222.
In the illustrated embodiment, the first element 21 and the contact element 221 of the second element 22 are equal in their design, an example is shown in fig. 2.
As shown in fig. 2, in order to provide part of the cooperating structure in the mechanism the first and the second element 21, 22 comprises a first contact surface 21a and a second contact surface 22b respectively, where the second contact surface 22a is opposite to and facing the first contact surface 21 a. Each of the contact surfaces 21a, 22a further comprises an inclined surface portion 21a', 22a' or ramp 21a', 22a', having a suitable pitch, these respective inclined portion may together form a contact surface pair. This pitch determines the axial load as will be elaborated on below. In other words, the mechanism in of the illustrated embodiment may also be referred to as an axial cam mechanism, where relative rotation between the first and second cam ring 21, 22 (i.e. the first and second element 21, 22 referred to above) results in a change in the axial distance between these rings.
Given the at least partly cylindrical shape of the first and second element 21, 22, where the first and second element 21, 22 each comprises at least a portion 21p, 22p having a substantially cylindrical shape, the respective contact surfaces are arranged along an outer portion OP of a respective substantially circular cross section CS of the first and second element 21, 22. In the illustrated embodiment, each element 21, 22 comprises five contact surface portions 21a, 22a, together forming five contact surface pairs.
To facilitate the interaction between the contact surfaces, more particularly the inclined surface portions, rolling element 24 are arranged between the contact surfaces 21a, 22a, more particularly between the inclined surface portions 21a', 22a' to engage the surfaces. In the illustrated embodiment, one rolling element is associate with each surface pair, and hence five rolling element 24 are comprised.
As mentioned above, depending on the relative position of the socket and the input shaft, the mechanism selectively generates an axial force proportional to the torque applied. This force pulls the bolt, i.e. acts in a direction away from the workpiece, and is in turn cleverly transferred to the housing such that the housing is being pressed against the work piece surface as will be explained in the following (with reference to fig. 1a and b).
In an initial state, shown in fig. 1a, the housing 11 has just contacted the work piece WP. Until this point, the first element 21 which engages, or is attached to, the input shaft 11 has driven the second element 22 (comprising the contact element 221 and the housing 222) and the socket 12 in ordinary rotation.
At this point, the mechanism 20 (or cam mechanism) is in what may be referred to as a top position, i.e. the rolling element 24 is arranged at a respective end of the respective inclined portion 21a', 22a' of each of the contact surfaces 21a, 22a and the axial distance between the first and the second element 21, 21, more particularly the contact element 221 of the second element, is at its maximum. But, as the housing 11 now has reached and bears against the work piece surface and can no longer follow the axial movement of the bolt B, as the bolt B continuous to rotate the rolling element 24 starts to roll downhill and the distance between the first element 21 and the contact element 221 of the second element decreases.
Turning to fig. 1b, it may be seen that the socket is now positioned closer to the work piece, i.e. the distance d2 is smaller than the distance d1 indicated in fig. 1a, and the rolling element 24 is now positioned at an intermediate position P along the inclined contact surface portions 21a', 22a' (i.e. the surface pair) and from this point, the axial force generated by the mechanism is proportional to the transferred torque and determined by the pitch of the inclined portions 21a', 22a'. The torque is however still very small, so not much force is generated. In the next phase however, when the screw head makes contact with the workpiece surface, the torque starts to increase and the axial force increases proportionally. The clamp force generated in the joint increases until the set torque is reached. The cam generates an axial force determined by its pitch, the force increases proportionally to the torque on the screw thereby maintaining a constant margin against sliding throughout the tightening process, assuming constant friction coefficients.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiment. The skilled person understands that many modifications, variations and alterations are conceivable within the scope of the invention as defined in the appended claims. In the claims, the word "comprising" does not exclude other elements or steps and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

Hand held power tool (1) adapted to apply a torque to a bolt (B), said tool comprising
a rotatable input shaft (10),

a housing (11) comprising a front portion (111) having an end surface (111a) oriented normal to said input shaft and adapted to selectively bear against a surface of a work piece (WP),

a radially protruding element (101) arranged on said input shaft, and bearing rotatably against a shoulder (111b) formed in an inner side of said housing such that an axial force may be transferred from said input shaft to said housing

a socket (12) adapted to engage under the head of a bolt protruding from said work piece surface such that a pulling force may be exerted on said bolt by said socket in an axial direction and to apply torque to said bolt, and

a mechanism (20) connecting said socket and said input shaft, said mechanism comprising:
a first element (21) engaging said drive shaft

a second element (22) engaging said socket, and

a connecting arrangement (23) arranged to provide a connection allowing a variable axial distance between said first and second element,
wherein said first and second element (21,22) are coaxially arranged with respect to said input shaft (10) and wherein said connecting arrangement is further arranged to provide a connection such that first and second element are further movably arranged between a first relative angular position and a second relative angular position, and

wherein said variable axial distance between said first and second element decreases as the first and second element move, in use when torque is applied, from the first relative angular position to the second relative angular position,

thereby providing an axial force acting on said input shaft and further transferred to said housing via said shoulder, pressing said end surface of said housing against said work piece surface whereby said axial force pressing said end surface of said housing against said work piece surface is proportional to the torque applied.
Power tool according to claim 1, wherein said first element comprises a first contact surface (21a) and said second element comprises a second contact surface (22b), and wherein said second contact surface is an opposite surface facing said first contact surface.
Power tool according to claim 2 wherein at least one of said first and second contact surface comprises an inclined surface portion (21a'; 22a'), and wherein said inclined surface portion has a first pitch
Power tool according to any of the preceding claims 1-3, wherein said mechanism is an axial cam mechanism such that said relative rotation between said first and second element results in a change in the axial distance between said first and second element.
Power tool according to claim 4, wherein said cam mechanism further comprises at least one rolling element (24) arranged to engage said first and said second contact surface (21a; 21b).
Power tool according to claim 5, wherein said at least one rolling element is arranged between said first and second contact surface.
Power tool according to any of the preceding claims1-6, wherein said first and said second element each comprises a portion (21p; 22p) having a substantially cylindrical shape, and wherein said respective contact surface are arranged along an outer portion OP of a respective substantially circular cross section CS of said cylindrical portions of said first and second element.
Power tool according to any of the preceding claims 1-7, wherein said second element is a clutch unit comprising a contact element (221) and a sleeve element (222), wherein said contact element comprises said second contact surface, wherein said sleeve element is arranged to connect said contact element and said socket and wherein said sleeve element encloses said first element and said contact element.
Power tool according to any of claims 4-8, wherein said mechanism comprises a plurality of rolling elements.
Power tool according to any of the preceding claims 1-3, wherein said mechanism is a screw mechanism such that said relative rotation between said first and second element results in a change in the axial distance between said first and second element.
Front part for a power tool adapted to apply torque to a bolt, said front part having an end surface (111a) oriented normal to said input axis and adapted to selectively bear against a surface of a work piece (WP), said front part comprising
an input shaft (10) adapted to be connected to a rotating output shaft of said power tool,

a front part housing comprising means for connection to a housing of said power tool,

a radially protruding element (101) arranged on said input shaft, and bearing against a shoulder (111b) formed in an inner side of said front part housing such that an axial load may be transferred from said shaft to said housing

a socket (12) adapted to engage a head of a bolt protruding from said work piece surface such that a pulling force may be exerted on said bolt by said socket in an axial direction and to apply a torque so said bolt, and

a mechanism (20) connecting said socket and said input shaft,
said mechanism comprising:
a first element (21) engaging said drive shaft

a second element (22) engaging said socket, and

a connecting arrangement (23) arranged to provide a connection allowing a variable axial distance between said first and second element,
wherein said first and second element (21,22) are coaxially arranged with respect to said input shaft (10) and wherein said connecting arrangement is further arranged to provide a connection such that first and second element are further movably arranged between a first relative angular position and a second relative angular position, and

wherein said variable axial distance between said first and second element decreases, in use when torque is applied, as the first and second element move from the first relative angular position to the second relative angular position,

thereby providing an axial force acting on said input shaft and further transferred to said housing via said shoulder, pressing said end surface of said housing against said work piece surface whereby said axial force pressing said end surface of said housing against said work piece surface is proportional to the torque applied.