WO2025237675A1

WO2025237675A1 - Power tool and two-speed gear assembly for a power tool

Info

Publication number: WO2025237675A1
Application number: PCT/EP2025/061724
Authority: WO
Inventors: Jonas MILLINGER; Daniel Hallberg
Original assignee: Atlas Copco Industrial Technique AB
Current assignee: Atlas Copco Industrial Technique AB
Priority date: 2024-05-16
Filing date: 2025-04-29
Publication date: 2025-11-20
Anticipated expiration: 2026-11-16
Also published as: SE547707C2; SE2430272A1

Abstract

The present specification relates to a power tool comprising a two-speed power transmission comprising a planetary gear (4) and a gear shift assembly for directing torque through or past the gear, comprising a driving member (2), a driven member (1), a movable coupling sleeve (6) arranged to, in a first position, intercouple the driving member (2) and the driven member (1) and in a second position the planetary gear (4) and the driven member (1) and comprising a magnet (7) and a linear actuator (10) arranged to effect a displacement of the sleeve (6), comprising a first and a second coil (11, 12) arranged to generate a first and second magnetic field to effect the axial movement of the sleeve such that the first and second magnetic fields, in use, have opposite directions The present specification also relates to a two-speed power transmission for such a power tool.

Description

POWER TOOL AND TWO-SPEED GEAR ASSEMBLY FOR A POWER TOOL

Technical field

The present invention generally relates to power tools, more particularly to a power tool comprising a two-speed gear assembly.

Technical Background

Different types of power tools are known to be used in various industries, where one common type is tightening tools used for tightening of screws or bolts.

One problem known to cause various design challenges in the field is that the working conditions and the requirements on the expected output tend to vary a lot during use for example with regards to the speed and torque required during different parts of a typical working operation. This is the case for example for the tightening tools mentioned above as during tightening of screws or nuts during the initial phase of the tightening, i.e. the so called run-down, the torque needed is low whereas the rotation speed should ideally be high in order to reduce the time required for the operation whereas during the actual tightening phase, i.e. during the actual tightening of the joint, the torque required is higher.

Power tools subjected to these types of varying requirements are however known to, in order to be able to provide the desired torque levels, include one or more transmissions such as for example one or more planetary gear steps connected in series.

But, using such transmissions to achieve the high torque levels required for actual tightening, the rotational speed provided decreases correspondingly thus resulting in an undesired low rotational speed also during run-down. This becomes a significant problem particularly in applications where screws are tightened to very high torque values, where commonly a very low RPM results from the provision of the desired high torque, which in turn commonly makes the rundown phase unreasonably slow.

In order to alleviate some of these problems, attempts have therefore been made to use two-speed power transmissions, i.e. transmissions where the force flow through the gear unit may be such that a higher rotational speed may be used during run-down and the high torque/low speed mode only when needed during actual tightening.

However, there are many problems associated with such transmissions remaining. For example, in designs utilizing solenoids to alter the force flow through the gear unit, known problems include that the strong position dependency associate with solenoids significantly impacts the reliability of gear switching and high current consumption. Other known problems are related to designs having a high complexity and requiring a lot of space.

Hence, there exists a need for improvement in the field of two-speed gear solutions for power tools.

Summary of the invention

Accordingly, it would be desirable to provide a power tool having an improved two speed transmission where energy consumption may be reduced. In particular, it would be desirable to provide such a transmission having a more simple and compact design.

To better address one or more of these concerns a power tool and a two-speed power transmission for such a power tool as defined in the independent claims is provided. Preferred embodiments are defined in the dependent claims.

According to a first aspect of the invention a power tool is provided comprising a motor, an input shaft drivingly connected to the motor, an output shaft and a two-speed power transmission, the two-speed power transmission comprising a planetary gear and a gear shift assembly for directing torque through the planetary gear in a high torque/low speed drive mode or past the planetary gear in a low torque/high speed drive mode, the gear shift assembly comprising a driving member drivingly connected to the input shaft, a driven member drivingly connected to the output shaft, an axially movable coupling sleeve arranged to, in a first position, intercouple the driving member and the driven member and to, in a second position, intercouple the planetary gear and the driven member, the coupling sleeve comprising a permanent magnet, wherein the gear shift assembly further comprises a linear actuator arranged to effect a displacement of the coupling sleeve between the first and second positions, wherein the linear actuator comprises a first coil and a second coil arranged to, in use, generate a first and second magnetic field, respectively, to effect the axial movement of the sleeve, and wherein the first and second coils are configured such that the first and second magnetic fields, in use, have opposite directions.

According to the first aspect, the two-speed transmission of the power tool provides an inventive solution to the concerns described above by means of a design incorporating a gear shift mechanism directing torque through a planetary gear step in a high torque /low speed drive mode or past the planetary gear in a lowtorque/high speed drive mode, having a simple compact design where reduced energy consumption is provided by means of the utilization of a coupling sleeve comprising a permanent magnet displaced by means of two coils generating two magnetic fields.

More particularly, the design comprising two coils configured such that the magnetic fields generated may have opposite directions acting on a magnetic coupling sleeve, results in a more linear force regardless of position of the coupling sleeve and lower peak currents, reducing power consumption and losses and improving the reliability of switching. Further, a low number of moving parts provides for a simple, compact design.

Further, as the magnetic field(s) generated by the coils acts on the permanent magnet comprised by the coupling sleeve, an even force may be effected on the coupling sleeve reducing the risk of the coupling sleeve getting stuck or misaligned when it is pulled or pushed, increasing friction and making it difficult to move smoothly (i.e. a drawer effect).

The first and second magnetic field may for example, in use, be described as one field pulling the coupling sleeve and one field pushing on the coupling sleeve. The linear actuator may also be referred to as a linear motor.

The permanent magnet(s) may for example be made from a rare-earth permanent magnet alloy, examples include Neodymium Iron Boron (NdFeB) or Samarium Cobolt (SmCo). The permanent magnet(s) may further have either one of an axial or radial magnetization pattern, a combination of them both (i.e., Halbach array), or sinusoidal magnetization.

In order to increase the efficiency of the linear motor, as much as possible of the magnetic flux from the permanent magnets should preferably pass through the first and second coil. This may for example be achieved by making the coupling sleeve of a nonmagnetic material. Examples include austenitic steel, aluminum, brass and plastic.

The power tool may further comprise a housing, such as a housing enclosing among others the linear actuator. The housing may in some embodiments be made of a soft magnetic material, such as carbon- or silicon steel, which may also contribute to improved efficiency.

The first coil and the second coil may for example be arranged close to/adjacent the coupling sleeve/permanent magnet such that the magnetic fields generated act directly on the sleeve to effect the axial movement. The first coil and a second coil may hence be arranged to, in use, effect the axial movement of the sleeve by generating a first and second magnetic field acting directly on the coupling sleeve, i.e. not by means of/via an intermediate component.

In some embodiments, the first and/or second coil are annular coils, the first and/or second coil may further be coaxially arranged with respect to the coupling sleeve (and/or the permanent magnet) which may contribute to an even resulting force being effected on the coupling sleeve.

According to one embodiment, the polarity of the first and/or the second coil may be changed in order to change shifting direction. I.e., in order to either switch from first to second position or the other way around.

According to one embodiment, the first and second coils are arranged axially offset such that, in use, one of the coils exerts a pulling force and the other a pushing force on the coupling sleeve. The coils may also be described as successively arranged (in the axial direction), in some embodiments as arranged adjacent one another. The axial direction may be a direction defined by the driving member, and/or by an axis of the coupling sleeve.

According to one embodiment, the first and second coils are wound in opposite directions, i.e. the winding direction is opposite. Such coils may also be described as opposing coils. According to one embodiment, the two coils are (electrically) connected in series. Such serially connected coils may be powered by the same current supply and may thus only need one DC-current to function.

According to one embodiment, the first and second coils are wound in the same direction and are further arranged to be powered by separate currents sources/supplies. Hereby, the direction of the magnetic field may be controlled by a respective first and second current each powering one of the coils separately.

According to one embodiment, the gear shift assembly is further arranged to generate a force for maintaining the coupling sleeve in the first and/or the second position. Such a force may be selectively generated, i.e. it may be generated when appropriate or needed, for example when the coupling sleeve is positioned in the first and/or the second position only.

According to one embodiment, the first and/or the second coil is/are arranged to generate a holding magnetic field for maintaining the coupling sleeve in the first and/or the second position. The holding field may be a weaker magnetic field compared to the respective first and second magnetic field utilized for displacing the coupling sleeve. In one embodiment, the power tool comprises a device arranged to exert a force for maintaining the coupling sleeve in the first and/or the second position, the device may be arranged to generate a holding current inducing a weaker magnetic field in the first and/or second coil for maintaining the coupling sleeve in the first and/or the second position.

According to one embodiment, the linear actuator is a bistable linear actuator. The bistable linear actuator may be an actuator which remains stable in both positions without relying on a constant power supply. For example, the bistable linear actuator may be configured for activation (e.g. moving from the first to the second position) by means of the coils, may be held in the second position without requiring energy to remain there until activated to be brough back to the first position by means of the coils.

According to one embodiment, the gear shift assembly (and/or the tool) further comprises a portion made from a ferrous material to which the coupling sleeve is attracted in the first and/or second position, more particularly the permanent magnet comprised by the coupling sleeve. The portion of ferrous material, such as steel, may for example be a portion of the tool body or housing. Hereby, the position of the coupling sleeve (in the first and second position) may be maintained by permanent magnetic forces, i.e. without additional current consumption. The amount of steel may be matched to the magnet force, such that a holding force strong enough to hold the sleeve in place and still be overcome by the coil magnetic field may be achieved. The gear shift assembly and/or the tool may in some embodiments comprise two magnetic portions for maintaining the coupling sleeve in the first and the second position respectively.

According to one embodiment, the permanent magnet is an annular magnet arranged on an outer side of the coupling sleeve. Hereby, the magnet may be positioned close to the coils. In other embodiments, the permanent magnet may be arranged on an inner side of the coupling sleeve or integrated in (forming part of) the coupling sleeve. An annular permanent magnet may also contribute to a more even resulting force to be effected on the coupling sleeve (magnet), reducing the risk of the coupling sleeve getting stuck or misaligned when it is pulled or pushed.

In some embodiment, the coupling sleeve comprises a plurality of permanent magnets. The magnets may be (evenly) distributed along a circumference of the coupling sleeve, which may contribute to a more even resulting force as described above.

A suitable engagement or coupling may be provided between the coupling sleeve and the driven member, allowing the coupling sleeve to slide axially along the driven member while rotating along with the driven member, for example by means of a provision of congruent surfaces.

According to one embodiment, the driven member comprises a first engaging portion adapted to engage a corresponding congruent first portion of the coupling sleeve, and the planetary gear may comprise a planet wheel carrier comprising a second engaging portion adapted to engage a corresponding congruent second portion of the coupling sleeve. For example, according to one embodiment, at least one of the first and second engaging portion have a polygonal shape where examples include a square- or hex shaped cross section.

According to one embodiment, the driving member comprises a first engaging portion adapted to, in the low torque/high speed mode, engage (and/or cooperate) with a matching first end portion of the coupling sleeve. The first engaging portion may be arranged on, comprised by or integrally formed with the driving member. The first end portion may be arranged on, comprised by or integrally formed with the coupling sleeve.

The coupling sleeve may be adapted to slide into engagement with the driving member when entering the first position. In some embodiments, the engagement (or cooperation) between the first engaging portion (device) and the first end portion is such that a limited relative rotation is allowed, i.e. it may involve a small play. This may facilitate the transition into engagement as the coupling sleeve moves into the first position.

According to one embodiment, at least one of the first engaging portion and the first end portion has a polygonal cross section.

According to one embodiment, the planetary gear comprises a planet wheel carrier comprising a second engaging portion adapted to, in the high torque/low speed mode, engage with a matching second end portion of the coupling sleeve. The second end portion may be arranged on, comprised by or integrally formed with the coupling sleeve.

The first and second end portion of the coupling sleeve may be end portions arranged on a first side (or end) of the coupling sleeve. The first side may be a rear side or end of the coupling sleeve, i.e. a side or end facing the motor of the tool. The first end portion may for example be arranged on an inner side of the coupling sleeve, and the second end portion may for example be arranged on an outer side of the coupling sleeve.

The first and second end portion of the coupling sleeve may further be arranged at substantially the same axial position of the coupling sleeve, i.e. they may be (in a sense) coaxially arranged. In some embodiments, the coupling sleeve is concentrically arranged with respect to the planetary gear.

According to one embodiment, at least one of the second end portion and the second engaging portion comprises teeth and/or splines. The second end portion may be adapted to slide into engagement with the planet carrier when entering the second position. In one embodiment, the first and second position is a first and a second end position, wherein the coupling sleeve is continuously movable there between and may be positioned at any position between the first and second end position.

According to one embodiment, the power tool further comprises a position sensor arranged to sense a position of the coupling sleeve. Examples include an internal hallsensor. Hereby, the position of the coupling sleeve may be continuously determined and monitored.

In one embodiment, the linear actuator and/or the tool further comprises, or may be connected to, a control unit configured to for example monitor and control the movement of the coupling sleeve and/or configured to communicate with for example an external control unit configured to control the power tool.

In one embodiment, the power tool further comprises means for monitoring the torque delivered by the tool. The means for monitoring a torque delivered by the tool may comprise for example a sensor such as a torque transducer. Other examples include circuitry adapted to monitor the motor current or other internally provided data related to the performance of the motor.

With regards to the power tool as such, according to one embodiment, the motor is an electric motor. The tool may for example be an electrical stationary or fixtured power tool chosen from the group comprising a screwdriver, a nutrunner, a drill and a grinder. The skilled person however realizes that only slight modification of the structure would be required for use with hand-held tools. In some embodiments, the power tool may be a battery powered tool.

The tool may further comprise, or may be connectable to, a (external) control unit operative to control the power tool.

According to a second aspect of the present invention, a two-speed power transmission for a power tool according to any of the embodiment described in the foregoing is provided, the transmission comprising a planetary gear and a two-speed gear shift assembly for directing torque through the planetary gear in a high torque/low speed drive mode or past the planetary gear in a low torque /high speed drive mode, the gear shift assembly comprising a driving member drivingly connected to the input shaft, a driven member drivingly connected to the output shaft, an axially movable coupling sleeve arranged to intercouple in a first position the driving member and the driven member and to intercouple in a second position the planetary gear and the driven member, the coupling sleeve comprising a permanent magnet, wherein the assembly further comprises a linear actuator arranged to effect a displacement of the coupling sleeve between the first and second position, wherein the linear actuator comprises a first and a second coil, arranged to in use generate a respective first and second magnetic field to effect the axial movement of the sleeve and wherein the first and second coil are further configured such that the first and second magnetic fields have opposite directions.

Objectives, advantages and features of the transmission 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.

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 1 is a cross sectional view of an exemplary power tool comprising a two-speed power transmission according to one embodiment.

Figure 2 is a cross sectional view of an exemplary power tool comprising a two-speed power transmission 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. 1 and 2 are cross sectional views of a portion of an exemplary power tool 1 according to one embodiment. The tool comprises a housing H, an input shaft and a motor (not shown), an output shaft OS and a two-speed transmission T arranged between the input shaft and the output shaft OS. Further, the tool may comprise an internal control unit or be connectable to an external control unit operative to control the tool 1. The housing may for example comprise or be made of a soft magnetic material.

The two-speed power transmission T of the embodiment shown in fig. 1 comprises a planetary gear 40 and a gear shift assembly for directing torque from the input shaft to the output shaft OS through the planetary gear 4 in a high torque/low speed drive mode or past the planetary gear 4 in a low torque /high speed drive mode. The transmission is shown in fig. 1 in the low torque /high speed drive mode and in fig. 2 in the high torque/low speed mode.

The planetary gear 4 comprises a ring gear (or gear rim) 42 secured in the housing H and a planet wheel carrier 40.

The gear shift assembly comprises a driving member 2, drivingly connected to the motor via the input shaft at a first end 2a, and a driven member 3 drivingly connected to the output shaft OS. A sun gear 23 adapted to engage the planets engaging the ring gear 43 may be arranged on the driving member 2.

An axially movable coupling sleeve 6 is axially movable between a first and a second position. The sleeve 6 may further be arranged to, in the first position, intercouple the driving member 2 and the driven member 3 and to, in the second position, intercouple the planetary gear 4 and the driven member 3. A suitable engagement or coupling may be provided between the coupling sleeve 6 and the driven member 3, allowing the coupling sleeve 6 to slide axially along the driven member 3 but rotate along with the driven member 3, for example by means of congruent surfaces. The coupling sleeve 6 may comprise and/or be made of a non-magnetic material.

The coupling sleeve 6 may further comprise a permanent magnet 7, for example an annular permanent magnet 7 arranged on an outer side of the sleeve 6. A linear actuator 10 may be used to displace the coupling sleeve between the first and second position, more particularly to effect (cause) a displacement of the coupling sleeve 6 between the first and second position.

The linear actuator 10 may comprise a first coil 11 and a second coil 12 arranged to, in use, generate a first and second magnetic field, respectively, to effect (i.e. cause or bring about) the axial movement of the sleeve 6, by attracting or repelling the permanent magnet 7.

The first and second coil 11, 12 may further be configured such the first and the second magnetic fields, in use, have opposite directions. Hereby, the first coil 11 may exert a pulling force on the permanent magnet 7 and thus the sleeve 6 while the second coil 12 exerts a pushing force, or the other way around. To this end, the coils 11, 12 may as can be seen from fig 1 be arranged axially offset, or axially successively.

To achieve the opposite first and second magnetic fields, the coils 11, 12 may be wound in opposite directions, and may further be connected in series and supplied by a common, or shared, power supply such as a DC-power supply (not shown). Alternatively, the coils 11, 12 may be wound in the same direction and each be supplied by a separate first and second current supply (not shown).

To ensure reliable switching and operation, the gear shift assembly may further be arranged to generate a force for maintaining the coupling sleeve 6 in the first and/or the second position. The first and/or second coils 11,12 may for example be arranged to generate a holding magnetic field for maintaining the coupling sleeve 6 in the first and/or the second position. Such a holding field may be a weaker field compared to the filed(s) used to displace the coupling sleeve 6. Alternatively, the power tool 1 may comprise a portion made of a ferrous material to which the permanent magnet 7 may be attracted in the first and/or second position. Such a ferrous portion may be realized for example by a bearing 8 or the planet carrier 40, or by a separate item such as a steel ring or the like.

The power tool 1 may further comprise a hall sensor (not shown) configured to sense a position of the coupling sleeve 6.

In the low torque/high speed drive mode shown in fig 1, the driving member 2 and the driven member 3 are intercoupled in direct drive where torque is directed past the planetary gear 4. The driving member 2 may for example comprise a first engaging portion 21 adapted to, engage or cooperate with a matching first end portion 61 of the coupling sleeve 6. As may be seen from fig. 1 the first engaging portion 21 may be a separate element 21 arranged on the driving member 2, or alternatively comprised by or integrally formed with the driving member 2. The first end portion 61 may be arranged on, comprised by or integrally formed with the coupling sleeve 6. The first end portion 61 may for example be arranged on an inner side of the coupling sleeve 6, at a back or rear side (end) 6b of the sleeve facing the motor side of the tool 1.

To provide a secure engagement, the first engaging portion 21 and the first end portion 61 may have polygonal cross sections. The engagement may further be such that a limited relative rotation is allowed, i.e. it may involve a small play. This may facilitate the transition into engagement as the coupling sleeve 6 moves into the first position.

In the high torque/low speed drive mode shown in fig. 2, the planetary gear 4 and the driven member 3 are intercoupled and the torque is thus directed through the planetary gear 4. The planet carrier 40 may for example comprise a second engaging portion 43 adapted to engage with a matching second end portion 62 of the coupling sleeve 6.

To provide a secure engagement, the second end portion 62 and the second engaging portion 43 may comprise teeth or splines, such that the second end portion 62 may be able to slide into engagement with the planet carrier 40 when entering the second position.

The second end portion 62 may be arranged on, comprised by or integrally formed with the coupling sleeve 6. The second end portion 62 may for example be arranged on an outer side of the coupling sleeve 6, at the back or rear side (end) 6b of the sleeve facing the motor.

The first and second end portions 61,62 of the coupling sleeve 6 may hence both be arranged on the first side 6b of the coupling sleeve 6, here the rear end 6b. Hereby, the first and second end portion 61,62 of the coupling sleeve may further be arranged at substantially the same axial position of the coupling sleeve.

In operation, the power tool of the illustrated embodiment may comprise an internal control unit (not shown) or form part of a system comprising an external control unit (not shown) operative to control the tool. The coupling sleeve 6 may initially be maintained in the first position to perform rundown - i.e. low torque/high speed mode, for example by means of a small holding current generating a weaker magnetic field in the coils 11, 12.

Run-down ends when snug is reached, this may be identified by the control unit for example by monitoring that a motor current limit threshold is passed or by means of a suitable sensor or the like. As the linear actuator receives a signal that run-down is complete, displacement of coupling sleeve 6 forwards towards the second position starts by generating a stronger magnetic field by means of a higher current through the coils 11, 12. With reference to fig 1, the fields may be such that the first field generated by the first coil 11 exerts a pulling force on the magnet 7 and hence on the sleeve 6 and the second magnetic field generated by the second coil 12 exerts a pushing force on the sleeve 6.

The position of the coupling sleeve 6 may be monitored by means of a hall sensor such that it may be determined when the coupling sleeve engages the planetary gear 4, i.e. when the sleeve 6 has reached the second position (shown in fig. 2). A signal may be sent to the control unit and the final and actual tightening phase (i.e. high torque /low speed mode) may start. The coupling sleeve 6 may be maintained in the second position for example by means of a small holding current or a ferrous portion of the tool as explained below.

During the final part of the tightening, the torque may be monitored by a torque transducer (not shown) and when the desired torque is reached the linear actuator may receive a signal, reverse the polarity (e.g. by changing the direction of the current) such that the field generated by the first coil 11 exerts a pushing force and the field generated by the second coil 12 a pulling force, and the coupling sleeve 6 moves back to the first position to engage high speed/low torque mode again (fig. 1).

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 as defined in the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, form a study of the drawings, the disclosure and 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

1. Power tool (1) comprising: a motor an input shaft (IS) drivingly connected to the motor; an output shaft (OS); and a two-speed power transmission (T), said two-speed power transmission comprising a planetary gear (4) and a gear shift assembly for directing torque through said planetary gear (4) in a high torque/low speed drive mode or past said planetary gear (4) in a low torque/high speed drive mode; said gear shift assembly comprising: a driving member (2) drivingly connected to said input shaft, a driven member (3) drivingly connected to said output shaft, an axially movable coupling sleeve (6) arranged to, in a first position, intercouple said driving member (2) and said driven member (3) and to, in a second position, intercouple said planetary gear (4) and said driven member (1), said coupling sleeve (6) comprising a permanent magnet (7), wherein said gear shift assembly further comprises a linear actuator (10) arranged to effect a displacement of said coupling sleeve (6) between said first and second positions, wherein said linear actuator (10) comprises a first coil (11) and a second coil (12) arranged to, in use, generate a first and second magnetic field, respectively, to effect said axial movement of said sleeve, and wherein said first and second coils are configured such that said first and second magnetic fields, in use, have opposite directions.

2. Power tool according to claim 1, wherein the polarity of the first and/or the second coil may be changed in order to change shifting direction.

3 . Power tool according to claim 1 or 2, wherein said first and second coils are arranged axially offset such that, in use, one of the coils exerts a pulling force and the other a pushing force on said coupling sleeve.

4. Power tool according to any one of the preceding claims, wherein said first and second coils are wound in opposite directions.

5. Power tool according to claim 4, wherein the two coils are connected in series.

6. Power tool according to any one of the preceding claims 1-3, wherein said first and second coils are wound in the same direction and are further arranged to be powered by separate currents sources/supplies.

7. Power tool according to any one of the preceding claims, wherein said gear shift assembly is further arranged to generate a force for maintaining the coupling sleeve in said first and/or said second position.

8. Power tool according to claim 7, wherein said first and/or said second coil is/are arranged to generate a holding magnetic field for maintaining said coupling sleeve in said first and/or said second position, said holding field being a weaker magnetic field compared to the respective first and second magnetic field.

9. Power tool according to claim 7, wherein said linear actuator is a bistable linear actuator.

10. Power tool according to claim 7, wherein said gear shift assembly further comprises a portion made from a ferrous material to which the permanent magnet is attracted in said first and/or second position.

11. Power tool according to any one of the preceding claims, wherein said permanent magnet is an annular magnet arranged on an outer side of the coupling sleeve.

12. Power tool according to any one of the preceding claims, wherein said driving member (2) comprises a first engaging portion (21) adapted to, in said low torque /high speed mode, engage with a matching first end portion (61) of said coupling sleeve.

13. Power tool according to claim 12, wherein at least one of said first engaging portion and said first end portion has a polygonal cross section.

14. Power tool according to any one of the preceding claims, wherein said planetary gear comprises a planet wheel carrier (40) comprising a second engaging portion (43) adapted to, in said high torque /low speed mode, engage with a matching second end portion (62) of said coupling sleeve.

15. Power tool according to claim 14, wherein at least one of said second end portion and said second engaging portion comprises teeth and/or splines.

16. Power tool according to any one of the preceding claims, further comprising a position sensor arranged to sense a position of said coupling sleeve.

17. Two-speed power transmission for a power tool according to any one of the preceding claims, said transmission comprising a planetary gear and a two-speed gear shift assembly for directing torque through said planetary gear in a high torque/low speed drive mode or past said planetary gear in a low torque/high speed drive mode; said gear shift assembly comprising a driving member (2) drivingly connected to said input shaft, a driven member (1) drivingly connected to said output shaft, an axially movable coupling sleeve (6) arranged to intercouple in a first position said driving member (2) and said driven member (1) and to intercouple in a second position said planetary gear (4) and said driven member (1), said coupling sleeve comprising a permanent magnet, wherein said assembly further comprises a linear actuator (10) arranged to effect a displacement of said coupling sleeve between said first and second position, wherein said linear actuator comprises a first and a second coil (11, 12), arranged to in use generate a respective first and second magnetic field to effect said axial movement of said sleeve, and wherein said first and second coil are further configured such that said first and second magnetic fields have opposite directions.