WO2023110230A1

WO2023110230A1 - Power tool and load-responsive power transmission for a power tool

Info

Publication number: WO2023110230A1
Application number: PCT/EP2022/081307
Authority: WO
Inventors: Johan RÖNBLOM
Original assignee: Atlas Copco Industrial Technique Ab
Priority date: 2021-12-17
Filing date: 2022-11-09
Publication date: 2023-06-22
Also published as: SE2130369A1

Abstract

The present specification relates to a power tool comprising a two-speed power transmission. The two-speed power transmission comprising a planetary gear assembly and a load responsive gear shift mechanism for directing torque through the planetary gear assembly in a high torque/low speed drive mode or past the planetary gear assembly in a low torque/high speed drive mode, wherein the planetary gear assembly is a two stage gear mechanism comprising a first and a second planetary gear stage, wherein the first planetary gear stage comprises a first sun wheel, wherein the second planetary gear stage comprises a second sun wheel, wherein the second sun wheel is a hollow sun wheel, and wherein the input shaft is arranged to extend through the hollow sun wheel thereby allowing for bypassing of second planetary gear stage in the low torque/high speed drive mode.

Description

POWER TOOL AND LOAD -RESPONSIVE POWER TRANSMISSION FOR A POWER TOOL

Technical field

The present invention generally relates to power tools , more particularly to a load-responsive transmission for a power tool .

Technical Background

Di f ferent types of power tools are known to be used in various industries , where one common type is power tools used for tightening of screw 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 di f ferent 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 j oint , the torque required is higher .

Power tools subj ected to these types of varying requirements are known to , in order to be able to for example provide the desired torque levels , include di f ferent types of transmissions . However, using transmissions to provide these higher torques , has the disadvantage that the rotational speed provided decreases correspondingly thus resulting in an undesired lower rotational speed also during run down . This becomes a signi ficant problem particularly in the field of power tools adapted to tighten large screws 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 comprising for example torque responsive gear shi ft mechanisms , i . e . transmissions where the force flow through the gear unit , and hence possibly the gear ratio provided, depends on the torque level such that a higher rotational speed may be used during run down and the high torque/ low speed mode only when needed .

However, there are many problems associated with such transmissions remaining . This include problems relating to heat generation which may be considerable , especially at the end of the tightening when the torque increases as the motor current may become very high . Such excessive heat generation is undesired not only for the motor but also for electronics and batteries where such are used . Hence , there exists a need for improvement in the field of torque-responsive gear units for power tools .

Summary of the invention

Accordingly, it would be desirable to provide an improved two- speed gear unit or power transmission for a power tool . In particular, it would be desirable to provide such a transmission, where heat generation is at least reduced . To better address one or more of these concerns a power tool comprising a two-speed power transmission and a power transmission is provided . Preferred embodiments are defined in the dependent claims .

According to a first aspect of the invention a power tool is provided, the tool comprising a housing, a motor, an input shaft connected to the motor, an output shaft and a two-speed power transmission . The two-speed power transmission comprises a planetary gear assembly and a load responsive gear shi ft mechanism for directing torque through the planetary gear assembly in a high torque/ low speed drive mode or past the planetary gear assembly in a low torque/high speed drive mode . Wherein the planetary gear assembly is a two stage gear mechanism comprising a first and a second planetary gear stage , wherein the first planetary gear stage comprises a first sun wheel connected to the input shaft , a first planet wheel carrier and at least one first planet wheel in engagement with the first sun gear and engaging a ring gear secured in the housing , wherein the second planetary gear stage comprises a second sun wheel connected to the first planet carrier, a second planet wheel carrier and at least one second planet wheel in engagement with the second sun gear and engaging the ring gear secured in the housing, wherein the second sun wheel is a hollow sun wheel , and wherein the input shaft is arranged to extend through the hollow sun wheel thereby allowing for bypassing of second planetary gear stage in the low torque/high speed drive mode .

According to the first aspect , the two-speed transmission and hence the power tool provides an inventive solution to the concerns described above by means of a design incorporating a load responsive gear shi ft mechanism automatically 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 depending on the torque level , where the problems described above relating to heat generation are mitigated by means of a two stage gear mechanism comprising a first and a second planetary stage , where the second stage comprise a hollow sun gear through which the input shaft extends . As the gear unit shi fts over two stages having a combined higher gear ratio , the motor torque at the end of the tightening is lowered correspondingly, which in turn lowers heat losses as the heat losses are proportional to the current level squared .

The skilled person reali zes that the two-stage planetary gear mechanism works equally well with any type of load responsive mechanism for directing torque through the planetary gear assembly in a high torque/ low speed drive mode or past the planetary gear assembly in a low torque/high speed drive mode , both manually controlled shi fting mechanisms and automatic mechanism including mechanically actuated mechanism as well as mechanisms actuated by an actuator based on sensor data or similar .

According to one embodiment , the second sun wheel and the first planet carrier are integrally formed . The second sun wheel may in such an embodiments be described as forming part of the first planet carrier, and may for example be formed in and extend from the center of the carrier . According to one embodiment , the first sunwheel forms part of the input shaft .

Further, as the first and second planetary gear engage the same ring gear, the number of cogs on the respective sun wheel will be decisive for the gear ratio . For example , according to one embodiment , the gear ratio of the first gear stage is larger or equal to 5 . In one embodiment , the gear ration lies in the interval 5 . 5- 6 . 5 . Further, according to one embodiment , the gear ratio of the second gear stage lies in the range 2-5 , preferably 3-4 . In one embodiment , the gear ration lies in the range 2 . 75-3 . 25 .

By the wording load-responsive should be understood a transmission where the mode of operation depends on the load under which the transmission operates . For example , according to one embodiment , the load responsive gear shi ft mechanism is a speed responsive gear shi ft mechanism . In one embodiment , such a speed responsive mechanism may comprise a centri fugal coupling or clutch . According to one embodiment , the load responsive gear shi ft mechanism is a torque responsive gear mechanism .

According to one embodiment , the torque responsive transmission is a first stage gear unit . I . e . , the input shaft in such an embodiment is directly connected to the motor, or even in some cases arranged in motor module of the tool . This is particularly advantageous in that a more compact and slim design of the tool may be achieved .

According to one embodiment , the power tool further comprises a front gear unit , or second stage gear unit . In such an embodiment the torque responsive transmission may be arranged between the motor and the front- or second stage gear unit . This is advantageous in that the front gear unit for such a tool may be an interchangeable front gear unit , such that the front gear unit may be changed depending on the desired torque while the torque-responsive transmission remains in or on the tool .

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 hand-held power tool chosen from the group comprising a screw driver, a nut runner, a drill and a grinder . The skilled person however reali zes that only slight modi fication of the structure would be required for use with a stationary or fixtured tools . In some embodiment , the power tool may be a battery powered tool . In one embodiment , the power tool is a tool providing a higher tightening torque , for example in the range 3500-4500 Nm, in some cases up to 8000 Nm .

According to one embodiment , the power tool further comprises means for monitoring a quantity indicative of a torque delivered by the tool and a control unit operative to control the rotational speed of the motor based on the monitored quantity indicative of the torque . In one embodiment , the means for monitoring a quantity indicative of a torque delivered by the tool may comprise at least one sensor for sensing a quantity indicative of a torque delivered . For example , the ring gear may be secured in the housing by means of a torque transducer such that readings of the torque trans ferred may be obtained . Further, the means for monitoring a quantity indicative of a torque may comprise circuitry ( i . e . a circuit arrangement ) adapted to monitor the motor current or other internally provided data related to the performance of the motor .

Hence , the control unit may be configured to control the tool by comparing the value of the quantity obtained from the sensor to a predetermined threshold value and reduce the rotational speed of the motor when the measured value approaches the threshold value . Hereby, the change between the drive modes of the transmission may be facilitated .

For example , in the low torque/high speed drive mode , the quantity sensed may be a motor current indicative of the torque , such that as the current approaches the threshold value and thus as the torque in this case increases , the rotational speed of the motor may be decreased such that the shi ft to the high torque/ low speed drive mode may be facilitated . Similarly, in the high torque/ low speed drive mode , the quantity sensed may be a torque value from a torque transducer, such that as the torque value approaches the threshold value and thus as the torque in this case decreases , the rotational speed of the motor may be decreased such that the shi ft back to the low torque/high speed drive mode may be facilitated . Accordingly, the at least one sensor may also be a sensor for sensing the motor current or a torque sensor, i . e . a torque transducer, or generally speaking even an internal feature of the control unit monitoring the current or other motor parameters . In one embodiment , the torque transducer is only active in the high torque/ low speed drive mode . The activation of such a transducer may be used as an indicator to the control unit that the transmission has switched to high torque/ low speed mode .

According to one embodiment , the gear shi ft mechanism comprises a driving member connected to the sun wheel of the first planetary gear stage , a driven member connected to the output shaft , and a number of coupling elements arranged to intercouple in a first position the driving member and the driven member and to intercouple in a second position the second planet wheel carrier and the driven member . The driven member comprising a number of axially extending grooves arranged to support the coupling elements for axial displacement of the coupling elements between the first and the second position, the driving member comprising an axially acting first cam means for cooperation with the coupling elements in the first position of the coupling elements . A first axially acting spring means is arranged for biasing the coupling elements toward the first position of the coupling elements , whereby the action of the first spring means counteracts the axial force developed by the first cam means upon the coupling elements , such that the coupling elements are maintained in the first position at torque values below a predetermined level but forced out of the first position by the first cam means at torque values above the predetermined level and the driven member further comprising second, axially acting cam means arranged to exert an axial shi fting force upon the coupling elements toward the second position of the coupling elements against the biasing action of the spring means as the coupling elements have left the first position at torque values above the predetermined level . The second planet wheel carrier is coupled to a coupling sleeve which provides a radial support for the coupling elements in the second position of the coupling elements and which is provided with a number of axially extending tracks for cooperation with the coupling elements in the second position of the coupling elements , and the driving member comprises a number of recesses , wherein the first axially acting cam element ( s ) each form part of respectively one of the recesses , and wherein each of the recesses are adapted for receiving and radially supporting one of the coupling elements in the first position of the coupling elements .

One advantage with this type of transmission is that , as the driving member of the inventive transmission incorporates a number of recesses adapted for receiving and radially supporting the coupling elements in the high speed/ low torque mode , the coupling elements are ef ficiently shielded, i . e . decoupled, from the components rotating at a di f ferent speed without the provision of additional components .

More particularly, the coupling sleeve may in some embodiments provide a radial support for the coupling elements in second position of said coupling elements only i . e . , such that each of said coupling elements are shielded and rotationally decoupled from the coupling sleeve in said high speed/ low torque mode . In one embodiment , the axially acting cam means may be provided in the axially extending grooves .

According to one embodiment , the coupling sleeve is arranged to be axially movable . Hereby, a slight play may be achieved which in turn may be utili zed to handle possible angular misalignment of the coupling elements and the coupling sleeve . Especially when the coupling elements move from the first position to the second position, any misalignment between the coupling elements and the axially extending grooves in the driven member may be handled by the axially movable sleeve instead of causing j amming between parts .

According to one embodiment , the two-speed power transmission further comprises a second axially acting spring means for biasing the axially movable coupling sleeve against of the coupling elements . Hereby, the misalignment may be even more ef ficiently handled as the coupling sleeve pushes slightly against the coupling element , thereby facilitating the return into the recesses by a slight force acting on the coupling elements in the right direction . The coupling sleeve commonly stays in the default position, i . e . adj acent to the driving member, but should the coupling elements become misaligned, the coupling sleeve may spring or move slightly in the axial direction under the bias of the spring allowing the coupling elements to slide into the right position .

According to one embodiment , the second planet wheel carrier comprises an outer sleeve , the sleeve extending in an axial direction and being rotationally locked to the coupling sleeve to intercouple in the second position the second planet wheel carrier and the driven member . In one embodiment , the outer sleeve is rotationally locket to the coupling sleeve by means of a number of balls or other rolling elements arranged in tracks formed on the outside of the coupling sleeve . Accordingly, the torque will in the low speed/high torque mode be directed via the planet carrier, the trans ferring balls , the coupling sleeve , and further via the coupling elements to the driven member . Further, the outer sleeve may in some embodiments be coaxially arranged with the driven and driving member . In one embodiment , there is an axial overlap between the outer sleeve and the portion of the driving member in which the recesses are arranged and/or at least a portion of the coupling sleeve . According to one embodiment , a first axial bearing is provided supporting the driving element against the second planet wheel carrier and a second axial bearing is provided to support the outer sleeve against the housing, such that the force from the first spring means acting on the coupling elements may be absorbed into the tool housing via the first and second bearing . Hereby, an advantageous decoupling between the spring ( i . e . the coupling elements ) and the motor axle is achieved, relieving the motor axle ( i . e . the input shaft ) from the axial force exerted by the spring . Instead, this force may be absorbed by the tool housing . Further advantages include the provision of a design having shorter tolerance chains , i . e . less accumulation of tolerances .

According to one embodiment , the driving member comprises an axial flange , the axial flange being arranged to radially support the coupling elements . This is an advantageous design in that improved radial support and rotational decoupling is provided . In one embodiment , the combined axial extent of the flange and the depth of the recesses is larger than the radius of the balls . Hereby, suf ficient radial support as well as a complete radial decoupling between the balls and the components rotating at a di f ferent rotational speed is ensured in high-speed mode .

In one embodiment , the first spring means comprises a first coil spring which is arranged in a coaxial relationship with the driven member and which exerts a biasing force upon the balls . This is advantageous for example in that a more compact design may be achieved . In one embodiment , the first coil spring bears directly against the coupling elements . In another embodiment , the first coil spring bears against the coupling element via a contact element which is in continuous contact with the coupling elements , examples include a flat ring element . Further, in some embodiment , there may be provided means for varying the setting of the bias force exerted by the spring .

According to a second aspect of the present invention, a power transmission for a power tool according to any one of the embodiments described above is provided .

According to a third aspect of the present invention, a method for controlling a power tool comprising a gear unit according to any of the embodiments described above is provided . According to one embodiment , the method comprises the steps of monitoring a quantity indicative of a first torque value , comparing the first torque value to a predetermined threshold torque value , and controlling the rotational speed of the motor when the measured value approaches the threshold value . According to one embodiment , the step of controlling the rotational speed of the motor involves reducing the rotational speed of the motor when the measured value approaches the threshold value . According to one embodiment , the monitored quantity is at least one of a motor current and a torque transducer value .

Obj ectives , advantages and features of the method conceivable within the scope of the second and third aspect of the invention are readily understood by the foregoing discussion referring to the second aspect of the invention .

Further obj ectives 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 reali ze that di f ferent features of the present invention can be combined to create embodiments other than those described in the following .

Brief description of the drawing

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 la is a cross sectional view of a portion of an exemplary power tool comprising two-speed power transmission for a power tool according to one embodiment , shown in the low torque/high speed drive mode .

Figure lb is a cross sectional view of a portion of the same exemplary power tool comprising two-speed power transmission for a power tool according to one embodiment , shown in the high torque/ low speed mode .

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 . la is a cross sectional view of a portion of an exemplary power tool according to one embodiment , in this case a handheld battery powered tool . The tool comprises a housing 10 , an input shaft 20 , a motor (not shown) connected to the input shaft , an output shaft 15 and a two-speed transmission arranged between the input shaft and the output shaft .

The two-speed power transmission 1 of the embodiment shown in fig . la comprises a planetary gear assembly 18 and a torque responsive gear shift mechanism 19 for directing torque from the input shaft 20 ( i . e . from the motor ) to the output shaft 15 throuqh the planetary gear 18 in a high torque/low speed drive mode or past the planetary gear 18 in a low torque/high speed drive mode . The transmission is shown in fig . la in the low torque/high speed drive mode .

The planetary gear assembly 18 of the illustrated embodiment is a torque-responsive two stage gear mechanism comprising a first and a second planetary gear stage 28 , 38 .

The first planetary gear stage 28 comprises a first sun wheel 281 connected to the input shaft 20 , in the illustrated embodiment three planet wheel in engagement with the sun gear 20 and a first planet wheel carrier 282 engaging a ring gear 22 secured in the housing 10 .

The second planetary gear stage 38 on the other hand comprises a second sun wheel 381 integrally formed with the first planet carrier 282 , planet wheels in engagement with the second sun wheal and a second planet wheel carrier 382 engaging the ring gear 22 . In the illustrated embodiment , the second sun wheel 381 and the first planet carrier 282 are integrally formed, the second sun wheel portion extending in the center of the carrier 282 in a forward direction as may be seen in fig . la . Further, the first sun wheel 281 forms part of the input shaft 20 .

As may be seen from fig . la, the second sun wheel 381 is more particularly a hollow sun wheel , designed such that the input shaft 20 may extend through the hollow sun wheel . Hereby, the planetary gear may be bypassed in said low torque/high speed drive mode .

In the high torque/ low speed drive mode , the torque is directed over the first and the second gear stage hence achieving a larger combined gear ratio - i . e . the shi ft is

RECTIFIED SHEET (RULE 91) ISA/EP over two planetary stages ( at the same time ) . In the illustrated exemplary embodiment, the gear ratio of the first gear stage is 6 , whereas the gear ratio of the second stage is the 3 . 25 .

The exemplary power tool of fig . la comprises a torque- responsive gear mechanism where coupling elements , in the illustrated embodiment three balls 30 , are arranged to intercouple in a first position the driving member 26 and the driven member 27 , i . e . in what is referred to above as the low torque/high speed drive mode, and to intercouple in a second position the second planet wheel carrier 382 and the driven member 27 , i . e . in what is referred to above as the high torque/ low speed drive mode .

Further, the driven member 27 comprises a number of axially extending grooves 36 arranged to support the balls 30 for axial displacement between the first and the second position, whereas the driving member 26 comprises an axially acting first cam means arranged in equally spaced recesses 39 for cooperation with the coupling elements 30 in the first position of the coupling elements 30 .

A first axially acting coil spring 31 is coaxially arranged with respect to the driven member for biasing the balls 30 towards the first position, whereby the action of the coil spring 31 thereby counteracts the axial force developed by the first cam means on the balls 30 . Hereby, the balls 30 are maintained in the first position at torque values below a predetermined level but forced out of the first position by the first cam means at torque values above the predetermined level . In the illustrated embodiment, the spring 31 bears directly against the balls 30 .

The driven member 27 in turn comprises second, axially acting cam means 36b arranged to exert an axial shi fting force upon the balls 30 toward the second position of the balls 30 against the biasing action of the coil spring 31 as the balls 30 have left the first position at torque values above the predetermined level .

Further, in the illustrated embodiment , the second planet wheel carrier 382 is coupled to an axially movable coupling sleeve 29 which provides a radial support for the balls 30 in the second position . Therefore , a number of axially extending tracks for cooperation with the balls 30 are arranged in an inner surface of the coupling sleeve 29 . The number of tracks

RECTIFIED SHEET (RULE 91) ISA/EP in the illustrated embodiment the coupling sleeve 29 is twice the number of balls 30 , i . e . six in the illustrated embodiment . Further, the coupling sleeve 29 is in the illustrated embodiment biased against the balls 30 by second axially acting coil spring 40 , the coil spring 40 being coaxially arranged with respect to the driven member 27 as well as to the first coil spring 31 .

In order to intercouple the second planet wheel carrier 382 and the driven member 27 in the second position, the planet wheel carrier 382 comprises an outer sleeve 32 . This sleeve 32 extends in an axial direction and is rotationally locked to the coupling sleeve 29 by means of a number of smaller balls 32a . Further, in order to handle the forces from the first coil spring 31 acting on the balls 30 , a first axial bearing 33 is provided supporting the driving element 26 against the second planet wheel carrier 382 and a second axial bearing 34 is provided to support the outer sleeve 32 against the housing ( 10 ) , such that the force may be absorbed by the housing .

In operation, the input shaft 20 is connected to an electrical motor, and the output shaft 15 is coupled to a screw j oint to be tightened via a nut socket . The functionality of the transmission and hence the power tool is achieved by the transmission selectively providing a connection between the driving member 26 and the driven member 27 , either bypassing- or via the two-stage planetary gear mechanism depending on the torque level .

When the tightening operation starts , the motor starts delivering a torque through the transmission . In a first stage , as the gear shi ft mechanism 19 occupies a high speed/ low torque drive mode , the balls 30 are seated in the recesses 39 of the driving member 26 and the torque delivered to the driving member 26 via input shaft 20 is trans ferred via the recesses , the balls 30 and the grooves 36 to the driven member 27 , i . e . directly from the driving member 26 to the driven member 27 bypassing, and hence without any influence by, the two stage gear assembly, by the input shaft 20 extending through the hollow sun gear 381 .

As the torque resistance in the screw j oint increases , the first axially acting cam elements apply increasing axial forces upon the balls 30 , and when a predetermined torque level is reached this force supersedes the biasing force of spring 31 and the balls 30 will start moving axially through the grooves 36 , where eventually the cam means 36b will apply an auxiliary axial force on the balls 30 as well , again eventually superseding the force of the spring 31 and thus allowing the balls 30 to complete their axial movement and occupy their second position . Examples of such cam means 36b include sloping side or diverging portions of the respective grooves 36 .

Now, the gear shi ft mechanism 19 has brought the transmission into its high torque/ low speed drive mode - i . e . the torque is now directed over the two-stage transmission . The same embodiment as in fig la is shown in the high torque/ low speed stage in fig . lb . This drive mode is maintained as long as the trans ferred torque is high enough to make the action of the second cam means 36b dominate over the biasing force of spring 31 . When the torque has decreased to that level , i . e . when the predetermined drive mode shi fting point is reached, the force exerted by cam means 36b will no longer dominate over the spring force , and the balls 30 are shi fted back to their first position .

In order to facilitate this intercoupling, more particularly to facilitate the gear change , the power tool as mentioned above comprises a sensor (not shown) , in this case a torque transducer . A control unit , either comprised by the tool or a separate unit , is operative to receive the sensed data from the torque transducer and control the rotational speed of the motor accordingly . More particularly, as the measured torque value approaches the predetermined threshold torque value , i . e . the value at which a gear change is to take place , the control unit reduces the rotational speed of the motor .

As in the illustrated embodiment , the torque transducer is arranged between the housing 10 and the gear rim 22 , it follows that the transducer is only active ( i . e . gives meaningful readings ) in the second drive mode , i . e . high torque/ low speed drive mode , when torque is actually directed over the ring gear the procedure described above using the data from the transducer to control the speed is hence only relevant when determining when to switch from the high torque/ low speed drive mode to the low torque/high speed drive mode .

As the transmission operates in the low torque/high speed drive mode , the control unit instead monitors the motor current by means of a suitable circuit arrangement (not shown) in order to determine that the torque is approaching the threshold value and that the rotational speed therefore should be decreased in order to facilitate the gear change . As an additional functionality, the notion that the torque transducer start delivering torque data may be used by the control unit to confirm that the transmission has switched to and is operating in the high torque/ low speed drive mode .

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 modi fications , variations and alterations are conceivable within the scope as defined in the appended claims . Additionally, variations to the disclosed embodiments can be understood and ef fected 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 di f ferent 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. A power tool comprising: a housing (10) ; a motor; an input shaft (20) ; an output shaft (15) and a two-speed power transmission, wherein said two-speed power transmission comprises a planetary gear assembly (18) and a load responsive gear shift mechanism (19) for directing torque through said planetary gear assembly (18) in a high torque/low speed drive mode or past said planetary gear assembly(18) in a low torque/high speed drive mode; wherein said planetary gear assembly is a two stage gear mechanism comprising a first and a second planetary gear stage (28, 38 ) ; wherein said first planetary gear stage (28) comprises a first sun wheel (281) connected to said input shaft (20) , a first planet wheel carrier (282) and at least one first planet wheel in engagement with said first sun gear and engaging a ring gear (22) secured in the housing 10, wherein said second planetary gear stage (38) comprises a second sun wheel (381) connected to said first planet carrier, a second planet wheel carrier (382) and at least one second planet wheel in engagement with said second sun gear and (381) and engaging said ring gear (22) secured in the housing 10; wherein said second sun wheel is a hollow sun wheel, and wherein said input shaft is arranged to extend through said hollow sun wheel thereby allowing for bypassing of second planetary gear stage in said low torque/high speed drive mode.

RECTIFIED SHEET (RULE 91) ISA/EP

2. Power tool according to claim 1, wherein said second sun wheel and said first planet carrier are integrally formed.

3. Power tool according to claim 1 or 2, wherein said first sun wheel forms part of said input shaft.

4. Power tool according to any one of the preceding claims, wherein the gear ratio of the first gear stage is larger or equal to 5.

5. Power tool according to any one of the preceding claims, wherein the gear ratio of the second gear stage lies in the range 2-5, preferably 3-4.

6. Power tool according to any one of the preceding claims, wherein said load responsive gear shift mechanism is a speed responsive gear shift mechanism.

7. Power tool according to any one of the preceding claims, wherein said load responsive gear shift mechanism is a torque responsive gear mechanism.

8. Power tool according to any one of the preceding claims, wherein said gear shift mechanism (19) comprises a driving member (26) connected to the sun wheel (281) of said first planetary gear stage (28) , a driven member (27) connected to said output shaft (15) , and a number of coupling elements (30) arranged to intercouple in a first position said driving member (26) and said driven member (27) and to intercouple in a second position said second planet wheel carrier (382) and said driven member (27) ; said driven member (27) comprising a number of axially extending grooves (36) arranged to support said coupling elements (30) for axial displacement of said coupling elements (30) between said first and said second position,

RECTIFIED SHEET (RULE 91) ISA/EP 17 said driving member (26) comprising an axially acting first cam means for cooperation with said coupling elements (30) in said first position of said coupling elements (30) ; a first axially acting spring means (31) is arranged for biasing said coupling elements (30) toward said first position of said coupling elements (30) , whereby the action of said first spring means (31) counteracts the axial force developed by said first cam means upon said coupling elements (30) , such that said coupling elements (30) are maintained in said first position at torque values below a predetermined level but forced out of said first position by said first cam means at torque values above said predetermined level; and said driven member (27) further comprising second, axially acting cam means (36b) arranged to exert an axial shifting force upon said coupling elements (30) toward said second position of said coupling elements (30) against the biasing action of said spring means (31) as said coupling elements (30) have left said first position at torque values above said predetermined level wherein said second planet wheel carrier (382) is coupled to a coupling sleeve (29) which provides a radial support for said coupling elements (30) in said second position of said coupling elements (30) and which is provided with a number of axially extending tracks (38) for cooperation with said coupling elements (30) in said second position of said coupling elements (30) , and wherein said driving member comprises a number of recesses (39) , wherein said first axially acting cam element (s) each form part of respectively one of said recesses, and wherein each of said recesses are adapted for receiving and radially supporting one of said coupling elements (30) in said first position of said coupling elements (30) .

RECTIFIED SHEET (RULE 91) ISA/EP 18

9. Power tool according to claim 8, wherein said coupling sleeve is arranged to be axially movable.

10. Power tool according to claim 9, further comprising a second axially acting spring means (40) for biasing said axially movable coupling sleeve (29) against said coupling elements (30) .

11. Power tool according to any one of the preceding claims 8-10, wherein said second planet wheel carrier (382) comprises an outer sleeve (32) , said sleeve extending in an axial direction and being rotationally locked to said coupling sleeve (29) to intercouple in said second position said second planet wheel carrier (382) and said driven member (27) .

12. Power tool according to claim 11, wherein a first axial bearing (33) is provided supporting said driving element against said second planet wheel carrier and wherein a second axial bearing (34) is provided to support said outer sleeve against said housing (10) , such that the force from said first spring means acting on said coupling elements may be absorbed into said housing via said first and second bearing.

13. Power tool according to any one of the preceding claims 8 12, wherein said driving member (26) comprises an axial flange, said axial flange being arranged to radially support said coupling elements.

14. Power tool according to any of the preceding claims, further comprising means for monitoring a quantity indicative of a torque delivered by the tool and a control unit operative to control the rotational speed of the motor based on said sensed quantity indicative of said torque.

RECTIFIED SHEET (RULE 91) ISA/EP