Abstract:
The invention relates to a power tool, in particular to an angle grinder, having a quick clamping device for fastening of a tool between a clamping flange and a counter flange. Such quick clamping devices are usually equipped with an actuation element, during the transfer of which from a clamping position into a release position the clamping is neutralized. In the quick clamping device according to the invention, the actuation element returns, on its own, and in a controlled way, when in the release position and, by accident, the motor of the power tool is turned on. For that purpose, an eccentric being mounted on the actuation element acts with its running surface onto a cam in such a way that it retains the eccentric due to the frictional force acting between the cam and the running surface. A rotation of the cam caused by starting the motor reduces the frictional force in such a way that the cam, being prestressed by a spring, moves, on its own, the actuation element out of the release position into the direction of the clamping position.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a power tool, in particular to an angle grinder, comprising a quick clamping device for fastening of a tool onto a spindle, comprising a clamping flange and a counter flange, between which the tool under the effect of an elastic force means is clamped, an actuation element being movable between a clamping position and a release position, wherein in the release position the clamping between the counter flange and the clamping flange against the effect of the elastic force means is neutralized, and a cam acting on the clamping flange which can, at least in the release position, be rotated by a motor of the power tool, and on which acts at least, when the actuation element is transferred from the clamping position in the release position, a running surface designed on the actuation element, wherein a movement of the actuation element is transferred into an axial shift of the clamping flange; and which, in the release position, when the motor is idle, holds the actuation element under the action of a frictional force acting between the cam and the running surface. 
     RELATED PRIOR ART 
     Such a power tool is known from EP 0 152 564 A2. This known power tool has a quick clamping device, by means of which disk-shaped tools, e.g. abrasive wheels or circular saw blades, can be exchanged quickly and comfortably. The quick clamping device comprises, for that purpose, a clamping flange constructed as a nut, and a counter flange, between which the disk-shaped tool is clamped. In this way, the clamping flange is screwed upon a clamping pin, which is attached for common rotation to a hollow spindle driven by the motor of the power tool, but movable in axial direction. The clamping pin is clamped, with respect to the hollow spindle, by means of a spring in such a way that the clamping pin is pulled toward the counter flange, due to the bracing effect of the clamping flange. 
     By transferring an actuation element, which is configured as a pivot lever, from a clamping position into a release position, the clamping between the clamping flange and the counter flange can be neutralized. To this end, a cylindrical socket is arranged on the pivot lever, said socket being screwed into the housing of the power tool. If the pivot lever is actuated, the socket is screwed further into the housing, until it, finally, acts upon the end of the clamping pin facing towards the socket and presses down the clamping pin, together with the clamping flange screwed thereon. In that way, the clamping between the clamping flange and the counter flange is neutralized, so that the clamping flange can be unscrewed from the clamping pin manually. After that, the tool can be exchanged against another tool. 
     In the operation of such power tools it has turned out that users sometimes switch on, carelessly or curiously, the motor of the power tool, although the pivot lever is still in the release position. It is true that also when the clamping flange is only loosely screwed upon the clamping pin, this cannot result in detaching the tool and, thus, in endangering the user. As, however, the dog is still in its declined position, it presses, with its bottom part, from the top onto the clamping pin, which now, after switching on the motor, rotates with high speed. Due to the relatively high forces, welding or deformation may occur in this case. 
     In the quick clamping device known from EP 0 152 564 A2, the friction conditions between the areas facing each other of the dog are selected in such a way that, when the motor is started, the friction force between the two areas is high enough to transfer the pivot lever into the clamping position. The pivot lever is, thus, if such an operating error occurs, returned into the clamping position. 
     It has turned out, however, that this return movement is relatively hard to monitor. On the one hand namely, by starting the motor, a relatively high torque is transmitted onto the pivot lever, as the adhesive friction between the two engaged parts, existing at the beginning, allows a high force transmission. Thus, the self-instructed return process starts with a very abrupt movement, which may result in accidents. 
     On the other hand, the friction conditions between the two engaged surfaces change after a while, as the force transmission is performed exactly by making use of the friction force and, thus, a wear of the surfaces is inevitable. The result is that also the force transmission and, in connection therewith, the kind of movement of the pivot lever changes after a while. 
     From EP 0 650 805 B1, another power tool is known, which is equipped with a similar quick clamping device. The actuation element is in this case, however, configured as a pivot lever, which is firmly connected with an eccentric. When the pivot lever is pivoted, the eccentric presses down a pressure head which is guided axially movable, until the eccentric rests on a thrust piece, into which the clamping flange is screwed via a threaded bolt. If the clamping lever is further pivoted, the pressure head finally presses down the thrust piece and, thus, also the clamping flange, against the action of cup springs. 
     In this known power tool, the pivot lever is connected via a shifter bar with a switch for starting the motor in such a way that the motor can be switched on only when the actuation element is in the clamping position. With this measure, it is prevented that, if the actuation element is in the release position, the lowered pressure head presses, with its bottom part, onto the thrust piece, which would rotate at high speed after starting the motor. Without such a measure, there would be welding or deformation between the pressure head and the thrust piece (or a friction plate fastened onto it), due to the relatively high forces. The mechanical connection between the actuation element and the switch for switching on the motor is, however, relatively complicated in design. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the invention to disclose an improved power tool which overcomes the drawbacks of the prior art. It is a further object of the invention to provide a power tool that reliably avoids failures when the motor of the power tool is switched on erroneously. The power tool shall be nevertheless simple in design, while allowing a cost effective manufacturing. 
     With respect to such a power tool as mentioned at the outset, this object is achieved by mating the cam and the running surface in such a way that, in the release position, a rotation of the cam caused by starting the motor reduces the frictional force between the cam and the running surface in such a way that the cam, which was prestressed by elastic force means, moves the actuation element from the release position into the direction of the clamping position on its own. 
     The solution principle underlying the invention is, consequently, distinguished by the fact that, for restoring the actuation element in its clamping position, not a torque generated by the motor is used, but, rather, the pressure exerted by the elastic force means via the cam onto the actuation element is used. As long as the cam rests, however, the adhesive friction force acting between the cam and the running surface keeps the actuation element in the release position. Only if the cam, when the motor is started, is set into rotation, the adhesive friction transitions into a distinctively smaller sliding friction, which is, then, not sufficient any more to keep the actuation element in the release position. The motor causes, thus, merely a modification of the friction conditions between the cam and the running surface; a force transmission from the motor to the actuation element, however,—at least in a significant amount—does not take place. 
     In comparison to the clamping device of EP 0 152 564 A2 mentioned at the outset, this principle has the decisive advantage that the speed of the actuation element, when being returned, is practically not dependent any more on the motor speed, but only depends on constructively determinable, mostly unchangeable parameters. 
     These parameters are, in particular, the pressure force caused by the elastic force means, the moment of inertia of the actuation element, the direction in which the cam acts on the actuation element via the running surface, and, of course, the friction conditions between the cam and the running surface. The latter depend, on their part, on the type of the tools used as well as on their surface quality. 
     In this regard, by the way, a clamping flange and a counter flange shall be regarded as any component suitable to clamp a tool onto a shaft. The clamping flange can be, in particular, a common nut, which is screwed onto the shaft. 
     In this regard, the terms clamping position and release position are not to be understood in a limiting sense, designating an exactly defined position. The clamping position shall rather be any position, i.e. also a greater position area of the actuation element, in which at least a partial clamping of the clamping flange and the counter flange is obtained. Correspondingly, all positions are designated as release position, in which there is no clamping between the clamping flange and the counter flange. So, if a movement of the actuation element from the release position in direction to the clamping position is the subject, this means, finally, that the actuation element is moved so far until clamping takes place at least partially. This results, on the other hand, in a relief of the components of the quick clamping device, so that additional forces generated by the motor can practically not result in damage any more. 
     A cam shall be any component, which engages, for the purpose of force transmission, a surface provided on another component, here called running surface. A special shape is not to be implied with the term cam. The running surface itself can be plain, but also be curved optionally, wherein the curves can also be considered as inclinations of a running surface unwound. 
     In this regard, it is particularly preferred, if the running surface has at least two sections of different inclinations. 
     This has the advantage that, by variation of the inclination of the running surface, the movement behavior of the actuation element, when returning on its own into the clamping position, can be influenced within certain limits by construction. Namely, the inclination influences the direction in which the cam acts upon the actuation element via the running surface. For instance, a setting of the running surface inclination is possible, in which the actuation element is transferred from the release position into the clamping position at approximately constant speed. 
     In a preferred improvement of this embodiment, the inclination of the running surface is smaller in the release position of the actuation element than the inclination of the running surface in the clamping position. 
     Thus, a small force transmission onto the actuation element at the beginning can be obtained, so that the return movement into the clamping position can be initiated slowly. Thereafter, the inclination of the running surface increases, so that the actuation element more accelerates. This can e.g. be advantageous, if the actuation element can be locked in the clamping position. The speed of the actuation element may then be sufficient to overcome the detent resistance. 
     In another advantageous embodiment of the invention, the running surface has at least two sections of different surface qualities. 
     This has the advantage that, in that way, also after the return movement has started, the friction conditions can be influenced. E.g. the running surface can be provided with a roughened section, which increases the sliding friction in such a way that a self-acting return pivoting of the actuation element is retarded, or at least a further acceleration is counterbalanced. The surface quality can also be modified by coating. 
     In a particularly preferred embodiment of the invention, the running surface is the circumferential unroll area of an eccentric arranged on the actuation element, mounted pivotably about a pivot axis. 
     This has the advantage that the running surface is so-to-speak rolled around the eccentric and, thus, has an essentially smaller “space requirement” than a plain surface, as it may be e.g. constructed on an actuation element constructed as a slider. For example, the actuation element can have a pivot knob arranged laterally on the eccentric, by which the eccentric can be pivoted around its axis. 
     In a preferred improvement of this embodiment, the actuation element, however, comprises a pivot lever, which is fastened on the eccentric and is pivotable about the pivot axis of the eccentric. 
     The use of a pivot lever has the advantage that much higher torques can be applied than possible with a rotation knob, for instance. Moreover, by the sweeping pivot movement of the pivot lever in the self-acting return movement from the release position into the clamping position, it is clearly shown to the user that he has omitted to bring the pivot lever, before actuating the power tool, into the clamping position. The fixation of the pivot lever on the eccentric can also be performed via a free running. When the actuation element is in the release position, only the eccentric moves back to the clamping position, but not the pivot lever, when the motor is started. 
     In another advantageous improvement of this embodiment, the eccentricity of the eccentric is between 1% and 20% of the largest diameter of the eccentric. 
     It has been found that with such a selected eccentricity of the eccentric a particularly reliable force transmission according to the principle of the invention is possible. 
     In another preferred embodiment of the invention, the eccentric is arranged laterally displaced with respect to the pivot axis of the cam, into the direction in which the pivot axis of the eccentric extends. 
     Such a transverse displacement results in that, when starting the motor in the release position, the rotating cam transmits an additional torque onto the eccentric, which is not due to the elastic force means, but to the rotation of the cam as such. Whether this additional torque supports or counteracts the torque produced by the elastic force means, depends on the side to which the eccentric is arranged, displaced, in the direction of the eccentric pivot axis. Preferably, the displacement is selected so that the torque produced by the elastic force means is supported for reaching an additional acceleration of the actuation element. 
     In a further preferred embodiment of the invention, the surfaces of the cam and of the running surface have a Vickers hardness of more than 54, preferably about 64, and a surface roughness R z  of 0.2 μm to 8 μm. 
     In that manner, due to the great hardness, a sufficient wear resistance is provided, so that the friction conditions between the cam and the running surface, which are decisive for returning the actuation element, remain constant in the course of time. On the other hand, a roughness selected in such a way provides for a sufficient adhesive friction force between the cam and the running surface, so that the actuation element, in the release position, when the motor is idle, can be held by the cam. For example, such a surface quality can be reached by hardening and grinding, or by tumbling steel parts (if necessary, sintered). 
     In another advantageous embodiment of the invention, the surfaces of the cam and of the running surface consist of a porous sinter material, the pores of which near to the surface are filled with a lubricant. 
     By this measure known per se, an emergency running lubrication between the cam and the running surface is guaranteed. When the surfaces are worn, namely, the pores open gradually, thus releasing the lubricant contained therein. 
     In another preferred embodiment of the invention, the spindle being driven by the motor is constructed as a hollow spindle. The cam acts on the clamping flange via a thrust piece, to which the clamping flange is fastened detachably, the thrust piece being arranged movably in axial direction in the hollow spindle by the cam engaging the thrust piece against the effect of the elastic force means. 
     By this measure known per se, a very reliable quick clamping device can be realized in a simple way. The cam can be part of the thrust piece or can be firmly connected thereto, so that each movement of the actuation element is transmitted directly onto the thrust piece and, thus, onto the clamping flange fastened thereon. In this case, however, a small distance should be kept, at least in the clamping position, between the running surface and the cam, so that, during operation of the power tool, there will not be permanent friction between the rotating cam and the running surface of the actuation element. 
     In an advantageous improvement of this embodiment, the cam is, however, not directly connected to the thrust piece. The cam is, rather, part of a pressure head, which is mounted rotatably in a sleeve and, when the actuation element is transferred into the release position against the effect of said elastic pressing means, which presses the cam of the pressure head against the running surface of the actuation element, the pressure head is moved in such a way in the direction of the thrust piece that the pressure head comes at least indirectly in friction contact with the thrust piece. 
     This has the advantage that, on the one hand, the cam is always in touch with the running surface of the actuation element. This gives a better feeling when operating the actuation element than if it would suddenly be placed with its running surface on the cam. The friction contact between the pressure head and the thrust piece facilitates in particular the exchange of the tool, since also the hollow spindle being connected to the thrust piece for common rotation is blocked against rotation due to the friction contact. In order to enlarge the friction between the pressure head and the thrust piece, the latter can be, on its end, connected to a friction plate. This friction plate has a surface with a sufficiently high coefficient of friction and can be, should it once be worn, exchanged relatively easily against a substitute plate. 
     In another advantageous embodiment of the invention, an elastic return means acts onto the actuation element, which, independently of a rotation of the cam exerts a force onto the actuation element, which supports a movement of the actuation element from the release position into the direction of the clamping position. 
     By means of such an additional return means, it is reached that the actuation element also occupies a clamping position definable by a stop, when the actuation element is in an interposition between clamping position and release position. This embodiment makes also sense, in particular, in such embodiments, in which no pressure head admitted by elastic force means acts upon the actuation element, as was described above. 
     It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are explained in the following in more detail with reference to the drawings. In the drawings: 
     FIG. 1 shows a simplified longitudinal section through a power tool of the invention in the area of its gear head, wherein an actuation element for the quick clamping device is in the clamping position, in which a tool is clamped between a clamping flange and a counter flange; 
     FIG. 2 shows the power tool of FIG. 1, in which the actuation element was transferred into the release position, so that the bracing between the clamping flange and the counter flange is counterbalanced; 
     FIG. 3 shows the power tool of FIGS. 1 and 2, in which in the release position of the actuation element the clamping flange is completely unscrewed; 
     FIG. 4 shows a schematic longitudinal section through an eccentric with a cam acting thereon while the actuation element is transmitted into the release position; 
     FIG. 5 shows a representation corresponding to FIG. 4, in which the actuation element is returned from the release position into the clamping position; 
     FIG. 6 shows a rear view onto the eccentric of FIG. 5, in which can be seen that the eccentric is arranged, in the direction of the eccentric pivot axis, displaced to the cam; 
     FIG. 7 shows an unrolling of an eccentric unroll area; 
     FIG. 8 shows an actuation element suitable for the power tool of the invention, in a perspective view; 
     FIG. 9 a  shows a highly simplified longitudinal section through another embodiment of an actuation element suitable for the invention, which is in the clamping position; 
     FIG. 9 b  shows the actuation element of FIG. 9 a  in top view; 
     FIG. 10 shows the actuation element of FIG. 9 a  in the release position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a first embodiment of a power tool according to the invention embodied as an angle grinder is designated in its entirety with  10 . Within a casing  12  of power tool  10  there is a motor-/gear unit  14 , which is, for the sake of clarity, represented in a dashed way. Motor-/gear unit  14  comprises an electric motor  16 , the motor shaft  18  of which carries a pinion  20  for a bevel gear unit. A drive gear  22  of the bevel gear unit is rigidly connected to a hollow spindle  24 , which is mounted in ball bearings—not shown in FIG.  1 —rotatably about a rotation axis  25 . On an end of hollow spindle  24  protruding outwardly out of casing  12 , a counter flange  26  is arranged to which a clamping flange  28  for clamping a tool  30  is assigned. Tool  30  is a grinding disk or a cutting disk in the embodiment shown. In order to allow a manual exchange of tool  30 , a quick clamping device is provided, which comprises, among other things, counter flange  26 , clamping flange  28  and an actuation element  31  with a pivot lever  32 . By actuating actuation element  31 , a bracing acting between counter flange  26  and clamping flange  28  can be overcome. 
     For that purpose, the quick clamping device has a thrust piece  34  being coaxially arranged in hollow spindle  24 . Thrust piece  34  has approximately the shape of a cup and is held, by means of guidings, which are not shown in detail, movably in axial direction for common rotation with hollow spindle  24 . On its inner side, thrust piece  34  is provided with an inside thread  36 , into which a clamping pin  40  having an outer thread  38  can be screwed. Clamping pin  40 , on its part, is firmly connected to clamping flange  28 , or is integral therewith. 
     The bracing creating a tight seat between counter flange  26  and clamping flange  28  is effected by a cup spring set  42  only schematically indicated, which is supported, on one side, by a step  44  rotating in hollow spindle  24 . On the opposite side, cup spring set  42  is supported by the bottom side of thrust piece  34 . As the upper side of thrust piece  34  gets in contact with a retaining ring  45 , which is inserted in a groove  46  rotating in hollow spindle  24 , the unit being formed by thrust piece  34 , clamping pin  40  and clamping flange  28  can be moved only against the effect of cup spring set  42  downwardly into the direction of the arrows designated with  48 . 
     A compensation of the bracing caused by cup spring set  42  is only possible by means of actuation element  31 , by transferring pivot lever  32  arranged therewith, in the direction represented by an arrow  50 , from the clamping position shown in FIG. 1 into a release position shown in FIG.  2 . 
     Pivot lever  32  is fixedly connected to an eccentric  52 , which is mounted onto a shaft  53  pivotably about a pivot axis  54 . A running surface  56  forming the unroll area of eccentric  52  acts on a cam  58 , which is configured integral with a pressure head  60 . Pressure head  60  is seated rotatably about the rotation axis  25  in a self-lubricating bearing sleeve  62  axially movable. By means of a helical spring  64 , which is held between a ring shoulder  66  of pressure head  60  and a projection  68  of casing  12 , pressure head  60  is pressed against running surface  56  of eccentric  52 , so that pressure head  60  is always in contact with actuation element  31 . 
     The function of the quick clamping device of the invention is now explained in more detail by means of FIG. 2 and 3. 
     If pivot lever  32  is moved into the direction of arrow  50 , due to the eccentricity, eccentric  52  presses down cam  58 , via its running surface  56 . By this measure, pressure head  60  moves down altogether, against the effect of helical spring  64 . Approximately after half of the pivot way of actuation element  31 , the conically shaped bottom end of pressure head  60  gets in touch with a friction plate  69 , which is fixed to the hollow spindle on the axial upper side of thrust piece  34  for common rotation therewith. 
     If pivot lever  32  is further pivoted into the release position, the descending movement of pressure head  60  is continued, which now presses thrust piece  34  against the effect of cup spring set  42  down into the direction of the arrows  48 . Clamping flange  28  being screwed into thrust piece  34  is moved down by the same amount. Thus, between clamping flange  28  and tool  30 , a small gap  70  is created, which shows that the bracing between counter flange  26  and clamping flange  28  is now compensated. This state is shown in FIG.  2 . 
     In this unstressed state, no significant friction forces act any more between tool  30  and clamping flange  28 . Clamping flange  28  can, thus, be unscrewed by hand in this position of thrust piece  34 , as indicated in FIG. 3 by an arrow  71 . In order to facilitate unscrewing, the clamping flange is provided with a circumferential knurl  72 . 
     In the embodiment shown in FIGS. 1 to  3 , clamping flange  28  does not rest directly on tool  30 . Rather, on the unit formed by clamping flange  28  and clamping pin  40 , an intermediate flange  74  is located rotatably, which consists of a thin disk  76  and of a dog  78  protruding from the center of disk  76 . A cylindrical bore runs through the center of disk  76  and dog  78 , through which clamping pin  40  is led. On its outer sides, dog  78  has the shape of a regular polygonal. This polygonal shape has its corresponding part in the section of hollow spindle  24  beneath cup spring set  42 , which has also, at its inner part, the form of a polygonal. Intermediate flange  74  with its dog  78  can thus be entered into this section of hollow spindle  24 , to reach a positive fit between hollow spindle  24  and intermediate flange  74 . A mutual rotation of hollow spindle  24  and intermediate flange  74  is then, even in the release position, not possible any more. 
     In the clamping position of actuation element  31 , tool  30  is clamped between counter flange  26  and clamping flange  28 . In order to guarantee a positive engagement between clamping flange  28  and counter flange  26  in the clamping position and, thus, to prevent detaching of clamping flange  28  in all operating conditions, the surfaces of clamping flange  28  and intermediate flange  76  facing each other are additionally provided with front gear teeth or the like. Thus, a continuous positive fit engagement between clamping flange  28  and hollow spindle  24  is reached. Consequently, it is not possible that clamping flange  28  together with clamping pin  40  during use of the power tool detaches on its own from thrust piece  34  by rotation. It is, thus, sufficient, after exchange of the tool, to tighten clamping flange  28  only slightly, until clamping flange  28  gets into touch with intermediate flange  76 , as, after bracing, clamping flange  28  is fixed in any case, and the bracing effected by cup spring set  42  is practically independent of how far clamping pin  40  is screwed into thrust piece  34 . 
     As clamping flange  28 , after compensating the bracing towards intermediate flange  76 , can be easily rotated, clamping pin  40  can be screwed out of thrust piece  34 , even if intermediate flange  74  is still held with its dog  78  nonrotatably with respect to hollow spindle  24 . A retaining ring  80  applied onto clamping pin  40  carries along intermediate flange  74 , when clamping pin  40  is unscrewed, until intermediate flange  74  reaches the position shown in FIG.  3 . Clamping flange  28  can now, together with intermediate flange  74  and clamping pin  40 , be completely pulled out into the direction of arrows  48 , whereby tool  30  is taken off from counterflange  26  and can be exchanged against a new tool. The assembly is performed the other way round. 
     In the quick clamping device of the invention, eccentric  52  acts together with cam  58  in such a way that, if actuation element  31  is in the release position shown in FIG. 2 and 3, actuation element  31  is automatically returned again into the direction of the clamping position, as soon as the motor  16  of power tool  10  is started. As in the release position pressure head  60  is pressed onto thrust piece  34 , if the motor is started by mistake, a movement of hollow spindle  24  is transmitted via an existing friction contact onto thrust piece  34  being arranged thereon and, thus, onto pressure head  60  and cam  58  being arranged thereon. The rotation of pressure head  60  caused thereby causes a transition of the adhesive friction, which acts between cam  58  and running surface  56  arranged on eccentric  52 , into sliding friction, which is much smaller. While the adhesive friction acting when pressure head  60  was standing still, kept actuation element  31  in the release position, now the torque, which pressure head  60 , being under pressure of cup spring set  42 , applies via cam  58  onto eccentric  52 , prevails over the torque acting into the other direction, which is caused by the sliding friction. Eccentric  52  begins, thus, together with pivot lever  32 , to move back into the direction indicated by dashed arrow  82  to the clamping position. 
     The processes acting between cam  58  and eccentric  52  are explained in the following by means of FIG. 4 and 5. 
     FIG. 4 shows in full lines eccentric  52  and cam  58  constructed on pressure head  60  in the clamping position shown in FIG.  1 . Pivot lever  32  is not shown for the sake of clarity. Running surface  56  of eccentric  52  describes in the embodiment shown in FIG. 4 the form of an arc. The eccentricity designated as e, i.e. the vertical distance between the center point  55  of the arc and pivot axis  54  of eccentric  52  in the clamping position, determines the measure by which in a rotation of eccentric  52  cam  58  is pressed down. In a rotation of eccentric  52  about 180°, this measure is  2   e.    
     Eccentric  52  is represented in dashes in FIG. 4 in a rotation of about 45° into the direction of the release position. In this process, it can be seen how running surface  56  of eccentric  52  moves to the bottom and transmits cam  58  into the position shown in dashes. Pressure head  60  moves, thus, into the direction indicated by arrow  82 . 
     FIG. 5 shows in full lines eccentric  52  in the release position. As pressure head  60  is pressed to the top by cup spring set  42  into the direction indicated by arrow  84 , a torque acts upon eccentric  52 , which strives to transmit eccentric  52  into the clamping position in direction of arrow  86 . This torque comes into being because cam  58  exerts a force onto eccentric  52 , the force not being directed centrally to the pivot axis of eccentric  52 . Rather, a mismatch or offset v exists between the contact line, along which eccentric  52  and cam  58  get in touch, and pivot axis  54  of the eccentric. From this mismatch v, a lever arm results, which leads to the torque mentioned. In the embodiment described above, the arc diameter is 12.6 mm, the eccentricity e 1.2 mm and the mismatch v 0.2 mm. 
     As long as pressure head  60  with cam  58  rests, the adhesive friction acting between running surface  56  of eccentric  52  and the upper side of cam  58  causes eccentric  52  and, thus, whole actuation element  31  to remain in its release position, in spite of the acting torque. 
     If, now, however, by operating the motor, pressure head  60  is, rotated together with cam  58 , the adhesive friction between running surface  56  and cam  58  transitions into sliding friction. If cam  58  and running surface  56  are manufactured of hardened polished steel, the sliding friction is smaller than the adhesive friction by approximately one dimension. The friction is now not sufficient any more to counterbalance the torque exerted by the pressure head, so that eccentric  52  moves back into the direction of the clamping position via the direction indicated by arrow  86 . 
     An eccentric  52  led back by approximately 60° is indicated in FIG. 5 in dashes. It can here be seen that pressure head  60  now moving to the top in the direction of arrow  84 , still exerts a torque onto eccentric  52  via cam  58  configured on pressure head  60 , so that actuation element  31  increasingly accelerates, until it is, finally, transferred into the clamping position shown in FIG. 4 (or into a position shortly before). 
     FIG. 6 shows a rear view onto eccentric  52 , wherein it can be seen that eccentric  52  is arranged displaced to cam  58  in the direction of pivot axis  54  of the eccentric. The size of this mismatch or offset equals the distance between a center plane  57  of eccentric  52  and of pivot axis  25  of cam  58 . Due to this displacement, the contact line between eccentric  52  and cam  58  is not arranged symmetrically any more with respect to pivot axis  25  of cam  58 . When cam  58  is rotated, an additional torque is exerted onto eccentric  52 , the direction of which depends on whether cam  58  rotates about its pivot axis  25  clockwise or anticlockwise. In the embodiment shown, cam  58  rotates in the direction indicated by arrow  59 , thus causing eccentric  52  to move back about its longitudinal axis  54  in the direction of arrow  61  into its clamping position. 
     A similar effect is caused, by the way, when eccentric  52  is slightly conically shaped with respect to the direction of pivot axis  54 . It is, thus, reached that the contact line between eccentric  52  and cam  58 , which is shortened in this case almost to a contact point, is not arranged symmetrically any more with respect to pivot axis  25  of cam  58 . The conical form of eccentric  52  can, in this regard, adopt values of approximately 0.1 to 1°, preferably of approximately 0.3°. 
     Preferably, however, both eccentric  52  and pressure head  60  with cam  58  consist of sintered tumbled steel parts, the Vickers hardness of which is in the range of about 64, and the surface roughness R z  of approximately 2 μm. This has the advantage, as already mentioned above, that in this manner a particularly high difference between the adhesive friction and the sliding friction is created. Apart from that, due to the high hardness, wear is low, so that the friction conditions between cam  58  and running surface  56 , which are decisive for the leading back of actuation element  31 , and also the lift generated while pivoting pivot lever  32  remain constant in the course of time. For such steel surfaces the adhesive friction force is approximately 300 N and the sliding friction force approximately 40 N, if cup spring set  42  generates a pressing force of approximately 3000 N. 
     In order to be able to influence the torque acting onto eccentric  52  while being pivoted back into the clamping position, an eccentric can be used, the running surface of which has no constant inclination, instead of an eccentric with arc-shaped running surface. 
     In FIG. 7, an unroll area of an eccentric modified in such a way over the pivot angle α is applied. It can be seen in this procedure that inclination α 2  of the unroll area is smaller in large pivoting angles (release position) than inclination α 1  of the unroll area in smaller pivoting angles (clamping position). This configuration of the unroll area causes actuation element  31  to leave the release position slowly first and then to accelerate faster after the motor is started. If desired, the final speed can be that high that actuation element  31  finally ends in an end position by overcoming a locking resistance, actuation element  31  being immersed in a recess in casing  12  of power tool  10 . With higher locking resistances, this may, however, not be useful, as this would possibly require such a high acceleration of actuation element  31  that an endangering of the user in the pivoting process cannot be excluded. 
     The speed in the automatic return pivoting of actuation element  31  may also be influenced by modifying the surface quality of running surface  56 . It can, for instance, be provided that the running surface is roughened such that between a pivot angle of 0° and 60° (seen from the clamping position, see FIG. 3) that, due to the adhesive friction being larger between this pivot angle, the automatic return pivot movement of actuation element  31  is retarded or, at least, a further acceleration is opposed to. 
     In the embodiment shown in FIGS. 1 to  3 , cam  58  is configured on pressure head  60 , which, not before actuation element  31  is pivoted, acts on thrust piece  34 . Alternatively, eccentric  52  may be configured to act directly onto thrust piece  34 . Thrust piece  34  has to be configured such that it projects out of the top of hollow spindle  24 . On thrust piece  34 , a cam  58  can additionally be configured. But cam  58  can be omitted in the sense that the whole thrust piece  34  and/or pressure head  60  are considered to be a cam, both in this example and in the embodiment mentioned above. 
     In order to prevent for this alternative embodiment even in the clamping position of actuation element  31 , a steady friction between thrust piece  34  and running surface  56  being configured on eccentric  52 , a small gap should remain in the clamping position between eccentric  52  and thrust piece  34 . In order to replace the effect of helical spring  64 , a restoring spring  92  acting on actuation element  31  may be provided, as shown in FIG.  8 . Restoring spring  92  prestresses actuation element  31  already in its clamping position with a small spring force and provides for the necessary distance to thrust piece  34 . 
     Finally, it is also possible to use a rotation lever instead of a pivot lever, as described in EP 0 152 564 A2 mentioned at the outset. The surfaces of the pivot lever and of the thrust piece (and/or of a thrust piece acting) touching each other in the release position, and the pitch of the thread by which the pivot lever is screw-connected with the casing, are then to be designed such that, if the motor is activated, the pivot lever is just not set in motion by a force that was transmitted by friction contact onto the pivot lever from the thrust piece. Rather, the parameters mentioned are to be determined such that the friction at the contact surface decreases, when the thrust piece rotates, so that the pivot lever is turned out of the thread exclusively under the effect of the elastic force means and, thus, is led back into its clamping position. In that way, the pivot lever can be led back into the clamping position in a much more controlled way, as if it was driven directly via friction contact by a motor. 
     In FIG. 9 a,    9   b  and  10 , an alternative embodiment for actuation element  31  is shown schematically. Instead of a pivot lever connected to an eccentric, a slider  100  is used, the beveled bottom part of which forms a running surface  102 , which acts together with a pressure head  104 . Different from pressure head  60  described above, pressure head  104  comprises a rounded top side  106 , so that pressure head  104  acts altogether as a cam. Slider  100  being provided with a passage  108  is guided such between two guidings  110 ,  112 , which are indicated only schematically, that it can be shifted into the direction indicated by arrow  116 . In FIG. 9 a,  slider  100  is located in the clamping position, which is shown in FIG. 9 b  in top view. When slider  100  is now shifted into the direction of arrow  116  by pulling at passage  108 , running surface  102  on the bottom part of slider  100  presses pressure head  104  down in direction of arrow  114 . In that way, the bracing between clamping flange  28  and counter flange  26  is compensated, as was described in detail with reference to FIG. 1 to  3 . 
     In FIG. 10, slider  100  is in the release position. If now pressure head  60  is set in motion, the friction force acting between top side  106  of pressure head  60  and running surface  102  of slider  100  is decreased, as already described above, whereby pressure head  60  is now able to push slider  100  back into the clamping position, as is indicated by arrow  118 . Also in this case, running surface  102  can, of course, comprise different inclinations, as was described above for FIG.  7 .