Abstract:
The invention is based on a power tool, in particular a hand power tool, with a drive motor ( 12 ) disposed in a housing ( 10 ) and at least one adjusting device ( 14, 16 ), which can be used to durably set at least one parameter of the drive motor ( 12 ) and/or of a transmission during an operating phase. 
     The invention proposes that the adjusting device ( 14, 16 ) have a reset unit ( 18, 20 ), which resets the adjusting device ( 14, 16 ) to a starting position depending on at least one operating parameter.

Description:
This is the national stage of PCT/DE01/00988 filed Mar. 15, 2001, which claims priority to German application 100 23 174.8 filed May 11, 2000. 
   BACKGROUND OF THE INVENTION 
   The invention is based on a power tool, in particular, a hand power tool. 
   For most power tools, in particular hand power tools, it is advantageous to be able to vary a drive speed of the drive motor. In hand power tools, it is known to use a control unit to keep the drive speed as constant as possible independent of the load, or to variably adjust the drive speed by means of an adjusting device. 
   For the adjustment, the operator is frequently provided with an adjusting wheel that can be used to durably set the speed and can limit the speed to a maximal value. The adjusting wheel can also be combined with a potentiometer in the switch trigger so that the adjusting wheel presets the upper limit of the trigger range. 
   The adjusting wheel can, for example, be used to match the speed to a machining procedure or to a material to be machined. An operator can preselect a speed that can be kept constant while machining a work piece, for example when sawing, grinding, etc. 
   SUMMARY OF THE INVENTION 
   The invention is based on a power tool, in particular a hand power tool, with a drive motor disposed in a housing and at least one adjusting device, which can durably set at least one parameter of the drive motor and/or of a transmission during an operating phase. 
   The invention proposes that the adjusting device have a reset unit, which resets the adjusting device to a starting position depending on at least one operating parameter. Operating errors can be reliably prevented, particularly if the power tool is used by a number of people. The reset unit can advantageously always reset the power tool to a standard operating position from which each operator can start in order to adjust the power tool in the usual manner for his specific intended use. This reliably prevents a new user from overlooking a special adjustment that is unsuited for a particular intended use. Depending on the power tool, it can be advantageous that in the starting position, the power tool can be operated with a maximal possible parameter, which can then be reduced by means of the adjusting device, that in the starting position, the power tool can be operated with a minimally adjusted parameter from which an adjustment should always be started for safety reasons, or that in the starting position, the power tool can be operated with a parameter that lies between a maximal and a minimal parameter. 
   In power tools, frequently a speed of the tool is crucial to an advantageous machining; to this end, the adjusting device can preferably set a maximal speed as the parameter, either directly by means of the drive machine and/or possibly also by means of a transmission. Besides the maximal speed, it can also be useful in principle to use the adjusting device to establish a minimum speed, which should not be undershot during use. 
   In addition to a speed, other parameters deemed appropriate by one skilled in the art can also be embodied so that they can be adjusted by means of the adjusting device, such as a minimal and/or maximal transmitted torque and/or an operating stroke, for example in a saber saw or an eccentric grinder, etc. 
   In another embodiment of the invention, the reset unit resets the adjusting device depending on a period of downtime. A resetting can be achieved in a structurally simple, properly functioning manner. 
   If the reset unit can be used to adjust the period of downtime after which the reset unit resets the adjusting device, then the power tool can also be adjusted for different fields of application and an unintentional resetting can be reliably prevented; the period of downtime is preferably between one and twenty minutes. 
   In another embodiment, the invention proposes that the reset unit reset the adjusting device when a power supply is interrupted, for example when a power plug is pulled or when a battery is being changed. Since in particular a power plug of a hand power tool is frequently unplugged or the power supply is interrupted before a change of operators, the change of operators can be detected with a particularly high degree of probability. 
   Furthermore, there are other conceivable operating parameters, which may be deemed appropriate by one skilled in the art and depending on which the reset unit resets the adjusting device, for example depending on temperature values. The adjusting device can be reset as soon as the power tool has cooled, for example in that a bimetal element releases a detent connection, and/or the adjusting device can be reset if a particular temperature is exceeded, for example in the event of an overload. 
   A mechanical reset mechanism can be embodied in a structurally simple and inexpensive manner, particularly through the use of a prestressed spring element, which is secured in its position during operation, for example by means of a detent mechanism that is coupled to an on-off switch and is released when the power tool is switched off. 
   If the reset unit has an electromagnetic adjusting element, the resetting procedure can simply be advantageously coupled to a power supply, for example in that when there is a supply of power, an electromagnet can—directly or by means of a detent mechanism—prevent a reset spring from resetting the adjusting device. If the power supply is interrupted and an attraction force of the electromagnet is neutralized, then the reset spring resets the adjusting device. Besides a mechanical reset spring, the electromagnetic adjusting element can also be combined with other mechanisms deemed appropriate by one skilled in the art, for example electrical and/or hydraulic actuators. 
   In another embodiment, the invention proposes that the adjusting device have at least one electronic unit, which is for setting the parameter and can be reset by the reset unit. The resetting process can advantageously be coupled to a power supply in the same way as with an electromagnetic unit. Furthermore, the electronic unit can be embodied in a particularly space-saving, lightweight, and inexpensive manner. A mechanical resetting of an adjusting element, e.g. of an adjusting wheel, can be advantageously avoided. 
   The embodiment according to the invention can be used in various power tools deemed appropriate by one skilled in the art, in particular in hand power tools that are used by a number of operators, for example hand power drills, percussion drills, hammer drills and/or chisel hammers, screwdrivers, right angle grinders, eccentric grinders, saws, planes, etc. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages ensue from the following description of the drawings. Exemplary embodiments of the invention are shown in the drawings. The drawings, the specification, and the claims contain numerous features in combination. One skilled in the art will also suitably consider the features individually and will unite them into other appropriate combinations. 
       FIG. 1  shows a schematically depicted hand power drill, 
       FIG. 2  shows an adjusting wheel with an electromagnetic reset unit, and 
       FIG. 3  shows an adjusting wheel with an electronic control unit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a hand power drill with an electric motor  12  disposed in a housing  10 . The hand power drill can be turned on and off with a switch  28 . For adjusting the speed, the hand power drill has an adjusting device  14  with an adjusting wheel  30 , which can be used to set a speed to a particular value. The switch can also be equipped with a potentiometer, which permits it to vary the speed, for example, from 0 to an end value preselected by means of the adjusting wheel. 
   According to the invention, the adjusting device  14  has a reset unit  18 , which resets the adjusting device  14  to a starting position depending on an operating parameter as soon as a power supply is interrupted ( FIG. 2 ). The adjusting wheel  30  is supported so that it can rotate in a housing part  34  by means of an axle  32 , which also permits a potentiometer, not shown, to rotate. The axle  32  is secured in the housing part  34  axially in relation to the adjusting wheel  30  by means of a gear  36  fastened to the axle  32  and is secured axially in relation to the gear  36  by means of a flange  38  on the adjusting wheel  30 . A helical spring  22  is supported on the flange  38  and, with a first leg  42 , engages in a form-fitting manner in the rotation direction between two pins  44 ,  46  fastened to the adjusting wheel  30  and with a second leg  48 , engages in a form-fitting manner between two pins  50 ,  52  fastened to the housing part  34 . 
   If the hand power drill is connected to a power supply by means of a power plug that is not shown and a power cable, then an electromagnet  24  is supplied with current and moves a ram  54  into a tooth gap of the gear  36 . If the adjusting wheel  30  is turned in order to limit a maximal speed that can be set by the switch trigger  28  to a particular value, then the ram  54  can be moved out of its detent position in the gear  36  i.e. out of a tooth gap, counter to the magnetic force of the electromagnet  24 . If a tooth gap of the gear  36  comes to rest over the ram  54 , then the magnetic force of the electromagnet  24  moves the ram  54  into the tooth gap. The adjusting wheel  30  can be moved in steps that are predetermined by the tooth gaps of the gear  36 . If a desired step or a desired speed limitation is achieved, then the adjusting wheel  30  and the potentiometer are immobilized by means of the axle  32 , the gear  36 , and the ram  54  of the electromagnet  24  engaging in the tooth gap of the gear  36 . Moving the adjusting wheel  30  places the helical spring  22  under stress. 
   If the power plug is pulled after an operating phase, and the power supply to the electromagnet  24  is interrupted, the helical spring  22  can move the ram  54  out of the tooth gap of the gear  36 . The helical spring  22  resets the adjusting wheel  30  and the potentiometer into their starting positions and the speed limitation is cancelled so that the hand power drill can be operated at a maximal possible speed by means of the switch trigger  28 . The gear  36  and the electromagnet  24  with the ram  54  advantageously serve to graduate the adjusting range of the adjusting wheel  30 , to lock the adjusting wheel  30  during an operating phase, and to advantageously embody a reset mechanism. 
     FIG. 3  shows an alternative adjusting device  16  with an adjusting wheel  56 . In principle, components that remain essentially the same are labeled with the same reference numerals. For features and functions that remain the same, the reader should refer to the description of the exemplary embodiment in  FIGS. 1 and 2 . 
   The adjusting wheel  56  is connected by means of a pulse generator  58  to an electronic control unit  26 , which provides a speed governor  60  with a set point value. Instead of a pulse generator, however, it is also possible to provide an endless potentiometer. If the hand power drill is connected to a source of current by means of a power supply  62  with a power plug and a power cable, then a line  64  supplies the control unit  26  with current independent of the actuation of the switch  28 . The adjusting wheel  56  can then be used to adjust the speed of electric motor  12  to a particular value. The adjusted speed is indicated on a display  66 . If the adjusting wheel  56  has reached an adjusting limit in one of its rotation directions, the adjusting wheel  56  can be rotated further without changing the speed setting. 
   If the power plug is pulled and the supply of current to the control unit  26  is interrupted or if a particular period of downtime has been reached, i.e. the hand power drill has not been actuated for a particular amount of time, then a reset unit  20  contained in the control unit  26  resets the control unit to its starting position and the previously selected speed is cancelled so that the hand power drill can be operated at a maximal possible speed. The period of downtime after which the reset unit  20  resets the control unit  26  can be adjusted by means of an adjusting wheel  40  to a period from two to twenty minutes. 
   Instead of an adjusting wheel, it is also conceivable to provide a different adjusting element deemed appropriate by one skilled in the art for adjusting the speed limitation, such as a switch rocker, one or more actuating knobs, etc. 
   REFERENCE NUMERAL LIST 
   
       
         10  housing 
         12  drive motor 
         14  adjusting device 
         16  adjusting device 
         18  reset unit 
         20  reset unit 
         22  spring element 
         24  adjusting element 
         26  electronic unit 
         28  switch 
         30  adjusting wheel 
         32  axle 
         34  housing part 
         36  gear 
         38  flange 
         40  adjusting wheel 
         42  leg 
         44  pin 
         46  pin 
         48  leg 
         50  pin 
         52  pin 
         54  ram 
         56  adjusting wheel 
         58  pulse generator 
         60  speed governor 
         62  power supply 
         64  line 
         66  display