Abstract:
The invention discloses a power tool comprising a clutch having a first clutch element and a second clutch element which are seated for rotation one relative to the other and between which a torque can be transmitted. The clutch comprises an actuating element engaging at least one of the first and second clutch elements for moving the clutch between an engaged position for effecting power transmission from a drive shaft to an output shaft, and between a released position wherein there is no power transmission to said output shaft. The actuating element comprises a nanotube actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority of German patent application No. 10 2004 059 814.2 filed on Dec. 6, 2004. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a power tool comprising a clutch movable between an engaged position in which power is transmitted from a drive shaft to an output shaft, and a released position in which there is no power transmission to the output shaft. The invention further relates to a clutch having an actuator for moving the clutch between an engaged position and a released position. 
   U.S. patent Ser. No. 5,159,522 discloses a clutch cooperating with a drive shaft and having a first disk-shaped clutch element from which the torque of a drive shaft can be transmitted via a friction lining to an associated second clutch element which latter is connected with a drive shaft. The clutch is designed as an electromagnetic clutch wherein a contact pressure is produced by a coil when the latter is suitably driven via a transistor circuit. 
   A clutch of that kind can be used for numerous operating functions. It is, however, a disadvantage of such a clutch that the power input required for achieving sufficient torque transmission is quite considerable. 
   There have further been known in the art numerous other clutch types comprising various actuating elements. In the automotive industry, for example, hydraulically operated clutches have been known for many years. Other clutches known include, for example, electric motor-operated clutches and, of course, mechanically operated clutches that are actuated by a lever or the like. 
   However, power tools, in particular hand held power tools, such as nut runners having a preset shut-off-torque, at which torque transmission is interrupted, need very compact clutches that allow for a high torque transmission, are very reliable, light-weight and have a low power consumption if electrically activated. 
   SUMMARY OF THE INVENTION 
   In view of this, it is a first object of the present invention to disclose a power tool comprising a clutch for interrupting torque transmission having a quick response and being of compact and light-weight design. 
   It is a second object of the invention to disclose a power tool comprising a clutch for interrupting torque transmission allowing for a high torque transmission. 
   It is a third object of the invention to disclose a hand-held power tool comprising a clutch which can be activated electrically. 
   It is a forth object of the invention to disclose a hand-held power tool comprising a clutch which can be activated electrically and has a low power consumption, in particular when powered by accumulators. 
   It is a fifth object of the invention to disclose a power tool comprising a clutch which can be activated very reliably for interrupting torque transmission e.g. when a predefined torque for tightening a screw has been reached. 
   It is a sixth object of the invention to disclose a clutch which can be moved by an actuating element between an engaged and a released position and which can be activated very quickly and reliably. 
   These and other objects of the invention are achieved by providing a clutch having an actuating element for moving the clutch between an engaged and a released position, wherein the actuating element comprises a nanotube actuator the volume change of which effected by a voltage is translated into a movement effecting an engagement or a release of the clutch. 
   According to the invention the nanotube actuator for the actuating element allows rapid actuation of the clutch with very low power input. Very low voltages are already sufficient to operate the clutch. The clutch can be operated with high precision and provides high reliability in continuous operation even after numerous actuating cycles. And the clutch is also suited for transmitting high torques of the order of 10 Nm or over, which are especially encountered with hand-held power tools. 
   The clutch according to the invention generally may have any desired structure. It may be configured, for example, as a disk clutch, as a cone friction clutch, as a jaw clutch, as a gear clutch, as a dog clutch or as a switchable overrunning-spring clutch. Also the clutch may be designed for force-locking and/or form-locking power transmission. 
   According to a preferred further development of the invention, the actuating element comprises a carbon nanotube actuator. 
   According to an advantageous further development of the invention, the nanotube actuator comprises at least one layer in which nanotubes are arranged with preferred orientation. 
   The nanotube actuator may in this case comprise at least one layer of single-wall or multi-wall carbon nanotubes or nanotubes consisting of other inorganic components, such as BN, MoS 2  or V 2 O 5 . 
   The use of carbon nanotubes permits considerably higher forces to be generated than the previously known polymer and piezo actuators. Also carbon nanotubes can be operated with a very low supply voltage of approximately 1 Volt, while polymer actuators require supply voltages of 70 to 300 Volts, and piezo actuators even require supply voltages of up to 1000 Volts. Further, no overshoot behavior is encountered with carbon nanotubes. 
   Due to the oriented arrangement of the nanotubes, with a preferred direction, improved activation characteristics can be achieved. 
   The nanotube actuator may comprise at least one layer comprising nanotubes or nanotube fibers structured by a CVD process. 
   According to another preferred embodiment of the invention, the nanotube actuator comprises at least one layer of nanotubes epitaxially grown between mutually parallel plates. 
   This allows especially high capacity and high efficiency of the actuator to be achieved. 
   Basically, however, a stacked-layer structure of the actuator is likewise possible. 
   Preferably, the nanotube actuator, together with its electrolyte, is sealingly received in a housing, and any expansion in volume of the electrolyte is converted via a diaphragm into an axial movement of at least one piston. 
   As any application of voltage will lead to both axial and radial expansion of the nanotube actuator, it is thus possible to convert the expansion in volume into an axial movement by hydraulic transmission, whereby an expansion of 1%, for example, in all three directions can be converted into an axial movement of approximately 3%. 
   The nanotube actuator may, for example, consist of a plurality of annular disk elements, stacked one upon the other and connected in series, which permits the required supply voltage to be reduced. The number of stacked disks (stacking height) depends on the displacement required for disengaging the clutch and on the hydraulic transmission ratio between the nanotube actuator and the surface area of the pistons on which the diaphragm acts. 
   According to an advantageous further development of the invention, the actuator is arranged for disengaging the two clutch elements and acts against the action of a biasing force by which active engagement of the clutch elements is achieved when the actuator is in its inoperative condition. 
   This provides the advantage that in the engaged condition of the clutch power transmission as such is not effected by the nanotube actuator but rather by a biasing force provided, for example, by a suitably sized spring. 
   A power tool according to the invention comprises a drive shaft and an output shaft, with a clutch of the kind according to the invention provided between them for power transmission. The actuator is designed in this case for axial movement of the clutch elements one relative to the other, and acts against a restoring force acting between the clutch elements. 
   Preferably, the clutch elements are urged into an engaged position by a restoring force, the actuator being designed to cause a relative movement between the two clutch elements into a disengaged position in which power transmission between the clutch elements is interrupted. 
   This provides the advantage that the nanotube actuator needs to be actuated only for interrupting the interaction between the two clutch elements. 
   According to an advantageous further development of the invention, voltage can be applied to the actuator via a sliding-contact connection. 
   The clutch may be configured as a disk clutch, for example, with at least one of the clutch elements comprising a friction lining for power transmission. 
   According to an advantageous further development of the power tool according to the invention, the output shaft has one end rotatably seated on the drive shaft, and comprises a first clutch element and an associated second clutch element, connected for common rotation with the drive shaft, that can be axially displaced via the nanotube actuator for disengaging the two clutch elements and for transferring them to an open position. 
   This guarantees a simple, compact and reliable structure. 
   Basically, the reverse action is of course likewise possible, in which case actuation of the nanotube actuator by application of a voltage causes the clutch to be engaged. 
   It is understood that the features of the invention 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. 
   Further features and advantages of the invention will become apparent from the description that follows of a preferred embodiment of the invention, with reference to the drawing. The single FIGURE shows a simplified representation of an electric tool according to the invention with a clutch according to the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The only FIGURE shows a simplified longitudinal section of a power tool according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the drawing a power tool according to the invention is indicated generally by reference numeral  10 . 
   The illustrated tool may be a hand-held screw runner, for example, provided with a torque-controlled electrically operated clutch, which is generally indicated by reference numeral  23 . Such a screw runner may be used, for example, for tightening a screw connection with a precise torque. A torque sensor (not shown) may be provided for this purpose for actuating the clutch  23  when a predetermined torque is reached so as to interrupt transmission of the torque. 
   The electric tool  10  according to the invention comprises a drive shaft  12 , which is driven by an electric motor indicated schematically by reference numeral  20 , if necessary via a gearing. The rotary movement of the drive shaft  12  can be transmitted via the clutch  23  to an output shaft  14  on which a tool can be attached, for which purpose a tool holder  16  is indicated schematically in the drawing. 
   The tool end of the output shaft  14  is seated in a bearing  18 , while its end facing the drive shaft  12  is connected with a flange  22  for common rotation with the latter. A first sleeve-like clutch element  24  of the clutch  23  is connected with the flange  22  by a screw connection. A second clutch element  30 , being connected with the drive shaft  12  for common rotation with it, is seated for axial displacement on the section of the drive shaft  12  facing the output shaft  14 . The second clutch element  30  likewise has the shape of a flange and is guided for axial displacement on the surface of the drive shaft  12 . 
   For realizing the connection for common rotation with the drive shaft  12 , there is provided a featherkey  36 , received on the drive shaft  12 , which slidably engages in a receiving groove  38  in the second clutch element  30 . The second clutch element  30  comprises a disk-shaped friction lining  32  which coacts with an associated friction lining  26  of the first clutch element for permitting force-locking power transmission between the two clutch elements  24 ,  30 . 
   A central bore of the first clutch element  24  rests on the outer surface of a cylindrical section of the first clutch element  30 , being thereby rotatably guided. A bushing  28  is attached to the end of the drive shaft  12  that faces away from the output shaft  14 . Between the bushing  28  and the end face of the second clutch element  30  facing it, there is provided a spring element  34  by means of which the second clutch element  30  is biased toward the friction surface  26  of the first clutch element  24 . The spring element  34  may for example take the form of a helical spring, as shown in the drawing, or of a disk spring or the like. The spring element  34  acts to urge the friction surface  32  of the second clutch element  30  against the coacting friction surface  26  of the first clutch element  24  so that the contact pressure required for transmission of the torque is predefined by the strength of the spring element  34 . 
   The clutch  23  comprises an actuating element, generally indicated by reference numeral  25 , which in the embodiment according to the invention comprises a nanotube actuator  56 . The nanotube actuator  56  is sealably received, together with an electrolyte, in a housing  54  which coaxially encloses that section of the drive shaft  12  that faces away from the output shaft  14 . 
   When a voltage is applied, the nanotube actuator  56  expands in all three directions, thereby displacing the electrolyte which is sealed by a seal (diaphragm  52 ) on its side facing the clutch. The diaphragm  52  is contacted by three pistons  44  the axial displacement of which is transmitted to a bearing housing  42 . A bearing  40  accommodated in that housing converts that movement into an axial displacement of the second clutch element  30  in a direction opposite to the action of the spring element  34 . Thus, expansion of the nanotube actuator  56  causes the second clutch element  30  to be axially displaced in a disengaging direction so that the interaction between the friction element  32  and the associated friction surface  26  is interrupted and the clutch  23  is disengaged. 
   The hydraulic transmission by means of the electrolyte has the effect to convert the expansion in volume of the nanotube actuator  56  in all three spatial directions into an axial movement. 
   The housing  54  for the nanotube actuator  56  is supported on a bearing  62 , on the side of the drive shaft  12  that faces away from the output shaft  14 , and comprises an annular space in which the nanotube actuator  56  is received. The nanotube actuator  56  comprises carbon nanotubes, which have been epitaxially grown by a CVD process in oriented arrangement between two end plates  58 ,  60 , or consists, as has been mentioned before, of stacked carbon nanotubes in the form of annular disks. The electrolyte may be configured as solution of alkaline-alkaline earth, aluminum and metal salts, of halides, nitrates, sulfates, phosphates, dihydrogen phosphates, hydrogen phosphates, halogenates, per-halogenates, hydroxides, acetates, oxalates or acids or mixtures thereof. 
   For example, an aqueous solution of Na 2 SO 4  or Na 2 HPO 4  or Na 2 PO 4  or KCl may be used, and the concentration may be varied between 0.1 mol and 1 mol, for example. High maximum forces are achieved, for example, with 1 mol of KCl and 1 mol of Na 2 SO 4 . 
   On the side facing away from the bearing  62 , the nanotube actuator  56  is sealed toward the outside by a seal  52  which acts as a diaphragm on the three associated pistons  44 . The housing  54  is screwed to a cover  50  in which the pistons  44  are received, with a seal  52  placed between the two elements. There may be provided, for example, three pistons  40  arranged at a uniform angular spacing of 120° and axially parallel to, and radially set off to the outside from, the lengthwise axis of the drive shaft  12 . In the upper half of the FIGURE, a piston  44  can be seen whereas a further piston  46  is indicated schematically in the lower part of the drawing. The pistons  44 ,  46  are received in axial bores of the cover  50  for longitudinal displacement, and bear against the end face of the nanotube actuator  56  or the seal (diaphragm)  52 , respectively. 
   The pistons  44  are sealed relative to their receiving bores in the cover  50  by O-ring seals  48 . Thus, generally complete sealing of the nanotube actuator in the housing  54  and the cover  50  toward the outside is achieved. The plates  58 ,  60  of the nanotube actuator  56  can be suitably connected to an external voltage source, via sliding contacts (not shown), for permitting actuation of the nanotube actuator. 
   The pistons  44 ,  46  transmit a voltage-induced displacement to the end face of the bearing housing  42  in which the second clutch element  30  is seated in a bearing  40  designed as ball bearing. 
   Thus, axial displacement of the pistons  44 ,  46  is directly converted into a disengaging movement of the second clutch element  30 . 
   Due to the fact that the nanotube actuator  56  responds very quickly even to minor voltage pulses of the order of 1 Volt, extremely precise and rapid response of the clutch  23  can thus be ensured in order to produce a disengaging movement or engagement of the clutch  23 .