Patent Publication Number: US-10315293-B2

Title: Electric power tool

Description:
The invention relates to an electric torque delivering impulse tool, such as e.g. a screw machine. In particular the invention relates to a tool with an interconnected electric motor and a torque impulse generating pulse unit. 
     In a conventional torque delivering impulse tool the motor and the torque impulse generating pulse unit are mounted with individually bearings and the motor and the pulse unit are interconnected by means of e.g. a hexagonal or quadratic male and female connection part, which are interconnected such that a play or allowance by necessity exists between them. The allowance between the interconnected parts is inevitable for assembly with respect to manufacturing tolerances of the parts. 
     A problem inherent in this conventional arrangement is that an increasing gap is formed between e.g. the hexagonal male and female connection parts. This gap will increase due to the joint work of the motor, on the one hand, and the partly opposed work of the pulse unit, on the other hand. In this procedure the connection will slowly degrade such that it will have to be replaced at one time sooner or later. 
     Further, this kind of connection has considerable backlash and elasticity. Therefore, there will be an irresolute transmission of the torque pulses generated in the system and as a consequence the contribution of torque from the energy stored in the motor part will not be optimal. 
     Hence, there is a need new of an improved connection arrangement between the motor and the pulse unit, which allows for a prolonged life time of the motor and the pulse unit. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an electric torque delivering impulse tool, which is more durable and more efficient than a conventional torque delivering impulse tool. A specific object of the invention is to provide an improved connection between the motor and the pulse unit, in order to achieve a higher efficiency, a reduced weight and/or a prolonged life time for the tool. 
     The invention relates to an electric torque delivering impulse tool comprising: a housing with a front end and a back end, an electric torque delivering motor with a rotor that is arranged to rotate with respect to a stator, an output shaft arranged at the front end of the housing, and a pulse unit intermittently coupling said motor to said output shaft, wherein the pulse unit comprises an inertia drive member that is connected to said motor rotor. The rotor and the inertia drive member are rigidly assembled to each other without play to form one integrated rotatable structure which is mounted as one single unit inside said housing. 
     With the tool according to the invention the possibility of movement between the interconnected parts of the tool is restricted, such that virtually no wear due to fatigue or repeated strokes will be present. 
     Further, the construction of the tool will be more compact with respect to that of prior art arrangements. This is an advantage as the tool may be made smaller, and because the tool may be arranged to absorb the forces produced by the motor and the pulse unit in a more efficient manner, which leads to an overall more agreeable manoeuvring of the tool for the operator. 
     In the prior art, the rotor and inertia drive member are individually journalled with respect to the housing, typically using three or more bearings. Due to manufacturing tolerances and the different axial locations of the journal bearings in the structure, such a system can never be truly coaxial. Any run-outs or misalignments of housing parts of the outer structure will inflict an angularity between rotor and inertia drive member. This angularity will in turn reduce the effective stiffness of the torque transmitting hexagonal joint that conventionally connects the rotor and the inertia drive member in such a way that a significant elasticity is introduced into the system in conflict with the desired rigidity. 
     The elasticity is increased by the fact that the hexagonal joint has small radial dimensions, necessary to allow the motor bearing to be assembled outside the shaft. Since the rotor and inertia drive members are assembled one at a time into the supporting structure, the hexagonal joint must have enough backlash to allow the parts to slide together during assembly and disassembly. Given the necessary manufacturing tolerances of such hexagonal joint parts and allowance for dimensional alterations during hardening processes, the angular backlash will have an initial value of typically some degrees. 
     The repetitive torque pulses travelling back and forth through the hexagonal joint during operation will gradually deteriorate the joint by wear and fatigue effects in such a manner that the backlash tends to increase over time. This reduces further the effective rigidity. Other fail modes like splintered or broken shafts often occur and limit the lifetime of the traditional system. 
     The idea of the invention, on the other hand, is that the rotor and the inertia drive member should be rigidly assembled to each other without a gap or play, so as to form one integrated rotatable structure which is mounted as one single unit inside said housing. With the inventive solution, any movement of the rotor and the inertia drive member with respect to the housing will be uniform, as opposed to the prior art, where the rotor and the inertia drive member are allowed to move individually with respect to each other. 
     One advantage of the tool according to the invention is that it will have a higher specific torque output than a conventional one. Another advantage is that due to the integrated rotatable structure of the rotor and the inertia drive member it is possible to exclude one or more journal bearings. This will reduce the size, weight and friction in the system. The friction is important to keep as low as possible as a system with low inherent friction generates less heat than a system with a higher inherent friction. 
     Additional objects and advantages of the invention will appear from the following specification and claims. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       In the following detailed description reference is made to the accompanying drawings, of which: 
         FIG. 1  is a cross sectional view of an electric torque delivering impulse tool according to a first embodiment of the invention. 
         FIG. 2  is a detailed view of a part of the tool shown in  FIG. 1 . 
         FIG. 3  is a detailed view of a part of an electric torque delivering impulse tool according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION 
     The electric torque delivering impulse tool schematically shown in  FIG. 1  comprises a housing  10  and a handle  11 . The handle  11  may include an actuator (not shown), preferably in the form of a trigger, for controlling the power of the tool. Further the handle  11  may include a connection to a battery or to an electric power net. The tool further comprises an electric motor  12  including a stator  13  and a rotor  14 , and a torque impulse generating pulse unit  15  with an output shaft  16  for connection to a socket (not shown). 
     The function of a torque impulse generating pulse unit  15  is well known to a person skilled in the art and is not described in detail in this application. A more detailed description of the function of a pulse unit is described in the international patent application WO 91/14541. 
     A detailed view of the motor  12  and the pulse unit  15  of the first embodiment of the invention is shown in  FIG. 2 . An advantage of the invention is that the motor rotor  14  and the pulse unit  15  are intimately assembled to form one single structure, such that there is no gap or play between the interconnected parts. This may be achieved in different manners whereof two possible embodiments are shown in  FIGS. 2 and 3 , respectively. 
     In the first embodiment, e.g. the embodiment shown in  FIGS. 1 and 2 , the stator  13  is arranged inside the rotor  14 . Typically the stator  13  comprises a conventional electrical winding  17 . The rotor  14  comprises a permanent magnet  35 , which is located on the inside of the rotor  14 . In a not shown alternative embodiment of the invention the rotor is arranged inside the stator, instead of outside it. 
     In the embodiment shown in  FIGS. 1 and 2  the rotor  14  is connected to a cylindrical inertia drive member  18  of the pulse unit  15  via a male and female connection part  20  and  22 , respectively. In the shown embodiment the connection of the male connection part  20  to the female connection part  22  consists of a splined coupling  21  between the interior of the female connection  22  and the exterior of the male connection part  20 . As discussed in the background part of this application this splined connection  21  would be the sole connection between the pulse unit and the motor in a conventional electric torque delivering impulse tool. 
     In the inventive arrangement a screw  19  is centrally arranged through the rotor  14  and into the male connection  20 . This arrangement creates a clamp force assures that the cylindrical inertia drive member  18  and the rotor  14  are both rigidly and fixedly assembled to each other, e.g. such that no mutual movement in either the axial, angular or radial direction is permitted between them. As alternative a screw could be arranged from the male part  20  into the female part where it could be fastened, e.g. by means of a nut. 
     By means of this screw attachment the rotor and the inertia drive member are assembled to each other so as to form one integrated rotatable structure which is mounted as one single unit inside said housing. This implies that the unit formed by the rotor  14  and the inertia drive member  18  may be mounted on joint bearings, and as a consequence only two bearings are needed in total for said unit. 
     In order to assure that both the rotor  14  and the inertia drive member  18  are stabilised with respect to the housing  10 , a central bearing  23 , e.g. a ball bearing, is clamped on the outside of the female part  22 . The outside of this central bearing  23  is attached via a support ring  36  to the inside of the housing  10 . Hence, by means of this central bearing  23  both the rotor  14  and the inertia drive member  18  are stabilised, both with respect to each other and to the housing  10 . 
     Apart from this central bearing  23 , only one additional bearing for stabilising the combined motor-pulse unit is needed inside the housing. This additional bearing could be arranged either at the back end  10   b  of the housing  10 , e.g. on the rotor, or at the front end  10   a  of the housing on the inertia drive member  18 . 
     In the shown embodiment, a front bearing  24 , a ball bearing, is arranged on the output shaft  16 . The front bearing  24  is arranged in a conventional manner such that it stabilises the output shaft  16  in both the axial and radial direction. Further though, it contributes to stabilise the inertia drive member  18  in the axial direction, such that no axial movement will be allowed between the inertia drive member and the output shaft  16 . 
     In the second embodiment, which is shown in  FIG. 3 , the interconnection between the rotor  14  and the inertia drive member  18  is arranged in a different manner. In this embodiment the rotor  14  is also arranged outside stator  13 . A first difference with respect to the first embodiment is the location of the bearings. In the second embodiment a rear bearing  25 , e.g. an axial bearing, is arranged at the rear of the housing  10 , behind the motor  12  and in coaxial alignment with the stator  13 . The rear bearing  25  is arranged inside a solid back end part  26 , which comprises a central bar  27  that is inserted into, and fixedly connected to, the stator  13 . The solid back end part  26  further includes a back plate  28  and a block ring  29  that extends forward from the back plate  28 . 
     The rear bearing  25  is arranged inside the block ring  29  of the solid back end part  26 . An S-shaped bearing connection part  30  is arranged with one end inside the rear bearing  25  and the opposed end attached to the inside of the rotor  14 . With this location, the rear bearing  25  stabilises the rotor  14  with respect to both the housing  10  and the stator  13 . This double stabilising effect is accomplished by means of the solid back end part  26 , which solidly connects both the stator  13  and the housing  10  to the rotor  14 . The connection to the rotor  14  is of course achieved via the rear bearing  25  and the bearing connection part  30 . 
     A further difference of this second embodiment with respect to the first embodiment lies in the connection between the rotor  14  and the inertia drive member  18 . In this second embodiment the rotor  14  is assembled to the cylindrical inertia drive member  18  by means of a splined coupling  31 . Apart from the splined coupling  31 , the front end  32  of the rotor  14  abuts a collar  33  on the rear periphery  39  of the inertia drive member  18 . This abutment ensures that the rotor  14  may not move forward with respect to the inertia drive member  18  and vice versa. 
     In order to prohibit mutual movement in the opposite axial direction, i.e. in the separating direction, a block  34  in the form of a solid plate has been provided. The block  34  restricts the movement of the splined coupling part  32  of the rotor  14  away from the splined coupling part  39  of the inertia drive member  18 . The block  34  is fastened to a solid portion of the inertia drive member  18  by means of at least three screws  38 . This arrangement provides a very solid connection between the rotor  14  and the inertia drive member  18  in both the axial and the radial direction. No central bearing, arranged around the connection of the rotor  14  and the inertia drive member  18 , is arranged in this second embodiment. 
     In the second embodiment a front bearing  24  is arranged on the output shaft  16 , in the same manner as in the first embodiment. Likewise, the front bearing  24  stabilises the output shaft  16  in both the axial and radial direction. In addition it stabilises the inertia drive member  18  in the axial direction, such that no axial movement will be allowed between the inertia drive member  18  and the output shaft  16 . 
     Both embodiments of the invention may include a resolver magnet  37  for detecting the rotational movement of the rotating parts of the torque delivering tool. By means of said detection, it is possible to calculate the retardation magnitude of said rotating parts. This arrangement per se is known to a skilled person and is described in e.g. EP 1 379 361 B1. 
     The optimal positioning of the resolver magnet  37  is not the same in both of the presented embodiments. In the first embodiment, which is illustrated in  FIG. 2 , the resolver magnet  37  is located around the rear end of the inertia drive member  18 , close to the central bearing  23 . 
     In the second embodiment, which is illustrated in  FIG. 3 , the resolver magnet  37  is instead located around the front end of the inertia drive member  18 , close to the front bearing  24 . Hence, in both embodiments the resolver magnet  37  is located close to a bearing. This is advantageous, because of the fixing action of the bearing that implies that the disturbance of the rotation of the resolver magnet  37  will be kept at a minimum. 
     In a third, not shown, embodiment the rotor  14  and the inertia drive member  18  are formed as a unit from one single block of metal. In such an embodiment the rotor  14  and the inertia drive member  18  will of course be absolutely rigidly assembled to each other, without any displacement or offset movement between them. Care will have to be taken to choose a material for the integrated unit that is hard enough to withstand the pulses that act on the inertia drive member  18 , but that at the same time is magnetic, such that the magnetic field of the permanent magnets  35  on the rotor  14  will not be negatively affected. It is, however, obvious to a person skilled in the art to select a material that may be given the properties desired for the purpose. Preferably, such an integrated rotor  14  and inertia drive member  18  will be journalled in two bearings only, either one front bearing and one back bearing, or one central bearing and one back or front bearing. 
     Above, by way of example, the invention has been described with reference to specific embodiments. The invention is however not limited to either of these embodiments. Instead, the invention is limited by the scope of the following claims.