Electric tool

An electric tool is provided with a rotary striking mechanism unit converting the rotational force of a brushless motor to a striking force and applying the striking force to a tip tool. The required rated input of the motor is 1000-1300 W, the motor speed under fixed speed control is 16800±10% (min−1), and the variable Ku, which relates to the motor, is defined by the following expression Ku={(stator core outer diameter)2×(stator core lamination thickness)×(total tooth width)×(rotor outer diameter)}÷{(rated input)×(motor speed under fixed speed control)}, wherein the stator core outer diameter, the stator core lamination thickness, the total tooth width and the rotor outer diameter are shown in mm, the rated input is shown in W, the motor speed is shown in min−1, and the Ku value of the motor is set to 14.6<=Ku<=21.8.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 application of an International PCT application serial no. PCT/JP2015/064754, filed on May 22, 2015, which claims the priority benefits of Japan Application No. 2014-112509, filed on May 30, 2014. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to an electric tool, such as a hammer drill, an impact drill, an impact wrench, or the like, where a brushless motor is used as a driving source.

Description of Related Art

Electric tools, particularly electric striking tools, such as hammer drill have a mechanical section that is complicated and has a significant amount of components. To maximize the performance of the mechanical section to its full extent, it is very important for the mechanical section to match motor performance.

For example, to solidly match the mechanical section with motor performance, motor speed control utilizing electronic control is applied to suppress variation of a target motor speed within a range of plus or minus a few percentage points. In addition, in order to continuously perform demanding operations, a rated power input (W) of the device is also important. Based on a target value, motor winding, thickness of motor core lamination, or the like may be modified.

Regarding the shape of motor core, a standard motor core capable of achieving average performance in various devices is adopted.

PRIOR ART LITERATURE

Patent Literature

Patent Literature 1: Japanese Patent Publication No. 2007-259513

Patent Literature 1 does not describe specific values as target performance, but includes recitation concerning the optimization of the shape of motor core. However, based on values of target performance (e.g., values of speed, torque, rated power input, cost, etc.), the optimized shape of the motor core may differ. For example, when a high speed motor is optimized, it is more suitable to adopt a reduced rotor outer diameter, so as to stand a centrifugal force. Also, when a high torque is needed, contrary to speeding up the motor, the rotor outer diameter needs to be increased to dispose a larger magnet. Moreover, when a high rated power input is needed, a stator core having larger slots is required to wind stator coils having a greater diameter to reduce a resistance. Furthermore, when the cost is to be reduced, a volume of the magnet disposed to the rotor needs to be reduced to increase a volume of a rotor core.

Thus, regarding the shape of the motor core of Patent Literature 1, where the shape is determined without setting the target performance of the electric tool, there is an issue that the shape may not be an optimal shape for an electric tool, particularly an electric striking tool.

SUMMARY OF THE INVENTION

With awareness of the situation, the invention is achieved to provide an electric tool where optimization is performed for the required target performance and the size allows excellent operability.

A mode of the invention is an electric tool. The electric tool includes an electric striking tool. The electric tool includes: a brushless motor having a stator where stator coils are wound around teeth of a stator core, and a rotor rotatably supported on an inner circumference side of the stator; and a rotary striking mechanism unit, converting a rotational force of the brushless motor into a striking force and applying the striking force to the tip tool. When the brushless motor has a rated power input ranging from 1000 to 1300 W and a motor speed under fixed speed control at 16800±10% revolutions per minute, and an optimization variable Ku relating to the brushless motor is defined by Expression as follows:
Ku={(stator core outer diameter)2×(stator core lamination thickness)×(total tooth width)×(rotor outer diameter)}÷{(rated power input)×(motor speed under fixed speed control)},
wherein the stator core outer diameter is a numerical value shown in mm, the stator core lamination thickness is a numerical value shown in mm, the total tooth width is a numerical value shown in mm, the rotor outer diameter is a numerical value shown in mm, the rated power input is a numerical value shown in W, and the motor speed is revolutions per minute,
a value of Ku of the brushless motor is set to 14.6≤Ku≤21.8.

In the mode, it is preferable that the rotor has plate magnets.

In the mode, it is preferable that the stator core has six slots.

Another mode of the invention is also an electric tool. The electric tool includes: a brushless motor having a stator where stator coils are wound around teeth of a stator core, and a rotor rotatably supported on an inner circumference side of the stator; and a transmission part transmitting a rotational force of the brushless motor to a tool maintaining element. The brushless motor has a rated power input ranging from 1000 to 1300 W and a motor speed under fixed speed control at 16800±10% revolution per minute.

When an optimization variable Ku relating to the brushless motor is defined by Expression as follows:
Ku={(stator core outer diameter)2×(stator core lamination thickness)×(total tooth width)×(rotor outer diameter)}÷{(rated power input)×(motor speed under fixed speed control)},
wherein the stator core outer diameter is a numerical value shown in mm, the stator core lamination thickness is a numerical value shown in mm, the total tooth width is a numerical value shown in mm, the rotor outer diameter is a numerical value shown in mm, the rated power input is a numerical value shown in W, and the motor speed is revolutions per minutes,
a value of Ku of the brushless motor is set to 14.6≤Ku≤21.8.

In the meantime, any combination of the above forming elements and a method, a system and the like converted from the expression of the invention are also effective as the modes of the invention.

According to the invention, an electric tool as follows is achieved. Namely, the electric tool has a motor size not damaging the operability, and is able to achieve the required target performance, namely the rated power output ranging from 1000 to 1300 (W) and the motor speed under the fixed speed control at 16800±10% (min−1).

DESCRIPTION OF THE EMBODIMENTS

In the following, the preferred embodiments of the invention are described in detail with reference to the accompanying drawings. Same or equivalent forming elements, components, processes, and the like shown in the respective figures are marked with the same reference symbols. In addition, repeated descriptions are appropriately omitted. Also, the embodiments merely serve as exemplary examples, instead of limitations of the invention. All the features described in the embodiments or combinations thereof are not necessarily the essence of the invention.

As an embodiment of the electric tool of the invention, an application of a hammer drill as an electrical striking tool is described.

As shown inFIG. 1, a hammer drill1includes: a brushless motor2configured as a driving force and stored in a housing17; a rotary striking mechanism unit19converting a rotational force of the brushless motor2into a striking force and applying the striking force to a tip tool (not shown), such as a drill, installed to a tool maintaining element16; and a control substrate18mounted with a control circuit operating the brushless motor2.

The control substrate18is disposed to a lateral side of the brushless motor2and stored in the housing17.

The rotary striking mechanism unit19includes: a striking part (including a first gear4, a crank shaft6, a conrod7, a piston pin8, a piston9, a striking piece10, and an intermediate piece11), and a rotary transmission mechanism (including a second gear12, a third gear14, a cylinder15, and the tool maintaining element16). The piston9, the striking piece10, and the intermediate piece11are slidably disposed in the cylinder15, and move reciprocally in the cylinder15.

The hammer drill1is configured to be able to perform a striking operation and a rotating operation. The striking operation is performed as follows: a driving shaft3rotates as driven by rotation of the brushless motor2, the rotation of the driving shaft3is transmitted to the crank shaft6having an eccentric pin5through the first gear4, the piston9is moved reciprocally through the conrod7rotatably installed to the eccentric pin5and the piston pin8, the striking piece10is moved reciprocally through an air spring intervening between the piston9and the striking piece10, and a substantial center of the striking piece10strikes the tip tool through the intermediate piece11. The rotating operation is performed as follows: the rotation of the driving shaft3is transmitted to an intermediate shaft13having a tooth part13athrough the second gear12, and then transmitted by rotating the cylinder15through the third gear14engaged with the tooth part13a, so as to rotate the tip tool by rotating the tool maintaining element16.

FIG. 2is a traverse cross-sectional view illustrating the brushless motor2, andFIG. 3a longitudinal cross-sectional side view of the same. In the figures, a fixed part of the brushless motor2fixed to the housing17shown inFIG. 1has a stator20, and a rotating part of the brushless motor2rotatably supported on an inner circumference side of the stator20by making use of the housing17has a rotor30.

The stator20has a stator core21laminated with an electromagnetic steel sheet. As shown inFIG. 4, the stator core21has a yoke22allowing a magnetic flux to flow in a circumferential direction and six teeth23disposed side by side in the circumferential direction to allow the magnetic flux to flow in a radical direction. A slot26is provided between adjacent teeth23. For each of the teeth23, a resin-made insulator24capable of electrical insulation and damage prevention is wound around a stator coil25.

As shown inFIG. 5, the rotor30has a rotor core31. A total of four plate magnets33are disposed in four gaps (slit holes)32of the rotor core31. The plate magnet33is magnetized such that, with respect to a wide width surface, one side is of N polarity, whereas the other side is of S polarity. The driving shaft3penetrates through a central part of the rotor core31and is fixed so as to integrally rotate with the rotor core31.

As shown in the cross-section ofFIG. 3, balance rings35made of metal are disposed on two ends of the rotor core31to weight-balance the rotor30. As coil end parts25a, the stator coils25protrude from two ends of the laminated stator core21. An insulator27is disposed between the core end part25aand the stator core21. A width of the laminated stator core21is defined as a stator core lamination thickness Ts.

FIG. 4illustrates a stator core outer diameter Rs and a tooth width Qt. The stator core outer diameter Rs is a diameter of an outer circumference part of the yoke22. A total tooth width Q is defined as follows: total tooth width=tooth width×tooth number. In an example for numerical values of the embodiment, the total tooth width Q is 10 mm×6=60 mm.

FIG. 5illustrates a rotor outer diameter Rr. The rotor outer diameter Rr represents a diameter of an outer circumference part of the rotor.

FIG. 6shows motor characteristics of the brushless motor2used in the hammer drill of this embodiment. Since the brushless motor2for the hammer drill is matched precisely with the rotary striking mechanism unit19shown inFIG. 1, even if a load is applied to the brushless motor2during an operation, the control circuit may still exert fixed speed control by having the brushless motor2rotate at a target speed. Since the rotary striking mechanism unit is formed by a plurality of components performing complicated operations, if the motor speed is deviated from the target value, the striking performance may be reduced. As a way of fixed speed control, it is common to feedback the motor speed while exerting duty control on a power source voltage.

Table 1 is a table showing optimized motor sizes with respect to the hammer drill.

TABLE 1Case 1Case 2Case 3Case 4Case 5Case 6Case 7Case 8Case 9rated power inputW115011501150115011501150115011501150motor speed under fixedrpm168001680016800168001680016800168001680016800speed controlouter diameter of stator coremm45.8248.4252.97558.56571.574.8876.50583.85stator core laminationmm80.572.160.249.44033.130.128.924.0thicknesstotal tooth widthmm42.344.748.954606669.1270.6277.4outer diameter of rotormm24.325.628.031.034.437.839.640.544.4S coil resistanceΩ0.950.820.700.520.460.520.720.790.90Ku—9.010.012.014.618.121.824.025.030.0
In Table 1, target performance of a motor corresponding to a 40 mm-level hammer drill is set as having a rated power input of 1150 W and a motor speed under the fixed speed control at 16800 min−1(RPM). Based on the respective stator core outer diameters Rs, the stator core lamination thicknesses Ts applicable for the device are respectively determined under a premise that even if the outer diameters Rs are different, the stator cores still have the same volume. Accordingly, the total tooth widths Q and the rotor outer diameters Rr (Case 1 to Case 9) are derived by having a resistance of the stator coils at the lowest. According to sizes of Case 1 to Case 9, values of an optimization variable Ku defined in Expression (1) below are calculated.
Ku={(stator core outer diameter)2×(stator core lamination thickness)×(total tooth width)×(rotor outer diameter)}÷{(rated power input)×(motor speed under fixed speed control)}  (1)
Here, the stator core outer diameter is a numerical value shown in mm, the stator core lamination thickness is a numerical value shown in mm, the total tooth width is a numerical value shown in mm, the rotor outer diameter is a numerical value shown in mm, the rated power input is a numerical value shown in W, and the motor speed is a numerical value shown in min−1(RPM).

Moreover, as shown inFIG. 7, if a curve diagram is illustrated using Ku as the horizontal axis and the resistance of the stator coils as the vertical axis, the resistance of the stator coils is most effectively lowered when the value of Ku of the brushless motor is set to 14.6≤Ku≤21.8 (i.e., the range of Case 4 to Case 6 of Table 1). When the resistance of the stator coils is lowered, a copper loss is reduced and an increase in temperature is also reduced. Thus, the rated power input of 1150 W may be easily achieved. Moreover, since a motor efficiency is facilitated, the motor speed is increased, making it easier to achieve the motor speed of 16800 min−1. Furthermore, since a design of magnetic properties is optimized, and the rotor outer diameter Rr is in a suitable size, when the plate magnets33having a low cost are disposed in the rotor30, high performance as well as low cost may be achieved at the same time. Also, the stator core outer diameter Rs and the stator core lamination thickness Ts are in suitable sizes. Thus, the size of the housing17shown inFIG. 1does not need to be increased to be able to store the brushless motor2, so a motor storage portion of the housing17does not need to be increased, either.

Besides, when a ratio between the outer diameters of the stator core21and the rotor30satisfies the value of Ku, if the number of slots of the stator core21is set at six, it is easy to wind the stator coils25having a greater diameter. Therefore, the resistance of the stator coils may be lowered most effectively. This is because that, if the number of slots of the stator is too few, the number of turns of the coil wound in one slot is increased, making it difficult to wind the coil in alignment and thus unable to wind the stator coil25having a greater diameter. Also, if the number of slots of the stator is increased, gaps for insertion of coil winding devices must be disposed in the respective slots26during a winding operation. Thus, the gaps for winding the stator coils25may be reduced, making it unable to wind the stator coils25having a greater diameter.

In addition, in the Table 1, the values of Case 1 to Case 9 are obtained by setting the rated power input at 1150 W and the motor speed under the fixed speed control at 16800 min−1. However, in cases where the rated power input ranges from 1000 to 1300 (W) and the motor speed under the fixed speed control at 16800±10% min−1, the same range of the value of Ku is also applicable.

With the embodiment, the following effects are achievable.

(1) An electric striking tool as follows is achieved. Namely, the electric striking tool has the brushless motor2of a size not damaging the operability, and is able to achieve the required target performance, namely the rated power output ranging from 1000 to 1300 (W) and the motor speed tinder the fixed speed control at 16800±10%(min−1).

(2) The brushless motor2is able to be designed such that the stator coils have the lowest resistance, so as to reduce the copper loss and reduce the increase in temperature in practical use. Accordingly, the motor efficiency is also increased.

(3) The rotor outer diameter Rr is in suitable size ranging from 31.0 mm to 37.8 mm. Therefore, when the low-cost plate magnets33are disposed in the rotor30, high performance as well as low cost may be achieved at the same time.

(4) If the number of slots of the stator core21is set at 6, it is easy to wind the stator coils25having a greater diameter in attempt to further reduce the resistance of the stator coils.

In view of the foregoing, the invention is described by taking the embodiments as examples. However, people having ordinary skills in the art should understand that various modifications may be made to the respective forming elements or treatment processes of the embodiments within the scope as recited in the claims. In the following, examples of such modifications are described.

In the embodiments, the plate magnets are inserted into the gaps of the rotor core to serve as the rotor. However, a rotor using a cylindrical magnet where N polarity and S polarity are alternately formed on an outer circumference surface.

In addition, in the embodiments, a hammer drill is exemplified. However, the invention may also be applied to an electric striking tool for striking and rotating that uses a brushless motor as the driving source, such as an impact drill, an impact wrench, or the like, or an electric tool without a striking mechanism, such as a driver drill.