HK1131364B - Electric power tool - Google Patents

Description

Electric tool

Technical Field

The present invention relates to an electric power tool, such as a drill driver, a circular saw, or the like, having a speed change function performed by a speed reduction mechanism.

Background

Conventionally, there is an existing electric power tool having a speed change function from the viewpoint of improving the work efficiency (see, for example, japanese patent Laid-open application No. s 63-101545).

One example of a power tool is shown in fig. 16. The electric power tool of this example includes: a motor 101 as a driving power source; a speed reduction mechanism 102 for transmitting the rotational power of the motor 101; a transmission unit (not shown) for transmitting the rotational power of the speed reducing mechanism 102 to the end tool; a resin-made housing 104 for accommodating the motor 101 and the reduction mechanism 102 in a main body portion thereof, the housing 104 being provided with a handle portion 104 a; shifting units 105 and 105a for changing the reduction ratio of the reduction mechanism 102, the shifting unit 105 being disposed at such a position as to allow an operator to manipulate it from outside the housing 104; a power switch 106 for turning on and off the power of the motor 101 held in the handle portion 104 a; and a battery pack 107 engaged with the housing 104 for supplying electric power to the motor 101.

When the load is high (i.e., when the workload is light), the shifting unit 105 is kept in the low-speed high-torque state as shown in fig. 17A, but when the load is low (i.e., when the workload is light), the shifting unit is switched to the high-speed low-torque state as shown in fig. 17B. This makes it possible to perform a suitable tightening operation according to the work load, thereby improving work efficiency.

In the case where the work load varies during the work, the speed changing unit 105 may be switched to change the reduction ratio during the work. However, this can present problems for power tools. Specifically, if the reduction ratio is changed during operation by the speed change unit 105, the rotary gears 102a of the reduction mechanism 102 are switched, at which time the gears 102a contact each other during their rotation and undergo wear or damage. This is the cause of problems in power tools. Generally, this problem is prevented by increasing the strength of the gear 102 a. In this case, the gear is formed of a high-strength metal member, or formed in a large size, which brings about disadvantages of high cost and weight increase.

Disclosure of Invention

In view of the above, the present invention provides an electric power tool capable of detecting the operation of a speed change unit and controlling the rotation of a motor even if the speed change unit is operated during the operation, thereby preventing the occurrence of a problem that would otherwise be caused by the wear or damage of gears of a speed reduction mechanism.

According to an embodiment of the present invention, there is provided a power tool including: a motor as a driving power source; a speed reduction mechanism for transmitting a rotational power of the motor, the speed reduction mechanism having two or more gear stages; a transmission unit for transmitting the rotational power of the speed reduction mechanism to the tip end tool; a bearing unit for rotatably supporting the transmission unit; a housing for accommodating the motor, the reduction mechanism, the transmission unit and the bearing unit in a main body portion thereof, the housing being provided with a handle portion; a speed change unit for changing a reduction ratio of the speed reduction mechanism, the speed change unit being provided at a position allowing an operator to operate it from outside the housing; a power switch for turning on and off a power of the motor; and an operation detecting unit for detecting a reduction ratio changing operation by the speed changing unit to control the electric power supplied to the motor.

With this structure, if the operator wishes to change the speed of the electric power tool, the operator operates the speed change unit, which is provided at a position that allows the operator to operate it from outside the housing. Even if the speed changing unit is operated to change the reduction ratio when the work load is changed during work, the operation of the speed changing unit is detected by the operation detecting unit. In response to the shift operation thus performed, the electric power supplied to the electric motor is controlled.

As a result, it is possible to prevent a problem from being generated in the electric power tool, which would otherwise be caused by the speed change operation performed during the operation. It is also possible to prevent the gears of the reduction mechanism from being worn or broken. In other words, when the shifting unit 5 is operated during operation, the operation of the shifting unit 5 is detected by the operation detecting unit 6. In response to the thus detected gear shift operation, the electric power supplied to the electric motor 1 is controlled by the control unit 7 before the gears of the reduction mechanism 2 are switched. Therefore, it is possible to reduce or prevent wear or damage of the gears of the reduction mechanism 2 when the gears are meshed with each other.

Preferably, the operation detecting unit operates in synchronization with the shifting unit. In this case, the operation of the operation detection unit can be effectively detected.

It is also preferred that the operation detecting unit is immovable relative to the housing and adapted to detect operation of the shifting unit by sensing a change in position of the shifting unit relative to the housing. In this case, it is possible for the operation detecting unit to easily and accurately detect the operation of the shifting unit.

The operation detecting unit may be adapted to detect the operation of the shifting unit without contacting the shifting unit. In this case, it is possible to reduce the wear of the operation detection unit and to extend the life span thereof, as compared with the case of using a general switch having a mechanical contact point.

In addition, when the reduction ratio of the reduction mechanism is changed, the operation detecting unit is preferably adapted to detect the gear shifted by the shifting unit. This ensures an improved detection accuracy and an improved reliability because the operation of the speed reducing mechanism is directly detected by the operation detecting unit.

Preferably, the electric power tool further includes a brake unit configured to apply a brake to the motor when the operation of the speed change unit is detected. In this case, the brake is applied to the electric motor when the operation of the speed changing unit is detected during operation. This makes it possible to additionally reliably prevent the occurrence of a problem in the electric power tool, which would otherwise be caused by wear or damage to the gears of the speed reduction mechanism.

The power tool may further include a detection unit configured to detect a rotation speed of the motor, and the power supplied to the motor is controlled only when the rotation speed of the motor is greater than a predetermined value at a time when the operation of the speed change unit is detected.

In this case, the predetermined value refers to a rotational speed of the electric motor at which the gears of the reduction mechanism are not worn or broken by the shifting operation of the shifting unit. The control of reducing the rotation speed of the motor is performed only when the rotation speed of the motor is greater than a predetermined value. Therefore, if the rotation speed of the motor is kept equal to or less than the predetermined value, although the speed change unit is operating, that is, if no problem is caused in the electric power tool, the electric power tool can be continuously used without stopping the operation.

In addition, the electric power tool may further include a holding unit configured to hold a task of controlling the electric power supplied to the motor by detecting the operation of the speed changing unit. In this case, it is possible to keep supplying electric power to the motor, that is, to control the rotation speed of the motor 1 to be equal to or less than a predetermined value, or to keep controlling the task of the motor 1 to be braked even after the shifting operation. Therefore, it is not necessary for the speed change unit to perform the speed change operation each time the electric power tool is stopped and restarted. In particular, even after the operation of the speed change unit, the electric power tool can be prevented from being restarted in an unstable state in which it is difficult for the operator to stably hold the tool with his hand. This ensures the safety of the work.

In addition, the electric power tool may further include a releasing unit configured to release the task of the control electric power being supplied to the motor. In this case, by performing the releasing operation according to the intention of the operator and controlling the electric power supplied to the motor, the work can be performed smoothly, and the efficiency is improved.

Preferably, the shifting unit is formed of a double lever operable from outside the housing, the double lever having an upper layer portion and a lower layer portion, the upper layer portion being movable toward the lower layer portion, the operation detecting unit including a switch or a sensor connected to the lower layer portion, the upper layer portion being adapted to be pushed down to press the switch or activate the sensor when the double lever is operated, so that the shifting operation can be detected. In this case, it is possible for the operation detecting unit to effectively detect the shifting operation using the movement of the double lever.

With the present invention, the electric power tool can detect the operation of the speed changing unit and control the electric power supplied to the motor even if the speed changing unit is operated during operation. This makes it possible to prevent problems from occurring in the electric power tool that would otherwise be caused by wear or damage to the gears of the speed reducing mechanism due to the speed changing operation. In addition, cost and weight increase can be avoided because the gears of the reduction mechanism do not need to be formed of high-strength metal parts, nor need to be formed in a large size to avoid wear or damage.

Drawings

The objects and features of the present invention will become apparent from the following detailed description of embodiments thereof, given with reference to the accompanying drawings, in which:

fig. 1 is a side sectional view showing a speed change unit and an operation detection unit of an electric power tool according to an embodiment of the present invention;

FIG. 2 is a block diagram of the power tool of FIG. 1;

fig. 3 is a view explaining a case where the tact switch of the operation detecting unit is installed in a lower layer portion of the double lever of the shifting unit;

fig. 4 is a view explaining a case in which the limit switch (or micro switch) of the operation detecting unit is installed below the speed changing unit;

fig. 5 is a view explaining a case in which the photointerrupter of the operation detecting unit is mounted on a lower layer portion of the double lever of the speed changing unit;

fig. 6 is a view explaining a case where the hall sensor of the operation detecting unit is mounted on the lower layer portion of the double-layer lever of the shifting unit;

fig. 7 is a view explaining a case where the pressure detecting switch of the operation detecting unit is installed in a lower layer portion of the double lever of the shift unit;

fig. 8 is a sectional view showing an example of a case in which a laser sensor of an operation detection unit is fixed to a housing, and a light reflection surface for reflecting laser light is formed on a gear of a speed change unit;

fig. 9 is a view explaining a state where the laser sensor shown in fig. 8 has received laser light reflected from the light reflecting surface of the gear of the shifting unit and has detected a shifting operation;

fig. 10 is a plan view showing an example of such a case, in which the operation direction of the speed changing unit is parallel to the rotation shaft of the motor;

fig. 11 is a plan view showing an example of a case in which the operating direction of the speed changing unit extends in a lateral direction perpendicular to the rotational shaft of the motor;

FIG. 12 is a control flow diagram of a circuit for maintaining the task of controlling the power supplied to the motor;

FIG. 13 is a control flow diagram of a circuit for releasing the task of controlling the power supplied to the motor;

fig. 14 is a view for explaining a case where the tact switch of the operation detecting unit is disposed beside the shifting unit;

fig. 15 is a view for explaining a case where the pressure sensor of the operation detecting unit is disposed beside the shifting unit;

fig. 16 is a side sectional view showing a general electric power tool; and

fig. 17A and 17B are views for explaining a switching operation of the general electric power tool between the low-speed high-torque state and the high-speed low-torque state.

Detailed Description

Embodiments of the present invention will now be described with reference to the accompanying drawings, which form a part hereof.

Fig. 1 shows an example of the electric power tool of the present embodiment. The electric tool includes: a motor 1 as a driving power source; a speed reduction mechanism 2 for transmitting the rotational power of the motor 1, the speed reduction mechanism 2 having two or more gear stages (stages); a transmission unit 3 for transmitting the rotational power of the reduction mechanism 2 to a tip end tool; a bearing unit for rotatably supporting the transmission unit 3; a housing 4 for accommodating the motor 1, the reduction mechanism 2, the transmission unit 3, and the bearing unit in a main body portion thereof, the housing 4 being provided with a handle portion 4 a; a speed change unit 5 for changing the reduction ratio of the reduction mechanism 2, the speed change unit 5 being provided at a position allowing an operator to operate it from outside the housing 4; a power switch 106 (see fig. 16) for turning on and off the power of the motor 1; and a battery pack 107 (see fig. 16) engaged with the housing 4 for supplying electric power to the motor 1.

In this regard, the electric power tool of this embodiment of the invention includes the operation detecting unit 6 for detecting the operation of the speed changing unit 5, which is the speed reduction ratio changing operation of the speed reducing mechanism 2 by the speed changing unit 5, the operation detecting unit 6 and controlling the electric power supplied to the electric motor 1.

The gear shift unit 5 of this example is formed by double-decker (stock) levers 5a and 5b that can move back and forth on the outer surface of the housing 4, the double-decker levers 5a and 5b having an upper section 5a and a lower section 5 b. The upper portion 5a can be depressed along the guide portion 5 c. The operation detection unit 6 includes a switch 6a connected to the lower layer portion 5 b. The upper section 5a is biased away from the lower section 5b (upwards) by a spring (not shown). The shifting operation of the double lever 5a and 5b is performed in two steps including the pressing operation of the upper section 5a and the moving operation thereof. At this time, the upper section 5a is pressed downward to press the switch 6a connected to the lower section 5b, thereby detecting the shifting operation.

In this embodiment, the direction of movement of the double lever 5a is such that: when the double lever 5a and 5b are moved along the rotation shaft 8 of the motor 1 (in the shift operation direction), the upper section 5a is moved perpendicular to the rotation shaft 8 of the motor 1, thereby driving the operation detecting unit 6 provided in the lower section 5 b. In response to this, the shifting operation is detected, and the power supplied to the motor 1 is controlled.

The operation detection unit 6 includes: a switch 6a for detecting a shift operation; and a control unit 7 (see fig. 2) for controlling the electric power supplied to the electric motor 1 depending on the thus detected shift operation.

The switch 6a of the operation detection unit 6 may be of a type having a mechanical contact. Examples of the switch 6a include a tact switch 11 and a limit switch 13 shown in fig. 3 or a micro switch having an elastic contact piece 13a that can be brought into contact with the protruding portion 5k of the shift unit 5 as shown in fig. 4. Preferably, the switch 6a is excellent in mechanical strength and environmental resistance. In fig. 3, an arrow a indicates the moving direction of the double-layer levers 5a and 5B, and an arrow B indicates the downward pushing direction of the upper layer portion 5 a.

The switch 6a of the operation detecting unit 6 may be of a type that detects a shifting operation while not requiring contact with the shifting unit 5. For example, the photo interrupter 10 shown in fig. 5 may be used.

In this regard, the opening portions 5f and the non-opening portions 5g are alternately arranged in the lower layer portions 5b of the double-layered bars 5a and 5b in the moving direction indicated by the arrow a. When the upper layer portion 5a is pressed downward in the direction of the arrow B, the photo interrupter 10 optically detects the opening portion 5f or the non-opening portion 5 g. The power supplied to the motor 1 is controlled based on the detection result. Reference numeral 5h in fig. 5 denotes a spring. The photo interrupter 10 is of a non-contact type and thus may have an extended service life. In addition, since the lead wire (lead line) through which the signal detected by the sensor is transmitted to the motor power supply line is kept stationary regardless of the operation of the speed changing unit 5, the photo interrupter 10 is extremely low in possibility of line breakage due to flexible deformation, and exhibits improved reliability.

As another example of the switch 6a of the operation detection unit 6, a hall sensor 12, which generates a voltage according to the intensity of a magnetic field as shown in fig. 6, may be used. The hall sensor 12 detects the intensity of magnetic field formed by the magnet 14, and the magnet 14 is provided in the upper layer portion 5a of the double-layer levers 5a and 5b, thereby controlling the power supplied to the motor 1. As another example of the switch 6a, a pressure sensing switch 15 may be used, as shown in fig. 7. With the pressure sensing switch 15, the pressure generated when the upper layer portion 5a of the double-layer levers 5a and 5b is pressed is converted into resistance. The electric power supplied to the motor 1 is controlled according to the resistance thus converted.

With the above-described structure, if the operator wishes to change the speed of the electric power tool, the operator operates the speed change unit 5, which is provided in a position that allows the operator to operate it from outside the housing 4. Even if the speed changing unit 5 is operated to change the reduction ratio when the work load varies during work, the operation of the speed changing unit 5 is detected by the operation detecting unit 6. In response to the shifting operation thus performed, the electric power supplied to the electric motor 1 is controlled by the control unit 7.

In other words, when the detected shift operation is used to convert the high-load operation into the low-load operation, the electric power supplied to the electric motor 1 is controlled so that the output of the electric motor 1 can be changed from the low-speed high-torque state to the high-speed low-torque state. In contrast, when the detected shift operation is used to shift the low-load operation to the high-load operation, the electric power supplied to the electric motor 1 is controlled so that the output of the electric motor 1 can be changed from the high-speed low-torque state to the low-speed high-torque state.

As a result, it is possible to prevent wear or damage of the gears of the reduction mechanism 2, which is usually caused by the gears contacting each other during rotation. It is also possible to prevent the generation of a problem that would otherwise be caused by the shifting operation performed during operation. In other words, when the shifting unit 5 is operated during operation, the operation of the shifting unit 5 is detected by the operation detecting unit 6. In response to the thus detected gear shift operation, the electric power supplied to the electric motor 1 is controlled by the control unit 7 before the gears of the reduction mechanism 2 are switched. Therefore, it is possible to reduce or prevent wear or damage of the gears of the reduction mechanism 2 when the gears are meshed with each other.

In addition, since the gear of the reduction mechanism 2 does not need to be formed of a high-strength metal member or formed in a large size to avoid abrasion or damage thereof, there is an advantage in that an increase in cost and weight can be avoided.

Fig. 8 and 9 show an example of a case in which the operation detecting unit 6 is kept immovable with respect to the housing 4, and the operation of the shift unit 5 is detected by sensing a change in the position of the element of the reduction mechanism 2 with respect to the housing 4. The operation detection unit 6 in this example detects the movement of the gear of the reduction mechanism 2, for example, the ring gear 2a, in the axial direction D. A light reflecting surface is formed on the outer peripheral wall portion 2b of the ring gear 2a, which is to be moved during the shifting operation. As the switch 6b of the operation detection unit 6, a laser sensor 40 is firmly fixed to the inner surface of the gear case that houses the ring gear 2 a.

If laser light is incident on the light reflection surface in the state shown in fig. 9, the laser light is reflected toward the laser sensor 40, and as a result, the position of the ring gear 2a is detected. At this time, a change in the relative positions of the ring gear 2a and the laser sensor 40 is detected to control the power supplied to the motor 1. If the speed changing unit 5 is operated during operation, the movement of the ring gear 2a is detected by the above unit, thereby controlling the rotation of the motor 1. Therefore, it is possible to prevent a problem from being generated in the electric power tool, which would otherwise be caused by wear or damage of the gears in the speed reduction mechanism 2.

In addition, the operation detecting unit 6 in this example detects the operation of the speed changing unit 5 by directly sensing the ring gear 2a, which is moved when the reduction ratio of the reduction mechanism 2 is changed. This ensures an improvement in detection accuracy and reliability. In addition, the operation detecting unit 6 operates in synchronization with the shifting unit 5. The advantage is that the operation detection unit 6 can efficiently perform its detection tasks.

As another example, it is preferable to provide a brake unit 70 for applying a brake to the motor 1 when the operation of the speed changing unit 5 has been detected by the operation detecting unit 6. In this case, the control unit 7 is provided with circuitry for: during operation, when operation of the speed changing unit 5 is detected, i.e., when the speed changing unit 5 is moved, the motor 1 is forced to decelerate and stop. This makes it possible to further reliably prevent the occurrence of a problem in the electric power tool, which would otherwise be caused by wear or damage to the gears of the speed reduction mechanism 2.

As another example, in the operation detecting unit 6, a detecting unit 80 may be provided for detecting the rotation speed of the motor 1. The electric power supplied to the electric motor 1 is controlled only when the rotation speed of the electric motor 1 is higher than a predetermined value at the time when the operation of the speed changing unit 5 is detected.

In this regard, the predetermined value refers to a rotational speed of the electric motor 1 at which the gears of the reduction mechanism 2 are not worn or broken by the shifting operation of the shifting unit 5. The control of reducing the rotation speed of the motor 1 is performed only when the rotation speed of the motor 1 is greater than a predetermined value. Therefore, if the rotation speed of the motor 1 is kept equal to or less than the predetermined value when the speed changing unit 5 is operated, that is, if no problem is caused in the electric power tool, the electric power tool can be continuously used without stopping the operation.

As another example, it is preferable to provide a holding unit 90, the unit 90 being configured to hold a task of controlling the electric power supplied to the electric motor 1 by detecting the operation of the speed changing unit 5. The holding unit 90 is adapted to store, for example, the shift information in the circuit of the control unit 7, and the control unit 7 receives the detection signal from the operation detecting unit 6.

Fig. 12 shows a flowchart of an example of a task for maintaining control of the electric power supplied to the motor 1, that is, a task for maintaining control of the rotational speed of the motor 1 equal to or less than a predetermined value, or a task for maintaining control of the motor 1 to be braked. First, the changeover operation of the shift unit 5 is detected by the operation detecting unit 6 at step S1. In response to the thus detected shift operation, the electric power supplied to the motor 1 is controlled so that the rotation speed of the motor 1 is equal to or less than a predetermined value (or the motor 1 is to be braked) at step S2. In steps S3 and S4, the supply of electric power to the motor 1 is controlled until the releasing (releasing) unit detects a releasing signal. In step S5, when the release signal is detected, the control of the electric power supplied to the motor 1 is released.

If the holding unit 90 is not present, the following situation will occur. When the operator operates the speed changing unit 5 during work, the electric power supplied to the electric motor 1 is controlled in such a manner that the operation of the electric power tool is stopped. If the operator finishes operating the speed change unit 5 in this state, the task of controlling the electric power supplied to the motor 1 is released, and the electric power tool is restarted. At this time, just after the operation of the speed changing unit 5, the electric power tool is restarted in an unstable state in which it is difficult for the operator to stably hold the tool with his hand. Therefore, the operator may be injured or may damage the workpiece.

The provision of the holding unit, which is configured to hold the task of controlling the electric power supplied to the electric motor 1 by detecting the operation of the speed changing unit 5, ensures that the electric power tool can be prevented from being restarted in a state where the operator holds the tool unstably by hand. This ensures safety in operation.

According to the intention of the operator, it is preferable to provide a releasing unit for releasing the task of controlling the electric power supplied to the motor 1. As the releasing unit, there may be a method in which the power switch 106 is reactivated, the switch 106 is used to turn on and off the power of the motor 1 held in the handle portion 4a, that is, the power switch 106 is completely released and pushed back, and a signal indicating the reactivation is sent to the circuit of the control unit 7, thereby releasing the task of controlling the power. The releasing unit may be a method in which the task of controlling the power supplied to the motor 1 is released when the power switch 106 for turning on and off the power of the motor 1 held in the handle portion 4a is in an off state.

Alternatively, a release switch (not shown) may be provided separately. Fig. 13 shows a flowchart of an example for releasing the task of controlling the electric power supplied to the electric motor 1. By performing the releasing operation according to the intention of the operator and controlling the electric power supplied to the electric motor 1 in this manner, the work can be performed smoothly. This helps to improve work efficiency without concern for operator injury or damage to the workpiece.

In the above embodiment, the speed shift unit 5 is operated back and forth in the axial direction D parallel to the rotary shaft 8 of the motor 1, as shown in fig. 10. Alternatively, the operating direction of the speed shift unit 5 may extend, for example, in a lateral direction perpendicular to the rotational shaft 8 of the motor 1, as shown in fig. 11.

Although the operation detecting unit 6 is disposed below the shift unit 5 in the above-described embodiment, the operation detecting unit 6 may be located beside the shift unit 5, for example, as shown in fig. 14 and 15.

Fig. 14 shows a case where the tact switch 60 (operation detecting unit 6) is provided beside the shifting unit 5. Fig. 15 shows a case where the pressure sensor 61 (operation detecting unit 6) is provided beside the speed changing unit 5 (along side). In these cases, the elastic projection 50, which is movable together with the shift unit 5, is formed on an extended surface of the shift unit 5, and the ridge-and-groove portions 4b and 4c are formed in a portion of the housing 4 for accommodating the shift unit 5.

With the structure shown in fig. 14, the operation of the shift unit 5 is detected in such a manner that the tact switch 60 is pushed by the elastic projection 50, which is normally pressed against the ridge-and-groove portions 4b and 4c, when the shift unit 5 is operated, the switch 60 being disposed in the ridge-and-groove portions 4b and 4 c.

With the structure shown in fig. 15, the operation of the shift unit 5 is detected in such a manner that the elastic protrusion 50 pushes the pressure sensors 61 provided in the ridge groove portions 4b and 4c when the shift unit 5 is operated. The operation detection unit 6 is not limited to the tact switch and the pressure sensor described above, but may be a limit switch, a micro switch, or the like.

While the invention has been shown and described with reference to the embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims (10)

1. A power tool, comprising:

a motor as a driving power source;

a speed reduction mechanism for transmitting a rotational power of the motor, the speed reduction mechanism having two or more gear stages;

a transmission unit for transmitting the rotational power of the speed reduction mechanism to the tip end tool;

a bearing unit for rotatably supporting the transmission unit;

a housing for accommodating the motor, the reduction mechanism, the transmission unit and the bearing unit in a main body portion thereof, the housing being provided with a handle portion;

a speed change unit for changing a reduction ratio of the speed reduction mechanism, the speed change unit being provided at a position allowing an operator to operate it from outside the housing;

a power switch for turning on and off a power of the motor;

an operation detection unit for detecting a reduction ratio changing operation by the speed changing unit; and

a control unit for controlling the electric power supplied to the motor in accordance with the detected shift operation, wherein when the detected shift operation is for switching a high-load operation to a low-load operation, the control unit controls the electric power supplied to the motor before the gear of the reduction mechanism is switched so as to change the output of the motor from a low-speed high-torque state to a high-speed low-torque state, and when the detected shift operation is switched from the low-load operation to the high-load operation, the control unit controls the electric power supplied to the motor before the gear of the reduction mechanism is switched so as to change the output of the motor from the high-speed low-torque state to the low-speed high-torque state.

2. The electric power tool as claimed in claim 1, wherein the operation detecting unit operates in synchronization with the speed changing unit.

3. The electric power tool as claimed in claim 1, wherein the operation detecting unit is immovable with respect to the housing and adapted to detect the operation of the shifting unit by sensing a change in position of the shifting unit with respect to the housing.

4. The electric power tool as claimed in claim 1, wherein the operation detecting unit is adapted to detect the operation of the speed changing unit without contacting the speed changing unit.

5. The electric power tool according to claim 1, wherein the operation detecting unit is adapted to detect the gear shifted by the speed changing unit when changing the reduction ratio of the reduction mechanism.

6. The power tool of any one of claims 1-5, further comprising a brake unit configured to apply a brake to the motor when operation of the speed change unit is detected.

7. The electric power tool as claimed in claim 1, further comprising a detection unit configured to detect a rotation speed of the motor, wherein the electric power supplied to the motor is controlled only when the rotation speed of the motor is higher than a predetermined value at a time when the operation of the speed changing unit is detected.

8. The power tool of any one of claims 1-5 or 7, further comprising:

a holding unit configured to hold a task of controlling the electric power supplied to the electric motor by detecting an operation of the speed changing unit.

9. The power tool of claim 8, further comprising a release unit configured to release the task of controlling the power supplied to the motor.

10. The power tool of claim 1,

the shifting unit is formed of a double lever operable from outside the housing, the double lever having an upper section and a lower section, the upper section being movable toward the lower section, and the operation detecting unit includes a switch or a sensor connected to the lower section, the upper section being adapted to be pushed down to press the switch or activate the sensor when the double lever is operated, so that the shifting operation can be detected.