This invention relates to power tools having at least one reciprocating blade, such as, non-exclusively, hedgetrimmers.

Hedgetrimmers and other power tools comprising at least one reciprocating blade driven by a motor (typically an electric motor or a petrol engine) are well known in the prior art. Other similar power tools include edging shears, shrub shears, pruning shears or secateurs. In each case, if a thick branch or other object is offered up to the blade or blades, the blade or blades can become blocked, which can lead to the motor overheating or otherwise becoming damaged. In addition, the user has to manually remove the offending object from the blades, and potentially find some other means for cutting it.

According to a first aspect of the invention, we provide a power tool comprising a motor, a blade set comprising at least one blade driven for reciprocating motion by the motor and a drive train connecting the motor to the blade set, the drive train having a coupled state in which motion of the motor is coupled to the blade set and an uncoupled state in which motion of the motor is not coupled to the blade set, the drive train being capable of sudden transition from the uncoupled state to the coupled state so as to transfer angular momentum built up in the motor or the drive train to the blade set.

As such, such a power tool allows for a hammer action to be provided to the blades, so that a strong impulse, larger than the force normally exerted by the blade set can be transmitted to the blade set for particularly stubborn obstacles.

The drive train may be arranged so as to switch from the coupled state to the uncoupled state dependent upon a decrease in a speed with which the blade set is moving. Thus, if the blade set is being impeded by some stubborn foliage, branch or other object, the drive train will uncouple the blade set from the motor. This will improve the life of the motor. Furthermore, given that the drive train can suddenly switch into the coupled state, that sudden switching can allow a strong impulse to be provided to the blade set to cut into the object impeding the blade set. If the first impact is insufficient, the blade set may again be moving so slowly that the drive train will switch to the uncoupled state, so that further successive sudden transitions into the coupled state can occur. The decrease in speed may represent the blade set moving at a speed less than a threshold, or may represent the blade set not moving. This represents the situation where the blade set is blocked by an object to be cut. The drive train may be arranged such that a decrease in the speed of the blade is detected by determining the rotational speed of an output shaft of the motor. As such, the drive train may comprise a clutch, arranged to engage suddenly once the rotational speed of the output shaft increases above a first threshold and to disengage once the rotational speed of the output shaft drops below a second threshold. Typically, the first threshold will be higher than the second threshold.

The clutch will typically comprise two rotating parts: a first rotating part operatively coupled to the output shaft and a second rotating part operatively coupled to the blade set, the first and second rotating parts being able to rotate relative to one another when in the uncoupled state, typically about a common axis, but not in the coupled state.

In one embodiment, the clutch will typically also comprise a centrifugal member fixed to one of the first and second rotating parts but which can move relative to the rotating part to which it is fixed under a centrifugal force as the rotating part rotates, the first and second rotating parts selectively engaging each other through the centrifugal member. In such a case, the clutch may be a centrifugal clutch.

The clutch may further comprise a biasing member, such as a spring, arranged so as to bias the centrifugal member in a direction opposing the centrifugal force.

The centrifugal member may selectively engage a radial protrusion on the rotating part to which it is not fixed, with the motion due to the centrifugal force selecting whether the centrifugal member engages the protrusion. Because the engagement of the centrifugal member and the other rotating part occurs through a radial protrusion, the centrifugal member will suddenly engage the protrusion as the rotating parts rotate relative to one another, thus providing a strong impulse to the blade set.

The centrifugal member will typically be fixed to the first rotating part for rotational movement about a pivot axis parallel to the common axis. The centrifugal member may comprise two arms extending away from the pivot axis: a long arm and a short arm, the long arm having a greater moment of inertia about the pivot axis than the short arm, such that, as the first rotating part is rotated, the long arm is forced radially outwards relative to the common axis by the centrifugal force and so the short arm is forced radially inwards relative to the common axis, the short arm engaging the protrusion on the second rotating part when the long arm is forced radially outwards.

In an alternative embodiment, the clutch may be arranged so that it suddenly engages dependent upon the relative rotational speed between the first and second rotating parts.

As such, the clutch may comprise:

a clutch member pivotably mounted on one of the first and second rotating parts for rotational movement about a pivot axis parallel to the common axis, pivoting motion of the clutch member causing engagement or disengagement of the clutch;

a cam mounted on the other of the first and second rotating parts on which the clutch member is not mounted;

the clutch member having a cam follower arranged to follow a circumference of the cam as the first and second rotating parts rotate relative to each other, the circumference having a step therein,

in which if the first and second rotating parts are rotating relative to one another at a first relative rotational speed, the cam follower will follow the step in circumference and cause the clutch member to pivot so as to not engage the clutch, and if the first and second rotating parts are rotating relative to one another at a second relative rotational speed higher than the first relative rotational speed, the cam follower will skip over the step so as to suddenly engage the clutch. In such a case, the clutch will depend upon a cam and follower rather than a centrifugal element, and so can be used where insufficient centrifugal force would be available. Should the blades become blocked, the first and second rotating parts will start to rotate relative to one another, initially slowly, and the clutch will disengage to permit such relative movement. Once sufficient angular momentum has been built up so that the cam follower skips over the step in circumference, the clutch will suddenly reengage, providing an impulse to whatever is causing the blade set not to move.

The clutch may comprise a biasing member, such as a spring, which is arranged to bias the clutch member out of contact with the other of the first and second members on which the clutch member is not mounted. The other of the first and second members on which the clutch member is not mounted will typically be provided with a protrusion arranged such that the centrifugal clutch engages the protrusion when it skips over the step in circumference to engage the clutch, but typically does not impart rotational motion to the other of the first and second members on which the clutch member is not mounted otherwise. Typically, the clutch member will be mounted on the first rotating part.

The circumference of the cam may be of the form of a spiral, with ends of the spiral joined by the step in circumference. The cam may be formed as a plate, typically mounted on the second rotating part. The cam follower may be a pin or other protrusion on the clutch member.

The power tool may be provided with a control circuit, which is arranged to selectively apply power (typically electrical power) to the motor, and to determine a speed with which the blade set is moving. The control circuit may be arranged so as to cease to apply power to the motor on determining that the speed of the blade set has dropped. The control circuit may also be arranged to reapply power to the motor subsequently; this can allow the sudden impulse to be applied.

The centrifugal clutch may be provided with an actuator arranged so as to selectively cause the centrifugal clutch to engage and disengage; the control circuit may be arranged so as to control the actuator so as to disengage the centrifugal clutch when the control circuit determines that the speed of the blade set has dropped, and can be arranged so as to control the actuator so as to engage the centrifugal clutch when power is reapplied to the motor.

Alternatively, the drive train may comprise a clutch of any type, the clutch providing selective engagement between the motor and the blade set, the tool being provided with an actuator which selectively engages or disengages the clutch, the control circuit being arranged so as to disengage the clutch when the speed of the blade set has dropped, and to subsequently suddenly engage the clutch after the power has been reapplied (and so angular momentum has been built up).

Typically, the power tool will be a hedgetrimmer, but could also be a set of edging shears, shrub shears, pruning shears or secateurs. Typically, the blade set will comprise two blades, arranged for reciprocating motion relative to one another. Where the tool is a hedgetrimmer, the blades will typically be arranged for reciprocating linear motion relative to each other, and may each comprise a plurality of teeth, the teeth of one blade passing over the teeth of the other blade during the reciprocating linear motion. There now follows, by way of example only, description of embodiments of the invention, described with reference to the accompanying drawings, in which:

Figure 1 shows a partial perspective view of a hedgetrimmer according to a first embodiment of the invention;

Figure 2 shows a perspective view of the centrifugal clutch of the hedgetrimmer of Figure 1 , in an uncoupled state;

Figure 3 shows an enlargement of area A of Figure 2;

Figure 4 shows a perspective view of the centrifugal clutch of the hedgetrimmer of Figure 1 , in a coupled state;

Figure 5 shows an enlargement of area B of Figure 4;

Figure 6 shows a partial cross section through the drive train of the hedgetrimmer of Figure 1 ;

Figure 7 shows a partial perspective view of a hedgetrimmer according to a second embodiment of the invention;

Figure 8 shows a flowchart depicting the method of operation of the hedgetrimmer of Figure 7; Figure 9 shows a partial perspective view of a hedgetrimmer according to a third aspect of the invention;

Figure 10 shows the same view as Figure 9, partially exploded; and Figures 11 to 15 each show a partially cut away view of the clutch of the hedgetrimmer of Figure 9, in different positions.

A hedgetrimmer 1 according to a first embodiment of the invention is shown in Figures 1 to 6 of the accompanying drawings.

The hedgetrimmer 1 comprises a pair of elongate blades 2, 3, each comprising a set of overlapping teeth 4. The hedgetrimmer 1 also comprises a motor 5, which drives the blades 2, 3 for reciprocating linear motion relative to one another through a drive train 6. The hedgetrimmer 1 is shown in the Figures without its housing, which would typically surround the motor and drive train, and would also house a user-operable switch by means of which the user can selectively apply electrical power from the battery to the motor in order to selectively drive the blades 2, 3. In case of a battery hedge trimmer, the housing would also house a power source such as a battery.

The drive train 6 comprises a pinion gear 7 mounted on an output shaft 8 of the motor 5. The drive train also comprises a wheel gear 9 which engages the pinion gear 7, and which contains a centrifugal clutch 10. The centrifugal clutch 10 selectively engages the wheel gear 9 to a shaft 11 coaxial with the wheel gear 9. The shaft 1 1 passes through the wheel gear 9 and, on its underside (shown in Figure 6 of the accompanying drawings) terminates in two mutually eccentric cams 12. These cams 12 each work in a linear slot 13 in one of the blades 2, 3, perpendicular to the length of the blades 2, 3. This arrangement is known as a Scotch yoke, and converts rotational motion of the shaft 1 1 into reciprocating (back-and-forth) linear movement of the blades 2, 3.

The centrifugal clutch 10 can be seen in more detail in Figures 2 to 5 of the accompanying drawings. The centrifugal clutch comprises a centrifugal member 14 mounted on the wheel gear 9 so as to be able to pivot about a pivot point 15. As such, the centrifugal member 14 can rotate or pivot about a pivot axis through pivot point 15 parallel to the common axis of the shaft 1 1 and wheel gear 9.

The centrifugal member 14 comprises two curved arms which extend away from the pivot point in opposing directions generally tangential to the axis of shaft 1 1. Of these arms, a long arm 16 extends further around the wheel gear 9 than a short arm 17. This gives the long arm 16 a greater moment of inertia about the pivot axis than the short arm 17. The centrifugal clutch 10 also comprises a spring 18 mounted on the wheel gear 9, which acts to bias the end of the long arm 16 distal from the pivot point 15 radially inwards towards the shaft 1 1.

The short arm 17 carries a tooth 19 which protrudes from the centrifugal member 14 radially inwards towards the shaft 1 1. The external circumference of the shaft 1 1 at this point spirals inwards, so as to define a radial step or protrusion 20. The protrusion 20 and the tooth 19 each define a face 21 , 22, the face of the tooth 19 and the protrusion 20 facing in opposing tangential direction relative to the axis of the shaft 1 1.

As such, when the wheel gear 9 is at rest or at low rotational speed, the spring 18 will bias the long arm 16 inwards towards the shaft and so the short arm 17 outwards. The faces 21 , 22 of the tooth 19 and the protrusion 20 do not engage, and so the shaft 1 1 is not turned, as shown in Figures 2 and 3 of the accompanying drawings.

As the rotational speed of the wheel gear 9 is increased (typically by the motor engaging), the centrifugal member will experience a centrifugal force relative to the wheel gear 9. Because it has a larger moment of inertia, the long arm 16 will experience a larger force F _{c l} forcing it radially outwards away from the shaft 1 1 than the corresponding force F _c2 on the short arm 17, so forcing the long arm outwards and the short arm inwards around the pivot 15. Eventually, the faces 21 , 22 of the tooth 19 and the protrusion 20 will engage suddenly, transferring an impulse of angular momentum stored in the motor 5 and the wheel gear 9 to the shaft for onwards transmission to the blades. The centrifugal clutch is engaged, and the rotational motion of the wheel gear 9 is transmitted to the shaft 1 1. This is the situation shown in Figures 4 and 5 of the accompanying drawings.

In use, when a user is cutting foliage of a size with which the blades 2, 3 can deal satisfactorily, the centrifugal clutch will be engaged and the motor 5 will drive the wheel gear 9 at a sufficient speed to ensure the engagement of the centrifugal clutch 10.

However, if a branch or other member that is too large becomes trapped between the teeth 4, the motor 5 will not be able to drive the blades 2, 3 (through the drive train 6) at any great speed, if at all. The rotational speed of the wheel gear 9 will therefore drop, causing the faces 21 , 22 to come out of engagement. The centrifugal clutch 10 is thereby disengaged, and the motor 5 will drive the blades 2, 3 for movement no more.

The motor 5, no longer under load, will increase in speed, and angular momentum will be built up in the motor and in the wheel gear 9. Eventually, the speed will have increased sufficiently for the faces 21 , 22 to reengage suddenly in an impact, transferring an impulse of angular momentum to the shaft, and onwards to the blades. This may be sufficient to clear the blockage; if not, the motor 5 speed will drop once more, and the disengagement and sudden reengagement of the centrifugal clutch 10 will successively occur until the blockage clears, in a similar manner to an impact wrench.

A hedgetrimmer according to a second embodiment of the invention is depicted in Figures 7 and 8 of the accompanying drawings. In this embodiment, equivalent integers to those of the first embodiment have been given corresponding reference numerals, raised by 50.

In this embodiment, rather than having a tooth formed in the short arm 67, the face 71 which contacts the face 72 of the protrusion 70 is formed in the end of the short arm 67. The short arm 67 thereby pushes, rather than pulls, the protrusion 70 to drive the shaft 61 when the faces 71 , 72 engage. This has been found to improve the stability of the centrifugal clutch 60.

In addition, a control circuit 80 is provided for the motor 55. This selectively applies power from an electric battery 81 or other electric power source to the motor 55. A user can operate a switch 82 to command operating of the hedgetrimmer, and the control circuit 80 will supply electric power to the motor 55 dependent on such command. The control circuit 80 is also capable of determining the speed with which the motor 55 is being driven. A flowchart showing the operation of the hedgetrimmer in this embodiment can be seen in Figure 8 of the accompanying drawings. At step 100, the user activates the switch 82. In response, the control circuit turns the motor 55 on at step 102 by applying power from the battery 81. The centrifugal clutch 60 is at this point disengaged, but as the motor 55 speeds up, the clutch will engage as the large end 66 of the centrifugal member 64 (referred to as the lever in the flowchart) moves outwards about pivot point 65 (step 104). This causes the faces 71 , 72 to engage (step 106), the blades to move (step 108) so that the user can then commence cutting branches or other foliage (step 110).

As long as only small branches are being cut (step 1 12), the centrifugal clutch 60 will remain engaged and so cutting can continue (step 1 14) until the user releases the switch 82 (step 1 16) and so the motor 55 stops (step 1 18), disengaging the centrifugal clutch 60.

However, if an attempt is made to cut overly large branches (step 120), the speed of the blade will decrease or stop (step 122). The control circuit 80 will detect that the motor speed or blade speed has dropped at step 124, typically by monitoring an increase in the current drawn by the motor 55 or by using a speed sensor on the motor output shaft or on the blades. The control circuit 80 then cuts power to the motor 55 (step 126) and the motor 55 stops. The spring 68 will disengage the faces 71 , 72 of the centrifugal clutch 60 and so the centrifugal clutch will be disengaged (step 128).

After a short period, the control circuit will reapply power to the motor 55 (step 130). The method will then recommence from step 104, with the angular momentum built up by the spinning of the motor 55 and the wheel gear 59 suddenly transferred to the blades 52, 53 as the centrifugal clutch 60 re-engages at step 106. This provides a hammer action on the overly-large branch, which may be enough to sever the branch. If not, the circuit can proceed through the loop via step 120 until the branch is severed.

This embodiment therefore has some electronic control over the operation of the centrifugal clutch, as the selective supply of power to the motor 55 by the control circuit 80 will influence whether the centrifugal clutch is engaged. In an alternative embodiment (not shown), an actuator, e.g. a solenoid, can be provided that disengages the centrifugal clutch on detection of a low-speed situation, and re-engages it suddenly once the motor has built up sufficient angular momentum.

A hedgetrimmer according to a third embodiment of the invention is shown in Figures 9 to 15 of the accompanying drawings. In this embodiment, equivalent integers to those of the first embodiment have been given corresponding reference numerals, raised by 100. In this embodiment, rather than using a centrifugal clutch, the clutch makes use of a cam and follower, and so is useful in situations where the moment of inertia of the centrifugal member, or the rotational speeds, would be insufficient for the centrifugal clutch to operate as desired.

As such, the drive train 106 comprises a pinion gear 107 on the output shaft of the motor (not shown). The drive train 106 also comprises a ring gear 109, having internal teeth, which engages the pinion gear 107, and which contains a clutch 1 10. The clutch 1 10 selectively engages the ring gear 109 to a shaft 1 1 1 coaxial with the ring gear 109. The shaft 1 1 1 drives the blades 102, 103, as in the first embodiment. The clutch 1 10 comprises a clutch member 1 14 mounted on the ring gear 109 so as to be able to pivot about a pivot point 1 15. As such, the clutch member 1 14 can rotate or pivot about a pivot axis through pivot point 1 15 parallel to the common axis of the shaft 1 1 1 and ring gear 109. The clutch member 1 14 comprises two curved arms which extend away from the pivot point in opposing directions generally tangential to the axis of shaft 1 1 1. Of these arms, a long arm 1 16 extends further around the ring gear 109 than a short arm 1 17. The clutch 110 also comprises a spring 1 18 mounted on the ring gear 109, which acts to bias the end of the long arm 1 16 distal from the pivot point 1 15 radially inwards towards the shaft 11 1.

The short arm 1 17 has an end face 121 (as in the second embodiment, rather than the tooth of the first embodiment). The external circumference of the shaft 1 1 1 spirals inwards, so as to define a radial step or protrusion 120. The protrusion 120 and the face 121 each face in opposing tangential directions relative to the axis of the shaft 1 1 1 , so that when the face 121 and the protrusion 120 engage, the clutch 1 10 is engaged and rotational motion of the ring gear can be transmitted from the ring gear 109 to the output shaft 1 1 1.

A cam plate 150 is provided on the shaft 1 1 1. This is fixed to the shaft 1 1 1 so as to rotate therewith. It is of the form of a flat plate, laying in a plane perpendicular to the common axis. Its circumference spirals inwards towards the common axis, so as to define a step 151 in the circumference.

The clutch member 1 14 is provided with a pin 152 that acts as a cam follower. The combination of the spring 1 18 and the pin 152 means that the pin 152 and so the clutch member 1 14 will generally follow the circumference of the cam plate 150 as the ring gear 109 and the output shaft 1 1 1 rotate relative to one another, as discussed below.

The operation of the clutch 1 10 can be demonstrated with reference to Figures 1 1 to 15 of the accompanying drawings. In these drawings, part of the cam plate 150 has been cut away (at 153) to show the workings beneath; however, the cam plate 150 will be of the shape shown in Figures 9 and 10 of the accompanying drawings.

As such, when the ring gear 109 is at rest or at low rotational speed, the spring 1 18 will bias the long arm 1 16 inwards towards the shaft 1 1 1 and so the short arm 1 17 outwards. The faces 121 , 120 of the clutch member 1 14 and the shaft 1 1 1 do not engage, and so the shaft 1 1 1 is not turned, as shown in Figure 1 1 of the accompanying drawings. The ring gear 109 is free to rotate about the shaft 1 1 1. The clutch 1 10 is therefore disengaged. As the ring member 109 (being driven by the motor) rotates around the shaft with the clutch disengaged, the pin 152 will follow the circumference of the cam plate 150, as shown in Figures 12 to 14. At relatively slow relative rotational speeds of the ring gear 109 relative to the shaft 1 1 1 , the pin 152 will simply follow the circumference all of the way around the shaft 1 1 1. Once the pin 152 reaches the step 151 in the circumference (as shown in Figure 15 of the accompanying drawings), it will be drawn down the step by the action of spring 1 18.

This has the effect of moving the long arm 1 16 radially inwards towards the common axis and so the short arm 1 17 outwards, thus lifting the faces 120, 121 out of engagement. It is to be noted that the position of the ring gear 109 relative to the shaft 1 1 1 where the pin 152 is coincident with the step 151 is the position where the faces 120, 121 would engage, if the clutch member 1 14 is in the correct position about its pivot axis. The ring gear 109 therefore continues to rotate relative to the shaft 1 1 1 , and so at lower relative speeds will cycle through the positions shown in Figures 1 1 to 14. However, as the relative rotational speed of the ring gear 109 relative to the shaft 1 1 1 increases, the pin 152 will begin to skip over the step, the moment of inertia of the clutch member 1 14 being sufficient that the spring cannot immediately draw the pin 152 down the step. Eventually, the position of Figure 15 is reached, where the pin 152 has moved negligibly radially inwards after having passed circumferentially over the step 151. This allows the faces 120, 121 to engage, thus suddenly engaging the clutch 1 10. The angular momentum built up in the ring gear 109 and motor is therefore coupled as an impulse exerted on the shaft 1 1 1 , which is transmitted as before to the blades 102, 103.

Once the clutch 1 10 engages, the ring gear 109 and shaft 1 1 1 will rotate together, and so there is no relative angular movement. As long as the ring gear 109 and shaft 1 1 1 are rotated together at speed, the higher centrifugal force on the long arm 1 16 as opposed to the short arm 1 17 will keep the clutch in engagement.

In use, when a user is cutting foliage of a size with which the blades 2, 3 can deal satisfactorily, the clutch 1 10 will be engaged, due to the differential centrifugal forces felt by the long 1 16 and short 1 17 arms respectively. If the blades 2, 3 stop due to a blockage, the differential centrifugal forces will no longer apply and so the clutch will disengage.

Once disengaged, the ring gear 109 is free to rotate relative to the shaft, and will do so until sufficient relative rotational speed is built up in order to reengage the clutch 1 10 as discussed above. If it is not possible to build up sufficient rotational speed to reengage the clutch 1 10, the hammer action will continually repeat until the blockage is cleared.

Whilst the control circuit 80 has been described only with reference to the embodiment of Figures 7 and 8 of the accompanying drawings, the skilled man would appreciate that it could be employed in any of the embodiments herein described.