WO2014038998A1

WO2014038998A1 - Power tools having at least one reciprocating blade

Info

Publication number: WO2014038998A1
Application number: PCT/SE2013/051025
Authority: WO
Inventors: Markus Maag; Wolfgang Kast; Dimitri DAUBERT; Muhammad Ibrahim Nailul MORAD; Florian RUOFF; Benedikt HILDEBRANDT
Original assignee: Husqvarna Ab
Priority date: 2012-09-04
Filing date: 2013-09-03
Publication date: 2014-03-13
Also published as: WO2014038987A1; CN104602510A; EP2892317A1; EP2892317A4

Abstract

A power tool, such as a hedgetrimmer, comprising a motor (5), typically electric, a blade set (2, 3) comprising at least one blade (2, 3) driven for reciprocating motion by the motor (5) and a drive train (6) connecting the motor (5) to the blade set (2, 3), the drive train (6) having a coupled state in which motion of the motor (5) is coupled to the blade set (2, 3) and an uncoupled state in which motion of the motor (5) is not coupled to the blade set (2, 3), the drive train (6) being capable of sudden transition from the uncoupled state to the coupled state so as to transfer angular momentum built up in the motor (5) or the drive train (6) to the blade set (2, 3). This 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 comprise a centrifugal clutch (10), arranged to engage suddenly once the rotational speed of an output shaft (7) of the motor (5) increases above a first threshold and to disengage once the rotational speed of the output shaft (7) drops below a second threshold.

Description

POWER TOOLS HAVING AT LEAST ONE RECIPROCATING BLADE

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 transmitted 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 at least one blade of the blade set moving at a speed less than a threshold, or may represent at least one blade of 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.

The clutch may be arranged such that the first and second rotating parts impact one upon the other during the sudden transition. As such, one of the first and second rotating parts may comprise an impact surface, against which the other of the first and second rotating parts impacts in the transition from the uncoupled to the coupled states. The first and second rotating parts may be coupled together through the impact surface in the coupled state.

In one embodiment, the clutch will typically also comprise a centrifugal member fixed to the first rotating part but which is movable relative to the first rotating part under a centrifugal force as the first 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 second rotating part, with the motion due to the centrifugal force selecting whether the centrifugal member engages the protrusion. The radial protrusion may form the impact surface. 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 radial protrusion may be angled outward relative to the common axis, so that it tends to deflect the centrifugal member against the centrifugal force. Thus, this can replace the need for a biasing member, as rotation of the centrifugal member past the point where it engages the radial protrusion can act to disengage the clutch.

The centrifugal member will typically be fixed to the first rotating part for rotational movement about a pivot axis parallel or generally 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 clutch will disengage and the first and second rotating parts will start to rotate relative to one another, initially slowly. 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 protrusion may be angled outward relative to the common axis, so that it tends 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. Thus, this can replace the need for a biasing member, as rotation of the centrifugal member past the point where it engages the radial protrusion can act to disengage the clutch.

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 clutch may be provided with a friction part, which is arranged to accelerate the first and second rotating parts relative to one another before the sudden transition at start up of the power tool, when there is no blockage of the blade set. Thus, the first and second rotating parts can engage each other through a sliding contact (thus allowing the first and second rotating parts to partially engage and to partially equalise their rotational speed) before the sudden transition to the coupled state. The friction part may comprise either or both of the cam and the cam follower; the cam or the cam follower may be biased against each other over at least part of their range of travel.

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 control circuit may also be arranged to only reapply power a predetermined number of times before a blockage is removed from the blade set; this prevents excessive wear on the motor by limiting the number of times it can be run whilst the blade set is blocked.

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 embodiment of the invention;

Figure 10 shows the same view as Figure 9, partially exploded;

Figures 11 to 15 each show a partially cut away view of the clutch of the hedgetrimmer of Figure 9, in different positions;

Figure 16 shows a partial perspective view of a hedgetrimmer according to a fourth embodiment of the invention;

Figures 17 and 18 shows a partial enlargement of Figure 16, in two different rotational positions of the clutch;

Figure 19 shows a partial perspective view of a hedgetrimmer according to a fifth embodiment of the invention; and

Figure 20 shows a partial enlargement of Figure 19.

A hedgetrimmer 1 according to a first embodiment of the invention is shown in Figures 1 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 or power mains 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 generally or substantially 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 tooth 19 and the protrusion 20 each define a face 21 , 22, the face of the tooth 19 and the protrusion 20 facing in opposing tangential directions relative to the axis of the shaft 1 1 , and the face 22 of the protrusion 20 defining an impact surface.

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 1 1 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 arm 66 of the centrifugal member 64 moves outwards about pivot point 65 (step 104). This causes the faces 71 , 72 to engage (step 106), and consequently the blades to move (step 108) so that the user can then commence cutting branches or other foliage (step 1 10). As long as only small branches are being cut, the centrifugal clutch 60 will remain engaged and so cutting can continue (step 1 12) until the user releases the switch 82 (step 114) and so the motor 55 stops (step 1 16), disengaging the centrifugal clutch 60. However, if an attempt is made to cut overly large branches, 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 and the motor 55 stops (step 126). 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.

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 1 10 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 1 1 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 and 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 11 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.

A hedgetrimmer in accordance with a fourth embodiment of the invention is shown in Figures 16 to 18 of the accompanying drawings. This functions in a similar manner to that of the previous embodiment; equivalent features have been assigned reference numerals of the equivalent features in the previous embodiment, raised by 100.

In this embodiment, a pinion gear 207 coupled to the output shaft of the motor (not shown) engages a ring gear 209 having internal teeth. A clutch 210 selectively engages the ring gear 209 to a shaft 21 1 coaxial with the ring gear 209. The shaft 21 1 drives the blades 202, 203, as in the previous embodiments.

The clutch 210 comprises a clutch member 214 mounted on the ring gear 209 so as to be able to pivot about a pivot point 215. As such, the clutch member 214 can rotate or pivot about a pivot axis through pivot point 215 parallel to the common axis of the shaft 21 1 and ring gear 209.

A cam plate 250 is provided on the shaft 21 1. This is fixed to the shaft 21 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 251 in the circumference. However, the step 251 in this embodiment is at the distal end of an arm 260 of the cam plate 210. The proximal end of the arm 260 is joined to the cam plate, such that whilst the arm 260 remains in the same plane as the remainder of plate, it can move within that plane; the possible movement is small but elastic in nature.

As with the previous embodiment, a cam follower 252 is provided which follows the outline of the cam plate. However, as it reaches the arm 260, it will act to compress the arm 260 against the cam plate 250. This acts to increase the frictional force between the cam follower 252 and the arm 260 (as the frictional force will be generally proportional to the reaction force of the arm 260 on the cam follower 252). Thus, there will be some frictional coupling at this point between the ring gear 209 and the shaft 21 1 , and so the shaft 21 1 can start to accelerate. Once the cam follower reaches the step 251 at the distal end of the arm 260, it will function as described with respect to the preceding embodiment; if the relative speed of the cam follower 252 relative to the cam plate 250 is sufficiently high, the cam follower will skip over the step 251 , thus engaging the clutch 210. What differentiates this embodiment from the previous embodiment is that there is a period of frictional engagement before the sudden impact-based engagement of the clutch.

This is useful in particular in the situation where the hedgetrimmer is started up without any object blocking the blades 202, 203. It allows the blades 202, 203 to partially accelerate before the sudden impact; as such, it reduces the noise made by the sudden impact of the clutch engaging at start up, which may otherwise alarm a user.

A hedgetrimmer according to a fifth embodiment of the invention is shown in Figures 19 and 20 of the accompanying drawings. This embodiment functions in a similar manner to that of the third embodiment of the invention shown in Figures 9 to 15 of the accompanying drawings; equivalent features have been assigned corresponding reference numerals, raised by 200.

In this embodiment, a pinion gear 307 coupled to the output shaft of the motor (not shown) engages a ring gear 309 having internal teeth. A clutch 310 selectively engages the ring gear 309 to a shaft 31 1 coaxial with the ring gear 309. The shaft 31 1 drives the blades 302, 303, as in the previous embodiments.

The clutch 310 comprises a clutch member 314 mounted on the ring gear 309 so as to be able to pivot about a pivot point 315. As such, the clutch member 314 can rotate or pivot about a pivot axis through pivot point 315 parallel to the common axis of the shaft 31 1 and ring gear 309.

The clutch member 314 comprises two curved arms which extend away from the pivot point in opposing directions generally tangential to the axis of shaft 31 1. Of these arms, a long arm 316 extends further around the ring gear 109 than a short arm 317. As discussed with respect to the first embodiment of Figures 1 to 6 of the accompanying drawings, this means that, when the ring gear 309 is rotating at high speeds relative to the shaft 31 1 , the long arm 316 will be thrust outwards, moving the short arm inwards in order to engage the clutch 310 as described below.

The short arm 317 has an end face 321 (as in the second embodiment, rather than the tooth of the first embodiment). The external circumference of the shaft 31 1 spirals inwards, so as to define a radial step or protrusion 320 with a face 322. The face 322 and the face 321 each face in opposing tangential directions relative to the axis of the shaft 31 1 , so that when the face 321 and the face 322 impact and engage, the clutch 310 is suddenly engaged and rotational motion of the ring gear can be transmitted from the ring gear 309 to the output shaft 31 1. However, rather than relying upon a spring to bias the clutch 310 out of engagement, this embodiment relies upon the relative angle 370 of the end face 321 and the face 322. The end face 321 and the face 322 have the same angle 370 relative to the radius of the shaft 31 1 , so that the face 322 faces outwards relative to the axis of the shaft 31 1. This angle 370 tends to deflect the end face 321 outwardly off the face 322 should there be further relative movement of the ring gear 309 relative to the shaft 31 1. Thus, in the steady state where the shaft and the ring gear are co-rotating, the clutch will remain coupled, but should the blades face significant resistance (such as a branch blocking the blades), the end face 321 will be deflected off the step 320, disengaging the clutch. Thus, this embodiment will still be able to provide the hammer mechanism of previous embodiments, without needing the spring to bias the clutch out of engagement.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although the foregoing descriptions and the associated drawings describe exemplary embodiments in the context of certain exemplary combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative embodiments without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims.

For example, the scotch yoke described above is only one example of an arrangement for converting rotational motion into linear reciprocating motion. An alternative solution may e.g. be to use one or more connecting rods that are allowed to rotate around the cams 12 and in relation to the cutting blades.

Further, 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. Correspondingly, the frictional part described in connection with the fourth embodiment may be employed in any of the other embodiments described herein.

Also, either a wheel gear or a ring gear may be used in any of the described embodiments. Alternatively, some other kind of gear may be used, such as for example an epicyclic gear or a belt gear.

In the first and second embodiments a pull and a push system respectively are described. For any of the embodiments described herein either a pull or a push system may be used. Finally, either a spring or an angled impact surface may be used with any of the embodiments described above for moving the clutch member against the centrifugal force.

Claims

1. A handheld 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 transmitted to the blade set and an uncoupled state in which motion of the motor is not transmitted 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.

2. The power tool of claim 1 , in which the drive train is 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.

3. The power tool of claim 1 or claim 2, in which the drive train comprises a clutch, arranged to engage suddenly once the rotational speed of an output shaft of the motor increases above a first threshold and to disengage once the rotational speed of an output shaft of the motor drops below a second threshold.

4. The power tool of claim 3, in which the clutch comprises 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, about a common axis, but not in the coupled state, and a centrifugal member fixed to the first rotating part but which can move relative the first rotating part under a centrifugal force as the first rotating part rotates, the first and second rotating parts selectively engaging each other through the centrifugal member.

5. The power tool of claim 4, in which the centrifugal clutch comprises a biasing member arranged so as to bias the centrifugal member in a direction opposing the centrifugal force.

6. The power tool of claim 4 or claim 5, in which the centrifugal member selectively engages a radial protrusion on the second rotating part, with the motion due to the centrifugal force selecting whether the centrifugal member engages the protrusion.

7. The power tool of claim 6, in which the centrifugal member is fixed to the first rotating part for rotational movement about a pivot axis parallel to the common axis and comprises 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.

8. The power tool of claim 6 or claim 7, in which the radial protrusion has an impact surface which is angled outward relative to the common axis, so that it tends to deflect the centrifugal member against the centrifugal force.

9. The power tool of claim 3, in which the clutch is arranged so that it suddenly engages dependent upon the relative rotational speed between the first and second rotating parts.

10. The power tool of claim 9, in which the clutch comprises:

11. The power tool of claim 10, in which the clutch comprises a biasing member, which is arranged to bias the clutch member out of contact with the other of the first and second rotating parts on which the clutch member is not mounted.

12. The power tool of claim 10 or claim 1 1 , in which the other of the first and second members on which the clutch member is not mounted is provided with a protrusion arranged such that the clutch engages the protrusion when it skips over the step in circumference to engage the clutch.

13. The power tool of claim 12, in which the protrusion is angled outward relative to the common axis, so that it tends 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.

14. The power tool of any preceding claim, provided with a control circuit which is arranged to selectively apply power to the motor and to determine a speed with which the blade set is moving or a speed with which the motor is running, the control circuit being arranged so as to cease to apply power to the motor on determining that the speed of the blade set or the motor has dropped.

15. The power tool of claim 14, in which the control circuit is arranged to reapply power to the motor subsequent to ceasing to apply power.

16. The power tool of claim 3 or any other claim dependent thereon, in which the clutch is provided with a friction part, which is arranged to accelerate the first and second rotating parts relative to one another before the sudden transition.

17. The power tool of claim 16 as dependent directly or indirectly from claim 10, in which the friction part comprises the cam and the cam follower; the cam and/or the cam follower being biased against the other of the cam and the cam follower over at least part of their range of travel.

18. The power tool of any preceding claim, being selected from the group comprising a hedgetrimmer, a set of edging shears, a set of shrub shears, a set of pruning shears and a set of secateurs.