Safety arrangement for a powered tool or implement

This invention relates to safety arrangements for powered tools and implements. The term "powered tools and implements" covers hand-held power tools, for example hedge trimmers, chain saws and angle grinders and the like, as well as implements which are not hand-held but whose operation is controlled by the hands of a user. Such implements include lawn mowers, scarifiers and cultivators.

Hedge trimmers, chain saws and angle grinders are usually provided with the two handles to enable the user to control the tool more easily. However, the user will sometimes hold and operate the tool with one hand only and thereby expose himself to the risk of injury either to the hand not holding the tool or to another part of his body because he has insufficient control over the tool.

To minimise the risk of injury some tools, for example electrically-powered hedge trimmers are fitted with two series-connected control switches both of which have to be actuated before the tool is energised. In a hedge trimmer, one switch is built into the main handle of the trimmer and is usually in the form of a trigger-operated switch and will be referred to as a primary switch. Another switch to be referred to as a secondary switch is connected in series with the primary switch and is associated with the other handle of the hedge trimmer in such manner that the secondary switch can be operated only when the user grasps the other handle. Thus, the hedge trimmer will be energised only when both switches are operated and this occurs only when the user grasps the main handle with one hand and the secondary handle with his other hand. Removal of either hand from the respective handle will release the appropriate switch and the tool is de-energised.

It is known to place the secondary switch directly on the other handle and to link the switch to a movable part of the handle that is actuated when a user grasps the handle. In another known arrangement, the secondary switch is mounted in the body of the trimmer and a lever associated with the other handle is linked to the secondary switch. When the user grasps the handle, he actuates the lever which, in turn, operates the secondary switch via the linkage (see DE-A-2839002).

Mounting the switch directly in the other handle exposes the user to the risk of electrical shock and is therefore undesirable from the point of view of safety.

However, if the switch is mounted in the body of the trimmer, the risk to the user of electrical shock is small but interconnecting linkage between the switch and the lever has to be provided and accommodated and this increases both the cost and the weight of the tool.

Both known arrangements are also inherently unsatisfactory because there is the possibility that the movable parts required to operate the secondary switch may jam with the switch operated, in which case the tool will not be de-energised when the user removes his hand from the handle. Such movable parts also require return springs which the user has to overcome to actuate the secondary switch and this gives rise to user fatigue.

It is the object of the invention to avoid these disadvantages.

According to the present invention there is provided a power tool or implement comprising an electric motor, a primary switch and a secondary switch so arranged that said electric motor can be actuated only when both said switches are actuated, characterized in that said secondary switch is actuable by fluid under pressure, and further characterized in that said power tool or implement further comprises a container manually deformable to provide fluid under pressure, and means for transmitting fluid under pressure from said container to said secondary switch.

In one embodiment of the invention the power tool or implement includes a single handle upon which said container is mounted, and means for actuating said primary switch mounted on said handle remote from said container to inhibit simultaneous actuation of said primary switch and deformation of said container with the same hand.

In another embodiment of the invention the power tool or implement includes a primary handle provided with means for actuating said primary switch, and a secondary handle upon which said container is mounted.

Preferably, the container is accommodated within its handle. Advantageously, such an embodiment includes a user actuable member mounted on said handle and movable relative thereto to deform said container. Preferably, the container is of elongate form and said user actuable member is substantially co-extensive therewith.

Preferably, the user actuable member protrudes beyond said handle and said handle is provided with inturned lips which retain said user actuable member in said handle.

Preferably, the power tool or implement includes delay means for preventing energisation of said electric motor if actuation of said primary switch and deformation of said container do not occur within a predetermined time. Advantageously delay means comprises a time delay operatively connected to both said primary switch and said secondary switch so as to be set in operation when either said primary switch or said secondary switch is actuated and which time delay is rendered inoperative in the event that deformation of the other of said primary switch and said secondary switch follows within a predetermined time.

Preferably, the time delay is connected to a switch which is actuated by said time delay to prevent actuation of said electric motor in the event that said primary switch and said secondary switch are not actuated within said predetermined time.

Advantageously, said fluid is air.

By way of example only, embodiments of the invention will now be described in greater detail with reference to the accompanying drawings of which:-

• Fig. 1 is a perspective view of a hedge trimmer embodying the invention,
• Fig. 2 is a section on the line II-II of Fig. 1 on an enlarged scale,
• Fig. 3 is a perspective view of a part of an actuator,
• Fig. 4 is a cross-section of a detail,
• Fig. 5 is an explanatory diagram,
• Fig. 6 is a perspective view of a chain saw embodying the invention, and
• Fig. 7 is a perspective view of a lawn mower embodying the invention.

Fig. 1 shows a hedge trimmer in perspective view. The trimmer includes a body 1 housing an electric motor and a drive mechanism for cutter members 2, 3 that extend forwardly from the body 1. Extending rearwardly from the body 1 is a main handle handle 4 with a trigger 5 for operating an electric switch housed within the main handle. A releasable "lock-off" button 6 projects sideways from the handle. Button 6 must be depressed before actuation of the trigger 5.

Fixed to the forward side of the body 1 is a secondary handle 7 of inverted U-shaped form. The vertical limbs of the U are inturned as at 8 to contact the side walls of the body 1. The handle 7 is secured to the body 1 by screws one of which is shown at 9 and which pass through the inturned portions 8 and into the body 1. A forward guard 10 with a horizontal part 11 secured to the underside of the body 1 has a vertical part 12 interposed between the secondary handle 7 and the adjacent ends of the cutter members 2, 3.

The cutter members 2, 3 are arranged one above the other, the lower member 3 being fixed in the body 1 while the upper member 2 extends into the body and is connected to a mechanism which converts the rotary motion of the rotor of the electric driving motor to a reciprocatory motion for reciprocating the upper member 2 relatively to the lower member 3. Both members 2, 3 have laterally extending teeth 13 and they operate in the conventional manner to cut hedges, trim bushes and the like. It us, of course, possible to drive both cutter members.

The secondary handle 7 is of channel form over most of its length, the channel accommodating an elongate, flexible-walled air sac 14 closed at one end and joined at the other end to a length of connector tube described in more detail below. The air sac 14 is indicated in Fig. 1 by the dotted line and as can be seen from that Figure the tube is co-extensive with the U-shaped part of the secondary handle 7.

The channel in the handle 7 has inturned lips 15 which retain an actuator member 16 also of generally U-shaped form as can be seen from Fig. 1 and having grooves 17 in its side walls which coact with the lips 15 to retain the actuator member in the channel in handle 7. The outer face 18 of the actuator member is gently rounded as indicated in Fig. 2 to form a head, the body of the actuator being movable into the channel in the handle as will be described below.

The air sac 14 is shown in more detail in Fig. 3 and, as has been described above, the sac is closed at one end as at 19 and connected at the other to a connector tube 20 which is housed in one of the inturned portions 8 of the handle 7.

The end wall 21 (Fig. 4) of the inturned part 8 housing the connector tube 20 is formed with a nozzle 22 over which the end of the connector tube 20 fits as can be seen in Fig. 4. Extending into the bore of the nozzle 22 is a hollow pin 23 that is a force fit in an aperture 24 in the wall 25 of the body 1. The pin 23 extends into the body 1 and is connected to a pneumatically-operated electric switch - the secondary switch - located inside the body 1 via a connecting tube 26.

A typical pneumatically-operated switch is shown in Fig. 5. Clamped between a base plate 27 and an inverted cup-shaped housing 28 is a flexible diaphragm 29 that supports a cup 30 movable within the housing 28 against the action of a helical spring 31. Fixed to the base of the cup 30 is a shorting bar 32 which, with fixed contacts 33, 34 forms the secondary switch referred to above. The contacts 33, 34 are mounted in but insulated from the housing 28.

As can be seen from Fig. 5, the fixed contacts 33, 34 are in series electrical connection with a primary switch 35 actuated by the trigger 5, supply terminals 36, 37, the field winding 38 and the brushes and commutator 39 of the electric motor housed within the body 1.

The input terminals 36, 37 are connected to the conductors of a supply cable 40 that extends from the main handle 4 as can be seen from Fig. 1.

To operate the hedge trimmer, a user grasps the main handle 4 in one hand and the secondary handle 7 in the other. The trigger 5 is actuated so closing switch 35, the "lock-off" button 6 having been depressed to allow the trigger 5 to be actuated. In addition, the actuator member 16 is depressed thereby compressing part at least of the air sac 14 and increasing the air pressure therein to a value such that the diaphragm 29 is flexed upwardly so moving cup 30 upwardly (as seen in Fig. 5) causing the shorting bar 32 to bridge the fixed contacts 33, 34 and thereby complete the energising circuit of the driving motor.

If, during use of the hedge trimmer, the user removes either hand from the tool, the motor will be de-energised because either the primary switch 35 will open or the secondary switch will open.

The air sac 14 and the connecting tubes 20 and 26 are made of an electrically-insulating material, for example a plastics or rubber material and thereby isolate the user from the secondary switch. Only a light spring loading of the diaphragm is required thereby requiring only a little pressure to compress the air sac 14 to operate the secondary switch and in this manner user fatique is prevented.

Moreover, because the secondary switch is operated pneumatically and the connecting tubes are flexible, the switch can readily be located in a position where it can easily be protected against ingress of moisture. In the present case, the secondary switch is located in the body 1 which is "splash-proofed" or protected against the ingress of moisture.

The pneumatic system is also of the "fail-safe" type because if a leak develops in the air sac or elsewhere in the pneumatic circuit, the secondary switch cannot be operated.

Furthermore, because of the elongate form of the air sac and because it extends round the greater part of the handle, the user will always operate the secondary switch irrespective of the exact point at which he grasps the handle.

Fig. 6 is a perspective view of a chain saw embodying the invention. Except as described below, the chain saw is of conventional form having a housing 41 accommodating an electric driving motor whose output shaft drives, via gearing contained in a gear case 42 joined to the housing 41, a cutter chain 43 supported in conventional manner on a cutter bar 44. Extending rearwardly from the gear case 42 is main handle 45 with a trigger 46 that actuates a primary switch (not shown) housed in the handle 45.

Projecting from the gear case 42 in a secondary handle 47 in whose upper surface is located an actuator member 48 of similar design to actuator member 16 described above. Member 48 is located un a channel in the handle 47 and the channel also houses an elongate air sac (not shown) joined by a connecting tube (not shown) to a diaphragm-operated switch (not shown) similar to that described above with reference to Fig. 5 and located in the motor housing 41. The diaphragm operated switch is the secondary switch and is series connected with the primary switch actuated by trigger 46.

The operation of the components just described is similar to that of the corresponding components of the hedge trimmer described above. A user must actuate the trigger 46 and the actuator member 48 before the driving motor can be energised. Refease of either trigger or actuator member results in de-energisation of the motor.

The actuator member 48 is, as can be seen from Fig. 6, co-extensive with the handle 47 and is, therefore, operated by the user regardless of the precise position of the user's hand on the handle.

Fig. 7 is a perspective view of a lawn mower embodying the invention. The mower shown is of the air cushion supported type but it could be a cylinder mower or a rotary mower.

Housing 49 accommodates the mower driving motor and is mounted upon a cutter cover 50 that accommodates the impeller and cutter bar of the mower, these latter components being driven by the motor.

Extending from the cover 50 is a handle 51 by which a user guides the mower over a grassed surface. Mounted on the handle 51 adjacent a horizontal portion 52 thereto is a housing 53 incorporating an electric switch - the primary switch - that controls the application of electric power to the motor via a connecting cable 54. The primary switch is operated by a lever 55.

Mounted in the horizontal portion 52 is an actuator member 56 similar to member 16 described above. The end of member 56 is spaced from the lever 55 by a distance sufficient to prevent a user operating both the lever 55 and the actuator member 56 by the same hand. If desired, the actuating member 56 may extend for a short distance down the vertical limb of the handle as indicated at 57.

Accommodated beneath the actuator member 56 in a groove in the portion 52 is an elongate resilient air sac similar to sac 14 described above. The air sac is joined by a connector tube located within the handle to a secondary switch located in the housing 53 and in series connection with the primary switch with that housing.

It will be appreciated that a user must employ both hands to operate the lever 55 and the actuator member 57 and the removal of either hand will result in de-energisation of the motor driving the mower. Again, the position of the hand operating the actuator member 57 is not limited to a precise position.

If desired, the safety arrangement according to the invention may be supplemented by a unit for controlling the flow of energy to the motor only if both switches are operated within a particular time, for example within fifteen seconds. The unit may be an electronic circuit incorporating a timer actuated when either the primary or secondary switch is closed and de-actuated when the secondary or primary switch is closed. Unless de-activated, the time will "time-out" after fifteen seconds and open the motor energising circuit. The unit is energised from the supply terminals and is shown schematically in dotted lines in Fig. 5.

The circuit incorporating the timer is shown as rectangle 58 with inputs from primary switch 35 and the secondary switch and an output controlling the operation of a switch 59.

In operation, a user may actuate the primary switch 35 before he compresses the air sac, or he may carry out both operations substantially simultaneously, or he may compress the air sac before he actuates the primary switch.

In the first and third cases, the timer is actuated when the first component is operated and deactivated when the second component is actuated provided the latter takes place within the fifteen seconds. In the second case, the timer is not activated.

If the timer is de-activated within the fifteen seconds or is not activated, a control signal is sent out at the expiry of the fifteen seconds which results in closure of switch 59 and energisation of the motor.

If both components are not activated within the fifteen seconds, switch 59 remains open.