GB2192252A - Improvements in or relating to braking mechanisms - Google Patents

Abstract

A brake mechanism for a rotary member comprises a helical spring (3) in frictional engagement with the member so as to be rotatable therewith. One end (5) of the spring (3) extends radially from the spring (3). A stop member (6) is movable into and out of the path of movement of the end (5). When the stop member (6) is clear of the path of movement of the end (5), the spring (3) rotates freely with the member. On movement of the stop member (6) into the path of the end (5), the latter is held against rotation. That causes a few turns of the spring (3) adjacent the end (5) to loosen but the remainder remain in frictional engagement with the member and exert a braking effect thereon. The member may be a shaft (1) or a cup- shaped container with the spring (3) in frictional engagement with the interior wall thereof. The brake mechanism may be employed in a powered tool such as a hedge trimmer or lawn mower. <IMAGE>

Description

SPECIFICATION Improvements in or relating to braking mechanisms This invention relates to braking mechanisms and has particular reference to braking mechanisms for rotating members, for example rotating shafts.

It is often required to bring a rotating member rapidly to a standstill. For example, in electrically powered tools and appliances, rotating members do not necessarily come rapidly to a standstill when the power to the motor is switched off and the continued rotation of such members, albeit for a short time, may present a hazard to the user of the tool or appliance.

Some form of braking mechanism may, therefore, be desirable but the limited space available to accommodate the mechanisms precludes the use of conventional braking mechanisms as may the cost thereof.

According to the present invention, a braking mechanism for a toatable member comprises a helical spring in frictional engagement with the member, and means for selectively holding one end of the spring against rotation with the member or allowing that end to rotate with the member.

Alternatively, a braking mechanism for a rotatable member according to the invention comprises a spring coiled round the member and in frictional engagement therewith, and means for selectively holding one end of the spring against rotation with the member or allowing that end to rotate with the member.

The mechanism may further comprise an enclosure round the spring for restricting movement of the spring or of a part thereof outwardly from the member.

The spring may have an end- portion that extends outwardly from the spring and co-operates with the means for holding the one end of the spring against rotation.

Preferably, the spring is an interference fit on the rotatable member.

The rotatable member may be a shaft in which case the enclosure is of tubular form.

The rotatable member may be the output shaft for example the output shaft of a driving motor which may be an electric motor.

The means may be operatively coupled to a switch controlling energisation of the electric motor, -the coupling being such that actuation of the switch to de-energise the motor results in the means holding the one end of the spring against rotation whilst actuation of the switch to energise the motor results in the means allowing such rotation.

The means for selectively holding one end of the spring may include a component rotatable with the member and so mounted that it occupies a first position clear of the means when the member is stationary and a second different position when the member is rotating, and in which means also includes a stop movable into the path of movement of a part of the component when the latter is in its second position.

The component may be pivotally mounted about an axis that is parallel to the axis of rotation of the member.

The component may be resiliently biassed into its first position.

An electromagnet may be provided for moving the stop.

The invention also comprises a power driven appliance including a driving motor, a tool holder driven by the driving motor and a braking mechanism as defined in one or other of the preceding paragraphs for applying a braking force to bring the tool holder to a standstill when the appliance is de-energised.

The power driven appliance may be a two-handed power tool in which the braking mechanism is operable to apply a braking force to bring a tool holder of the appliance to a standstill, the braking mechanism being effectively coupled to a first actuator associated with one of the handles and to a second actuator associated with the other of the handles, the arrangement being such that the braking force is released only when both actuators are operated, operation of either actuator alone being insufficient to release the braking force.

The first and second actuators may be operatively connected to a device through which the braking force is released and which effects release of that force only when both actuators are operated.

The device may be a lever, operation of one actuator producing movement of the lever that is insufficient to effect release of the braking force, operation of both actuators producing additional movement of the lever sufficient to release the braking force.

The invention also comprises a powered appliance including a braking mechanism as defined in any of the preceding paragraphs, the mechanism being effective to stop movement of the working element of the tool.

By way of example only, embodiments of the invention will now be described in greater detail with reference to the accompanying drawings of which: Figure 1 is an explanatory side elevation of a braking mechanism embodying the invention, Figure 2 is a section on the line ll-ll of Fig. 1, Figure 3 is an explanatory view from below of the part shown in Fig. 1, Figure 4 is a perspective view in exploded form of part of an alternative form of braking mechanism embodying the invention, Figures 5 and 6 are, respectively, a vertical section of part of another alternative form of braking mechanism embodying the invention, and a perspective view of a component of the part shown in Fig. 5, Figure 7 is a side view partly in section of a powered appliance embodiment of the invention, Figure 8 is a plan view of a part of the appliance shown in Fig. 7, Figure 9 is a side view of another powered appliance embodying the invention with a casing part removed, Figure 10 is a side view of a further powered appliance embodying the invention with a casing part removed, Figure 10A is a scrap view on the line X-X of Fig. 10, Figures 11, 12 and 13 are, respectively, a view from above, side elevation partly in section, and side view of selected components of another powered appliance embodying the invention.

Figs. 1, 2 and 3 show a rotatable shaft 1 driven, for example, by an electric motor (not shown) and which is connected to a driven member (also not shown).

Coiled round a portion of the shaft 1, which may be located between positioning collars 2, 3 on the shaft, is a spring 4. The spring 4 is dimensioned with respect to the shaft 1 to fit sufficiently tightly round the shaft 1 to maintain frictional engagement and preferably an interference fit therewith.

The spring 4 is of rectangular transverse cross-section wire to provide a relatively large contact area per spring coil with the surface of the shaft 1.

Coilars 2 and 3 locate the spring 4 on the shaft 1 and hoid it in position.

One end portion 5 of the spring 4 projects radially from the shaft as shown, whilst the other end portion lies round the circumference of the shaft.

Associated with the spring 4 is a component 6 movable towards and away from the shaft between a position A in which the end of the component is clear of the end portion 5 and a position B in which the end of the component lies in the path of movement of the end portion 5.

When component 6 is in position A and the electric motor driving the shaft 1 is energised, spring 4 rotates with the shaft as does the end portion 5 of the spring.

On de-energisation of the motor, the inertia in the rotating components thereof together with that of the shaft 1 and of the member driven thereby would, normally, maintain the rotation for a short period. However, if on de-enerigisation of the motor, component 6 is moved to position B, the end portion 5 of the spring is held against rotation. That causes the upper (as seen in Fig. 1) few coils of the spring to loosen somewhat on the shaft but the remaining coils do not loosen and, because of the interference fit between these remaining coils and the shaft 1 and because the end portion 5 and thus the entire spring is held against rotation, a decelerating torque is applied to the shaft which is rapidly brought to a standstill.

The lower collar 3 prevents any downward movement of the spring 4 that might occur when the decelerating torque is applied, there then being a tendency for the spring to "screw down" the shaft 1.

The magnitude of the decelerating torque and hence the rapidity with which the shaft 1 is brought to a standstill depend upon a number of factors including the extent of the interference fit between the spring 4 and the shaft 1, the coefficient of friction between the spring 4 and the shaft 1, the spring design including cross-sectional shape, the number of turns and spring rate and the inertia of the rotating components. By appropriate choice of the various factors, a required stopping time will be achieved.

In one particular embodiment of the invention, the following dimensions and characteristics were used: Diameter of shaft 1 - 25 mm Internal diameter of spring 4 in its free state - 24.2 mm Rotational speed of shaft - 6000 rpm Coefficient of friction between shaft 1 and spring 4 - 0.12 Wire guage of spring 4 - 2 mmx 2 mm Number of turns of spring 4 - 5 Tensile strength of spring 4 > 482N/mm2 Interia of rotatable components of motor and shaft 1 - 2.5 kg/cm2 Inertia of component driven by shaft 1 - 16.0 kg/cm2 With those values, a decelerating torque of 0.4 N metres is produced giving a stopping time of 3 secs.

It will be appreciated that the component 6 may take many different forms as may the mechanism for moving the component. For example, the component may be a pivotally mounted lever or a linearly-movable member actuated by a mechanical linkage. The linkage may comprise a Bowden cable.

Alternatively, an electromechanical system may be used in which the armature of a solenoid is used directly or indirectly to engage the end portion 5 of the spring and bring it to a standstill.

To prevent excessive outward movement away from the shaft 1 of the upper few coils of the spring 4, a collar (not shown) may be positioned round the spring 4. The collar allows the upper few turns to loosen on the-shaft 1 but limits the extent of such loosening and hence the outward movement of the turns. Such a collar may be needed in embodiments of the invention in which outward movement of the few coils may bring them into contact with adjacent surfaces or create permanent deflection- of the spring.

The mechanism may be associated with the operating member of an electric switch controlling energisation of the motor rotating the shaft so that termination of energisation of the motor also applies the decelerating torque to the shaft.

It will be appreciated that, in the embodiment just described, arm 6 must be moved out of engagement with end 5 on energisation of the motor in order to release the braking action of spring 4. Whilst the release is easy with a mechanical linkage, it is sometimes found with mechanisms involving the electromechanical operation of the arm 6, that disengagement of the arm from the end 5 is not complete before rotation of the shaft 7 commences. Such rotation results in the application of pressure on the arm 6 and continued disengaging movement of the latter may be prevented. A more powerful electromagnet would provide sufficient force to overcome the pressure but it is not always possible to provide the additional space required by the more powerful electromagnet.

Fig. 4 shows in exploded form one form of mechanism which overcomes the problem just described and does not require the use of a more powerful electromagnet.

A helical spring 7 wrapped ih frictional engagement round a shaft 8 has one end 9 projecting radially of the shaft. The spring 7 rests upon a collar 10 secured to the shaft 8. Secured to the upper (as viewed in Fig. 4) end of shaft 8 by means of a screw 11 are upper and lower caps 12, 13. Upper cap 12 has a cutaway portion 14 in its side wall 15. Lower cap 13 has a central boss 16 which holds the caps in spaced relationship when assembled. Lower cap 13 also has a cutaway 17 in its side wall 18. On top of the-upper face of cap 13 is a lever 19 attached at one end to the cap at 20 and pivotal about an axis parallel to that of shaft 8. About midway of its ends, lever 19 has an arcuate recess 21 in that edge facing boss 16.Recess 21 allows the lever, in its non-operative position, to lie closely against the boss 16, the other end of the lever 19 lying just within the periphery of the cap. That position is shown in Fig. 4 in full lines. As will be explained below, the lever 19 has a second position, shown in dotted lines, in which it projects beyond the periphery of the lower cap 13 and through the cutaway 14.

Lever 19 is biassed into its full line position adjacent the boss 16 by a spring 22 acting between the lever and either the screw 11 or a part of the upper cap 12.

When the shaft 8 is at rest, lever 19 is biassed into its full line position but when the shaft 8 is rotated about its longitudinal axis by a driving motor (not shown) the caps 12, 13 rotate with it and lever 19 is moved by centrifugal force away from boss 16 against the action of spring 23 and into contact with the side wall 15 of cap 12 and into the position shown in dotted lines. In that position, the other end of lever 19 projects beyond the confines of cap 13.

Co-operating with lever 19 is a mechanism, for example a pivotally mounted arm (not shown), that is movable by means of an electromagnet (not shown) into and out of the path of movement of the projecting end of lever 19.

If the mechanism is now brought into the path of the projecting end of lever 19, cap 13 and the upper turns of spring 7 are held against rotation, and the lower turns of the spring effect a braking action on the shaft 8 in the manner described above.

The direction of rotation of the shaft 8 is indicated by arrow A and it will be appreciated that in order to stop rotation of the upper turns of the spring 7, the mechanism or a part thereof must engage the projecting end of lever 19 on that side adjacent the cut-away 17.

Once the shaft 8 has been brought to a standstill, there will be some anti-clockwise movement of the upper and lower caps and lever 19 as the upper turns of spring 7 tighten up round the shaft 8 due to the release of stored energy in the spring. That movement allows the projecting end of lever 19 to move away from all mechanism or the part thereof and also allows spring 22 to return the lever to its full line position adjacent boss 16. In that position, the lever 19 does not project from the caps.

When next the shaft is rotated, the mechanism or the part thereof can be withdrawn easily and there is no risk of the withdrawal being impeded by the lever 19. Withdrawal can be effected before the lever 19 re-assumes its dotted line position as the shaft 8 starts to rotate.

Collar 10 serves to prevent any downward movement of the spring 7 that might otherwise ocur because of a "screwing" action as the upper turns of the spring are held against rotation.

The side wall of the lower cap 13 limits an outward expansion of the upper turns of the spring as they loosen.

It is not essential that the spring brake be located on the external surface of the rotatable component. Figs. 5 and 6 show an alternative form in which a spring brake 23 is located within a cup-shaped member 24 rotatable in the direction indicated by arrow B. The spring 23 is in frictional engagement with the inner curved face of the member 24 and has a radially extending end 25 at its upper (as veiwed in Fig. 5) end.

Mounted above the member 24 is a lever 26 pivotally mounted at 27 and with a downturned end 28. The lever 26 is linked to an operating mechanism (not shown). When the mechanism is actuated, lever 26 is pivoted in a clockwise direction to bring end 28 into the path of movement of the spring end 25.

The spring 23 operates in the manner described above with the result that, when the lever end 28 contacts spring end 25 and holds the latter against rotation, the spring 23 applies a braking force to the member 24 and the latter can rapidly be brought to a standstill.

It is not essential that the shaft be of circular transverse cross-section. A shaft of polygonal transverse cross-section may be used in which case it is preferred that the spring be of a corresponding configuration although this is not essential.

Furthermore, whilst it is preferred to employ the rectangular cross-section material referred to above for the spring, this is not essential. Circular cross-section material could be used and, whilst this provides a smaller contact area per coil with the shaft, the use of a greater number of coils on the spring will increase the contact area to a required value.

The braking mechanism described above may be embodied in any device that requires the application of a decelerating torque to a rotating member to bring to a standstill the working member of the device.

Examples are powered tools which may be drills, jig saws, sanders, chain saws, powered saws, powered files, circular saws, and powered appliances which may be kitchen appliances such as food processors, tin openers, or garden appliances such as mowers, edgers, scarifiers.

In each of the examples just quoted, there is an exposed working member which represents a user hazard. In the case of a drill, the working member is the drill bit and the chuck, in the case of the saws, it is the saw blade or saw chain. In a food processor, the working member may be a knife blade or cutter. The working member of a powered mower is the cutter bar or cylinder, that of a scarifier is the tine assembly, as is the case with a lawn edger.

As an example, Figs. 7 and 8 show parts of an air-cushion supported mover embodying the invention.

The mower is of generally conventional construction comprising a cutter cover 29 on which is mounted an electric motor 30 whose armature shaft 31 carries, at its lower end (as seen in Fig.

7), an impeller 32 and a cutter bar 33.

The motor is an a.c./d.c. motor with a commutator 34 and brushes 35, only one of which is shown, supported upon a brush support 36.

The armature shaft 31 extends upwardly beyond the commutator 34 and has a portion 37 round which is wrapped a coiled spring 38 similar to spring 4 described above and which has an end part 39 projecting radially with respect to the portion 37.

Extending from an end housing 40 of the motor 30 is a bracket 41 between the limbs of which a lever 42 is pivotally mounted. One end of the lever 42 is down-turned as at 43 while the other end is connected to the cable 44 of a Bowden cable 45 whose outer sheath terminates on an extension -46 of the bracket 41. A return spring 47 is located between the extension 46 and that end of the lever 42 to which the cable 44 is attached. The spring 47 biasses the lever 42 into the position shown in full lines, i.e; the- position in which the down turned end 43 of the lever 42 lies in the path of rotation of the end part 39 of the spring 38. In that position, the end part 39 is held against rotation.

The cable 44 of the Bowden cable 45 is connected at its other end to a trigger (not shown) that operates a switch controlling energisation of the motor 30.

When the trigger is actuated to operate the switch to energise the motor, there is a downward (as seen in Fig. 7) pull on cable 44 and this pivots lever 42 into the dotted position shown in Fig. 7 and in which the down-turned end 43 of the lever 42 is clear of the path of rotation of the end part 39 of the spring 38. The spring 38 thus rotates freely with the shaft portion 37.

When the trigger is released to de-energise the motor, return spring 47 moves lever 42 back into the position shown in full lines. Down-turned end 43 of lever 42 moves back into the path of rotation of the end part 39 of the spring 38 and holds that part against rotation. That causes the lower coils of the spring 38 to apply a decelerating torque to the shaft portion 37 and the motor shaft is brought rapidly to a standstill as is the impeller 32 and, more important, the cutter bar 33. Thus, the risk of injury to a user from the rotating cutter bar is considerably reduced if not eliminated.

The operation of the spring 38 is exactly the same as that of the spring 4 described above with reference to the embodiment shown in Figs. 1, 2 and 3.

Fig. 8 shows a hedge trimmer embodying the invention. The trimmer has a body of clam shell construction split along the length of the trimmer. The body is made up from two clam shell halves of which only one, referenced 48, is shown in Fig. 9, the other half having been removed to reveal the major mechanical components of the trimmer.

An electric driving motor 49 is mounted in clam shell half 48, the armature shaft 50 of the motor being vertical as seen in Fig. 9. The lower end of the armature shaft 50 is rotatably supported in bearings 51 mounted in the upper wall 52 of a gear box housing to which the motor 49 is secured.

The lower end of the armature shaft 50 is formed as a pinion 53 that meshes with a gear wheel 54 secured to a stub shaft 55 rotatably mounted in bearings 56 in the wall 52. Gear wheel 54 mounts two downwardly-extending driving pins 57, 58 which locate in driving slots in upper and lower cutter blades 59, 60 only parts of which are shown in Fig. 9. The blades 59, 60 are supported by a support bar 6f bolted to an extension of the gear box upper wall 52.

Rotation -of the gear wheel 54 reciprocates the blades 59, 60 backwards and forwards relatively to the support bar 61.

Energisation of the motor 49 is controlled by a switch shown diagrammatically at 62 and mounted in the rear handle portion of the casing. The switch is operable by a trigger lever 63.

Trigger lever 63 has an extension 64 that extends into the trimmer housing into close proximity with one end of a brake operating lever 65 pivotally mounted at 66 in clam shell half 48. A part 67 of the lever 65-extends downwardly into contact with the upper end of an actuating pin 68 that is biassed upwardly by a helical spring 69 that encircles the upper part of the pin 68 and acts between the wall 52 and an enlarged head 70 of the pin 68. The lower end of the pin 68 is turned at right angles to form a short arm 71.

Gear wheel 54 has a concentric annular recess 72 on its upper (as seen in Fig. 9) face.

Located in the recess 74 is a collar 75 inside which is a helical spring 76 that is in frictional engagement with the inner wall of the recess 74. The upper end of the collar 75 is outwardly flanged as indicated at 77 and, at a position aligned with the arm 71, the flange has a notch 78 large enough to receive the'arm 71 as will be described below.

The upper (as seen in Fig. 9) end of the spring 76 extends radially as indicated at 79 and is engaged in a hoie in the side wall of the collar 75.

When in use, the swich 62 is operated by upward pressure on the trigger lever 63. The extension- 64 iifts the adjacent end of the brake operating lever 65 and this results in downward pressure being exerted on the pin 68 by the part 67 of the lever. That downward pressure pushes the pin 68 downwardly and the arm 71 disengages the notch 78 so allowing the collar 75 and the spring 76 to rotate freely with the gear wheel 54.

When the upward pressure on the trigger lever 63 is released, the swtich 62 is actuated to its 'OFF' position and the motor 49 is de-energised. Release of the upward pressure on the trigger lever 63 allows spring 69 to surge pin 68 upwardly and the arm 71 moves into the notch 78 when the latter is next aligned therewith. The collar 75 and the end 79 are thus held against rotation. The upper few turns of the spring 76 loosen slightly but the remainder of the spring retains its frictional engagement with the inner wall of the recess 74 and this effects a braking action on the gear wheel which rapidly brings the latter and thus the blades 59, 60 to a standstill.

The collar 75 limits the outward movement of the upper few turns of the spring as such turns loosen.

When next the trimmer is brought into use, upward pressure on the trigger lever 63 results in downward movement of the actuating pin 68 and disengagement of the arm 71 from the notch.

Thus, the collar 75 and spring 76 are allowed to rotate freely with the gear wheel, and the braking action of the spring 76 is removed.

A fuller explanation of the action of the spring 76 is set out above.

In the embodiment just described with reference to Fig. 9, the braking action is brought into operation on release of the trigger lever only. This is acceptable if the blades 59, 60 are of the so-called safety type but further precautions are required if the blades are not of that type.

Such further precautions must ensure that release of either the rear handle or the forward handle of the trimmer results in de-energisation of the driving motor and application of the brake.

A hedge trimmer embodying such further precautions will now be described with reference to Figs. 10 and 10A of the accompanying drawings.

The general construction of the hedge trimmer shown in.Figs. 10 and 10A is generally similar to that of the trimmer described above with reference to Fig. 9 and similar components have the same reference numbers as in Fig. 9.

Energisation of the driving motor 49 of the trimmer shown in Figs. 10 and 10A is controlled by a switch 62. Switch 62 is actuated by a trigger lever 80 operated when a user grasps the rear handle 81 of the trimmer. The lever 80 is pivotally attached to the rear handle~at 82 and is operated upwardly against the action of a return spring 83.

Trigger lever 80 has forward extension 64 that projects into the housing of the trimmer into close proximity with the lower face (as viewed in Fig. 10A) of one arm 84 of a pivot lever 85 shown in more detail in Fig. 10A. The pivot lever 85 has a bifurcated end 86 and located between the branches of this end is one end of a forward handle lever 87 pivotally mounted at 88 on a spindle that is rotatable when a movable portion 89 of a forward handle 90 is grasped and squeezed by a user against the action of a return spring 91.

Resting upon the upper (as viewed in Fig. 10) face of the pivot lever 85 and centrally of the ends thereof is the brake operating lever 65. Lever 65 is pivoted at 66 and has an upturned end 92 for actuating the switch 62.

As before, part 67 of lever 65 extends downwardly into contact with the upper end of actuating pin 68 biassed upwardly by the helical spring 69. The pin 68 operates to allow or prevent rotation of the collar 75 and spring 76- in the manner described above.

To use the trimmer shown in Figs. 10 and 10A, a user must grasp both the rear handle 81 and the forward handle 90. As the rear handle 81 is grapsed, trigger lever 80 is pivoted upwardly against spring 83 and extension 64 causes pivot lever 85 to pivot in a clockwise direction as- seen in Fig. 10A about lever 87. Although lever 87 is pivoted as a result, the movement is insufficient to release the braking action of spring 76.

However, if, at the same time, the user also grasps the forward handle 90 so moving portion 89 against spring 91, lever 87 is pivoted in a clockwise direction about its pivotal axis 88. Such movement pivots lever 85 about the point of contact of the latter with extension 64 and this results in further pivotal movement of lever 65. That further movement of lever 65 is sufficient to disengage arm 71 of pin 68 from the notch 69 and the braking action of the spring 76 on the gear wheel 54 is released.

If a user now releases his grasp on either the rearward handle 81 or the forward handle 90, the resuitant movement of extension 64 or of'lever 87 results in sufficient pivotal movement of lever 85-to allow lever 65 to permit upward movement of pin 68 and its arm 71 to bring the latter into engagement with the notch 69 and this results in the application of the braking action of spring 75 as described above.

It will be appreciated that, in the embodiments described, arm 71 of the actuating pin 68 must be moved out of -engagement with notch 69 on energisation of the motor in order to release the braking action of spring 78. Whilst the release is easy with the mechanical linkages described above, it is sometimes found with mechanisms involving the electromechanical operation of the actuating pin 68, that disengagement of the arm 71 from the notch 69 is not complete before rotation of the gear wheel 54 commences. Such rotation results in the application of pressure on the arm- 71 and continued disengaging movement of the latter may be prevented. A more powe-rful electromagnet would provide sufficient force to overcome the pressurn -but it is not aiways-possible to provide the additional space required by the more powerful electromagnet.

Fig. 4 shows in exploded form one form of mechanism which overcomes the problem just described and does not require the use of a more powerful electromagnet. That mechanism could- used in the embodiment of Figs. 10 and 10A with appropriate modification of cooperating parts.

Figs. 11, 12 and 13 show part of an embodiment of the invention incorporating the mecha nism des:c(lbed above with reference to Fig. 4. The embodiment is an air cushion supported lawnmover.

Figs. 11, 12 and 13 show only that part of: the lawnmover directly associated with the mechanism.

Housed within a motor casing 94 mounted upon a cutter casing (not shown) is an electric driving motor part of which is shown at 95. The armature shaft 96 extends upwardly through an end cap 97 of the motor and carries a drum 98 round which is wound a helical spring 99 corresponding with spring 7 described above with reference to Fig. 4.

Mounted upon the upper (as seen in Fig. 12) face of the drum 98 are upper and lower caps 100, 101 of a form identical with that of the caps 12, 13 described above with reference to Fig. 4. In Fig. 11, the upper cap 100 has been removed to reveal a pivoted lever 102 that corresponds with lever 19 referred to above.

The mechanism for holding lever 102 against rotation compises an electromagnet 103 whose armature is coupled to a lever 104 pivotally mounted at 105 to a suitable part (not shown) of the end cap 97. The lever 104 is stepped as at 106 to provide a stop 107 movable by the electromagnet into and out of engagement with the end of the lever 102 when that end projects beyond the confines of the cap 101.

Electromagnet 103 is connected in series with the motor 95 so that both are energised and de-energised together. In the de-energisation condition, the armature of the electromagnet and the lever 104 occupy the positions shown in full lines in Fig. 12. In those positions, the stop 107 lies in the path of movement of the projecting end of lever 102.

On energisation of the motor 95 and of the electromagnet 103, armature shaft 96 commences to rotate and lever 104 is pivoted into the dotted line position shown in Fig. 12. Such pivotal movement is not impeded by the lever 104 because the latter is clear of the stop 107 and has not yet assumed the outward position shown in full lines in Fig. 11.

By the time that the shaft 96 has reached its maximum rotational speed and the lever 104 is in its full line position, the stop 107 is clear of the path of movement of the projecting end of lever 104.

On de-energisation of the-motor 95 and the electromagnet 103, lever 104 pivots into the full line position shown in Fig. 12. The stop 107 then lies in the path of movement of the projecting end of lever 104 and this results in the application of the braking force to the shaft as explained above.

It will be appreciated that the braking arrangements described above have applications additional to those set out above. The arrangements may be embodied in other power tools and in power appliances generally where it is desirable to bring a rotating member to a standstill usually as quickly as possible.

Claims (22)

1. A braking mechanism for a rotatable member comprising a helical spring in frictional engagement with the member, and means for selectively holding one end of the spring against rotation with the member or allowing that end to rotate with the member.

2. A braking mechanism for a rotatable member comprising a spring coiled round the member and in frictional engagement therewith, and means for selectively holding one end of the spring against rotation with- the member or allowing that end to rotate with the member.

3. A braking mechanism as claimed in claim 1 or 2 and further comprising an enclosure round the spring for restricting movement of the spring outwardly from the member.

4. A braking mechanism as claimed in claim 1, 2 or 3 in which the spring has an end portion that extends outwardly from the spring and co-operates with the means for selectively holding the one end of the spring against rotation.

5. A braking mechanism as claimed in any one of the preceding claims in which the spring is an interference fit on the rotatable member.

6. A braking mechanism as claimed in any one of the preceding claims in which the rotatable member is a shaft.

7. A braking mechanism as claimed in clairns 3 and 5 in which the enclosure is of tubular form.

8. A braking mechanism as claimed in any one of the preceding claims in which the rotatable member is the output shaft of a driving motor.

9. A braking mechanism as claimed in claim 8 in which the motor is an electric motor.

10. A braking mechanism as claimed in claim 9 in which the means is operatively coupled to a switch controlling energisation of the motor, the coupling being such that actuation of the switch to de-energise the motor results in the means holding the one end of the spring against rotation whilst actuation of the switch to energise the motor results in the means allowing such rotation.

11. A braking mechanism as claimed in any one of claims 1-10 in which the means for selectively holding one end ofthe spring includes a component rotatable with the member and so mounted that it occupies first position clear of the means when the member is stationary and a second different position when the member is rotating, and in which means also includes a stop movable into the path of movement of a part of the component when the latter is in its second position.

12. A braking mechanism as claimed in claim 11 in which the component is pivotally mounted about an axis that is parallel to the axis of rotation of the member.

13. A braking mechanism as claimed in claim 11 or 12 in which the component is resiliently biassed into its first position.

14. A braking mechanism as claimed in claim 11, 12 or 13 in which an electromagnet is provided for moving the stop.

15. A power driven appliance including a driving motor, a tool holder driven by the driving motor and a braking mechanism as claimed in any one of claims 1-13 for applying a braking force to bring the tool holder to a standstill when the appliance is de-energised.

16. A two-handled power tool including a driving motor, a tool holder driven by the driving motor and a braking mechanism as claimed in any one of claims 1-13 for applying a braking force to bring the tool holder to a standstill and in which the braking mechanism is operatively coupled to a first actuator associated with one of the handles and to a second actuator associated with the other of the handles, the arrangement being such that the braking force is released only when both actuators are operated, operation of either actuator alone being insufficient to release the braking force.

17. A power tool as claimed in claim 16 in which the first and second actuators are operativley connected to a device through which the braking force is released and which effects release of that force only when both actuators are operated.

18. A power tool as claimed in claim 17 in which the device is a lever and in which operation of one actuator produces movement of the lever that is insufficient to effect release of the braking force, operation of both actuators producing additional movement of the lever sufficient to release the braking force.

19. A braking mechanism substantially as herein described with reference to and as illustrated by Figs. 1, 2 and 3, or Fig. 4, or Figs. 5 and 6 of the accompanying drawings.

20. A powered appliance including a braking mechnaism as claimed in any one of claims 1-14 in- which the mechanism operates to stop movement of the working element of the tool.

21. An electrically powered lawnmover substantially as herein described with reference to and as illustrated by Figs. 7 and 8, or Figs. 11, 12 and 13 of the accompanying drawings.

22. An electrically powered tool substantially as herein described with reference to and as illustrated- by Fig. 9 or Fig. -10 of the accompanying drawings.