Abstract:
The invention provides a clutch mechanism ( 200 ) having a driven pulley ( 32 ) and a drive pulley ( 34 ) to which a load is applied, the clutch mechanism ( 200 ) having an engaged position in which torque applied in use to the driven pulley ( 32 ) is transmitted to the drive pulley ( 34 ), and a disengaged position in which torque applied in use to the driven pulley ( 32 ) is not transmitted to the drive pulley ( 34 ), wherein the clutch mechanism ( 200 ) also has override means ( 238, 242 ) by which at least a proportion of the torque applied to the driven pulley ( 32 ) is absorbed when the load applied to the drive pulley ( 34 ) exceeds a predetermined value. The override means comprising a pair of detent plates ( 238, 242 ), and at least one ball-bearing ( 248 ) being held captive between the detent plates ( 248, 242 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a clutch mechanism. Particularly, but not exclusively, the invention relates to a clutch mechanism for use in conjunction with the brush bar of a vacuum cleaner and the drive therefor. 
     BACKGROUND OF THE INVENTION 
     Upright vacuum cleaners have downwardly directed dirty-air inlets arranged in the cleaner head through which dirty air is sucked into the vacuum cleaner. In the vast majority of cases, a brush bar is rotatably arranged in the mouth of the dirty air inlet so as to agitate the fibres of a carpet over which the vacuum cleaner is passed to release dirt and dust trapped in the carpet. The brush bar is normally rotated by the motor of the vacuum cleaner via a drive belt. 
     Upright vacuum cleaners are commonly convertible into cleaners which can be used as cylinder cleaners for above-floor cleaning. In this mode of operation, the main part of the cleaner, including the cleaner head, is often left stationery for a period of time with the motor running whilst dirty air is sucked into the vacuum cleaner via a hose or wand. If the brush bar is allowed to continue to brush against the carpet during this time, the carpet can become unnecessarily worn. Many cleaners incorporate devices for automatically lifting the cleaner head and brush bar away from the carpet when the cleaner is put into cylinder mode, but the rotation of the brush bar is not normally stopped since the lifting of the cleaner head prevents the unnecessary brushing of the carpet. A clutch mechanism has been proposed in which the drive belt used to drive the brush bar is shifted onto an idler pulley to disengage the drive when the cleaner is put into the cylinder mode of operation, but this type of mechanism is bulky, expensive to produce and unreliable. 
     Another difficulty with upright vacuum cleaners is that, on occasion, the brush bar will become jammed. When this happens, the motor can easily overheat and/or the drive belt can become damaged. Some machines are equipped with devices for automatically cutting out the motor when it overheats, but this is not always sufficient warn the user of the cause of the cut out and, when the motor has cooled, the cleaner is switched on again but the problem remains. Furthermore, there is always a risk, with machines of this type, that foreign objects such as children&#39;s fingers can be inserted into the dirty air inlet and serious injury can be caused by a rotating brush bar. 
     An object of the present invention is to provide a clutch mechanism, particularly a clutch mechanism suitable for use with a rotating brush bar of a vacuum cleaner, which is compact and reliable when used to disengage the drive of the brush bar. Another object is to provide a clutch mechanism which reduces the risk of overheating of the motor or mechanical failure in the event of the torque required to turn the brush bar exceeding a predetermined level. A further object is to provide a clutch mechanism which encourages the user of a vacuum cleaner to remove the cause of the problem when the brush bar becomes jammed. 
     SUMMARY OF THE INVENTION 
     The invention provides a clutch mechanism. The clutch mechanism includes override means, and is capable of allowing the brush bar to be driven in the engaged position, of disengaging the drive to the brush bar when above-floor cleaning is being carried out, and also of allowing the motor to continue running without overheating or destroying the drive belt (or belts) if the torque required to turn the brush bar exceeds a predetermined level. A vacuum cleaner in which the clutch mechanism is fitted is thereby safer to operate than known cleaners and is less prone to damage or faults which require maintenance or spare parts to be fitted. The cleaner is therefore cheaper to run and more user-friendly. 
     A preferred embodiment of the invention includes a feature in which belts remain carried by their respective pulleys in the same axial position means that the clutch mechanism can be put into the disengaged position without the need for bulky, unreliable means for transferring one or more belts to an idler pulley. Removing the need for axial movement of either pulley heads to less wear and tear on the components involved and also reduces the likelihood of malfunction. 
     A further preferred embodiment of the invention produces an override signal when override occurs draws the attention of the user of a vacuum cleaner in which the mechanism is fitted to the fact that the torque required to turn the brush bar exceeds a predetermined value and the need for the cause of the problem to be removed. The override signal is given at all times when the brush bar is jammed or retarded and is not triggered by the operating temperature of the motor. The user of the vacuum cleaner is therefore encouraged to remove the cause of the problem before recommencing cleaning, which is not always the case with cleaners in which the motor cuts out when its operating temperature exceeds a predetermined value when the brush bar is jammed. The invention discourages the user from continuing to use the machine under abnormal or strained conditions. A further advantage is that, when the override signal is audible, an audible warning is given immediately a foreign object such as a child&#39;s fingers is introduced into the dirty air inlet thereby minimizing any delay between an accident occurring and its discovery. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described and fully explained with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic side view of the cleaner head of a vacuum cleaner illustrating the relative positions of a motor, a brush bar and a clutch mechanism according to the invention; 
     FIGS. 2 and 2A are side and front views of a clutch mechanism according to the invention omitting the drive belts for reasons of clarity; 
     FIG. 3 is an exploded isometric view of the components of the mechanism of FIGS. 2 and 2A; 
     FIG. 4 is a cross-sectional view of the mechanism of FIGS. 2 and 2A shown in the normal driving or engaged position including the drive belts carried by the pulleys; 
     FIG. 5A is a cross-sectional view similar to FIG. 4 but illustrating the static and rotating parts of the mechanism, again in the engaged position; 
     FIG. 5B is a cross-sectional view similar to FIG. 5A but showing the mechanism in the disengaged position; and 
     FIG. 5C is a cross-sectional view similar to FIG. 5A but showing the override means in operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates schematically the cleaner head  10  of a vacuum cleaner  100 . The cleaner head  10  is rotatably mounted on a motor housing  12  located at the lower end of a main body  14  in which dust separating apparatus (not shown), in the form of a dust bag, cyclonic separator or other filter, is housed. A pair of wheels  16  are also mounted on the motor housing  12 , from which the cleaner head  10  extends in a forward direction. 
     The cleaner head  10  has a dirty air inlet  18  located at its forward end and facing downwardly so that, in use, the dirty air inlet  18  rests on the surface  20  to be cleaned, usually a floor or carpet. A brush bar  22  is rotatably mounted in a known manner by means of bearings (not shown) so that the brush bar  22  extends across substantially the entire width of the dirty air inlet  18 . The brush bar  22  protrudes slightly out of the dirty air inlet  18  so as to agitate the fibres of a carpet being cleaned and so enhance the cleaning process. 
     A motor  24  is housed within the motor housing  12 . Normally, the motor  24  would be used to drive the brush bar  22 , either directly or via a gearing mechanism. Prior art machines have been known to include clutch mechanisms having idler pulleys for receiving one of the drive belts and mechanisms for transferring the relevant belt to the idler pulley as described above, but these mechanisms are not reliable. In the embodiment shown, the clutch mechanism  200  according to the invention is located between the motor  24  and the brush bar  22  and drive belts  26 , 28  are arranged so as to transfer torque from the motor  24  to the clutch mechanism  200  and from the clutch mechanism  200  to the brush bar  22  respectively. The motor  24  can be any motor suitable for use in domestic vacuum cleaners. It must be able to receive a drive belt  26  and therefore carries a drive pulley  30  for receiving the drive belt  26 . The drive belt could equally be carried directly on the motor shaft. The drive belt  26  is also carried by a driven pulley  32  forming part of the clutch mechanism  200  (to be described more fully later). The clutch mechanism  200  also has a drive pulley  34  which carries the drive belt  28 , which is also carried by a pulley  36  on the brush bar  22 . The drive belts  26 , 28  are preferably high strength, reinforced drive belts having an expected life of 10 years under normal operating conditions. They can be ridged or toothed belts, but are preferably flat. The relative diameters of the pulleys  30 ,  32 ,  34 ,  26  are designed to gear down the rotational speed of the motor (commonly 30-40 k rpm) to a suitable rotation speed for the brush bar  22 . A suitable rotational speed for a brush bar is typically 3.5-5 k rpm. The brush bar  22  can be of any known design, preferably having a flared bristle arrangement at the edges thereof to facilitate edge-to-edge cleaning. The precise features and dimensions of the brush bar  22 , the cleaner head  10  and the motor  24  are not significant to the present invention and will be described no further here. 
     The specific features of the clutch mechanism  200  will now be described in s detail with reference to FIGS. 2,  2 A,  3  and  4 . Looking firstly at FIGS. 2 and 2A, it can be seen that the clutch mechanism  200  is a self-contained unit having a main housing  202  and an actuator  204 . The actuator  204  is connected to the main housing  202  so as to be rotatable with respect thereto, as will be described below. An actuator tag  206  extends outwardly from the actuator  204  at one circumferential location, and a manual actuator grip portion  208  extends outwardly from the actuator  204  at another circumferential location. The functions of these parts will be described fully below. The main housing  202  incorporates moulded projections  210  having through-holes for receiving screws or bolts by means of which the clutch mechanism  200  can be mounted onto a suitable part of the cleaner head of a vacuum cleaner, or in any other suitable y position for operation. The main housing  202  also incorporates openings  212 ,  214  positioned and dimensioned so as to allow the drive belts  26 ,  28  to pass through the main body  202  to the pulleys  32 ,  34 . 
     The components of the clutch mechanism  200  are shown in an exploded view in FIG.  3 . The clutch mechanism  200  has a base cover  220  which is essentially formed as an annular plate  220   a  having an annular lip  220   b  around the outer circumference thereof. Three eyelets  220   c  are spaced around the circumference of the annular plate  220   a  and incorporate through-holes for receiving screws or bolts for securing the base cover  220  to the main housing  202 . An upstanding cylindrical wall  220   d  is located around the inner circumference of the base cover  220  and the upstanding wall  220   d  has an inwardly extending lip  220   e  around its base. A first cylindrical bearing  222  is dimensioned to fit without play into the recess formed by the upstanding wall  220   d  and the inwardly projecting lip  220   e  acts as a stop to prevent unlimited axial movement of the first cylindrical bearing  222  through the said recess. 
     A second cylindrical bearing  224  in the form of a needle bearing receives a spindle  226  and runs freely thereon. The spindle  226  has an annular groove  226   a  for receiving a circlip  228  therein. The second cylindrical bearing  224  is dimensioned so as to fit inside the first cylindrical bearing  222  leaving a small annular gap between inner wall of the first cylindrical bearing  222  and the outer wall of the second cylindrical bearing  224 . 
     The driven pulley  32  consists of a generally cylindrical outer pulley surface  32   a  adapted to receive the drive belt  26 . Strengthening ribs  32   b  are spaced around the inner circumference of the cylindrical surface  32   a . An axially extending circumferential wall  32   c  is dimensioned to fit without play into the annular gap defined between the first cylindrical bearing  222  and the second cylindrical bearing  224 . The driven pulley  32  is thereby supported between the cylindrical bearings  222 ,  224 . 
     The driven pulley  32  carries a plurality of axially extending lugs  32   d  which are generally rectangular in shape and extend axially away from the driven pulley  32  on the side remote from the base cover  220 . In the embodiment illustrated, six equispaced lugs  32   d  are shown. A plurality of friction plates  230  and a corresponding number of clutch plates  232  are located within the circumference defined by the lugs  32   d . In the embodiment shown, two friction plates  230  and two clutch plates  232  are shown, although more friction plates and clutch plates can be provided if desired. Provision of only a single friction plate and a single clutch plate is also possible. The clutch plates  232  are positioned between the friction plates  230  so that the friction plates  230  alternate with the clutch plates  232 . Each friction plate  230  has a plurality of lugs  230   a  extending outwardly from the outer circumference thereof. The lugs  230   a  are dimensioned so as to project into the spaces between the lugs  32   d  of the driven pulley  32 . In this way, when the driven pulley  32  is rotated, the friction plates  230  must rotate with the driven pulley  32 . The interior circumference of each friction plate  230  is circular. Each clutch plate  232  has a circular outer circumference dimensioned to sit inside the lugs  32   d  but is provided with a plurality of inwardly projecting lugs  232   a  which project inwardly beyond the inner circumference of the friction plates  230 . The thickness of the friction plates  230  and clutch plates  232  is such that, when all of the friction plates  230  and clutch plates  232  are placed against one another, the lugs  32   d  of the driven pulley  32  extend at least as far as the friction plate  230  which is furthermost from the driven pulley  32 . 
     A support member  234  is generally dish-shaped in construction. The base of the support member  234 , i.e. the side facing the base cover  220 , is generally cylindrical and has a plurality of outwardly projecting lugs  234   a  which are arranged and dimensioned to project into the spaces between the lugs  232   a  around the inner circumference of each clutch plate  232 . In this way, the support member  234  is rotatable with the clutch plates  232 . A circumferential lip  234   b  projects outwardly from the support member  234  in order to allow an axial force to be applied to the clutch plate  232  furthermost from the base cover  220 . A central aperture  234   c  extends through the support member to allow the spindle  226  to pass therethrough. The support member  234  is press-fitted on the spindle  226  so that the support member  234  rotates with the spindle  226 . On the side of the support member  234  remote from the lugs  234   a  are further lugs  234   d  which extend parallel to the spindle  226  and away from the base cover  220 . These further lugs  234   d  are shown as being fewer in number than the lugs  234   a , although the relative sizes and spacings are immaterial. 
     A compression spring  236  is seated in the dish-shaped interior of the support member  234 . The compression spring  236  bears against a first detent plate  238  which is mounted on the spindle  226  by means of a bearing  240 . The first detent plate has outwardly projecting lugs  238   a  which are spaced and dimensioned to project into the spaces between the lugs  234   d  of the support member  234 . Thus, the first detent plate  238  is rotatable with the support member  234 . The first detent plate  238  is maintained at an axial distance relative to the support member  234  by the compression spring  236  such that, if sufficient force were to be applied to the first detent plate  238 , the first detent plate  238  could move axially towards the support member  234 . 
     A second detent plate  242  is mounted on the spindle  226  by means of a bearing  244 . The second detent plate  242  also has outwardly projecting lugs  242   a , but these do not project into the spaces between the lugs  234   d  of the support member  234 . Between the first and second detent plates  238 ,  242  a bearing cage plate  246  is sandwiched. The bearing cage plate  246  is also mounted on the spindle  226  and is freely rotatable about the spindle  226 . The bearing cage plate  246  has a plurality of through-holes  246   a  (three are shown in this embodiment) in which three ball-bearings  248  are held captive. Each detent plate  238 ,  242  has a plurality of inclined grooves  242   b  which are shaped such that, when the first and second detent plates  238 ,  242  rotate with respect to one another, the relative rotation of the ball-bearings  248  forces the detent plates  238 ,  242  apart thereby compressing the compression spring  236 . The grooves  242   b  also include lips and recesses which also allow the detent plates  238 ,  242  to be pressed together under the action of the compression spring  236 . The shaping of the grooves  242   b  ensures that the ball-bearings  248  are pressed into the recesses relatively quickly thereby causing an audible sound each time a ball-bearing  248  falls into a recess. The detent plates  238 ,  242  also carry small ramp-like projections  242   c  over which the bearing cage plate  246  rides when there is relative rotation between the detent plates  238 ,  242 . This ensures that the bearing cage plate  246  remains in a position relative to the ball-bearings which discourages them from rolling out of the through-holes  246   a , i.e. in a position substantially equidistant from the detent plates  238 ,  242 . 
     The drive pulley  34  includes on its side facing the base cover  220  a plurality of lugs  34   a  which are designed to project into the spaces between the lugs  242   a  on the second detent plate  242 . Therefore, the drive pulley  34  is rotatable with the second detent plate  242 . The drive pulley  34  includes a cylindrical surface  34   b  for receiving the drive belt  28  leading to the brush bar  22 . An outwardly extending lip  34   c  retains the drive belt  28  on the cylindrical surface  34   b . The drive pulley  34  is mounted on the spindle  226  by means of a bearing  250 . 
     A third cylindrical bearing  252  is arranged on the spindle  226  adjacent the drive pulley  34 . The third cylindrical bearing  252  is mounted on the spindle  226  via a bush  253  so as to support the spindle  226  but also to allow axial movement between the third cylindrical bearing and the spindle  226 . A fourth cylindrical bearing  254 , which is mounted on the spindle  226  near the cirelip  228 , also supports the spindle  226  although a small gap of approximately 1.5 mm is left between the circlip  228  and the face of the fourth cylindrical bearing  254  adjacent the circlip  228 . 
     The main housing  202  is designed to cover and enclose the interior components of the clutch mechanism  200 . The main housing  202  has a cylindrical portion  202   a  shaped and dimensioned so as to enclose the driven pulley  32 . Three bosses  202   b  which incorporate through-holes are spaced around the cylindrical portion  202   a  and co-operate with the projections  220   c  on the base cover  220 . Screws or bolts can thereby be used to secure the main housing  202  to the base cover  220 . The main housing  202  also has a central portion  202   c  dimensioned and designed to enclose the friction and clutch plates  230 ,  232 , the support member  234 , the compression spring  236 , the detent plates  238 ,  242 , the bearing cage plate  246  and ball-bearings  248 , and also the drive pulley  34 . The arrangement of the components allows a good air seal to be maintained within the clutch mechanism, particularly in the area of the base of the drive pulley  34 , which prevents dirt and dust entering vulnerable parts of the mechanism. The main housing  202  also has an end portion  202   d  which projects into the interior of the actuator  204 . The end portion  202   d  is dimensioned so as to accept and house the third and fourth cylindrical bearings  252  and  254  and also the end of the spindle  226   a  remote from the base cover  220 . The end portion  202   d  incorporates a shoulder  202   e  against which the fourth cylindrical bearing  254  abuts in order to prevent unlimited axial movement. The end portion  202   d  also incorporates an outwardly extending lip  202   f  which includes a recess for receiving one end of a torsion spring  256 . The other end of the torsion spring  256  is retained by the actuator  204 . 
     The main housing  202  and the actuator  204  also include cam surfaces  258 . By means of these cam surfaces  258 , the axial position of the actuator  204  with respect to the housing  202  is altered when the actuator  206  is rotated about the spindle  226 . As has been previously mentioned, the actuator  204  includes an actuator tag  206  and also an actuator grip portion  208  so that the actuator  204  can be rotated with respect to the housing  202  either by hand or automatically by rotation of the vacuum cleaner cleaning head with respect to the main body and/or motor housing. Finally, an opening in the top of the actuator  204  is closed by means of a cap  260 . 
     FIG. 4 shows the clutch mechanism  200  in its assembled form. It will be appreciated that the main housing  202  is fixedly connected to the base cover  220  by screws or bolts. The torsion spring  256  is acting so as to press the actuator  204  towards the housing  202 . The spindle  226  is pressed to the left by means of the spring  236  and the clutch plates  232  are thus pressed firmly against the friction plates  230 . This pressing contact means that any rotation of the friction plates  230  causes the clutch plates  232  to rotate. Furthermore, the ball-bearings  248  are also pressed by the compression spring  236  into the recesses  242   b  in the detent plates  238 ,  242  and therefore any rotation of the first detent plate  238  causes the second detent plate  242  to rotate. 
     This is then the normal driving position of the clutch mechanism  200 . When the driven pulley  32  is rotated by the drive belt  26 , the friction plates  230  are rotated and, because of the pressing force between the friction plates  230  and the clutch plates  232 , the clutch plates  232  also rotate. This causes the support member  234  to rotate and therefore also the spindle  226  and the first detent plate  238 . The pressing action of the compression spring  236  maintains the ball-bearing  248  within the recesses  242   b  in the second detent plate  242  which then also rotates. This causes rotation of the drive pulley  34  and the torque is transmitted via the belt  28  to the brush bar  22 . FIG. 5A, in which the cross-hatched parts are the rotating parts, illustrates this normal driving position of the clutch mechanism  200 . 
     When the vacuum cleaner  100  is to be used for above-floor cleaning, the handle of the vacuum cleaner will be brought into the upright position. The relative movement between the main body  14  and the cleaner head  10  can be used to actuate the actuator tag  206  automatically and cause the actuator  204  to rotate with respect to the main housing  202 . The rotation of the actuator  204  with respect to the main housing  202  of the clutch mechanism  200  causes interaction of the cam surfaces  258 . The cam surfaces  258  cause the actuator  204  to lift slightly with respect to the main housing  202 . This brings the clutch mechanism  200  into the declutched position shown in FIG.  5 B. As can be seen, the lifting of the actuator  204  causes the fourth cylindrical bearing  254  to lift the spindle  226  by abutting against the circlip  228 . The entire spindle  226  is thereby lifted by a distance of not more than 1.5 mm. The spindle  226  slides in the bearing  253  on which the third cylindrical bearing  252  is mounted. It also slides in the bearing  250  carrying the drive pulley  34  and in the bearings  240 , 244  carrying the first and second detent plates  238 , 242  respectively. However, the support member  234  is lifted with the spindle  226  against the action of the spring  236  so that the pressing force acting between the friction plates  230  and the clutch plates  232  is released. The clutch plates  232  are therefore no longer pressed against the friction plates  230  and torque is thereby not transmitted therebetween. 
     In this position, the driven pulley  32  is still driven by the drive belt  26 . Although the friction plates  230  are rotated with the driven pulley  32 , the clutch plates  232  are not driven by the friction plates  230 . The support member  234  therefore remains static, along with the spindle  226 , the detent plates  238 ,  242  and the drive pulley  34 . In this position, the torque transmitted to the driven pulley  32  by the drive belt  26  is not transmitted to the drive pulley  34  by the belt  28 . FIG. 5B illustrates the disengaged position, again with the rotating parts shown cross-hatched. 
     The actuator grip portion  208  is provided so that the user of the vacuum cleaner  100  can put the clutch mechanism  200  into the declutched position at any time. This facility is useful in a number of situations, particularly when the vacuum cleaner is being used on an un-carpeted floor and the brush bar is not required. 
     The clutch mechanism  200  also has override means in the form of the first and second detent plates  238 ,  242 . It sometimes happens that the brush bar  22  becomes jammed and therefore the torque required to turn the brush bar is drastically increased. There can also be an appreciable increase in the torque required to turn the brush bar if the vacuum cleaner is used on a carpet having a very long pile. When the torque is increased beyond a predetermined level, there can be a serious risk of the motor  24  overheating or one of the drive belts  26 ,  28  becoming worn. If the brush bar  22  becomes jammed or the torque required to turn it is too great, the drive pulley  34  should not be made to rotate. In this situation, there is forced relative rotation between the first and second detent plates  238 ,  242 . The ball-bearings  248  ride around the grooves  242   b  in the detent plates  238 ,  242  whilst remaining held captive by the bearing cage plate  246  which is prevent from twisting with respect to the detent plates by means of the ramp-like projections  242   c . This relative rotation is allowed by the enforced pressing of the first detent plate  238  against the action of the compression spring  236 . The compression of the spring  236  also increases the force applied via the support member  234  to the friction plates  230  and the clutch plates  232  which reduces the risk of slip occurring therebetween immediately before or during override. The action of the compression spring  236  presses the ball-bearings  248  back into the recesses in the detent plates  238 ,  242  at every opportunity and the speed of the relative rotation is such that the ball bearings  248  produce a loud clattering noise during any override. This audible signal alerts the user of the vacuum cleaner  100  to the fact that the brush bar  22  is jammed and requires to be cleared before normal use of the vacuum cleaner  100  resumes, or to the fact that the brush bar is being retarded, perhaps by the length of pile of the carpet. The override operation of the clutch mechanism  200  is shown in FIG. 5C with the rotating parts shown cross-hatched. 
     The advantages of the clutch mechanism described above will be clear to a skilled reader. In particular, the mechanism is lightweight and compact with a construction which will not be prone to failure. The moving components are housed within a protective housing and, under normal operating conditions, there will be only minimal movement in the axial direction of only a few components since there is no need to transfer drive belts from one pulley to another as in the prior art. In the event of a failure, the clutch mechanism can be removed from the vacuum cleaner as a single unit and either serviced or replaced as desired. 
     The provision of an override signal, which is preferably audible and uncomfortably loud in the normal operating environment of a home, encourages the user to ensure that the cause of the override is removed before continuing to use the vacuum cleaner, very often merely by removing material which has become wrapped around the brush bar causing it to jam. This results in the vacuum cleaner being operated under proper working conditions for a higher percentage of the time and extends the life of the cleaner. It also results in better customer satisfaction and lower running costs due to less maintenance and fewer spare parts being required. 
     Purely for illustrative purposes, a clutch mechanism as described above can be fitted in the cleaner head of a vacuum cleaner having a 600W motor. The power required to turn the brush bar under normal operating conditions would be around 15W, or perhaps less. If the brush bar became jammed or retarded so that the power required to apply sufficient torque to turn it increased to around 45W, the override mechanism would operate causing the ball bearings to run around the recesses and give an audible signal to the user. The power required to drive the override means is only 15W and therefore the mechanism will only resume driving the brush bar when the power required to do so is less than 15W. The characteristics of the mechanism can be adjusted by altering the characteristics of various components, notably the compression spring.