Patent Publication Number: US-8966709-B2

Title: Cleaner head for a vacuum cleaner

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of United Kingdom Application No. 1214420.0, filed Aug. 13, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a cleaner head for a vacuum cleaner. 
     BACKGROUND OF THE INVENTION 
     The cleaner head of a vacuum cleaner typically comprises an agitator driven by a drive assembly. The drive assembly may include a turbine for generating the torque necessary to drive the agitator. The turbine may be driven by dirty air that is drawn in through the main suction inlet. Alternatively, the turbine may be driven by clean air that is drawn in through an inlet distinct from the suction inlet. 
     A clean-air turbine has the advantage that the turbine is not exposed to the dirty air, which might otherwise obstruct the turbine. However, should the path carrying the dirty air become obstructed, the flow of clean air through the turbine will increase. As a result, the speed of the turbine will increase. The increase in speed may be several orders of magnitude greater than the normal operating speed of the turbine. As a result, components of the drive assembly, such as bearings, may be damaged. 
     Schemes for preventing overspeed of a clean-air turbine are known. However, such schemes tend to be complicated, thereby increasing the cost, size and/or weight of the cleaner head. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cleaner head for a vacuum cleaner comprising a suction inlet for admitting a first airflow, a turbine inlet for admitting a second airflow, an outlet for discharging the first airflow and the second airflow, a first airflow path for carrying the first airflow from the suction inlet to the outlet, a second airflow path for carrying the second airflow from the turbine inlet to the outlet, an agitator for agitating a surface to be cleaned, a drive assembly for driving the agitator, the drive assembly comprising a turbine that is driven by the second airflow, and a baffle located in the second airflow path and movable between an open position in which the second airflow path is unrestricted and a closed position in which the second airflow path is restricted, wherein the baffle is biased in the open position, the second airflow exerts a lift force on the baffle when in the open position, and the baffle moves to the closed position when the dynamic pressure of the second airflow at the baffle exceeds a threshold. 
     The baffle therefore closes automatically when the dynamic pressure of the second airflow exceeds a threshold. The baffle closes so as to restrict the second airflow path, thereby reducing the mass flow rate of the second airflow. Since the second airflow is responsible for driving the turbine, overspeed of the drive assembly may be avoided. In comparison to existing schemes for preventing overspeed, the baffle is less complex, cheaper to implement and lighter. Moreover, since the baffle is located within the second airflow path, overspeed protection may be achieved without increasing the size of the cleaner head. 
     The baffle may restrict the second airflow path completely such that, other than incidental leaks, the second airflow is prevented from flowing from the turbine inlet to the outlet. As a result, the drive assembly and the agitator are stopped. Alternatively, the baffle may restrict the second airflow path partially such that the second airflow continues to flow from the turbine inlet to the outlet. The mass flow rate of the second airflow may be sufficient to drive the drive assembly and the agitator. However, in restricting partially the second airflow path, the mass flow rate is reduced such that overspeed of the drive assembly may be avoided. 
     The baffle may be biased in the open position by a biasing force, such as that provided by a spring or an elastic tether. The baffle then moves to the closed position when the lift force exerted by the second airflow exceeds the biasing force. 
     When in the closed position, the baffle may restrict the second airflow path such that the mass flow rate of the second flow is insufficient to drive the drive assembly and the agitator. This then has the benefit that, when a blockage occurs within the first airflow path, a user is provided with a clear indication (namely the stopping of the agitator) that a blockage has occurred. 
     The baffle may be coupled to a user-operable actuator, such as a dial or knob. A user is then able to move the baffle between the open and closed positions, irrespective of the dynamic pressure of the second airflow. Where the baffle restricts completely the second airflow path, the user-operable actuator may be used to turn off and on the agitator. Alternatively, where the baffle restricts partially the second airflow path, the user-operable actuator may be used to switch between two different power settings for the agitator. 
     The first airflow and the second airflow are generated in response to suction at the outlet. The baffle may be configured such that, when in the closed position, the suction retains the baffle in the closed position, and the baffle returns to the open position when the suction is removed. This then has the benefit that, when employed with a vacuum cleaner, the baffle remains closed until such time as the vacuum cleaner is turned off or the cleaner head is detached from the vacuum cleaner. The cleaner head is intended primarily to be used with the agitator turned on (when the baffle restricts completely the second airflow path) or in high-power mode (when the baffle restricts partially the second airflow path). By ensuring that the baffle returns to the open position once the suction is removed, the agitator is again turned on or returned to the high-power mode when the vacuum cleaner is later turned on or the floor tool is later reattached to the vacuum cleaner. 
     When the baffle moves to the closed position, one or more small gaps may exist between the baffle and a wall of the second airflow path. As the second airflow is drawn through these gaps, unwanted noise or vibration may be generated. The baffle may therefore comprise a flexible seal that deforms to create a seal within the second airflow path when in the closed position. As a result, such gaps may be avoided or minimised. 
     The baffle may comprise a body to which the seal is attached. The body is then made of a stiffer material than that of the seal. For example, the body may be formed of a thermoplastic, whilst the seal may be formed of natural or synthetic rubber which is overmoulded onto the body. During use, suction at the outlet is intended to retain the baffle in the closed position. If the baffle were formed only of a compliant material, the baffle would most likely deform when in the closed position. By having a baffle that comprises a stiff body and a flexible seal, the baffle is able to both retain its position when in the closed position and deform to create a seal within the second airflow path. 
     The baffle may lie generally parallel to a wall of the second airflow path when in the open position and normal to the wall when in the closed position. As a result, a relatively compact arrangement may be achieved. By lying parallel to the wall, the baffle does not interfere adversely with the second airflow when the baffle is in the open position. 
     The present invention also provides a cleaner head for a vacuum cleaner comprising a suction inlet for admitting a first airflow, a turbine inlet for admitting a second airflow, an outlet for discharging the first airflow and the second airflow, a first airflow path for carrying the first airflow from the suction inlet to the outlet, a second airflow path for carrying the second airflow from the turbine inlet to the outlet, an agitator for agitating a surface to be cleaned, a drive assembly for driving the agitator, the drive assembly comprising a turbine that is driven by the second airflow, and a baffle located in the second airflow path and movable between an open position in which the second airflow path is unrestricted and a closed position in which the second airflow path is restricted, wherein the first airflow and the second airflow are generated in response to suction at the outlet, the baffle is biased in the open position, the second airflow exerts a lift force on the baffle when in the open position, the baffle moves to the closed position when the dynamic pressure of the second airflow at the baffle exceeds a threshold, the suction subsequently retains the baffle in the closed position, and the baffle returns to the open position when the suction is removed. 
     The baffle therefore closes automatically when the dynamic pressure of the second airflow exceeds a threshold. The baffle closes so as to restrict the second airflow path, thereby reducing the mass flow rate of the second airflow. Since the second airflow is responsible for driving the turbine, overspeed of the drive assembly may be avoided. 
     The first airflow and the second airflow are generated in response to suction at the outlet. When the baffle closes, the suction retains the baffle in the closed position. The baffle then returns to the open position when the suction is removed. This then has the benefit that, when employed with a vacuum cleaner, the baffle remains closed until such time as the vacuum cleaner is turned off or the cleaner head is detached from the vacuum cleaner. The cleaner head is intended primarily to be used with the agitator turned on (when the baffle restricts completely the second airflow path) or in high-power mode (when the baffle restricts partially the second airflow path). By ensuring that the baffle returns to the open position once the suction is removed, the agitator is again turned on or returned to the high-power mode when the vacuum cleaner is later turned on or the floor tool is later reattached to the vacuum cleaner. 
     The present invention further provides a cleaner head for a vacuum cleaner comprising a suction inlet for admitting a first airflow, a turbine inlet for admitting a second airflow, an outlet for discharging the first airflow and the second airflow, a first airflow path for carrying the first airflow from the suction inlet to the outlet, a second airflow path for carrying the second airflow from the turbine inlet to the outlet, an agitator for agitating a surface to be cleaned, a drive assembly for driving the agitator, the drive assembly comprising a turbine that is driven by the second airflow, a baffle located in the second airflow path and movable between an open position in which the second airflow path is unrestricted and a closed position in which the second airflow path is restricted, and a user-operable actuator coupled to the baffle such that a user may move the baffle between the open and closed positions, wherein the baffle is biased in the open position, the second airflow exerts a lift force on the baffle when in the open position, and the baffle moves to the closed position when the dynamic pressure of the second airflow at the baffle exceeds a threshold. 
     The baffle therefore closes automatically when the dynamic pressure of the second airflow exceeds a threshold. The baffle closes so as to restrict the second airflow path, thereby reducing the mass flow rate of the second airflow. Since the second airflow is responsible for driving the turbine, overspeed of the drive assembly may be avoided. 
     The baffle may additionally be moved between the open and closed positions using the user-operable actuator. The baffle may restrict completely the second airflow path. In this instance, the user-operable actuator may be used to turn off and on the agitator. 
     Alternatively, the baffle may restrict partially the second airflow path. In this instance, the user-operable actuator may be used to switch between two different power settings for the agitator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be more readily understood, an embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an exploded view of a cleaner head in accordance with the present invention; 
         FIG. 2  is an isometric view from above of the cleaner head; 
         FIG. 3  is an isometric view from below of the cleaner head; 
         FIG. 4  is an exploded view of a drive-control mechanism of the cleaner head; 
         FIG. 5  is a sectional slice through the baffle of the drive-control mechanism; 
         FIG. 6  is a sectional slice through the cleaner head along the plane A-A; 
         FIG. 7  is a sectional slice through the cleaner head along the plane B-B, wherein the baffle of the drive-control mechanism is in an open position; and 
         FIG. 8  is a sectional slice through the cleaner head in the plane B-B, wherein the baffle of the drive-control mechanism is in a closed position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The cleaner head  1  of  FIGS. 1 to 8  comprises a housing  2 , an agitator  3 , a drive assembly  4 , and a drive-control mechanism  5 . 
     The housing  2  comprises several different sections  2   a - 2   c  that, when assembled, define a suction inlet  6 , a turbine inlet  7 , an outlet  8 , a first airflow path  9  for carrying a first airflow from the suction inlet  6  to the outlet  8 , and a second airflow  10  path for carrying a second airflow from the turbine inlet  7  to the outlet  8   
     The suction inlet  6  is formed on an underside of the housing  2 . During use, dirty air is drawn in through the suction inlet  6  and carried by the first airflow path  9  to the outlet  8 . 
     The turbine inlet  7  is formed at the rear of the housing  4 . Owing to its location, clean air rather than dirty air is drawn in through the turbine inlet  7  during operation. From there, the clean air is carried by the second airflow path  10  to the outlet  8 . 
     The first and second airflow paths  9 , 10  are defined by walls of the housing  2 . The first airflow path  9  comprises an agitator chamber  11 , and the second airflow path  10  comprises a turbine chamber  12 . The two airflow paths  9 , 10  merge at the outlet  8 , which is located at the rear of the housing  2 . 
     The agitator  3  comprises an elongate body  13  to which bristles, flicker strips or other means  14  for agitating a cleaning surface are attached. The agitator  3  is rotatably mounted with the agitator chamber  11 . More specifically, the agitator  3  is mounted at one end to a bushing  15  seated within a removable cap  16 . The opposite end of the agitator  3  is mounted to a dog  19  forming part of the drive assembly  4 . 
     The drive assembly  4  comprises a turbine  17  and a transmission  18  for transmitting torque generated by the turbine  17  to the agitator  3 . The transmission  18  comprises, among other things, a dog  19  to which one end of the agitator  3  is mounted. The turbine  17  is mounted within the turbine chamber  12  and is driven by the second airflow. The details of the drive assembly  4  are not pertinent to the present invention, and drive assemblies suitable for use in the present cleaner head  2  are well known. By way of example, the cleaner head of the DC12 vacuum cleaner sold by Dyson Limited, and the mini turbine head sold by Dyson Limited as an accessory for removing pet hair, each include a drive assembly suitable for inclusion in the present cleaner head  2 . 
     The drive-control mechanism  7  comprises a shaft  20 , a baffle  21 , a retainer  22 , a torsion spring  23 , and a user-operable actuator  24 . 
     The baffle  21  is semi-circular in shape and is attached to the shaft  20  along its straight edge. The baffle  21  comprises a body  25  formed of a relatively stiff material over which a cover  26  of a more compliant material is moulded. The cover  26  projects beyond the body  25  so as to define a flexible seal  27 . 
     The retainer  22  comprises a disc-shaped body having a hook on one side. The retainer  22  is attached to and projects radially outward from the shaft  20 . 
     The spring  23  is mounted to the shaft  20  at the retainer  22 . Moreover, one arm of the spring  23  is retained by the hook of the retainer  22 . 
     The user-operable actuator  24  comprises a cylindrical dial attached to one end of the shaft  20 . Turning the actuator  24  thus causes the shaft  20  to rotate about its axis. 
     The shaft  20 , the body  25  of the baffle  21 , and the retainer  22  are formed as a single moulded component. This then simplifies the manufacture of the drive-control mechanism  5 . Equally, however, the various parts may be formed separately and then attached to one another. 
     With the exception of the user-operable actuator  24  and that part of the shaft  20  attached to the actuator  24 , the drive-control mechanism  5  is located within the housing  2 . 
     The retainer  22  is seated within a recess in the housing  2 . As noted above, one arm of the spring  23  is retained by the retainer  22 . The other arm of the spring  23  is retained by the housing  2 . Consequently, as the shaft  20  rotates, the spring  23  is twisted and exerts a biasing force in the opposite direction. 
     The baffle  21  is located within the second airflow path  10  at a position downstream of the turbine chamber  12  and upstream of the outlet  8 . The baffle  21  is movable between an open position and a closed position. In the open position, as shown in  FIG. 7 , the baffle  21  lies flat against a wall of the second airflow path. In the closed position, as shown in  FIG. 8 , the baffle  21  lies normal to the wall. The baffle  21  is shaped and sized such that, when in the closed position, the baffle  21  closes the second airflow path  10 . In the present embodiment, the cross-sectional shape of the second airflow path  10  at the baffle  21  is generally semi-circular, hence the shape of the baffle  21 . The wall of the second airflow path  10  includes a small stub  28  against which the baffle  21  abuts when in the closed position. The stub  28  prevents the baffle  21  from moving beyond the closed position, as is explained below in more detail. 
     Turning the user-operable actuator  24  causes the baffle  21  to move between the open position and the closed position. The actuator  24  may therefore be regarded as having an ON position in which the baffle  21  is in the open position, and an OFF position in which the baffle  21  is in the closed position. Owing to the biasing force of the torsion spring  23 , the actuator  24  is biased in the ON position. 
     The cleaner head  1  is intended to form part of a vacuum cleaner. For example, the cleaner head  1  may form the primary cleaner head of the vacuum cleaner. Alternatively, the cleaner head  1  form of an accessory that is intended to be attached to a wand, hose or the like of the vacuum cleaner. 
     During use, the vacuum cleaner generates suction at the outlet  8  of the cleaner head  1 . The suction causes a first airflow to be drawn in through the suction inlet  6 , and a second airflow to be drawn in through the turbine inlet  7 . The first airflow is carried by the first airflow path  9  to the outlet  8 , and the second airflow is carried by the second airflow path  10  to the outlet  8 . 
     The turbine  17 , which is located in the second airflow path  10 , is driven by the second airflow. The torque generated by the turbine  17  is transmitted to the agitator  3  by the transmission  18 . As the agitator  3  rotates, the bristles or other means  14  project beyond the suction inlet  6  and agitate the cleaning surface. The agitation encourages dirt to be lifted from the cleaning surface, whereupon the dirt is drawn in through the suction inlet  6 . The first airflow path  9  is therefore intended to carry dirty air, whereas the second airflow path  10  is intended to carry clean air. In order that dirt is prevented from being carried inadvertently by the second airflow, the turbine inlet  7  may be covered by a fine mesh or other screen. 
     The baffle  21  is biased in the open position by the torsion spring  23 . When in the open position, the second airflow path  10  is open and thus the second airflow is free to flow from the turbine inlet  7  to the outlet  8 . As a result, the turbine  17  is driven by the second airflow and thus the agitator  3  is caused to rotate. 
     Rotation of the agitator  3  may be stopped by turning the user-operable actuator  24  from the ON position to the OFF position. Turning the actuator  24  to the OFF position causes the baffle  21  to move from the open position to the closed position. When the baffle  21  is in the closed position, the second airflow path  10  is closed and thus the second airflow is prevented from flowing from the turbine inlet  7  to the outlet  8 . As a result, the turbine  17  and thus the agitator  3  stop rotating. 
     When the baffle  21  is in the closed position, the suction generated by the vacuum cleaner creates a partial vacuum on one side of the baffle  21 . The opposite side of the baffle  21 , on the other hand, is at ambient. As a result, there is a net force that exceeds the biasing force of the spring  23  and maintains the baffle  21  in the closed position. The baffle  21  is returned to the open position in one of two ways. First, the user-operable actuator  24  may be turned to the ON position. The actuator  24  may therefore be used to turn the agitator  3  both off and on. Second, the suction generated by the vacuum cleaner may be removed from the cleaner head  1 , e.g. by turning off the vacuum cleaner or by detaching the cleaner head  1  from the vacuum cleaner. As a result, the baffle  21  returns to the open position under the biasing force of the spring  23 . The cleaner head  1  is intended to be used primarily with the agitator  3  turned on. Ensuring that the baffle  21  returns to the open position once the suction is removed has the benefit that, when the vacuum cleaner is subsequently turned on or the cleaner head  1  is subsequently reattached to the vacuum cleaner, the agitator  3  is already turned on. 
     During use, the first airflow path  9  may become obstructed, e.g. by dirt or some other object inadvertently drawn in through the suction inlet  6 . The obstruction may lead to a decrease in the mass flow rate of the first airflow. Since the suction at the outlet  8  is generally unchanged, the mass flow rate of the second airflow increases. As a result, the speed of the turbine  17  increases. Overspeed of the turbine  17  may damage components of the drive assembly  4 , such as bearings. However, as will now be explained, the baffle  21  moves automatically to the closed position whenever the mass flow rate of the second airflow reaches a level that would otherwise result in overspeed. 
     When the baffle  21  is in the open position, the second airflow exerts a lift force on the baffle  21 . When the mass flow rate of the second airflow lies within normal operating limits, the biasing force of the spring  23  exceeds the lift force of the second airflow. Consequently, the baffle  21  is held in the open position during normal use of the cleaner head  1 . When an obstruction occurs in the first airflow path  9 , the mass flow rate of the second airflow increases. As a result, the dynamic pressure of the second airflow at the baffle  21  increases and thus the lift force exerted by the second airflow on the baffle  21  increases. When the dynamic pressure at the baffle  21  exceeds a threshold, the lift force exceeds the biasing force of the spring  23  and the baffle  21  moves to the closed position. The baffle  21  therefore moves automatically to the closed position whenever the mass flow rate of the second airflow reaches a level that would otherwise result in overspeed of the drive assembly  4 . 
     When the baffle  21  moves automatically to the closed position, the agitator  3  stops rotating. A user is therefore provided with an indication that an obstruction has occurred in the first airflow path. This is particularly useful since any obstruction is likely to compromise the cleaning performance of the cleaner head  1 . Without the baffle  21 , a user would not necessary know that an obstruction had occurred. This is particularly true since the agitator  3  would continue to rotate. 
     When an obstruction occurs, the user may attempt to clear the obstruction whilst the vacuum cleaner is turned on and continues to apply suction at the outlet  8 . As noted above, the suction generated at the outlet  8  maintains the baffle  21  in the closed position. Accordingly, on clearing the obstruction, the agitator  3  remains stationary. Potential harm to the user (e.g. the hand used to remove the obstruction) is therefore avoided. 
     As noted above, the baffle  21  moves to the closed position whenever the dynamic pressure of the second airflow at the baffle  21  exceeds a threshold. Several factors influence the magnitude of this threshold. Chief among them are the cross-sectional area of the second airflow path  10 , the spring constant of the torsion spring  23 , the mass of the baffle  21 , and the lift coefficient of the baffle  21 . The lift coefficient is determined by, among other things, the shape and angle of attack of the baffle  21 . Adjusting any one of these factors will influence the dynamic-pressure threshold at which the baffle  21  moves to the closed position. So, for example, decreasing the spring constant of the torsion spring  23  will cause the baffle  21  to move to the closed position in response to a lower dynamic pressure and thus a lower mass flow rate. 
     When the baffle  21  is in the closed position, the baffle  21  abuts the stub  28  in the wall of the second airflow path  10 . This then prevents the baffle  21  from moving beyond the closed position. Without the stub  28 , the suction at the outlet  8  would cause the baffle  21  to move beyond the closed position. The second airflow would then continue to flow and thus overspeed protection of the turbine  17  would no longer be provided. In the present embodiment, the baffle  21  is stopped by a sub  28 . However, other means for preventing travel of the baffle  21  beyond the closed position might equally be used. For example, the wall of the second airflow path  10  may include a ridge or the wall may narrow slightly. As a further alternative, the shaft  20  or the retainer  22  may include a feature that permits rotation through 90 degrees only. 
     The baffle  21  comprises a body  25  formed of a relatively stiff material over which a cover  26  of a more compliant material is formed. The cover  26  projects beyond the body  25  to create a flexible seal  27  at the periphery of the baffle  21 . When in the closed position, the seal  27  deforms to form a seal against the wall of the second airflow path  10 . As a result, potential leaks around the baffle  21 , which might otherwise generate noise and/or vibration, may be avoided. 
     If the baffle  21  were made only of a compliant material, the baffle  21  would most likely deform under the suction of the vacuum cleaner when in the closed position. As a result, the second airflow would continue to flow past the baffle  21 . Indeed, the baffle  21  may deform to such an extent that overspeed protection and/or manual on/off control is no longer provided. By having a baffle  21  that comprises a body  25  formed of a relatively stiff material, the baffle  21  has the necessary stiffness to ensure that the baffle  21  does not deform under the suction of the vacuum cleaner when in the closed position. The combination of a stiff body  25  and flexible seal  26  therefore ensures that the baffle  21  retains its position when in the closed position whilst deforming at the periphery to create a seal within the second airflow path  10 . 
     In spite of the aforementioned advantages, the seal  27  is not regarded as essential since any leaks around the baffle  21  are unlikely to be sufficient to drive the drive assembly  4  and the agitator  3 . Even if the drive assembly  4  and the agitator  3  were to rotate, the speed would be relatively slow. Moreover, any noise generated by leaks around the baffle  21  may actually serve to provide the user with a further indication that an obstruction has occurred in the first airflow path  9 . Accordingly, the seal  27 , and the cover  26  used to create the seal  27 , may be omitted. 
     In the embodiment described above, the baffle  21  is sized and shaped such that, when in the closed position, the baffle  21  closes completely the second airflow path  10 . Consequently, other than incidental leaks, the second airflow is prevented from flowing from the turbine inlet  7  to the outlet  8 , and thus the drive assembly  4  and the agitator  3  are stopped. Conceivably, the baffle  21  may be sized and shaped such that the baffle  21  closes only partially the second airflow path  10 . In this instance, the second airflow would continue to flow from the turbine inlet  7  to the outlet  8 . Moreover, the mass flow rate of the second airflow may be sufficient to drive the drive assembly  4  and the agitator  3 . However, in closing partially the second airflow path  10 , the mass flow rate of the second airflow is reduced. As a result, overspeed of the drive assembly  4  may be avoided. Additionally, the user-operable actuator  24  may be used to switch between two different power settings for the drive assembly  4  and the agitator  3 . For example, when the baffle  21  is in the open position, the second airflow has a higher mass flow rate and thus the drive assembly  4  and the agitator  3  are driven at higher power. In contrast, when the baffle  21  is in the closed position, the second airflow has a lower mass flow rate and thus the drive assembly  4  and the agitator  3  are driven at lower power. Since the baffle  21  may be configured to partially or fully close the second airflow path  10 , the second airflow path  10  may be said to be restricted (e.g. partially or fully closed) when the baffle  21  is in the closed position and unrestricted (e.g. open) when the baffle  21  is in the open position. 
     The drive-control mechanism  5  provides two functions. First, it provides a user with the means to control the agitator  3 . In particular, the actuator  24  may be used to turn the agitator  3  off and on, or switch between different power settings. Second, it provides overspeed protection of the drive assembly  4  by automatically restricting the second airflow path  10  in response to an excessive mass flow rate. Conceivably, the user-operable actuator  24  may be omitted such that the drive-control mechanism  5  provides overspeed protection only. Additionally, the shaft  20  and retainer  22  may be omitted and the baffle  21  may be directly attached to the housing  2 . For example, the baffle  21  may include a pin at each end of the straight edge which is held by the housing  2  such that the baffle  21  is free to pivot within the second airflow path  10 . Accordingly, in its most simplest sense, the drive-control mechanism  5  may be regarded as a baffle  21  located within the second airflow path  10 , which is movable between an open position in which the second airflow path  10  is unrestricted (e.g. open) and a closed position in which the second airflow path  10  is restricted (e.g. partially or fully closed). The baffle  21  is then biased (e.g. by means of a spring, elastic tether or other means) in the open position and moves to the closed position when the dynamic pressure of the second airflow at the baffle  21  exceeds a threshold. 
     In the embodiment described above, the user-operable actuator  24  comprises a dial that is coupled to the baffle  21  by means of a shaft  20 . However, the actuator  24  may comprise alternative means (e.g. knob, lever or the like) which, when coupled to the baffle  21 , can be operated by a user in order to move the baffle  21  from the open position to the closed position. 
     The agitator  3  of the above embodiment comprises an elongate body  13  having bristles, flicker strips or other means  14  for agitating the cleaning surface. This type of agitator is generally referred to as a brush bar or beater bar. However, alternative types of agitator capable of being driven by the drive assembly  4  might equally be used. By way of example, the agitator  3  may comprise a pair of rotary discs, as described in US2012/0144621. 
     The present invention provides a relatively simple mechanism  5  for protecting against overspeed of the drive assembly  4 . In comparison to existing mechanisms, the baffle  21  is relatively cheap and lightweight. Moreover, since the baffle  21  is located within the path  10  used to carry the airflow for driving the drive assembly  4 , overspeed protection may be achieved without increasing the size of the cleaner head  1 .