Abstract:
The invention provides a drive train for a hedge trimmer, the drive train being arranged, in use, to provide power to a downstream mechanism, the drive train comprising: a motor having an output shaft; a pinion coupled to the output shaft; a reduction gear arranged to be driven by the pinion, the reduction gear contained within a gear housing; a flywheel engageable to the output shaft, wherein the flywheel is arranged within the gear housing.

Description:
[0001]    The present invention relates to a drive train for a hedge trimmer, a hedge trimmer and to a method of controlling a hedge trimmer. 
       BACKGROUND OF THE INVENTION 
       [0002]    Hedge trimmers are well known tools for cutting hedges and, more generally, vegetation from plants. Typically, a hedge trimmer includes a motor forming part of a drive train arranged to drive the motion of one or more suitably shaped blades, thereby to cut vegetation. Typically two toothed blades are provided and they are driven in relative reciprocating motion such that stems or leaves caught between a pair of teeth on one of the blades are cut as the teeth blade shear with a pair of teeth on the other of the blades. Since it is the movement of the blades relative to each other that is important, in some cases, one axially reciprocating blade is provided disposed adjacent a stationary blade, the teeth of the stationary blade providing a reaction force for the teeth of the reciprocating blade. In another example, both blades are caused to move so as to generate the required relative motion. 
         [0003]    The motor for driving movement of the blade(s) may itself be powered by any appropriate power source. In one example, the motor is arranged to receive power from a battery such as a Li ion battery provided within the tool housing. In another example an AC mains connection is provided to the tool which supplies power to drive the motor. In the case of the power source being a battery, clearly the available power to the motor will be more limited given the physical limitations on battery power that derive from size and weight and the fact that the tool (containing the battery) needs to be light enough so that it can be used. 
         [0004]    Whilst having a powerful motor is desirable, the more powerful a motor is, the larger it will tend to be. Accordingly, since the motor is for use in a hand-held tool, in terms of weight and the ease with which the tool can be manipulated, it is clearly desirable to have as small a motor as possible. 
         [0005]    Known systems for powering hedge trimmers are disclosed in, for example, GB-A-1,406,244, GB-A-1,427,077, GB-A-848,325 and GB-A-884,387. In GB-A-884,387 a clutch is provided in a tool (an example of which is a hedge trimmer). The clutch also functions as a flywheel to provide the benefit of stored kinetic energy. The clutch, and therefore the flywheel, is arranged within the tool housing and is thus exposed to the air within the cavity defined by the housing. In use, it will generate and experience windage as a consequence. Windage may be considered the force of the air acting on the flywheel due to the relative movement between them or, conversely, the wind generated by the flywheel as it rotates. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the present invention, there is provided a drive train for a hedge trimmer, the drive train being arranged in use, to provide power to a downstream mechanism, the drive train comprising: a motor having an output shaft; a pinion coupled to the output shaft; a reduction gear arranged to be driven by the pinion, the reduction gear contained within a gear housing; a flywheel coupled or coupleable to the output shaft, wherein the flywheel is arranged within the gear housing. 
         [0007]    The invention provides a drive train for a hedge trimmer that includes a flywheel arranged within a gear casing. This has the effect of reducing windage whilst still providing the benefits of increased inertia from the flywheel. Thus, a smaller motor can be used without sacrificing inertia and without encountering the problem of windage. Furthermore, the presence of the flywheel within the gear housing means also that windage generated by the gears and the flywheel is minimised and thus has no deleterious effects on the drive train as a whole. 
         [0008]    It will generally be understood that the presence of a flywheel enables the storage of energy which may be transferred, when required, to the blades of the hedge trimmer. The inertia of a flywheel is generally proportional to its radius squared (r 2 ). The energy stored by a flywheel is generally proportional to the inertia and the square of the angular velocity of the flywheel. 
         [0009]    In one embodiment, the flywheel is selectively engageable and disengagable from the output shaft so as to selectively vary the inertia provided by the motor. 
         [0010]    The invention, in this embodiment, provides a drive train for a hedge trimmer that includes a motor and an output shaft with a flywheel that is selectively engageable and disengagable from the output shaft so as to selectively vary the inertia provided by the motor. In other words, without disengaging the output shaft from the motor, and so as to continue to be able to derive power from the motor, the flywheel can be engaged or disengaged. This has the effect that the drive torque is decoupled from the inertia or kinetic energy provided by a motor. 
         [0011]    The drive torque is the continuous torque required to drive the mechanism at the required speed. The inertia however is the stored energy required to be converted into cutting force at the blades. By providing a flywheel that is selectively engageable or disengageble from the output shaft of the motor, the kinetic energy delivered by the drive train can be varied. 
         [0012]    Thus, the ability is provided by which a motor can simply be chosen to drive the blades at a “no-load” speed and the amount of inertia required to cut can be chosen independently of this. The heavier or larger the flywheel, the greater the inertia. Furthermore in some examples, the flywheel is selectively engageable or disengageable during use such that different power levels of cutting can be obtained from the same device. 
         [0013]    This enables a smaller motor to be used, which is particularly advantageous in the case of use of a battery-powered motor. By providing a smaller motor, the rate of use of the power from the battery is reduced. Consequently, the required time between charges is increased. 
         [0014]    In one example, the flywheel is arranged at the input side of a gear reduction. In other words, the output gear forms part of a gear reduction with the pinion. Since the flywheel is located at the input side of the gear reduction some significant advantage is provided in terms of stored energy. For example, and without any limitation, considering a flywheel weighing 76 g and having a diameter of, say 50 mm, at a typical motor speed of 24,000 rpm, about 80J of energy would be stored. In contrast, if the flywheel were provided at a typical gear-reduced output speed of, say, 1500 rpm, the stored energy would be about 0.3 J. Thus, by providing the flywheel at the input to the gear reduction, significantly improved energy storage is enabled. It will be appreciated that the actual values given here are merely exemplary, but demonstrate well the advantages of providing the flywheel at the input to a gear reduction. 
         [0015]    In one embodiment, in use, the downstream mechanism can be driven irrespective of whether or not the flywheel is engaged. This provides the advantage that the motor can be used to drive a mechanism at two alternative power levels. In one case, the flywheel would be engaged and therefore the inertia, i.e. stored energy, would be relatively high. This would translate into an increased energy transfer from the blades and therefore greater cutting power. 
         [0016]    In an alternative, the mechanism can be driven with the flywheel disengaged and thus be driven at relatively low power. Accordingly, the power required to drive the motor can be controlled correspondingly, which means that battery power will not be wasted when only low power output is required from the motor. This contrasts markedly with the device of for example, GB-A-884,387, in which with the clutch, i.e. the flywheel, disengaged, no power can be drawn from the motor to drive the associated tool. 
         [0017]    In one embodiment, the flywheel is supported axially along its axis of rotation on both sides of the flywheel. This provides additional support and stability to the flywheel in use. 
         [0018]    In one embodiment, the pinion and the flywheel are integrated as a single unitary component. The provision of an integrated flywheel/pinion component enables a simplified manufacturing process to be achieved. A single sintered component may be used, thus reducing part count and simplifying manufacture. 
         [0019]    According to a second aspect of the present invention, there is provided a hedge trimmer, comprising a drive train according to the first aspect of the present invention. 
         [0020]    Thus, a hedge trimmer is provided that has all the advantages described above with reference to the drive train. 
         [0021]    In one embodiment, the hedge trimmer comprises: two blades arranged for relative reciprocating motion so as to enable the cutting of vegetation therebetween; and a drive train arranged to provide power to the blades and control motion of the blades relative to each other, wherein the drive train is a drive train according to the first aspect of the present invention. 
         [0022]    Thus, a hedge trimmer is provided that has all the advantages described above with reference to the drive train. 
         [0023]    In one embodiment, a hedge trimmer is provided including a drive train according to the first aspect of the present invention which includes the selectively engageable and disengageable flywheel. The tool also comprises an outer tool housing including one or more handles for a user to hold and activate the tool; and a flywheel actuator provided on the outer tool housing to enable a user to selectively engage or disengage the flywheel. 
         [0024]    Thus, a simple and easy-to-use mechanism is provided by which a user can manually control the inertia of the hedge trimmer. The actuator may include one or more switches or levers. 
         [0025]    In an alternative embodiment, instead of providing a manual actuator, an automatic sensor is provided which detects automatically the resistance to cutting and when some threshold level of resistance is detected, the flywheel is automatically engaged. Conversely, when the resistance drops below the threshold, the flywheel is automatically disengaged. An appropriate control circuit may be provided that will be within the knowledge of the skilled man. 
         [0026]    Thus a user is able to use the hedge trimmer without concern as to the difficulty of cutting that might be encountered. Quite simply, if the resistance to cutting is such that the sensor detects the need for greater inertia, the flywheel is automatically engaged. Conversely, and so as to limit power consumption when possible, if the resistance then falls below the threshold, the flywheel is automatically disengaged. 
         [0027]    According to a third aspect of the present invention, there is provided a method of enabling control of a hedge trimmer, the method comprising in a hedge trimmer, providing a drive train to provide power to a downstream mechanism, the drive train comprising: a motor having an output shaft; a pinion coupled to the output shaft; a reduction gear arranged to be driven by the pinion, the reduction gear contained within a gear housing; a flywheel coupled or coupleable to the output shaft, wherein the flywheel is arranged within the gear housing. 
         [0028]    Thus, a method is provided that has all the advantages described above with reference to the drive train. 
         [0029]    According to a further aspect of the present invention, there is provided a drive train for a hedge trimmer, the drive train being arranged in use, to provide power to a downstream mechanism, the drive train comprising: a motor having an output shaft; and a flywheel coupled or coupleable to the output shaft, wherein the flywheel is arranged within a flywheel housing that conforms closely to the shape of the flywheel. Preferably, the flywheel is substantially in the form of a disc having opposed major surfaces, wherein the clearance between the flywheel and the housing is less than 5 mm on both major surfaces of the flywheel. This provides the advantage of reduced windage generated or experienced by the flywheel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which: 
           [0031]      FIG. 1  shows an example of a motor from a drive train for a hedge trimmer; 
           [0032]      FIG. 2  shows the motor of  FIG. 1  with an engagement wheel arranged thereon; 
           [0033]      FIGS. 3A and 3B  show, from opposed sides, views of a flywheel for use on the motor of  FIG. 1 ; 
           [0034]      FIGS. 4 and 5  show a motor with a flywheel engaged and disengaged, respectively; 
           [0035]      FIG. 6  shows a schematic representation of the housing and internal components of a hedge trimmer; 
           [0036]      FIG. 7A  shows a schematic representation of the drive train within a hedge trimmer; 
           [0037]      FIG. 7B  shows a simplified schematic representation of a section through part of the drive train of a hedge trimmer; and 
           [0038]      FIG. 8  shows a schematic representation of the drive train within a hedge trimmer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    Referring first to  FIG. 1 , and before describing the arrangement of the drive train in situ within a hedge trimmer, the components that would typically be included within a drive train according to an exemplary embodiment of the present invention will now be described. 
         [0040]      FIG. 1  shows a motor  2  having an output shaft  4 . In this example, the motor is a battery-powered motor that may run off a Li ion battery or other suitable type of battery. The shaft  4  is of a length so as to be able to support both a pinion and a flywheel, to be described in detail below. Shoulders  6  are provided which enable accurate positioning of the pinion and/or the flywheel on the shaft  4  to be achieved. In view of the requirement to support both a pinion and flywheel, the shaft  4  is extended somewhat as compared to the length that might be included were no flywheel included. In use the motor  2  would be arranged within the housing of a hedge trimmer. The flywheel is arranged within the gear casing (not shown) which serves to encase the pinion and the output gear. By providing the flywheel within this housing, the windage experienced or generated by the device is reduced. 
         [0041]      FIG. 2  shows the end of the motor  2  of  FIG. 1  with an engagement wheel  8  arranged thereon. The engagement wheel has a shape and configuration that enables it to lock with another appropriately shaped component, to be described. In this example, the wheel  8  has diametrically opposed recessed sectors  10 , and extended sectors  12  arranged therebetween. The recessed sectors  10  are shaped so as to be able to receive and lock rotationally with a component having similar recessed and extended sectors. The engagement wheel  8  is fixed rotationally with respect to the shaft  4 . In other words, the engagement wheel  8  cannot rotate relative to the shaft  4 . This can be achieved by keying the engagement wheel to the shaft or by any other suitable means. 
         [0042]      FIGS. 3A and 3B  show, from opposed sides, views of a flywheel for use on the motor of  FIGS. 1 and 2 . The flywheel  14  has a cylindrical axial bore  16  arranged to fit on to the shaft  4  of the motor  2 . Two extended sectors  18  are provided that are shaped correspondingly to the recessed sectors  10  of the engagement wheel  8 . Thus, it will be appreciated that the flywheel  14  can be rotationally locked to the engagement wheel  8 , by relative axial movement along the shaft  6 . In an engaged position, the extended sectors  18  of the flywheel fit into the recessed sectors  10  of the engagement wheel  8 , whilst the extended sectors  12  of the engagement wheel  8  fit into the recessed sectors defined between the extended sectors  18  of the flywheel. By controlling the relative axial position of the engagement wheel  8  and the flywheel  14 , it can be determined whether or not the flywheel  14  is engaged with the output shaft  6 . 
         [0043]    A coupler  24  is provided, as shown in  FIG. 3A , engaged with the rear side of the flywheel  14 . The rear side  22  of the flywheel has an enlarged central axial opening  20  shaped to receive the coupler  24 . The coupler  24  has an extended cylindrical extension  26  arranged to fit tightly within the opening  20  on the rear side  22  of the flywheel and connect thereto. The connection between the coupler  24  and the flywheel must be secure such that movement of the flywheel can be controlled by control of the coupler  24 . This can be achieved by use of one or more of welding, adhesive, press fit or any other appropriate means of coupling. Indeed, in one example, the coupler and the flywheel are formed and/or provided as an integrated component. This is particularly advantageous as it leads to a reduced parts count and can be formed conveniently by a sintering or molding process. 
         [0044]    As can be seen, the coupler  24  has an extended annular rim  28 .  FIG. 4  shows a schematic representation of the drive train in which the flywheel  14  is engaged with the engagement wheel  8 . The extended and recessed annular sectors of the flywheel and the engagement wheel  8  are engaged such that the flywheel is rotationally fixed to the engagement wheel  8 . Accordingly, since the engagement wheel is rotatably fixed to the output shaft, the flywheel is also rotatably fixed to the output shaft of the motor. A pinion  36  (not shown in  FIG. 4  or  5 , but seen clearly in, say  FIGS. 7A and 8 ) is typically provided on the output shaft  6  for engagement with a gear  38  for driving movement of a downstream mechanism such as the blades of a hedge trimmer. A clutch is also preferably provided within the drive train, e.g. to enable rotational movement of the motor to be coupled to the down stream mechanism, as will be understood by a skilled person. 
         [0045]    In this example, a control arm  30  is provided that is sized and arranged to fit in the gap between the extended annular rim  28  of the coupler  24  and the rear side  22  of the flywheel  14 . By control of the position of the control arm  30 , the flywheel  14  can be made to move axially along the shaft  4  thus moving it between an engaged position as shown in  FIG. 4  and a disengaged position as shown in  FIG. 5 . Thus, a user is able to selectively control the flywheel either to be engaged or disengaged from the output shaft  4  of the motor  2 . It will be appreciated that other forms of coupler may be used so long as it enables connection between the control arm  30  and the flywheel. 
         [0046]    By doing this a user is able to control the inertia of the device and thus the energy which may be imparted to the downstream mechanism, i.e. the blades of a hedge trimmer. Accordingly, the motor could be run with low inertia when cutting one type of material and then have the flywheel engaged so as to increase the inertia and be able to cut a different harder-to-cut type of material. The same motor is used at all times and this means that a smaller motor can be used which can then have its inertia selectively increased by engagement of the flywheel with its output shaft. 
         [0047]    Preferably, in the unengaged position the flywheel is free to rotate relative to the output shaft. 
         [0048]    Conventionally, if a high inertia cutting device was required, a correspondingly sized large motor would be required. Using the present system, a low power and inertia motor can be used for most cutting, but, when difficult to cut material is to be cut, the inertia can be selectively increased. When running at low inertia, less battery power will be consumed as compared to when running at high inertia, i.e. if the motor were larger, as in a conventional hedge trimmer. In a battery-powered device, this is particularly advantageous since it means that a relatively small motor can be used, requiring a correspondingly smaller and therefore lighter battery, which is nevertheless able to deliver high inertia when required. The system will thus benefit from the low power consumption that goes with a small motor most of the time and need only increase power consumption when required to cut harder-to-cut material. 
         [0049]    The control arm  30  will typically be connected to a control device (not shown) on the external housing of the tool. Thus, a user can easily and conveniently switch to the required inertia level. Furthermore, by providing the flywheel within a housing, windage is reduced, thereby increasing efficiency. 
         [0050]    It will be appreciated that in the example described above, the flywheel is selectively engageable or disengageable from the output shaft of the motor. However, in some examples, the flywheel is fixedly engaged with the output shaft. The advantage of providing a significantly increased inertia from a smaller motor is still obtained and the benefits of arranging the flywheel within the gear housing or its own dedicated housing ensures that the problem of windage is addressed, as explained above. 
         [0051]    The materials for forming the engagement wheel  8  and the flywheel  14  can be selected from any appropriate materials which will be well known to a skilled man. In one example, the engagement wheel is a sintered component formed on the output shaft  4  of the motor  2 . 
         [0052]    Referring now to  FIG. 6 , the internal components of a hedge trimmer are shown. The hedge trimmer comprises an external or outer tool housing  32  within which are arranged a motor  34  having an output shaft with a pinion  36  arranged thereon. A reduction gear  38  is provided, which is arranged to drive, preferably via a clutch mechanism (not shown), the relative movement of the blades (also not shown). The pinion gear has a length that enables a significant area of overlap with the reduction gear  38  so as to enable a press fit between the two. The inertia of the reduction gear is relatively small due to its relatively slow angular velocity as compared to that of the driving pinion directly coupled to the motor output shaft. 
         [0053]    As explained above, in the case of a cordless motor such as a Li ion battery-powered motor, if sufficient inertia is to be provided so that energy can be transferred to the cutting blades to enable hard-to-cut vegetation to be cut, then a powerful and correspondingly large motor (and appropriate battery) would be required. Given the effect that the use of such a motor would have on battery life and weight of the tool overall, this is not desirable. 
         [0054]      FIG. 7A  shows a schematic representation of the internal components of a hedge trimmer similar to that of  FIG. 6 . However, in this case, a flywheel  40  is provided coupled to the output shaft  42  of the motor  34 . In the example shown in  FIG. 7A , the flywheel is fixedly mounted to the output shaft. However, as explained above with reference to  FIGS. 4 and 5 , in one example, the flywheel is selectively engagable or disengagable from the output shaft. In the case that the flywheel is selectively engagable or disengagable from the output shaft, preferably an actuator is provided on the outer tool housing, which enables a user to manually control the position and state of engagement of the flywheel. The actuator may be a switch or lever configured to control the position of the arm  30 , as shown in  FIGS. 4 and 5 . 
         [0055]    In one embodiment, a sensor is provided which detects the resistance to cutting and when some threshold level of resistance is detected, the flywheel is automatically engaged. Conversely, when the resistance drops below the threshold, the flywheel is automatically disengaged. An appropriate control circuit may be provided that will be within the knowledge of the skilled man. 
         [0056]    A housing (not shown) is provided to encase the gears or, at the very least, the flywheel  40 , so as to minimise windage that will occur due to the fast rotation of the flywheel. It will be appreciated that since the flywheel, when engaged, is rotating at the same rate as the output shaft of the motor, significant windage could be experienced or generated. This is avoided by providing the flywheel within the gear housing or within its own dedicated housing. The end  45  of the output shaft  42  is preferably supported within a boss (not shown) or some other such fitting provided on an inner surface of the housing of the tool. This provides additional stability to the output shaft and the flywheel in use. 
         [0057]      FIG. 7B  shows schematically a longitudinal section through a part of the power train shown in  FIG. 7A . The motor  34  has output shaft  42  having arranged thereon a flywheel  40 . A housing  43  is arranged to surround the flywheel  40 , pinion  36  and reduction gear  38 , and thereby reduce windage generated thereby. In one example, a dedicated housing to surround the flywheel may be provided. The housing is formed to conform closely to the dimensions of the flywheel (in both positions if it is disengageable as described above) whilst ensuring sufficient clearance so as not to hinder movement of the flywheel or risk collision therewith. 
         [0058]    Referring to  FIG. 8 , the blades  44  of the hedge trimmer can be seen. A transmission  46  is provided between the gear  48  and the pinion  50 . A housing  52  is provided around the flywheel such that the windage generated or experienced is minimised or entirely avoided. The housing conforms closely to the dimensions of the flywheel as explained above. Typically, the flywheel may be substantially in the form of a disc having opposed major surfaces, wherein the clearance between each of the major surfaces of the flywheel and the housing is less than 5 mm. Preferably, the clearance would be between 1 and 5 mm. This also enables a relatively small flywheel housing to be provided. 
         [0059]    Thus, a drive train for a hedge trimmer is provided in which an output or reduction gear  38  and a pinion are provided within a gear housing, together with a flywheel coupled or couplable to the output shaft of a motor. The flywheel is arranged within a housing which may be the same as the gear housing or a separate, dedicated flywheel housing. In both cases, windage is reduced, thereby increasing efficiency of the device whilst providing the benefits of the flywheel described above. This is particularly advantageous in the case of a battery-powered motor since a smaller motor can be powered whilst not sacrificing power of the device overall. 
         [0060]    Embodiments of the present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention as defined by the appended claims.