Abstract:
A transmission is disclosed for a wheeled vehicle. The transmission has a rotary input ( 14 ) to be driven by a rotary power source, e.g. an internal combustion engine. It has a rotary output ( 16 ) for driving a vehicle wheel. A transmission unit is coupled between the rotary input and the rotary output to transfer drive between them at a speed ratio which is continuously variable through a range which includes both negative ratios (providing a reversal of rotational direction between the rotary input and the rotary output), positive ratios (in which the rotary input and the rotary output rotate in the same direction), and a geared neutral ratio, in which the rotary output is stationary. In accordance with the invention, the transmission further comprises a brake ( 20 ) for braking the rotary output. The brake is provided with a control arrangement adapted to apply it when the speed ratio is set to the geared neutral ratio and to release the brake as the speed ratio is adjusted away from the geared neutral ratio.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a national phase application under 35 U.S.C. §371 of International Application No. PCT/US2009/031275 filed Jan. 16, 2009, which claims priority to Great Britain Application No. 0800826.0 filed Jan. 17, 2008, both of which are incorporated by reference without disclaimer. 
     BACKGROUND 
     The present invention is concerned with improvements to continuously variable transmissions which are able to provide infinite speed reduction (“geared neutral”). 
       FIG. 1  represents, purely by way of example and in highly schematic form, one such transmission. Box  10  represents a variator—that is, a device having a rotary variator input  6 , a rotary variator output  8 , and a mechanism for transferring drive between the two at a speed ratio (the “variator ratio”) which can be continuously (steplessly) varied over a finite range. Variators of numerous different types are known in the art. The variator may for example be a toroidal race rolling traction device as described in Torotrak (Development) Ltd&#39;s published UK application GB2423122 and its PCT counterpart WO2006084905. Box  12  represents an epicyclic (planetary) gear. The general construction of such gearing is well known and will not be described herein beyond noting that it comprises, in conventional manner, a central sun gear engaging with planet gears which are mounted upon a carrier and engage in their turn with an outer annular gear. The carrier is coupled through gearing R 1  to a rotary input  14  of the transmission. The sun gear is coupled through the variator  10  and gearing R 2  to the same transmission input  14 . The annular gear is coupled via gearing R 3  to a rotary output  16  of the transmission. 
     The transmission input  14  would typically be coupled to some source of rotary drive, such as the internal combustion engine of a motor vehicle. The transmission output  16  would be connected to some point of power usage, such as the driven wheels of the vehicle. The transmission serves to transfer drive between the two. The ratio of transmission output speed to transmission input speed is a function of the variator ratio, and is thus continuously variable. 
     Gearing in such a transmission is typically designed such that there is a certain variator ratio (the “geared neutral variator ratio”) at which the speeds of the carrier and sun gears cancel each other out, leaving the annular gear, and the transmission output  16 , stationary, despite the fact that they remain mechanically coupled to the rotating transmission input  14 . This is the infinite speed reduction referred to above, and transmissions having this facility are sometimes referred to as “infinitely variable transmissions”. Variator ratios to one side of the geared neutral variator ratio provide rotation of the transmission output  16  in one direction (e.g. forward drive for a motor vehicle). Variator ratios to the other side of geared neutral provide transmission output rotation in the opposite direction (reverse drive). 
     If one defines the speed ratio of the transmission to be the rotational speed of its output divided by that of its input, and takes rotation in one direction to be positive and rotation in the other direction to be negative, then speed ratio is positive when the transmission input rotate in the same direction, and negative when they rotate in opposite directions. 
     Other vehicle transmissions typically require a clutch or other means to mechanically de-couple the engine from the wheels when the vehicle is brought to rest, and to accommodate an initial mismatch of speeds between engine and transmission during vehicle launch. An infinitely variable transmission, however, makes it possible to halt the vehicle, and to move off from rest, without need of any such launch device, merely by appropriate adjustment of variator ratio. 
     It should be noted that the particular layout represented in  FIG. 1  is presented merely in order to illustrate the general principles and is by no means the only arrangement suitable for implementing an infinitely variable transmission. 
     A problem can arise in relation to changes of engine speed. Suppose that the vehicle is stationary, the transmission is in geared neutral and the engine is idling. The driver then abruptly raises engine speed, in preparation for vehicle launch, but does not yet adjust the variator ratio. Since the ratio remains at its neutral setting, the driver&#39;s expectation is that no torque will yet be exerted on the vehicle wheels. However, inertias within the transmission must accelerate along with the engine. These inertias are mechanically coupled to the transmission output  16 . The torque required to accelerate the inertias is reacted partly to the transmission output, and consequently a torque is briefly experienced at the vehicle wheels, potentially causing the vehicle to jerk forwards or backwards. 
     Another problem can arise if the transmission is not set precisely to geared neutral when the driver requires it, in which case unwanted creep of the vehicle may result. 
     SUMMARY 
     In accordance with a first aspect of the present invention, there is a transmission for a wheeled vehicle, the transmission having a rotary input to be driven by a rotary power source, a rotary output for driving a vehicle wheel, and a transmission unit coupled between the rotary input and the rotary output to transfer drive between them at a speed ratio which is continuously variable through a range which includes negative ratios, providing a reversal of rotational direction between the rotary input and the rotary output, positive ratios, in which the rotary input and the rotary output rotate in the same direction, and a geared neutral ratio, in which the rotary output is stationary, the transmission being characterised in that it further comprises a brake for braking the rotary output and a control arrangement adapted to apply the brake when the speed ratio is set to the geared neutral ratio and to release the brake as the speed ratio is adjusted away from the geared neutral ratio. 
     Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of major components of an infinitely variable transmission; 
         FIG. 2  is a view of a brake mechanism embodying the present invention; 
         FIGS. 3 and 4  are both views of part of the same brake mechanism, showing alternative profiles of a guide slot; 
         FIG. 5  is a view of a development of the brake mechanism, incorporating a parking brake function, viewed from above; 
         FIG. 6  illustrates the mechanism of  FIG. 5 , viewed from one side; 
         FIG. 7  is a view from above and to one side of a vehicle transmission embodying the present invention and incorporating the brake mechanism of  FIG. 2 , and additionally includes a mechanical arrangement for controlling both transmission and brake; 
         FIG. 8  shows one side only of the  FIG. 7  arrangement, to a larger scale; and 
         FIG. 9  is a view from one side of the  FIG. 7  arrangement. 
     
    
    
     DETAILED DESCRIPTION 
     The mechanism  20  illustrated in  FIGS. 2-4  forms a disc brake which is automatically applied when the transmission is in geared neutral and released at other times. 
     The brake mechanism  20  has been developed for use in a constructionally simple agricultural vehicle, specifically a “ride-on” lawnmower. In this vehicle the left and right hand wheels are driven from a common engine through respective transmissions whose speed ratios (i.e. the ratio of engine speed to wheel speed) are independently and continuously variable. Such a vehicle may be steered merely by adjustment of the transmissions&#39; ratios, and may be highly manoeuvrable. For instance by setting the transmissions to cause the vehicle wheels to rotate in opposite directions at equal speeds the vehicle can be caused to spin on the spot, an operation referred to as a “zero turn”. 
     Both of the vehicle&#39;s transmissions incorporate, in accordance with the present invention, a brake mechanism  20 , which is to be mounted adjacent to a driven vehicle wheel (not shown) and incorporates a wheel shaft  22  which carries the vehicle wheel and is the final output of the transmission. The brake mechanism is mounted on a main housing  24 , seen in  FIG. 6  to have at its front a mounting surface  26  to which a continuously variable transmission unit is to be secured with its output shaft protruding through an opening  28  in the housing wall to engage with a first gear  30  rotatably mounted within the main housing  24 . The transmission unit is not shown in  FIGS. 2 to 6  but  FIGS. 7 and 8  show the main housing  24  mounted on the transmission unit  70  in this manner. 
     Concealed within main housing  24  are a second, idler, gear (not seen) which meshes with the first gear  30 , and a third, output, gear (not seen) which meshes with the idler gear and drives the wheel shaft  22 . The first to third gears are of ascending sizes and form a gear train which provides a speed reduction at the wheel shaft  22 . 
     A brake disc  32  is seen in  FIG. 2  to have a splined central bore through which it is mounted upon a shaft (omitted from the drawing for clarity) which extends into and rotates along with the idler gear. Braking the disc  32  thus brakes the aforementioned gear train, and hence the wheel shaft  22 . The brake could in principle act on a different transmission component, but in the particular example illustrated braking the idler gear provides the best compromise, neither rotational speed nor torque being excessive at this point. Brake pads are mounted on both sides of the brake disc  32 , although both are hidden in the drawings. One is carried on a plate  34  secured to the main housing  24 . The other is carried by an “L” shaped block  36  forming part of a brake lever  38 , moving which causes the brake to be applied and released. The brake lever  38  further comprises an arm  40  secured to the block  36  e.g. by welding. It has a fulcrum formed by a pin  42  received by the “L” shaped block  36 . 
     The arm  40  is elbowed at  44  to pass through a clearance opening in a fixed pivot plate  46  secured to the main housing  24 , and terminates in an upstanding end plate  48  which mounts a pivot pin  50 . The pivot pin  50  extends through a bore in the pivot plate  46  and is rendered captive by a nut  52  on the opposite side of the pivot plate  46  from the end plate  48 . Carried upon the pivot pin  50 , between the fixed pivot plate  46  and the end plate  48 , are (i) a sleeve part  53  of a control lever  54  and (ii) a helical spring  56 . The spring  56  is pre-stressed in compression between the pivot plate  46  and the control lever  54 , and its force can be transferred through the control lever to the end plate  48  to apply the brake. However the spring force can be selectively relieved, to release the brake, as will now be explained. 
     The control lever  54  cooperates with a cam pin  58  mounted in and radially protruding from the pivot pin  50  to form a cam mechanism controlling the brake lever  38 . Its sleeve part  53  has a cam surface  59  (see  FIGS. 3 and 4  in particular), formed in this embodiment within a slot  60 , which runs upon the cam pin  58  and is biased against it by the action of the spring  56 . The surface  59  may for example be formed with a détente  62 , as in  FIG. 3 , or with a “V” shape, as in  FIG. 4 . In either case it will be apparent that turning the control lever  54  causes the control lever  54  to move along the pivot pin  50 , releasing the spring biasing from the end plate  48  and permitting the brake lever  38  to turn slightly and so release the brake. The profile of the cam surface  59  is chosen to provide a desired brake characteristic. The profile of  FIG. 3 , for example, gives a relatively abrupt application of the brake at geared neutral. The “V” profile at  FIG. 4  gives more progressive brake application and release. 
     An adjuster  68  is threadedly received in the L shaped block  36  and acts on its adjacent brake pad, enabling the pad to be advanced e.g. to accommodate wear. 
     The brake mechanism  20  is controlled by a mechanical arrangement in coordination with the vehicle&#39;s transmission. The control arrangement will now be described with reference to  FIGS. 7 to 9 . This arrangement has been developed for use in the agricultural vehicle mentioned above, although it must be understood that the invention could be applied in any of a wide range of vehicles. This particular vehicle is controlled through right and left driver operable hand levers  76 ,  76 ′ each rotatable through a limited range about respective pivots  78 ,  78 ′. Through the hand levers the driver controls the drive ratio provided by right and left continuously variable transmission units  70 ,  70 ′. Movement of either hand lever  76 ,  76 ′ is transmitted through a respective first push rod  80 ,  80 ′ to a respective crank  82 ,  82 ′ rotatably mounted in a respective bearing block  84 ,  84 ′. Cranks  82 ,  82 ′ are each coupled to a respective transmission push rod  86 ,  86 ′ which in its turn is coupled to the corresponding transmission unit  70 ,  70 ′ to control its ratio. 
     Placing either hand lever  76 ,  76 ′ at the end of its travel in the forward direction (arrow  86  in  FIG. 7 ) provides maximum forward drive ratio and hence maximum forward rotation speed (for a given engine speed) at the corresponding wheel shaft  20 ,  22 ′. Moving either hand lever to the end of its travel in the opposite direction causes a progressive and continuous adjustment of the transmission ratio until it achieves maximum reverse rotation speed. In between these extremes is a hand lever position which corresponds to geared neutral—i.e. to zero rotation speed of the wheel shaft  22  or  22 ′. When the hand lever is placed in this position, the brake  20  is automatically applied. 
     To this end, each of the aforementioned control levers  54 ,  54 ′ of the brake mechanisms  20 ,  20 ′ is coupled to a respective brake push rod  88 ,  88 ′, and each push rod is coupled to the corresponding crank  82 ,  82 ′ to be moved as the crank turns. Push rod lengths are chosen such that the brake is applied when the corresponding hand lever  76 ,  76 ′ is in the geared neutral position. 
     By automatic application of the brake when the transmission is at geared neutral, the problem explained above of unwanted movement of a hitherto stationary vehicle upon sudden engine acceleration is avoided. Any tendency for the vehicle to suffer “creep”—i.e. slow movement when in geared neutral, due to failure to select the appropriate variator ratio with sufficient precision, can likewise be resisted by the brake. 
     Note that release of the brake as the transmission is moved away from geared neutral may be gradual, so that the brake would be applied over a limited range of hand lever positions on either side of geared neutral. 
     The brake mechanism can be adapted to additionally provide a user operable “parking brake” function. That is, a separate user operable control can be provided to apply the brake regardless of transmission ratio. In the  FIG. 7  embodiment this control is formed as a brake lever  100  carried on a rotatably mounted brake bar  102  which further carries actuating levers  104 ,  104 ′ coupled to respective park brake push rods  106 ,  106 ′. In  FIGS. 5 and 6  park brake push rod  106  is seen to be coupled to one limb of an “T” planform lever  108  pivotally mounted on the brake&#39;s main housing  24 , its other limb being coupled to a pull rod  110  which passes through the brake lever  38  and acts upon it through a helical spring  112  retained on the pull rod by its enlarged head  114 . Note that the brake lever  38  differs from the one depicted in  FIG. 2  in that it has been extended upwardly to meet the pull rod. To apply the parking brake the driver raises brake lever  100 , causing pull rods  110  of both brake mechanisms to move to apply both brakes. 
     The transmission units  70 ,  70 ′ may take any of a number of different forms. They may for example be of “expanding pulley” type. However the preferred type of transmission unit uses a variator of toroidal race, rolling traction type, having a movable control member such as a lever whose position determines the variator&#39;s ratio. A suitable unit is described in GB 2423122 and in WO 2006084905, and the attention of the reader is directed thereto for information on the construction of such a variator. 
     The invention is however applicable particularly to transmissions whose variator is of “ratio control” type. That is to say that the ratio adopted by the variator is directly determined by its control system. Certain variators are instead “torque controlled”—they receive an input indicative of the torque to be reacted through the variator casing, and changes in ratio take place automatically as a result of the action of the resultant torque on system inertias. 
     The aforegoing embodiments are described by way of example only. Numerous other ways of implementing the invention are possible. For example the mechanism of  FIGS. 7 to 9  provides the user with hand levers  76 ,  76 ′ to control the transmissions. Other “zero turn” vehicles use a steering wheel and speed control pedal instead, but may equally well be modified to incorporate the brake of the present invention.