This invention relates to rotary drive transmissions incorporating a hydrostatic pump and motor. Hydromechanical drive transmissions employing hydrostatic pumps and motors are known in a variety of forms. Reference can be made to FR 1231667, FR 2315043, O ^' 89/11129, DE 2144063 and DE 2910184. These drive transmissions allow the drive speed to be varied but they have limited application because of functional limitations and/or because of their complexity and therefore high cost.

According to one aspect of the present invention, there is provided a drive transmission means comprising rotary input and output drive members, one of said members being coupled to a rotary housing which is coupled to a pair of hydrostatic units rotated with the housing, said units having fluid connections between them to operate as pump and motor, means being provided that are operable to prevent or restrict relative rotation between said units, and a differential drive mechanism comprising two rotary input elements and an output element driven by the rotary input elements to rotate at a speed dependent on the speeds of said input elements, said three elements being coupled respectively to the pump, the motor and the other of said drive members.

In a preferred arrangement at least one of said hydrostatic units comprises a rotor that is located

within a hollow to define therewith an annular space surrounding a rotary axis of said at least one unit, pivotable vane elements at angular intervals around said space to divide it into a series of chambers, the volumes of said chambers varying circumferentially about said rotary axis, whereby said unit operates as a pump or a rotor. It is possible to achieve a compact arrangement in which the housing or a carrier body secured to the housing provides respective hollows for the two hydrostatic units, each of which comprises a series of pivotable vane elements to operate as multiple-chamber pump and motor mechanisms.

According to another aspect of the invention, there is provided a drive transmission means comprising rotary input and output drive members, a housing containing a hydrostatic pump, a hydrostatic motor coupled to said pump and a hydraulic fluid reservoir in circuit with said pump and motor, said housing being mechanically connected to one of said drive members, the pump and motor being mechanically connected to respective rotary transmission elements and the other said drive member being mechanically connected to at least one further rotary transmission element so coupled to said transmission elements of the pump and motor as to cause said further element to rotate at a speed to be determined by the relative rotary speeds of said pump and motor transmission elements, control means being provided that are operable to prevent or restrict relative

rotation between the pump and the motor from standstill . In a further aspect of the invention a compact form of rotary drive transmission is provided in which a hydrostatic pump and motor are arranged coaxially within a rotatable housing mechanically coupled to an input or an output shaft of the transmission, the pump and motor drives being mechanically connected to respective elements of a differential or epicyclic gear mechanism, the other element of said gear mechanism being mechanically connected to the other of said input and output shafts, and means being provided that are operable to prevent or restrict relative rotation between the pump and the motor from standstill. Conveniently, the housing is disposed coaxially within an outer casing coupled .to the other of said shafts.

The means preventing or restricting relative rotation between the pump and motor are arranged to operate at start-up to establish a drive through the transmission means and the freedom for relative rotation can then be re-established, preferably in a progressive manner. In one form of the invention valve means are provided in the path of the fluid flow circulating through the pump and motor to obtain the required blocking or restriction of that flow. Thus, a valve in the fluid flow path and operated by pressure or flow rate can be held closed when the transmission means is stationary and be opened by an increasing output from the pump or motor.

The invention will be further described by way of example with reference to the accompanying schematic drawings, in which:

Fig. 1 is an axial sectional view of a first embodiment of the invention,

Fig. 2 is an end view of a hydrostatic unit of the first embodiment with its cover plate removed,

Fig. 3 is a detail illustration of the control valve of the first embodiment, Figs. 4 to 6 are axial sectional views of further embodiments of the invention, and

Figs. 7 and 8 are detail views of alternative flow restriction valves that can be incorporated in the illustrated embodiments of the invention. Figs . 1 to 3 illustrate an embodiment of a rotary drive transmission according to the invention in a configuration intended to form the hub of a wheel, such as a pedal cycle wheel, to drive that wheel.

An outer casing 2 forms an outer rim of the hub and is secured to the spoked wheel structure (not shown) at its flanged ends 4,6. Supported coaxially within the outer casing by bearings 7 to be rotatable relative thereto is an inner casing or housing 8 in which respective hydrostatic units in the form of a pump 10 and a motor 12 are arranged in tandem on a common central rotary axis A. The inner casing 8 is driven from the cycle pedals through a conventional chain drive (not shown) .

Concentric inner and outer shafts 14,16 on the axis A project from the housing. The inner shaft 14 is secured to a rotor 18 of the pump 10 and the outer shaft 16 to a rotor 20 of the motor 12. Each shaft 14,16 carries at its opposite end a bevel crown gear 22,24 respectively. The crown gears face each other to form respective crown wheels of a differential gear assembly. For this purpose the crown wheels are coupled together by two or more bevel pinions 26 mounted to be freely rotatable on radial shafts 28 journalled in a cage 30 fixed to the outer casing 2.

The pump and motor rotors 18,20 are mounted in eccentric hollows 32,34 formed in opposite ends of a carrier 36 which, with an inner casing 38 to which it is secured, and an end cover 40 forms the housing 8. The carrier 36 is sandwiched between an end wall 42 of the inner casing and an end plate 44 which forms a fixed assembly with the carrier and shell . The open eccentric recesses in the carrier are separated by a partition wall 46 and provide respective chambers for the pump and motor.

The arrangement of the rotor is shown in Fig. 2 and is the same for both hydrostatic units 10,12. The rotor, as is further illustrated in Fig. 3, has a main body 52 concentrically mounted on the respective pump or motor shaft 14 or 16 and on which a number of vanes 54 are pivotally mounted. The body 52 is formed with a regularly spaced series of arcuate recesses 56, each

extending rather more than a semi-circle. The vanes are held by their circular bosses 54a in the recesses 56 so as to be pivotable on the main body. Springs 62 held in pockets 64 in the rotor body urge the vanes outwards into sliding contact with the peripheral wall 66 of the respective chamber 32,34 across an annular fluid-filled volume of the pump or motor defined between the rotor body 52 and the peripheral wall 66 contacted by the vanes. The fluid-filled volumes are sealed from the shafts 14,16 by seals 68 sealed in the rotor bodies.

The space within the inner casing 38 between end cover 40 and end plate 44 provides the main volume of a reservoir 72 for the fluid that fills the pump and motor chambers. Inlet porting from the reservoir into the pump is provided by apertures 74 (Fig. 2) in the end plate 44. Further apertures 76 (Fig. 2) in the partition wall 46 connect the pump to the motor. Outlet ports 78 are formed in the peripheral wall of the motor chamber and communicate through a channel 80 with an axial conduit 82 which leads via a control valve 84 in the carrier back to the reservoir 72.

The pump and motor chambers have an asymmetrical form relative to the central axis of rotation of the shafts 14,16, as Fig. 2 illustrates, and the asymmetry of the two chambers is angularly coincident. In the case of the pump the rotor 18 turns anti-clockwise relative to the carrier 36 so that the volumes of the spaces defined between successive vanes 54 are increasing as they pass the inlet ports 74. The fluid thus drawn into the pump

is expelled to the rotor through the transfer ports 76, where the spaces are contracting. In the motor the rotor 20 can thereby rotate relative to the carrier 36 in the opposite direction, ie. clockwise. The volumes of the spaces between the vanes increases as they pass the transfer ports 76 from the pump and contract as they pass the outlet ports 80.

The volume of the motor chambers is larger than the pump chambers, because of their different axial lengths, so that the speed of rotation of the motor is less than that of the pump. It is also possible to obtain an effective non-unity speed ratio in the transmission by giving the crown gears 22,24 unequal diameters and thereby different numbers of teeth. The control valve 84 is provided to check the fluid flow in the circuit through pump, motor and reservoir or start up until at least a minimum fluid pressure is generated. The control valve 84 is further illustrated in Fig. 3. It comprises a valve body 86 bound by a spring 88 to close conduit 80a forming a final section of the outlet channel 80 to the reservoir. A connection 92 out of the plan of Fig. 3 connects the conduit 80a ahead of the valve body to a space 94 behind head 86a of the valve body so that the motor exit pressure acts against the force of the spring 88. On start up the valve prevents circulation of the flow through the pump and motor to the reservoir space, but the fluid pressure increases at the motor outlet, to

displace the valve body against the spring force. Because of a connection 96 from the spring chamber to the reservoir space, the fluid pressure remains unchanged in the spring chamber. As necked portion 86b of the valve body reaches the conduit 80a the circulatory flow is established through the pump and motor and the transmission is fully operative. A bleed passage 98 through the valve body to the enclosed space that forms behind it as it moves against the spring provides damping for the valve movements.

In use, a drive is input through the pedals attached to the inner casing 8 carrying the pump and motor. The bevel gears 22,24,26 operate as a differential gear mechanism to drive the cycle wheel fixed to the outer casing 2 in which the bevel pinions 26 are mounted. The outer casing rotates at the mean of the speeds of the pump and motor rotors 18,20.

Initially, the fluid flow circuit is blocked by the spring-loaded valve 84, so that at start up the transmission operates as a mechanical transmission with the pump and motor locked together. Because of the resistance of the driven wheel, however, the pressure within the hydrostatic units immediately begin to increase, so progressively opening the valve and establishing circulation of the fluid through the pump and motor. The pump and motor are now able to rotate at different speeds so as to share the torque load between them, as dictated by the relative capacities of the pump

and motor and by the differential gear mechanism. For a constant torque input, as the torque load on the driven wheel varies, the input:output speed ratio will be varied inversely by the drive transmission apparatus. Particular features of the embodiment that has been described are its very light and compact nature, and the ease with which it can be manufactured. In its use as- a cycle drive, it can be arranged that when running at a constant speed on the level, there is little or no fluid flow, so that there can be correspondingly very low energy losses.

The sealing enclosure provided by the housing 8 also simplifies the sealing of the pump and motor. Small leakages within the housing, eg. between pump and motor, can be tolerated without any significant loss of efficiency. The device can therefore be produced at relatively low cost.

Although this first example shows a two-chamber vaned pump and motor it will be understood that other positive displacement devices can be used, with the number of chambers chosen to suit each particular application.

Fig. 4 illustrates an alternative embodiment of the invention. Input and output shafts 102,104 are journalled at opposite ends of a casing 106. A housing 108 containing a hydrostatic pump 110 and motor 112 is supported concentrically and rotatably within the casing by bearings 114. The input shaft 102 is secured to one

end of the housing 108 to rotate it and the pump and motor within it. The pump mechanism comprises meshing rotors 120,122, the first 120 freely rotatably supported in the casing and the rotor 122 connected by its shaft 122a to an external gear 124 engaging a ring gear 126 fixed to the housing casing.

The motor similarly has meshing rotors 128,130, the first 128 freely rotatable and the rotor 130 having fixed to its shaft 130a an external gear 132 engaging the sun wheel 134 of an epicyclic gear 134,136,138. The epicyclic gear planet wheels 136 are journalled on an end plate 140 fixed to the output shaft 104 and they mesh with the sun wheel 134 and an annulus 136 fixed to the casing 106. The sun wheel is freely rotatably supported in the housing 108 and the end plate 140.

Similarly to the first example, fluid circulates through the hydrostatic units 110,112 via a sealed reservoir 144 formed between the casing 106 and the housing 108. In a similar manner also, the flow through the fluid circuit can be blocked by pressure-sensitive means (not shown) .

In the further embodiment shown in Fig. 5, parts corresponding to those described with reference to Fig. 4 are indicated by the same reference numbers. Thus, pump 110 and motor 112 are contained in the rotary housing 108 journalled in the outer casing 106. A sealed reservoir chamber 144 is similarly formed between the housing and the casing and is in circuit with the pump and motor

through a pressure-responsive blocking valve.

The pump rotor 122 is connected to the casing through the gears 124,126 while the motor rotor 128 is connected to the sun wheel 134 by the gear 132. The sun wheel again forms part of an epicyclic gear comprising an annulus 138 fixed to the casing and planet wheels 136 meshing with the sun wheel annulus and journalled on an end plate 140. In this instance, the end plate 140 has an integral gear ring 146 for an external drive connection.

The housing 108 with its pump 110 and motor 112 are arranged in the casing 106 in Fig. 6 identically to Fig. 4. Rotary shafts 122a,130a similarly extend from the pump and motor respectively to mesh with the annular gear 126 on the casing and the sun wheel 134. The embodiment of Fig. 6 differs from that in Fig. 4 in the arrangement of the epicyclic gear mechanism 134,136,138 so that the planet wheels 136 are carried by the casing 106 and the annulus 138 is on the end plate 140. The manner of operation in each of these further examples is generally similar to that already described with reference to the first embodiment. The interconnection of the pump and motor through the housing and the epicyclic gear mechanism allows the torque ratios and speed ratios of the input and output shafts to be varied in a similar manner. The pump and motor again have different displacement volumes to give a non-unitary speed ratio between them, as required by the function of

the transmission means.

Fig. 7 shows a non-return valve which can be incorporated in any of the preceding examples as a means of blocking or restricting the flow through the hydraulic circulation path at start-up. The connection between conduits 152,154 is blocked by valve body 156 under the influence of spring 158 until pressure is applied through the conduit 152 by the operation of the pump. The valve body is then lifted by the pressure to complete the fluid circulation path through the pump and motor.

As an alternative to the valve of Fig. 7, Fig. 8 shows a valve body 162 is urged into a conduit 164 by spring 166 to restrict the flow through the conduit. Passages 168,170 are connected to a valve chamber 172 above and below a head 174 of the valve body so that the valve body experiences a force proportional to the pressure drop in the conduit 164 across the valve. As flow in the passage increases, so does the pressure differential to produce a force on the valve body which raises it to allow free circulation of the fluid through the pump and motor. The arrangement is analogous to that described with reference to Fig. 3.