WO1988001953A1

WO1988001953A1 - Vehicle retarder

Info

Publication number: WO1988001953A1
Application number: PCT/GB1987/000651
Authority: WO
Inventors: Keith Foulds; Vincent D'olier
Original assignee: Avon Transmission Services Limited
Priority date: 1986-09-19
Filing date: 1987-09-18
Publication date: 1988-03-24
Also published as: AU7914987A

Abstract

Two embodiments of vehicle retarder (10, 30). The second embodiment, disclosed in Figures 4 to 6, comprises a housing (33), a shaft (34) rotatably mounted in the housing, retarder plates (38) mounted on the shaft (34) to rotate therewith, stator retarder plate (39) mounted in the housing, a piston (41, 52) slidably mounted in the housing to bring the retarder plates into braking co-operation with each other, a pump (45) in fluid communication with the interior of the housing (33) to provide a supply of hydraulic fluid which cools, lubricates and transmits drive between the plates, and a hydraulic control circuit (Figure 6) which interconnects the pump (45), a dump (48), a piston (52) and the retarder plates (38, 39), the control circuit including a check valve (53) for maintaining a predetermined minimum pressure supply to the piston when the retarder (30) is in its operative mode.

Description

VEHICLE RETARDER

This invention relates to a vehicle retarder.

It is well known to use so-called retarders on heavy duty road vehicles, the purpose of which is to supplement the braking action provided by the vehicle brake system. The brake system of heavy duty vehicles, and especially vehicles which are reguired to stop and start at regular and short intervals, such as buses, are subject to repeated application which gives rise to a relatively short service life of brake linings, and substantial maintainance costs. Therefore, in order to lower maintainance costs, and also to reduce the out-of-service time of heavy duty vehicles, retarders are often installed as components which are separate from the vehicle braking system proper, and which usually exert a retarding action upon the vehicle drive train i.e. the transmission or propellor shaft, whereas the braking system proper usually acts directly to brake the vehicles wheels.

Retarders are usually employed to control speed^' whilst the vehicle is descending a hill, but they also give a useful additional means of bringing the vehicle to rest gradually without operating the foundation brakes. Thus, the initial application of the brake pedal normally brings the retarder into operation, but if further retardation is required, then further application of the brake pedal brings the vehicle braking system proper into operation. All retarders adopt various means e.g. hydraulic or electrohydraulic, to apply a torsional braking action on a rotary part of the vehicle drive train, but existing designs of retarder are complicated and also expensive, a typical cost being of the order of £3,000 or more. Further, at least some of the existing designs have the disadvantage of applying substantial drag forces even when they are in an inoperative i.e. non-retarding mode. There exists, therefore, a need to provide a design of retarder which is less complicated and of simpler design, and which can be manufactured more cheaply.

The concept of so-called "wet clutches" is known, in which packs or stacks of clutch plates are brought into positions of cooperation with each other,- upon application of a clutch pedal, and which transmit torque therebetween via a special type of hydraulic fluid which actually transmits the torque. This fluid performs a dual function, in that It transmits torque between respective pairs of clutch plates, and also functions as a cooling agent in order to prevent over-heating of the clutch plate surfaces. It is, perhaps, a surprising fact that torque can be transmitted between clutch plates which, in the clutch- operation mode, do not actually make contact with each other, in that the torque is transmitted via the hydraulic fluid in the interspace between the facing surfaces of the respective sets of clutch plates. The hydraulic fluid is subjected to shear forces, but is of such a composition that it can readily withstand these f.orces without deterioration and over a substantial service period. One particularly advantageous application of wet clutch is via its incorporation in the so-called Maxwell gearbox.

The present invention has been derived from the technology involved in wet clutches in order to provide a novel, technically advantageous and relatively simple construction of vehicle retarder.

According to one aspect of the invention there is provided a vehicle retarder which is adapted to be mounted in a drive train of a vehicle and which comprises: a housing; a shaft rotatably mounted in the housing and having an input for coupling with an upstream portion of the vehicle drive train and an output for coupling with a downstream portion of the vehicle drive train; a first stack of axially spaced retarder plates coupled with the shaft for rotation therewith; a complementary second stack of axially spaced retarder plates non-rotatably mounted in the housing and co-operable with the first stack of plates, in which the plates of the first stack are maintained spaced apart from corresponding plates of the second stack when the retarder is inoperative and are movable relatively towards each other for braking cooperation in the retarding mode of the retarder; a piston slidably mounted in the housing for movement, under the action of a fluid pressure medium applied thereto, in order to bring the plates of the first and second stacks into braking-cooperation with each other; a pump in fluid communication with the interior of the housing and operable to supply a pressurized supply of hydraulic fluid to the housing in order to flow into the interspaces defined between the plates of the first and second stacks for cooling, lubricating and transmitting drive therebetween when the plates are moved to the positions of braking cooperation in the retarder mode; an idler or free wheeling circuit coupled with the interior of the housing and with the pump for circulating the supply of hydra lic fluid when the retarder is in the inoperative mode without creating undue pressure; and a cooler unit rigidly attached to the housing and having fluid communication therewith for cooling the hydraulic fluid via hydraulic lines extending internally of the housing and the cooler units.

The piston may be slidably moved within the housing under the action of any suitable fluid pressure medium applied thereto, such as pneumatic pressure, in which case a suitable pneumatic supply line will be coupled with the exterior of the housing in order to supply pneumatic pressure to one side of the piston. Alternatively, means may be provided for actuating the piston under hydraulic pressure action, in which case conveniently hydraulic pressure supply may be derived from the pumped supply of hydraulic fluid to the interior of the housing for engagement with the retarder plates.

According to a further aspect of the invention there is provided a vehicle retarder which is adapted to be mounted in a drive train of a vehicle and which comprises: a housing; a shaft rotatably mounted in the housing and having an input for coupling with an upstream portion of the vehicle drive train and an output for coupling with a downstream portion of the vehicle drive train; a first stack of axially spaced retarder plates coupled with the shaft for rotation therewith; a complementary second stack of axially spaced retarder plates non-rotatably mounted in the housing and co-operable with the first stack of plates, in which the plates of the first stack are maintained spaced apart from corresponding plates of the second stack when the retarder is inoperative and are movable relatively towards each other for braking cooperation in the retarding mode- of the retarder; a piston slidably mounted in the housing for movement, under the action of a fluid pressure medium applied thereto, in order to bring the plates of the first and second stacks into braking-cooperation with each other; a pump in fluid communication with the interior of the housing and operable to supply a pressurized supply of hydraulic fluid to the housing in order to flow into the interspaces defined between the plates of the first and second stacks for cooling,- lubricating and transmitting drive therebetween when the plates are moved to the positions of braking cooperation in the retarder mode; an idler or free wheeling circuit coupled with the interior of the housing and with the pump for circulating the pressurized supply of hydraulic fluid when the retarder is in the inoperative mode; and a hydraulic control device interconnecting the pump. the piston and a pressure supply to the retarder plates, said control device including a pressure line extending from the output of the pump to the pressure supply to the retarder plates, a restrictor arranged in said pressure line, a branch line from said pressure line upstream of the restrictor and leading to the piston to actuate the latter hydraulically, and a by-pass line across the restrictor and communicating with said branch line, in which the by-pass line includes a controllable flow controller which determines pressure feed-back to the piston in the retarder mode of the retarder.

A vehicle retarder according to the invention may be supplied as original equipment on newly manufactured vehicles, especially heavy duty vehicles, and particularly vehicles up to 12 ton weight. Alternatively, the vehicle retarders may be applied to already existing vehicles. In either event, conveniently the vehicle retarder will be mounted at a suitable location in the vehicle drive train, and particularly advantageously by replacing the normal "centre bearing" provided in a prop shaft, whereby the vehicle retarder will be located, in some instances, approximately midway along the length of the vehicle.

Conveniently, the pump employed in the vehicle retarder is a constant displacement pump, and one type of hydraulic fluid which is suitable, though to which the invention is not exclusively restricted, is so-called "tractor" oil of the type used in "wet brakes" of agricultural tractors, such oil having necessary stick/slip properties.

In a practical embodiment of the invention, power dissipation up to 50kw can be achieved, and prolonged operation of the retarder can be achieved without overheating or other impairment of the retarder for retardation forces of up to 7% of vehicle weight i.e. the retardation force required to maintain substantially constant speed of a vehicle descending a 7% gradient. For intermittent operation of the retarder, which will be the more general use of the retarder, higher retardation forces will be possible without any appreciable risk of damage.

According to a further aspect of the invention there is provided a vehicle retarder which is adapted to be mounted in a drive train of a vehicle and which comprises: a housing; a shaft rotatably mounted in the housing and having an input for coupling with an upstream portion of the vehicle drive train and an output for coupling with a downstream portion of the vehicle drive train; a first stack of axially spaced retarder plates coupled with the shaft for rotation therewith; a complementary second stack of axially spaced retarder plates non-rotatably mounted in the housing and co-operable with the first stack of plates, in which the plates of the first stack are maintained spaced apart from corresponding plates of the second stack when the retarder is inoperative and are movable relatively towards each other for braking co-operation in the retarding mode of the retarder; a piston slidably mounted in the housing for movement, under the action of a fluid pressure medium applied thereto, in order to bring the plates of the first and second stacks into braking co-operation with each other; a pump in fluid communication with the interior of the housing and operable to supply a pressurised supply of hydraulic fluid to the housing in order to flow into the interspaces defined between the plates of the first and second stacks for cooling, lubricating and transmitting drive therebetween when the plates are moved to the positions of braking co-operation in the retarder mode; and a hydraulic control circuit interconnecting the pump, a dump, the piston and the retarder plates, the control circuit including a check valve for maintaining a predetermined minimum pressure supply to the piston when the retarder is in its operative mode.

Conveniently, the hydraulic circuit includes a shuttle valve arranged to receive the output from a pump driven from the vehicle drive train, and operable to route the pressure fluid to the dump, or to a pressure line leading to the check valve and the piston. The check valve is operable, when pressure fluid is supplied thereto, to allow pressure fluid to pass to the retarder plates, but also maintains the pressure supply to the piston at or above a predetermined minimum value, whereby the piston can maintain a braking action on the retarder plates even when the vehicle speed falls below a level sufficient to generate an adequate pumping pressure from the pump.

Embodiments of vehicle retarder according to the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a sectional view of a vehicle retarder - according to the invention;

Figure 2 is an end view of the vehicle retarder, illustrating a pump thereof;

Figure 3 is a schematic hydraulic diagram of a control device for use with the vehicle retarder;

Figure 4 is a longitudinal sectional view of a further embodiment of vehicle retarder according to the invention, this view showing the upper half of a housing of the retarder and the components mounted therein;

Figure 5 is a view of the lower half of the housing of the vehicle retarder shown in Figure 4; and

Figure 6 is a schematic hydraulic circuit diagram of a modified arrangement for controlling the routing of hydraulic fluid to the operating components of the vehicle retarder shown in Figures 1 and 2, or Figures 4 and 5.

Referring now to Figures 1 and 2 of the drawings, a vehicle retarder according to the invention is designated generally by reference 10 and is adapted to be mounted in a drive train of a vehicle at any convenient location, and particularly conveniently by replacement of the centre bearing of a propeller shaft. The retarder 10 comprises a housing 11 in which a shaft 12 is rotatably mounted and has an input 13 for coupling with an upstream portion of the vehicle drive train and an output 14 for coupling with a downstream portion of the vehicle drive train.

A first stack of axially spaced retarder plates 15 are coupled with the shaft 12 for rotation therewith, so as to form "rotor" plates. A complementary second stack of axially spaced retarder plates 16 is non-rotatably mounted in the housing 11 for co-operation with the plates of the first stack 15. In the illustration of Figure 1, the respective pairs of plates of the first and second stacks are axially spaced apart from each other, and this is the inoperative mode of the retarder. However, upon actuation of the retarder, in a manner to be described in more detail below, the plates of each co-operating pair of plates are relatively moved towards each other (though without-coming Into contact with each other) to a position of braking- cooperation, whereby retardation forces are applied from the plates of the second stack, which act as "stator" plates, to the rotating plates of the first stack 15. These retardation forces are applied solely through the intermediary of the hydraulic fluid which is pumped into the interspaces defined between the pairs of plates, as also will be described below.

A piston 17 is slidably mounted in the housing 10 for movement, under the action of a fluid pressure medium applied thereto, in order to displace the cooperating pairs of plates of the first and second stacks 15 and 16 towards the position of braking-cooperation. Although not shown, a fluid pressure medium supply line will be coupled with an end plate 18 of the housing 11 and communicates with the interior of the housing via a suitable port in order to apply fluid pressure medium action on the adjacent face of the piston 17 in order to drive the latter to the left, as seen in Figure 1. The fluid pressure supply line may comprise a hydraulic pressure line, or a pneumatic pressure line, depending upon the required fluid pressure medium for actuation of the retarder.

Referring to Figure 2, a pump for supplying hydraulic fluid under pressure to the interior of the housing 11, when the retarder is in the retarder mode, comprises a constant displacement pump in the form of a gear pump 19. This delivers a constant supply of hydraulic fluid under pressure to the interior of the housing 11, and this issues via a passage 20, as shown in Figure 1. If the retarder is operating in its inoperative mode, the hydraulic fluid returns to a sump and then to the pump via an idler or free wheel circuit (not shown) and without coming into contact with the retarder plates. However, when the retarder is actuated, this hydraulic fluid is delivered under pressure along passage 21 and to the right, as seen in Figure 1, so as to enter the spaces defined between the retarder plates. The hydraulic fluid is of the type used in "wet clutches", and is capable of cooling and lubricating the facing, though not contacting, surfaces of the retarder plates, and also can transmit retardation torque therebetween solely through the fluid.

Prior to passage through the spaces between the retarder plates, the fluid is pumped through a regulating valve and then through a cooler unit 22 which is rigidly attached to one side of the housing 11, so as to form a composite assembly. By this means, any necessary supply lines, pipes, conduits, ducts or the like can be located solely within the confines of the retarder housing 11 and the cooler unit 22, which is a safer arrangement than existing arrangements in which there are external pipes carrying hydraulic fluid under pressure and which are vulnerable to damage by stones or other road debris, which could result in loss of pressure fluid. The hydraulic circuit also includes a filter 23.

As indicated above, the piston 17 can be actuated by hydraulic or pneumatic pressure media. In the case of a requirement for hydraulic pressure actuation, conveniently the same hydraulic fluid is employed as within the interior of the housing 11, in which case a branch line feeds hydraulic pressure from a main line interconnecting the pump and the interior of the housing, in order to deliver hydraulic pressure to actuate the piston. A hydraulic control device for controlling the operation of the retarder will now be described with reference to Figure 3.

In Figure 3, a hydraulic control device is shown, for controlling the manner by which hydraulic fluid is supplied under pressure, in the retarder mode, to the retarder plates, and also how the pressure supplied to the piston 17 is controlled. A main pressure supply line is shown diagramatically by reference 24 which extends from pump P (19) to the retarder plates 15, 16-, and this pressure line incorporates a restrictor 25. The purpose of the restrictor is to allow fluid pressure supply therethrough, but to exert a pressure feed-back upstream thereof. A branch line 26 extends from the upstream portion of the supply line 24 and to the piston 17. Further, a by-pass line 27 extends across the restrictor 25 and includes an adjustable flow controller 28. Depending upon the setting of the flow controller 28, this controls the feed-back pressure generated in the upstream portion of the supply line 24, and also in the branch line 26, whereby the pressure action on, and extent of movement of the piston 17 is controlled. Therefore, this provides a simple means whereby the pressure action exerted on the piston 17 can be adjusted, and without affecting fluid (oil) flow through the retarder plates, given that the pump P is a constant displacement pump.

The illustrated embodiment of vehicle retarder is particularly suitable for use with vehicles up to 12 tons in weight, and can generate up to 50kw of retardation power. Also, the retarder can operated on a continuous basis with a retardation force sufficient to retard a vehicle descending a 7% gradient. However, for interraitent operation of the retarder, it will be possible to generate higher retardation efforts without damage.

As an alternative to the restritor arrangement of Figure 3, it may be possible to use a variable orifice in place of the restrictor. The variable orifice will be designed to maintain a substantial constant pressure on the operating piston with an increase in oil flow from the pump (and therefore to the retarder plates) .

Referring now to Figures 4 and 5 of the drawings, there is shown a further embodiment of vehicle retarder according to the invention which is designated generally by reference 30. The retarder 30 has a piston and cover assembly 31 having cooling ribs 32 and bolted to a main housing 33 of the retarder. A shaft 34 is rotatably mounted in the housing and has an input coupling 35 at an upstream end and an output coupling 36 at a downstream end, the couplings 35 and 36 being adapted to be coupled at a suitable position along the drive train of a vehicle, conveniently substantially mid-way along the propeller shaft.

In use, the shaft 34 is driven by the propeller shaft, and a pump (not shown in detail) is also driven thereby, so as to provide a pressure supply of fluid to the operating components of the retarder. A rotor 37 is secured to the shaft 34 to rotate therewith, and carries a stack of rotor plates 38, between which are interleaved stator plates 39 carried by a fixed annulus 40. The plates 38 and 39 are normally maintained spaced-apart from each other, and are generally similar to the plates 15 and 16 described above with reference to Figure 1. A piston 41 can be operated by a supply of pressure fluid thereto in order to engage the plates 33 and 39 with each other via the special fluid, so as to impart a retardation action on the shaft 34.

Pressure supply to the piston 41, and for cooling, lubricating and transmitting torque between the plates 38 and 39 is derived from internal supply pipe 42, and routing of the fluid also takes place along supply pipe 43 to integrally attached cooler unit 44.

Referring now to Figure _r there is shown schematically a hydraulic control circuit for controlling the supply of pressure fluid to the various components of the retarder. The pump 45 is driven from the propeller shaft, or other component of the vehicle drive train, and its output passes to a shuttle valve 46 which is controlled so as to route the presure fluid along line 47 to dump 48 when the retarder is inoperative. A suitable retarder control will be provided (not shown) in the cab of the vehicle, and this may comprise an independently operated control, such as a hand control, or it may comprise a connection with the bcake pedal so as to respond to the initial path of travel of the brake pedal from its inoperative position. Thus, when a retardation action is required, the shuttle valve 46 is then operated to route the pressure fluid from the pump 45 along line 49. Line 49 has a branch line 50 which delivers pressure fluid via regulator 51 to piston 52, which may comprise the piston 41 of Figures 4 and 5, or the piston 17 of Figures 1 and 2. This then brings the retarder plates into braking co¬ operation with each other, so as to apply a retardation force to the shaft of the retarder. There is also provided a check valve 53 in a line 54 leading to cooler 55 and the retarder stack of plates 56. The purpose of the check valve 53 is to maintain a predetermined minimum pressure in the line 50 which leads to the piston 52, so that the retarder plates can be maintained in engagement with each other, even when the vehicle speed falls below a sufficient level for the pump 45 to deliver an adequate quantity of pressure fluid to operate the piston 52. Therefore, this feature of the check valve 53 enables the retarder plates to exert a retarding action down to much lower vehicle speeds than is possible with existing retarders, which tend to cut-out at higher speeds, thereby leaving the normal brake system to be operated in order to effect further braking of the vehicle.

The parameters of the components shown in Figure 6 are set up for a datum position of approximately twenty miles per hour, at which a substantial retarding action can be achieved, and there may be a marginal decrease in retarding action above this datum, and a marginal increase in retarding action below this datum.

The check valve 53 therefore enables satisfactory retarding action to be exerted by the retarder alone down to quite low speeds, and while it may temporarily starve the line 54 of pressure fluid, this will only be for a very short period during which the vehicle is being finally slowed-down.

In practice, the shuttle valve 46 may comprise a valve body having a restricted passage through it, the outlet of which can be controlled by a solenoid operated valve 57 controlled from the cab of the vehicle, in order to convert the shuttle valve from the dumping mode to the operating mode. However, the combined action of the shuttle valve 46 and check valve 53 is to prevent pressure fluid being supplied to the plate stack 56 when the retarder is in its inoperative mode, thereby avoiding drag being applied to the drive train of the vehicle when the retarder is inoperative.

In tests with a retarder as shown in Figures 4 and 5, and having a control circuit as shown in Figure 6, it has been found that a vehicle only occasionally requires actual operation of the brake system, and possibly up to 90% of the braking requirements of the vehicle can be derived from the retarder. This significantly enhances the operating life of the brake system, which is a major cause of "down- time" of lorry and bus operators, as well as substantial cost. By contrast, the retarder, provided that a careful check is kept on the provision of the special fluid to operate it, can operate satisfactorily over extended periods, with virtually no wear taking place on the retarder plates, it only being necessary to change the special fluid at regular intervals, which is only a small routine task.

In order to protect the retarder against overheating, a temperature switch (not shown) is provided in the sump, and which controls a further solenoid valve 58 between the regulator 51 and the piston 52. In the event of a renewed operation of the retarder, or continued operation, when the temperature of the operating fluid exceeds a predetermind level liable to cause damage to the plates, the valve 58 is operated to prevent operation of the piston 52, and the pumped supply along line 49 passes only through the check valve 53 and the cooler 55 (and to the plate stack 56 which thereby applies a certain level, of drag) , so as to cool the fluid. When the fluid has cooled sufficiently, the pressure supply to the piston^' commences, or resumes, to enable the retarder to apply retardation to the vehicle.

Claims

1. A vehicle retarder (10) which is adapted to be mounted in a drive train of a vehicle and which comprises: a housing (11) ; a shaft rotatably mounted in the housing and having an input (13) for coupling with an upstream portion of the vehicle drive train and an output (14) for coupling with a downstream portion of the vehicle drive train; a first stack (15) of axially spaced retarder plates coupled with the shaft for rotation therewith; a complementary second stack (16) of axially spaced retarder plates non-rotatably .mounted in the housing and co-operable with the first stack of plates, in which the plates of the first stack are maintained spaced apart from corresponding plates of the second stack when the retarder is inoperative and are movable relatively towards each other for braking cooperation in the retarding mode of the retarder; a piston (17) slidably mounted in the housing (11) for movement, under the action of a fluid pressure medium applied thereto, in order to bring the plates of the first and second stacks into braking-cooperation with each other; a pump (19) in fluid communication with the interior of the housing (11) and operable to supply a pressurized supply of hydraulic fluid to the housing in order to flow into the interspaces defined between the plates of the first and second stacks for cooling, lubricating and transmitting drive therebetween when the plates are moved to the positions of braking cooperation in the retarder mode; an idler or free wheeling circuit (not shown) coupled with the interior of the housing (11) and with the pump (19) for circulating the supply of hydraulic fluid when the retarder is in the inoperative mode without creating undue pressure; and a cooler unit (22) rigidly attached to the housing (11) and having fluid communication therewith for cooling the hydraulic fluid via hydraulic lines extending internally of the housing and the cooler units.

2. A retarder according to Claim 1, in which the piston (17) is arranged for actuation under hydraulic pressure action, and means is provided for supplying hydraulic pressure from the pump supply of hydraulic fluid- to the interior of the housing for engagement with the retarder plates (15, 16).

3. A vehicle retarder (10) which is adapted to be mounted in a drive train of a vehicle and which comprises: a housing (11) ; a shaft rotatably mounted in the housing (11) and having an input (13) for coupling with an upstream portion of the vehicle drive train and an output (14) for coupling with a downstream portion of the vehicle drive train; a first stack (15) of axially spaced retarder plates coupled with the shaft (12) for rotation therewith; a complementary second stack of (15) axially spaced retarder plates non-rotatably mounted in the housing (11) and co-operable with the first stack (15) of plates, in which the plates of the first stack are maintained spaced apart from corresponding plates of the second stack when the retarder is inoperative and are movable relatively towards each other for braking cooperation in the retarding mode of the retarder; a piston (17) slidably mounted in the housing (11) for movement, under the action of a fluid pressure medium applied thereto, in order to bring the plates of the first and second stacks into braking-cooperation with each other; a pump (19) in fluid communication with the interior of the housing (11) and operable to supply a pressurized supply of hydraulic fluid to the housing in order to flow into the interspaces defined between the plates of the first and second stacks for cooling, lubricating and transmitting drive therebetween when the plates are moved to the positions of braking cooperation in the retarder mode; an idler or free wheeling circuit (not shown) coupled with the interior of the housing (11) and with the pump (19) for circulating the pressurized supply of hydraulic fluid when the retarder is in the inoperative mode; and a hydraulic control device (Figure 3) interconnecting the pump (19), the piston (17) and a pressure supply (8) to the retarder plates, said control device including a pressure line (24) extending from the output of the pump to the pressure supply to the retarder plates, a restrictor

(25) arranged in said pressure line (24), a branch line

(26) from said pressure line upstream of the restrictor (25) and leading to the piston (17) to actuate the latter hydraulically, and a by-pass line (27) across the restrictor and communicating with said branch line, in which the by-pass line includes a controllable flow <• controller (28) which determines pressure feed-back to the piston in the retarder mode of the retarder.

4. A retarder according to Claim 1 or Claim 3, in which the pump (19) is a constant _Usp1.dC__.nent type pump.

5. A vehicle retarder (30) which is adapted to be mounted in a drive train of a vehicle and which comprises: a housing (33) ; a shaft (34) rotatably mounted in the housing and having an input (35) for coupling with an upstream portion of the vehicle drive train and an output (36) for coupling with a downstream portion of the vehicle drive train; a first stack (38) of axially spaced retarder plates coupled with the shaft (34) for rotation therewith; a complementary second stack (39) of axially spaced retarder plates non-rotatably mounted in the housing (33) and co-operable with the first stack of plates, in which the plates of the first stack are maintained spaced apart from corresponding plates of the second stack when the retarder is inoperative and are movable relatively towards each other for braking co-operation in the retarding mode of the retarder; a piston (41, 52) slidably mounted in the housing (33) for movement, under the action of a fluid pressure medium applied thereto, in order to bring the plates of the first and second stacks into braking co-operation with each other; a pump (45) in fluid communication with the interior of the housing (33) and operable to supply a pressurised supply of hydraulic fluid to the housing in order to flow into the interspaces defined between the plates of the first and second stacks for cooling, lubricating and transmitting drive therebetween when the plates are moved to the positions of braking co-operation in the retarder mode; and a hydraulic control circuit (Figure 6) interconnecting the pump (45) , a dump (48) , the piston (52) and the retarder plates (38, 39),.-the control circuit including a check valve (53) for maintaining a predetermined minimum pressure supply to the piston when the retarder is in its operative mode.

6. A retarder according to Claim 5, in which the hydraulic circuit includes a shuttle valve (46) arranged to receive the output from a pump (45) driven from the vehicle drive train, and operable to route the pressure fluid to the dump (48) , or to a pressure line (49) leading to the check valve (53) and the pistons (52) .

7. A retarder according to Claim 6_r in which the check valve (53) is operable, when pressure fluid is supplied thereto, to allow pressure fluid to pass to the retarder plates (38, 39), but also to maintain the pressure supplied to the piston (52) at or above a predetermined minimum value, whereby the piston can maintain a breaking action on the retarder plates even when the vehicle speed falls below a level sufficient to generate an adequate pumping pressure from the pump.