1246 Hydraulic type retarder Voith

The design of a hydraulic retarder is similar to that of a fluid coupling. Basically, the retarder consists of two saucer-shaped discs, a revolving rotor (or impel-lor) and a stationary stator (or reaction member) which are cast with a number of flat radial vanes or blades for directing the flowpath of the fluid. The rotor is bolted to the flange of the internally splined drive shaft hub, which is itself mounted over the external splines formed on both the gearbox mainshaft and the flanged output shaft, thereby coupling the two drive members together. Support to the drive shaft hub and rotor is given by a roller bearing recessed in the side of the stator, which is in turn housed firmly within the retarder casing.

Theory of operation (Fig. 12.37) The two half-saucer members are placed face to face so that fluid can rotate as a vortex within the cells created by the radial vanes (Fig. 12.37).

When the transmission drives the rotor on overrun and fluid (oil) is introduced into the spaces between the rotor and stator, the fluid is subject to centrifugal force causing it to be accelerated

Electric Retard
Fig. 12.35 (a-c) Electric eddy current type retarder
Fig. 12.36 Principle of electric eddy current retarder

radially outwards. As the fluid reaches the outmost periphery of the rotor cells, it is flung across the junction made between the rotor and stator faces. It then decelerates as it is guided towards the inner periphery of the rotor cells to where the cycle of events once again commences. The kinetic energy imparted to the fluid passing from the revolving rotor to the fixed stator produces a counter reaction against the driven rotor. This counter reaction therefore opposes the propelling energy at the road wheels developed by the momentum of the moving vehicle, causing the vehicle to reduce speed.

The kinetic energy produced by the rapidly moving fluid as it impinges onto the stator cells, and the turbulance created by the movement of the fluid between the cells is all converted into heat energy. Hence the kinetic energy of the vehicle is converted into heat which is absorbed by the fluid and then dissipated via a heat exchanger to the cooling system of the engine.

The poor absorption capacity of the hydraulic retarder increases almost with the cube of the propeller shaft speed for a given rotor diameter.

When the retarder is not in use the rotor rotates in air, generating a drag. In order to keep this drag as low as possible, a number of stator pins are mounted inside and around the stator cells. These disc-headed pins tend to interfere with the air circulating between the moving and stationary half-cells when they have been emptied of fluid, thereby considerably reducing the relatively large windage losses which normally exist.

Output torque control (Fig. 12.37) In order to provide good retardation at low speeds, the retarder is designed so that maximum braking torque is reached at approximately a quarter of the maximum rotor speed/propeller speed. However, the torque developed is proportional to the square of the speed, and when the vehicle speed increases, the braking torque becomes too great and must, therefore, be limited. This is simply achieved by means of a relief valve, controlling the fluid pressure which then limits the maximum torque.

The preloading of the relief valve spring is increased or reduced by means of an air pressure-regulated servo assisted piston (Fig. 12.37). The control valve can be operated either by a hand lever when the unit is used as a continuous retarder, or by the foot control valve when it is used for making frequent stops.

When the retarder foot control valve is depressed, air from the auxiliary air brake reservoir is permitted to flow to the servo cylinder and piston. The servo piston is pushed downwards relaying this movement to the relief spill valve via the inner spring. This causes the relief valve spool to partially close the return flow passage to the sump and to open the passage leading to the inner

Fig. 12.37 Hydraulic type retarder

periphery of the stator. Fluid (oil) from the hydraulic pump now fills the rotor and stator cells according to the degree of retardation required, this being controlled by the foot valve movement. At any foot valve setting equilibrium is achieved between the air pressure acting on top of the servo piston and the opposing hydraulic pressure below the spool relief valve, which is itself controlled by the hydraulic pump speed and the amount of fluid escaping back to the engine's sump. The air feed pressure to the servo piston therefore permits the stepless and sensitive selection of any required retarding torque within the retarder's speed/torque characteristics.

Should the oil supply pressure become excessively high, the spool valve will lift against the control air pressure, causing the stator oil supply passage to partially close while opening the return flow passage so that fluid pressure inside the retarder casing is reduced.

12.4.7 A comparison of retarder power and torque absorbing characteristics

Retarders may be divided into those which utilize the engine in some way to produce a retarding effort and those which are mounted behind the

Propeller shaft speed [%)

Fig. 12.38 Typical comparison of power absorption of various retarders relative to propellor shaft speed

Propeller shaft speed [%)

Fig. 12.38 Typical comparison of power absorption of various retarders relative to propellor shaft speed gearbox, between the propeller shafts or in front of the final drive.

Retarders which convert the engine into a pump, such as the exhaust compression type or engine compressed air Jacobs type retarders, improve their performance in terms of power and torque absorption by using the gear ratios on overrun similarly to when the engine is used to propel the vehicle forwards. This is shown by the sawtooth power curve (Fig. 12.38) and the family of torque curves (Fig. 12.39) for the engine pump Jacobs type retarder. In the cases of the exhaust compression type retarder and engine overrun loss torque curves, the individual gear ratio torque curves are all shown merged into one for simplicity. Thus it can be seen that three methods, engine compressed air, exhaust compression and engine overrun losses, which use the engine to retard the vehicle, all depend for their effectiveness on the selection of the lowest possible gear ratio without over speeding the engine. As the gear ratio becomes more direct, the torque multiplication is reduced so that there is less turning resistance provided at the propeller shaft.

For retarders installed in the transmission after the gearbox there is only one speed range. It can be seen that retarders within this classification, such as the multi-friction plate, hydrokinetic and electrical eddy current type retarders all show an increase in power absorption in proportion to propeller shaft speed (Fig. 12.38). The slight deviation from a complete linear power rise for both hydraulic and electrical retarders is due to hydrodynamic and eddy current stabilizing conditions. It can be seen that in the lower speed range the hydraulic retarder absorbs slightly less power than the electrical retarder, but as the propeller shaft rises this is reversed and the hydraulic retarder absorbs proportionally more power, whereas the multiplate friction retarder produces a direct increase in power absorption throughout its speed range, but at a much lower rate compared to hydraulic and electrical retarders because of the difficulties in dissipating the generated heat.

When considering the torque absorption characteristics of these retarders (Fig. 12.39), the electrical retarder is capable of producing a high retarding torque when engaged almost immediately as the propeller shaft commences to rotate, reaching a peak at roughly 10% of its maximum speed range. It then declines somewhat, followed by a relatively constant output over the remainder of its speed range. However, the hydraulic retarder shows a slower resisting torque build-up which then gently exceeds that of the eddy current resisting torque curve, gradually reaching a peak followed by a very small decline as the propeller shaft speed approaches a maximum. In comparison to the

Fig. 12.39 Typical comparison of torque produced by various retarders relative to propeller shaft

other retarders, the multiplate friction retarder provides a resisting torque the instant the two sets of friction plates are pressed together. The relative slippage between plates provides the classical static high friction peak followed immediately by a much lower steady dynamic frictional torque which tends to be consistant throughout the retarder's operating speed range. What is not shown in Figs 12.38 and 12.39 is that the electrical, hydraulic and friction retarder outputs are controlled by the driver and are generally much reduced to suit the driving terrain of the vehicle.

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