Rotary vane type vacuum pump rotary exhauster

(Fig. 11.47(c)) When the rotor revolves, the cell spaces formed between the drum blades on the inlet port side of the casing increase and the spaces between the blades on the discharge port side decrease, because of the eccentric mounting of the rotor drum in its casing.

As a result, a depression is created in the enlarging cell spaces on the inlet side, causing air to be exhausted (drawn out) directly from the brake vacuum servo chamber or from a separate vacuum reservoir. However on the discharge side the cells are reducing in volume so that a positive pressure is produced.

The drive shaft drum and vanes require lubricating at pressure or by gravity or suction from the

Vane Type Vacuum Pump

(cl Rotary vane type vacuum pump

Fig. 11.47 (a-c) Types of vacuum pumps

(cl Rotary vane type vacuum pump

Fig. 11.47 (a-c) Types of vacuum pumps engine oil supply. Therefore, the discharge port returns the oil-contaminated air discharge back to the engine crank case.

11.8.4 Hydraulic servo assisted steering and brake system

Introduction to hydraulic servo assistance (Fig. 11.48) The alternative use of hydraulic servo assistance is particularly suited where emission control devices to the engine and certain types of petrol injection system reduce the available intake manifold vacuum, which is essential for the effective operation of vacuum servo assisted brakes. Likewise, diesel engines, which produce very little intake manifold vacuum, require a separate vacuum source such as a vacuum pump (exhauster) to operate a vacuum servo unit; therefore, if power assistant steering is to be incorporated it becomes economical to utilize the same hydraulic pump (instead of a vacuum pump) to energize both the steering and brake servo units.

The hydraulic servo unit converts supplied fluid energy into mechanical work by imposing force

Vacuum Brake Exhauster
Fig. 11.48 Hydraulic servo-assisted and brake system (ATE)

and movement to a power piston. A vane type pump provides the pressure energy source for both the power assisted steering and for the brake servo. When the brake accumulator is being changed approximately 10% of the total pump output is used, the remaining 90% of the output returns to the power steering system. When the accumulator is fully charged, 100% of the pump output returns via the power steering control unit to the reservoir. Much higher operating pressures are used in a hydraulic servo compared to the vacuum type servo. Therefore the time needed to actuate the brakes is shorter.

The proportion of assistance provided to the pedal effort is determined by the cross-sectional area ratio of both the power piston and reaction piston. The larger the power piston is relative to the reaction piston, the greater the assistance will be and vice versa.

In the event of pump failure the hydraulic accumulator reserves will still provide a substantial number of power assisted braking operations.

Pressure accumulator with flow regulator and cutout valve unit (Fig. 11.49(a and b)) The accumulator provides a reserve of fluid under pressure if the engine should stall or in the event of a failure of the source of pressure. This enables several brake applications to be made to bring the vehicle safely to a standstill.

The pressure accumulator consists of a spherical container divided in two halves by a rubber diaphragm. The upper half, representing the spring media, is pressurized to 36 bar with nitrogen gas and the lower half is filled with the operating fluid under a pressure of between 36 and 57 bar. When the accumulator is charged with fluid, the diaphragm is pushed back, causing the volume of the nitrogen gas to be reduced and its pressure to rise. When fluid is discharged, the compressed nitrogen gas expands to compensate for these changes and the flexible diaphragm takes up a different position of equilibrium. At all times both gas and fluid pressures are similar and therefore the diaphragm is in a state of equilibrium.

Accumulator being charged (Fig. 11.49(a)) When the accumulator pressure drops to 36 bar, the cutout spring tension lifts the cut-out plunger against the reduced fluid pressure. Immediately the cut-out ball valve opens and moves from its lower seat to its uppermost position. Fluid from the vane type pump now flows through the cut-out valve, opens the non-return conical valve and permits fluid to pass through to the brake servo unit and to the under side of the accumulator where it starts to compress the nitrogen gas. The store of fluid energy will therefore increase. At the same time, the majority of fluid from the vane type pump flows to the power assisted steering control valve by way of the flutes machined in the flow regulator piston.

Accumulator fully charged (Fig. 11.49(b)) When the accumulator pressure reaches its maximum 57 bar, the cut-out valve ball closes due to the fluid pressure pushing down the cut-out plunger. At the same time, pressurized fluid in the passage between the non-return valve and the rear of the flow regulating piston is able to return to the reservoir via the clearance between the cut-out plunger and guide bore. The non-return valve closes and the fluid pressure behind the flow regulating piston drops. Consequently the fluid supplied from the pump can now force the flow regulator piston further back against the spring so that the total fluid flow passes unrestricted to the power assisted steering control valve.

Hydraulic servo unit (Fig. 11.50(a, b and c)) The hydraulic servo unit consists of a power piston which provides the hydraulic thrust to the master cylinder. A reaction piston interprets the response from the brake pedal input effort and a control tube valve, which actuates the pressurized fluid delivery and release for the servo action.

Brakes released (Fig. 11.50(a)) When the brake pedal is released, the push rod reaction piston and control tube are drawn towards the rear, firstly causing the radial supply holes in the control tube to close and secondly opening the return flow hole situated at the end of the control tube. The pressurized fluid in the operating chamber escapes along the centre of the control tube out to the low pressure chamber via the return flow hole, where it then returns to the fluid reservoir (container). The power piston return spring pushes the power piston back until it reaches the shouldered end stop in the cylinder.

Brakes normally applied (Fig. 11.50(b)) When the brake pedal is depressed, the reaction piston and control tube move inwards, causing the return flow hole to close and partially opening the control tube supply holes. Pressurized fluid from either the accumulator or, when its pressure is low, from the pump, enters the control tube central passage and passes out into the operating chamber. The pressure buildup in the operating chamber forces the power piston to move away from the back end of the cylinder. This movement continues as long as the control tube is being pushed forwards (Fig. 11.50(b)).

Holding the brake pedal in one position prevents the control tube moving further forwards. Consequently the pressure build-up in the operating chamber pushes the power piston out until the radial supply holes in both the power piston and control tube are completely misaligned. Closing the radial supply holes therefore produces a state of balance between the operating chamber fluid thrust and the pressure generated in the tandem master cylinder.

The pressure in the operating chamber is applied against both the power piston and the reaction piston so that a reaction is created opposing the pedal effort in proportion to the amount of power assistance needed at one instance.

Fig. 11.49(a and b) Flow regulator with pressure accumulator
Fig. 11.50 (a-c) Hydraulic servo unit

Braking beyond the cut-out point (Fig. 11.50(c)) When the accumulator cut-out pressure is reached, the control tube touches the power piston, causing the radial supply holes in the control tube to fully align with the power piston. Under these conditions, the accumulator is able to transfer its maximum pressure to the operating chamber. The power piston is therefore delivering its greatest assistance. Any further increase in master cylinder output line pressure is provided by the brake pedal effort alone, as shown in Fig. 11.51, at the minimum and maximum cut-out pressures of 36 and 57 bar respectively.

Rear brake circuit pressure regulator and cut-out device (Fig. 11.52(a, b and c)) The rear brake pressure regulator and cut-off device provide an increasing front to rear line pressure ratio, once the line pressing in the rear pipe line has reached some predetermined minimum value. In other words, the pressure rise in both front and rear pipe lines increases equally up to some pre-set value, but beyond this point, the rear brake pipe line pressure increases at a much reduced rate relative to the front brakes. An additional feature is that if the front brake circuits should develop some fault, then automatically the pressure regulator is bypassed to ensure that full master cylinder fluid pressure is able to operate the rear brakes.

Low brake fluid pressure (Fig. 11.52(a)) When the brakes are lightly applied, the pressure in the front pipe line circuit pushes the cut-off piston over against the opposing spring force. Simultaneously, fluid from the master cylinder enters the inlet port, passes through the open pressure reducing valve, then flows around the wasted cut-off piston on its way out to the rear brake pipe line circuit.

Pedal pressure/servo pressure

Fig. 11.51 Hydraulic servo action pressure characteristics

Pedal pressure/servo pressure

Fig. 11.51 Hydraulic servo action pressure characteristics

High braking fluid pressure (Fig. 11.52(b)) With increased foot pedal effort, the fluid pressure entering the inlet port and passing through the pressure reducing valve, on the way to the rear brake circuit outlet port, rises proportionally. Eventually the resultant force imposed on the stepped piston, caused by the fluid pressure acting on the large and small surface areas of the piston, pushes it outwards against the resistance of the preload spring until the pressure reducing valve closes. Further master cylinder generated pressure acting on the annular face of the stepped piston forces the piston to move in the opposite direction, thereby increasing the rear brake pipe line fluid pressure on the large surface area side of the piston, but at a reduced rate to that of the master cylinder output fluid pressure. The pressure reducing valve is immediately dislodged from its seat. The pressure reducing valve opens and closes repeatedly with rising master cylinder output fluid pressure until the reduced pressure on the large surface area output side of the piston has adjusted itself. These pressure characteristics are shown in Fig. 11.52(d).

Front brake circuit fail condition (Fig. 11.52(c)) If the front brake circuit should fail, the pressure imposed on the cut-off piston collapses, enabling the spring at the opposite end to push over the cutoff piston so that the left hand side of the shuttle valve opens and the right hand side closes. The pressure reducing valve passage to the rear brake line is immediately cut off and the direct passage via the left hand shuttle valve is opened. Pressure from the master cylinder is therefore transmitted unrestricted directly to the rear brake pipe line. The effect of failure in the front brake circuit will be a considerable increase in foot pedal movement.

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