118 Brake servos

11.8.1 Operating principle of a vacuum servo (Fig. 11.45)

The demand for a reduction in brake pedal effort and movement, without losing any of the sensitivity and response to the effective braking of cars and vans, has led to the adoption of vacuum servo assisted units as part of the braking system for most light vehicles. These units convert the induction manifold vacuum energy into mechanical energy to assist in pressurizing the brake fluid on the output side of the master cylinder.

A direct acting vacuum servo consists of two chambers separated by a rolling diaphragm and power piston (Fig. 11.45). The power piston is coupled to the master cylinder outer primary piston by a power push rod. The foot pedal is linked through a pedal push rod indirectly to the power piston via a vacuum-air reaction control valve.

Fig. 11.45 (a and b) Operating principle and characteristics of a vacuum servo

When the brakes are in the 'off' position, both sides of the power piston assembly are subjected to induction manifold pressure. When the brakes are applied, the vacuum in the front chamber remains undisturbed, whilst the vacuum in the rear chamber is replaced by atmospheric air closing the vacuum supply passage, followed by the opening of the air inlet passage to the rear chamber. The resulting difference of pressure across the power piston causes it to move towards the master cylinder, so that the thrust imposed on both the primary and secondary pistons in the master cylinder generates fluid pressure for both brake lines.

The operating principle of the vacuum servo is best illustrated by the following calculation:

Example (Fig. 11.45(a)) A direct acting vacuum servo booster has a 200 mm diameter power piston suspended on both sides by the induction manifold vacuum (depression), amounting to a gauge reading of 456 mm Hg, that is 0.6 bar below atmospheric pressure.

The foot pedal leverage ratio is 4:1 and the master cylinder has 18 mm diameter.

Determine the following when a pedal effort of 300 N is applied and the rear power piston chamber which was occupied with manifold vacuum is now replaced by atmospheric air (Fig. 11.45(a)).

a) The push rod thrust and generated primary and secondary hydraulic brake line pressures due only to the foot pedal effort.

b) The power push rod thrust and the generated fluid pressures in the pipe lines due only to the vacuum servo action.

c) The total pedal push rod and power piston thrust and the corresponding generated fluid pressure in the pipe lines when both foot pedal and servo action are simultaneously applied to the master cylinder.

F1 = pedal push rod thrust (N) F2 = power piston thrust (N) P1 = pressure in the rear chamber

P2 = manifold pressure (kN/m2) P3 = fluid generated pressure (kN/m2) A1 = cross-sectional area of power piston (m2) A2 = cross-sectional area of master cylinder bore (m2)

a) Pedal push rod thrust F\

Master cylinder fluid pressure P3

4715.7 kN/m2 or 47.2 bar b) Power piston thrust F2 = A\(P\ — P2)

= 12103.635kN/m2 or 121.04 bar

11.8.2 Direct acting suspended vacuum-assisted brake servo unit (Fig. 11.46(a, b and c)) Brake pedal effort can be reduced by increasing the leverage ratio of the pedal and master cylinder to wheel cylinder piston sizes, but this is at the expense of lengthening the brake pedal travel, which unfortunately extends the brake application time. The vacuum servo booster provides assistance to the brake pedal effort, enabling the ratio of master cylinder to wheel cylinder piston areas to be reduced. Consequently, the brake pedal push rod effective stroke can be reduced in conjunction with a reduction in input foot effort for a given rate of vehicle deceleration.


Brakes off (Fig. 11.46(a)) With the foot pedal fully released, the large return spring in the vacuum chamber forces the rolling diaphragm and power piston towards and against the air/vac chamber stepped steel pressing.

When the engine is running, the vacuum or negative pressure (below atmospheric pressure) from the induction manifold draws the non-return valve away from its seat, thereby subjecting the whole vacuum chamber to a similar negative pressure to that existing in the manifold.

When the brake pedal is fully released, the outer spring surrounding the push rod pulls it and the relay piston back against the valve retaining plate. The inlet valve formed on the end of the relay piston closes against the vac/air diaphragm face and at the same time pushes the vac/air diaphragm away from the vacuum valve. Negative pressure from the vacuum chamber therefore passes through the inclined passage in the power piston around the seat of the open vacuum valve where it then occupies the existing space formed in the air/ vac chamber to the rear of the rolling diaphragm. Hence with the air valve closed and the vacuum valve open, both sides of the power piston are suspended in vacuum.

Brakes applied (Fig. 11.46(b)) When the foot pedal is depressed the pedal push rod moves towards the diaphragm power piston, pushing the relay piston hard against the valve retaining plate. Initially the vac/air diaphragm closes against the vacuum valve's seat and with further inward push rod movement the relay piston inlet seat separates from the vac/air diaphragm face. The air/vac chamber is now cut off from the vacuum supply and atmospheric air is now free to pass through the air filter, situated between the relay piston inlet valve seat and diaphragm face, to replace the vacuum in the air/vac chamber. The difference in pressure between the low primary vacuum chamber and the high pressure air/vac chamber causes the power piston and power push rod to move forward against the master cylinder piston so the fluid pressure is generated in both brake circuits to actuate the front and rear brakes.

Brake held on (Fig. 11.46(c)) Holding the brake pedal depressed momentarily continues to move the power piston with the valve body forward under the influence of the greater air pressure in the air/vac chamber, until the rubber reaction pad is compressed by the shoulder of the power piston against the opposing reaction of the power push rod. As a result of squeezing the outer rubber rim of the reaction pad, the rubber distorts and extrudes towards the centre and backwards in the relay piston's bore. Subsequently, only the power piston and valve body move forward whilst the relay piston and pedal push rod remain approximately in the same position until the air valve seat closes against the vac/air diaphragm face. More

Master cylinder fluid pressure P3

c) Total power piston and pedal push rod thrust

Total master cylinder fluid pressure P3

Fig. 11.46 (a and b) Vacuum-assisted brake servo unit

Fig. 11.46 (a and b) Vacuum-assisted brake servo unit atmospheric air cannot now enter the air chamber so that there is no further increase in servo power assistance. In other words, the brakes are on hold. The reaction pad action therefore provides a progressive servo assistance in relation to the foot pedal effort which would not be possible if only a simple reaction spring were positioned between the reaction piston and the relay piston.

If a greater brake pedal effort is applied for a given hold position, then the relay piston will again move forward and compress the centre region of the reaction pad to open the air valve. The extra air permitted to enter the air/vac chamber therefore will further raise the servo assistance proportionally. The cycle of increasing or decreasing the degree of braking provides new states of hold which are progressive and correspond to the manual input effort.

Brakes released (Fig. 11.46(a)) Releasing the brake pedal allows the pedal push rod and relay piston to move outwards; first closing the air valve and secondly opening the vacuum valve. The existing air in the air/vac chamber will then be extracted to the vacuum chamber via the open vacuum valve, the power piston's inclined passage, and finally it is withdrawn to the induction manifold. As in the brakes 'off position, both sides of the power piston are suspended in vacuum, thus preparing the servo unit for the next brake application.

Vacuum servo operating characteristics (Fig. 11.45(b)) The benefits of vacuum servo assistance are best shown in the input to output characteristic graphs (Fig. 11.45(b)). Here it can be seen that the output master cylinder line pressure increases directly in proportion to the pedal push rod effort for manual (unassisted) brake application. Similarly, with vacuum servo assistance the output line pressure rises, but at a much higher rate. Eventually the servo output reaches its maximum. Thereafter any further output pressure increase is obtained purely by direct manual pedal effort at a reduced rate. The extra boost provided by the vacuum servo in proportion to the input pedal effort may range from 1%:1 to 3:1 for direct acting type servos incorporated on cars and vans.

Servo assistance only begins after a small reaction force applied by the foot pedal closes the vacuum valve and opens the air inlet valve. This phase where the servo assistance deviates from the manual output is known as the crack point.

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