DESCRlPTlON
The Girling Vacuum Servo Unit is installed in the brake hydraulic system between the master cylinder and the brake assemblies on the radials with master cylinder pressure pipe connected to the vacuum servo unit and the servo hydraulic outlet is connected to the brake assemblies. The force required to augment the driver s effort is obtained by admitting atmospheric pressure to a vacuum cylinder containing a piston.
The pressure difference thus created across the piston produces a thrust load which is used to increase the hydraulic pressure available at the brake assemblies.
In the Girling Vacuum Servo Unit the piston in the vacuum cylinder is normally subjected to vacuum on both sides and this principle is known as Suspended Vacuum System . The vacuum is obtained from the inlet manifold of the engine and therefore servo assistance is only available while the engine is running.
Between the engine inlet manifold and the vacuum servo unit is a non-return valve which prevents air or petrol fumes entering the servo unit.
OPERATION (Fig. 10)
When air at atmospheric pressure is admitted to the vacuum cylinder (19) by a composite control valve the piston (5) drives the piston rod (18) and the output piston (25) down the hydraulic output cylinder (28) providing a considerable increase of hydraulic pressure to the brake assemblies on the radials.
The control valve operated by hydraulic pressure from the master cylinder exercises a precise control over the pressure increase and the brakes are applied exactly in proportion to the pressure applied to the foot pedal.
When the servo unit is at rest with no pressure in the hydraulic system, the vacuum valve (22) of the control valve is open and permits vacuum from the inlet manifold to temporarily communicate with the end cover side of the piston (5) In the vacuum cylinder (19) thus vacuum of both side of the piston (5) is equal. |
When pressure is applied to the brake pedal, hydraulic pressure is exerted throughout the braking system and equally on both ends of the valve control piston (10 and 13). As one end of this control piston is larger than the other, an equal pressure per square inch on both its ends causes a proportionally greater thrust to be exerted on the larger end moving the control piston and results in the Tee shaped lever (11) opening the air valve (31) to the atmosphere and closing the vacuum valve (22). Air is admitted to the end cover side of the piston (5) destroying the temporary vacuum driving the piston (5) forward.
The piston rod (18) seals the center hole in the output piston (25) and continued movement applies additional pressure to the hydraulic pressure proceeding to the brake assemblies and also to the small end of the valve control piston (13).
The movement of the output piston (25) continues until the thrust on the small end of the valve control piston (13) by the higher pressure to the brake assemblies overcomes the thrust on the large end of the control piston (10) by the lower pressure from the master cylinder. The valve control piston (10 and 13) is thus moved back and closes the air valve (31); at this point both valves are closed and the brakes held on.
When the brake pedal is released the lower pressure from the master cylinder is reduced at the large end of the valve control piston (10) which moves back towards its rest position causing the Tee shaped lever (11) to open the vacuum valve (22) drawing the air from the end cover side of the piston (5) in the vacuum cylinder
(19) the piston (5) returns to its rest position and with it the output piston (25) relieving the hydraulic pressure in the brake assemblies. The piston rod (18) is withdrawn from the center hole in the output piston (25) allowing a flow of hydraulic fluid between the brake assemblies and the fluid reservoir of the master cylinder.
If the brake pedal pressure was Increased instead of being decreased the control valve operates to give additional assistance until the thrust on each end of the valve control piston is balanced or until the limit of available vacuum is reached. Conversely if the pedal pressure is only reduced the control valve operates to reduce the hydraulic pressure in the brake assemblies until again. a state of balance of the valve control piston is reached. |
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