1154 Compensating port type tandem master cylinder Fig 1126ad

Tandem master cylinders are employed to operate dual-line hydraulic braking systems. The master cylinder is composed of a pair of pistons functioning within a single cylinder. This enables two independent hydraulic cylinder chambers to operate. Consequently, if one of these cylinder chambers or part of its hydraulic circuit develops a fault, the other cylinder chamber and circuit will still continue to effectively operate.

Brakes off (Fig. 11.26(a)) With brakes in the 'off' position, both primary and secondary pistons are pushed outwards by the return springs to their respective stops. Under these conditions fluid is permitted to circulate between the pressure chambers and the respective piston recesses via the small compensating port, reservoir supply outlet and the large feed ports for both primary and secondary brake circuits.

Brakes applied (Fig. 11.26(b)) When the foot pedal is depressed, the primary piston moves inwards and, at the same time, compresses both the intermediate and secondary return springs so that the secondary piston is pushed towards the cylinder's blanked end.

Initial movement of both pistons causes their respective recuperating seals to sweep past each compensating port. Fluid is trapped and, with increased piston travel, is pressurized in both the primary and secondary chambers and their pipe line circuits, supplying the front and rear brake cylinders. During the braking phase, fluid from the reservoir gravitates and fills both of the annular piston recesses.

Brakes released (Fig. 11.26(a)) When the foot pedal effort is removed, the return springs rapidly expand, pushing both pistons outwards. The speed at which the swept volume of the pressure chambers increases will be greater than the rate at which the fluid returns from the brake cylinders and pipe lines. Therefore a vacuum is created within both primary and secondary pressure chambers.

As a result of the vacuum created, each recuperating seal momentarily collapses. Fluid from the annular piston recess is then able to flow through the horizontal holes in the piston head, around the inwardly distorted recuperating seals and into their respective pressure chambers. This extra fluid

Tandem Master Cylinder Compensated Port
Fig. 11.26 (a-d) Tandem master cylinder

entering both pressure chambers compensates for any fluid loss within the brake pipe line circuits or for excessive shoe to drum clearance. But, if too much fluid is induced in the chambers, some of this fluid will pass back to the reservoir via the compensating ports after the return springs have fully retracted both pistons.

Failure in the primary circuit (Fig. 11.26(c)) Should a failure (leakage) occur in the primary circuit, there will be no hydraulic pressure generated in the primary chamber. When the brake pedal is depressed, the push rod and primary piston will move inwards until the primary piston abuts the secondary spring retainer. Further pedal effort will move the secondary piston recuperating seal beyond the compensating port, thereby pressurizing the fluid in the secondary chamber and subsequently transmitting this pressure to the secondary circuit pipe line and the respective brake cylinders.

Failure in the secondary circuit (Fig. 11.26(d)) If there is a failure (leakage) in the secondary circuit, the push rod will move the primary piston inwards until its recuperating seal sweeps past the compensating port, thus trapping the existing fluid in the primary chamber. Further pedal effort increases the pressure in the primary chamber and at the same time both pistons, separated by the primary chamber fluid, move inwards unopposed until the secondary piston end stop contacts the cylinder's blanked end. Any more increase in braking effort raises the primary chamber pressure, which accordingly pressurizes the primary circuit brake cylinders.

The consequence of a failure in the primary or secondary brake circuit is that the effective push rod travel increases and a greater pedal effort will need to be applied for a given vehicle retardation compared to a braking system which has both primary and secondary circuits operating.

11.5.5 Mecanindus (roll) pin type tandem master cylinder incorporating a pressure differential warning actuator (Fig. 11.27(a-d)) The tandem or split master cylinder is designed to provide two separate hydraulic cylinder pressure chambers operated by a single input push rod. Each cylinder chamber is able to generate its own fluid pressure which is delivered to two independent brake pipe line circuits. Thus if one hydraulic circuit malfunctions, the other one is unaffected and will provide braking to the wheel cylinders forming part of its system.

Operation of tandem master cylinder

Brakes off (Fig. 11.27(a)) With the push rod fully withdrawn, both primary and secondary pistons are forced outwards by the return springs. This outward movement continues until the central poppet valve stems contact their respective Mecanindus (roll) pins. With further withdrawal the poppet valves start opening until the front end of each elongated slot also contacts their respective roll pins, at which point the valves are fully open. With both valves open, fluid is free to flow between the primary and secondary chambers and their respective reservoirs via the elongated slot and vertical passage in the roll pins.

Brakes applied (Fig. 11.27(b)) When the brake pedal is applied, the push rod and the primary return spring pushes both pistons towards the cylinder's blank end. Immediately both recuperating poppet valves are able to snap closed. The fluid trapped in both primary and secondary chambers is then squeezed, causing the pressure in the primary and secondary pipe line circuits to rise and operate the brake cylinders.

Brakes released (Fig. 11.27(a)) Removing the foot from the brake pedal permits the return spring to push both pistons to their outermost position. The poppet valve stem instantly contacts their respective roll pins, causing both valves to open. Since the return springs rapidly push back their pistons, the volume increase in both the primary and secondary chambers exceeds the speed of the returning fluid from the much smaller pipe line bore, with the result that a depression is created in both chambers. Fluid from the reservoir flows via the elongated slot and open poppet valve into the primary and secondary chambers to compensate for any loss of fluid or excessive shoe to drum or pad to disc clearance. This method of transferring fluid from the reservoir to the pressure chamber is more dynamic than relying on the collapse and distortion of the rubber pressure seals as in the conventional master cylinder.

Within a very short time the depression disappears and fluid is allowed to flow freely to and fro from the pressure chambers to compensate for fluid losses or fluid expansion and contraction caused by large temperature changes.

11.5.6 Operation of the pressure differential warning actuator

As a warning to the driver that there is a fault in either the primary or secondary hydraulic braking circuits of a dual-line braking system, a pressure differential warning actuator is usually incorporated as an integral part of the master cylinder or it may be installed as a separate unit (Fig. 11.27).

The switch unit consists of a pair of opposing balance pistons spring loaded at either end so that they are normally centrally positioned. Mounted centrally and protruding at right angles into the cylinder is an electrical conducting prod, insulated from the housing with a terminal formed at its outer end. The terminal is connected to a dashboard warning light and the electrical circuit is completed by the earth return made by the master cylinder.

Operation (Fig. 11.27(b)) If, when braking, both hydraulic circuits operate correctly, the opposing fluid pressure imposed on the outer ends of the balance piston will maintain the pistons in their equilibrium central position.

Simple Tandem Master Cylinder
Fig. 11.27(a-d) Tandem master cylinder with pressure differential warning actuator

Should one or the other of the dual circuits develop a pressure drop fault due to fluid leakage (Fig. 11.27(c and d)), then if the pressure difference of 10 bar or more exists between the two circuits, an imbalance of the fluid pressure applied against the outer ends of the pistons will force both pistons to move in the direction of the faulty circuit. The sideways movement of the pistons will cause the shoulder of the correctly operating circuit balance piston to contact the protruding prod, thus automatically completing the dashboard warning light electrical circuit, causing it to illuminate. Removing the brake pedal effort causes the fluid pressure in the effective circuit to collapse, thereby enabling the balance pistons to move back to their central positions. This interrupts the electrical circuit so that the warning light switches off.

11.6 Apportioned braking

11.6.1 Pressure limiting valve (Fig. 11.28) The object of the pressure limiting valve is to interrupt the pressure rise of fluid transmitted to the rear wheel brakes above some predetermined value, so that the rear brakes will be contributing a decreasing proportion of the total braking with further increased pedal effort and master cylinder generated line pressure. By imposing a maximum brake line pressure to the rear brake cylinders, the rear wheels will be subjected to far less overbraking when the vehicle is heavily braked. It therefore reduces the tendency for rear wheel breakaway caused by wheels locking. Note that with this type of valve unit under severe slippery conditions the rear wheels are still subjected to lock-up.

Operation (Figs 11.28 and 11.29) Under light brake pedal application, fluid pressure from the master cylinder enters the valve inlet port and passes through the centre and around the outside of the plunger on its way to the outlet ports via the wasted region of the plunger (Fig. 11.28(a)).

When heavy brake applications are made (Fig. 11.28(b)), the rising fluid pressure acting on the large passage at the rear of the plunger displaces the plunger assembly. Instantly the full cross-sectional area equivalent to the reaction piston is exposed to hydraulic pressure, causing the plungers to move forward rapidly until the plunger end seal contacts the valve seat in the body of the valve unit. The valve closing pressure is known as the cut-off pressure. Under these conditions the predetermined line pressure in the rear pipe line will be maintained constant (Fig. 11.29), whereas the front brake pipe line pressure will continue to rise unrestricted, according to the master cylinder pressure generated by the depressed brake pedal.

11.6.2 Load sensing pressure limiting valve

To take into account the weight distribution between the front and rear wheels between an unladen and fully laden vehicle, a load sensing valve may be incorporated in the pipe line connecting the master cylinder to the rear wheel brakes. The function of the valve is to automatically separate the master cylinder to rear brake pipe line by closing a cut-off valve when the master cylinder's generated pressure reaches some predetermined maximum. This cut-off pressure will vary according to the weight imposed on the rear axle.

Front Brake Limiting Valve

To rear brakes

Fig. 11.28(a and b) Pressure limiting valve

To rear brakes

Fig. 11.28(a and b) Pressure limiting valve

Fig. 11.29 Pressure limiting valve front to rear brake line characteristics

Operation (Figs 11.30 and 11.31) This valve device consists of a plunger supporting a rubber face valve which is kept open by the tension of a variable rate leaf spring. The inward thrust on the plunger keeping the end face valve open is determined by the leaf spring pre-tension controlled by the rear suspension's vertical deflection via the interconnecting spring and rod link. When the vehicle's rear suspension is unloaded, the leaf spring will be partially relaxed, but as the load on the rear axle increases, the link spring and rod pulls the leaf spring towards the valve causing it to stiffen and increase the inward end thrust imposed on the plunger.

With a light brake pedal application, fluid pressure generated by the master cylinder enters the inlet port and passes around the wasted plunger on its way out to the rear brake pipe line (Fig. 11.30(a)).

If a heavy brake application is made (Fig. 11.30(b)), the rising fluid pressure from the master cylinder passes through the valve from the inlet to the outlet ports until the pressure creates a force at the end of the plunger (Force = Pressure x Area) which opposes the spring thrust, pushing back the plunger until the face valve closes. Any further fluid pressure rise will only be transmitted to the front brake pipe lines, whereas the sealed-off fluid pressure in the rear brake pipe lines remains approximately constant.

If the load on the rear axle alters, the vertical deflection height of the suspension will cause the leaf spring to stiffen or relax according to any axle load increase or decrease.

A change in leaf spring tension therefore alters the established pressure (Fig. 11.31) (at which point the cut-off valve closes) and the maximum attainable pressure trapped in the rear brake pipe lines.

11.6.3 Load sensing progressive pressure limiting valve (Fig. 11.32)

The load sensing progressive pressure limiting valve regulates the fluid pressure transmitted to the rear brake cylinders once the master cylinder's generated pressure has risen above some predetermined value corresponding to the weight carried on the rear axle.

Fig. 11.30 (a and b) Load sensing pressure limiting valve

Fully laden

Fully laden

Front line pressure (bar!

Fig. 11.31 Load sensing pressure limiting valve front to rear brake line characteristics

Front line pressure (bar!

Fig. 11.31 Load sensing pressure limiting valve front to rear brake line characteristics

The reduced rate of pressure increase, in proportion to the pedal effort in the rear brake pipe line, provides a braking ability for both the front and rear brakes which approximately matches the load distribution imposed on the front and rear wheels, so that the tendency for the rear wheels to be either under or over braked is considerably reduced.

Operation (Fig. 11.32) When the foot pedal is applied lightly (Fig. 11.32(a)), pressure generated by the master cylinder will be transferred through the centre of the stepped reaction piston, between the cone and seat and to the rear brake pipe lines.

If the brake pedal is further depressed (Fig. 11.32(b)), increased fluid pressure acting on the large piston area produces an end force, which, when it exceeds the opposing link spring tension and fluid pressure acting on the annular piston face, causes the stepped piston to move outwards. This outward movement of the piston continues until the valve stem clears the cylinder's blanked end, thereby closing the valve. The valve closure is known as the cut-off point since it isolates the rear brake pipe lines from the master cylinder delivery.

Further generation of master cylinder pressure exerted against the annular piston face produces an increase in force which moves the piston inwards, once again opening the valve. The hydraulic connection is re-established, allowing the rear brake pipe line fluid pressure to increase. However, the pressure exerted against the end face of the piston immediately becomes greater than the spring force and hydraulic force pushing on the annular piston face, and so the piston moves outwards, again closing the valve.

Every time the valve is opened with rising master cylinder pressure, the rear brake pipe line pressure increases in relation to the previous closing of the valve. Over a heavy braking pressure rise phase the piston oscillates around a position of balance, causing a succession of valve openings and closings. It subsequently produces a smaller pressure rise in the rear brake pipe line than with the directly connected front brake pipe lines.

(a) Low deceleration

Fig. 11.32(a and b) Load sensing progressive pressure limiting valve

(a) Low deceleration

Fig. 11.32(a and b) Load sensing progressive pressure limiting valve

From line pressure Ibart

Fig. 11.33 Load sensing and progressive pressure limiting valve front to rear brake line characteristics

From line pressure Ibart

Fig. 11.33 Load sensing and progressive pressure limiting valve front to rear brake line characteristics

The ratio of the stepped piston face areas determines the degree of rear brake pipe line increase with respect to the front brake pipe lines (Fig. 11.33).

The cut-off or change point depends on the ten-sioning of the pre-setting spring which varies with the rear suspension deflection. The brake force distribution between the front and rear brakes is not only affected by the static laden condition, but even more so by the dynamic weight transference from the rear to the front axle.

11.6.4 Inertia pressure limiting valve (Fig. 11.34) The inertia pressure limiting valve is designed to restrict the hydraulic line pressure operating the rear wheel brakes when the deceleration of the vehicle exceeds about 0.3 g. In preventing a further rise in the rear brake line pressure, the unrestricted front brake lines will, according to the hydraulic pressure generated, increase their proportion of braking relative to the rear brakes.

Operation (Figs 11.34 and 11.35) The operating principle of the inertia valve unit relies upon the inherent inertia of the heavy steel ball rolling up an inclined ramp when the retardation of the vehicle exceeds some predetermined amount (Fig. 11.34(b)). When this happens, the weight of the ball is removed from the stem of the disc valve, enabling the return spring fitted between the inlet port and valve shoulder to move the valve into the cut-off position.

At this point, the fluid trapped in the rear brake pipe line will remain constant (Fig. 11.35), but fluid flow between the master cylinder and front brakes is unrestricted and therefore will continue to rise with increased pedal force. As a result, the front brakes will contribute a much larger proportion of the total braking effort than the rear brakes.

When the vehicle has slowed down sufficiently or even stopped, the steel ball will gravitate to its lowest point, thereby pushing open the cut-off valve. Fluid is now free again to move from the master cylinder to the rear wheel brakes (Fig. 11.34(a)).

To rear brakes

To rear brakes

(a) Low deceleration (b) high deceleration

Fig. 11.34(a and b) Inertia pressure limiting valve

(a) Low deceleration (b) high deceleration

Fig. 11.34(a and b) Inertia pressure limiting valve

Inertia Brake Pressure Limiting Valve
Fig. 11.35 Inertia pressure limiting valve and inertia progressive pressure limiting front to rear brake line

11.6.5 Inertia and progressive pressure limiting valve (Fig. 11.36)

The inertia and progressive pressure limiting valve unit enables the braking power between the front and rear wheels to be adjusted to match the weight transference from the rear wheels to the front wheels in proportion to the vehicle's deceleration rate. This two stage valve unit allows equal fluid pressure to flow between the front and rear brakes for light braking, but above some predetermined deceleration of the vehicle, the direct pressure increase to the rear brakes stops. With moderate to heavy braking, the front brake line pressure will equal the master cylinder generated pressure. The rear brake line pressure will continue to increase but at a much slower rate compared to that of the front brakes.

Operation (Figs 11.35 and 11.36) This inertia and progressive valve unit differs from the simple inertia pressure limiting valve because it incorporates a stepped piston (two piston dimeters) and the ball performs the task of the cut-off valve.

If the vehicle is lightly braked (Fig. 11.36(a)), fluid will flow freely from the master cylinder inlet port, through the dispersing diffuser, around the ball, along the piston central pin passage to the outlet port leading to the rear wheel brakes.

As the brake pedal force is increased (Fig. 11.36(b)), the vehicle's rate of retardation will cause the ball to continue to move forward by rolling up the inclined ramp until it seals off the central piston passage. This state is known as the point of cut-off.

Further foot pedal effort directly increases the front brake pipe line pressure and the pressure in the ball chamber. It does not immediately increase the rear brake pipe line pressure on the output of the valve.

Under these conditions, the trapped cut-off pressure in the rear brake lines reacts against the large piston cross-sectional area, whereas the small piston cross-sectional area on the ball chamber side of the piston is subjected to the master cylinder hydraulic pressure.

As the master cylinder's generated pressure rises with greater foot pedal effort, the input force produced on the small piston side (Input force = Master cylinder pressure x Small piston area) will increase until it exceeds the opposing output force produced on the large piston area (Output force = Rear brake line pressure x Large piston area).

A further rise in master cylinder pressure which will also be experienced in the ball chamber pushes the stepped piston backwards. Again, the rear brake line pressure will start to rise (Fig. 11.35), but at a reduced rate determined by the ratio of the small piston area to large piston area, i.e. AS/AL. For example, if the piston area ratio is 2:1, then the rear brake line pressure increase will be half the input master cylinder pressure rise.

where P; = input pressure Po = output pressure

To safeguard the rear brake pipe lines, should the piston reach its full extent of its travel, the centre pin will stand out from the piston. Consequently the ball is dislodged from its seat so that fluid pressure is permitted to pass to the rear brake pipe lines.

If there are two separate rear brake pipe line circuits, each line will have its own rear brake pressure reducing valve.

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Responses

  • balbo
    Why inlet port is smaller than compensating port?
    3 years ago

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