CHAPTER 1 - INTRODUCTION

The Load Conscious Pressure Reducing Valve (LCRV) also known as the load

sensing proportioning valve (LSPV) is installed on systems with front disc and rear

drum braking systems. It is installed in the hydraulic line leading to the rear drum

brakes and helps prevent rear wheel lock up. Most proportioning valves are mounted

directly in the master cylinder outlet to the rear brakes. They provide balanced

braking during panic braking situations or when the wheel is loaded by restricting

fluid pressure to the rear brakes. This helps prevent rear wheel lock up as the

vehicle’s weight is shifted towards the front wheels.

The typical inline LCRV remains open till a panic braking situation occurs. When

sudden pressure from the master cylinder moves the large piston it closes the valve

restricting fluid pressure.

The LCRV is usually found on trucks and is located between the vehicle’s chassis

and the rear axle. When a driver brakes hard, the rear end tends to lift up and

forward. This lightens the load on the rear tires increasing the chance of lock up.

This valve incorporates a lever attached to a spring that moves the valve to restrict

fluid pressure as the vehicle’s chassis rises away from the axle.

1.1 SCOPE OF THE PROJECT

The aim of this project is to study the construction and working principles of a Load

Conscious Pressure Reducing Valve system, perform suitable calculations for inlet

and outlet pressures that will approximate the behaviour of the system, and exposit

on a modification proposed to the system that will accommodate manual changes in

vehicle ride height.

1.2 METHODOLOGY

A case study approach to the project was adopted, which encompassed a two-day

visit to TAFE ACCESS LIMITED, TRICHY ROAD, COIMBATORE, to study the

mounting methods, materials and working of LCRV systems in TATA vehicles (the

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SUMO VICTA, SAFARI DICOR 3.0 and XENON particularly). A comprehensive

survey of literature available in the public domain was done and appropriate

conclusions were arrived at.

1.3 MOTIVATION

When a vehicle is braked at high speeds, weight transfer happens. The vehicle

weight shifts to the front axle. There will be relatively very less weight in the rear

axles. If the brake fluid pressure is same for both front and rear axles, it will lead to

“Rear Wheel Lock” condition. A locked rear wheel will lead to vehicle instability and

skidding/Tail swing. This makes it imperative that a pressure control system be

installed on the rear brake lines so that when weight on the rear wheels becomes

lesser, application of high pressure to the rear brake drum is prevented, thereby

preventing wheel lock-up. The main advantages of an LCRV system are

Suitable for X and V split systems

Dynamic sensing of load variation

Simple hydraulic installation

Optimum laden / un-laden cut-in pressure

Can cope with wide variation in ride height

ABS compatible

An advantage of the reducing valve is that if the position sensing springs fails, the

bias spring moves the piston to maintain it in a position in which communication

between the inlet and the outlet is interrupted until applied inlet pressure rises to a

predetermined value, and is operative, when the inlet pressure is higher than the

predetermined value to provide a pressure at the outlet which is smaller than the

pressure at the inlet.

This mini project aims at studying the operational principles of an LCRV braking

system and proposing a modification in the system so that changes in ride height

can be accommodated.

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1.4 DIFFICULTIES FACED

Current LCRV braking systems have a cumbersome provision for accommodating

changes in the ride height of the vehicle. If any alteration is done in the Rear

suspension the performance of this valve will be impaired. The valve stem gap

should be adjusted whenever rear spring is changed.

1.1 LCRV adjustment screw

The changes in ride height complicate the LCRV assembly, and to adjust for them,

the bellcrank is mounted higher up by installing a new plate.

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CHAPTER 2 – CONSTRUCTION AND WORKING

2.1 CONSTRUCTION

2.1 An LCRV assembly CAD model

An LCRV system has been designed for varying the amount of brake fluid pressure

and thus the braking torque at the rear brakes of a passenger car. The system

includes a load sensor oriented between a suspension component, such as a

supporting spring. The vehicle frame or body and includes a cavity for

incompressible fluid in which variation in load will vary the volume of the cavity for

providing increased pressure and volumeric flow of an incompressible fluid from the

cavity to a valve. The valve is incorporated into the hydraulic brake lines extending

from the master cylinder to the rear brakes of a passenger car for regulating the

braking pressure and thus braking torque to the rear wheels. This is to reduce or

eliminate premature rear wheel locking when applying brakes of a lightly loaded

vehicle and to maintain adequate braking force for the same vehicle when heavily

loaded. A flexible line interconnects the load sensor and the valve to compensate for

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relative movement between the vehicle frame and suspension system. The valve

includes an actuator in the form of a control piston engageable with the proportioning

piston in the proportioning valve to regulate the movement of the proportioning piston

and thus regulate the proportioning valve in response to variations in load applied to

the rear suspension components of the vehicle. Thus relative changes in ride height

are gradually transmitted to the control piston, which can then reposition itself so that

the proportion of line pressure received by the rear brakes will be either increased or

decreased to reflect loading conditions on the rear axle. The valve is connected to

the vehicle's rear axle by pivotally connected linkages, a bell crank, a position

sensing spring, and a piston actuating lever. The system comprises of a link

connecting the rear axle housing to another connecting link, which is connected

pivotally to the first connecting link. The second connecting link is connected to a bell

crank, which is pivotally connected to the valve. A lever is pivotally attached to the

valve to control its operation through a position sensing spring. The movement of the

lever is controlled in response to the movement of the connecting links and the bell

crank.

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LCRV unitLoad

sensing spring

First connecting

link

Second connecting

link

Bell crankRear axle housing

2.2 Construction

2.3 LCRV drawing and CAD model

P1 – Inlet Pressure from master cylinder

P2 – Outlet Pressure to brake cylinder

A1 – Fluid flow area

A2 – Area of piston

A3 – Area of piston rod

The valve 10 comprises a body 11 which is fixed to the sprung part 19 of a vehicle

and which houses a control piston 12 carried by a piston rod 13. The piston rod 13 is

acted upon by a position sensing spring 20 which reacts against a sprung part 21 of

the vehicle. The piston 12 houses a valve 14 which is normally held open by a pin 15

secured to the valve body 11 to provide communication between a master cylinder

26 connected to inlet 16 and a brake cylinder 27 connected to outlet 17. The valve

14 closes as the piston 12 moves to the right, and thereby interrupts communication

between the inlet 16 and outlet 17. An internal bias spring 18 applies a constant

force K to the piston tending to close the valve 14. The bias spring 18 acts in

opposition to the position sensing spring, but the position sensing spring is designed

and set so that it always applies to the control piston a foroe S which is greater than

the constant force K of the bias spring 18.

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2.2 WORKING

The position sensing spring applies to the control piston a force (F) determined by

the relative position of the sprung and the unsprung parts. The sensing spring force

(F) tends to move the control piston in the direction to open the valve, and is

opposed by an internal bias spring which tends to close the valve. By suitable choice

of piston areas A1 and A2 and spring forces increase (above cut-in pressure) in input

pressure (P1) will always produce an increase in output pressure (P2) despite the

reduction in sensing spring force (F) which will occur due to weight transfer.

The bias spring applies to the control piston an approximately constant force which

tends to move the control piston in the backward direction as it opposes the force

applied to the control piston by the position sensing spring. The bias spring becomes

operational when the external spring fails to maintain the control piston in a position

in which communication between the inlet and the outlet is interrupted until the inlet

pressure rises to a predetermined value. It also becomes operational when the inlet

pressure is higher than the predetermined value to provide a pressure at the outlet

which is smaller than the pressure at the inlet.

2.3 INLET AND OUTLET PRESSURE VARIATIONS

The cut-in pressure of the brake pressure reducing valve is given by the formula,

PcA3 = S - K

Where

Pc – cut-in pressure

A3 – area of the piston rod of the control piston

K – force applied to the control piston by the bias spring

S – force applied to the control piston by the position sensing spring

At pressures above cut-in, the outlet pressure is given by the formula,

P2A2 = P1A1 + S - K

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where

P1 – Inlet pressureP2 – Outlet pressureA1 – area of the control piston exposed to inlet pressureA2 – area of the control piston exposed to outlet pressure

2.4 The driver only characteristics OA'B' and fully laden characteristics OC'D' of the valve.

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CHAPTER 3 – SERVICING

3.1 Setting Method:

3.1 Setting Method

1. Install the connecting rod (a) to the No. 1 hole. 2. Adjust the clearance “X” to be 0 mm and tighten the bolt (c) using the lock nut

(b) (Tightening Torque = 14 to 18 Nm).3. Remove the connecting rod (a) from the No. 1 hole and reinstall it to the No. 2

hole (Tightening Torque = 14 to 18 Nm).4. Place alignment marks between lock nut (b) and adjusting screw (c) after the

valve setting.5. LCRV setting should be performed with unloaded vehicle condition.

3.2 Inspection:

When a malfunction occurs in the LCRV, the sensor spring, valve body assembly, rear axle or rear spring may be replaced. For the replacement, the following inspection should be performed:

Symptom Possible Cause ActionPoor braking performance

Air in brake system BleedingPoor adjustment of sensor spring

Adjust

Damaged sensor spring ReplaceFluid leaking from LCRV Replace

Rear wheels lock early Poor adjustment of sensor spring

Adjust

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Internal fluid leaking of LCRV Replace

Notice:

Fluid leaking from LCRV results from poor valve open/close or sealing wear caused by foreign materials in LCRV.

When the LCRV is changed or repaired, it may affect braking performance. Thus it must be ensured that only the appropriate part is used.

3.3 Removal and Installation:

1. Remove the nut mounting the stud from the LCRV connecting rod. 2. Remove the brake pipe retaining the LCRV.3. Remove the LCRV mounting bolt from the LCRV bracket and remove the

LCRV. 4. Installation should follow the removal procedure in reverse order.

Tightening Torque values:

Valve body mounting nut = 14 to 18 Nm

Bracket mounting bolt = 10 to 13 Nm

Stud mounting nut = 14 to 18 Nm

Note: Bleed the brake system.

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CHAPTER 4 - CALCULATIONS

Data Needed:

Assuming a typical vehicle weighing 2500 kg and taking approximate values for diameters of springs and piston of an LCRV setup based on the values seen on the vehicles at TAFE ACCESS LIMITED,

Total weight = 2500 kg = 24525 N Weight Distribution (static-40%front, 60%rear) Static weight on Front =1000 kg = 9810 N Static weight on rear =1500 kg = 14715 N Speed of the vehicle = 180 km/hr = 50 m/s Height of CG = 290 mm & Wheelbase = 1700 mm Disc diameter front and rear = 215 mm Wheel cylinder size = 1963.5 mm2 (d=50mm) Leverage ratio = 5

Static Axle Load Distribution:

Mr

M=ψ

Where

Mr – static rear axle load (kg)M – Total vehicle mass (kg)Ψ – Static Axle load distribution

Ψ = 270460 = 0.5869

Relative Centre of Gravity Height:

hwb

=X

Where

h – vertical distance from C of G to ground on level (mm)wb – wheelbase (mm)X – relative centre of gravity height

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X = 270

1700=0.1588

Dynamic Axle Loads:

Mfdyn ¿¿ 0.5869)+(0.1588*0.82))*2500 = 1358.315 kg

Braking Force:

BF ¿1358.351*0.82*9.81 = 10926.56 N

FA = 1358.315*9.81*0.4 = 5330.00844 N

Dynamic Axle Load:

Weight Transfer during braking = mfhL

Where

mf – inertial force of the vehicleh – height of CGL – wheelbase

Weight Transfer during braking = 2500∗0.82∗9.81∗270

1700 = 3194.021 N

Dynamic front axle load (W1) = 9810 + 3194.021 = 13004.02174 N

(Contributes 53.02% of the total weight)

Dynamic front axle load per wheel = 6502.011 N

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Dynamic rear axle load (W2) = 14715 - 3194.021 = 11520.979 N

(Contributes 46.98% of the total weight)

Dynamic front axle load per wheel = 5760.4895 N

Brake Torque:

T = µ*Fal*R

Where

µ - Coefficient of friction between friction pad and lining

Fal – Dynamic axle load (N)

R – static laden radius of the tyre (m)

Tfront = 0.6*6502.011*0.270 = 1053.325 Nm

Trear = 0.6*5760.4895*0.250 = 864.0734 Nm

Stopping Distance:

Deceleration a = 0.82g

V=u+at (u=0) and s=ut+12a t 2

At 180 km/hr, Stopping time t = u/a= 6.21 sec Stopping distance s = u2/2a = 155.391 m

Brake Force:

Kinetic Energy = 0.5mv2 = Fs = 0.5*2500*502 = F*155.391

F = 20110.559 N

Braking force required on front axle = 10662.618 N (53.02% of total braking force)

Braking force required on rear axle = 9447.94 N (46.98% of total braking force)

Braking forces required on each of the front and rear wheels are 5331.309 N and 4723.97 N respectively.

Clamp Load:

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C = Fb2µ

Where

Fb – Braking Force

µ - Coefficient of Friction between pad and drum / disc

Cfront = 5331.309

2∗0.6 = 4442.7575 N

Crear = 4723.972∗0.6 = 3936.6417 N

Line Pressure:

Line Pressure created from master cylinder = F PlpηpAmc

= 2000×5×0.851963.5 = 4.2676 MPa =

618.96 psi

where

Leverage ratio of brake pedal lp = 5

Pedal lever efficiency ηp = 0.85

Pedal force Fp = 2000 N

Line Pressure required required for front braking = Clamp Load

Area of wheelcylinder=4442.7575

1963.5=2.2626MPa = 328.07 psi

Line Pressure required required for rear braking = Clamp Load

Area of wheelcylinder=3936.6417

1963.5=2.005MPa = 290.78 psi

During heavy braking:

Deceleration = 2.5g

The axial load is 9737.87 N

Dynamic front axle load W1 = 19547.87 N (81.01%)

Dynamic rear axle load W2 = 4582.32 N (18.99%)

Tfront = 0.6*9773.935*0.270 = 1583.377 Nm

Trear = 0.6*2291.16*0.250 = 343.674 Nm

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Stopping distance s = 50.968 m

Braking forces required on front and rear axles are 16291.56 N and 3818.99 N.

Cfront = 16291.56

2∗0.6 = 13576.3 N

Crear = 3818.992∗0.6 = 3182.2959 N

Pressure from master cylinder = 8.534 MPa = 1237.75 psi

Line pressure required for front braking = 6.914 MPa = 1002.84 psi

Line pressure required for rear braking = 1.62 MPa = 235.0655 psi

Cut-in pressure:

The cut-in pressure (Pc) under fully-laden conditions:

PcA3 = S - K

A3 = 314.16 mm2

For the bias spring,

The coil diameter is 30 mm and the wire diameter is 5 mm.

Assuming modulus of rigidity to be 75 GPa,

Spring stiffness = Gd

8C3n=¿

75000×58×63×5

= 43.402 N/mm

The axial load is 50 N under 2.5g braking.

Displacement = 8PC 3nGd

= 8×150×63×1075000×5

=¿6.912 mm

Force applied to the control piston by the bias spring K = Spring stiffness x displacement = 150 N

For the position sensing spring,

The coil diameter is 20 mm and the wire diameter is 4 mm.

Assuming modulus of rigidity to be 75 GPa,

Spring stiffness = Gd

8C3n=¿

75000×48×53×10

= 30 N/mm

The axial load to the sensing spring is 100×9737.87

2500=389.5148N under 2.5g braking.

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Displacement = 8PC 3nGd

= 8×389.5148×53×1075000×4

=¿12.9838 mm

Force applied to the control piston by the load sensing spring S = 389.5148 N

Cut-in pressure Pc = 389.5148−150

314.16 = 0.76239 MPa = 110.5 psi

Outlet pressure:

The general equilibrium equation of the valve lat pressure above cut-in is:

P1A1 + S - K = P2A2

where

P1 – Inlet pressure

P2 – Outlet pressure

A1 = 700 – 314.16 = 385.84 mm2

A2 = 700 mm2

P2 = (8.534×385.84 )+389.5148−150700 = 5.046 MPa = 731.860 psi

Thus here the decompression ratio is P1:P2 = 1.691:1

Variation of Output pressure with input pressure before and after cut-in:

P1 P2 P1 P20 0.550735 4.25 2.67905

0.25 0.688535 4.5 2.816850.5 0.826335 4.75 2.95465

0.75 0.964135 5 3.092450.7623

90.756679 5.25 3.23025

1 0.88765 5.5 3.368051.25 1.02545 5.75 3.505851.5 1.16325 6 3.64365

1.75 1.30105 6.25 3.781452 1.43885 6.5 3.91925

2.25 1.57665 6.75 4.057052.5 1.71445 7 4.19485

2.75 1.85225 7.25 4.332653 1.99005 7.5 4.47045

3.25 2.12785 7.75 4.608253.5 2.26565 8 4.74605

3.75 2.40345 8.25 4.88385

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4 2.54125 8.5 5.02165

0 12.25 3.5

4.75 67.25 8.5

0

1

2

3

4

5

6

Inlet Pressure (P1) Vs Out-let Pressure (P2)

Outlet Pressure P2 (MPa)

Inle

t Pre

ssur

e P1

(MPa

)

Cut-inpressure

4.1 Calculated fully laden characteristics of the valve

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CHAPTER 5 – DESIGN OF ROTARY SLOT FOR ADJUSTMENT ACCORDING TO RIDE HEIGHT

5.1 Description

The second connecting link, which is connected to the bell crank, is to be connected to a rotary slot. The cap of the rotary slot is designed such that the second connecting rod pivots about the top mount, thus enabling the rotary slot to perform the operation of the bell crank, effectively replacing it. The rotary slot consists of two concentric cylinders, mated by threads on their inner and outer circumferences. A cap mounted to the inner cylinder is free to rotate inside it through a disc-like protrusion. This cap is connected to the inner cylinder by means of two bolts. It is to the top of this cap that the second connecting rod is bolted.

5.2 Function

When ride height is adjusted at the shock absorbers by any means (shims, etc.), the LCRV is usually adjusted by removing the bell crank, and mounting it higher up by installing a new plate. In this assembly, the adjustment is made by screwing the inner cylinder into the outer one. The procedure is to remove the two bolts connecting the cap to the inner cylinder, and the bolt that fixes the inner cylinder to the outer one. Consequently the inner cylinder becomes free to screw inside the outer one, and thus the adjustment is made. The pitch of the threads has been designed to be 1 mm, so that adjustments of accuracy 1 mm can be made. Thus the leverage is altered.

F

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5.1 The assembly with the rotary slot

5.2 Sectional view of the rotary slot

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5.3 Cylinder Top Drawing

5.4 Inner Cylinder Drawing

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5.5 Outer Cylinder Drawing

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5.1 FINITE ELEMENT ANALYSIS

On cylinder top,

Stiffness of bias spring = 43.402 N/mm

Stiffness of sensing spring = 30 N/mm

Stiffness of spring controlling ball valve = = Gd

8C3n=¿

75000×28×53×4

= 37.5 N/mm

Force acting on the top of the cylinder cap is calculated to be 1032 N at the two bolts.

5.6 Static nodal stress – cylinder top cap

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5.7 Static displacement analysis – cylinder top cap

CHAPTER 6 – CONCLUSIONThus the construction and working principles of a Load Conscious Pressure

Reducing Valve system have been studied, suitable calculations for inlet and outlet

pressures that will approximate the behaviour of the system have been performed,

and the modification proposed to the system that will accommodate manual changes

in vehicle ride height has been exposited upon, based on the knowledge gained of

the concerned component through the visit to TAFE ACCESS LIMITED, TRICHY

ROAD, COIMBATORE. A comprehensive survey of literature available in the public

domain was done and the advantages of a newly designed modification to adjust for

variations in ride height have been expounded.

6.1 REFERENCES:

1. Ssangyong Rexton service manual2. United States Patent “Vehicle with load conscious brake pressure reducing

valve” by Glyn. P. Farr, patent number 4,669,788.3. United States Patent “Load sensing proportioning valve for brake system” by

Cole et al., patent number 4,986,6094. Brake line pressure calculations – BAJA BHAIS and PEGASUS RACING.5. Design of Machine Elements by V. B. Bhandari

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