VARIABLE DAMPING FORCE SHOCK ABSORBER · of the shock absorber. The fluid can pass through both piston nozzles and through grooves for average shocks. Once the piston gets past the

VARIABLE DAMPING FORCE SHOCK

ABSORBER D.Mohankumar1, R. Sabarish1, Dr. M. PremJeyaKumar2.

1 Research Scholar, Department of Automobile engineering, BIST, BIHER, Bharath University, Chennai.

2 Supervisor, Department of Automobile engineering, BIST, BIHER, Bharath University, Chennai.

[email protected], [email protected], [email protected],

Abstract: The automobile chassis is mounted on the axles, not

directly but through some form of springs. This is done to isolate the

vehicle body from the road shocks which may be in the form of

bounce, pitch, roll or sway. These tendencies give rise to an

uncomfortable ride and cause additional stress in the automobile

frame and body. All the parts which perform the function of isolating

the automobile from the road shocks are collectively called a

suspension system. It includes the springing device used and various

mounting. The present shock absorber damping force will remain

same for shocks of different magnitude. The result of using the

present shock absorber is more vibration for shocks of smaller

magnitude, which result in lesser comfort for the passengers. The aim

of the project is to provide better comfort for the passengers, when the

vehicle experiencing shocks. The comfortability can be increased by

varying the damping force. In order to achieve better comfort, shock

absorber damping is increased by having grooves in the inner cylinder

of the shock absorber. The fluid can pass through both piston nozzles

and through grooves for average shocks. Once the piston gets past the

grooves, the shock absorber will behave like a conventional shock

absorber. Since the force experienced by a piston varies, the name has

given as variable damping force shock absorber. This result in a

cushioning of average shocks and at the same time because of friction

experienced by a fluid in grooves, damping of vibrations will be faster.

The analytical solution has been given for the vehicle. The cutting of

grooves indirectly reduces the unsprung weight of the vehicle which

also result in a reduced vibrations. The testing of the modified shock

absorber shown better results while, comparing with the present shock

absorber.

I INTRODUCTION

Inventions are made to ease the effort of

people. Among the inventions, the most important in the last

century is a vehicle. In order to ease the road transport it has

been invented. Since its invention lot of changes has been

made to increase the comfort of passengers. A vehicle contains

lot of systems to add comfort to the passengers. The vital one

is a suspension system, which is used to connect chassis and

the vehicle wheels. A suspension system is used to suspend the

chassis from responding to road irregularities. In the

suspension system the most important one is damper and

helical spring[1-4].

A lot of research has been going on over optimization of

suspension system. Each researcher has their own ideas in

enhancing the performance. A.M.A. Soliman proposed an

adaptation algorithm to maintain optimal performance over the

wide range of input conditions typically encountered by a

vehicle. Richard van kasteel, proposed a new shock absorber

model with an application in vehicle dynamics. Peter Holen

and Boris Thorvald given an analytic expression with

simulations of a 3-d truck model to study roll and bounce

damping for heavy vehicles to illustrate the limits in

performance resulting from the choice of dampers and

mounting positions.

The damper is used to absorb shocks, when vehicles run over a

pit or irregular surface. This is done to avoid fatigue to the

passengers. The energy of road shocks causes the spring to

Oscillate[5-9]. These oscillations are restricted to a reasonable

level by a damper, which is more commonly called a shock

absorber.

Objects of the suspension are:

a. To prevent road shocks from being transmitted to the

vehicle components.

b. To safeguard the occupants from road shocks.

c. To preserve the stability of the vehicle in pitching or

rolling in motion.

Nowadays, the most used damper is twin tube shock absorber.

To modify the present damper, description and working should

be known. So it has been given below with the dimensions.

II PRINCIPLE BEHIND SHOCK ABSORBER

Dampers / Shock absorber are designed to work in

concert with the spring to keep the tyre contact patch on the

racing surface. In bump, the damper compresses to help

control the wheel travel and prevent "overshoot", and in

rebound, the damper helps absorb the energy stored in the

spring.

Good damper control is the most significant contributor to the

"mechanical grip" we hear so much about. Mechanical grip is

all about keeping the tyre patch in contact with the racing

surface with as little excitation as possible. It is the task of the

damper to dissipate that excitement. Thus Shock absorber can

be better called as an energy-absorbing device that works on

the conversion of energy principle for stopping moving load

with minimum load rebound and shock to the load and to

surrounding equipment. To stop a moving load smoothly, is

necessary in motion control. Different types of instruments like

rubber snubber, a compression spring, and a dashpot is used

for stopping the moving load. These instruments accomplished

their tasks by absorbing energy[10-15].

In spring and snubber, energy is stored and when they are

compressed the energy is released thereby resulting in a

rebound. In a dash pot on the other hand if a force acts against

International Journal of Pure and Applied MathematicsVolume 119 No. 7 2018, 2241-2251ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

2241

the piston, it encounters high resistance from the fluid at the

beginning of the stroke, then much less as the piston retracts.

However there is a limitation in working of spring, snubber

and dashpots. These instruments do not dissipate the energy

uniformly. The energy is transferred to he load uniformly only

in the case of shock absorber. Take the case when in all the

above mentioned instruments (snubber, dashpots, springs and

shock absorber) the same amount of kinetic energy is

absorbed. In this situation the energy will be dissipated at

differing rates[16-20].

The kinetic energy of the load is converted into heat by the

Shock absorbers which is transferred into the atmosphere.

There is no rebound in shock absorbers. The potentially

dangerous shocks are prevented from reaching to equipment.

The design of a normal shock absorber is quite simple to

understand. Generally speaking, a shock absorber contains

double-walled cylinder. There is a space between the

concentric inner and outer walls, a piston, some means of

mechanical return for the piston, and a mounting plate. The

piston can be mounted externally around the piston rod or

internally on the inside of the cylinder body. In inner cylinder

wall many orifices are drilled. The cylinder contains the fluid

which is devoid of air as the bubbles may reduce the efficiency

of the shock absorber. The movement of the piston inside,

forces the fluid through the orifices in the inner cylinder wall.

The orifice is closed as the the piston retracts thereby reducing

the effective metering area, and maintaining a uniform

deceleration force as the load loses its energy.

The pressure of the fluid remains constant which provides

constant resistance to the load. Since the kinetic energy of the

load becomes zero, the load slows to a stop. Also as the shock

absorber stores no energy, there is no rebound. The shock

absorber returns to its position after the load is removed. The

piston is pushed by the spring outward and open a check valve.

This permits the flow of fluid from behind the piston to the

space the piston was in its retracted position.

While mounting care must be taken to to bolt the shock

absorbers to a non-flexing mounting structure. External stop is

also necessary for providing a firm positioning point, and for

preventing the shock absorber piston from bottoming out at the

end of its deceleration stroke. Usually an external stop is

required to prevent damage both to their product and to the

user's equipment. Shock absorber can be mounted through a

drilled hole. The mounting can be secured by using stop collar.

Shock absorbers work on the principle of fluid displacement as

you consider them a working piston, having hydraulic fluid in

it. The hydraulic fluid in the piston, is forced through tiny

holes -which are called 'Orifices'- in the piston as the

suspension travels through jounce and rebound. However, the

orifices let only a small amount of fluid through the piston,

which in turn slows down spring and suspension movement.

Shock absorbers are velocity sensitive hydraulic damping

devices, meaning the faster the suspension moves, the more

resistance the shock absorbers provide. Because of this feature,

shock absorbers adjust to road conditions. As a result, shock

absorbers reduce bounce, roll or sway, brake dive and

acceleration squad

The basic principle of a shock absorber is that as the unit

compresses or rebounds, valves within the oil-filled tube

restrict the flow of oil to reduce the movement of the piston.

This reduces oscillation of the road spring, keeping the tyre in

contact with the road and improving ride comfort.

Monotube and twin-tube shock absorbers perform the same

tasks but differ in design.

A monotube gas shock is filled with oil and gas at

25-30 bar pressure, and a movable separator piston separates

the two substances. A piston valve attached to the piston rod

controls oil flow and damping effect[21-26].

Monotube shock absorber schematic

A twin-tube shock absorber has two concentric

chambers: the oil-filled working chamber housing the piston

rod and piston valve; the compensation chamber formed of the

space between the working cylinder and the outer tube; this is

filled with two-thirds oil and one third air. In a gas-pressurised

shock, gas at 6-8 bar pressure replaces the air. The piston valve

and a valve in the base of the working chamber control oil

flow and damping effect[27-31].

International Journal of Pure and Applied Mathematics Special Issue

2242

Left: Twin-tube shock absorber schematic

Twin-tube shock absorber with control groove/hydraulic

bypass

III TWIN TUBE-DAMPER

A.Principle

In hydraulic dampers, pumping fluid through an

orifice converts energy to heat which can then be dissipated

into atmosphere. The objective in a damper valve design is to

maintain consistent laminar flow characteristics through

operating range of loads.

B.Description

The outer tube is connected at the bottom of the axle

or suspension member with the help of an eye. The inner tube

has an end blank at the bottom. It acts as a non- return valve. It

allows the oil to pass from the cylinder to reservoir during

rebound stroke. The compression disc, washer, orifice disc,

conical springs are all riveted to the end blank. The upper part

has a dirt excluder, a bearing with an oil return channel within

it, a seal for piston rod and piston have two non-return valves

with an eye welded to upper end of the piston rod. The

cylinder is fully filled with hydraulic fluid while reservoir is

partially filled. The piston rods are chrome plated and super

finished for improved wear corrosion resistance. The piston is

of sintered iron which has got good self lubricating properties

and reduces wear due to friction[32-36].

C.Working

When the piston descends, causing the central

rebound valve to close and the piston bump valve opens. So

the fluid is transferred from the lower to upper cylinder


2243

chamber. At the same time, the outer base rebound valve

closes and the central base bump valve opens, displacing a

quantity of fluid to the outer reservoir. The flow of fluid

through the orifice provides the necessary damping force.

Fig 1 Schematic representation of a Damper

During rebound stroke, the piston is pushed up

towards the cylinder. The fluid above the piston passes through

to lower part of cylinder as rebound valve opened. Due to

volume occupied by the piston rod, there is not enough fluid

above the piston to completely fill the volume of the cylinder

below the piston. Hence the lower portion of the cylinder

develops the slight vacuum and extra fluid flow from the

reservoir to lower portion of the cylinder. This happens only

when foot valve opens.

In this way, the shock absorbers successfully perform two

main functions. They are

1. To control quick bouncing of wheels on road

surface.

2. To control slow bouncing of the body on the

suspension springs.

IV DESIGN OF VARIABLE DAMPING FORCE SHOCK

ABSORBER

To vary the dampers damping characteristics grooves

can be cut in the inner cylinder of damper.

A.Determination of shape and size of grooves

To increase the comfort of passengers for average

shocks of smaller magnitude, nature and dimensions of

grooves are to be determined. So, the grooves can be easily

machinable. These constraints lead to v-groove.

Apart from these, there are some more constraints like

a. Inner cylinder thickness (of1mm.)

b. Depth and width of grooves.

c. Position of grooves.

The above three constraints will directly affect the working of

damper. On taking the account of the first two, three v-

grooves of 0.5x0.5x25 are taken. Since the grooves should be

placed without affecting the strength of the damper cylinder.

So the grooves are placed at an angle of 120 degree difference.

The sectioned view of the modified damper is shown in the

figure below.

Fig 2 Schematic representation of Modified Damper

To design the variable damping force shock absorber

the following assumptions are made through the guidelines of

EUROPEAN SHOCK ABSORBER ASSOCIATION.

Design procedure consist of following determination

Determination of Diameter of the piston.

Determination of Diameter of the rod.

Determination of the Length of the cylinder.

Determination of Outer diameter of the shock

absorber.

Determination of working Temperature of the fluid.

Determination of Damping forces

During Rebound

During Compression

Determination of damping co-efficient.

Conventional shock absorber

Variable damping force shock absorber

Assumptions taken for the design are

Piston velocity = 1m/s

Density of fluid (Turbine oil) = 9000N/m3

Rebound pressure = 5 MPa

Compression pressure = 1.25MPa

Ambient temperature = 30oC

Heat transfer co-efficient = 60 W/m2K

Flow co- efficient = 0.5

Flow, Fc = d *10-6 m2

DESIGN CALCULATION

Diameter of the piston (dp)

According to EuSAMA (European Shock Absorber

Manufactures Association)

the standard piston diameters for light vehicles are:

18 mm

22 mm

27 mm

30 mm

Here we have selected a piston of diameter, dp = 30 mm.

Diameter of the rod (dr) The diameter of the rod, dr = (3 to 7)* dp


2244

The diameter of the rod, dr = 0.4* 30

The diameter of the rod, dr = 12 mm.

Outer diameter of shock absorber (D) (Dust cover) D0.3 = (3.4[V0.7 (air)])/ Kt

D = (3.4 *[(60 * 1000)/3600)0.7])/60

Diameter (D) = 0.0498m.

Diameter (D) = 50mm.

Chosen viscosity of the fluid The viscosity of the fluid with temperature variation is given

below.

Table 1: Variation of viscosity with respect

to Temperature

S.NO Temperature

(oC)

Time

(Seconds)

Kinematic viscosity(Cs)

1. 90 32 6.8

2. 80 47 10.0

3. 70 54 11.5

4. 60 62 13.0

Determination of design Temperature of working fluid N/427 = Kt *SO*(T – Ta)

T = Ta+[N/427 *Kt* So]

= 30+[fmax*Vp/427*Kt*dp*π]

Temperature, T = 360C

Determination of design compression force – F(r)

Fc = [Ap – Ar]* Pr

= π/4[dp2 – dr2]*Pr

Fc = 2388N

Determination of design rebound force – F( c ) Fr = Ap* Pr

= π/4[ dp2]*Pr

Fr = 716N

Determination of damping coefficient

(conventional)

Conventional

Compression Kc =

Kc = 2092.92 N-s/m

Rebound Kr =

Kr = 7276 N-s/m

Determination of damping coefficient (variable)

Compression

Where, Fc = Fc1+ (b*d)

b = 5*10-3m.

d = 0.5 *10-3m.

Fc = 30*10-6 m2

Kc =

Kc = 1932 N-s/m

Rebound

Fr = Fr1+ (b*d)

b = 5*10-3m.

d = 0.5 *10-3m.

Fr = 12*10-6 m2

Kr =

Kr = 7136 N-s/m

Fig3 Schematic arrangement of set up

The experimental set up for testing the shock absorber

is shown in figure4.1 consists of frame connected to the axle

through damper, an AC motor coupled to the axle and a

variable transformer to maintain speed of motor.

MOTOR

The motor used for the project is a three phase

induction type, 7.5 HP motor with 1500 rpm. It is used to

transmit rotary motion to the wheels of the set up for

simulation.

VARIABLE TRANSFORMER

Variable Transformer is used to maintain constant

power supply to a motor to maintain constant speed. It consists

of primary and secondary coils. Primary coils are connected to

230V regular power supply and the secondary coils are

stepping up the voltage to run the motor. The multiplication

factor of 60 is maintained.

VIBRATION ANALYZER

It is used to measure and analyze the vibration

characteristics like displacement, acceleration and velocity

with respect to the frequency of it. The sensing material is a

piezo-electric transducer, which will send electric signal to the

instrument after sensing vibrations.

EXPERIMENTAL PROCEDURE

The experimental set up is shown in the above figure

6.1. The power supply is given to the variable transformer and

multiplication factor is increased still the motor speed is

constant to take on the load of set up. Using vibration analyzer

the readings are taken for various loads and the displacement,

velocity and acceleration are noted down.

From the experimental data bound damping co-efficient and

rebound damping co-efficient are calculated. Graphs are drawn

by substituting the values in the proposed model equation.

g

P

Fc

CpA2

42

gFr

AA RP

2

Pr(4 )2

gFc

Ap

2

Pr42

gFr

AA RP

2

Pr(4 )2


2245

CALCULATION OF DAMPING CO-EFFICIENT Based on the experimental set up, model has given

below to find the damping co-efficient through the principle of

forced damping.

Fig-4 schematic representation for calculating damping co-

efficient

By Newton law

ctedcanbenegleC

XXCXXKXCXKXMMg

XM

XXCXXKXCXKXMFF

XCXKXMF

2

12112122222221

121121222222221

1111111

4

Where,

M1 = sprung Mass of the vehicle (Kg)

C1 = Damping co-efficient (N-s/m)

K1 = Stiffness of spring (N/m)

By knowing the above parameters of sprung and

unsprung masses we can calculate the damping co-efficient of

the damper using the above equation (i).

RESULTS AND DISCUSSIONS

In this chapter the experimental readings are tabulated,

graphs are drawn using these data and the discussions are

carried out for the obtained results.

DISPLACEMENT

Displacement of the damper found to be increasing

during rebound and decreasing during bound for various loads.

VELOCITY

Velocity of the damper found to be increasing during

rebound and decreasingduring bound for various loads.

ACCELERATION

Acceleration of the damper found to be increasing

during rebound and decreasing during bound for various loads.

REBOUND DAMPING CO-EFFICIENT

Rebound Damping co-efficient of the damper find to

be increasing during rebound with increase in loads.

Fig. 2 shows the variation of Rebound Damping co-efficient

with respect to different frequency for both present and

modified shock absorber at no load. The maximum rebound

damping co-efficient is of present shock absorber is 1819N-

s/m. where as for modified damper is1800.90 N-s/m.




damping co-efficient is of present shock absorber and it is

2601.50N-s/m. where as for modified damper is 2550.85N-s/m.










5512.75 N-s/m. where as for modified damper is 5462.80N-

s/m.

BOUND DAMPING CO-EFFICIENT

Bound Damping co-efficient of the damper found to

be increasing during bound with increase in loads.

Fig. 1 shows the variation of bound Damping co-efficient with

respect to different frequency for both present and modified

shock absorber at no load. The maximum bound damping co-

efficient is of present shock absorber and it is 1790.30N-s/m.

where as for modified damper is 1787.89N-s/m.

Fig. 3 shows the variation of bound Damping co-

efficient with respect to different frequency for both present

and modified shock absorber at no load. The maximum bound












2888.20N-s/m. where as for modified damper is 2861.19 N-

s/m.

GRAPHS

The readings are noted down while testing the conventional &

variable shock absorber for different weights and by using

these readings, graphs are plotted with bound & rebound

damping co-efficient in y-axis.

)(2

)(1

)(21

)(2

)(2

)/(2

)/(2

)(1

)(1

)/(1

)/(1

2

2

KgssunsprungmaM

KgsprungmassM

NtiongroundreacF

NshockforceduetoF

mofthebodyntDisplacemeX

smthebodyVelocityofX

smyonofthebodAcceleratiX

NmasstheappliedforceduetoF

mofthebodyntDisplacemeX

smthebodyVelocityofX

smyonofthebodAcceleratiX


2246

FOR No Load

At 25Km/hr

Fig5

Fig6

FOR 5 Kg

At 25Km/hr

Fig7

Fig8

FOR 10 Kg

At 25Km/hr

Fig9

Fig10

FOR 15 Kg

At 25Km/hr

Rebound

Damp co-eff. Vs Frequency

0

500

1000

1500

2000

2500

3000

3500

4000

4500

2 4 6 8 10

Frequency(hertz)

Da

mp

co

-eff

. (N

-s/m

)

Present shock

absorber

Modified shock

absorber

Bound

Damp.co-eff. Vs Frequency

0

500

1000

1500

2000

2500

3000

12 14 16 18 20

Frequency(hertz)

Da

mp

.co

-eff

.(N

-s/m

)

Present shock

absorber

Modified shock

absorber

Rebound

Damp.coeff Vs Frequency

0

500

1000

1500

2000

2500

3000

2 4 6 8 10

Frequency(hertz)

Da

mp

.co

eff

.(N

-s/m

)

Present shock

absorber

Modified shock

absorber

Bound

Damp.co-eff Vs Frequency

0

500

1000

1500

2000

2500

12 14 16 18 20

Frequency(hertz)

Da

mp

.co

-eff

.(N

-s/m

)

Present shock

absorber

Modified shock

absorber

Bound

Damp.coeff Vs Frequency

0

200

400

600

800

1000

1200

1400

1600

1800

2000

12 14 16 18

Frequency(hertz)

Da

mp

.co

eff

.(N

-s/m

)

Present shock

absorber

Modified shock

absorber


2247

Fig11

Fig11

Fig12

Fig.-5, Fig-7, Fig-9, Fig-11 shows the variation of

Bound Damping co-efficient with respect to frequency for 0, 5,

10, 15 Kg in order for both present & modified damper.Bound

Damping co-efficient for a modified damper found to be less

when compared to the present one.

Fig.-6, Fig-8, Fig-10, Fig-12 shows the variation of

Rebound Damping co-efficient with respect to frequency for 0,

5, 10, 15 Kg in order for both present & modified damper.Re

Bound Damping co-efficient for a modified damper found to

be less when compared to the present one.

CONCLUSION& FUTUREWORK

The modified damper has more displacement, less

compression damping co-efficient& less rebound damping co-

efficient when compared to the present damper for average

shocks. It’s all due to reduced unsprung mass, friction of

grooves, increased oil passages and increased heat dissipation

due to the machined grooves. The modified damper provides

better comfort, stability to the vehicle with reduced vibrations.

Further modification can be done by drilling holes

circumferentially to vary the damping co-efficient.

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Rebound

Damp. co-eff. Vs Frequency

0

1000

2000

3000

4000

5000

6000

2 4 6 8 10

frequency(hertz)

da

mp

. c

o-e

ff.

(N-s

/m)

Present shock

absorber

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absorber

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Damp. co-eff. Vs Frequency

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frequency(hertz)

da

mp

. c

o-e

ff.

(N-s

/m)

Present shock

absorber

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absorber

Bound

Damp.co-eff. Vs Frequency

0

500

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1500

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frequency(hertz)

da

mp

.co

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.(N

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)

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absorber

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absorber


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