Szybkobieżne Pojazdy Gąsienicowe (39) nr 1, 2016

Piotr WOCKA, Stanisław TOMASZEWSKI - Ośrodek Badawczo – Rozwojowy Urządzeń Mechanicznych

OBRUM sp. z o.o., Gliwice

Piotr WOCKA

Stanisław TOMASZEWSKI

HYDROPNEUMATIC SUSPENSION OF A TRACKED VEHICLE WITH

FRICTION DAMPERS

Abstract. The paper presents the concept of a hydropneumatic suspension of a tracked vehicle based on

the hydropneumatic cylinder arrangement and friction damper integrated with the suspension arm holder. The

purpose, configuration, design of single suspension unit and the kinematics and design concept of the suspension

system of a tracked vehicle are discussed.

Keywords: suspension, damper, hydropneumatic, tracks, toad wheels, traction system.

1. INTRODUCTION

The technology of hydropneumatic components in the suspension systems of wheeled

and tracked vehicles has been used and developed for several decades. The basic principle of

hydropneumatic components, where the role of the spring element is fulfilled by compressed

gas separated from the oil-filled space, has never changed. The main progress in the field of

vehicle suspension with hydropneumatic components is reflected in improved durability,

extended range of control and installation options. In military vehicles, including tracked

vehicles, hydropneumatic suspension components were introduced to improve shock

absorption and traction properties, especially on dirt roads, rough roads and off road. In tracked

combat vehicles the hydropneumatic suspension components are also an important part of the

vehicle that absorb forces during firing from medium and large calibre guns, cannons. Recoil

forces generated during the shot are transferred by gun trunnions, onto the hull, chassis, and

finally the suspension of the vehicle. These forces should be damped in the most efficient way

to reduce stresses transferred onto the vehicle structure and to ensure vehicle stability, which is

important for the accuracy and effectiveness of the conducted fire.

Today, in the era of modernization of equipment of the Polish Armed Forces and in

view of the lack of available domestic solutions, it is necessary to acquire or develop units,

components, systems for suspensions of tracked vehicles, in particular for suspensions with

hydropneumatic components. This paper presents the concept of a hydropneumatic suspension

of a tracked vehicle based on the hydropneumatic cylinder arrangement and friction damper

integrated with the suspension arm holder.

2. CONCEPT OF HYDROPNEUMATIC SUSPENSION WITH A FRICTION

DAMPER

Piotr WOCKA, Stanisław TOMASZEWSKI

A suspension system according to the solution concept is based on the suspension arm

assembly integrated with a friction damper and an external hydropneumatic cylinder. Both

these components form a single suspension unit.

The suspension arm assembly with the integrated friction damper is installed on the

outside to side hull skirt. The space inside the hull is occupied by the friction damper assembly.

The damper, positioned along the axis of rotation of the suspension arm, includes a pack of

discs with friction lining and an internal control unit, which changes and adapts the damping

force in relation to the movement of the suspension arm by changing the clamping force in the

disc pack and the resulting change in friction force. In addition, the damper control of the

damper enables connecting an external control system, which may change the characteristics of

damping and enables locking the rotation of the suspension arm, thereby blocking the

movement of the suspension.

Fig. 1. OBRUM's hydropneumatic suspension unit

Fig. 2. OBRUM's hydropneumatic suspension unit

Hydropneumatic suspension of a tracked vehicle with friction dampers

Fig. 3. Installation of OBRUM's hydropneumatic suspension system

Table 1. Parameters of the hydropneumatic suspension system with friction damping

Parameter Manufacturer

OBRUM

Type HYDROPNEUMATIC

SUSPENSION

Product status (manufacture) R&D work (Poland)

Scope of supply Complete system

Type Hydropneumatic

Configuration Separated

Installation External

Damping method Friction

Range of vehicle clearance

adjustment in the vertical plane (mm) +196 / -255

Weight of 1 unit (kg) ~200

Load on 1 unit (kg) ~3500

The number of suspension units in a tracked vehicle depends on the weight of the

vehicle and on traction requirements specified for the given vehicle. The suspension system

works together with the track tensioning system. 5, 6 or 7 suspension units may be used in

tracked vehicles weighing 35 to 50 tonnes. Hydropneumatic cylinder, as the basic elastic

element of the suspension, ensures elastic suspension of road wheels. In addition it enables

adjusting the clearance and longitudinal/transverse angle of the hull in relation to nominal

Piotr WOCKA, Stanisław TOMASZEWSKI

position by means of a hydraulic control and feed system. The hydropneumatic cylinders

installed in the suspension system enable changing the position of suspension arms, and thereby

the angle of longitudinal/transverse position of the vehicle, and the clearance of the vehicle.

This has a direct effect on the ability to negotiate vertical walls and on the traction properties of

the vehicle. The friction dampers enable changing the characteristics of suspension behaviour

by changing damping and enable locking the suspension in any position.

3. BASIC ASSUMPTIONS

The following values were adopted for calculations:

vehicle weight M = 35 t,

number of suspension units n = 12,

clearance 450 mm.

The elastic element used was a gas spring placed in the cylinder assembly and

functionally linked with the friction damper installed along the suspension arm. In the adopted

system the average load on one wheel in the normal (static) position is:

(1)

The figure below shows the adopted setting parameters of the suspension unit in the

normal (static) position.

HYDRAULIC CHAMBER

GASCHAMBER

Fig. 4. Suspension in the normal (static) position

Hydropneumatic suspension of a tracked vehicle with friction dampers

4. COMPUTATIONAL ANALYSIS OF THE GAS SPRING

In the computational analysis two cases related to the position of the floating piston

relative to the surface A were adopted.

4.1. Case 1

In this case, in the lower position of the suspension unit, the hydraulic chamber of the

cylinder is filled with oil and the floating piston separates the gas chamber from the hydraulic

chamber. The piston is not moved away from surface A. The gas chamber is filled with gas

under the pressure of 16 MPa. The position is shown in the drawing.

FLOATING PISTON

Fig. 5. Suspension in the lower position

Assumptions:

diameter of piston in the cylinder (hydraulic part): d1 = 80 mm,

diameter of piston in the cylinder (gas part): d2 = 100 mm,

diameter of piston rod in the cylinder (hydraulic part): d3 = 60 mm.

The effective surface area of the piston, equal to the difference between 80 mm and 60

mm sections:

(2)

Denoting the force acting on the piston of the gas spring as Rs and the vertical force

acting on the wheel as Rk, static pressure in the cylinder is calculated:

Piotr WOCKA, Stanisław TOMASZEWSKI

(3)

In the lower position the suspension arm is in a position where the piston rod is fully

extended, as shown in the drawing below.

Fig. 6. Suspension in the lower position

In the static position the volume of the gas chamber is decreased by the volume of liquid

displaced from the hydraulic chamber as the wheel moves from the lower position to the static

position.

Volume of the displaced liquid (decreased volume of the gas chamber):

(4)

Next the translation of the floating piston is calculated:

(5)

When the volumes of gas in static and lower positions and the required static pressure

are known, the pressure in the lower position can be calculated,

Volume of gas chamber in lower position (maximum value):

Hydropneumatic suspension of a tracked vehicle with friction dampers

(6)

Volume of gas chamber in normal (static) position

(7)

The relationship between the two states in an adiabatic process is described by:

. (8)

ϗ - adiabatic index, adopted value 1.4

Minimum pressure pmin in the lower position is calculated.

(9)

The force system in the cylinder and the vertical force acting on the wheel were

calculated based on the calculated values and the developed 3D geometric model of the

suspension using Solid Works Motion software.

The drawings below show a conceptual geometric 3D model adopted in the calculations

and the force diagrams.

Fig. 7. Conceptual model of the suspension unit

DRIVE

FORCE

REACTION FORCE

Piotr WOCKA, Stanisław TOMASZEWSKI

Fig. 8. Diagram of the changing force acting on the wheel

Fig. 9. Diagram of the changing reaction force of the cylinder

For a given initial pressure pmin, characteristics of differing slope are obtained by

changing the volume of the chamber.

4.2. Case 2

In this case the hydraulic chamber is filled so that the floating piston is moved away

from surface A. Three distances between the piston and surface A were adopted: 20 mm, 40

mm and 60 mm. In every case the gas chamber is filled with gas under the pressure of 16 MPa.

a. 20 mm distance

Hydropneumatic suspension of a tracked vehicle with friction dampers

Fig. 10. Suspension in the lower position

(distance L=20mm)

Fig. 11. Vertical force on wheel (distance L=20mm)

Fig. 2. Cylinder force (distance L=20mm)

b. 40 mm distance

Piotr WOCKA, Stanisław TOMASZEWSKI

Fig. 13. Vertical force on wheel (distance L=40mm)

Fig. 14. Cylinder force (distance L=40mm)

c. 60 mm distance

Fig. 15. Vertical force on wheel (distance L=60mm)

Fig. 16. Cylinder force (distance L=60mm)

The values calculated for case 1 (piston to surface distance = 0 mm) and case 2 (distance

20 mm, 40 mm and 60 mm) are given in Table 2.

Hydropneumatic suspension of a tracked vehicle with friction dampers

In all cases the gas filling pressure is the same and equal to pmin =16 MPa, and the

vertical force acting on the wheel in the normal (static) position is the same and equal to Pkstat

=29,166 N.

Table 2. Hydropneumatic cylinder specifications

Case

Piston to

surface

distance

(mm)

pmin

(MPa)

Pkstat

(N) pmax

(MPa)

pkmin

(N)

pkmax

(N)

pmax /

pmin

pkmax /

pkmin

1 0 16 29166 32 21171 73683 2 2.52

2

20 16 29166 34.5 21169 79699 2.15 2.73

40 16 29166 38.2 21167 88019 2.38 3.01

60 16 29166 43.4 21165 100244 2.71 3.44

5. COMPUTATIONAL ANALYSIS OF THE FRICTION DAMPER

5.1. Version 1

Based on analyses of solutions, the design of the friction damper illustrated in Fig. 14

was adopted.

The active pack consisted of 9 active discs and 10 lined discs. Clamping was effected by

means of 4 coil springs. In order to increase the clamping force it is possible to install 12

springs. Additionally, the clamping force can be increased by pumping oil into the feed

chamber. By changing pressure, it is possible to change the total friction moment of the damper.

Fig. 17. Construction of the friction damper

In the adopted construction design, during the turning of the suspension arm there is a

constant moment of friction, which depends on the set pressure and on the pressure of springs.

The following specifications of the friction damper were adopted:

Piotr WOCKA, Stanisław TOMASZEWSKI

number of friction pairs: z =18,

outer diameter of friction lining: Dz =220 mm,

inner diameter of friction lining: 122 mm,

piston rod diameter: drod = 25 mm,

force of spring under no load (length 25 mm): 118 N/mm,

force of spring under maximum compression of 6.2 mm: 736,1 N.

Based on the inner and outer diameters of the friction discs, the mean radius of friction

can be calculated:

(10)

Then the moment of friction is determined:

(11)

In the nominal position the springs are compressed to the length of 19.7 mm. Based on

the adopted parameters of the spring and deflection of 5 mm, it is possible to calculate the force

of one spring:

(12)

Adopted friction coefficient:

Table 3. Spring specifications and number

Number of

springs μ

Clamping force

P (N)

Moment of friction Mt

(Nm)

Force on

suspension arm Rk

(N)

4 0.11 2360 410 1026

8 0.11 4720 821 2052

12 0.11 7080 1231 3078

When springs are boosted by plungers, their force increases. The tables below list

calculation results with clamping force of springs, moment of friction and force on the

suspension arm perpendicular to arm axis and applied along the wheel axis.

Table 4. Specifications of system with no springs

No springs

P (MPa) P (N) Mt (Nm) Fk (N)

Hydropneumatic suspension of a tracked vehicle with friction dampers

2 3927.0 683.0 1707.5

4 7854.0 1366.0 3415.0

6 11781.0 2049.0 5122.5

8 15708.0 2732.0 6830.0

10 19635.0 3415.0 8537.5

12 23561.9 4098.0 10244.9

14 27488.9 4781.0 11952.4

16 31415.9 5464.0 13659.9

20 39269.9 6830.0 17074.9

25 49087.4 8537.5 21343.6

Table 5. Specifications of system with 4 springs

4 springs

P (MPa) P (N) Mt (Nm) Fk (N)

2 6286.99 1093.45 2733.64

4 10213.98 1776.45 4441.13

6 14140.97 2459.44 6148.62

8 18067.96 3142.44 7856.11

10 21994.95 3825.44 9563.60

12 25921.94 4508.43 11271.10

14 29848.94 5191.43 12978.58

16 33775.93 5874.42 14686.07

20 41629.91 7240.42 18101.05

25 51447.39 8947.91 22369.77

Table 6. Specifications of system with 8 springs

8 springs

P (MPa) P (N) Mt (Nm) Fk (N)

2 8647.0 1503.9 3759.8

4 12574.0 2186.9 5467.3

6 16501.0 2869.9 7174.8

8 20428.0 3552.9 8882.3

10 24355.0 4235.9 10589.7

12 28281.0 4918.9 12297.2

14 32208.0 5601.9 14004.7

16 36135.0 6284.9 15712.2

20 43989.0 7650.9 19127.2

25 53807.4 9358.4 23395.9

Table 7. Specifications of system with 12 springs

12 springs

P (MPa) P (N) Mt (Nm) Fk (N)

2 11007.0 1914.4 4785.9

4 14934.0 2597.4 6493.4

6 18861.0 3280.4 8200.9

Piotr WOCKA, Stanisław TOMASZEWSKI

8 22788.0 3963.4 9908.4

10 26715.0 4646.4 11615.9

12 30641.9 5329.4 13323.4

14 34568.9 6012.4 15030.9

16 38495.9 6695.3 16738.4

20 46349.9 8061.3 20153.3

25 56167.4 9768.8 24422.1

The obtained calculation results indicate that the maximum clamping force is obtained

with a set of 12 springs and under a pressure of 25 MPa.

Calculation of maximum force acting on friction linings.

(13)

Maximum allowable force acting on friction linings:

(14)

Based on the results of calculations made, the following is true:

(15)

5.2. Version 2

The same friction pack was used as in version 1. Instead of the clamping disc, a cylinder

was used. When the suspension arm turns, the cylinder changes its width and presses the

plungers and springs. In this design the moment changes with the angle of suspension arm

deflection. After complete clamping of the springs it is possible to lock the suspension arm by

creating an appropriate pressure in the hydraulic chamber, and also to unlock the suspension

arm by reducing the pressure in the chamber to zero.

Four plungers 32 mm in diameter and four springs 51 mm long in the unloaded state

were used in this version providing the strength of 128 N per millimetre of deflection.

Maximum deflection of the spring is 13 mm under which the strength provided is 1,664 N.

Hydropneumatic suspension of a tracked vehicle with friction dampers

Fig. 18. Friction damper (Version 2)

In this case, when the pressure in the feed chamber is close to zero, and the slack in the

friction pack is eliminated, the moment of friction in the static position is zero.

Upward or downward deflection of the suspension arm causes compression of the

springs by the cylinder dependent on the suspension arm deflection angle. The calculated

values of the friction moment, force acting on the wheel, pressure on the friction pack are given

in the table.

Zero angle corresponds to normal (static) position of the suspension arm (arm turned

downwards by an angle of 17 degrees in relation to horizontal position).

Table 8. Damper specifications with no initial pressure

Initial pressure 0 bar

Suspension arm angle (°)

P (N) Mt (Nm) Fk (N)

44 2560.0 445.2 1113.1

30 1664.0 289.4 723.5

20 1024.0 178.1 445.2

8 256.0 44.5 111.3

4 0.0 0.0 0.0

0 0.0 0.0 0.0

-4 0.0 0.0 0.0

-8 653.4 113.6 284.1

-16 1960.2 340.9 852.3

-25 3430.4 596.6 1491.6

Piotr WOCKA, Stanisław TOMASZEWSKI

Fig. 19. Relationship between friction moment and suspension arm angle (initial pressure

0 bar)

Upon increasing the pressure to 10 bar, springs undergo initial deflection by 6 mm.

When the suspension arm is deflected, the springs are loaded in accordance with the changing

width of the cylinder in proportion to the angular position of the suspension arm.

The calculated values of the friction moment, force acting on the wheel, pressure on the friction

pack are given in the table.

Table 8 Damper specifications with initial pressure of 10 bar

Initial pressure 0 bar

Suspension arm angle (°)

P (N) Mt (Nm) Fk (N)

44 5632.0 979.5 2448.0

30 4736.0 823.7 2059.3

20 4096.0 712.4 1781.0

8 3328.0 578.8 1447.0

4 3072.0 534.3 1335.7

0 3072.0 534.3 1335.7

-4 3072.0 534.3 1335.7

-8 3725.4 647.9 1619.8

-16 5032.2 875.2 2188.1

-25 6502.4 1130.9 2827.3

Hydropneumatic suspension of a tracked vehicle with friction dampers

Fig. 20. Relationship between friction moment and suspension arm position (initial

pressure 10 bar)

Application of initial pressure in the range of 0 to 10 bar changes the characteristics

shown in Fig. 16 by moving the graph upwards until it coincides with the graph shown in Fig.

20.

Higher initial pressures may by applied only when the vehicle is stopped in order to lock

the suspension.

6. SUMMARY AND CONCLUSIONS

Based on the computational analysis and the developed concept and 3D model, design

documentation was drawn up allowing fabrication of one suspension unit (assembly). This

documentation was used to make a functional suspension unit, which was then subjected to

testing.

The results of experimental testing of the suspension unit have confirmed design

presumptions and the possibilities of manufacturing such units.

At a further step of experimental testing the functionality of the suspension unit may be

extended by introducing control effected by varying the pressure in the friction damper unit.

Under real conditions this will allow adjusting the operating characteristics of the suspension to

road conditions.

7. REFERENCES

[1] Burdziński Z. Teoria ruchu pojazdu gąsienicowego. Warsaw, Wydawnictwo

Komunikacji i Łączności WKiŁ, 1972. [2] Szargut J. Termodynamika techniczna. Gliwice, Wydawnictwo Politechniki Śląskiej,

2013. ISBN: 9788378801801.

[3] Walczak J. Wytrzymałość materiałów oraz podstawy teorii sprężystości i plastyczności.

Warsaw, Państwowe Wydawnictwo Naukowe PWN, 1973. [4] Testing Department. Results of suspension unit testing – unpublished texts.