Usage:
Hydraulic System
1. Hydraulic Pump - small cartridge
2. Hydraulic Pump - large cartridge
3. Suspension Control Valve
4. Front Suspension Cylinders
5. Accumulator
6. Hoist Control Valve
7. Steering Control Valve
8. Restrictor
9. Flow Control Valves
10. Bleed Valves
11. Suspension Cylinders - centre axle
12. Suspension Cylinders - rear axle
13. Restrictors
Front and Rear Suspension System Schematic - Early Arrangement (Factory fit to DJB serial numbers 6102-6969 and 6979)
The front suspension is made up of a control valve (3), accumulator (5) and suspension cylinders (4). Oil is supplied to the system from the small cartridge of the hydraulic pump (1). When the suspension control valve (3) is in the spring centered mid-position ("level ride") oil flows through the valve to the hoist control valve (6). Oil in the front suspension cylinders (4) and accumulator (5) is trapped.
When the valve is in the "raise" position oil flows past integral check valve (3a) to the front suspension cylinders.
When the valve is in the "lower" position oil in the front suspension cylinders flows to tank.
The position of the suspension control valve is determined by the flow of pilot air from the suspension rotary valve (see later). If the suspension is too high or too low pilot air flows to the suspension control valve and the spool is offset into the "lower" or "raise" position as appropriate. The early arrangement has restrictors (8) and (13) which allow the suspension to rise or fall at a controlled rate, thus preventing overshoot.
1. Hydraulic Pump - small cartridge
2. Hydraulic Pump - large cartridge
3. Suspension Control Valve
4. Front Suspension Cylinders
5. Accumulator
6. Hoist Control Valve
7. Steering Control Valve
9. Flow Control Valves
10. Bleed Valves
11. Suspension Cylinders - centre axle
12. Suspension Cylinders - rear axle
14. Restrictor/Check Valve Assembly
15. Restrictor/Check Valve Assembly
Front and Rear Suspension System Schematic - Later Arrangement (Factory fit to DJB serial numbers 6970-6978, 6980-7611 and Caterpillar Product Identification Numbers 8XC1-Up, IMD1-Up)
The later arrangement has two restrictor/check valve assemblies (14 & 15) which replace the fixed restrictors used in the early arrangement. When the suspension control valve (3) is in the "raise" position restrictor/check valves (14 and 15) allow full flow into the cylinders (4) and accumulator (5). When in the "lower" position flow is restricted by the restrictor/check valves (14 & 15). In this way the suspension lowers under control without slowing down the "raise" operation. Restrictor/check valve (14) is externally mounted while restrictor/check valve (15) is housed in the accumulator end cap.
The front suspension cylinders (4) are connected to the accumulator (5) and the control valve (3). The accumulator contains a precharged nitrogen filled chamber. The compressibility of the nitrogen provides the necessary "spring". The suspension control valve is equipped with an integral relief valve (3b) to limit the circuit supply pressure.
The suspension cylinder piston rods are fixed to the front axle mounting brackets by pivot pins and the cylinders themselves are fixed to the frame by pivot pins. The front axle is mounted in a suspension frame which pivots about the main frame.
The three positions of the suspension control valve spool are:
- 1. Level Ride
- 2. Raise
- 3. Lower
- 2. Raise
Level Ride
The control valve spool is spring centered and pump oil is directed to the hoist control valve (6). Oil in the suspension cylinders (4) is blocked by the spool.
When the front wheels hit a bump the piston rods move into the cylinders forcing oil into the accumulator. The nitrogen in the accumulator is compressed and its pressure rises. When the disturbing force is removed from the front wheels the nitrogen expands and pushes oil back into the cylinders.
When the front wheels fall into a hole the piston rods move out of the cylinders. The nitrogen in the accumulator expands and forces oil into the suspension cylinders. The nitrogen and oil pressures fall and the weight of the truck pushes the piston rods back into the cylinders. Oil then flows from the cylinders back to the accumulator.
When driving over rough terrain the piston rods will be continually moving in and out of the suspension cylinders and the compressibility of the nitrogen allows oil to flow into and out of the accumulator, thus absorbing shock.
Raise
When the truck is loaded the additional weight forces the piston rods to move into the cylinders. The ride height of the suspension reduces and the oil and nitrogen pressure rises. As the suspension ride height reduces pilot air will flow to the suspension control valve (3). Pilot air on the other side of the spool is connected to exhaust. The spool of the suspension control valve offsets so that pump oil is directed into the suspension cylinders and the ride height increases to the 'level ride' position.
Lower
When the truck load is dumped the additional volume of oil which was pumped into the cylinders to compensate for the weight of the payload causes the ride height to increase. As the ride height increases pilot air flows to the opposite end of the suspension control valve. Pilot air on the other side of the spool is connected to exhaust. The spool of the suspension control valve offsets so that the suspension cylinders are connected to tank and the ride height falls to the 'level ride' position.
Rear Suspension
The main components of the rear "hydroflex" suspension system are flow control valves (9), centre axle suspension cylinders (11) and rear axle suspension cylinders (12). The rear suspension cylinders are arranged in pairs, each pair consisting of the centre axle cylinder and rear axle cylinder on the same side of the truck. Each pair of cylinders is connected to a flow control valve (9) which is normally closed so that the rear suspension operates as a pair of closed systems.
Oil flows from cylinder to cylinder according to the relative positions of the wheels. The suspension cylinder pistons feature a number of through holes which allow transfer of oil between the full bore and annular sides of the cylinder. If, for example, the right hand centre axle wheel rides over a bump, the effective surface area of the piston forces a greater proportion of the oil from the full bore side of that cylinder into the full bore side of the rear axle cylinder. Thus as the centre axle wheel is pushed upward the rear axle wheel moves downward. The piston through holes pass oil at a controlled rate thus providing a damping effect on the suspension cylinder action.
The rear suspension system is connected to the front suspension system by flow control valves (9) which allows the rear suspension height to be adjusted periodically without the need for an external charging pump.
The suspension cylinders (12) are fixed to the rear frame by pivot pins (16) while the rods are fixed to the A-frame assemblies, also by pivot pins (17).
The A-frames (18) are fixed by spherical bearings (19) at the head ends and by panhard rods (20) at the leg ends. This provides the machine with lateral flexibility.
12. Suspension Cylinder - rear axle
16. Pivot Pin
17. Pivot Pin
18. A-Frame
19. Spherical Bearing
20. Panhard Rod
Rear Suspension Arrangement - (Early 6x4 arrangement illustrated)
Accumulator - Early Type
Early Type Accumulator
Accumulator - Later Type
1. Nitrogen Charging Valve
2. End Cap
3. Nitrogen Chamber
4. Cylinder Tube
5. Piston
6. Pressure Oil Chamber
7. End Cap
8. Port
9. Restrictor/Check Valve
10. Holes
Later Type Accumulator
The accumulator has a sealed piston (5) that moves along the bore of the cylinder tube (4). Chamber (3) is charged with nitrogen gas through the charging valve (1). The accumulator receives pressure oil through port (8) which compresses the nitrogen in chamber (3). During normal operation the compressed nitrogen acts as a source of stored energy to provide the necessary 'spring' and absorb shock.
The accumulator is similar to the early type described previously except that the end cap (7) has an integral restrictor/check valve (9). When oil is flowing into the accumulator the restrictor/check valve lifts off its seat and allows free flow into the oil chamber (6). When oil is flowing out of the accumulator the restrictor/check valve is seated and oil can only escape at a controlled rate through the two holes (10).
When the front wheels hit a bump the resulting high pressure in the suspension cylinders unseats restrictor/check valve (9) and allows free flow of oil into the accumulator. The suspension therefore reacts very quickly to the disturbing force resulting from the bump. When the disturbing force is removed the increased nitrogen pressure forces oil out of the accumulator. Restrictor/check valve (9) closes so that oil can only escape at a controlled rate through holes (10).
Suspension Air Filter
1. Inlet Port
2. Louvres
3. Quiet Zone
4. Element
5. Valve
6. Outlet Port
Air Filter
The air filter ensures a supply of clean pressurised air to the suspension rotary valve and control valve. The air flow from the truck main air system enters the filter through inlet port (1). The air flow passes through louvres (2) which force it into a whirling flow pattern. Liquid particles are thrown against the inside wall of the bowl by centrifugal force, and run down the wall into the bottom of the bowl into a 'quiet zone' (3). The air then passes through the sintered bronze element (4) where any remaining solid particles are removed. Air flow through the filter creates a pressure differential above and below the diaphragm of valve (5). After a period of time the difference in pressure is sufficient to cause the diaphragm to lift valve (5). Thus any liquid accumulated in the bottom of the bowl is automatically drained.
External Restrictor/Check Valve
1. Cap
2. Body
3. Spring
4. Poppet
5. Inlet Port
External Restrictor/Check Valve
When the suspension control valve is in the "raise" position oil flows into the suspension cylinders and accumulator through the external restrictor/check valve. Oil enters inlet connection at port (5) and lifts the poppet (4) off its seat allowing full forward flow. When the suspension valve is in the "lower" position oil flowing out of the cylinders and accumulator must pass through the hole in poppet (4). The poppet itself is seated.
This action allows the suspension height to increase quickly when "raise" is selected and to reduce relatively slowly (and under control) when "lower" is selected. When "raise" is selected the suspension cylinders have to overcome the weight of the tractor unit bearing down on them. When "lower" is selected the weight assists the levelling action which would be very rapid if the release of oil from the cylinders was not restricted.
Suspension Control Valve
Oil from the small cartridge of the hydraulic pump enters the suspension control valve through port (6) on early trucks and through port (3) on later trucks. On early trucks port (3) is plugged. On later trucks port (6) is used as a test point. The valve spool (15) is an open centre type so that inlet oil flows past the spool, to the hoist control valve when in the "level ride" position. Outlet port (21) is blocked.
The position of the spool (15) is controlled by the air cylinder (13) and spring (11). When the front suspension is high or low, air under pressure is fed to one side of the piston (14). The other side of the piston is connected to exhaust. The piston is connected to the spool by screw (12), thus the spool moves one way or the other against the effect of spring (11). When the suspension height is correct ("level ride") both sides of the piston (14) are connected to exhaust and spring (11) takes over to move the spool to the centre position.
When the front suspension is too low air pressure is directed to annulus (10). Annulus (9) is connected to exhaust and spool (15) offsets. Inlet oil flows past check valve (4), along passages (17) and (16) to the spool at passage (26). Passage (26) is connected to passage (7) by the offset spool allowing oil to pass through port (21) to the suspension cylinders and accumulator. This is the "raise" position where oil is pumped into the cylinders until the "level ride" position is reached.
When the front suspension is too high air pressure is directed to annulus (9). Annulus (10) is connected to exhaust and spool (15) offsets. Port (21) is connected to tank port (5) by way of passages (7), (8) and (20). On later trucks port (5) is plugged and an alternative tank port on the opposite side of the valve is used. This is the "lower" position where oil is bled out of the cylinders until the "level ride" position is reached.
Relief Valve Cartridge
1. Spring
2. Piston
3. Holes
4. Pressure connection
5. Valve seat
6. Tank connection
Inlet oil flows from pressure connection (4), through holes (3) and is felt against the underside of piston (2). When the pressure is high enough the effect of spring (1) is overcome and piston (2) lifts off its seat (5). Oil then flows into tank connection (6). The cartridge is factory set and is not adjustable.
Suspension Rotary Valve
1. Rotor
2. Inlet Port
3. Bearing
4. Passage
5. Orifice
6. Passage
7. Exhaust Port
8. Passage
9. Reservoir
10. Outlet Port (lower)
11. Outlet Port (raise)
12. Reservoir
13. Exhaust Port
14. Arm
15. Adjusting Link
Air flow from the main air system enters the rotary valve at inlet port (2). If the truck is in the 'level ride' position the air is blocked at inlet port (2) by rotor (1). The rotor is connected to the front suspension cradle by arm (14) and adjusting link (15).
Suspension Rotary Valve
If the suspension height rises (when the load is dumped) the rotor (1) offsets and air at main system pressure flows past the rotor into reservoir (9) by way of passage (4), orifice (5) and passage (6). After a pre-determined time period air pressure in reservoir (9) reaches a set level. Outlet port (10) is connected to the air cylinder on the suspension control valve. When the air pressure in reservoir (9) reaches the set level, the control valve is piloted across to lower the suspension height. The time delay for the air pressure to reach the set level is determined by orifice (5) and reservoir (9). When the suspension reaches the 'level ride' position, inlet port (2) is again blocked by rotor (1). The air in reservoir (9) is then open to exhaust by way of passage (8) and exhaust port (7). Passage (8) is connected to the outlet from the reservoir by an axial drilling.
When the suspension height falls (when the dump body is loaded) the rotor (1) offsets in the opposite direction. The process is repeated but on the 'raise' side of the rotary valve.
The time taken for the air pressure to build up in reservoirs (9) or (12) is such that the suspension will level correctly during loading or unloading but will not react to every bump or hollow in the haul road.