Systems Operation

Usage:

Hydraulic System

Front Suspension System Schematic - Early Arrangement (Factory fit to DJB serial numbers 6113-6968)

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. Restrictor

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 (9) which allow the suspension to rise or fall at a controlled rate, thus preventing overshoot.

Front Suspension System Schematic - Later Arrangement (Factory fit to DJB serial numbers 6970 - last built with rigid rear axle © and Caterpillar Product Identification Numbers 9YC1-9YC300)

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

10. Restrictor/Check Valve Assembly

11. Restrictor/Check Valve Assembly

The later arrangement has two restrictor/check valve assemblies (10 & 11) which replace the fixed restrictors used in the early arrangement. When the suspension control valve (3) is in the "raise" position restrictor/check valves (10 and 11) allow full flow into the cylinders (4) and accumulator (5). When in the "lower" position flow is restricted by the restrictor/check valves (10 & 11). In this way the suspension lowers under control without slowing down the "raise" operation. Restrictor/check valve (11) is externally mounted while restrictor/check valve (10) is housed in the accumulator end cap.

The font 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 on 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

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

Current trucks have both front and rear axle suspension. The front suspension is made up of the first stage of control valve (3), accumulator (5), suspension cylinders (4) and two restrictor/check valve assemblies (10 and 11). Oil is supplied to the system from the small cartridge of the hydraulic pump (1). When the first stage spool is in the spring centered mid-position ("level ride") oil flows through the first stage to the second stage spool. Oil in the front suspension cylinders (4) and accumulator (5) is trapped.

When the valve first stage spool is in the "raise" position oil flows past integral check valve (3a) to the front suspension system. In the "lower" position oil in the system flows to tank. The front suspension operates in the same way as described previously.

The rear suspension system is made up of the second stage of control valve (3), high pressure accumulator (14), low pressure accumulator (13), and rear suspension cylinders (12). The valve second stage spool operates in the same way as the first stage spool. The three positions of the spool are:

1. Level Ride

2. Raise

3. Lower

Level Ride

In the "level ride" position the spool is spring centered and oil is directed to the hoist control valve (6). Oil in the rear suspension cylinders (12) and accumulators (13 and 14) is blocked by the spool. The operation of the rear suspension is the same as the front except that there are two accumulators instead of one. The two accumulators are charged to different precharge pressures to allow for the laden and unladen weights of the truck.

When the truck is unladen the low pressure accumulator (13) operates. The high nitrogen precharge in accumulator (14) exceeds the static pressure in the system so the accumulator piston is fully extended. When driving over rough terrain the piston rods will be continually moving in and out of the suspension cylinders. Oil flows between the cylinders and low pressure accumulator to absorb haul road shock. At times pressure pulses will be generated which will move the piston in the high pressure accumulator but generally it is the low pressure accumulator which operates when the truck is unladen. When the truck is loaded the static pressure in the system increases considerably to balance the increased weight. The low pressure accumulator piston is then almost completely compressed and the high pressure accumulator operates to absorb haul road shock.

Front and Rear Suspension System Schematic (All trucks with rear suspension)

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

10. Restrictor/Check Valve Assembly

11. Restrictor/Check Valve Assembly

12. Rear Suspension Cylinders

13. Low Pressure Accumulator

14. High Pressure Accumulator

Raise

When the truck is loaded the additional weight forces the piston rods to move into the cylinders. The suspension ride height reduces and the oil and nitrogen pressures rise. As the ride height reduces pilot air flows to the second stage spool which offsets so that oil flows to the suspension cylinders. The ride height increases to the "level ride" position and the pressure in the cylinders will be considerably higher than before.

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 second stage spool which offsets so that oil in the rear suspension cylinders flows to tank. The ride height reduces to the "level ride" position and the pressure in the cylinders will be considerably reduced.

Accumulator - Early Type

Early Type Accumulator

Accumulator - Later Type

Later Type Accumulator

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. Holes

10. Restrictor/Check Valve

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 (10). 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 (9).

When the wheels hit a bump the resulting high pressure in the suspension cylinders unseats restrictor/check valve (10) 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 (10) closes so that oil can only escape at a controlled rate through holes (9).

Suspension Air Filter

Air Filter

1. Inlet Port

2. Louvres

3. Quiet Zone

4. Element

5. Valve

6. Outlet Port

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

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 truck 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 (Trucks With Rigid Rear Axle)

Oil from the small catridge 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 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 Control Valve (Trucks With Rear Suspension)

Oil from the small cartridge of the hydraulic pump enters the first stage of the suspension control valve through port (3). This oil flows along passage (19) to relief valve (18) and through an internal passage to check valve (4) and to spool (15). The valve spool is an open centre type so that inlet oil flows past the spool and through adaptor (28) to the second stage inlet port (32) when in the "level ride" position. Outlet port (21) is blocked by the spool.

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 rid") 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 passage (20) by way of passages (7) and (8). This is the "lower" position where oil is bled out of the cylinders until the "level ride" position is reached.

If the suspension valve first stage spool is in the "raise" position when the front wheels hit a bump the pressure generated in the cylinders is fed back through port (21), passages (7), (26), (16) and (17) to check valve (4). The check valve then closes preventing excess pressure being felt by the pump.

Oil enters the second stage through port (32). The valve spool (45) 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 (53) is blocked.

The position of the spool (45) is controlled by the air cylinder (50) and spring (46). When the rear suspension is high or low air under pressure is fed to one side of piston (51). The other side of the piston is connected to exhaust. The piston is connected to the spool by screw (49), thus the spool moves one way or the other against the effect of spring (46). When the suspension height is correct ("level ride") both sides of the piston (51) are connected to exhaust and spring (46) takes over to move the spool to the centre position.

When the rear suspension is too low air pressure is directed to annulus (48). Annulus (47) is connected to exhaust and spool (45) offsets. Inlet oil flows along passages (55) and (54) to the spool at passage (33). Passage (33) is connected to passage (34) by the offset spool allowing oil to pass through port (53) to the suspension cylinders and accumulators. This is the "raise" position where oil is pumped into the cylinders until the "level ride" position is reached.

When the rear suspension is too high air pressure is directed to annulus (47). Annulus (48) is connected to exhaust and spool (45) offsets. Port (53) is connected to tank passage (39) by way of passages (34) and (35). 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

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 the 'level ride' position the air is blocked at inlet port (2) by rotor (1). The rotor is connected to the suspension frame by arm (14) and adjusting link (15).

Front Suspension Rotary Valve

Rear Suspension Rotary Valve - if fitted.

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.