308C CR Excavator Hydraulic System – Pump Control (Main Hydraulic)

Flow Control Under No Load

Illustration 1 g00753234

Regulation of pump flow (no-load condition)

(1) Spring

(2) Spring

(3) Blade pump

(4) Piston

(5) Piston

(6) Piston

(7) Piston

(8) Piston

(9) Swashplate

(10) Pin

(11) Piston

(12) Pilot Line

(13) Pilot line

(14) Port

(15) Port

(16) Passage

(17) Passage

(18) Passage

(19) Passage

(20) Passage

(21) Passage

(22) Passage

(23) Passage

(24) Orifice

(25) Piston

When the joysticks and/or the travel levers/pedals are in the NEUTRAL position or when the joysticks and/or the travel levers/pedals are partially moved from the NEUTRAL position, orifices in the main control valve control the negative flow control pressure. Pilot oil enters ports (14) and (15) through pilot lines (12) and (13). Therefore, the pressure in the pump delivery lines increases.

When all of the hydraulic controls are in the NEUTRAL position, the main pump destrokes in the following manner.

Pilot pressure acts on both ends of piston (8) through passage (17) and passage (18). Piston (8) shifts by the higher pressure and the low pressure oil flows through orifice (24), passage (23), passage (16), and piston chambers (25). Also, the negative flow control pressure in pilot line (12) and pilot line (13) are equal. Both pressure oils travel through orifices (24), passage (23), and passage (16) and to the chambers of piston (25) .

The pressures of the chambers for piston (25) act on the right end of pistons (4) and (5). Piston (4) and piston (5) shift to the left, which pushes piston (6) and piston (7) to the left. Because of the mechanical linkage with pin (10), swashplate (9) is rotated against spring pressure (1) and (2) the swashplate angle decreases. As a result, the main pump destrokes.

Horsepower Control Characteristic (Period Before Decreased Pump Stroke)

When any of the hydraulic controls are activated, the pump flow control changes. A change occurs from a destroked position of the pump (no-load condition) to an upstroke position of the pump (load condition) in the following manner.

  1. Pump flow that passes orifice (24) decreases, as negative flow control pressure decreases.
  1. Pressure that acts on the right end of piston (4) and piston (5) decreases. Pistons (4) and (5) are applying no force through pin (10) against swashplate (9). The angle of swashplate (9) begins to increase by the forces of spring (1) and (2) for pump upstroke.

The angle of swashplate (9) increases in proportion to the decrease in negative flow control pressure.

Main pump delivery pressure is directed to passages (19) and (20). The main pump delivery pressure goes through the inner passage of piston (4) and piston (5). This pressure acts on the surface area of pistons (6) and (7) with the same area as piston (4) and piston (5). The main pump delivery pressure acts on pin (10) and attempts to decrease the angle of swashplate (9) against the force of springs (1) and (2). Rotating direction of swashplate (9) is determined by the relationship between the forces of springs (1) and (2) and the total main pump delivery pressures. The total of the delivery pressures of the pump that act on pistons (6) and (7) do not compress spring (1) when a smaller load is placed on the machine. This causes swashplate (9) to attain the maximum angle for the pump upstroke. The main pump provides the maximum output flow.

Horsepower Control Characteristic (Period After Start of Decreased Pump Stroke)

Due to an increase in the main pump delivery pressure in passage (19) and passage (20), the forces that act on piston (6) and piston (7) are more than the forces of spring (1) and spring (2). The angle of swashplate (9) decreases and the swashplate compresses spring (1) and spring (2). This results in a destroked position of the pump.

Swashplate (9) is kept at the balanced position due to the following forces: spring (1), spring (2) and the total main pump delivery pressures. These three factors cause pump output flow to remain almost constant.

Total Horsepower Control (Variable Horsepower Control with Blade Pump Pressure)

The main pump senses the delivery pressure from the blade pump. If the delivery pressure of the blade pump increases too high, the main pump will destroke in order to keep an optimum engine speed. The ability of the main pump to destroke will allow the main pump and the blade pump to maintain a constant horsepower. This will also prevent the engine from becoming overloaded.

When delivery pressure of blade pump (3) increases, the pressure oil will flow to passage (21). This oil flow will act on piston (11). As a result, piston (11) acts on swashplate (9) through pin (10) against pin (10). The angle of swashplate (9) starts to decrease for a destroked position of the pump at a lower main pump delivery pressure.

Because the force of piston (11) is combined with the forces of piston (6) and piston (7), the total forces that are acting on swashplate (9) through pin (10) compress spring (1). When the delivery pressure of blade pump is high, the angle of swashplate (9) will start to decrease fast. When the delivery pressure of the blade pump is low, swashplate (9) will have a slower destroke. The main pump now provides output flow that is low and the total horsepower is kept.

Pressure/Flow Characteristic Curves

Illustration 2 g00753369

P-Q Characteristic Curves

(1) Point (start of destroked position of the pump)

(2) Point

The swashplate of the main pump is activated by the combined delivery pressures from the right pump and the left pump. At a given pressure, the right pump delivery pressure “P1” and the left pump delivery pressure “P2” may be equal or different. The output flow from each pump will be equal at the particular combined delivery pressure.

A value for pump flow is determined only after both pump delivery pressures have been set at a given value. A pressure/flow characteristic curve represents a set of flow rates that are based on the following conditions.

  • Right pump delivery pressure “P1” is added to left pump delivery pressure “P2”.

  • Right pump delivery pressure “P1” equals left pump delivery pressure “P2”.

After a pump starts to operate, each pump has a set of pressure/flow characteristic curves. The P-Q curve represents a set of flow rates for different pump circuit pressures. Point “1” represents the start of the destroked position of the pump. The swashplate angle is the maximum position. At Point “2” on the P-Q curve, the ratio of decrease in flow rate to the rate of pressure increase becomes smaller. Each point on the characteristic curve represents the respective flow rate and pressures in order to maintain constant hydraulic horsepower output.

Note: Keep the delivery pressure of the right pump equal to the delivery pressure of the left pump during the pump flow test. This will ensure the correct measurement of pump flow.