4 minute read
Open loop pump............................................................5
Open loop pump troubleshooting
The pump used on this machine is a axial piston pump with load sense and pressure compensation control. Maximum displacement is 165 cc/ rev (10 cubic inches) and maximum service pressure is 400 bar (6,090 psi). There is a secondary gear pump mounted to the rear of the main pump that supplies cooling flow to the cases of the main pump and the conveyor drive motor. In an open loop configuration, there is no charge pump. The supply to the pump is a direct suction line to the hydraulic oil reservoir.
A pressure transducer is located at the pump inlet to monitor for a blocked suction line, however, it does not guard against aeration caused by a leaking suction line or low oil level in the reservoir. If either of these occur, the pump will make an erratic grinding noise and will run at elevated temperatures. The pump will fail quickly at these conditions and should be shutdown immediately. Investigate oil level and suction hose integrity and identify and correct the issue before operating the pump.
At startup, the pump will be on full stroke to prime itself. Once the pump is at full speed, its control will de-stroke the displacement to a minimum and reduce system pressure to approximately 30 bar (435 psi). This mode is referred to as “standby”or “margin”and is typical of load sense controlled pumps. In standby, the system pressure gauge will read 30 bar (435 psi) and the load sense pressure gauge will read 0 bar. The actual output of the pump is at low pressure and only at a flow rate required to make up for internal system leakage. In this mode, the pump is waiting for a signal to provide higher pressure and flow to meet a specific task. That signal is in the load sense line which connects the pump control to the main valve bank. When valves are shifted to run a function of the machine. For example, the conveyor control valves will connect the load pressure of the conveyor motor circuit to the load sense line. The pump will quickly respond by increasing its displacement to create the pressure to meet that conveyor load. At this point, the load sense gauge will read the load of the conveyor function and the system pressure gauge will reflect that load pressure plus the margin pressure. If the conveyor requires 250 bar (3,625 psi) to operate, the system pressure will be 280 bar (250 bar +30 bar) (4,061 psi).
The adjustment of the load sense margin pressure is located on the control block at the top rear of the pump. It is the adjusting screw located at the front of the block.
The pump control includes a second stage for pressure compensation. This feature limits the maximum output pressure of the pump. When the pressure compensation setting is reached, the pump will quickly destroke to a minimum output. This condition is referred to as “dead head”or “stall”. The pump will continue to hold this pressure until the function is stopped. In the conveyor function, stall may arise from an obstruction in the breaker and this condition is monitored by a pressure transducer. The pump itself can operate in this mode indefinitely due to the operation of the pressure compensator.
The pump itself can operate in this mode indefinitely due to the operation of the pressure compensator. There is no excess hydraulic heat generated because relief valves are not opening to control the pressure limitation. If there is significant heating during a stall or high pressure operation, check the setting of the system relief located in the inlet of the main valve bank. This relief valve is preset to 370 bar (5,365 psi). To verify its setting, the pump compensator must be raised to 380 bar (5,510 psi). Once the relief is correctly adjusted, reduce the pump compensator to 350 bar (5,075 psi). If the relief is set lower than the pressure compensator, the relief valve will open, causing system heating.
The adjustment of the pressure compensator setting is located on the control block at the top rear of the pump. It is the adjusting screw located at the rear of the block. The pressure compensator must be adjusted while the pump is “loaded”. This is best accomplished with the conveyor function engaged.
The main pump is an efficient device: displacement efficiency when new is approximately 98%. The pump will produce nominal 74.75 gpm at 1,750 rpm. This means internal leakage is only 1.5 - 2.0 gpm. As a pump ages and wears, the efficiency will decrease, resulting in increased flow from the case drain port. Because the flow from the main pump case is a combination of internal leakage and the cooling flow, the case drain flow from a new pump will average 6 gpm. If case drain leakage exceeds 10 gpm, the main pump should be changed out. An acceptable method of measuring case drain leakage is to use a five gallon bucket and a stop watch. If the five gallon bucket is filled in 30 seconds, the pump needs changed.
When removing a used pump, use the plugs from the replacement pump and duct tape to seal off all ports on the used pump.
If a pump is replaced, it is critical that the pump case be filled before start up. It is often difficult to do this well underground, so start with filling and plugging the pump before taking underground. Pumps stored on site should be filled with oil. The pump has two case drain ports the highest on should be used for filling.
Upon installation, top up oil in pump case at the highest case drain port. Jog the pump motor at least five time before starting up. If the pump is properly filled and priming, it will run and sound smoothly on startup.
