--- title: "Piston pump of an injection molding machine" url: "https://www.imspares.com/injection-molding-machine-parts/hydraulics/piston-pump/" --- # Piston pump of an injection molding machine How the variable axial piston pump of an injection molding machine works: pressure control, case drain, Yuken A and ARL1 codes, signs of wear and replacement. A piston pump on an injection molding machine is usually a **variable-displacement axial piston pump**: pistons in a rotating cylinder block stroke against a tilted swashplate, and the swashplate angle sets how much oil the pump delivers. Because it reduces its own flow when the machine needs less, it wastes less energy than a fixed pump whose surplus oil goes back to the tank over a valve. The Yuken A and ARL1 series are common examples. ## How an axial piston pump works The drive shaft turns a **cylinder block** holding a ring of **pistons**. Each piston ends in a **shoe** that slides on the **swashplate**, a plate set at an angle to the shaft. As the block turns, the tilt makes each piston move out of its bore for half a revolution, drawing oil in through the suction port of the **valve plate**, and back in for the other half, pushing the oil out through the pressure port. The stroke of the pistons depends on the swashplate angle. A large angle gives a long stroke and full flow; as the angle approaches zero, the flow approaches zero. A spring holds the swashplate at its largest angle, and a control piston, fed by the pump's control valve, pushes it back to reduce the flow. ## Pump controls The control valve mounted on the side of the pump decides when the swashplate moves: - A **pressure compensator** keeps full flow until the outlet pressure reaches its setting, then reduces the displacement so the pump delivers only what is needed to hold that pressure. During hold pressure and cooling it delivers almost no flow. - Other controls add a remote pressure setting, two pressure levels switched by a solenoid, or electro-hydraulic control of both pressure and flow from the machine controller. Pumps of the same size with different controls are not interchangeable, so the control is always part of the model code. ## The case drain Some oil leaks past the pistons, shoes and valve plate on purpose: it lubricates them and carries heat away. It collects in the pump housing and returns to the tank through a separate **case drain** line. Pump makers generally require this line to run to the tank on its own, ending below the oil level, without filters or valves that could restrict it, and the housing must be filled with oil before the first start. A restricted case drain raises the pressure in the housing and can push out the shaft seal. ## The piston pumps we supply Our [piston pump](https://www.imspares.com/parts/hy-pw-07/) listing covers the **Yuken A series**, from A10 to A145, and the smaller **ARL1** pumps (6 to 16 cm³/rev in our list, rated to 7 MPa), together with the AR16 and AR22. They are listed for Haitian MA and Jupiter series and Chen Hsong SM series machines. In a Yuken A code such as **A56-F-R-01-C-S-K-32**, **A56** is the series and the maximum displacement in cm³/rev, **F** is flange mounting (**L** is foot mounting), **R** is the direction of rotation, **01** is the control type, the next letter gives the pressure adjustment range, and the remaining characters give options and the design number. Copy the full code from the nameplate when you order. ## Signs of wear or failure - Pressure that hunts or will not settle, or a pump that does not reduce its flow during hold (control fault). - Slower movements at the same settings. - A louder whine, rattle or vibration. - The housing running hotter than usual, and more oil coming out of the case drain. - Oil leaking at the shaft seal. - Metal particles in the suction filter or on the drain plug magnet. ## Why piston pumps fail - **Dirty oil.** The clearances between pistons and bores, shoes and swashplate, and cylinder block and valve plate are small. Particles score them quickly, and fine dirt makes the control valve stick. Piston pumps are less tolerant of dirty oil than vane or gear pumps; see the [oil filter](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-filter/). - **Dry start.** A housing that was not filled with oil damages the shoes and bearings soon after start-up. - **Restricted case drain.** Pressure in the housing damages the shaft seal and the shoes. - **Cavitation.** A blocked suction filter, low oil level or cold oil starves the pistons. - **Overheated oil.** Thin oil leaks more and lubricates the shoes less. - **Misalignment.** A worn [pump coupling](https://www.imspares.com/injection-molding-machine-parts/hydraulics/pump-coupling/) loads the shaft bearings. ## Inspecting, repairing and replacing 1. Check the outlet pressure with a calibrated gauge and compare it with the setting in the machine manual. 2. Measure the case drain flow under load with a flow meter or a measuring container, following the pump maker's procedure. Compare it with the maker's limit or with a figure recorded when the pump was new. 3. A worn pump can be rebuilt by a hydraulic workshop: pistons and shoes, cylinder block, valve plate, swashplate, bearings and seals. 4. When fitting a new or rebuilt pump, fill the housing with clean oil through the highest drain port, connect the case drain directly to the tank, check the direction of rotation and start it unloaded. 5. Set the pressure control to the values in the machine manual, and change the filters. ## What to send for a replacement quote The full model code from the nameplate, including the control type, plus the machine model and serial number and photos of the pump, its control valve, shaft, flange and ports. For the differences with other types, see [hydraulic pump types](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-pump/) and [vane pump vs piston pump](https://www.imspares.com/blog/vane-pump-vs-piston-pump-injection-molding/). Then [request a quote](https://www.imspares.com/request-quote/). For the rest of the circuit, see the [hydraulic system guide](https://www.imspares.com/injection-molding-machine-parts/hydraulics/). ## Questions ### What is the advantage of a variable piston pump on an injection molding machine? It delivers only the flow the machine needs. When the pressure reaches the set value, the pump reduces its displacement instead of sending surplus oil over a relief valve, which saves energy and heats the oil less than a fixed pump. ### Why does a piston pump have a case drain? A small amount of oil leaks past the pistons and the valve plate to lubricate them and collects in the pump housing. The case drain line returns it to the tank; if the line is restricted, pressure builds in the housing and can damage the shaft seal and the pump. ### How do I check a piston pump for wear? Compare the outlet pressure with the setting, and measure the case drain flow under load. A case drain flow that has grown compared with a new pump shows internal wear; the pump maker gives the acceptable limit. ### What does A56 mean in a Yuken pump code? In the Yuken A series, the number after the A is the maximum displacement in cm³ per revolution, so an A56 delivers up to about 56 cm³ each revolution. The letters and numbers that follow give the mounting, direction of rotation, control type and pressure range. ## Replacement parts - [HY-PW-07 Hydraulic Piston Pump](https://www.imspares.com/parts/hy-pw-07/): Variable displacement hydraulic piston pump for high-pressure injection molding machine applications.