Valves are a mechanical device that controls the flow and pressure of the fluid or gas within a hydraulic or pneumatic system. A valve is essentially a pass with at least two ports that allows the liquid or gas to enter and to exit. When the ports are all closed, the flow path of the fluid is discontinued and the circulation of the fluid inside a system is stopped. As for the valve with multiple ports, the direction of the flow can be changed with certain ports open and the others closed. Based on the configuration a valve has, the purpose of it varies. The most common valves types include ball valves, butterfly valves, prefill valves, check valves, pressure control valves, and directional control valves. In this article, we will talk about what a prefill valve is, how it works and what it is for.
The prefill valves are a type of drain valve that is particularly used to drain oil between a cylinder and a tank. The prefill valves essentially serve as a two-way pass; a prefill valve channels the oil from a tank into a cylinder while it also allows the oil to drain from the cylinder to the tank under certain circumstances. In addition to letting the oil pass, the prefill valves also prevents the oil from leaking out of the hydraulic system. The prefill valves are a common part in a hydraulic press which assists the action of the ram of the machine.
The prefill valves in a hydraulic pump are used to fill and exhaust a cylinder as it travels to engage and disengage with the work pieces. In such device, there is a subsidiary cylinder on the tank side and a main cylinder at the bottom of the device. A main flow poppet that attaches to the main cylinder seals the path between the tank and the main cylinder. When the pilot piston in the subsidiary cylinder descends, the main cylinder is driven downward accordingly and the poppet seal is open; the oil is hence allowed to flow from the tank into the cylinder through the prefill valve.
When the main cylinder reaches its end of the downward motion, the oil is directed to the main cylinder and a pressing force is generated. The press machine applies pressure onto the work piece when the the pressing force is released. After the force is released, the main cylinder pulls up again with the subsidiary cylinder and pushes the oil back to the tank through the prefill valve. The prefill valves are similar to the check valves since they serve as a one way pass during a single action. When the oil flows into the cylinder, the prefill valve prevents it from flowing back to the tank; when the oil flows back to the tank, the valve prevents it from staying in the main cylinder.
The check valves are similar to the prefill valves since they both control the flow of the working fluid inside a hydraulic circuit. The prefill valves and the check valves are both the one way valves, they only allow the fluid to pass from one end to the other during a single action. Without these valves, the backflow of the working fluid will lead to the damage of the system and the components within the system. Some considered that the prefill valves are a subtype of check valves except that the prefill valves come in a larger size than the check valves.
Generally speaking, when the functioning of a press requires a relatively high force, a larger diameter cylinder is needed to generate the force. With a larger diameter cylinder, larger pumps, pipes, and valves are needed consequently. However, the components in larger size may not be available and even if they are, the consumption of power will be huge. Therefore, the small pumps that incorporate with prefill valves are needed. The prefill valves are able to maintain the cycle time and pressing force while reduce the size of the machine at the same time. This very feature makes the prefill valves widely accepted in the application of pressing operations.
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