In the first part of this blog we covered a range of common hydraulic symbols, explaining how they formed part of a circuit diagram along with their various functions. We continue where we left off, focusing on common hydraulic valves. Pressure control valve The pressure control valve comes in two basic forms; direct acting and pilot operated and the main function of these is to control the flow rate or its pressure. As there are a number of different types of pressure control valve which deal with variations in pressure their symbols can appear very similar. A good way to check which variant is used in a system is by the location of the valve in a hydraulic circuit. Directional Control Valve The directional control valve is responsible for controlling hydraulic fluid flow. The spool of the valve works with the valve body which opens and closes the internals to control fluid flow. Different types of actuator supply power to move the spool with the spool being returned when the actuator is de-energised using springs. In order to successfully read the symbol for a control valve you must understand the following: - Every box in the circuit symbol represents a different valve condition where the conditions are controlled by the actuators - The number of individual ways shows you the potential number of hydraulic connections that can be added to the valve - Actuators are always responsible for the push and never the pull of spool Check valve The hydraulic check valve works to prevent flow in a certain direction. A spring in the check valve enables the valve to open but only when the pressure is exceeded. Reverse flow can be attained by the valve opening under the influence of pilot pressure. This is usually the case if one was looking to hold the pressure in the hydraulic cylinder. Tanks/Reservoirs In order for a hydraulic system to store its fluid a reservoir must be employed and these come in various forms including closed and vented tank forms. Vented tanks are more commonly used in general applications with the closed variety mainly used on offshore and aviation industry applications. Hydraulic Cylinders A hydraulic cylinder operates by generating mechanical force through hydraulic power. The cylinder illustrated above is a typical double acting welded end variety and, having two ports, can be powered in and out. The alternative, single acting variant are usually found in lift based applications where only one port would be needed for operation. Other common types of hydraulic cylinder include tie rod, telescopic and hydraulic jack variants with each manufacturer of these cylinders having their own production guidelines but all having to follow strict guidelines on cylinder construction; for example, those of the NFPA (National Fluid Power Association). At Hydraproducts we work closely with our customers to ensure that their bespoke needs are met whilst their power unit is designed and for those not familiar with hydraulic symbols and meanings, our qualified staff will help to answer any questions and offer cost effective solutions where necessary.