Hydraulic Terms D

Hydraulic Terms D speaks to hydraulic phrases D and hydraulic glossary D used by those involved in the hydraulics industry.

Detent A spring device which maintains the spool of a directional control valve in position.

Detent Release A mechanical, hydraulic or electrical device for releasing the detent.

Directional Control Valve This type valve consists of a body with internal passages which are connected and disconnected by a movable part. In directional valves, and in most industrial hydraulic valves the movable part has been a spool. Spool valves are by far the most common type of directional valve used in industrial hydraulics.

Directional Control Valve 4 Way This valve is known as 4 way because it has four distinct passages within its body and may consist of a pump passage, tank passage and two actuator passages.

Directional Control Valve 3 Way A 3 way valve consists of three passages within a valve body, pump passage, tank passage and one actuator passage.

Directional Control Valve 2 Way A 2 way directional valve gives an on-off function which is used in many systems to serve as a safety interlock and to isolate and connect various system parts.

Dirt in Oil The biggest problem with hydraulic oil in service is that it can easily be contaminated. The source of contamination can be water or air but more frequently it is dirt. Dirt in a hydraulic fluid can plug sensing orifices, cause moving parts to stick and wear excessively and act as a catalyst to oxidize oil.

Dirt Test Trying to determine the dirt level of a fluid with the unaided eye is many times impossible. Holding a beaker of hydraulic oil up to a light and inspecting for dirt is an inaccurate means of determining dirt contamination. Many harmful dirt particles for a hydraulic system are not visible. Determination of dirt contamination is best performed in a lab.

Displacement The amount of liquid transferred from the inlet of the pump to the outlet in a single revolution. Displacement can be fixed or variable. Dissolved Air Hydraulic fluid at sea level is made up of approximately 10% air. This air is dissolved in the liquid and cannot be seen and does not apparently add to the liquid volume.

The capability of hydraulic fluid, or any liquid, to hold dissolved air decreases as the pressure acting on the liquid decreases.

Drains In order for a pressure valve to operate properly the area above the spool must be continuously drained so that the liquid does not impair the movement of the spool. A drain is the passage within the valve body which is connected to the reservoir.

Drain Internal If the secondary passage of a pressure valve is connected to the reservoir, as in relief valve and counterbalance valve application, the drain passage is internally connected to the valve’s secondary or tank passage.

Drain External If the secondary passage of a pressure valve is a pressure line as in a sequence valve and pressure reducing valve applications, the drain passage is connected to the tank by means of a separate line.

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