Words of Hydraulics glossary

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Air breather

Installed on a tank to avoid contamination from air inlet.

Air-cooled oil cooler

An oil cooler that cools oil by using air. It consists of a radiator and a fan motor and is generally best for environments where cooling water cannot be supplied.

Allowable differential pressure

Max differential pressure that does not break a filter element when applied instantly. It is 0.7MPa for our standard line filter.


Bare tube

A row tube without fin thread processing. Generally, it is used when fluids at shell side and tube side are same.

Beta (β) ratio

= filtration ratio



Unusable condition of filter element due to accumulated particles on an element surface. It can be detected by differential pressure between inside and outside of element. We define a condition when differential pressure is over 0.3MPa as a clogging.

Cooling area

Same as heat transfer area. Outside surface area of cooling tube for heat exchanging (contact surface with high temperature fluid). The larger cooling area, the larger heat amount it can exchange.

Cracking pressure

The pressure at which a relief valve opens to protect filter element from breakdown.


Element (Filter element)

A filtration part installed inside a filter. It is generally constructed of filtration media and metal parts such as end plate.



A device which prevents the passage of impurities within fluids and gases.


To remove particles from fluids using filtrating material such as wire gauze and paper filters.

Filtration media

A filtration part of a filter element. Various type is available: paper, C-fiber, wire gauze, notch wire, and wedge wire.

Filtration rating

An index to describe filtration level of element. It is described with a particle diameter that can be trapped at a specific efficient.

Filtration ratio

Filtration ratio is estimated by the multi-pass test method. It is calculated by (the number of particles at upstream of a filter) ÷ (the number of particles at downstream of a filter) for particles that is larger than a specific diameter. For example, if the β value is 100, there are 100 dusts at upstream and 1 dust at downstream.

Fin tube

A cooling tube with thread shaped fin on its outer surface that enables increase of cooling area. There are several height types of fin: high, middle, low, and we use low fin tube for our standard heat exchanger.

Flange connection

It describes that pipe connection to a component is flange type. there is another connection: “thread” type.

Fluid code

A code to be put at the beginning of model code of a filter or a heat exchanger, and it describes available fluid type. sealing material and surface treatment are different depending on fluid type (ex: ”F”= phosphate ester fluid, sealing material is FKM).


G thread

It describes parallel pipe threads (JIS B0202).


Heat exchange amount

Heat amount that a heat exchanger can exchange. Normally, it is estimated by deducting exhaust heat value from total heat generation value of all working component.

Heat exchanger

A device that efficiently transfers heat from high temperature objects to low temperature objects.

High pressure element

An element with allowable differential pressure of 21MPa. It is appropriate when using line filter with max working pressure of over 14MPa and relief valve is not available.


Impulse test

A test to confirm fatigue strength of product by applying repetition pressure. Since it is regarded that strength reduction is generally saturated after ten million times, products that stand the durability test running ten million times are put on the market.


An equipment to detect differential pressure between upstream and downstream and indicates signals.
Visual observation type (I): Color of signal changes from green to red at setting value.
Electric contact type (E): microswitch is activated to output electric signal at setting value.

Initial pressure drop

A pressure drop value of a filter with an unused element. Pressure loss value is measured based on the standard method of JIS B8356.


Low fin tube

A fin tube that height of fin does not exceed a diameter of raw tube. Even if any fault in one of installed tubes, it is replaceable individually.


Max flow rate

Max flow rate is determined by the following main factors each.
・Shell side: When flow rate increases, wear on outside of cooling tubes increases and service life of a heat exchanger decreases.
・Tube side: When flow rate increases, erosion corrosion of cooling tubes might be induced.

Max working pressure

Max allowable pressure that oil filter or heat exchanger can be used. In case of a hydraulic circuit that undergoes repeated pressure fluctuation, max working pressure should be not a design value but a peak value.

Mesh (Filtration rating)

The number of meshes per inch and it describes a filtration gap of wire gauze (ex: 5 Mesh = 5 meshes in an inch). Since there are several types of wire diameter, filtration gap can be different even if Mesh number is same.
Mesh 40 60 80 100 150 200 250 325 500 1000 1500 2400
Micron 420 250 185 150 105 75 60 50 30 20 10 5
* The above values are for TAISEI wire gauze

Metallic particle abrasion

Metallic particles generated by the sliding action within the equipment.

Micron (Filtration rating)

A value for the particle size, measured in microns, of the filtration target. As for our C-fiber filter element, the value is determined at when β-ratio is over 200. For wire gauze, the value is determined by filtration gap between wires.

Min flow rate

When flow rate of cooling water at the tube side is low, scale accumulation occurs easily and this decreases service life of cooling tubes. In addition, low flow rate of cooling water enhances reduction of heat transfer efficiency at both shell side and tube side.

Modular type filter

A filter installable to a manifold block or a subplate conforming to an integrated value standard.

Multi-pass test method

A test method to measure filtration performance of filter element. efficiency of the element. Refer to the “Multi-pass test for filtration performance evaluation”.

Multi-tube heat exchanger

= Shell and tube type heat exchanger


Notch wire

The element inside the filter is constructed from notch wire that is made from tightly-coiled stainless steel wire. The element filters via the gaps between the stainless steel wires. The wire is also manufactured in-house and achieves a higher filtration than wire mesh. Notch wire elements can be washed and reused in the same manner as wire mesh elements. The element also has a flat surface which makes cleaning easy.

Number of fin threads

The number of fin threads per unit length of the fin tube. The more the number of fin threads, the larger its cooling surface area gets. Normally, it is described the number fin threads per inch.


Pass number

The number of turns of cooling water at low temperature side (tube side) in heat exchanger. Generally, the more pass number, the less cooling water quantity, however, efficiency of heat exchanger increases.

Phosphate ester fluid

One of the fire-resistant hydraulic fluids used in aircrafts, etc. Sealing parts material should be carefully checked when use.

Pressure gauge

An instrument used to measure fluid (liquids and gases) pressure.

Pressure switch

A device that detects fluid pressure and outputs electric signal when a certain set pressure has been reached.


Rc threaded

It describes taper pipe thread (JIS B0203).

Relief valve

A safety valve to prevent a filter element from breakdown when an element is clogged and differential pressure increases. Cracking pressure of relief valve is normally set at 0.35Mpa.


Scale coefficient

Heat exchanging performance can decrease gradually in long term use due to dirt accumulation in a heat exchanger. Scale coefficient is a value multiplied to provide a margin to a heat transfer coefficient when selecting a heat exchanger.


A part that scrapes off dust attached to the surface of the element of the AK type and CFK type filters.

Shell and tube type heat exchangers

Heat exchanger mainly composed of a shell for high temperature fluid and cooling tubes for low temperature fluid. It is the most popular and reliable structure among industrial heat exchangers.

Spiral serrated finish

Fine spiral unevenness is formed on sealing surface of a flange. This machining process is based on the standard method of ANSI B16.5.


Water-glycol fluid

One of the fire-resistant hydraulic fluids widely used in hydraulic equipment. It contains 30-50% of water.

Welding spatter

Particles with µm- mm in size that spatter when welding.