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Glossary of Terms - All Terms

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Glossary Subsets

Back EMF

Refers to the voltage produced across a motor's windings. This is due to the winding turns being cut by the magnetic field during motor operation. Also referred to as counter EMF (CEMF).

Back Connected

A condition where port connections are on normally unexposed surfaces of hydraulic equipment.

Back Pressure

Usually refers to pressure existing on the discharge side of a load, it adds to the pressure required to move the load.


In a mechanical system, backlash is the relative motion between two connected devices, either by a coupling, gear, etc.


A device, usually a plate, installed in a reservoir to separate the pump inlet from return lines. The baffle also aids in fluid cooling.

Ball Complement

The number of balls used in a ball bearing.

Ball Bearing

A bearing using balls as the rolling elements.

Ball Contact

The area of contact between the raceway and ball.


A spherical rolling element.

Ball Diameter

The dimension measured across the ball center.

Ball Pocket

A drilled, stamped, or molded receptacle that holds the ball in a cage.


A unit of pressure based on 105 Newtons per square meter, equal to 100kPa (kilopascals), approx. equal to 14.5 psig.

Base Speed

The speed a DC motor develops at the rated armature and field voltage with the rated load being applied.

Basic Dynamic Load Rating

Basic dynamic load rating (Cr) is the calculated constant which a group of identical bearings with stationary outer rings can theoretically endure for a rating life of 1 million revolutions of inner ring.

Bearing Seal

A soft washer, usally made of synthetic rubber, which contacts the inner and outer rings to retain lubricant and keep out contamination.

Bearing Shield

A steel washer which helps to retain lubricant and keep out larger particles. Not recommended for applications involving small particulate contamination or washdown applications since the inner race does not come in full contact with the shield.

Beta Rating

The method of comparing filter performance based on efficiency. This is done using the multi-pass test which counts the number of particles of a given size before and after a fluid passes through a filter.

Bernoulli Principle

An increase in velocity causes a decrease in pressure.


To divert a specific controllable portion of pump delivery directly to reservoir.

Block Grease

A very firm grease manufactured in block form to be applied to certain large open plain bearings or other system components operating at high temperatures and slow speeds.


The smallest internal dimension of inner or outer ring or separator. Also, the surface of the inner ring that fits against the shaft.

Boundary Dimensions

Refers to the basic dimensions for bore, width, outside diameter.

Boundary Lubrication

A state of lubrication characterized by partial contact between two metal surfaces. Due to metal-metal contact, severe wear can take place during boundary lubrication. This is countered by additives in certain lubricants, which will minimize wear under

Bridge Rectifier

A section of the DC drive that converts the AC power supply into DC.

Braking Torque

The torque required to stop a running motor. Also may refer to the torque developed by a motor during dynamic braking.


A carbon or graphite piece which contacts the commutator of a DC motor and conducts current to the armature windings from the power supply.


An device that stops a running motor and/or holds a load.


Permanent deformation of the bearing surfaces where the rollers or balls contact the races. Brinelling results from excessive load or impact on stationary bearings.

Brookfield Viscosity

The viscosity, in centipoises, as determined on the Brookfield viscometer (ASTM D2983). The operating principle for the Brookfield viscometer is the torque resistance on a spindle rotating in the fluid being tested. Most frequently associated with low

Breakdown Torque

The maximum torque that a motor develops without a significant drop in speed.


A device which permits air to move in and out of a component to maintain atmospheric pressure.


British Stantard Parallel Pipe


British thermal unit. The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.


A secondary passage for fluid flow.


British Standard Pipe Thread

Bypass Filtration

A system of filtration in which only a portion of the total flow passes through a filter at any instant.