Air Gap

The air space between the stationary and moving pole face in a solenoid assembly.

Ampere Turns

The absolute value of magnetic flux in a solenoid assembly determined by the number of turns of copper wire and the magnitude of the current.

Anchor Plate

A thin formed sheet of metal fitted over the mounting studs of a rotary solenoid to provide containment for a scroll type return spring.

Armature Assembly

The armature assembly of a rotary solenoid comprises of the shaft, hub and armature plate. This is the complete moving element of a rotary solenoid.

Armature Plate

The armature plate is the large diameter plate forming the main rotating element of a rotary solenoid, it contains the 3 coined ball races used in the Ledex principle to convert linear motion to rotary movement. The coined tracks are hardened for wearability.


The outer case of a solenoid assembly which provides the main flux path. In the case of a rotary solenoid, the case also contains the 3 hardened ball races which convert linear motion to rotary movement using the Ledex principle.


Copper windings provide the electrical element for all solenoids, current is passed through the coil to create a magnetic field. Coils can be precision wound, random wound or bobbin wound, performance is optimised when the maximum amount of copper for the space available is achieved.

Duty Cycle

The duty cycle is defined as the ratio of on + off time in a given cycle. The lower the duty cycle the higher the output that can be obtained from the solenoid, however, this can only be achieved in short bursts.

Gross Torque

The starting torque available from a rotary solenoid before any deductions for return springs etc.

Heat Rise

The rise in coil temperature which results from operating the solenoid. You should expect the temperature rise to be up to 80°C above ambient when mounted on a suitable heat sink.

Heat Sink

The maximum allowable watts for each solenoid are specified based on an unrestricted flow of air mounted on a heat sink equivalent to that specified. An inadequate heat sink or restricted air flow can result in the solenoid coil becoming overheated.

Holding Torque

The torque required to free the armature from the energised position whilst under power.


Part of the armature assembly forming the moving pole face of a rotary solenoid.

Lead Wires

Standard temperature rated coils use UL listed PVC insulated lead wires. Class ‘H’ coils use PTFE insulated leads.

Ledex Principle

Our range of rotary solenoids use the Ledex principle of operation. This employs the inclined plane of 3 ball races to convert linear motion to rotary developed by G H Leland during the 1940’s. It provides almost frictionless conversion of the motion and is widely adopted throughout the industry.


The ball races of our standard range of rotary solenoids are lubricated with a lithium based grease with temperature capabilities of -54°C to in excess of 90°C, endurance engineered models are lubricated in a similar manner with a grease suitable for temperatures of -29°C to +121°C


Solenoids provide the interface between electrical control and mechanical movement.

Permissible Temperature Rise

The permissible temperature rise of a solenoid coil limits the magnitude of the power and therefore the available ampere turns. Heat makes the coil less efficient because it increases resistance reducing ampere turns, flux density and torque or force output.

Residual Magnetism

The magnetism which remains in effect between 2 pieces of magnetic material after the electro-magnetic field created by the coil has been removed. An air gap is usually maintained between 2 magnetic poles to minimise the effects of residual magnetism.

Rotary Solenoids

Our range of rotary solenoids use the Ledex principle of operation with 3 inclined ball races, as a result they have a small axial stroke which must be allowed for in any application.


A cylindrical coil of wire acting as a magnet when electric current passes through it, converting electrical energy to mechanical energy.

Starting Torque

The torque which is produced by a rotary solenoid in the first 2 degrees of movement from its de-energised position.

Stator Assembly

The portion of any solenoid which contains the coil, case and base, the items which remain stationary during operation.

Temperature Rise

The increases in coil temperature above ambient resulting from operating the solenoid. A temperature rise of 80°C is typical for a standard solenoid.


A unit of magnetic flux.

The NSF Controls product experts are available to help you select the most suitable product to meet your specific requirements and, if required, will work with you to design and produce a customised solution. 

To find out more, please contact our specialist Design & Engineering Team +44 (0)1535 661144 or email 

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