Solenoid Components

This post details the components used in a typical solenoid design.

Solenoid Coil and Magnet wire

Solenoid Coil and Magnet wire

Solenoid construction: Coil & Magnet wire

Strength of the solenoid comes from its coil.

Coil is made of magnet wire, nearly pure copper wire with a very thin insulation. The insulation is usually less than .001 inch thick.

Many different insulations are used in magnet wire. For a transmission solenoid ,the insulation is polyester with a nylon overcoat, rated at 200 c.

The resistance of magnet wire decreases with size.

Low ohm coils use larger wire, and have lower resistance. PWM, EPC solenoids 1-3 ohms may have wire of.020 while higher ohm 15- 20 on/off solenoids use wire at .010 dia.

Because more electricity can travel through the larger wire, the low ohm solenoids are stronger, faster, also give off more heat.

Solenoid construction: Bobbin

The bobbin holds the magnet wire in place. It also provides a location to anchor the lead wires and to provide strain relief. The bobbin will often locate and guide the moving components of the solenoid.

The bobbin will also help determine or control the solenoid stroke.

Special engineering thermal plastics are used in molding the bobbin. The molded bobbin must withstand the harsh chemical environment, heat, and provide for a durable low friction surface.

A solenoid can (or enclosure)

A solenoid can (or enclosure)

Solenoid construction: Solenoid can

The solenoid can will provide for the means of attachment to the transmission valve body.

The can will locate all of the internal components and securely hold them together.

The solenoid can details will assist in determining the solenoid stroke. Close tolerances are needed to guarantee the concentricity of internal components.

The material composition and can geometry’s will carry and direct the magnetic flux to the proper areas of the coil for optimum operational performance.

Solenoid construction: Port Assembly and Seat

Port, locates and seals solenoid in VB.

Controls the flow of fluid. NC design, ball stays shut against port, until solenoid is energized. And lets fluid flow through.

Sealing area on the back of the seat prevents debris from entering sensitive areas of the solenoid.

Durable materials are used in the seat area. The material must be impact resistant and hold up well to ATF and high temperatures.

Often on/off solenoid seats will be plastic. PWM are usually metal This will depend on the solenoid cycle rate and expected service life. Rostra solenoids are designed for a minimum of 80,000 miles operation. Most are tested to the equivalent of 120,000 miles.

Solenoid flux ring

Solenoid flux ring

Solenoid construction: Flux ring

This washer shaped ring fits snuggly into the can, and adjacent to the port.

The flux ring carries the magnetic flux from the can to the center of the solenoid.

The gap between the flux ring and the armature must be carefully controlled. Too wide, not enough flux will jump the gap. Too small, armature will not want to move.

Speed and reaction time of the solenoid can be optimized by varying the design details of the flux ring.

Proper materials must be selected that can carry the magnetic flux.

Next add the wave spring washer:

Washer absorbs dimensional changes due to heat/ cold. Keeps components properly located.

Solenoid armature

Solenoid armature

Solenoid construction: Armature

The armature will be used to hold ball in place.

It will move upwards when solenoid is energized.

The design sizes, location & tolerances are critical.

Must move freely under all conditions, and temperatures.

It is desirable to balance the mass with the magnetic saturation of the steel.

Controls the sealing of the solenoid.

The armature is directly responsible for determining solenoid stroke.

Solenoid spring

Solenoid spring

Solenoid construction: Spring

The return spring keeps the armature pressed against the ball and provides a force that will keep pressure from opening up the solenoid seal ( Zero Flow).

Non-magnetic materials desired (300 stainless, slightly magnetic often used)

Must be designed to overcome spring fade.

Great for trapping debris.

solenoid insulator

Solenoid insulator

Solenoid construction: Insulator

Protects lead wires from an electrical short to the solenoid can

Diodes are often used to provide transient spike protection. When a magnetic field collapses, a voltage spike of 700 Volts or more may be created. The diode dampens this voltage spike to prevent computer damage.

Solenoid construction: Stop

The stop directs magnetic flux into solenoid, and provides the crimping surface for the solenoid can.

It is fixed in place and does not move.

The armature is attracted to the stop, and will halt the motion of the solenoid. This is the click you hear when turning on a solenoid.

The stop sets the working gap (stroke) of the solenoid.

The stop is often vented to remove debris, or extra fluid from the solenoid internals.

Some stops are adjustable to fine tune solenoid performance, then locked in place.

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