Application/Use

Solenoids are used in applications where a short linear force is needed to push or pull. Rotary solenoids are also available where a short rotary force is required. Runton Engineering current manufactures only linear action solenoids.

Stroke

Stroke length is generally limited to less than 0.4 times the outside diameter of the solenoid. Unlike air and hydraulic pistons a solenoid force is generated by magnetic line of force between the plunger and the fixed plunger seat. As the stroke is increased the force lines become weaker thus good solenoid applications fall into a relatively short stroke area. Information relating to the stroke and force can be found on the individual Data Sheets.

Force

The force produced by a solenoid is proportional to its ampere turns produced by its coil and the efficiency of its magnetic circuit. AC solenoids characteristically have a flat force curve with a dropping current flow.

The reason for this is that as the laminated pole pieces close, the inductance increases causing the "back EMF" to limit the current flow. The initial current is limited mostly by the DC resistance of the coil. The final current is limited by the full closed inductance of the device as well as the coil resistance.

DC solenoids have a steady current flow for any fixed voltage and at a given temperature. The current is limited by the DC coil resistance. This produces a rising force as the pole pieces close in distance.

More force can be generated with DC solenoids by "over-driving" the coil initially and then reducing the current flow to a holding current after the plunger has done its work. Runton Engineering has available power controllers for this purpose. Or, if the solenoid needs to operate for a short time, it can simply be supplied with an increased amount of power for a short operating cycle.

Push, Pull, and Push-Pull Solenoids

Most solenoids shown by the Data Sheets show push type solenoids. Pull type solenoids can be made by having the shaft emerge from the reverse end of a push type solenoid.

A push-pull type solenoid requires two coils, one for each motion. It is constructed with one case, but with two coils having essentially the same electrical and physical characteristics as a push type solenoid. Select a push type solenoid and specify Push-Pull.

Coil Resistance

The resistance of a copper wound coil changes as it heats up. The change is at a rate of 0.00393 times the resistance per degree Centigrade. The formula is:

where R is the resistance of the coil in ohms at operating temperature, Ro is the room temperature resistance of the coil in ohms, s=0.00393, and T is degrees Centigrade.

As resistance increases the current falls for a given voltage thus the ampere turns decrease causing the force output to decrease. Allowances must be made for the heating of the coil by establishing the potential maximum running temperature and being certain to provide sufficient voltage. It is then necessary to select a coil resistance for the solenoid which will produce the desired force output under all conditions.

Additional Options

• Return Springs - Return springs are often added internally to return the shaft to the non-energized position.
  
• Shaft Modifications - The shaft connects to the load and it is often necessary to modify it with slots, thread, holes, pads, different lengths or other configurations in order for the shaft to do its job. It is usually less expensive to do this while the solenoid is at the assembly stage.
  
• Mounting Variation - Every solenoid design is shown with a standard mounting. A customer's application often requires a modification of some sort in order for the solenoid to fit into the application properly.
  
• Electrical Connection - An electrical connection is a necessity. To fit the application different grades of lead wire, different lengths and termination types can be supplied.
  
• Seals - Solenoids can be made waterproof in various ways depending on the application and the solenoid type. Also solenoids can be sealed so that it can be used in a "wetted" application. This is where the plunger and possibly the coil become immersed in a fluid associated with its purpose such as valve application.
  
• Explosion-Proof Designs - Runton Engineering manufactures explosion-proof solenoids such as the ones shown on Data Sheet 10.  For this type of solenoids there are size limitations because of the necessity of having a "flame path" length built into any opening into the solenoid interior. This produces solenoids which are fairly large.
  
• Position Indicators - Internal position can be installed to provide an indication of the plunger position, either energized or non-energized.