Solenoids in Locking Mechanisms

Considerations for Solenoids in Locking Mechanisms

Fail-safe Condition – it may be required that solenoid driven locking systems revert to a default condition (either open or closed) in the case of power failure. This may preclude the use of bistable solenoids.

Security Level – security needs may vary, from a solenoid driven locking device used to reduce chance of human error (e.g. dispensing pills at correct interval) to locking devices subject to attack with sophisticated tools.

Power – power consumption may be limited in solenoid locking mechanisms by availability (e.g. battery or line-powered application), or by heat dissipation in doors made with insulating material.

Shock Resistance – Solenoid driven locking systems may have to withstand large physical shocks without changing the locking state.

Physical Envelope – dimensions may be constrained by the physical space into which solenoids in locks are required to fit.

Size and Shape Constraints in Solenoid Driven Locking Systems

Door locking systems tend to be produced in standard sizes. These can vary in different countries but a thickness of ½” (12.7mm), or ¾” (19.5mm) is common in order to fit within a standard lock cutout.

There are exceptions (such as solenoid or electromagnet driven hotel door locks) which may incorporate an external housing to contain card readers or electronics.

Other applications, such as safe boxes, may not be subject to the same size constraints

Space constraints may dictate the use of small size solenoids such as tubular solenoids or open-frame solenoids. Open-frame solenoids may also be constructed narrow in one axis.

Rotary Solenoids in a Locking Mechanism

The 3-Ball rotary solenoid (right) combines a high force linear solenoid design with a set of rotary ball races, which transform a short linear displacement into a larger rotary movement.

The high magnetic attraction of this solenoid makes it very resistant to ‘unlatching’ in a locking mechanism due to shock or vibration when energized.

The compact and efficient actuation and the action of the rotary ball races in this solenoid makes this resistant to closing due to shock or vibration when de-energized – important features for many locking applications.

Bistable rotary solenoids (left) have no axial displacement – provided load inertia is balanced about the axis of rotation, these solenoids can be used in locking systems with very high resistance to linear applied shock.

Rotary Solenoids in a Keyhole Slot Mechanism   

The picture on the left shows a linear locking mechanism using a rotary solenoid.

The solenoid shaft is flatted on both sides and passes through a slot with one or more ‘keyhole’ features where the bolt is to be stopped.The shaft of the solenoid can be supported on both sides to support high loads.

When the shaft is turned so the flats align with slot as drawn, the bolt can move along its axis. When rotated from this position, it is locked.

This concept can be used for locking other mechanisms like a rotary gimbal plate as shown to the right.

Solenoids in an Over-Center Locking Mechanism

An over-center locking mechanism can be used to support very large loads.

When the axis of the solenoid and the pivot points of crank and linkage are all aligned in a straight line, the locking mechanism can support high compression forces acting along this axis, with minimal torque exerted by the solenoid.

When this mechanism is employed, it may be advantageous to energize the rotary solenoid with ‘pick and hold’ mode of excitation.

In the case of the solenoid device pictured, a groove is incorporated in the front face for an O-ring seal. This seals against the component to which the solenoid is mounted. There is no seal between the front face and the solenoid shaft, so fluid or dust could enter here when the solenoid is loose as shown. The addition of a sliding seal here would add friction and impair the solenoid’s performance, so should be avoided if possible.

When the solenoid is mounted against another component with a seal, provided the back portion is sealed against dust and moisture ingress, the application is sealed. In this case, the cable exit is sealed into the body of the solenoid with resin.

Solenoids in a Toggle Locking Mechanism

A toggle mechanism based on a high-force push-pull solenoid is shown.

This is an ‘over-center’ mechanism. In the straight condition shown to the right, it can support very high loads. A stop pin allows the toggle to pass a controlled distance past the center position.

When energized, the solenoid pushes the center join past the ‘center’ position. The mechanism then collapses, pushed by the load. The mechanism is not connected to the solenoid so a large displacement is possible – however this mechanism needs to be reset manually.

Solenoids in a Geneva Locking Mechanism

The drawing shows front and back views of a linear Geneva locking mechanism as it runs through five successive points of a movement cycle.

The top and bottom views show successive ‘locked’ positions where the drive cam body securely locks the slide.

In the intermediate views, the cutout in the drive cam allows the drive pin to engage a slot in the slide and advance it to the next position.

With continuous rotation, the drive cam can advance or withdraw the slide through a number of securely locked positions.

The behavior may be more easily understood with reference to a rotary mechanism, as show here.

Still have questions? Let us know below; we’re happy to help!