Relay assembly: critical dimensions and adjustment

A relay is a mechanism before it is a component

A relay built for a specific application is an assembly of a coil, a magnetic circuit, an armature, a contact set and a spring. Its electrical rating depends on the geometry of that mechanism. If the armature sits too far from the pole face, the coil cannot pull it in at the minimum voltage. If the contact gap is too small, the arc may not extinguish. If the follow is too small, the contact force falls away as the faces wear.

So the critical dimensions on a relay drawing describe a mechanism in motion: a travel, a gap, a deflection and a force. A dimension that looks generous on paper can still leave no adjustment window, because two settings share the same spring.

Armature travel and the residual gap

Armature travel is the distance from the rest stop to the seated position on the pole face. It sets the stroke available to the contact set and the energy the coil has to deliver. A long travel gives a larger contact gap and more arc clearance, but it needs more ampere turns and produces a harder impact at the pole face.

The residual gap is the small non-magnetic separation left at the pole face when the relay is energised. It prevents sticking through residual magnetism when the coil is switched off. Too small a gap delays drop-out; too large a gap costs pull-in force, because the magnetic circuit now includes a permanent air gap. As an illustration, 0.6 mm of travel with a 0.05 mm residual gap leaves 0.55 mm of stroke. If the pivot adds 0.03 mm of side play, the armature can rock and close one corner of the pole face early. The figures are illustrative.

Contact gap, follow and force

The make gap is the separation between the normally closed contacts at rest. The break gap is the separation between the normally open contacts when energised. Both are arc clearance dimensions: large enough to break the load without restriking, small enough to leave stroke for the rest of the mechanism.

Contact follow is the distance the contact spring deflects after first touch. It builds the final force and provides the reserve that absorbs wear. Follow converts to force through the spring rate, so a stiff spring builds force over a short follow and a soft spring needs more follow for the same force. The conflict is direct: increasing the gap uses travel and leaves less follow, while increasing follow reduces the gap. The window narrows as the coil heats and the available force falls, which is why the force at end of life, not at first article, is the acceptance value.

How the setting is made

Most lines follow the same order, because each setting disturbs the one before it. Locate the armature, set the gaps, set the force, then lock and verify.

1. 01

   Seat the armature

   Set the rest stop so the armature has full travel without touching the pole face, and check side play at the pivot.

2. 02

   Set the residual gap

   Fit or adjust the non-magnetic shim, then confirm clean release at the minimum drop-out voltage.

3. 03

   Set the make gap

   Adjust the normally closed contact or its stop until the gap meets the drawing at rest.

4. 04

   Set follow and force

   Deflect the spring past first touch until the force at rest is met, then confirm the break gap is still in its window.

5. 05

   Lock and verify

   Stake, weld or tighten the setting, then re-check gap, follow, force, pull-in and drop-out on the same fixture.

The critical dimensions in one view

The table separates the dimensions that locate the mechanism from the ones that set its electrical behaviour. Several appear as a single callout but behave as a chain in production.

Adjustment methods and measurement

A bent tab is the cheapest adjustment and the least stable. The tab is a spring in its own right, and bending work hardens the material and leaves residual stress. The setting moves after thermal cycling or a few thousand operations. It suits a low duty product checked at end of line, where drift has a small consequence.

A screw adjuster gives a continuous setting and can be reworked, but it adds parts and a locking step. The thread can back out under vibration, and the adjuster changes the stiffness of the contact support. Staking and laser welding fix the setting by deforming or fusing the parts. They are stable under vibration and temperature and suit high volume, but there is no correction after the operation, so the incoming parts must be controlled tightly enough for the fixed setting to land inside the window.

Whichever method is used, the setting belongs before the cover is sealed, and the fixture has to hold the relay by the same features the application uses. Force and travel are best measured together: a force displacement trace shows first touch, the spring rate, the follow at the seated position and any hysteresis on the return stroke. Establish gauge repeatability before the fixture accepts product, then check operation at the minimum and maximum coil voltage.

Holding the setting in production

The settings that matter belong on a control chart, not only on a first article. Gap and force drift with tool wear, spring lot, plating thickness and operator technique. A chart of force against batch number shows a trend before the parts leave the window, which gives time to change the spring lot or service the fixture.

Every change to the mechanism deserves a re-validation. A new spring supplier, a different contact plating thickness, a revised armature stop or a new coil winding all move the force balance. Re-establish the window with the same test that accepted the original.