Vending machine lock solenoid: how to size stroke and force

Why this happens

A lock solenoid is usually selected from a datasheet peak force, and a lock is almost never limited by peak force. It is limited by the force available at the worst position in the stroke while the return spring is at its stiffest.

Two curves run in opposite directions. Solenoid force falls steeply as the air gap opens, because the working force varies roughly with the inverse square of the gap. The return spring, meanwhile, gets stiffer as the stroke advances, because you are compressing it. The worst case therefore sits at the end of travel, where the solenoid is weakest and the load is highest. Selecting on the peak force at closed gap is selecting the one number that describes a position the mechanism never works in.

The second mechanism is structural and it is the one that is usually missed. At the end of every stroke the plunger decelerates against something. The reaction load travels back into the mounting plate. A plate that is flat on the bench is not necessarily flat under that load, and the deflection shows up as a slightly larger residual air gap on the next stroke. Because force depends on the square of the gap, this is not a small effect:

Air gap	Relative force	Change
0.30 mm	1.00	reference
0.33 mm (+10%)	0.83	−17%
0.40 mm (+33%)	0.56	−44%

A tenth of a millimetre is easily lost in a plate that deflects, in a stop face that mushrooms, or in a gap-setting shim that creeps. And a tenth of a millimetre costs more force than going up a whole wire size buys back, at the cost of more current and more heat.

The dominant failure mode on these locks is not an undersized coil. It is an end-of-stroke impact that was never specified. The datasheet cannot warn you about it, because it is a property of your mechanism, not of the solenoid.

Check these in order

1. Measure the force at the largest working gap, not at closed gap. Put the plunger at the position where the load is highest — normally full travel against the spring — and measure there. If you only have a peak figure, treat it as an upper bound and do not design against it.

2. Measure the return spring at the same position. Spring force peaks at the end of the stroke. If you specified the spring by free length rather than by force and rate, this number is unknown, and an unknown load is the most common reason a lock works on the bench and not in the field.

3. Define the end stop explicitly. There should be a positive mechanical stop with a defined contact surface, and a stated allowance for the impact. If the plunger stops against the bobbin, the end cap, or the customer’s door frame, you have no control over the gap that results.

4. Re-measure the gap after cycling, not on a fresh build. Take a unit, run it for a few thousand cycles, then measure the residual gap again. Growth here is the whole story. Also inspect the stop face for fretting and check whether the mounting fasteners are still at the specified torque.

5. Confirm how the residual gap is set, and what holds it. A shim, a machined step, a press fit, a glued spacer — each has a different creep behaviour. Anything relying on a polymer in a load path is a spring, not a dimension.

6. Only then check the coil. Measure resistance at the actual ambient inside the machine and work back to the current at voltage. A lock in a closed cabinet with electronics around it is not at room temperature.

What actually to change

Finding	What to change	Why not the other thing
Force marginal at end of stroke	Add a controlled end stop and budget the residual gap	A bigger coil adds current and heat and does not fix a growing gap
Gap grows after cycling	Move the load path off the plate: add a stop boss that takes the reaction	Tightening tolerances on the plate does not help if it deflects under load
Spring force high at end of stroke	Specify spring force and rate; reduce preload	Re-specifying the solenoid is more expensive and changes the whole envelope
Gap set by a polymer feature	Replace with a machined metal step	Polymers creep under sustained load, and this one is in the impact path
Force fine cold, weak when warm	Reduce duty cycle, or add copper window	Raising supply voltage shortens insulation life
Latch will not release with power off	Look at geometry and friction before the magnetics	The release path does not involve the coil at all

When it IS the harder problem

The stroke was specified with no allowance for impact. This is the case that only shows up in the field, and it is worth recognising early. The design drills down to a clean force margin on paper, the first samples pass, and then complaints start at a few thousand cycles with no electrical fault to find. The remedy is not in the magnetics: it is to give the impact load a defined path into a structure that does not deflect, and to state the allowable residual gap as a specification item, so that the mechanism, not the solenoid, carries the tolerance.

The complaint is about the latch, not the solenoid. Door frames warp, hinges sag, and the latch line changes with the load in the machine. A solenoid that meets its curve can still fail to release a latch that has moved. This is why the useful question is not “is the solenoid strong enough” but “at what position does the mechanism need force, and how much”.

The unit has to hold for a long period rather than pulse. Holding changes the problem completely: the limit stops being force and becomes thermal, because the allowable temperature rise in a closed cabinet can be a third of what the same coil is allowed on the bench. If the design holds the coil energised for minutes rather than milliseconds, size it from the temperature budget first and the force second.