In elevator and rail locking, the first specification decision is not force — it is which state the mechanism must reach when power is lost. A door interlock and a brake release are both solenoids, but one must lock without power and the other must release without power, and getting that direction wrong produces a mechanism that fails dangerously rather than safely. Everything else follows from that choice.
Why Does Failure Direction Precede Attraction Force?
| Function | State on power loss | What that implies |
|---|---|---|
| Landing or car door interlock | Must stay locked | Spring-applied lock; solenoid energised only to release |
| Brake release on a hoist motor | Brake must apply | Spring-applied brake; solenoid energised to hold it off — continuous duty |
| Emergency release for evacuation | Must unlock | Energised to hold locked; loss of power releases |
| Rail vehicle door isolation | Defined by the operating rule, not by convention | Confirm with the governing standard before specifying |
Two of these are continuous-duty positions: the coil is energised for the whole time the system is running, not momentarily. That single fact reshapes the thermal budget, and it is the reason many of these applications need a larger frame or a higher insulation class than the force alone would suggest. The duty cycle article works through that arithmetic.
How to Comprehensively Evaluate Holding Force, Release, and Track Environment?
Holding Force Is Not Pull-In Force
A locking solenoid spends almost all its life at one end of travel, either fully seated or fully released. That means the number that matters in service is the holding force at the seated position — which is much higher than the force available at the start of stroke — while the number that decides whether the mechanism engages at all is the pull-in force across the working gap.
Specifications that quote a single force figure conflate the two. A unit that holds a heavy door adequately may still fail to pull the bolt across if a slightly misaligned mechanism increases the working gap. State both: the force required to travel the full stroke, and the force required to hold once seated, including any side load the mechanism imposes on the plunger.
The Release Problem in Safety Mechanisms
In a mechanism that must unlock on power loss, residual magnetism is not a nuisance — it is a safety defect. A plunger held by remanent flux after the coil de-energises does exactly what the design was built to prevent, and it does so intermittently, usually after long energised periods, which is precisely when the emergency case arises.
The engineering answer is the same as elsewhere but the tolerance is different: a defined minimum air gap at the stop face, a core material with low remanence, and a return force sized against remanent holding rather than against friction alone. The release-failure article covers the mechanism and how to test for it. In a safety function, that test belongs in acceptance rather than in troubleshooting.
Vibration, Temperature and Duty in Rail Environments
Rail rolling stock imposes conditions that most industrial specifications never encounter: sustained vibration across a wide frequency band, wide ambient swings between depot and service, and long continuous energisation. Vibration matters twice — it accelerates wear at the plunger bearing surface, and it can cause a marginally-held plunger to creep out of position.
Temperature matters because continuous-duty coils operate near their thermal budget by design, and a coil sized at 25 °C ambient loses margin quickly in an enclosure that reaches 55 °C or more. The insulation class article sets out how ambient consumes headroom. Both effects argue for specifying against the worst case rather than the nominal case, and for confirming behaviour at temperature rather than on a cool bench.
What Does Certification Cover and Not Cover?
Elevator and rail locking are governed by safety standards that apply to the complete mechanism and its control system, not to a solenoid in isolation. A component supplier can state materials, insulation class, tested electrical characteristics and dimensional conformity; it cannot certify that a mechanism satisfies a safety standard, because that assessment covers the assembly, the control logic and the installation.
This distinction matters at specification time. Ask a component supplier for the parameters they actually measure and can evidence — coil resistance, pull force, withstand voltage, temperature rise — and take responsibility for the safety assessment at the level where it belongs, with the notified body or authority governing your application. Anything else risks a certification gap discovered late.
Confirm These Six Items for Each Mechanism
The safe states of different mechanisms within the same equipment may be opposite, so each of the following items must be stated mechanism by mechanism, rather than assuming a single state for the entire project.
- Should this mechanism lock or release when de-energized: The safe state for door interlocks is locked (spring-locked, energized-to-release); emergency release for evacuation is the exact opposite. Both will exist in the same equipment.
- Is the coil continuously energized or pulse-actuated: Functionally, brake release operates continuously without cool-down rest periods; the thermal budget must be determined by continuous losses at the worst-case ambient temperature, not by average values.
- Ambient temperature inside the enclosure: Not the vehicle ambient temperature. Combined with “whether the coil is continuously energized,” this item usually determines whether a standard frame can withstand the thermal budget.
- Vibration conditions: This affects the plunger sliding surface as well as the margin required to keep the engaged plunger held in place; it is an item that must be specified separately beyond conventional parameters.
- Return force margin must cover residual magnetic holding force: Residual magnetism may still hold the plunger in the engaged position after de-energization, which is most pronounced after prolonged energization and at elevated temperatures. A minimum air gap defined by the stop surface and a return force calculated based on residual magnetism are standard countermeasures.
- Which agency are you seeking safety certification from: Component test reports substantiate the measured characteristics of the components themselves. Safety certification applies to the complete mechanism, control system, and installation—component data serves as input for that assessment, not a substitute.
How Shih Shin Technology Supports These Applications
When selecting a safety mechanism, the first priority to determine is not the attraction force, but the failure direction: whether this mechanism should lock or release when power is cut. Both can exist within the same equipment, which is why our first question addresses each mechanism individually. As for what component-level verification can substantiate—coil resistance, attraction force, withstand voltage, and temperature rise: the four piece-by-piece measured data items—they are inputs for your mechanism evaluation, not a substitute for it.
One Sentence Determines the Entire Design: Should It Lock or Release on Power Failure?
The selection process for safety mechanisms is the reverse of general applications: determine the fail-safe state first, then discuss pulling force. This is because the state it must rest in when power is cut determines whether the mechanism is spring-locked and energized-to-release, or vice versa—and both types may exist within the same piece of equipment.
So, clarify this from the very first sentence: when power is lost, must this mechanism remain locked or released? Next, provide the force required to complete the full stroke, the holding force required after pull-in, the worst-case ambient temperature at the mounting location, and whether the coil operates continuously or intermittently.
Write to info@solenoids.com.tw, or submit via our RFQ Form; we typically reply within 24 to 48 hours on business days. If this is a safety-rated application, please also let us know which standard applies so the verified parameters align with your evaluation. For general selection, you can simply start with the Solenoid Selection Guide Overview.
