FIRGELLI engineering guide
Actuator overload protection is not one device. It is a layered design problem involving the motor, controller, wiring, mechanism, user access and the structure being moved.
This guide separates the common protection methods so you know what each one can protect, what it cannot protect, and what still needs to be handled by the machine design.
Key Engineering Takeaways
- Motor current rises when mechanical load rises, but inrush current must not be mistaken for an obstruction.
- A thermal cutout protects the actuator from heat; it does not make the moving mechanism safe.
- A clutch can limit overload damage, but repeated slipping creates wear and heat.
- Limit switches stop normal end travel; they are not a substitute for pinch-point guarding.
- A safety nut can add secondary support in some designs, but it does not remove the need for proper load sizing.
- Check electrical limits with check continuous stall current, estimate motor starting current and check controller current headroom.
Engineering Guide
1. Start with the failure mode
An overload can mean several different things: the actuator is undersized, the mechanism binds, a user blocks the moving part, the actuator reaches a hard stop, a bracket bends, or the controller commands motion after travel is complete. Each case needs a different protection layer.
The first design rule is to avoid using the actuator as the fuse for the mechanism. Size the actuator, controller, power supply, relay, wiring and structure as a system.
For wiring and controller protection, start with calculate DC motor current draw and size a high-current actuator relay before specifying the control circuit.
2. Current limits and force shutoff
Current sensing is useful because DC motor current increases as load increases. A controller can stop motion when current exceeds a programmed threshold. The threshold must allow normal startup current, cold grease, expected load variation and momentary friction without nuisance trips.
Current limiting is not a calibrated safety force unless the whole mechanism is characterized. Linkage geometry, friction and speed change the relationship between actuator current and external pinch force.
The use the FCB-2 actuator controller can be part of a controlled actuator system, but the design still needs mechanical stops, guards and commissioning tests.
3. Clutches, thermal cutouts and limit switches
A mechanical clutch can slip when load exceeds its set point. That can help protect gears and structure during a jam, but it should not be used as a normal operating stop. Repeated clutch slip turns mechanical energy into heat and wear.
Thermal protection reacts to heat buildup. It may prevent motor damage after repeated overloads, but it reacts after the actuator has already been stressed. Internal limit switches are different again: they stop the actuator at normal end-of-stroke positions.
Use review safe actuator motion principles to plan the safety behavior around the actuator rather than relying on one internal feature.
4. Actuator example: powered lid with public access
A powered lid near people needs more than a force rating. The design should limit pinch points, set a conservative current threshold, use a controller rated for stall current, test obstruction behavior, and make sure the structure cannot be overloaded by a stronger replacement actuator.
Where synchronized or timed control is required, compare the application against review FCB-2 controller functions and then validate with real obstruction and cycle tests.
Specification Checks
| Check | What it means | Engineering note |
|---|---|---|
| Fuse | Protects wiring from excessive current. | Does not control normal actuator force precisely. |
| Current limit | Can stop motion when motor current rises. | Must allow inrush and real load variation. |
| Thermal cutout | Protects against heat buildup. | Usually reacts after abuse has occurred. |
| Clutch | Can slip under overload. | Repeated slipping is not a normal operating mode. |
| Limit switch | Stops normal end-of-travel motion. | Does not protect every obstruction along the stroke. |
Field Checklist
- Normal loaded current
- Startup and stall current
- Controller current limit and relay rating
- Fuse size and fuse location
- Mechanical hard stops and limit-switch behavior
- Obstruction or pinch-point exposure
- Thermal duty cycle under repeated operation
- Bracket and frame strength
- Manual release or service method if required
- Commissioning test for overload behavior
Common Mistakes
| Mistake | Why it causes problems | Better practice |
|---|---|---|
| Setting current limit too low | The actuator trips during cold starts or high-friction normal motion. | Measure startup and running current before setting the threshold. |
| Using a clutch as a stop | The clutch slips every cycle and wears prematurely. | Use limit switches or controlled stops for normal travel. |
| Ignoring structure | A stronger actuator bends the machine before the controller trips. | Design brackets and frames for worst-case actuator force. |
Frequently Asked Questions
What is the best overload protection for a linear actuator?
There is no single best device. A good system uses correct sizing, fusing, controller current limits, limit switches, mechanical design and obstruction testing together.
Can current sensing stop an actuator before damage occurs?
It can help, but it must be calibrated around startup current and real load variation. A current threshold is not automatically a precise external force limit.
Does a thermal cutout make an actuator safe?
No. A thermal cutout helps protect the actuator from heat. It does not guard pinch points or guarantee that the moving load is safe for users.
What does an actuator clutch protect?
A clutch can help protect the drivetrain during overload or jamming. It should not be used as a repeated end stop because slipping creates heat and wear.
Do internal limit switches protect against overload?
Internal limit switches stop normal end travel. They do not detect every obstruction or side load during the stroke.
Can I use the FCB-2 for overload protection?
The use the FCB-2 actuator controller can support controlled actuator operation and current-related features depending on setup, but the machine still needs proper mechanical design, wiring protection and testing.
How should I test overload behavior?
Measure current during normal motion, then test blocked or obstructed cases in a controlled way. Confirm the controller stops motion before wiring, brackets or the actuator are damaged.
