The Belimo LF120-S US is a compact spring-return (fail-safe) damper actuator delivering 35 in-lb [4 Nm] of torque with on/off (two-position) control on AC 120 V power. It direct-couples to round damper shafts from 3/8 in. to 1/2 in. (to 3/4 in. with the K6-1 accessory clamp) and, on loss of power, its internal spring returns the damper to a fail-safe position in under 25 seconds, making it a member of the LF platform for small commercial HVAC dampers that must fail safe. A built-in SPDT auxiliary switch signals damper position or interlocks equipment.
Specifications sourced from the official Belimo LF120-S US technical data sheet (rev 2026-06-17).
Overview
Belimo LF-Series actuators are compact spring-return drives for small commercial HVAC control dampers requiring fail-safe operation. The LF120-S US produces 35 in-lb [4 Nm] and mounts directly to round damper shafts from 3/8 in. to 1/2 in. (to 3/4 in. with the K6-1 accessory clamp). On power loss the spring returns the damper to its fail-safe position in under 25 seconds; the fail-safe direction is reversible by CW/CCW mounting. Two-position on/off control drives the damper, and the spring returns it on signal or power loss.
Best for
- Small commercial HVAC dampers needing fail-safe (spring-return) operation
- Compact damper and zone control (35 in-lb)
- Outdoor-air and exhaust dampers that must fail safe
- End-position signaling or interlock (1x SPDT)
- Line-voltage systems (120 VAC)
Key Features
Spring-Return Fail-Safe
A mechanical spring drives the damper to a safe position on power or signal loss (under 25 seconds) — no external power needed to fail safe.
Compact 35 in-lb Actuator
Sized for small dampers; direct-couples to round damper shafts from 3/8 in. to 1/2 in. (to 3/4 in. with the K6-1 accessory clamp).
On/Off Control
Accepts two-position on/off control from a controller, relay or switch.
Reversible Fail-Safe Direction
The spring-return (fail-safe) direction is selectable clockwise or counterclockwise by mounting orientation.
Built-In SPDT Aux Switch
One adjustable SPDT auxiliary switch (1 mA-3 A) signals damper end position or interlocks other equipment.
Line-Voltage Power
Runs directly on 120 VAC line power — no separate 24 V transformer required.
Technical Specifications
| Parameter | Specification |
|---|---|
| Manufacturer | Belimo |
| Part Number | LF120-S US |
| Actuator Type | Spring-Return (Fail-Safe) Damper Actuator |
| Nominal Voltage | AC 120 V |
| Nominal Frequency | 50/60 Hz |
| Power Consumption (Running) | 3.5 W |
| Torque (Motor & Spring) | 35 in-lb [4 Nm] (minimum) |
| Control Signal | On/Off (two-position) |
| Shaft Range | 3/8 in. to 1/2 in. (to 3/4 in. with K6-1 clamp) |
| Angle of Rotation | 95° (adjustable mechanical stops) |
| Running Time (Motor) | 75 s / 90° |
| Spring Return Time | < 25 s for fail-safe |
| Behavior on Power Loss | Spring returns damper to fail-safe position |
| Fail-Safe Direction | Reversible (CW/CCW) by mounting orientation |
| Auxiliary Switch | 1x SPDT, adjustable 0-95°, 1 mA-3 A (0.5 A inductive) |
| Electrical Connection | 18 GA plenum cable, 3 ft [1 m], 1/2" NPT conduit connector |
| Overload Protection | Electronic, full 0-95° rotation |
| Electrical Protection | Double insulated (Class 2) |
| Enclosure Rating | NEMA 2 / UL Type 2 |
| Ambient Temperature | -22 to 122°F [-30 to 50°C] |
| Agency Listing | cULus (UL60730-1A/-2-14, CAN/CSA) |
| Quality Standard | ISO 9001 |
| Plenum Rated | UL 2043 (suitable for air plenums) |
| Weight | 3.6 lb [1.6 kg] |
| Warranty | 5 Years |
Documentation
Related Products
Accessories & Mounting
Common Belimo accessories for mounting, linkage, signaling and powering the LF120-S US, drawn from the LF actuator datasheet. All are stocked by ZOT Supply as an authorized Belimo distributor.
Mounting & Linkage
Electrical Add-Ons
Frequently Asked Questions
What is the Belimo LF120-S US?
The Belimo LF120-S US is a compact spring-return (fail-safe) damper actuator for small commercial HVAC dampers. It produces 35 in-lb [4 Nm] of torque, uses on/off (two-position) control on AC 120 V power, and on power loss its spring drives the damper to a fail-safe position in under 25 seconds.
Is the LF120-S US a spring-return (fail-safe) actuator?
Yes. A mechanical spring drives the damper to its fail-safe position when power or the control signal is lost — no external power is needed to fail safe.
What happens to the LF120-S US on power loss?
Its internal spring returns the damper to the fail-safe position in under 25 seconds. The fail-safe direction (CW or CCW) is set by the mounting orientation.
What shaft sizes does the LF120-S US fit?
It direct-couples to round damper shafts from 3/8 in. to 1/2 in. (to 3/4 in. with the K6-1 accessory clamp) using its universal clamp. For shafts up to 3/4 in., use the K6-1 accessory clamp.
What control signal does the LF120-S US use?
It uses on/off (two-position) control. Two-position on/off control is driven from a controller, relay or switch.
Does the LF120-S US have an auxiliary switch?
Yes. It has one adjustable SPDT auxiliary switch (rated 1 mA to 3 A) for end-position signaling or interlocking other equipment.
What voltage does the LF120-S US run on?
The LF120-S US is a line-voltage actuator running on 120 VAC, so no separate 24 V control transformer is required.
Is the Belimo LF120-S US available from an authorized Belimo distributor?
Yes. ZOT Supply is an authorized Belimo distributor and carries the LF120-S US. Every unit ships factory new with Belimo's full 5-year warranty and application support.
- Actuator Type:
- Spring-Return (Fail-Safe)
- Torque:
- 35 in-lb
- Control Signal:
- On/Off
- Voltage:
- 120 VAC
- Auxiliary Switch:
- 1x SPDT
- Series:
- LF
