Description
Automatic Torque Limiter for PTO Shaft | Cam-Type Slip Clutch
The Automatic Torque Limiter — also known as a cam-type slip clutch, spring-cam safety coupling, or automatic-reset friction clutch — represents the premium tier of PTO overload protection. Engineered with a spring-pack and cam mechanism, this limiter delivers smooth, progressive slip when torque exceeds the preset threshold and automatically re-engages the instant conditions normalize. Spanning torque ratings from 1,600 Nm to 4,500 Nm across two frame sizes (SC151 and SC176), plus flange-mount SCF variants, it is the go-to solution for heavy-duty implements requiring both high torque capacity and automatic reset without operator intervention.
Specifications — SC151 Frame (⌀152 mm)
| Series | Torque (Nm) | Spline | A (mm) | B (mm) | C (mm) |
|---|---|---|---|---|---|
| 4SC151 / 4SC151.D | 1600 | 1 3/8"Z6 / Z21 | 207.5 | 28 | ⌀152 |
| 5SC151 / 5SC151.D | 2100 | 1 3/8"Z6 / Z21 | 207.5 | ||
| 6SC151 – 6SC151.E | 2600 / 2100 | Z6 / Z21 / 1 3/4"Z6 / Z20 | 213 | ||
| 7NSC151 – 7NSC151.E | 2800 / 2600 | Z6 / Z21 / 1 3/4"Z6 / Z20 | 213 | ||
| 8SC151 – 8SC151.E | 2800 / 2600 | Z6 / Z21 / 1 3/4"Z6 / Z20 | 220 | ||
| 36SC151 – 36SC151.E | 2800 / 2600 | Z6 / 1 3/4"Z6 / Z20 | 213 |
Specifications — SC176 Frame (⌀176 mm)
| Series | Torque (Nm) | Spline | A (mm) | B (mm) | C (mm) |
|---|---|---|---|---|---|
| 8SC176 – 8SC176.E | 3000 / 3500 | Z6 / Z21 / 1 3/4"Z6 / Z20 | 224 / 233 | 28 | ⌀176 |
| 9SC176 – 9SC176.E | 4000 / 4500 | Z6 / Z21 / 1 3/4"Z6 / Z20 | 233 |
Specifications — SCF (Flange Mount)
| Series | Torque (Nm) | Spline | A (mm) | B (mm) | C (mm) |
|---|---|---|---|---|---|
| SCF151 – SCF151.E | 1600 – 2800 | Z6 / Z21 / 1 3/4"Z6 / Z20 | 171 | 29 | ⌀168 |
| SCF176 – SCF176.E | 3000 – 4500 | Z6 / Z21 / 1 3/4"Z6 / Z20 | 175 | 28 | ⌀176 |
Component list: Outer housing + yoke (or flange), Bushing, Spring pack, Hub with taper pin and bushings, Bushing, Sealing ring, O-ring, Locking plate, Snap ring, Bushing (Qty.=4), Cam (Qty.=4), Taper pin (with nuts).
Why Choose Automatic Torque Limiters
Cam-and-spring mechanism slips smoothly under overload and locks again automatically — no manual intervention or replacement parts needed.
Up to 4,500 Nm capacity handles the most demanding PTO implements including large disc mowers and industrial shredders.
Available in 1 3/8" Z6, Z21, 1 3/4" Z6, and Z20 profiles covering 540 RPM and 1000 RPM PTO systems worldwide.
O-ring and sealing ring protect internal components from dust, moisture, and crop residue intrusion.
SC series with yoke for universal joint connections; SCF series with flanged housing for direct bolting to implement gearboxes.
Unlike abrupt shear-bolt disconnection, cam-type slip dissipates energy gradually, reducing shock loading on both tractor and implement.
How the Cam Mechanism Works
Four hardened steel cams are positioned around the hub, each riding on the hub’s cam profile. A powerful spring pack applies axial force through bushing surfaces to maintain cam engagement. During normal operation, the cams wedge into detent pockets in the outer housing, transmitting full torque. When an overload occurs, the transmitted torque forces the cams to ride up their ramps against the spring pack. This creates a controlled slip: the outer housing rotates relative to the hub while the spring pack absorbs energy. As soon as the overload drops below the spring’s holding force, the cams re-seat in their pockets and the drive connection restores. This cycle can repeat indefinitely, making the automatic torque limiter ideal for continuous-duty, high-reliability applications.
Implement Compatibility
High-inertia cutter bars with frequent stone-strike events benefit from smooth, auto-resetting slip rather than bolt fracture.
Embedded rocks and compacted soil layers cause heavy shock loading that the cam mechanism absorbs progressively.
Variable-density material feeding creates unpredictable torque spikes; automatic reset means the chipper self-recovers without stopping.
Type Comparison
| Criterion | Ball Detent | Friction Disc | Shear Pin | Automatic (Cam) |
|---|---|---|---|---|
| Max Torque | 3200 Nm | 3000 Nm | 4500 Nm | 4500 Nm |
| Slip Characteristic | Detent pop-out | Continuous friction | Bolt fracture | Progressive cam ride |
| Reset | Auto | Continuous | Manual | Automatic |
| Optimal Use | Moderate shock | Gradual overload | Catastrophic events | Repeated heavy shock |
Installation Procedure
- Verify the spline type and frame size match your PTO shaft and implement.
- Mount the hub onto the PTO shaft, ensuring the taper pin is fully seated and the nut torqued to specification.
- Slide the outer housing over the hub; confirm the cams engage in their pockets.
- For SCF models, bolt the flange to the implement gearbox using the correct bolt pattern.
- Check the sealing ring and O-ring for proper seating to prevent contamination.
- Apply grease and test by hand-rotating — the unit should lock in the drive direction and slip smoothly when forced beyond the setting.
Important: Never weld on or near the limiter housing. Heat will damage the spring pack calibration and void the warranty.
Selection Recommendations
Choose the SC151 frame (⌀152) for applications up to 2,800 Nm and the SC176 frame (⌀176) for 3,000–4,500 Nm. For implements with flange-mounted inputs, select the corresponding SCF model. Match the spline variant to your tractor’s PTO output: Z6 for 540 RPM standard, Z21 for 540 RPM economy, or 1 3/4" Z6/Z20 for 1000 RPM. Always confirm that the “A” dimension (overall length) is compatible with your driveline geometry.
Quality Assurance
Produced in an ISO 9001:2015 certified facility. CE marked for European compliance. Spring packs are fatigue-tested to 100,000 cycles. Cam surfaces are case-hardened to HRC 58–62 for maximum wear resistance. Backed by a 12-month limited warranty and responsive technical support.
Frequently Asked Questions
Can the automatic torque limiter handle continuous slip?
Short-duration slip events are handled without issue. Continuous slip generates heat that can degrade the spring pack; clear the obstruction promptly if the limiter does not reset within a few seconds.
What is the difference between SC and SCF?
SC features a yoke output for connection to a universal joint or cross kit. SCF features a flanged housing for direct bolting to an implement gearbox input flange. Internal torque ratings are identical.
How do I know if the spring pack needs replacement?
If the limiter begins slipping at noticeably lower torque than rated, the spring pack may have fatigued. Remove the unit and measure free length against the factory specification.
Is the D suffix important?
The suffix indicates the spline profile: no suffix = 1 3/8" Z6; D = 1 3/8" Z21; C = 1 3/4" Z6; E = 1 3/4" Z20. Choose according to your tractor’s PTO output specification.
Can I retrofit this onto an existing PTO shaft?
Yes, provided the spline and dimensional requirements are met. The automatic torque limiter mounts inline in place of a standard yoke on any compatible PTO shaft.
Customer Stories
“Running the 9SC176 on a 150-HP rotary tiller in heavy clay. The cam clutch slips smoothly when we hit rock and re-engages instantly. No downtime.”
“The SCF151 bolts directly onto our Comer gearbox. Perfect fit, smooth operation, and built to last.”
“Upgraded from shear bolts to the automatic limiter. Best decision I made — zero bolt changes all season on our disc mower.”
“We run 8SC176 units across our sugarcane operation. Reliable protection in extremely harsh conditions.”
“The automatic reset saves labor. Our operators no longer need to leave the tractor cabin to replace broken bolts.”
Explore Our Product Line
We manufacture complete PTO driveline solutions including shear bolt limiters, friction clutches, ratchet clutches, overrunning clutches, wide-angle joints, SD-double joints, yokes, safety shields, and adapters. Every product is designed for seamless interchangeability within our driveline ecosystem.
