Bareco PTO Safety Clutches and Overrun Clutches: What They Do and How to Choose

Last updated June 2026

A PTO driveline running without overload protection is an expensive gamble. When a slasher strikes a hidden stump, a post hole digger hits rock, or a blocked implement stalls — the shock load transmitted back through an unprotected driveline can destroy a PTO shaft, shear a gearbox input shaft, or damage the tractor's own PTO in an instant. The repair bill typically runs to many times the cost of the clutch that would have prevented it.

Bareco 9982 Series PTO safety clutches and overrun clutches protect the driveline by slipping when torque exceeds a set limit, absorbing the shock rather than transmitting it to the implement gearbox, PTO shaft, and tractor. The range covers 35HP to 220HP in standard friction plate and yoke-type configurations, with and without overrun capability.

Browse Bareco PTO clutches →

Safety Clutch vs Overrun Clutch — What's the Difference

A safety clutch (also called a slip clutch or shear clutch) protects the driveline from overload by slipping when the torque load exceeds the clutch's rated threshold. When a blocked implement stalls or a sudden impact occurs, the clutch slips rather than transmitting the full shock through the shaft. Once the blockage clears, the clutch re-engages and normal operation resumes. Safety clutches protect in both drive directions.

An overrun clutch adds a second function: it allows the implement to continue rotating under its own inertia after the tractor PTO is disengaged, without that rotation feeding back through the driveline. This matters on high-inertia implements — particularly slashers and mulchers, where the heavy rotor can take 30 seconds or more to spin down. Without overrun protection, the rotating implement drives back through the PTO shaft to the stopped tractor, causing driveline stress and potential joint damage. Overrun clutches address this — they disengage when the implement speed exceeds the drive shaft speed, allowing free coasting.

For most slashers and mulchers, an overrun clutch (or a clutch with both safety and overrun capability) is the correct specification. For post hole diggers and implements with low rotational inertia, a standard safety clutch is generally sufficient.

How to Identify the Right Clutch

Clutch selection depends on four factors:

1. HP rating — match to your tractor's PTO output at 540 or 1000 RPM
2. Spline specification — match to both the drive shaft spline and the implement input shaft spline
3. Safety only or safety with overrun — based on the implement type (see above)
4. Clutch type — standard friction plate or yoke type (see below)

Standard Friction Plate Clutches — B Series

The B-series friction plate clutches fit between the PTO drive shaft and the implement input shaft. Available from 35HP to 220HP with a range of spline combinations to match common tractor and implement shaft specifications.

Key variants by HP range

Additional variants with mixed spline configurations (e.g. 1-3/4" x 20 spline drive side / 1-3/8" x 6 spline implement side) are available for cross-spline applications. See the full product listing for all configurations.

Note on B4166: This variant uses a round bore with keyway on the implement side, specifically suited to post hole digger gearboxes with a keyed input shaft rather than a splined connection.

View B-series safety and overrun clutches →

Yoke Type Clutches — AK/AC Series

Yoke type clutches mount within the PTO drive shaft assembly rather than as a separate inline unit. Identification requires matching the clutch series to the drive shaft series, which is done by measuring the universal joint cap OD, cap-to-cap OD, and yoke OD on the existing shaft. The clutch slips when torque exceeds the rated NM threshold, protecting the shaft and connected components.

To identify the correct yoke type clutch, measure the universal joint cap OD, cap-to-cap OD, and yoke OD (ear to ear) on the drive shaft. These three measurements determine the drive shaft series, which then determines the correct clutch. Reference dimension diagrams for each series are available on the product page. Before first use, compress the springs fully and back off two turns to set the slip torque.

View yoke type clutches →

Which Clutch Type Do I Need?

Slashers and mulchers: Always use a clutch with overrun capability. The high rotational inertia of a heavy rotor means significant reverse torque when the PTO is disengaged. A standard safety-only clutch does not address this; an overrun clutch does.

Post hole diggers: A standard safety clutch is generally sufficient. The B4166 is specifically configured for post hole digger gearboxes with a keyed input shaft. A clutch with overrun is not required for this application.

Cultivators and tillage implements: A standard safety clutch at the appropriate HP rating. Overrun is generally not required.

Adding a clutch to a Bareco complete shaft: Bareco 9985 Series complete drive shafts are available with integral safety clutches already fitted (SB-prefix part numbers). If you are building a driveline from components, a separate inline safety clutch can be added. Call 1300 027 886 to confirm the correct clutch for your shaft and implement combination.

Browse all Bareco PTO clutches →  |  PTO shafts guide →  |  Full Bareco range →

Frequently Asked Questions

Do I need a safety clutch on my slasher?

Yes — and specifically one with overrun capability. Slasher rotors carry significant rotational inertia and continue spinning for 20–40 seconds after the PTO is disengaged. Without an overrun clutch, the still-spinning rotor drives back through the driveline to the stopped tractor, causing driveline stress and joint damage. A clutch with overrun capability disconnects when implement speed exceeds drive shaft speed, allowing the rotor to coast freely.

What is the difference between a safety clutch and an overrun clutch?

A safety clutch slips when torque exceeds a set threshold in the drive direction, protecting against overload and shock loads. An overrun clutch additionally disconnects when the implement rotates faster than the drive shaft — specifically protecting against reverse torque from high-inertia implements coasting after PTO disengagement. Many Bareco clutches provide both functions in one unit.

How do I know which spline my PTO shaft uses?

Check your tractor operator manual under PTO specifications. Most tractors up to approximately 100HP use 1-3/8" diameter with 6 splines. Larger tractors from around 100HP and above may use 1-3/4" diameter with 6, 20, or 21 splines. The implement input shaft specification is found in the implement manual. Both ends of the clutch must match the respective shaft specifications.

How do I set the slip torque on a yoke type clutch?

Before first use, compress the springs fully then back off two turns. This sets the clutch to the rated slip torque. Do not over-tighten the springs beyond the rated setting — this defeats the overload protection. Spare parts are available on request for all clutch types.