The Difference Between “Automotive-Style” and “Over-Center” PTOs
This Tech Tip focuses on the basics of spring- loaded automotive style clutches as used in wood chippers.
Many of our engine customers operate diesel driven power units that have industrial PTO clutches. To support them we sell PTOs, clutch packs and replacement parts. Like with everything else, clutches have a different terminology than engines. There are two main kinds of industrial clutches. The most common kind is the traditional “over-center power takeoff clutch” or PTO. These are manufactured by; Twin Disc®, Rockford™, NACD®, WPT Power® and others. These units rely on a flywheel that accommodates a drive ring, into which are nested friction disks, when the clutch is engaged, drive the PTO output shaft, but when disengaged spin freely. We stock WPT Power, Rockford, NACD and aftermarket Twin Disc to make sure our customers can get what they need when they need it. We discuss these over-center PTO’s elsewhere on our site in
- Tech Tip #48: Maintaining your Rockford and Twin Disc PTO,
- Tech Tip #67: How to Remove A Troublesome Pilot Bearing,
- Tech Tip #93: What Do I Need To Find The Right Parts For My PTO Clutch, and
- Tech Tip #147: Not Sure Why Your PTO Keeps Wearing Down? Want It To Last Longer?
The other style clutch is called the “automotive style”. The design of these units utilizes a spring-loaded assembly similar to what you might find in a pickup truck with a splined shaft. They rely on a flat faced engine flywheel that the PTO friction disc presses against to transfer rotational force to the PTO output shaft or, when disengaged, releases pressure from the flywheel face to stop the PTO shaft from spinning. It is often used on wood chippers, tub grinders and leaf vacumes manufactured by Bandit Industries®, Morbark®, Vermeer® and ODB™. Manufacturers tend to select automotive style clutches because the spring-loaded clutch is a little easier to engage and disengage than the traditional over center PTO and the units are slightly more compact allowing equipment manufacturers to fit them into a smaller area. While Twin Disc, Rockford, NACD and WPT Power do have automotive style clutches (in fact, most Ford® engine powered chippers run Rockford automotive style clutches) the prominence of these brands with equipment manufacturers ebbs and flows. More recent companies like Auto Clutch and Stein gained traction with diesel powered chippers, however in the recent past we believe that their focus on supporting the equipment manufacturer instead of the end user has made getting the correct components quickly and cost effectively more of a challenge than it ought to be. Give us a call an we can help you find what you need.
Common Clutch Nomenclature For Automotive Style PTOs.
Clutch Cover . This is an outer shell that is usually constructed from stamped steel. The clutch cover contains four main components. The pressure plate, the diaphragm spring/springs, pivot ring, and drive straps. The pivot ring provides the pivot point for the diaphragm and when combined with the diaphragm springs and drive straps connects the assembly to the cover and allows the pressure plate to be moved in or out. The pressure plate then acts as one of the friction surfaces (i.e. pressure plate on one site, flywheel on the other and the friction disc in the middle) pressing the clutch disc between the pressure plate and the face of the flywheel to transfer the engine’s rotation to the PTO drive shaft or, when disengaged, the pressure plate releases by springing back, releasing the disc from the flywheel and thereby stopping the transfer of the engine’s rotational force to the PTO output shaft.
Clutch Disc. This is the most critical part of the system. The transmission input shaft goes into the splines of the disc and slides backwards and forwards as the clutch is engaged. Clutch discs have a hub in the center with a number of internal teeth or “splines” to mate with the transmission input shaft which is also splined. The clutch disc also contains a number of torsion springs or idle-stage dampers. Most clutch discs are round and made from an organic material. Recently though, drag racing technology has migrated to the industrial clutch field. Increasingly clutch discs are shaped in paddle format that contain a Kevlar friction material on each paddle.
Clutch Kits. Most of our customers replace automotive-style clutches as a complete kit assembly. This includes the pressure plate, the disc, and the pilot bearing. As a complete assembly everything will mate together well for many thousands of hours of use. We recommend this approach as opposed to piecemealing the assembly so that newer wear components don’t wear out pre-maturely due to maintaining older worn components. This also tends to avoid some of the individual component variations that don’t work with certain separate components which are typically not well labeled if at all by the manufacturers.
How to order? When ordering a clutch kit from us simply give us the numbers from the inspection plate on the cover to the PTO. Older Rockford PTO’s will have a number beginning with PTA and newer Rockford or NACD PTO’s will have an Assembly Number resembling 4-25754 or 42574AM that starts with a “4”. Auto Clutch numbers are similar to HD1300 which comes with a 1.75″ diameter output shaft or a 2.25″ output shaft and should have a PTO spline diameter of 1.375″ (tip of spline to opposite tip). The HD1300XT150 should have a 2.25″ output shaft and a 1.5″ spline diameter and the HD1300XT175 should have a 2.25″ output shaft diameter with a 1.75″ spline diameter and all will use a 13″ disc, however discs may have differing paddle configurations and dampening springs depending on the unit. If the plate is gone, give us a call, let us know the equipment, brand of engine, the outside diameter of the clutch disc and the inside diameter of the splined bore (from bottom of groove to bottom of the opposite groove) of on the friction disc. We can take care of the rest!
Manufacturers names, symbols and numbers are for reference purposes only and do not imply manufacturing origin.
