|Articles > Tractor technical articles|
Power Take-Off (PTO) Types
The simpliest, and earliest, form of PTO is the transmission PTO. The PTO shaft is directly connected to the tractor's transmission. The PTO is only working when the tractor's clutch is relased, so if you take the tractor out of gear while slowing down the PTO will stop working. This is a disadvantage in applications such as mowing.Live (two-stage clutch)
A live PTO works with the use of a two-stage clutch. Pressing the clutch half-way will disengage the transmission while pressing it fully will disengage the transmission and the PTO. This allows the operator to slow down or change gears while the PTO is still operating.Independent
An independent PTO means that the PTO shaft is controlled with a separate clutch. As with a live PTO, this allows for full control over the tractor while separately controlling the PTO. There are two major types of independent PTO; mechanical and hydraulic. A mechanical-independent PTO uses a separate on-off selector, in addition to the PTO control lever. Often the tractor must be stopped or off to change this selector position. A hydraulic-independent PTO uses a single selector.The power take-off (PTO) drive, found on most farm tractors, is a major power source for agricultural equipment. The PTO system efficiently transfers mechanical power from the tractor to the implement, but it may present serious hazards to persons who are unfamiliar with its operation.
When hooked up to a piece of equipment, the complete PTO system or implement input driveline may connect either directly to the tractor spline or stub, or indirectly through a pedestal connection as illustrated in figure 1. Coupling devices are used to attach the shaft to the tractor and to the implement.
A spline collar slides over the tractor's spline, which extends from the tractor differential. The collar is held in place by a spring-loaded pin that latches into a recess on the spline. The telescoping feature of the shaft allows the collar to slide easily onto the spline. Together with the universal joints, the telescoping shaft allows the PTO system to flex and adjust when the tractor turns or travels over uneven terrain.
Figure 1 is a diagram of component parts of an implement PTO to better understand PTO hazards, guarding, and injuries. The upper drawing is of a PTO system involving a pedestal connection as found on many types of pulled machinery (e.g., hay balers, forage choppers, large rotary mowers, etc.). The bottom drawing is of a PTO system where the implement’s input driveline connects directly to the tractor PTO stub. Examples of this type of connection include three-point hitch mounted equipment (e.g., post hole diggers, small rotary mowers, etc.) and augers. The flexible universal joint or “U joint” makes the connection from the tractor to the implement. U Joints are connected by a square rigid shaft which turns inside another shaft.
The tractor’s stub shaft, often called the PTO, transfers power from the tractor to the PTO-driven machine or implement. Power transfer is accomplished by connecting a drive shaft from the machinery to the tractor’s PTO stub shaft. The PTO and drive shaft rotate at 540 rpm (9 times/second) or 1,000 rpm (16.6 times/second) when operating at full recommended speed. At all speeds, they rotate in proportion to the speed of the tractor engine. Note: 1000 rpm speed PTO shafts have more splines on the shaft.
Most incidents involving PTO stubs result from clothing caught by an engaged but unguarded PTO stub. The reasons a PTO stub may be left engaged include: the operator forgetting or not being aware of the PTO clutch is engaged; seeing the PTO stub spinning but not considering it dangerous enough to disengage; or, the operator is involved in a work activity requiring PTO operation. Boot laces, pant legs, overalls and coveralls, and sweatshirts are clothing items that can become caught and wrapped around a spinning PTO stub shaft. In addition to clothing, additional items that can become caught in the PTO include jewelry and long hair.
The PTO driveline is identified as a mechanical wrap point hazard and is one of the oldest and most common farm machinery hazards, referring specifically to the part of the implement (machine) drive shaft that connects to the tractor. This drive shaft is known as the implement input driveline (IID). The entire IID shaft is a wrap point hazard if the IID is completely unshielded.
If the IID shaft is partially guarded, the shielding is usually over the straight part of the shaft, leaving the universal joints, the PTO connection (front connector), and the Implement Input Connections (IIC, the re
PTO power machinery may be engaged while no one is on the tractor for several reasons. Some PTO powered farm equipment is operated in a stationary position so the operator only needs to start and stop the equipment. Examples of this type of equipment include elevators, grain augers, and silage blowers. At other times, adjustments or malfunction of machine components can only be made or found while the machine is operating.
Although PTO entanglement incidents have decreased over time compared with other causes of farm fatalities, Pennsylvania statistics over a recent ten year period records five fatalities showing that attention to PTO safety continues to be important.
Guarding a PTO system includes a “master shield” for the tractor PTO stub and connection end of the implement input driveline (IID) shaft, an integral-journal shield which guards the IID shaft, and an implement input connection (IIC) shield on the implement. The PTO master shield is attached to the tractor and extends over and around the PTO stub on three sides. This shield is designed to offer protection from the PTO stub and the front joint of the drive shaft of the connected machine. Many tractors, particularly older tractors, may no longer have PTO master shields. Master shields are removed or are missing from tractors for several reasons including: damaged shields that are never replaced; shields removed for convenience of attaching machine drive shafts; shields removed out of necessity for attaching machine drive shafts; and shield missing when used tractors are sold or traded.
These examples of PTO injury incidents involving Pennsylvania farmers will help illustrate the serious nature of PTO hazards:
Case #2: A teenager was helping her family load corn onto a grain elevator when her jacket sleeve because entangled by the elevator PTO shaft. Her body was flung around the shaft and her arm was torn from its socket before the tractor could be turned off.
Case #3: A small child was killed when as an “extra rider” on his father’s tractor; he slipped off the tractor and became entangled by a spinning PTO shaft. The father grabbed for the boy as he began to slip but was unable to hold him out of the shaft.
Case #4: An operator’s clothing was near a spinning shaft, pulled him in, flung him around the shaft a couple of times, and then threw him clear. He sustained injuries to his head, leg, right arm, and shoulder.
Though not always convenient or easy, there are several ways to reduce the risk of PTO injury incidents. These safety practices offer protection from the most common types of PTO entanglements.