PTOs & GEARBOXES
gearboxes PTOS —
The Power Take-Off, most commonly referred to by its acronym, PTO, is a common form of mechanical power delivery in the mobile machine market. The PTO is a method of transferring high power and torque from the engine (usually through the transmission) of trucks and tractors. In combination with gearboxes and pump mounts, nearly any type of mechanical power transmission is possible. There are three common power take-off methods in the mobile machine market: tractor style, truck transmission style and engine crankshaft-driven, although the latter is not commonly referred to as a PTO. The crankshaft-driven method of power transmission is often used for hydraulic pumps mounted to the front of an on-highway truck, such as a plow/spreader or cement mixer. In this drive configuration, a small shaft with U-joints attaches to a yoke coupler to turn the pump.
The tractor PTO goes back pretty much as far as tractors do. Most early PTOs were driven from the transmission, which was located at the back of the tractor, to allow for easy location of an output shaft. The transmission type of PTO is only engaged when the transmission clutch is also engaged, and is coupled directly to the transmission, so that when the clutch is depressed, the PTO isn’t driven. If the transmission is driving the wheels, then the transmission PTO is turning. This also means the implement can backward-power the transmission as well as when the clutch is depressed, such as down a hill or if the attachment has a mechanism with high rotational inertia, resulting in surging of the drive wheels. This can be avoided by adding a dedicated overrunning clutch for the PTO, which prevents torque from being applied in the opposite direction. A live PTO often uses a transmission clutch with two stages. The first stage of the clutch operates the driven portion
DOUBLE GEAR type power take-off Image courtesy of Muncie Power Products Inc.
of the transmission, and the second stage of the clutch controls the engagement of the PTO. This method allows independent control of the transmission, so that the PTO maintains operation regardless of transmission clutch activity, including stopping of the tractor itself. For a tractor with a mower attachment, for example, this is a minimum requirement; you can’t have the mower turn off when you feather the clutch up a hill and around a tree. The independent PTO has an entirely separate engagement clutch and works altogether exclusively of the transmission clutch. To engage the PTO, a button or lever is pushed; it’s that simple. Usually, a lever operated clutch is required to be activated before a separate PTO switch. Not only is the system easy, the independent PTO can be activated while the tractor is stopped or moving. Independent PTOs are available in both mechanical and hydrostatic 6 • 2016
FLUID POWER WORLD
6/17/16 9:37 AM
Fluid Power World Handbook 2016