What are PTOs/gearboxes

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 via 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. A small shaft with U-joints attaches to a yoke coupler to turn the pump. This configuration of drive is not generally referred to as a PTO, however.

The tractor PTO goes back pretty much as far as tractors. Most early PTOs were driven from the transmission, which being located at the back of the tractor, allows 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 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 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 was avoided by the addition of 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 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 exclusive 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 configurations, and as you can imagine, the hydraulic type is highly popular with the proliferation of hydrostatic drives.

The PTO shaft extending from back of the tractor is a male-splined structure. There are three types of PTO shafts, and their usage is loosely defined by the power output of the tractor. Each of the shafts runs at one of two standard speeds: 540 and 1000 rpm. Also, each of the shafts comes in one of two diameters; 13⁄8 or 13⁄4 inches. Finally, none of the three shafts share a number of splines:

540 rpm—6 spline—1 3⁄8-in. shaft
1000 rpm—21 spline—1 3⁄8-in. shaft
1000 rpm—20 spline—1 3⁄4-in. shaft

The two 1000-rpm shafts are known as the small (13⁄8-in.) and large (13⁄4-in.) version. Each shaft is designed for an appropriate range of torque and horsepower transmission, and the differing dimensions ensure they’re not used interchangeably. Some tractors can operate more than one of the speed and power ranges of shafts, but it is important to remember that the implements they drive are often limited to a design speed. So if a grain auger is designed to run at 540 rpm, it might not be a good idea to try to crank it up to 1000 rpm.

Safety is serious business with PTO shafts, because unfortunately, they are infamous for causing both injury and death. Implements should only be operated with the PTO safety covers in place, and if there is a reason they don’t exist, extra caution needs to be practiced.

All modern heavy-duty trucks come with the capability to run a power take-off from the transmission. The truck’s transmission will have a cover plate (or two), which can be removed, exposing a spur gear able to drive a PTO. The truck mounted PTO is versatile, although unlike tractor PTOs, they are not quite as universal—mostly because of the countless transmissions on the market.

Regardless, few transmissions have no PTO available. The simplest form of PTO is with a single gear to mesh with the transmission’s output gear. These types are somewhat restricted in both their speed and horsepower range, but are more economical due to their simplicity.

PTOs with two or three gears are more common, and are also available for heavy-duty applications requiring high horsepower, torque and speed. The multiple gears allow for a wider range of speed or torque multiplication.

Gearboxes
There are a few general types of gearboxes, including planetary, spur/helical gear and worm gear. Each has their advantages and applications that they are best suited for. The most common type for mobile and off-highway machinery is the planetary drive. These consist of a center “sun” gear with three or more circumferential “planet” gears, which in turn are surrounded by a ring gear. Planetary gearboxes are very robust, and are also capable of very high radial loads, especially when the load is correctly centered. This is why they’re often found in wheel drive applications.

Spur- or helical-gear style gearboxes use two or more gears meshed side by side, and are often used in industrial applications, such as a rolling mill. Their size and possible gear ratios vary widely, with the upper end of torque capacity only limited by the size of gears able to be manufactured. These types of gearboxes can have input and output shafts on either the same or opposing sides.

Also quite popular are worm gear style gearboxes. They employ a helical axial input shaft, which directs force across a perpendicular gear ring attached to the output shaft. Operation is always with input and output 90° from each other. Worm gear units are generally light to medium duty.

Gearboxes are applied to either increase torque or increase speed, and it should be clear that increasing one decreases the other. When used to increase torque, they can achieve this at ratios anywhere from 2:1 relative to input torque, to 10,000:1 or more…which of course would slow the output speed drastically. When used to increase speed, sometimes planetary or helical/spur gearboxes can be used in reverse. A common use for overdriven gearboxes is for pump drives. A 1000-rpm PTO speed doesn’t take advantage of a hydraulic pump capable of 3000 rpm, so a speed-increasing gearbox/pump mount would allow for the use of a smaller pump. Please note, worm gear reducers do not allow input energy at their output shaft.

Selecting a PTO
When selecting a PTO, you will need to provide your supplier with various parameters, as these units are not universal. You will need to know your transmission model number, your output speed, your torque and horsepower required, and the shifter style desired. The shifter engages and disengages the PTO, and is available with air shift, electric shift and mechanical (cable) shift. Some newer PTOs are operated via a clutch instead of sliding gears, but the application information changes little otherwise.

You will also need to know what pad mount is necessary for the pump you’re running off the PTO, such as SAE 2- or 4-bolt flange, and the series, such as SAE B or SAE C 4-bolt flange. Special consideration should be paid when mounting a pump to the PTO, as port location could interfere with chassis components or the transmission itself. These issues are often resolved by installing an intermediate shaft, allowing the pump to be mounted in a clear location a couple feet behind the transmission.

Even with the high torque able to be transmitted by tractor PTOs, there are occasions when even more torque is demanded than a tractor can transmit (or slower speed than a standard 540 or 1000 rpm PTO). Although implements often employ pulleys and chain drives, which can be used for torque multiplication, the heaviest of demands require a gearbox for reliable and efficient operation. Of course, the use of gearboxes are not limited to farm applications, and are most often seen in wheel drives and other massive-torque low-speed applications.