Power take-off

thumb|A PTO at the rear end of a farm [[tractor]]

thumb|A PTO (in the box at the bottom) in the center of the [[three-point hitch of a tractor]]

A power take-off or power takeoff (PTO) is one of several methods for taking power from a power source, such as a running engine, and transmitting it to an application such as an attached implement or separate machine.

Most commonly, it is a splined drive shaft installed on a tractor or truck allowing implements with mating fittings to be powered directly by the engine.

Semi-permanently mounted power take-offs can also be found on industrial and marine engines. These applications typically use a drive shaft and bolted joint to transmit power to a secondary implement or accessory. In the case of a marine application, such as shafts may be used to power fire pumps.

In aircraft applications, such an accessory drive may be used in conjunction with a constant speed drive. Jet aircraft have four types of PTO units: internal gearbox, external gearbox, radial drive shaft, and bleed air, which are used to power engine accessories. In some cases, aircraft power take-off systems also provide for putting power into the engine during engine start. See also Coffman starter.

History

thumb|Yellow shaft provides PTO drive to a [[flail mower on this International Harvester tractor]]

Various power transmission methods were available before power take-offs became common, but there were applications which would benefit more from some of the attributes that PTOs would provide. Flat belts were generally only useful for applications where the engine was stationary, such as factory steam engines, portable stationary engines, or traction engines parked in front of the work. For moving vehicles such as a traction engine or early tractor towing a farm implement, the implement could receive rotary power by taking it from one of its own wheels (whose turning was imparted by the towing) and distributing it via roller chains (to a sickle bar's crank, for example), but such a transmission ceases if the vehicle stops traveling, and the workload's resistance tends to make the wheel skid rather than turn, even if cleated. The concept of a shaft drive with easily connected and disconnected couplings, and flexibility for driving at changing angles (such as when an articulated tractor-and-trailer combination turns), was a goal to pursue.

Experimental power take-offs were tried as early as 1878, and various homemade versions were constructed over the subsequent decades.

International Harvester Company (IHC) was first to market with a PTO on a production tractor, with its model 8-16, introduced in 1918. Edward A. Johnston, an IHC engineer, had been impressed by a homemade PTO that he saw in France about a decade before, improvised by a French farmer and mechanic surnamed Gougis. Despite much work to reduce the frequency and severity of agricultural injuries, these events still occur.

Some implements employ light free-spinning protective plastic guards to enshroud the PTO shaft; these are mandatory in some countries.

In the UK, Health and Safety Executive guidance is contained in a leaflet.

Technical standardization

Agricultural PTOs are standardized in dimensions and speed. The ISO standard for PTOs is ISO 500,

Two newer types, supporting higher power applications, operate at 1000 rpm and differ in shaft size.

Inch-denominated shafts are round, rectangular, square, or splined; metric shafts are star, bell, or football-shaped.

{|class="wikitable sortable

! Type

! RPM

! Diameter

! Splines

| 1

| 540

| 6 straight

| 2

| 1,000

| 21 involute

| 3

| 1,000

| 20 involute

| 4

| 1,300

| 22 involute

Use on commercial vehicles

thumb|A hydraulic PTO mounted on a truck gearbox

thumb|Rear crossbar with PTO hydraulic connectors and coupling on a [[Unimog 421]]

Truck transmissions have one or more locations which allow for a PTO to be mounted. The PTO must be purchased separately and care is required to match the physical interface of the transmission with a compatible PTO. PTO suppliers will usually require details of the make, model and even serial number of the transmission. Care is also needed to ensure that the physical space around the transmission allows for installation of the PTO. The PTO is engaged and disengaged using the main transmission clutch and a remote control mechanism which operates on the PTO itself. Typically, an air valve is used to engage the PTO, but a mechanical linkage, electric or hydraulic mechanism are also options.

Most Unimogs come with front and/or rear PTOs and hydraulics as well as three point hitch systems.

Units will be rated according to the continuous and intermittent torque that can be applied through them and different models will offer different "PTO shaft rotation to engine RPM" ratios.

In the majority of cases, the PTO will connect directly to a hydraulic pump. This allows for transmission of mechanical force through the hydraulic fluid system to any location around the vehicle where a hydraulic motor will convert it back into rotary or linear mechanical force. Typical applications include:

Running a water pump on a fire engine or water truck
Running a truck mounted hot water extraction machine for carpet cleaning (driving vacuum blower and high-pressure solution pumps)
Powering a blower system used to move dry materials such as cement
Powering a vehicle-integrated air compressor system
Raising a dump truck bed
Operating the mechanical arm on a bucket truck used by electrical maintenance personnel or cable TV maintenance crews
Operating a winch on a tow truck
Operating the compactor on a garbage truck
Operating a Boom/Grapple truck
Operating a truck mounted tree spade and lift-mast assembly

Split shaft

A split shaft PTO is mounted to the truck's drive shaft to provide power to the PTO. Such a unit is an additional gearbox that separates the vehicle's drive shaft into two parts:

The gearbox-facing shaft which will transmit the power of the engine to the split shaft PTO;
The axle-facing shaft which transmit the propelling power to the axle.

The unit itself is designed to independently divert the engine's power to either the axle-facing shaft or the additional PTO output shaft. This is done by two independent clutches like tooth or dog clutches, which can be operated at total driveline standstill only. Because the main gearbox changes the rotation speed by selection of a gear, the PTO cannot be operated while the vehicle is moving.

On 4x4 vehicles, only the rear drive shaft is used by the split shaft PTO gearbox, requiring the vehicle's 4x4 drive scheme to be of the selectable 4WD type to keep the front axle drive shaft completely decoupled during PTO operation.

It is also possible to connect something other than a hydraulic pump directly to the PTO: for example, fire truck pumps.

"Sandwich" split shaft

A "sandwich" type split shaft unit is mounted between engine and transmission and used on road maintenance vehicles, fire fighting vehicles and off-road vehicles. This unit gets the drive directly from the engine shaft and can be capable of delivering up to the complete engine power to the PTO. Usually these units come with their own lubricating system. Due to the sandwich mounting style, the gearbox will be moved away from the engine, requiring the driveline to accommodate the installation.

References

External links

Hayneks Hydraulic — Manufacturer and supplier of power take-off (PTO) systems and hydraulic pumps for industrial vehicles.