Power take-off

A power take-off (PTO) is a splined driveshaft, usually on a tractor or that can be used to provide power to an attachment or separate machine. It is designed to be easily connected and disconnected. The power take-off allows implements to draw energy from the tractor's engine.

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

Safety

The PTO and its associated shafts and universal joints are a common cause of incidents and injury in farming and industry. According to the National Safety Council, 6 percent of tractor related fatalities in 1997 in the USA involved the PTO. When a piece of clothing, which can be as small as a single thread, touches a spinning part it can be pulled around the part. The clothing and the person wearing it are pulled into the shaft often resulting in loss of limb or death. Some implements do use plastic guards to try to keep a person from becoming entangled in a PTO shaft, but even with guards people need to exercise caution around PTO shafts when they are plugged into a tractor or truck . In some countries it is illegal to operate a PTO without the shaft guard correctly fastened.^[1][2]

Technical Standardization

Agricultural PTOs are standardized in dimensions and speed. The ISO standard for PTOs is ISO 500, which as of the 2004 edition was split into three parts ISO 500-1 (General specifications, safety requirements, dimensions for master shield and clearance zone), ISO 500-2 (Narrow-track tractors, dimensions for master shield and clearance zone), and ISO 500-3 (Main PTO dimensions and spline dimensions, location of PTO). The original type calls for operation at 540 revolutions per minute (RPM). A shaft that rotates at 540 rpm has 6 splines on it, and a diameter of 1⅜". Two newer types, supporting higher power applications, operate at 1000 RPM and differ in shaft size. The larger shaft has 20 splines (1¾" diameter), while the smaller has 21 (1⅜" diameter). All three types rotate counterclockwise when viewed from the tractor. A 10 spline type was used with some early equipment such as the 1948 Land Rover, a six spline adapter was usually supplied. It is customary for agricultural machines manufacturers to provide the nominal PTO power specification, an indication of the available instantaneous power at the shaft.

Commercial Vehicle PTOs

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/disengaged using the main transmission clutch and a remote control mechanism which operates on the PTO itself. Typically an air valve is use to engage the PTO, but a mechanical linkage, electric or hydraulic mechanism are also options.

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
• Powering a blower system used to move dry materials such as cement
• Raising and lowering 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

It is also possible but less common to connect something other than a hydraulic pump directly to the PTO.^[3]

References