Partnering ASABE and ISO Tractor PTO Shaft Standards

Resource, Feb 2008 by Mayhew, Roger D

The need for standards of interchangeability between the tractor PTO shaft and implement drive lines began in the 1920s, when there were many cases of farmers not being able to connect implement drive lines to tractor PTO shafts. Tractors often had different PTO shaft splines or diameters that were not the same as the implement drive line yokes. When this occurred, the farmer would have to trade the implement in or buy another implement that would mate with his tractor. Many times, this forced farmers to buy their tractors and implements from the same company, which limited the farmers' options. When farmers are working out in the field, having the ability to change implements can be critical. This lack of interchangeability between tractors and implements began a movement among agricultural engineers to standardize the PTO shaft.

S203 - a venerable standard

The resulting ASAE standard (S203: Rear power takeoff for agricultural tractors) is the Society's oldest standard. The original draft document was developed in 1926 by the Farm Equipment Institute (FEI), now called the Association of Equipment Manufacturers (AEM), and proposed to ASAE, which adopted the standard in 1927. The rotational speed was specified as 536 ±20 rpm. In 1931, the standard specified the shaft and the hubs. It was not until 1941 that the 6B spline was specified as SAE 6B.

Two become one

The 1,000 rpm rear PTO shaft was proposed by FEI to ASAE in 1958. The 1,000 rpm standard was originally S204, which was combined with S203 in 1976. The ASAE standard was gradually improved over the years to have minimal interchangeability problems between the tractor and the implement. One example was the tighter-fitting spline on the 1,000 rpm shaft to reduce vibration of the drive line. However, there were a number of differences between ASAE S203 and the equivalent ISO tractor PTO shaft standards. Industry manufacturers requested that the two PTO shaft standards be combined into one worldwide PTO shaft standard.

Consolidation

Consolidating the ASAE and ISO standards was difficult due to the different cultures and methods of enforcing the standards. Engineering standards in the United States are voluntary, but the shadow of product liability lawsuits becomes a force. In the European Union (EU), the governments adopt engineering standards, and the standards become almost like federal laws. The Committee for European Normalization (CEN) writes the engineering standards for EU member countries. Usually, CEN adopts the appropriate ISO standard, and it becomes a European Norm (EN) standard if it meets their requirements. The EU governments enforce the standards, which in this case include onfarm inspection of tractors and implements. Farmers can be fined for not adhering to the standards. Some EU countries were reluctant to change any ISO or EN standard because that meant they would have to petition CEN.

Meeting of cultures

One of the biggest problems in the consolidation was the width of the master shield. Some large constant-velocity joints would not fit in the narrow master shields. In addition, one European country uses narrow hitches that do not accept a wider master shield, and those hitches are not used outside of that country. Another area of considerable discussion was the use of the 1-3/8 6B spline at 1,000 rpm in Europe. In North America, that spline is only used on a 540-rpm shaft, and the implements are designed for that speed. Any implement that is designed for 540 rpm will have serious problems when attached to a 1,000 rpm PTO shaft.

The height of the PTO shaft above the ground was another area of long discussions. The height above the ground is affected by the tire diameter, which varies considerably. Nevertheless, after many hours of meetings in Europe, the final balloting of the new ISO 500 was finished, and the standard was approved in 2004. ASABE is in the process of adopting ISO 500 and withdrawing S203. The benefits of adopting one worldwide standard are great.

ASABE member Roger D. Mayhew, P.E., consultant, is retired vice president of engineering at Weasler Engineering, Inc, West Bend, Wis., USA; mayhewr7@alexssa.net.