The Power Take-Off: Sending the Tractor's Engine Back Down a Shaft

Clamp two bearings in line on a board and fit an input shaft with a paper protractor disc. Join it through two universal joints and a short intermediate shaft to an output shaft carrying a second protractor disc, in bearings set so each joint works at 20 degrees: the input and output shafts parallel but offset, like a tractor and an implement at different heights. **In phase.** Line up the two yokes on the intermediate shaft in the same plane. Turn the input in 15-degree steps and read the output disc each time. Output follows input to within the reading error. **Out of phase.** Loosen one yoke, turn it a quarter turn on the intermediate shaft, retighten, and repeat. Now the output leads and lags the input. Tabulate output minus input against input angle and compare with the notebook: the error peaks at 45, 135, 225 and 315 degrees. Mark the yokes of a real PTO shaft the same way before you pull its halves apart, and put them back in line.

**International Harvester** put a PTO on its **8-16** tractor in 1918 and designed implements to use it. Engineer **Edward A. Johnston** had seen a home-made PTO in France about ten years earlier. In **April 1927** the ASAE adopted the first PTO standard: 536 ± 10 rpm (later 540), a 1 3/8-inch six-spline shaft on the tractor's centre line, 14 inches ahead of the drawbar pin hole. A 1000 rpm standard with a 21-spline shaft was added later for larger implements. **Honest limits.** An unguarded PTO shaft is one of the most dangerous parts on a farm: the notebook shows it winds in clothing faster than anyone can react. Always keep the tractor's master shield and the shaft's own guard in place, and stop the engine before going near the shaft. Cardan joints also limit the working angle; wide-angle constant-velocity joints are used where the implement turns sharply.