Editing Power take-off (section)

==Technical standardization==
Agricultural PTOs are [[Standardization|standardized]] in dimensions and speed. The [[International Organization for Standardization|ISO]] standard for PTOs is ''ISO 500'',<ref name="iso"/> 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
#''ISO 500-3'' Main PTO dimensions and spline dimensions, location of PTO.

The original type (designated as Type 1) calls for operation at 540 [[revolutions per minute]] (rpm). A shaft that rotates at 540 rpm has six splines on it, and a diameter of {{convert|1+3/8|in}}.<ref name="agproud">{{cite web | url=https://www.agproud.com/articles/32127-equipment-hub-understanding-power-takeoff-drivelines | title=Equipment Hub: Understanding power takeoff drivelines - Progressive Forage &#124; Ag Proud }}</ref>

Two newer types, supporting higher power applications, operate at 1000&nbsp;rpm and differ in shaft size.<ref name="agproud"/> Farmers typically differentiate these two types by calling them "large 1000" or "small 1000" as compared to the Type 1 which is commonly referred to as the "540". All new types (2, 3, and 4) use [[involute]] splines, whereas Type 1 uses straight splines.<ref name="iso">{{Cite web |last=International Standard |date=April 1, 2014 |title=Agricultural tractors - Rear-mounted power take-off types 1, 2, 3 and 4 |url=https://cdn.standards.iteh.ai/samples/56698/4acca3e1ee5246fa8218c17d65da045f/ISO-500-1-2014.pdf |access-date=September 29, 2023 |website=International Standard}}</ref>

Inch-denominated shafts are round, rectangular, square, or splined; metric shafts are star, bell, or [[American football|football]]-shaped.<ref name="tulsa">{{cite web | url=https://www.driveshaftsoftulsa.com/blogs/news/the-different-types-of-pto-shafts | title=The Different Types of PTO Shafts | date=15 February 2021 }}</ref>

{|class="wikitable sortable
! Type
! RPM
! Diameter
! Splines
|-
| 1
| 540
| {{convert|1+3/8|in|disp=or|abbr=on}}
| 6 straight
|-
| 2
| 1,000
| {{convert|1+3/8|in|disp=or|abbr=on}}
| 21 involute
|-
| 3
| 1,000
| {{convert|1+3/4|in|disp=or|abbr=on}}
| 20 involute<!-- The "agproud" citation references this entire table, except for Type 4 which is reffed to the ISO standard -->
|-
| 4
| 1,300
| {{convert|57.5|mm|in}}
| 22 involute
|}<ref name="agproud"/><ref name="iso"/>

Due to ever-increasing horsepower requirements from farm implements, and higher horsepower engines being installed in farm tractors, a still larger type (designated as Type 4) has been added to ISO 500. It operates at a higher rotational speed of 1300&nbsp;rpm in order to allow for power transfer at reduced levels of torque. The shaft has 22 splines with a major diameter of 57.5 [[millimeters]] (mm). It is meant to handle PTO powers up to 450 [[kilowatts]] (kW), or roughly 600 [[horsepower]] (hp).

All four types rotate counterclockwise when viewed looking back from inside the tractor's cab; when standing behind the tractor and looking directly at the shaft, it turns clockwise.<ref name="agproud"/>
 
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. Newer tractors may come equipped with 540/540E and/or 1000/1000E options that allow the tractor to power certain low-power-demand implements like [[hay rake]]s or [[Tedder (machine)|tedder]]s using lower engine speeds to maintain the revolutions per minute needed, using less fuel and placing less stress on the engine – thereby improving efficiency and reducing costs. 

The first industry [[technical standard|standard]] for PTO design was adopted by ASAE (the [[American Society of Agricultural and Biological Engineers|American Society of Agricultural Engineers]]) in April 1927. The PTO rotational speed was specified as 536 ± 10&nbsp;rpm; the direction was clockwise. The speed was later changed to 540&nbsp;rpm.<ref>{{Cite magazine |first1=Carroll |last1=Goering |first2=Scott |last2=Cedarquist |date=October 2004 |url=http://www.asabe.org/imis/StaticContent/4/October_2004/p_31_last_word.pdf |title=Last Word: Why 540? |magazine=Resource Magazine |volume=11 |number=8 |page=29 |publisher=[[American Society of Agricultural and Biological Engineers]] |access-date=2009-05-03 |archive-url=https://web.archive.org/web/20110722060605/http://www.asabe.org/imis/StaticContent/4/October_2004/p_31_last_word.pdf |archive-date=2011-07-22 |url-status=dead }}</ref>