Why 540?

[mikester]

Why not 500 or 632 or whatever?

There must be some mechanical basis for standardizing on 540 rpm for PTO speed.

Let's light it up, tractor gurus!

I think its due mostly to engine RPM. Early motors were designed to run at 1800 rpm max. Put on a 3:1 gear box and that gives you 600 rpm at the PTO. Most engine torque curves peak before the max rpm then drop off. 540 PTO rpm is the closest to max engine torque curve peak for maximum power output without redlining the motor.

 

[CobyRupert]

I think its due mostly to engine RPM. Early motors were designed to run at 1800 rpm max. Put on a 3:1 gear box and that gives you 600 rpm at the PTO. Most engine torque curves peak before the max rpm then drop off. 540 PTO rpm is the closest to max engine torque curve peak for maximum power output without redlining the motor.

Seems like a good answer.

....but then why not a 2:1 gearbox (~810 rpm) or a 4:1 (~405 rpm)? Does torque vs. horsepower vs rpm and the universal joint technology of the day come into play? It seems that one would try to find a good balance in not having too bulky pto shafts and universals required for a higher torque/lower rpm ratio versus having them spin too fast, and wearing and being out of balance in a high rpm/low torque ratio.
Maybe 3:1 (540 rpm) was the sweet spot in balancing torque/rpm and the material construction of universals for the horsepowers they were dealing with in the 1920's or 30's, etc..?
...I'm just guessing...

 

[jeff9366]

International Harvester introduced the first practical pto on a farm tractor in 1918.

I have read in WWI histories that tractor PTOs were on at least some, perhaps many, French internal combustion farm tractors during the 1914 - 1918 war. American soldiers could see how PTO reduced labor with French men conscripted into the forces and women and children doing a great deal of the heavy farm work.

Was the flat belt (thresher) connection considered a PTO in this era?

Now, how practical these early PTOs were, rotational speed, location on the tractor (No Ferguson Three Point Hitch until 1930s) and how standardized, I have no idea.

 

[jeff9366]

It the shaft is too fast, then you might need a gearbox on the implement to slow it back down. Theres the odd 540 but then the nice even 1000. I once saw this awesome snowblower at a show, Videos showed it was truly amazing! What they didn't seem to think was worth mentioning, is that it ran at 1000 rpm!

Subcompact and compact tractor mid-PTOs, commonly used to power MMMs and front snowblowers, often rotate at 1,000 rpm.

Ag tractors with Category 4 and Category 5 Three Point Hitch installations revolve at 1,000 rpm, not 540. No need to gear PTO down for ample power on these powerful tractors.

 

[Atchuuu]

I have read in WWI histories that tractor PTOs were on at least some, perhaps many, French farm tractors during the 1914 - 1918 war. American soldiers could see how they reduced labor with French men conscripted into the forces and women and children doing a great deal of the heavy farm work.

Was the flat belt (thresher) connection considered a PTO in this era?

Now, how practical these early PTOs were, rotational speed, location on the tractor (No Ferguson Three Point Hitch until 1930s) and how standardized, I have no idea.

Yes, IH certainly did not invent the PTO but improved it to the point that it was "practical" - Having control over the implements that would make it useful, profitable and practical probably helped too.

From WIKI - "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.[2]"

 

[LouNY]

Subcompact and compact tractor mid-PTOs, commonly used to power MMMs and front snowblowers, often rotate at 1,000 rpm.

Ag tractors with Category 4 and Category 5 Three Point Hitch installations revolve at 1,000 rpm, not 540. No need to gear PTO down for ample power on these powerful tractors.

For Ag tractors;
The main reason for going to 1000 rpm was the ability to pass the available hp to the pto the torque for 100+ hp at 540 was to much for reasonably sized
u-joints to handle. The same physical sized and rated u-joints for 50 hp at 540 rpm can transmit a 100 hp at 1000 rpm.
The increasing hp is one reason that electrically powered implements are being developed and tested. PTO shafts are becoming unwieldy because of the physical size and weight to transmit the available and desired power.

 

[buickanddeere]

Subcompact and compact tractor mid-PTOs, commonly used to power MMMs and front snowblowers, often rotate at 1,000 rpm.

Ag tractors with Category 4 and Category 5 Three Point Hitch installations revolve at 1,000 rpm, not 540. No need to gear PTO down for ample power on these powerful tractors.

1000rpm is not fast enough for those HP levels but that is sort of where they are stuck with . In hind sight instead of 1000rpm they should have chosen 1800rpm.

 

[WranglerX]

Why do we have Metric and SAE....... Because there is having a standard better than everyone flying on their own... 540? does not really matter, it was just selected as a "standard"... Actually could have been anything but one engineering group decided 540 was good enough and it was adopted universally...

Dale

 

[CobyRupert]

Sidetrack: Why 78, 45, AND 33 RPM?

You're probably either old or a young hipster if you know what these rpm's refer to.

 

[LouNY]

Sidetrack: Why 78, 45, AND 33 RPM?

You're probably either old or a young hipster if you know what these rpm's refer to.

Well I certainly don't qualify as a young hipster, but I also don't consider myself old thats for them that are 75-80.:laughing:

 

[WranglerX]

Sidetrack: Why 78, 45, AND 33 RPM?

You're probably either old or a young hipster if you know what these rpm's refer to.

You forgot 16-2/3...

And then there was the wire recorder at 2 ft. a second (3,600 feet per spool)...

Dale

 

[CalG]

I can稚 recall any electric motors running at 540 rpm.

Those old timers were not opposed to applying "factors" such as belt pulleys or chain wheels.

 

[RickB]

Those old timers were not opposed to applying "factors" such as belt pulleys or chain wheels.

Irrelevant. 540 is not an even factor of 1725 or 3450.

 

[mikester]

Why do we have Metric and SAE....... Because there is having a standard better than everyone flying on their own... 540? does not really matter, it was just selected as a "standard"... Actually could have been anything but one engineering group decided 540 was good enough and it was adopted universally...

Dale

You can blame the French for metric...the main goal of metric is to standardize weights and measures to more meaningful units. Imperial units are a confusing mish-mash of oddities and localized "standards".

Seems like a good answer.

....but then why not a 2:1 gearbox (~810 rpm) or a 4:1 (~405 rpm)? Does torque vs. horsepower vs rpm and the universal joint technology of the day come into play? It seems that one would try to find a good balance in not having too bulky pto shafts and universals required for a higher torque/lower rpm ratio versus having them spin too fast, and wearing and being out of balance in a high rpm/low torque ratio.
Maybe 3:1 (540 rpm) was the sweet spot in balancing torque/rpm and the material construction of universals for the horsepowers they were dealing with in the 1920's or 30's, etc..?
...I'm just guessing...

It's all a balance of strength of materials and cost of manufacturing. The 3:1 was likely the best compromise at the time and fit a niche need. Everybody started using that standard and here we are today still using it.

BETA might have been a better technology but everyone used VHS. Then came DVD, blueray, blah blah blah.

For all we know they went to 3:1 because somebody's gear hobbler was set up to cut certain angle of teeth.

 

[Industrial Toys]

Well, us dumb Canadians never got screwed worse than when they started selling fuel in litres. It's easeir to jack up and manipulte prices when things are sold in smaller units. Never mind the theiving policians that got rich when Petroscam was formed!

The French are responsible for all kinds of low and disgusting things! lol

 

[CobyRupert]

Well, us dumb Canadians never got screwed worse than when they started selling fuel in litres. It's easeir to jack up and manipulte prices when things are sold in smaller units. Never mind the theiving policians that got rich when Petroscam was formed!

The French are responsible for all kinds of low and disgusting things! lol

There’s that one thing, I forget how to pronounce it,
.... it was just right on the tip of my tongue too..

 

[buickanddeere]

Well, us dumb Canadians never got screwed worse than when they started selling fuel in litres. It's easeir to jack up and manipulte prices when things are sold in smaller units. Never mind the theiving policians that got rich when Petroscam was formed!

The French are responsible for all kinds of low and disgusting things! lol

The Mob runs the construction industry in Quebec and is handed fat government contracts . The bureaucracy and contractors in the nuclear industry is particularly corrupt .

 

[timbrr]

Carroll Goering and Scott Cedarquist, American Society of Agricultural Engineers (ASAE), 2004:

Experimental power take-offs were tried as early as 1878, but International Harvester Company (IHC) was first (in 1918) to install a pto on a production tractor. In 1920, IHC offered this option on their 15-30 tractor, and it was the first pto-equipped tractor to be submitted for a Nebraska Test.

Walter Jones, writing in a 1922 agricultural engineering journal, explained the reasoning for equipping tractors with a pto. A sprocket attached to a ground-driven bullwheel on horse-drawn machines could provide rotary power. A tractor with no pto could replace the horses but not the bull wheel; slippage of the tractor drive wheels and bull wheel reduced the power transmission efficiency far below that of a pto shaft.

Industry leaders quickly saw the need for standardizing the pto. Three pto features needed standardization: the direction; speed; and the size, shape and location of the pto shaft. On Dec. 14, 1926, the world's first pto standard was drafted by industry engineers meeting in Chicago. They quickly agreed the shaft should rotate clockwise. Standardizing the speed proved more contentious.

Pto shaft speed must suit the needs of implements (initially grain binders), but should the pto speed be linked to speed of the engine or the tractor drive wheels? Experience in rice states, where the 1925 crop was heavy and traction was poor, demonstrated advantages of linking pto speed to engine speed. Grain binders could run at full speed while the travel speed was reduced to accommodate the heavy crop. The fastest shaft on binders (the pitman drive) ran at 500 to 600 rpm at normal travel speeds.

The 1926 draft standard was adopted by ASAE in April 1927. The pto rotational speed was specified as 536 ± 10 rpm; W.L. Zink reported that this speed seems to work out satisfactorily for all installations known. Thus, it is likely that the choice was a compromise related to the needs of early pto-driven machines.

Acceptance took time. A 1929 study of 35 popular tractors showed pto speeds ranging from 515 to 745 rpm with an average of 549 rpm. Through numerous revisions, the pto speed stayed at 536 rpm as late as 1948. By 1958, when a new 1,000 rpm pto standard was developed, the 536 rpm standard speed had been changed to 540 +- 10 rpm. Probably, the speed was simply rounded up to a more convenient value.

The pto shaft, originally conceived to run reapers and binders, has become an important part of the tractor and has enabled many implements to come into being the corn picker, for example. It was invented before 1920, but only became practical with the development of the pto.

To best and succinctly answer the question that prompted this article, it appears the speed was initially chosen to be compatible with needs of early implements, especially binders. A range of speeds would have worked, but the first pto standard drafters settled on 536 ± 10 rpm. Sometime between 1948 and 1958, the standard speed was rounded up to 540 ± 10 rpm.

...

The original article text has been preserved by the Internet Archive Wayback Machine. I'd post a link but am prevented from doing so by the forum software until I amass three posts.