Tractor Operating RPM

[Kmo]

There is a lot of discussion about engine RPM. What am I missing?
I have been around tractors all my life. I was an IH mechanic in the late '60s. I have tested tractors on a dynamometer. In general tractors have a specified full throttle No Load RPM and a Full Load RPM.
In general, a tractor produces full HP output when loaded to full throttle Full Load RPM. Full Load RPM on a tractor with 540 PTO would be when the PTO is loaded down to an RPM of 540.
For load operations such as mowing or pulling heavy loads such as plowing, run at full throttle. If the engine pulls below the 540 mark on the tach, reduce the ground speed.
If you load the engine below 540 at full throttle, you are are exceeding the max output and overloading the engine.
If you adjust the throttle to a PTO RPM of 540, you will never get full output from the engine.

K-Mo

 

[dmanlyr]

Well said. High speed lugging is death to any engine. Diesel, gasoline, water or air cooled. Two or four cycle, None of them like to be operated outside of there design specs.

And some people do not nor wil not operate differently designed engines properly, no they actualy expect the engine to operate to there desires, with no desire to even spend five minutes reading a operators manual to see how it was designed to be operated.

What most people who are not mechanicaly informed think is that higher rpm is bad for a engine, which is only the case if the engine is not rated for it.

A good automotive example is the air cooled VW engine. Most people do not realize, and even if they are told will not follow that advice to run it flat out. VW even specifies that the maximum speed of the automobile is the recommended cruising speed. (flat ground)

Guess what the top speed of a 74 bug is - 84 mph. Guess what cruising speed VW recommended - 84 mph. And it is right there in black and white in the owners manual. Kinda sucked that you had to damage your engine with the low freeway speeds this country had/has, especialy the double nickel.

This gives a 3800 to 4400 engine operating rpm depending upon the year, and that incidently is where the air cooling fan is designed to operate at maximum efficency and cooling.

I cannot tell the times that I would go up a mountain pass in 3rd gear full throttle at 45 mph in my old 71' VW westy bus, passing other VW buses going 40 mph in 4th gear, with the heat just radiating of the exhaust from the high speed lugging that 40 mph in 4th gear climbing a grade causes. The cylinder heads were just baking in that heat as well. Just not enough cooling fan volume for that loading.

But you could not tell any of those people that higher rpm equates directly to longer engine life in this case, nope, all you would hear from them is how the VW air cooled engine runs hot, overheats and has short engine life...... when in truth it was ALL due to their incorrect operation of the engine!

Something about leading a horse to water but you can't make it drink comes to mind. Can't fix ignorant people who refuse to listen either!

David

 

[Russell King]

I have posted on PTO speeds and am curious/confused by the operators manual and its statements now that I read them carefully again.

In the SPECIFICATIONS section it states PTO speeds (540, 700 and 1000 RPM) at engine RPMs of 2430.

In the operations section the manual states to run the "power driven equipment such as rotary mowers, snow thrower, rotary tiller, etc at full engine RPM unless otherwise specified in the equipment operator's manual."

So while there is not much difference between the stated 2430 RPM and full engine speed, what engine RPM would you select if the equipment stated to run it at 540 RPM?

For what I am doing it is basically an academic question since the grass is not that thick and the engine speed rarely/never slows much during mowing.

 

[Stumpy]

Either or. The difference between PTO and rated HP speeds are mostly negligible unless you need that little bit of HP the extra RPM provides.

Horse Power = Torque x (RPM/5252)

Say you've got a mower who's input shaft is being spun by a tractor engine doing 2500 RPM which in this machine equates to exactly 540RPM at the PTO. Now the mower is consuming 10HP and by the formula the torque the PTO shaft is bearing is now about 97 ftlbs. We'll also say the blades are being spun at 3600RPM at this speed.

If we now slow the engine down so it's turning 2300RPM the PTO is turning 497RPM to transmit the same HP the load on the shaft is now 106ftlbs. This is reasonable because while the blades are spinning slower (3300RPM) and consuming less power the mower now has to work harder to cut the same amount of grass.

Likewise if we speed the engine up to 2700 so the PTO is doing 583RPM our transmitted torque drops to 90ftlbs for the same 10HP. However our blades are now doing 3900rpm.

As we turn slower than rated speed the danger is the torque being too high for some components of the system and you end up with stripped splines or slipping belts. As we turn faster the limiting factor becomes the RPM, the bearings may overheat or the centrifugal forces on the blades becomes too much and they begin cracking and failing. In the cases above both numbers are likely within the mower's expected operating range and thus safe to run at.

The next part of the equation is the torque and horse power curve of the engine. Most tractor engines I've seen are designed to achieve peak torque well before rated RPM and maintain roughly the same torque out to maximum RPM. Horse power is a fairly smooth upward curve right out to max RPM where it then drops off sharply due to the cam design. Rated PTO speed is usually very close to the maximum power speed and to the implement the speed difference is likely to be negligible but to the engine it can mean a whole extra HP. Unless you're running something on the edge of your tractor's capacity I don't think the difference is significant.

Now running much slower than rated speed for fuel efficiency is a little different. Lets turn he same setup at an engine speed of 1600RPM. The PTO is now doing 346RPM for a torque of 152ftlbs and a blade rpm of only 2300 RPM. This is likely going to be more torque than the implement was designed to handle without chewing up something. A mower is going to cut crappy at such speeds anyway and the transmitted horse power is probably going to change due to a variety of variables I'm not going to try and speculate on but bear with me.

Assume the mower magical and cuts well and handles the torque without issue. Say you've been turning it with a 30HP tractor that is good for 15HP at 1600RPM. The engine is now operating 66% load vs the 33% it was at rated RPM, you're still only moderately using the engine. Except as you slow down you're increasing the load on the engine while decreasing it's speed. The main bearing's load capacity is dependent on RPM and as anyone who's familiar with manual transmissions can tell you if you try and accelerate in too high a gear you'll be rewarded with an unpleasant rumbling from the engine and a lack of acceleration. That is the lubrication film failing allowing the crank and bearings to make contact and scrape away your precious bearing metal. In this case you'd probably still be ok but the lugging point will be a little different for every setup. It's up to the operator to determine that.

So what's all that gibberish mean? If I was lightly utilizing an implement that isn't picky about it's rpm (such as a backhoe pump) I'd feel comfortable reducing engine speed a bit while keeping alert for signs of lugging. If I'm working even moderately I'd run it at rated RPM lest I damage the implement or lug the engine without realizing it.