What limits maximum RPM

CONSERVATIONMAN · Jan 8, 2022

What factor(s) limit the maximum RPM that a classic tractor was allowed to run at? Most tractors are limited to 1500 RPM or less. Was it the metallurgy of the time? Manufacturing tolerances? Fuel limits? Or ???

Not looking to change my governor setting just wondering
Thanks
wally

old · Jan 8, 2022

Don't know if this is really true but I was told decades ago some of the 2 cylinder hand start tractor could have the flywheel come apart if the RPMs got up to high

Jeff B. · Jan 8, 2022

On most of the older gasoline tractors, my guess is the limitations of the stroke of the piston rods. Manufacturers used engine designs with smaller pistons and very long strokes. Engines of that sort by design do not run very fast, but produce large amounts of torque, which is what they were after. Governors were installed to prevent engine damage due to over-revving. The max RPM would have been decided by the manufacturers, depending on the particular design.

J Hamilton · Jan 8, 2022

That's my thinking as well

jon f mn · Jan 8, 2022

Any tractor flywheel can do that. It's why flywheels need to be covered with a blast blanket or steel plate for most modified tractor pulls. There are may YouTube videos of tractors being cut in two by blown flywheels.

john in la · Jan 8, 2022

To keep it simple.....

A motor run at 3000 rpm will have more heat generated and twice the friction wear as a motor run 1500 rpm.
IE a motor run at 1500 rpm will last longer.

In a car engine they will sacrifice the engine to faster wear because they need more horsepower in a car.
In a tractor setting torque is more important than horsepower and they can achieve this at lower rpm's.

RCP · Jan 8, 2022

In my opinion, the biggest limiting factor was inertia forces on the rods and the bearings from the cast iron pistons. Later
engines and most replacement pistons for these old engines are aluminum with a weight about 1/2 of the cast iron originals which
reduces the inertia loads by 75%.

wore out · Jan 8, 2022

Many of the OLDER slow-speed engines didn't even have counterweighted crankshafts.

MSD · Jan 9, 2022

Natpa allows 1800 for a G in Div. 5+. I've turned that many times with no problems. I do have a 3/8ths steel cover around and over the flywheel as this is what the rules call for.

moresmoke · Jan 9, 2022

Design for the intended purpose. A tractor engine does
not need quick throttle response, higher torque at a
constant rate is desirable. Bigger heavier components for
longer life. A little extra weight in the design was not a
penalty.

Slower engine speeds usually lead to higher efficiency.
So the engineers designed something that would fill the
needed purpose, for the best cost, and provide the most
customer satisfaction.

Steve@Advance · Jan 9, 2022

The design engineers of the day knew more than we sometimes give them credit for.

So many factors go into building an engine, and so many prototypes, failures, re design. And still lemons got through, somebody would find a better way (usually the competition), the concept evolved.

Machining techniques and available metal alloys were major factors.

And not just engines, the rest of the drive train also had to be considered. I don't remember which tractor it was, but one of the behemoths was basically taken out of production after only a few were built because the final drive could not take the stress and there was no practical solution.

Really amazing what they did back then with what they had to work with!

560Dennis · Jan 9, 2022

I have to say the engineers of antique tractors did a good job of reliability,didnt they?There are 100 plus year tractors still running fields
today. There designed Governor gave the tractors an d equipment gave the user the best possible set up to last at the least cost of
operations.

teddy52food · Jan 9, 2022

The 150 HP Case steamer comes to mind.

CONSERVATIONMAN · Jan 9, 2022

Long life products seems to go against the grain of "modern" production.
What were some of the common failures on these tractors?
If broken rods were a common failure would the replacement rods manufactured today be better by default? Just because todays metals are better.

Janicholson · Jan 9, 2022

Knowledge was based less on science, and more on keeping it operational. With a power band from 800 RPM to 1600 RPM and 3 or 4
gears the tractor would work. With a powerband of 2500 to 2800 many more transmission gears are needed to keep Horse power on the
curve. But efficiency improves with RPM, short stroke better cylinder pumping (multi valve Pent Roof combustion chambers, Fuel
injection and compression ratio). If this were not true, modern tractors would have 3 gears and non-balanced long stroke 2 valve
engines, and they don't. Jim

IaLeo · Jan 9, 2022

I remember the flywheel spin pit at the design center. A big electric motor, variable speed. Deep concrete pit with an overhead gantry to put the heavy cover back over the test, of course with remote instrumentation.

JMOR · Jan 9, 2022

In addition to many good responses, you will note that even in 2022, that there are few 4 cylinder engines larger than 2 liters. Reason is that the design is inherently unbalanced and the vibrations will tear a larger engine apart. Keep it slow enough and/or small enough & it will live. The modern 4 cylinder engines have counterbalance crankshafts, and one or two balance shafts & thus will survive the higher rpms. Even some old tractors that were closer to 4+ liters used balance shafts (MF & Ford, for two examples). Yes, I am aware that all tractors are not 4 cylinder engine.

wisbaker · Jan 9, 2022

Additionally a car doesn't often need to run for a long time producing it's full rated power, except maybe a car in Germany or one that is racing. I always thought a lot of the low RPMs on tractor motors was so they could run at their rated power/RPM for hours on end without self destructing. Look at the evolution of gasoline tractors in the 50's and early 60's. Often the New model was the same basic engine as the previous model but had been tweaked to run at a higher RPM. We had a late model A John Deere, the engine in it was actually a little smaller displacement than the neighbor's model 60 but their 60 ran a but more RPMs. Look at Farmalls, what was the model H evolved into a 3 plow tractor while what was the the model C became a 2 plow tractor. More power, less displacement and more RPMs and gears. The 5.7 Hemi in my Ram truck makes horsepower and torque similar to the big block engines of my youth, but if I stomp on my Ram to pass something that tach jumps up to 4,000 RPMs or more pretty quick, it was a much longer journey to see 4,000 rpms in a Buick or Oldsmobile 455.

MarkB_MI · Jan 9, 2022

The short and simple answer is they didn't need to turn any faster. They were conservative designs in a day before finite element analysis and other engineering tools that allowed the design of engines with high power-to-weight ratios, and they were marketed in a time when there was no demand for high power-to-weight ratio tractors. Engineers back the made sure their designs had wide safety margins, because experience taught them that things broke when they didn't over-engineer them.

Certainly it was possible to build engines back then that would turn fast and put out lots of power. The Allison and Merlin aircraft engines had very high power-to-weight ratios, but they were complicated, expensive and maintenance-intensive. Tractors needed to be simple, cheap and reliable. Note that the standard for pre-war high-horsepower engines, the V-12 Liberty engine, only turned about 1800 rpm.

It's also important to consider piston velocity rather than rpm when comparing antique versus modern engines. An engine with a six inch stroke turning 1500 rpm has the same piston velocity as a engine with a three inch stroke turning 3000 rpm.

fixerupper · Jan 9, 2022

(quoted from post at 19:07:26 01/08/22) To keep it simple.....

A motor run at 3000 rpm will have more heat generated and twice the friction wear as a motor run 1500 rpm.
IE a motor run at 1500 rpm will last longer.

In a car engine they will sacrifice the engine to faster wear because they need more horsepower in a car.
In a tractor setting torque is more important than horsepower and they can achieve this at lower rpm's.

I read years ago a hard working engine cant exceed a certain piston speed because the cylinder walls cant absorb the heat any faster. There is also a certain cubic inch per horsepower rule for the same reason. I am thinking the limit was a half horsepower per cubic inch. Anything above that and the piston could not get rid off heat fast enough. In recent years we have seen fewer cubes per horsepower in farm tractors but the RPMs have not increased. The longer the stroke the slower the RPMs because of piston speed.

dhermesc · Jan 12, 2022

Agreed, long stroke leads to more torque and the engine is governored at peak torque not peak HP. As mentioned by another with engines that worked HARD for hours on end running at peak torque gave them years of reliability - had they run at peak HP for hours on end (double or even triple the RPMs) the engines wear twice as fast - if not more.

What limits maximum RPM

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