Pto speed linear?

Sprocket1 · Sep 4, 2003

Does ground speed and pto speed remain linear across the full range of engine rpm's? I've never had my tractor fully up to rated pto speed (engine rpm) while using my rotary cutter. It seems to me a waste of fuel and wear and tear on the equipment to run the engine at rated rpm. You must assume standard transmission and not a Hydrostat regarding this question. Thanks

MikePA · Sep 4, 2003

By linear I assume you mean does the relationship between engine speed and PTO speed remain constant across the entire engine rpm range, i.e., does the transmission get more or less efficient as rpms change. I vote that the relationship stays constant. If it doesn't stay constant the change in relationship is so minor as to be irrelevant.

 I've never had my tractor fully up to rated pto speed (engine rpm) while using my rotary cutter. 
What do you run it at if not 540 PTO rpm?

 It seems to me a waste of fuel and wear and tear on the equipment to run the engine at rated rpm. 
Based upon what evidence? The attachments are designed and built to run at 540 PTO rpm. Running a cutter at less rpm means the blades are not spinning as fast, meaning it's not cutting as easily as it could.

When using an implement attached to the PTO, I run the engine at whatever rpm is needed to achieve 540 rpm on the PTO. That's usually what attachment instructions call for.

normde2001 · Sep 4, 2003

We ain't rocket scientists, just tractor owners. /forums/images/graemlins/confused.gif

chrisjbell · Sep 4, 2003

It is my belief that you should run the tractor at the speed required to generate the correct PTO RPM for the implement. It was designed to run at that speed, the tractor was designed to run it at that speed, and quite frankly, fuel is a pretty insignificant factor when dealing with CUTs.

Wear and tear will probably be worse when you are operating your machine outside of the designed tolerances. The parts are still moving when you're running the tractor and implement, but not moving at the speeds they were designed to. Even if that isn't the case (I'm not a mechanical engineer - just a software engineer), they are still wearing and you aren't using them the way they were designed (and probably modified, based on real usage) to work.

normde2001 · Sep 4, 2003

"Does ground speed and pto speed remain linear across the full range of engine rpm's?"

If you are asking if the wheels and the pto are both driven from the same engine, the answer is "yes" /forums/images/graemlins/tongue.gif. An increase in ground speed = an increase in pto speed.

Kelvin · Sep 4, 2003

( Does ground speed and pto speed remain linear across the full range of engine rpm's? )

I would say that in most cases they would be close through SOME of the engine RPM range, particularly around the engine's rpm that the PTO speed is rated at, but not through the ENTIRE RPM range.

I will try to re-word the question in order to eliminate the word "linear" so that everyone can throw in their 2 cents worth (I'm trying to make use of my Mechanical Engineering degree that I haven't used in a while...).

Another way to word the question would be:

When the engine's RPM is doubled (or tripled), is the tractor's ground speed and PTO speed also doubled (or tripled)?

A similar question that most tractorbynet readers could relate to deals with automotive applications: Take a pickup truck with manual transmission in 2nd gear (or any other gear); if the engine RPM is doubled (from 1000 rpm to 2000 rpm), does the ground speed double? If the engine RPM is increased by a factor of "4" (from 1000 rpm to 4000 rpm), does the ground speed increase by a factor of "4"?

A Related Story? Many years ago I purchased a push lawn mower with a Hi-Low engine speed adjustment. I always mowed the lawn using the lower engine speed in an attempt to "save wear and tear on the engine". Later I noticed that newer designs of the same type lawn mower did not have the lower speed option and thus only had "Full Speed". When I enquired as to the purpose of the design change, the answer I got was that the air cooled engines depended upon the higher engine speed to create proper air flow across the engine, and that customers that were using the low speeds were having more problems with their engines overheating. While I never had any problems with my engine, I took that as a lesson learned and now I operate equipment using design parameters.

As stated in another post, I agree that by trying to "save wear and tear on your tractor" and operating at lower engine speeds you may actually be creating additional wear and tear.

Kelvin

chim · Sep 4, 2003

I believe it is linear. I brought a mechanical tach home once to check what engine RPM's would be needed to produce 540 PTO RPM's if the PTO lever was in the "960" position. I wanted to know because I use the side-discharge RFM to blow leaves in the Fall. The same practice also works well if the grass was way too long when mowed. It's easy to get it into a couple windrows by going over it the next day after it has dried a bit in the sun. The area where this takes place is level, and the tractor speed is low, so I figured if the 'Bota can run happily at 1500RPM, why not?

There are cases where less than 540RPM works better. Just last night I mowed some tall wet grass, and dropping the engine/PTO speed caused less clumping. This is quite the opposite for dryer grass that gets cut when it should. For that, 540RPM almost turns the clippings to dust and distributes it nicely....................chim

AndyMA · Sep 4, 2003

Assuming a gear driven system, it has to be linear. Since most PTO's are driven from a gear with in the transmission, the gear ratios and hence the relationship between the PTO output shaft speed and the output shaft to the drive differential remain constant, with only the input rpm of the input shaft varing. Gear drive systems don't slip the way belt and hydraulic systems can.

Assuming you stay in the same forward gear ratio, the ground speed and the PTO shaft speed will have a linear relationship at ALL engine rpm's.

PTO driven equipment is rated at 540 (or sometimes 1000 rpm) and should be driven at that speed for implements like mowers where blade tip speed is important to the quality of the cut. Tractors are rated to operate at this speed for extended periods of time. I know the first time you bring up the RPM's to mow, you may be suprised at the sound, but it is the way they are designed to operate. It's one of the reasons why tractors with more speed ranges allow you to choose a proper forward speed for each implement. Having said that, I will say that for some implements, such as those with hydraulic PTO pumps, I do not run at rated speed if the implement operates too quickly.

Andy

JerryG · Sep 5, 2003

The higher RPM, the higher the PTO speed as well as ground speed. Select the gear that will produce the ground speed you want, at the RPM that you want to run and not lug the engine. If you are getting the desired cut and the engine is not lugging, run it at that RPM. You will be fine. It won't hurt a thing. I hardly ever run at 540 unless it is called for in heavy grass or brush to get the desired cut.

herbenus · Sep 5, 2003

There was a thread here a while back that had to do with running brush hog at lower rpms. The high majority vote here was that you should run at rated pto rpm with no deviation. But I have to confess, and I'll probably be corrected here (again), that I seldom do. If my mower cuts fine at a lower rpm, what's the problem? When my grass is short it cuts good at about 80%. But if I cut this weekend, with grass tall and wet, I'll have to run at the "required" 540 pto rpm. I compare it to my lawnmower: I adjust it's rpm to the task required.