Pto speed linear?

normde2001 · Sep 5, 2003

Unless you are talking about some kind of "hokus-pokus" tire or clutch slippage, the answer is still "yes". On a stickshift car, if 3000rpm =70mph in high gear, 6000rpm=140mph, or 9000rpm=210mph in same gear.

nomad · Sep 5, 2003

Unless there is a floating gear in a gear transmission system, the degree of freedom of gear systems is zero, that is, a gear system can be considered as one single rigid body, hence they are linear. However, there are clearances, tolerances between the gears and in other connections in the gear system. These small clearances can cause some small non-linearities, but they can be significant only at very high speeds. But also these small nonlinearities at higher speeds may also cause the system results in failing. Between the engine and PTO, the rigidity is more (more linearity) than that between the engine and the wheels because of more joints and gears between the engine and the wheels.
Hydraulic system is usually nonlinear except at very very low speeds. But at higher speeds, hydraulic systems usually nonlinear, even chaotic sometimes.
As for at which speed of PTO should run when pulling an attachment; usually 540 rpm of PTO is designed to get the designed speed at the blades. But the blade speed can not be theoretical, - must be an emphirically designed speed. That is, many experiments on many different grass types are made and from the data collected, an average blade speed is determined. According to this average blade speed, the gear system is designed to connect to 540 rpm of PTO. Tolerances are given to the gear system to cover different grasses which are tested before. If your grass type deviates much from the tested grasses by designers, then you can run your PTO at another speed to obtain proper speed at the blade for your special grass.

herbenus · Sep 5, 2003

( If your grass type deviates much from the tested grasses by designers, then you can run your PTO at another speed to obtain proper speed at the blade for your special grass. )

Thank you.

rozett · Sep 5, 2003

Let me take a different approach to your question...

PTO speed is directly linked (linear) to engine RPM. The higher the engine RPM, the higher the PTO speed. For my tractor (CC 7360), the PTO is running at 540 RPM when the engine is running at 2484 RPM.

For my geared tractor, the ground speed is indirectly linked (not linear) to the PTO speed in that a given engine RPM (and therefor PTO speed) will result in a ground speed deteremined by the gear selection of the tranmission.

So, will an increase in ground speed mean an increase in PTO speed? Well, yea, sort of... but not because you're moving faster, but because the engine RPM is higher. For example: if I am running at 2500 RPM in first gear, the PTO is about 540 RPM and ground speed maybe 3 MPH. At 1500 RPM in second gear, my PTO may be running about 300 RPM and my ground speed about 6 MPH. I'm moving faster but a slower PTO speed.. because I have a lower engine RPM.

I think we're all saying the same thing but different thinkers sometimes understand better with different language. /forums/images/graemlins/smile.gif

Kelvin · Sep 5, 2003

( Unless you are talking about some kind of "hokus-pokus" tire or clutch slippage, the answer is still "yes". On a stickshift car, if 3000rpm =70mph in high gear, 6000rpm=140mph, or 9000rpm=210mph in same gear. ) )

One thing that I do know: An engine's RPM versus HORSEPOWER curve in NON-LINEAR over the entire RPM range, but is LINEAR in certain segments.

To back this up, I will refer to a 1995 Dodge truck sales brochure that contains RPM versus torque and horsepower curves for the V10 and the 5.9L diesel (I would have scanned the image and posted it but it has a black background and they don't usually show very well). For the V10 engine, the horsepower curve is fairly linear up to the peak 300 hp at approximately 4000 rpm, then starts decreasing rapidly such that it reaches 200 hp at 5000 rpm. The diesel engine horsepower curve has a similar fall-off after the peak horsepower is reached.

Now for the issue that I'll have to put more thought into before I take a stand; in the meantime, I'll listen to suggestions from other users:

I have no problem with the concept of the linear relationship between engine rpm and output speed as long as there is little or no resistance. But will a truck operating at 3000 rpm in 2nd gear be travelling at the same speed going up a hill empty as it will going up the same hill when pulling a 7,000 lb trailer? I would think that it would take more torque and/or horsepower to maintain the same ground speed, and that an increase in either torque or horsepower requires more rpm (according to the aforementioned engine torque and hp curves).

I'll try to study this issue some more.

Kelvin

GaryM · Sep 6, 2003

( But will a truck operating at 3000 rpm in 2nd gear be travelling at the same speed going up a hill empty as it will going up the same hill when pulling a 7,000 lb trailer? I would think that it would take more torque and/or horsepower to maintain the same ground speed, and that an increase in either torque or horsepower requires more rpm (according to the aforementioned engine torque and hp curves).
)

3000 RPM in second gear will give the same road speed regardless of load. The issue in your example is if the engine has enough power at 3000 RPM to drive the load.

If it's a manual transmission, the speed to RPM relationship can't change.

normde2001 · Sep 7, 2003

What you say about horsepower is correct, however the issue is rpm vs. ground speed, and that is constant, (linear, whatever), in a given gear.

shady1701 · Sep 7, 2003

not running the engine at enough rpm is worse than running too high. The lower the rpm the more pressure is on the pistons,rods,crank.. the pressure event is longer vs higher rpm which the pressure event is quicker and over more cycles.

also running the tractor lower rpm will wear the trans and everything else due to hydralic flow and increases stress normaly.

Pto speed should be linear if not every different gear would require fueling changes to keep engine rpm at the rated speed. If you notice the pto speed indicaters are all on tachs so engine rpm governs pto speed not transmission speed.

Soundguy · Sep 8, 2003

( 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...).
)

I think what he meant was the ratio between engine speed and the pto speed... not the actual speeds themselves, which would not increase linearly.. though the ratios would.. on that gear unit.

Soundguy

old cowboy1 · Aug 14, 2013

I have a massey 65,and assume my pto is a 540, the brush cutter seems to turn slower than I would like for it to.I do not race the engine at a high rpm, and was wondering if I should leave the pto alone,or to change it to a 1000 rpm. How do you know what the old brush cutter is rated to turn at,or run the older tractor at higher rpm"s , thanks for any reply