Help with PTO RPM calculation

[coldsteelva]

I'm new here and have a MT180D with a 3 speed PTO. It's mostly used to haul around a 60-inch Woods finish mower (540 rpm).

1st PTO gear makes 623 RPM @ 2700 (max engine rpm) and 540 rpm is marked off at 2340 engine rpm. So ~86-87%. Second gear (PTO) is rated at 919 rpm @ 2700 (max engine rpm). Can someone confirm, recalculate or pat me on the head and let me know if the following is true? Since it should be linear, if 540 desired rpms is ~58% of 919 max PTO rpms in 2nd PTO gear, then to achieve 540 pto rpms in 2nd PTO gear, I should be able to run at 1566 engine rpms?

Running in a higher drive gear and higher PTO gear at lower rpms should be better all around, no? This assumes that the lower rpms wouldn't stall me out.


Is there any reason why I shouldn't run in the higher gear?

 

[vsteel]

I wouldn't use percentage, I would use the RPM, I think it is simpler. So the 1st gear is 2700/623 = ~4.3 so you get about 4.3 engine RPM to 1 rotation of the PTO.

1st gear = 4.3 -> 540 = 2340 engine RPM
2nd gear = 2.93 -> 540 = 1587 engine RPM (so you are really close)

So if you are running in second gear you need to set your engine to 1587 RPM to get 540 at the PTO.

As long as you are not lugging or stalling the motor the higher gear should be fine. It will be a little quieter and save a tiny bit of wear on the tractor. Fuel consumption is going to be almost the same because almost the same amount of work is being done.

 

[Rock knocker]

Most tractors PTO rpm is constant at the rated engine rpm independent of the gear the trans is in. If you are correct, what you have is unusual for any tractor.

 

[coldsteelva]

Thanks for that. With the analog tach and the throttle lever, anywhere in the ballpark of 1550-1600 is about all I can hope for. Thankfully, I know what 540 sounds like on the mower so I can feather it by ear. On a 30 year old machine, less stress is better in my eyes. On the other hand there are still less than 1300 hours on it so it hasn't had too hard a life. Have a great day.

 

[coldsteelva]

Most tractors PTO rpm is constant at the rated engine rpm independent of the gear the trans is in. If you are correct, what you have is unusual for any tractor.

You have never seen one of these Mitsus then. I have an 8 speed drive gear box R-1-2-3 in Low and High range and I also have a 3 speed PTO gear shift 1-2-N-3

 

[paulfun9]

Most tractors PTO rpm is constant at the rated engine rpm independent of the gear the trans is in. If you are correct, what you have is unusual for any tractor.

He is not talking about travel gears but rather different selections for the PTO itself.

 

[Rock knocker]

I'm new here and have a MT180D with a 3 speed PTO. It's mostly used to haul around a 60-inch Woods finish mower (540 rpm).

1st PTO gear makes 623 RPM @ 2700 (max engine rpm) and 540 rpm is marked off at 2340 engine rpm. So ~86-87%. Second gear (PTO) is rated at 919 rpm @ 2700 (max engine rpm). Can someone confirm, recalculate or pat me on the head and let me know if the following is true? Since it should be linear, if 540 desired rpms is ~58% of 919 max PTO rpms in 2nd PTO gear, then to achieve 540 pto rpms in 2nd PTO gear, I should be able to run at 1566 engine rpms?

Running in a higher drive gear and higher PTO gear at lower rpms should be better all around, no? This assumes that the lower rpms wouldn't stall me out.


Is there any reason why I shouldn't run in the higher gear?

Your math should work out correctly.

To get 540 rpm in higher gears you will have to run the engine at rpms which may be hard on the engine. And if you run the mower too fast, that can clearly be hard on the mower and the resultant increase in work and power could kill the engine.

 

[Rock knocker]

He is not talking about travel gears but rather different selections for the PTO itself.

Don't think so

 

[Gary Fowler]

I'm new here and have a MT180D with a 3 speed PTO. It's mostly used to haul around a 60-inch Woods finish mower (540 rpm).

1st PTO gear makes 623 RPM @ 2700 (max engine rpm) and 540 rpm is marked off at 2340 engine rpm. So ~86-87%. Second gear (PTO) is rated at 919 rpm @ 2700 (max engine rpm). Can someone confirm, recalculate or pat me on the head and let me know if the following is true? Since it should be linear, if 540 desired rpms is ~58% of 919 max PTO rpms in 2nd PTO gear, then to achieve 540 pto rpms in 2nd PTO gear, I should be able to run at 1566 engine rpms?

Running in a higher drive gear and higher PTO gear at lower rpms should be better all around, no? This assumes that the lower rpms wouldn't stall me out.


Is there any reason why I shouldn't run in the higher gear?

My tractor (LS P7010) has a 3 speed PTO gear box, 540,750 &1000. Each PTO speed reaches the rated speed at the same RPM of 1950 engine. I do most times run my bushhog in the 750 gear and just throttle back to maintain the 540 PTO speed. Since 750 is 72% of 540 I reduce the engine speed to 72 % of the 1950 which is 1400. I do increase it to 1500 so the engine is not lugging in heavy material.
I don't agree that the same amount of fuel is used when reducing engine speed when doing the same amount of work. If the engine is running 1950 under almost no load and I reduce it to 1500 still under almost no load, there has to be an improvement in fuel consumption. This would be similar to a car shifting to OD when at cruise speed. The same work is being done but at reduced RPM so fuel is saved. Also there is less wear on the motor at reduced speed.

 

[Gary Fowler]

Don't think so

Think again. Lots of tractors have multi-speed PTO gear boxes in them. My Yanmar 4220 had a 4 speed box, my LS has a 3 speed box. I think many tractors made outside of the USA and sold in Europe and other overseas countries have multi-speed PTO gear boxes.

 

[MHarryE]

Your calculations are right so the speed will be correct, but realize available power will be only a fraction of what you have at rated engine speed. For example, the chart for the engine on my L5740 says 57 HP at rated and 30 HP at 1500 rpm. That, of course, is before parasitics. If you tractor will handle it at the lower speed, fine. Our Ford 7200 has such a ePTO position but it can't be used for things like running our MoCo disc mower conditioner because of not enough power. It works fine for the tedder which uses a tiny fraction of the 80 available horsepower.

 

[Rock knocker]

Think again. Lots of tractors have multi-speed PTO gear boxes in them. My Yanmar 4220 had a 4 speed box, my LS has a 3 speed box. I think many tractors made outside of the USA and sold in Europe and other overseas countries have multi-speed PTO gear boxes.

I have been wrong in the past, so this wouldn't be the first.

 

[paulfun9]

Don't think so

:confused3:

 

[Rock knocker]

What's there to be confused about? I have already been corrected that these machines have a three speed PTO output.

I've stated I was wrong.

I now apologize and will take any more lumps.

 

[vsteel]

I don't agree that the same amount of fuel is used when reducing engine speed when doing the same amount of work. If the engine is running 1950 under almost no load and I reduce it to 1500 still under almost no load, there has to be an improvement in fuel consumption. This would be similar to a car shifting to OD when at cruise speed. The same work is being done but at reduced RPM so fuel is saved. Also there is less wear on the motor at reduced speed.

Cars are a different beast, they are very concerned about noise levels and vibration levels. Yes they are very concerned about fuel use and that is why they take into account things that can get them another .1 mpg. Also don't forget that almost all cars in the US are gasoline. Gas has a much smaller air fuel ratio window for combustion so it is less flexible to deal with the extremes in the power requirement.

Think of it this way, you have to use a certain amount of energy to get your work done. Fuel going into your engine is using that energy to do work. Anything you are doing at the PTO is still requiring the same amount of work so the only change energy consumption wise is at your tractor. You are still pulling the same load moving the same speed so that energy is constant. The only difference is how fast the engine is turning. What gains do you see with a lower RPM, less parasitic drag of the accessories but you are also farther away from your ideal engine RPM where it is most efficient. Every tractor is going to be different based on the torque curves and the gearing to get the 540 PTO speed. Go ahead and try it but don't expect miracles.

You will get a little less wear on the engine, but with engines rated at many thousands of hours, the economy setting it more about not having to listen to an engine drone as loud.