3 speed PTO and my finish mower

[ctown75]

I have a 5' finish mower on my MT160D. My tractor has the 3 speed PTO, I believe the speeds are 540(1), 720(2), and 900(3). I have been mowing this summer on 1 at 2400 RPMs like the tach indicates for the 540 RPM. Can I switch up to the second speed (720 RPM) and run my tractor a little slower? Obviously as long as the power is there would be the answer, but what would my correct RPMs be to keep the deck at the same speed? If my math is correct, I come up with 1800 RPMs. Am I right? Thanks in advance.

Scott

 

[KennyV]

You are right 1800, will work fine but do not run the rpm up while in that faster pto speed. Your mower will not like over speed KennyV.

 

[greg_g]

First, I'll preface this by assuming your mower manufacturer specifies 540 RPM at the transmission input shaft. Second, I'll state that yours is a common misconception by first-time tractor owners. You wouldn't do that on a riding mower - or even a push mower - so why do you think it will be of any benefit on a tractor?

Two reasons not to; engine life and blade tip speed. Tractor engines - particularly diesels - are engineered to operate most efficiently in the upper RPM range. You should consult your owner's manual to find out at what RPM the engine develops max torque. Any actual work performed by the tractor should be done at no less than the point at which max torque is developed. Slower than that, you can actually wear the engine out faster. What little fuel may be saved by working the tractor below max torque RPMs will usually be offset over time - by the amount of money spent on premature engine and cooling system problems.

Blade tip speed is a design factor considered by the mower designers. They go through a mathematical process to first determine how fast they blades they use need to spin for the best cut. Then they assume the tractor PTO will be putting out a true 540 RPM at/above max engine torque. Below max torque, the engine bogs down too quickly in thick grass, blade tip speed drops, the cut suffers. Anyway - given a design blade speed and 540 PTO rpm - they then design a mower transmission in the middle to make that happen. Over and above the premature tractor wear mentioned above, fooling with the blade tip equation can often result in a less than satisfactory cut.

The time tested approach is simply to follow the manufacturer's advice. When mowing, run the tractor engine at the RPM necessary to produce the mower-specified PTO speed (in this case apparently the typical 540) - then control ground speed with the transmission.

That said - if 1800 RPMs on your tractor is at/above the torque peak, then it will mow fine in the light stuff. Just expect a poorer cut when the engine bogs down.

//greg//

 

[Ranch_Hand_Supp]

Which PTO out put speed of the tractor is determined by the PTO rated speed of the gear box.



Ronald
Ranch Hand Supply

 

[ctown75]

First off thanks for all the speedy replies

Second, I'm far from a first time tractor owner, but thanks for pointing that fact out. The only first about this mitsubishi is it's the first tractor I have owned that has a multiple speed PTO.

Third- since when have diesel engines been designed to operate best at high RPMs? That's poppycock. You ever looked at a diesel tach VS at gasoline tach? My diesel truck redlines around 3200 RPM and runs down the road about 1500 where my gas truck's redline is nearly double that and runs down the road about 2300. Those are trucks obviously- but just for sake of proving a point. Of course the motor likely generates the most horsepower near the rated RPM, otherwise the rated RPM would not be there, because the place of the rated RPM is based on power specs.

The huge advantage of a 3 cyl diesel at 18 HP vs and 18HP single cylinder gas is torque- and torque is what keeps everything spinning. As far as HP goes, that 18 HP is probably up at 2400 RPM. The highest torque is probably made closer to 2000 RPM or lower. EVERY engine's peak torque is different(and almost always lower RPM) than HP- and that's fact. It is likely that this tractor makes MORE torque at 1800 than at 2400, so as far as rotational force is concerned, the 1800 RPM is likely easier for the tractor to turn than the 2400. Now obviously the strain is more when you go from the 540 speed PTO and increase that load by 1/3 to 720- so that's more work needed to be done.

I'm not stupid, so obviously if the tractor is not operating as well on the higher PTO speed and is barely chugging along at the lower RPM, lord help me I will go back to lower PTO and higher RPM. I wasn't concerned as much with saving fuel as much as saving wear on the engine. There are a reason big rigs chug down the road at 1500 RPMs- because they are aiming for 500,000 miles without a major overhaul. I haven't mowed with this new mower much yet, so I don't even know how it will act at will act in the situation at hand. I probably won't get to until next year either. I just purchased it when I sold my riding mower(which was gas and I always operated at full throttle because the book said to LOL)

I really do appreciate your input on the situation. It got me thinking about it a little more this morning. Obviously it's near impossible for you all to judge the situation via the internet- you're going 100% off of what I tell you.

All that being said- the main reason for this post was to know if I was correct on the PTO speeds on this tractor. All the selector says is :1 - 2 - 3. I was assuming on those pto speeds above. Thanks!

 

[greg_g]

Well, you asked - and I've got no dog in this hunt. So since my answer isn't the one you wanted to read, ignore it.

WRT to my statement about diesel RPMs, this is an agricultural forum (read off-road). My statement about diesels working most efficiently at/above the peak torque RPM was made accordingly. And I'm not stupid either. Having >20 years experience with both, I know quite well the differences between on-road and off-road diesels. FWIW, I'm currently operating and maintaining one on-road, two off-road, and five PTO-powered implements.

But for the record, standard PTO output RPMs are in fact 540/720/1000. So despite what you consider poppycock, I guess you got what you came for after all.

//greg//

 

[ctown75]

Now I wasn't trying to be rude, I was just saying that I think you're wrong in one of your statements. I guess I shouldn't have asked a "Can I" question, because I obviously can do whatever pleases me, the same as anybody else. I think the question shouldve been "should I". I realize now I knew the answer, just needed to talk it through in my head. I guess my point will be this: There wouldn't be a 3 speed PTO if the manufacturer's intentions were for every implement to be ran at 2400 RPM and at the first PTO setting. Am I correct there at least?

I have two diesel trucks and three diesel tractors and I know as well as you do that having 100 diesels doesn't mean you know a darn thing about them- I've known guys that've owned diesels their whole life and couldn't change a fuel filter.

Thank you for answering my question about the PTO speeds, that is exactly the information I didn't have.

Thanks and good day!

 

[radioman]

now this discussion is making me think - wait a second here -- uhh what PTO implements needs higher rpms. from there we can discuss it further. why did manufactuers put those higher RPMS if most of the equipment is 540 ? call me ignorant or whatever - i am just curious here . anyone care to enlighten me ?

 

[ctown75]

Well I thought I was on the right track but who knows! LOL

My guess is that because most of the tractors with the multi speed PTO are foreign made, that there are implements over seas(and likely here as well) set up for higher RPM. My new Agco has the option of midmount PTO for a mower deck, if i got that it would be rated at 2000 RPM- but it's rear PTO is only 540. Mowers and things like that need to spin fast(obviously). They probably keep the 540 on there for things like augers, etc.

 

[wushaw]

Doesn't CAT use on-road diesels in many of their tractors? Like the Challanger line.

On-road/off-road, does it really matter just as long as you follow the manufactures recommendations on what to do with what they built.