mower rpm

eddieirvine · May 26, 2018

I am sure this has been asked before, I have a Kubota 3940 and use a rear woods mower rear discharge. I don't mow real fast if I don't get done there will hopefully be another day to mow. My question is what rpm is best to run the engine at? also they always say 540rpm on drive shaft? is that what max it will handle? Just curious can you have rpm to high I am sure it has to mow better but do you need it high? Thanks. Eddie

diesel85 · May 26, 2018

BTS (blade tip speed) is where it's at along with what type of mower (mulching/discharge), type of blades, grass height. Some mowers are geared better than others.

I think the MMM on my Kubota doesn't have a fast enough BTS, especially being it's a side discharge (or can be converted to a mulcher/bagger). I think the engineers went into the middle of the spectrum for it. Which fine, but sometimes it cuts my grass a little ragged (tears it) even with new Kubota OEM blades. Versus my lawn tractor that I know spins them blades a heck of a lot faster and it cleanly cuts the grass.

My neighbor who has a KingKutter mulcher also has a very good BTS when cutting normal height grass. If it is too high,the BTS obviously slows, and starts tearing the grass up. This is on a NH TC35 with a KK 72" mulcher.

I know in a year or two when I go to upgrade from my BX, this is one of the key aspects I am going to be looking at when I buy a rear mount for my bigger tractor. Woods and KingKutter are great brands, but I liked the results from the KingKutter versus the Woods.

My Woodmaxx flail mower does a great job too, again, as long as the grass is at normal height. I put the Kingkutter first, the Woodmaxx Flail second, and the Kubota MMM a third.

Now about cranking the RPMs higher, if you can, 100 more RPMs than 540 (640) won't do too much harm. But change your gearbox oil out routinely and grease them u-joints on the PTO shaft. Just keep an eye on things, and don't do anything that you will regret.

eddieirvine · May 26, 2018

Thanks Diesel

Dave M7040 · May 26, 2018

eddieirvine said:
I am sure this has been asked before, I have a Kubota 3940 and use a rear woods mower rear discharge. I don't mow real fast if I don't get done there will hopefully be another day to mow. My question is what rpm is best to run the engine at? also they always say 540rpm on drive shaft? is that what max it will handle? Just curious can you have rpm to high I am sure it has to mow better but do you need it high? Thanks. Eddie

eddie.

The 540 pto speed is designed into the tractor with the tractor engine's torque curve foremost in mind.

Think of the engine's torque curve like a mountain. An up slope, a peak and then a down slope past the peak.

Running at the recommended engine rpm to produce 540 pto rpm means you are just past the peak of the torque curve.

Where this becomes important is when there is a sudden increase in load on the pto. If you are running just past the torque peak, as the rpm's begin to slow, the torque actually increases and the engine can recover from the sudden load increase.

On the other hand, if you are running before the torque peak by using lower rpm's, a sudden load slows the engine and it has even less torque. A sudden stall is possible.

The V belts also are designed to transmit their power at higher speeds.

540 or even 700 pto rpm wont hurt anything.

Dave M7040

chim · May 27, 2018

With the RFM's I've been using since '92, there have been times when reducing the rev's a bit distributed the clipping better. The grass isn't always cut when the conditions are exactly the same. It is sometimes 'juicier" , taller, shorter, etc.

TomIre · May 27, 2018

Dave M7040 said:
eddie.

The 540 pto speed is designed into the tractor with the tractor engine's torque curve foremost in mind.

Think of the engine's torque curve like a mountain. An up slope, a peak and then a down slope past the peak.

Running at the recommended engine rpm to produce 540 pto rpm means you are just past the peak of the torque curve.

Where this becomes important is when there is a sudden increase in load on the pto. If you are running just past the torque peak, as the rpm's begin to slow, the torque actually increases and the engine can recover from the sudden load increase.

On the other hand, if you are running before the torque peak by using lower rpm's, a sudden load slows the engine and it has even less torque. A sudden stall is possible.

The V belts also are designed to transmit their power at higher speeds.

540 or even 700 pto rpm wont hurt anything.

Dave M7040

Great explanation. Thanks! Had not thought about it quite that way before.

JWR · May 27, 2018

eddieirvine: If you hunt around on the web there are specs for various mowers (incl.bush hogs) that list blade tip speed (BTS as diesel calls it.) For example the BTS specified for my 7ft bush hog is 15,268 ft/minute at 540 rpm. [Of course the "rpm" they are talking abut in these specs is ENGINE rpm, and 540 pto rpm, NOT blade shaft rpm.] Obviously there is a good bit of variation. The BTS is going to be in the teens of thousands of feet per minute for any grass/brush cutter. You can Google search on "lawn mower tip speed" or similar wording and get more than you ever want to read on the topic. One source says there is a"government mandated maximum speed of 19,000 fpm" which may well be true. BadBoy zero turns have a You tube video out saying they use about 18,500 ft/min tip speed and consider that critical to the quality of their cutting.

I have a couple of Kubota B2150's that have a two speed PTO. One is the normal 540rpm and the other is 780 rpm (or something just over 700.) If I were to use the 780 setting at full engine rpm I'd be running my blade tip speed on the order of 40% above spec which is way too high for prudent behavior. What I often do (esp. in light grass) is run in the higher PTO speed setting but then run the engine less than full engine rpm. It is a "listen and try it and see how it sounds" kind of thing but that allows me to mow in a higher gear for laid back slightly faster mowing speed yet keep the mower at the same approx tip speed it is spec'd for. Versatility in an older machine.

In my experience blade sharpness is the big smooth cutting factor once you have BTS within a reasonable "normal" range for your machine.

moship · May 27, 2018

Something you may want to be concerned about is the PTO hydraulic pressure (used to clamp PTO clutches) varies with RPM. I know this is true with the -3 models but not certain if the earlier Grand L40’s are the same.

Not saying you will have a problem if running under 2,000 engine RPM, but you won’t be generating as much clamp load on the PTO clutch disks. Another forum member had a problem running a salt spreader (very little PTO load) at low rpm. Clutches went bad prematurely and had to be replaced.