540 rpms or less?

BrettW · Dec 5, 2004

I just read, in the attachments discussion area, that mowing at less than 540 rpms is bad for the mower gears, as is mowing above 540. Is this true? I have a Kubota B7800 and 5' Midwest finish mower. I usually run it much lower than 540 rpms as it is just really noisy and does a good job cutting at the lower speed. So, is it OK to run it at the lower speed? Anyone using a Midwest finish mower out there? thanks, Brett w

DK35vince · Dec 5, 2004

( I just read, in the attachments discussion area, that mowing at less than 540 rpms is bad for the mower gears, )

Can't imagine why, I do it all the time.

kayssupply · Dec 5, 2004

FYI I used to sell a brand of mower that had a loud gear box. I would use a 50/50 mix with Lucas oil additive. It made a big difference in noise and it also kept the oil from foaming.

Bob_Skurka · Dec 5, 2004

Personally I can't see how it could be too bad for the gears, what I think it could be bad for is the cut of the grass. The implement, if designed properly, would be optimized to be run at the PTO speed of 540rpm. By optimized, I mean that it will spin the blade tips at proper speed to efficiently and effectively cut the grass. Now if you run at slower speeds your blade tip speeds are lower, you may (or may not depending on many conditions) be tearing the grass instead of cutting it. But I don't see how that would be hard on the gears.

thcri · Dec 5, 2004

I don't think it has any effect on the gears. The only thing it might effect is the quality of mowing. The mower is designed to run at 540 for maximum performance. If it mows ok at a slower speed then go for it.

But you could add to this question, what about the performance or longevity of the motor. Diesel motors are designed to operate at about 2240 or 540 at the PTO. I would be more concerned about the motor than the mower. Now just being close is ok but how far off are you going?

murph

OkieG · Dec 5, 2004

usually run it much lower than 540 rpms...and does a good job cutting at the lower speed. 

As mentioned in previous responses, I agree that lower rpm should have little, if any adverse impact on your gearbox.

The greater the tip speed, the more likely a blade will cut vs tear. Does it take 540 to cut? Will grass blades always tear at less than 540? If you look at different models of decks, their tip speeds will likely not be the same at 540rpm.

I believe a dull mower blade could tear a blade of grass, even at 540 rpm...while a sharp blade may cut your yard nicely at a speed below 540? Even after mowing once or twice, if you feel the edge of the mower blades, you will be able to tell that the sharpness is beginning to dull.

Then there's the question- So what if some grass blades are torn instead of cut? More prone to disease? etc. What diseases have you seen? I don't know. Most everybody has their yard cut regularly during summer. Right? Have you ever heard someone say, "Wow, look at that diesased yard. That guy must be mowing with dull blades at less than 540 rpm!!"

Anyway, my point is, keep your blades in decent shape, and if the cut of your yard is OK at less than 540 rpm, stay with it. I don't think it will hurt your gearbox.

OkieG

BrettW · Dec 6, 2004

Thanks for the replys,
From an earlier post, I learned that on my Kubota B7800, 540 rpm= 2600 on the engine. I think I've been running the mower at about 1800 or so and it is less noisy and seems to cut welll at that speed. I do run it faster if I'm doing more leaf grinding than grass cutting. So, is there a possibility of engine damage, or less engine life, running it the mower at a lower rpm?
thanks again, bw

Glenn9643 · Dec 6, 2004

It would seem to me that the inertia of the heavy stumpjumper and blades spinning at 540 would help to absorb some of the sudden loads encountered if cutting something more brushy, unlike grass where the load would generally be about the same. That built-up energy might relieve a load on the gearbox.

theboman · Dec 6, 2004

Why would you want to run below 540 RPMs? I can't think of any reason to run below 540 RPMs.

The mowers need to run at 540 RPMs for the mower to work properly. The blades LIFT the grass to cut. 540 RPMs is the desired level to LIFT and CUT.

rswyan · Dec 6, 2004

( I can't think of any reason to run below 540 RPMs. )

Here's two: 1. Noise 2. Fuel consumption

When I first got my B2910 (72" MMM) back in May I was cutting at around 1600 - 1800 RPM. When I mentioned this to my dealer he got a little excited and warned me not to lug the engine. From what I could tell the engine wasn't lugging at (wasn't like I was cutting 4" off 8" tall heavy grass) - it ran fine. And given the factory edge on the mower blades (dull as the edge of door) the cut was surprisingly good (the dealer had mentioned I might want to sharpen the blades cause the factory edge was so dull) I never did sharpen the blades, although that's on the list for this winter, and the mower still cut great all season. I'm interested in seeing what it will be like once the blades are actually sharp.