960 PTO speed - B7800?

[mathey]

My FIL in has a B7800 gear model that we use for mowing in between Christmas tree rows pulling a 40, 42 or 48" rotary mower. The dilema is with the gearing and ground speed.

Rear PTO speed #1: 540rpm pto @ 2600rpm engine
Rear PTO speed #2: 960rpm pto @ 2554rpm engine

If you use the 540 PTO, 2600 rpm, and hi range, 1st gear is too slow and 2nd gear is too fast...in lo range all gears are too slow. So this weekend i switched to 960 PTO speed, hi range and second gear, but dropped the RPMs down to about 1800, the ground spped was perfect, and it seemed to cut fine...my guess is that i'm basically using the 960 selection but running it at 540...make sense?

anyway...is there a reason why this is bad or is there a better way to handle this situation?

 

[skipperbrown]

My thoughts (as probably yours) is that if you aren't over rpm-ing the cutter and the engine is not laboring, everything is fine.

 

[Bob_Young]

Hi Mathey,
Seems like a good solution to me and one I considered for my B7600. However, doing a quick ratio calculation (what I call it) shows that with the 960 RPM selection, you'll get 540 RPM at the PTO at an engine speed of about 1436 RPM. So at 1800 RPM you're probably over speeding the mower a bit. In fact another ratio calculation shows that at 1800 engine RPM with the 960 PTO RPM setting, you're actually turning the PTO at about 676 RPM.

So, as usual, the devil's in the details. It appears you're pushing the mower pretty hard. If you can find a gear that lets you run comfortably at around 1400RPM without laboring the engine; you'll be good on all counts.

Though I've not yet tried it, it appears to me that the only thing the 960 RPM setting would be good for on my tractor is running a 3pt. seeder. Other than that, I haven't come up with anything.
FWIW
Bob

 

[mathey]

ok, i'll give that a shot this weekend...not really "knowing", I was trying to base it more on the sound...but now I have a more accurate number to shoot for...

 

[biker128pedal]

I thought the B7800s were all HSD/ hydrostatic with a mechanical cruise control? Shouldn't you just run the PTO at 540 in Mid or Hi and copntrol the speed wiht the pedal. The mechanicla cruise controls only have a few setting and maybe one will surfice.

 

[mathey]

oops, my bad...

looking at the Kubota website, its actually a B7510DTN...but its definitely a gear model...

do you think the RPM-PTO speeds are the same as for the B7800? anyone have one that can check the owners manual?

 

[Bob_Young]

Yep, just checked the B7610 manual which also covers the B7510DTN. It's the same as the numbers you gave above.....960 PTO RPM @ 2554 Engine RPM.

I wondered about that 'gear B7800' too, biker. Just assumed they might've made one in the past.
Bob

 

[mathey]

sorry about that...you'd think as much time as I spend on that dang thing, you'd think I'd know that...but i reckon since i din't buy it and I don't own it, its ok!

 

[biker128pedal]

Well the label is not visable from the operators station. Shows you spend more time driving it than working on the engine.

Mike

 

[3RRL]

I agree with Bob Young and his calculations.
At 1800 rpm in the higher pto range, you might be spinning that mower too fast? This may not be the best condition for the mower. My calcs are the same as his... that you need to run @ 1436rpm in the 960pto range to generate 540 pto speed. (540 ÷ 960 = .5625 x 2554 = 1436.325 rpm)

However, as far as using the 2nd pto range, I do that all the time when light/moderate cutting. Although my 2nd range pto speed is different than yours, it allows me to cruise along with better fuel economy and still have plenty of power. Mine is an 8 speed gear tractor so I can select appropriate (ground) speed pretty easily. It also happens to put my tractor right at its maximum torque range. I have found for most instances, running the engine rpm near where maximum torque is made is quite adequate (and desireable) for most needs. I only run the rpm up when I need the extra horses and then I switch back to the 540 pto range.

I'd say it's perfectly Ok as long as you're not lugging the tractor and can match your desired ground speed. Take every advantage of your tractor's features to make it as useful as possible without damaging it.