Correct PTO RPM to Bush Hog

[Farmwithjunk]

Does your tractor have a 2 speed PTO ... maybe 540 and 720? If so, you could put it in 720 PTO speed and run at lower engine rpm if your tractor has the hp to run the mower. That's what I do. For example, my tractor engine runs at 2300 rpm to get 540 PTO rpm. But when I put the PTO in the 720 range, I can run the engine at 1725 rpm and generate 540 PTO rpm that way. In your case (if you have a 720 PTO range) you could run your engine at 1950 rpm to get 540 PTO rpm. (I used 2600 engine rpm to get 540 PTO rpm), then assumed you have a 720 PTO speed.

Advantages for me are:
- less engine rpm so less fuel use
- my peak torque falls right at that (1725) rpm range so little "engine bog down"
- I can navigate terrain at a slower speed in each gear since my engine rpm's are less
- less noise and vibration
- less wear and tear on the engine and drive train ... regardless that you can run a diesel at full rpm

As usual, others may disagree, but that's what I do.
So, do you have a 2 speed PTO?

My older tractors pre-date the relatively new concept of 2-speed pto's (Not including the idea of 1000rpm pto, but that applies to larger tractors) My 150 Massey has the 540rpm and "ground speed" pto option, but that's a completely different concept altogether.

The 6430 Deere has enough of a mower behind it that it needs full rated rpms in all the mowing conditions where we've been using it.

A good many larger tractors (larger than most subs and compacts) will have engines with more cubes and a little wider powerband. They will facilitate using them at various engine speeds and still have enough hp/torque to do their thing without running at full "pto rated rpms". Regardless of the fact that some people just don't see it that way, a great deal of the time, mowing conditions will allow lower blade speeds and still you'll get the quality of cut that higher speeds will offer. In some conditions slower blade speed simply works BETTER. (Where trash and hidden obsticals are present comes to mine first)

As a generality, most manufacturers will suggest running at full rated rpms as a way of preventing the user from lugging their diesel engines. That's NOT GOOD on diesels. In no event would I suggest running any diesel slow enough to cause it to lug down.

I have a New Holland 451 sickel bar mower that has a "better idea"! They have (I hope I can spell this'n right) a variable speed sheave (pulley) on the belt drive that allows you to change it to "over-drive" the mower. That allows up to a 33% reduction in pto speed, while all along letting the mower run at the same speed as it would with a constant 540 rpm pto speed. Combine that with the fact that it can be used with cutterbar speeds just under "full tilt", and a 451 user can run their engine at just over 1/2 the rated pto speed. Sickel bars don't require very much hp. This option lets you cruise around the hayfield with your engine barely above idle and still get the job done.

As our mowing business takes off, I'm shopping for ideas to equip it better for more economic operation. (Yes, I'm a mizer) One idea I'm really interested in is a hydraulic driven mower. (Alamo Group.....Rhino) Most larger modern tractors will supply enough hydraulic flow and pressure at mid rpms to run these mowers at full speed. With diesel not too far off gas prices of more than $3 a gallon, it wouldn't take long to realize a significant savings I'd suspect.

Simply running my fuel mizer 150 Massey at a reduced engine speed will save me a quite a bit of fuel over the course of an 8 hour day of bush hogging. (At full pto rated rpms this little gem will power a 6' mower in heavy weeds, cutting 2-1/2+ acres per hour, and not burn more than 3/4 gallon per hour. And we're talking 1960's technology here too)

 

[browns40]

Wow I remember this very thread from last year, which went very similarly, although I think it's a shame you can forecast the downhill slide from sharing personal opinions & practices to detracting from others...

Anyway, my personal opinion, is that you can run any implement at PTO speeds that produce the desired result, with less force/speed on machinery equating to less wear. I've seen a lot of big Honda outboards around the bay on the salmon cages, and own one myself--it's the same basic motor as the Civic--they wear out much faster running at 4-5000 RPM planing off boats than they do at 2000 RPM getting people around at highway speeds, even though they are rated to do it.

Heck, I can cut through grass with a pitching wedge swinging about 60%, probably the bushog can too

I run full or near for genuine brush, and 3-4 hundred RPM above the peak of the torque curve for regular mowing of the front field... commercial guys that need to bang it out & get to the next job ought to run full, finish the job fast, and move to the next, as they can go at a higher ground speed & get the same number of turns on the blades per distance. Speed pays them.

If you fetch up your bushog with the tips going at 14000 fps, or 10000 fps, the proper slip clutch will provide driveline protection--I set mine to "break traction" a little low though... in a perfect world I'd love to get it just enough to get the implement turning at rated speed before letting go, but I'm not that good!

Experiment with it a little, do what you like the results of, avoid lugging your engine--heck it's your stuff, and it's fun!

 

[greg_g]

Uh, I'm also running less than full RPMs -- I'm running at max PTO RPMs, not max tractor RPM's.

Me too. On the KM454 for example, engine revs to a governor-limited 2650 rpm. PTO revs are in the 2200-2250 band

//greg//

 

[scott_vt]

Despite your unnecessarily rude delivery, I'll thank you anyway for reinforcing my point.

//greg//

Mornin Greg,
If youve ever seen any of Bills' equipment you would realize quite quickly that its far from junk !

 

[Farmwithjunk]

Mornin Greg,
If youve ever seen any of Bills' equipment you would realize quite quickly that its far from junk !

Howdy Scott,

Thanks. Unfortunately I think your comment fell upon deaf ears. I don't let insults from guys like that bother me anyway. Life's too short.

Bill

 

[greg_g]

RPM has nothing to do with the torque at which your clutch will slip.

Yes. It does. There's a mathematical relationship between horsepower and torque; [SIZE=-1]HP=TORQUE * RPM / 5252. Note that[/SIZE] one of the variables is RPM. Using that formula, you can arrive at approximately how much torque is being applied on the tractor side of the slipclutch. The rotary cutter applies torque on the opposite side of the slipclutch.

See attached chart. It contains settings for one of the most common slip clutch shafts around, the Eurocardan 60hp model; 8 adjustable springs pressing a pair of friction discs on either side of a smooth steel friction plate. The far right column lists how much torque it takes to overcome the spring tension for any given setting. When torque from the tractor side PLUS torque from the mower side EXCEED that value, a properly adjusted Eurocardan 60hp slipclutch - slips.

If your slip clutch is set correctly, it will slip as it is supposed to at any reasonable RPM.

No. It won't. When you change the PTO output RPMs, the amount of torque you're applying to the tractor side of the slip clutch changes. Yet the spring tension doesn't change, hence the amount of torque to slip the clutch doesn't change. So by running the tractor at less than recommended PTO revs upsets the torque split at the slipclutch. Less torque from the tractor side means it takes MORE torque from the mower side to slip the clutch. Damage may ensue.

On the matter of the JD slip clutch; the manner of setting may be different, but the principle is the same. I believe the JD design works in a very narrow horsepower window, whereas the Eurocardan design is engineered to fit a multitude of PTO outputs.

//greg//

 

[Gatorboy]

See attached chart.

I can't see it.

 

[N80]

But the initial concern was that at low pto rpms the shock load would not be sufficient to cause the slip clutch to 'slip'. If that is true then the shock load should be insufficient to damage the unit or the tractor unless there is a major mismatch between the tractor the slip clutch or the cutter. This might be a problem for someone adding a slip clutch, but again, on my JD there is no recommended adjustment other than a 4mm space. This suggests to me that within the recommended use and within the recommended hp rating, it should function just fine at any rpm up to but not beyond the recommended pto rpm of 540.

 

[greg_g]

If that is true then the shock load should be insufficient to damage the unit or the tractor unless there is a major mismatch between the tractor the slip clutch or the cutter.

Exactly. Which is why I specifically pointed out that the OP has a 120hp mower tranny behind a 45hp PTO

//greg//

 

[N80]

Exactly. Which is why I specifically pointed out that the OP has a 120hp mower tranny behind a 45hp PTO

//greg//

I don't see how that matters unless that tranny is attached to a rotating mass (blades, spindle, stump jumper) that is too large for the 45 hp tractor to handle. (And there is no indication that this is the case). I wouldn't think the gear box, regardless of how high it is rated, adds anything to a low velocity shock load. I'm not trying to argue, just trying to get this all sorted out in my head.