engine RPM when engaging rear PTO implements

[peterc38]

Looking for other members opinions on this. When bush hogging, I run engine throttle at mark indicating rear PTO RPM of 540. If memory serves correct, this is at about 2300 engine RPM. Normally, what I do is when I get to the end of a mowing pass, I push in clutch, disengage rear PTO and then lower engine RPM to about 1600. Once I get turned around for the next pass, I push in clutch, engage rear PTO, raise engine RPM and off I go.

The tractor is a Kubota B7610 and the bush hog is a SQ148 squealer. The reason I ask is this week a broke a shear bolt and I had not lowered engine RPM when engaging. There was no other explanation for shear bolt breaking, because I was on flat level ground and there were no obstructions underneath.

For what it is worth, the bush hog manual provides ambiguous instruction. It states to set tractor throttle for appropriate RPM speed (540). Then it states to place tractor in gear and move forward and advance throttle to 540 RPM. Using this instruction, how can it be advanced to 540 RPM when the previous step already said to put it there?

I guess my main question is, is it a good practice to lower engine RPM before engaging and dis-engaging rear PTO driven implements?

 

[Tig]

I usually forget to lower the RPM but I do pay attention when I am releasing the clutch to avoid shocking the PTO and the implement. It's probably not essential but lowering the RPM would be a good practice.

 

[VABlue]

I'd say lower it - my JD manual wants RPM running about 1/3 - 2/3 PTO speed while engaging. It's easier on the PTO and the engine. If I engage at PTO speed the engine takes a beating, but hardly skips when I engage at the lower revs. This is with an electric clutch, though, so threading a manual clutch should help offset the shock.

In any case, I can't see it breaking a good shear pin by just engaging it. The pin was probably weak to begin with...

 

[RobJ]

I like to start at a lower rpm.

But when hogging, I also like to set a pattern where I don't have to stop...at all.

 

[CDsdad]

You can shear a bolt if engaging the pto at high rpm if the bushhog is where it can torque up, such as in high grass, brush etc..or if you have a little more bushhog than you do tractor. I've seen it both ways and done the second.

I engage at just above idle and then don't generally turn off the pto until I'm done mowing.

 

[North Dakota]

I always engage at a low RPM. When mowing with my 3 Point 6 ft mower I turn around with the mower still engaged and on the ground. The only reason I see for disengaging the mower on the turn is if the angle of the PTO shaft becomes too sharp. The only way I see the angle becoming to sharp would be with a pull behind or by raising the mower up.

 

[peterc38]

It seems like the consensus is to engage at lower RPM's which makes sense to me so that is what I will do.

Also, the manual says the shear bolt size should be 1/2" x 3". The shear bolt that broke was only 3/8" diameter. This is what was in it since I bought it last summer, so that is what either mfr or dealer installed. The hole in the u-joint seems right about 1/2" but there is not enough clearance to accept a 1/2" bolt ( I tried). Maybe 7/16 diameter would be a better choice? Seems to me there should be as tight as possible of a fit.

 

[RoyJackson]

I'm with RobJ...try to plan your mowing so you don't need to shift the PTO in and out.
That said, I do cut forward and reverse quite a bit. That requires a fair amount of shifting.

When I first engage the PTO (cutter blades are not in motion at all), I engage at about 1200-1400 RPM. I just keep my foot near the throttle pedal if the tractor starts to stall.
However, when the cutter is up to speed, I don't mind shifting gears. With a 2-Stage clutch, the cutter doesn't slow down and there is no shock to the tractor or cutter drive train. There's a lot of inertia in those heavy blades and they take quite a while to stop spinning. As long as they're moving, shifting the PTO on and off shouldn't be too much of a shock load.

 

[SacandagaBrad]

1/2" sounds pretty hefty. I don't run a brush hog, but do run a rear snow blower on the same size tractor as yours, a TC-24D. That calls for a 5/16" on the drive shaft. Just be sure to use the proper grade bolt. My manual has a typo calling for a grade 8 which is way too stong. Normal is a grade 5, I even run grade 2 sometimes. It came with a grade 8, stalled the tractor instantaneously running at 540 rpm at the pto, never sheared. Try the 5/16", I'll be willing to bet that is the size. Manuals do have typos!!

Brad

 

[crashz]

I engage at a lower speed as well. My 5 foot Bush Hog is a beast to engage, so I start it by lifting it up and engage at about 1500 rpm slowly with the clutch. As I begin moving, I increase engine speed and then drop the cutter down to the approximate height that I want it.

Like other said, keep it going as best you can. Don't be afaraid of raising it up a little and slowing dow for a turn. I haven't quite gotten used to the 2 stage clutch like Roy, so I try to stay mowing at all times in the same gear in a loop or continuous pattern.