sudden and violent PTO engagement with new Kubota MX5800

[ovrszd]

It probably only stalled because there was too much load on the engine at that moment ... (While the blades were sorting themselves out)

Yep...

 

[rbtjr]

Good thoughts and info. Thanks to everyone. I will for sure check out the possibility of replacing a part that could slow the flow of fluid and thereby soften the PTO start. IMO, it stands to reason that the problem is that the hydraulic flow to the PTO is turned on suddenly and has immediate significant force. So, if I can dampen this flow to start, it could be a solution. Yes, the fluids are all good. I checked them all. The PTO shaft on the mower has a slip clutch in line, so presumably that's not the problem. Yes, I raise the mower so that the PTO shaft is in about a straight line with the PTO before I engage the PTO. However, I did notice some shaking of the PTO shaft, so I might need a longer one (or a PTO extension, which I have) for use with the mower. I am aware of the inertia effect of the blades being slung out or not. In fact, when the tractor shut off on one PTO engagement and the tractor wouldn't restart regardless of what else I did, including of course turning off the electric PTO switch, I figured that somehow the "PTO engaged" sensor was somehow still in the engaged mode. So, I disconnected the mower PTO shaft from the tractor PTO and rotated the tractor PTO shaft both ways. Then the tractor started. I took the tractor to the shed, raised the mower, and saw that the mower blades had become stuck together, one on top of the other. I had that happen once before but only when I was mowing and hit an obstacle that caused a blade to bent a little--never when just engaging the PTO. I used a light sledge hammer and got the blades separated. I then went back out and engaged the PTO again, and this time the mower did get up to speed but only with again the violent start and shaking. Once the PTO got up to speed the mower worked well.

 

[ovrszd]

Since this is totally a hypothetical discussion I'll add this.

The PTO clutch is a wet clutch, meaning it's running in oil and is activated by oil pressure. If the activation mode is softened the clutch discs will slip longer before complete engagement is achieved. So, wouldn't this slippage cause premature clutch disc failure?

 

[rbtjr]

Not to sound condescending, your tractor doesn't "know" if the PTO shaft of the cutter is attached.

No condensation taken, and never would be. To me, there is no such thing as a bad thought when we are trying to contribute to a solution. And agreed with respect to the electrical circuitry. That is why I couldn't understand why the tractor wouldn't start after it shut down following a violent PTO engagement. I turned the PTO electrical switch off, on and off several times. No go. So, I wondered if there were a mechanical component to the "all is clear" electrical circuit. Stated differently, I thought that if the PTO shut down suddenly, there might be a hydraulic pressure feedback loop or some such to prevent the tractor from restarting when there was a problem with the PTO. So, I disengaged the mower PTO shaft from the tractor PTO, and the tractor started. I don't know why this worked, but it did. I haven't investigated it but to me a logical possible answer is that there may be in the PTO-engagement circuitry a pressure-sensor switch that senses when there is still hydraulic-oil pressure trying to rotate the PTO even though the electric PTO engagement switch is turned off. That's why I tried disconnecting the mower PTO shaft and rotating the tractor PTO shaft both ways. It worked, but I am for sure not saying that the reason is what I thought it might be. Interesting, though.

 

[rbtjr]

Since this is totally a hypothetical discussion I'll add this.

The PTO clutch is a wet clutch, meaning it's running in oil and is activated by oil pressure. If the activation mode is softened the clutch discs will slip longer before complete engagement is achieved. So, wouldn't this slippage cause premature clutch disc failure?

Another good and pertinent thought. Based on my limited experience to date, I would probably run the risk of premature failure of the discs in order to eliminate the violent PTO engagement of the attachments, especially with the mower, which is my most often used attachment that requires the PTO power.

 

[ovrszd]

No condensation taken, and never would be. To me, there is no such thing as a bad thought when we are trying to contribute to a solution. And agreed with respect to the electrical circuitry. That is why I couldn't understand why the tractor wouldn't start after it shut down following a violent PTO engagement. I turned the PTO electrical switch off, on and off several times. No go. So, I wondered if there were a mechanical component to the "all is clear" electrical circuit. Stated differently, I thought that if the PTO shut down suddenly, there might be a hydraulic pressure feedback loop or some such to prevent the tractor from restarting when there was a problem with the PTO. So, I disengaged the mower PTO shaft from the tractor PTO, and the tractor started. I don't know why this worked, but it did. I haven't investigated it but to me a logical possible answer is that there may be in the PTO-engagement circuitry a pressure-sensor switch that senses when there is still hydraulic-oil pressure trying to rotate the PTO even though the electric PTO engagement switch is turned off. That's why I tried disconnecting the mower PTO shaft and rotating the tractor PTO shaft both ways. It worked, but I am for sure not saying that the reason is what I thought it might be. Interesting, though.

I don't think your tractor is that smart. But what do I know.

 

[ovrszd]

Another good and pertinent thought. Based on my limited experience to date, I would probably run the risk of premature failure of the discs in order to eliminate the violent PTO engagement of the attachments, especially with the mower, which is my most often used attachment that requires the PTO power.

My Kubota uses a cable operated lever to engage the PTO hydraulic clutch. It specifically states to not soften the start by feathering the lever, fully engage without hesitation. I believe that is to minimize clutch slippage.

 

[ericm979]

I have a Massey Ferguson with a similar electric PTO clutch. Starting my brush mower....it is almost scary given how bad it sounds. In contrast, starting my PTO wood chipper is about as smooth as can be. Go figure.

I find the same with my Branson's electric PTO. The chipper starts smooth and the rotary cutter makes gnarly noises and vibrates for a while. I think there's a few things going on- the wood chipper has a 24" 200 lb flywheel turning at twice PTO speed (making it effectlvely a 400 lb flywheel). The rotary cutter has a small diameter hub, stump jumper and two blades that weigh about 10lb each. Much less flywheel effect. Yet the tractors' PTO clutch engagement has to be able to handle the wood chipper without slipping too much. That means the rotary's engagement is abrupt.

I think the reason the rotary vibrates when it starts is that the blades are swinging on their pivots, which means the entire assembly is unbalanced. It's made to be balanced when the blades have swung out to the farthest position, not when they're swiging back and forth like a pendulum. A slower start like you can do with a manual clutch would reduce the pendulum effect.

It's unnerving but I figure that the cutter sees a lot worse when it's cutting brush and the forces are low so it's not going to damage the tractor. But if the tractor shuts down, then that's a problem.

 

[RoyJackson]

I'm afraid that's the nature of the beast. Do your engagements at engine idle to minimize the shock to the cutter or any other PTO attachment.

Same applies to disengaging an implement...idle down for 5-10 seconds before disengaging the PTO. Makes for a much smoother transition and, undoubtedly, will extend the life of the implement and tractor.

 

[RoyJackson]

I think the reason the rotary vibrates when it starts is that the blades are swinging on their pivots, which means the entire assembly is unbalanced. It's made to be balanced when the blades have swung out to the farthest position, not when they're swinging back and forth like a pendulum. A slower start like you can do with a manual clutch would reduce the pendulum effect.

That's right...until those pivoting blades extend and balance out, a cutter will make a heck of a racket. It helps if you keep the RPM low until the blades pivot into position (keep your foot near the throttle if the tractor starts to stall).