Question about PTO Operation

[kpbeddin]

Hey guys,
I found the shear pin on my rotary mower broke as soon as I finished mowing a couple of weeks ago. I had not hit anything but I had just made the dumb mistake of turning the ignition key instead of the PTO switch when I was ready to stop the mower. It occurred to me that the PTO shaft must have stopped abruptly once the engine shut down and caused the sheer pin to snap. I'm not sure this is the reason, but if it is, it seems like a fairly unforgiving design. Anyway, no harm done and lesson learned, but hopefully this knowledge will save someone some unnecessary grief and downtime.

 

[MikeInEburg]

Sounds to me like the shear pin did it's job! If it hadn't sheared, think what may have happened to your driveline...
Mike

 

[kpbeddin]

Sounds to me like the shear pin did it's job! If it hadn't sheared, think what may have happened to your driveline...
Mike

Yes, absolutely and I am not blaming the shear pin; however, my point was that on a tractor with a manually engaged PTO, the PTO will normally coast down if the engine stops for any reason. This doesn't appear to be the case here.

 

[LittleBlueTractor]

Yes, absolutely and I am not blaming the shear pin; however, my point was that on a tractor with a manually engaged PTO, the PTO will normally coast down if the engine stops for any reason. This doesn't appear to be the case here.

The PTO on my S3010 is of the "live" variety and I've always assumed it would freewheel if the engine dies for some reason. It's hard for me to imagine why they would design a PTO to work in any other way these days, but there may be some reason I'm unaware of. I'm wondering if maybe there's a brake involved in your "independent/manual" PTO system that mine doesn't have. I got that idea from here:

Power Take Off School

In any case, I'm not likely to test my theory by shutting down with the PTO engaged just to find out. Thanks for posting about this.

 

[rocknrod]

Yes, absolutely and I am not blaming the shear pin; however, my point was that on a tractor with a manually engaged PTO, the PTO will normally coast down if the engine stops for any reason. This doesn't appear to be the case here.

Think about what you did for a moment.
The engine and PTO drive train is cooking along at 24 or 2500 RPM Engaged.
You stop everything by turning off the key - Engine engaged to the PTO.
Your were expecting something different to happen?
That is an Abnormal Operation for any tractor.
Normally - It does coast down.

 

[kpbeddin]

Think about what you did for a moment.
The engine and PTO drive train is cooking along at 24 or 2500 RPM Engaged.
You stop everything by turning off the key - Engine engaged to the PTO.
Your were expecting something different to happen?
That is an Abnormal Operation for any tractor.
Normally - It does coast down.

Actually, I had slowed the RPMs way down because I was (so I thought) about to disengage the PTO. There may have been some coast down had the engine died for another reason (loss of fuel for example) since the switch would still be on. I don't know, I just thought this was worth posting because seeing as we are all human, something similar could happen to anyone under the right set of circumstances.

 

[Localmotion]

Actually, I had slowed the RPMs way down because I was (so I thought) about to disengage the PTO. There may have been some coast down had the engine died for another reason (loss of fuel for example) since the switch would still be on. I don't know, I just thought this was worth posting because seeing as we are all human, something similar could happen to anyone under the right set of circumstances.

I totally agree with your last sentiment Bedding :thumbsup:

While it is logical that the PTO should always disengaged before shutting off the engine, and the other posts in this thread are also right that that is one of the reasons a shear pin is fitted - it never hurts to remind ourselves of the consequences of cutting corners

Any machine (tractors included) are only as good as their operator - hence why shear pins and the like were invented!!!! :laughing:

 

[beenthere]

I wouldn't worry about it, as said the shear pin worked as it should.
But I also wouldn't jump to the conclusions that shutting off the tractor did the number on the shear pin (happened one time? ). It may have been border-line ready to fail.

Some tractors have a free-spin clutch to stop the rotary cutter on the PTO from 'pushing' back through the transmission. This tractor apparently doesn't.

 

[LittleBlueTractor]

Think about what you did for a moment.
The engine and PTO drive train is cooking along at 24 or 2500 RPM Engaged.
You stop everything by turning off the key - Engine engaged to the PTO.
Your were expecting something different to happen?
That is an Abnormal Operation for any tractor.
Normally - It does coast down.

Hey Rod,

I'm still wondering why it wouldn't be designed to just freewheel like a bicycle -- no matter how fast you are pedaling, when you stop, the driving wheel freewheels. That's very old technology and I cannot think of any reason for it to do otherwise on a modern PTO shaft. As the PTO never changes is rotational direction, what reason could there be for it to remain engaged when the driving force stops? I'm not necessarily expecting you to answer this question, but if there is anybody out there that can ....

I really don't like to think it's simply the cost involved unless the expense would be pretty substantial. I just don't think that could be it. Frankly, I'm hoping there's another explanation.

 

[beenthere]

Littleblue
Cost is involved.
The older gear tractors had direct drive PTO out of the tranny. Some of the "new" tractors have that same technology. And cost is involved, or people wouldn't buy the tractors that they do.