Engine compression on PTO?

[Snoho3]

I am sure this has been discussed before but I couldn't find it... In any case it is a very basic question.

I used my PTO for the first time the other day for blowing snow and broke a shear pin. No problem. But as I was getting ready to replace the pin I realized I didn't know if there was any danger that the snowblower might turn over due to residual engine compression. I had disengaged the PTO lever and shut off the engine. I also spun the snowblower blades a few times using a stick.

Was I putting my hands at risk when I then reached into the unit to replace the shearpin? I considered detaching the blower from the PTO, but did not do so.

Thanks for your comments.

- Snoho3

 

[George2615]

I have replaced PTO shear pins while still hooked up (engine off of course) but if you don't feel safe disconnect it before replacing the pin. If you have a PTO disconnect switch or lever you can disengage the PTO from the engine then you shouldn't need to unhook the shaft.

 

[PILOON]

I change shear pins practically every snow storm and have never given 'residual compression' the slightest thought.
Matter of fact I only disengage PTO and leave the engine running while I change the pin as I want to keep the cab warm to thaw out my pinkies.

Don't know why but I sure shear a lot of pins, so much so that I am contemplating going from 1/4" to 5/16" on the blower fan.
Rarely do I shear auger pins.

 

[DrainPondDesign]

That is a very good call Snoho3, thanks for waking me up & making me think about it.

With the engine off if you disengage the PTO switch or lever & the blower drive shaft turns easy then it is disconnected from the engine so it shouldn't matter if the engine turns a little from residual compression.

I originally thought that just seeing if the auger & impeller turned easy would be enough but then that's why you're replacing shear pins.

Stay warm

 

[Soundguy]

leave her in gear and brake locked.. and if ya got a pto brake.. even better... lotsa force to overcome..

I'd think residual compression problems would have caused havoc with older diesels with no electric fuel cutoff... and mag driven vehicles. we havn't heard mass stories of tractors starting up ontheir own just instantaniously spinning over and starting.. so .. I'd say you were as safe as getting in your car and driving at road speeds with a couple dozen gallons of highly flammable liquid a few feet from you, down a road driven by others that may or may not have a drivers license, insurance.. or may be using drugs or drinking...

feel any better?

soundguy

 

[JMynes]

Haven't seen one chug over yet, at least on multi cylider engines.
For the one cylinder that might be near the top of a compression stroke, there's likely at least one that is at the bottom of the compression stroke.
Single cylinder model airplane engines have rapped my knuckles, though.

 

[Jerry/MT]

I am sure this has been discussed before but I couldn't find it... In any case it is a very basic question.

I used my PTO for the first time the other day for blowing snow and broke a shear pin. No problem. But as I was getting ready to replace the pin I realized I didn't know if there was any danger that the snowblower might turn over due to residual engine compression. I had disengaged the PTO lever and shut off the engine. I also spun the snowblower blades a few times using a stick.

Was I putting my hands at risk when I then reached into the unit to replace the shearpin? I considered detaching the blower from the PTO, but did not do so.

Thanks for your comments.

- Snoho3

When you shut your engine down, it can no longer do work since there is no energy input from fuel to cause any motion. The sum of the pressure forces on the piston is reacted by the system friction and the engine stops. it doesn't matter whether there is a pressure greater than atmospheric in a cylinder. The rings and valves are not perfect and the pressure will quickly leak down. So don't worry about working on an engine that is shut down suddenly rotating the pto. Ain't gonna' happen.

 

[plastikosmd]

True, but in the push snowblower types, some degree of tension is maintained on the belts as the engine stalls in combination with the jam, the ice is removed etc and fingers come off. I usually get about 3-8 of these people per storm to put back together.

 

[Jerry/MT]

True, but in the push snowblower types, some degree of tension is maintained on the belts as the engine stalls in combination with the jam, the ice is removed etc and fingers come off. I usually get about 3-8 of these people per storm to put back together.

I don't want to negate your obsevation on snowblower problems but that's a different problem than the tractor pto/engine compression concern voiced in the intial post.
Your concern has to do with belt tension not engine compression which was the intial posters concern.

 

[plastikosmd]

My only comment is that your statement "When you shut your engine down, it can no longer do work since there is no energy input from fuel to cause any motion" can be misread as to believe that no motion can take place and that there is no energy in the system if the engine is shut down. Motion can happen and fingers do drop to the ground, just not the way the OP was postulating. I agree, we are talking about different beasts.