Chain fell off and cooked some drivers

[BobL]

Though you'all might like to see what happens when a chain comes off at WOT and you don't quite drop the revs quickly enough.

Normally I'm pretty attentive, watching the tacho and temp, aux oiler and wheels etc. This time I was just daydreaming and the chain came of about 3 ft down the log. It was probably just too loose to start with and overheated.

I was milling in the shade and the first thing I saw was sparks coming out where the sawdust normally comes out, must have been like 1 second before I woke up and let the throttle go, but it was enough to cook 6 drive links like this.

Near new 8 pin sprocket was barely scratched.
I replace the short length of chain containing 6 of the drivers and filed the cutters and rakers down to suit.

 

[huskyhank]

Did you fall asleep in your chair while it was auto-cutting?

 

[mtngun]

Is this the same derailment that happened during the milling holiday, or is this yet another derailment ?

Thanks for the pic.

 

[BobL]

Is this the same derailment that happened during the milling holiday, or is this yet another derailment ?

Thanks for the pic.

Yep same one (didn't take a pic at the time).

It wasn't auto milling but it could well have been and that's another reason why I don't autp run all that often.

 

[splitpost]

:jawdrop:not good bob ,but tell us what really happened ,were you auto milling when the sound of the 880 comming of load and four stroking up high woke you up:hmm3grin2orange:we've all seen the auto milling pic,just can't tell if ur eyes are open

 

[BobL]

:jawdrop:not good bob ,but tell us what really happened ,were you auto milling when the sound of the 880 comming of load and four stroking up high woke you up:hmm3grin2orange:we've all seen the auto milling pic,just can't tell if ur eyes are open

It was this log.

If you look back you will see I mention it in this post.

The log itself was not on a natural slope and a bit big for lifting to mill on a slope so it was not auto milling.

The most common time for the chain to come off on a long bar is entering or leaving a cut. Entering is probably because the chain heats up faster than the bar. Leaving is when the chain can get pinched even when using wedges because the wide slabs can still dip a little in the middle. I have gotten around this by driving a 6 ft long 1/4 x 2" wide hardwood slat through the kerf just behind bar.

The other time the chain can come off is close to the end of a slab when the chain needs sharpening and it is pushed too hard and probably just stretches too far.

This was the first time the chain came off just 3 ft into the cut. Normally I check chain looseness before starting a cut and I probably just forgot this time.

 

[splitpost]

yes bob ,i was just taking the piss outta you

 

[BobL]

yes bob ,i was just taking the piss outta you

I wasn't sure but I do know there's plenty of piss to get rid of.

 

[splitpost]

your to far away to help out with that beer problem

 

[BobL]

your to far away to help out with that beer problem

I dunno, say you'll have one for me and then I'll have one more for you ?

 

[splitpost]

That'il work

 

[billstuewe]

BobL, I am very disapointed in you--6 drive links! Your reflexs are way too slow. You should have been able to let up on the gas before 2 drivelinks went by!! Why, at 10000RPM with an 8 pin sprocket that's 80000 dl's/minute or 1333/sec. With 3/8 chain that's almost 60MPH---and it took you 6 DL's to release the trigger!! That's 1/222 of a second--over here we call that a "split second".
(It is nice to poke a little fun at a friend that saws from his recliner)

For those that haven't seen this--some interesting trivia:

Chainsaw Facts - Did You Know That....
A chainsaw piston goes up and down in the cylinder 20,000 times per minute.
The piston ring speed on the average chainsaw travels 2,500 surface feet per minute.
A chain slides across the surface of the sawbar rail at around 5000 feet per minute.
The chain is moving at 55 to 60 m.p.h. or a mile per minute ( 88 feet per second ).
The drive links impact the sprocket an average of 1,300 times per second.
The chainsaw bar sprocket rotates nearly 1 million times daily when sawmilling so grease that sprocket daily .
Chain Speed for Milling Lumber
The higher the chain speed, the faster the sawmilling speeds and to calculate this:

Take the chain pitch and multiply by 2. Multiply the above number by the number of teeth of the saw sprocket.
Multiply that number by the saw R.P.M.
The final answer will be the chain speed in inches per minute, just divide by 12 to convert to feet per minute.

 

[BobL]

BobL, I am very disapointed in you--6 drive links! Your reflexs are way too slow. You should have been able to let up on the gas before 2 drivelinks went by!! Why, at 10000RPM with an 8 pin sprocket that's 80000 dl's/minute or 1333/sec. With 3/8 chain that's almost 60MPH---and it took you 6 DL's to release the trigger!! That's 1/222 of a second--over here we call that a "split second".
(It is nice to poke a little fun at a friend that saws from his recliner)

Maybe I should change my name to recliner_bob

I agree with the math but that's not what I think happened. When the chain came off the drivers popped out of the sprocket, enough so the drive links did not to continue to be driven (which can make a real mess as well) but still enough for the tips of six drivers to still make contact with the sprocket. I saw sparks and for a second wondered if I had hit something and did not realize the chain had come off. During that second each driver tip was hit with 167 impacts from the sprocket - hence the sparks.

This was also the first time a 60" chain had come off while running an 8 pin sprocket and being a little larger it might have been harder for the drivers to stay out of the way of the sprocket.

 

[billstuewe]

Sounds very plausible. Centrifigual force usually throws the drivers out of the sprocket almost instantly, But???

 

[820wards]

Did you fall asleep in your chair while it was auto-cutting?

I was going to ask the same question...

jerry-