Hot Saw technique to reduce the risk of chain shot

[BIG JAKE]

Good post-never really thought much about this but it makes sense although the penetrate steel thing I'd like to see for myself. Not saying it can't happen would just like to see it. But I've seen cables and chains snap when pulling logs or stumps-just don't be standing near the cable at any time after connections are made. Good way to get killed.

 

[Crofter]

I am sure that you do shock load a chain when you deliberately jam it into the wood at 14 or 15 thousand rpm. You want to dump the free stored enery of the rotating crank/flywheel mass into the first 2 inches of cut, so that is pretty much the routine. To me it is a bit scary making switchovers from upcut to downcut at full rpm where you move the bar sideways as quickly as possible and hit the wood again, knowing that the chain is standing up entirely out of the bar groove. Does not take much to derail. EHP had one wrap around his leg and right throug the chaps each tooth punched a ragged hole in his his hide with a lot of bruising. Woould like to hear more of how the Corolis Effect applies here.

 

[Oregon Engineer]

I think two physical effects are getting confused, one is the problem of rotating the bar, the other is the dynamic whip action that generates a chain shot.

The rotation of the bar (with the motor at full speed) causes the chain to be pulled out of the bar grooves. It is obviously bad to have the chain out of the bar grooves, but it may not break the chain.

To create a chain shot, the chain loop must first break so it is no longer a loop and has an end to whip around. Then the dynamic whip action creates the second break which releases pieces of the chain.


Timberwolf & sawn_penn,
The parts come off the end of the chain because of a dynamic whip action that occurs in the chain, which is identical to what the tip of a bull whip experiences. Rotax Robert posted a link to a high speed video showing an actual chain shot where the end piece of the chain comes off due to the dynamic whip action.
The energy from the chain is dynamically imparted to the last few pieces of chain during the whip. The rivets in the last piece are sheared off (for .404 harvester chains it takes 2700lbs load to shear the rivets). The pieces only weight 5 to 8 grams yet as they undergo the very last part of the dynamic whip the acceleration imparts a force greater than 2700lbs.
High speed video in a Swedish test lab and a Finish test lab both concur the broken pieces go supersonic. Search this website for “chain shot” and you should see some previous posts on the speed. Search the entire internet web for chain shot and you will see more information.

Rotax Robert,
I agree that when you rev up the saw, the chain comes out of the bar groove on the top side of the bar only. As a chain goes from stationary to full speed the centrifugal forces on the chain cause it to elastically stretch. The bottom chain is pulled tight so that all the slack in the chain is forced to the top side. The area of concern is when the bar is not simply lowered onto the log but instead rotated. The rotation of the bar will cause the chain to come out of the grooves. I’m working on getting a video posted that shows this effect.
What is the link to your promotional oil video? I recall the video quality was good enough to see the bar rotate towards the cut and the chain came out of the groove.

Crofter,
You are right, if the saw handle and bar tip are lowered onto the log at the same speed there is no force pulling the chain out of the grooves. It is only when your saw technique causes the bar to rotate. You can lower the chain onto a log (the bar orientation stays the same to the ground) or you can rotate the bar into the log (the angle of the bar changes to the ground). Then the bar rotates you increase the risk of the chain jumping.

Regarding the figures for tension and force on the chain. 20 years ago I saw an analysis on determining the actual forces. I’ll look for it.

BIG JAKE
Two of the three sheets of steel were the side of a steel walled shop building. The third sheet was a stainless steel sheet. I don’t know how thick the steel sheets were. BUT, chain shots have gone through ½ thick polycarbonate windows. Recently chain shot testing was conducted to determine how thick a laminated (similar design as bullet resistant windows) window needs to be to stop a chain shot from a harvester machine. When the results are formally presented to the industry I will post the information.

 

[Oregon Engineer]

Enough words!! It's time for a demo.

The video is 1.8 meg so it may take some time to down load. The quality is not soo good because it had to be converted to a mov file. The bystander just happen to be filming when the chain broke and a chain shot was generated.

The hot saw is 500cc.

Watch the chain on the bottom of the bar as the bar is rotating downward.

Here is the sequence: (at least as I see it from a higher quality DVD that was slowed down to better see the action, it's way too big to download)

1) As the bar tip rotates downward the chain comes out of the grooves.
2) Sparks are seen (one second into the video) at the drive sprocket (the nose just blew out from the increase in tension).
3) The chain re-enters the grooves (just after the 2 second mark)
4) the chain opens up the bar nose more and runs in between the nose laminates, this creates slack chain so again the chain comes out of the groove for a second time.
5) the chain re-enters the bar groove
6) the chain jams in the nose
7) the chain breaks and one end shoots off a chain shot that is way toooo fast to see on this video while the other end slaps onto the log.

When you view the video find the A/V controls and run the video at 1/2 speed.

 

[ozflea]

Makes one wonder as to the effects chain lightning has on producing a weaker side link and seeing radiculy filed chain thats been dogboned and had the rivet heads ground back and the gullets cleaned out to the extreme no wonder chains break and some use an older chain which has already seen a lot of service as they prefer to use the back of the tooth just before the witness mark plus the top plate is set a lesser angle 15 to 20 degrees and with possibly a bit too much hook.

I'd call that stretching the friendship myself.

Mc Bob.

 

[Oregon Engineer]

ozflea,
The hot saw competitors reduce the chain strength and at the same time max out the supplied power to increase the cutting speed. When the system is sooooo pushed to the max (way beyond the design envelope), the saw handling techniques need to be refined to minimize any additional stress to the cutting systems. I'd like to slow the chain speeds down to a safer speed, but that's a decision each man makes as they weigh the danger risk. However, I can assist with information on better ways to understand the danger risk and then techniques to reduce it.

 

[ozflea]

Yes when i see some of the chains that have been modified it horrifies me the chain components have been ground leaving grinding marks scores in what was once a smooth factory finish thereby uping the stress levels placed on chain components.

Personally i start with a brand new chain for strength and hand file not grind to preserve strength but my chains might be as fast as some but i preferr the assurance of keeping my body parts intact and in working order.

 

[sawn_penn]

oregon_engineer,

Cool video. The hires version must be really something.

In my mind at least, I think I see what might be happening.

The saw tip is dropped, and this creates an excess of chain on the underside of the bar.

This excess allows the chain to run "higher" and "higher" (well, further out from the centre anyway) on the sprocket. This might cause a failure due to tension, or alternately the drive links might lift sufficiently to twist slightly in the sprocket.

Is there any damage to the sprocket/drive links? What do the broken ends of the chain look like?

Am I on the right track?

Thanks!

 

[Oregon Engineer]

sawn_penn,
Its not the dipping of the tip that causes the chain to come out of the grooves, it is forcing the bar to rotate with the chain at full speed and the resulting acceleration that bar rotation puts on the chain. I'll admit it is tough to understand there is a force pulling the chain out of the groove when you rotate the bar with the chain at full speed.
The best part of this phenomenon is that you can see it with any gas powered chain saw. We first noticed it 20+ years ago when the technicians could throw a chain off a new bar design anytime they wanted without striking any objects. No matter how tight we tensioned the chain the technicians threw the chain. Viewing the bar from the side, we were astonishhed to see the chain come out of the bar grooves when the technican rotated the bar, and then a little flick of the wrist and off the chain would come. Took a while to figure out the physics responsible for pulling the chain out of the groove.

 

[husky394]

Oregon Engineer:

I was at the Cherry Springs, Pa show maybe 25 years ago, and somebody was demonstrating a "big" saw on a "big" log before the regular hotsaw contest. (they were using 8x8's or 10x10's for the contest) The chain came apart on it, and a spectator, who was sitting in the stands behind the sawyer, was injured seriously by an apparently small piece of that chain. Although I was some distance away, I recall hearing that the link went through that individual. Does anybody else remember that?

I thought we were safe now with the Plexiglas or wire mesh screens between the contestants. They would hardly slow down chain links at the velocity you mention. Do I need a flack jacket?

Chuck

 

[slowsuki]

anybody ever wipped a fly of when fly fishing.i haven't

 

[Lakeside53]

anybody ever wipped a fly of when fly fishing.i haven't

I have, many times, before I figured out how to do it right... I even lost the lower portion of the hook by snapping it back. Thought they were bad hooks, but it was me.

 

[TimberPig]

anybody ever wipped a fly of when fly fishing.i haven't

It's not hard to do, all it takes is snapping your wrist back too quickly and your line whips and snaps the fly off. It's very frustrating when you only discover the fact because your perfectly placed cast now has no hook on the end of the line to catch the fish :bang: .

 

[Oregon Engineer]

anybody ever wipped a fly of when fly fishing.i haven't

That's because YYYOOOOUUUUU know to hold your fly rod still until the line has fully uncurled to the fly. IF you give any pull on the line before the line is fully uncurled, the fly will snap off.

Now, if we could only train the drive sprockets not to pull and the other end of the chain to stop, we'd have no worries.

 

[JohnL]

sawn_penn,
Its not the dipping of the tip that causes the chain to come out of the grooves, it is forcing the bar to rotate with the chain at full speed and the resulting acceleration that bar rotation puts on the chain. I'll admit it is tough to understand there is a force pulling the chain out of the groove when you rotate the bar with the chain at full speed.
The best part of this phenomenon is that you can see it with any gas powered chain saw. We first noticed it 20+ years ago when the technicians could throw a chain off a new bar design anytime they wanted without striking any objects. No matter how tight we tensioned the chain the technicians threw the chain. Viewing the bar from the side, we were astonishhed to see the chain come out of the bar grooves when the technican rotated the bar, and then a little flick of the wrist and off the chain would come. Took a while to figure out the physics responsible for pulling the chain out of the groove.

I'm having a hard time getting my mind around how this works. I can see what appears to be the effect you are describing in the video. How does rotating the bar place a force on the chain. It only happens while the chain is in motion? Is the force in proportion to chain speed, at what chain speeds does it become noticeable?

 

[Oregon Engineer]

How does rotating the bar place a force on the chain. It only happens while the chain is in motion? Is the force in proportion to chain speed, at what chain speeds does it become noticeable?

Well don't feel alone if you don't understand. I can see the effect very clearly but don't fully understand all the aspects. Yes the chain must be in motion. The force is proportional to the chain speed and rotational speed of the bar. The effect can be seen at low to moderate chain speeds if the chain tension is lowered. It is easiest to see on a bar with a very straight profile (no "belly"). The operator will not see much but bystanders will have the best and safest point of observation.


When a bar is rotated down the chain will come out of the grooves, when it is rotated up the chain be pulled into the grooves. This is one of those "don't try it at home" as I can do it on a machine and show you the video. Or check out good hot saw videos and watch for bar rotation before the chain starts into the log.

 

[sawn_penn]

Well don't feel alone if you don't understand. I can see the effect very clearly but don't fully understand all the aspects. Yes the chain must be in motion. The force is proportional to the chain speed and rotational speed of the bar. The effect can be seen at low to moderate chain speeds if the chain tension is lowered. It is easiest to see on a bar with a very straight profile (no "belly"). The operator will not see much but bystanders will have the best and safest point of observation.


When a bar is rotated down the chain will come out of the grooves, when it is rotated up the chain be pulled into the grooves. This is one of those "don't try it at home" as I can do it on a machine and show you the video. Or check out good hot saw videos and watch for bar rotation before the chain starts into the log.

OR_Eng,

As I said before, this is pretty cool. A really neat effect. I understand weird stuff that happens if you roll the saw around an axis that runs along the bar, but I still don't understand the effect during rotation about an axis running through the crankshaft.

I think about weird failure modes for a job. I don't work in the mechanical engineering field, but I find these effects fascinating. I try and string together enough maths to be happy I understand what's going on. I'm sure you've given me more than a few nights of head-scratching!