1/2" thick polycarbonate windows and "chain shot"

[Oregon Engineer]

Anybody out there operating a mechanized tree harvester (falling, delimbing, bucking trees in 20 seconds or less) with 1/2" thick polycarbonate windows? Or is this the wrong forum for that audience?

 

[Newfie]

No, but why? Are the windows not as protective as they are supposed to be?

 

[Oregon Engineer]

Recent "chain shot" injuries have occured where a piece of chain (drive link and two tiestraps) pierced a 1/2" thick polycarbonate window and seriously injured the operator. The chain was used on a mechanized tree harvester saw that typically operates in the 20 to 40+ HP range. Past designs have considered 1/2" thick polycarbonte able to prevent such an accident. But the recent injuries show that thicker windows are necessary.

This is not related to hand held chain saws.

 

[NYCHA FORESTER]

I have no experience in large scale harvesting... with that said; would attaching 1" chain link fence in front of the windows do the job?

 

[ray benson]

Do you know if the 1/2" polycarbonate was the original manufacturers part or a replacement that possibly had lower impact resistance? Ran across this in an OSHA publication -
Polycarbonates represent a family of various polymers, each of which possess different impact resistance characteristics at the same thickness and surface area. Various polycarbonates include Macrolux, Lexan, Relex, Replex, Dynaglass, Exolite, Verolite, Cyrolon, and Makrolon. These materials have different impact-resistance characteristics for different thicknesses and/or surface areas. It is important to note that increasing the thickness beyond a certain level does not always improve or increase the impact resistance characteristics.
Ray

 

[Oregon Engineer]

The window was a replacement but I don't know the brand name used.
You are correct, all polycarbonates are not the same. They are formulated to meet a criteria set by the plastic manfacturer. However, the main issue is 1/2" was considered safe but not anymore. There have been several accidents with original 1/2" thick windows from the harvester cab manufacturers. The windows that had been specified by the machine manufacturers after having met their criteria.

This thread was started in hopes of informing harvester operators about "chain shot" and how to reduce the risk by understanding the problem and doing the following.

- Chain Shot -
Chain shot is a phenomenon that occurs in mechanical harvesting and processing applications. It is the high velocity separation of a piece or pieces of cutting chain from the end of a broken chain. Chain shot occurs when the end of a broken chain is rapidly accelerated due to either an impact with a portion of the cutting head or to a whip-like motion of the chain end. This typically occurs near the drive end of the cutting system (Chain Shot Zone) but can also come from the bar tip area.

Chain shot exposes both machine operators and bystanders to a risk of serious injury or death from thrown components.

To reduce the risk of chain shot:
1) Operators and bystanders must never be in the plane of the bar when the chain is moving.
2) Appropriate windshield material must be installed.
3) Chain speed should be 40 m/s (8000 ft/min) or less for .404 pitch chain and 35 m/s (7000 ft/min) or less for ¾ pitch chain.
4) An energy absorbing guard (chain shot guard) should be installed in the chain path prior to the whip lash or rigid guarding.
5) Guarding should be designed to minimize the chance of the loose end of a broken chain encountering a protruding edge.
6) Used chains should be inspected and damaged or cracked chains removed from service.
7) Never reuse old chain parts.
8) Industry groups recommend that chains should be discarded after the second break.
9) Remove dull chains from service and sharpen to factory specifications.
10) Maintain proper bar and chain lubrication.
11) Replace the drive sprocket when it has visible signs of wear. Worn sprockets damage chain.

 

[begleytree]

Could chemicals (hyd fluid/gas, or solvents) have played a factor? Do they reduce impact resistance?
-Ralph

 

[Crofter]

Oregon Engineer, That is interesting about the velocity increase that can be given by a snapped chain. Kind of like being snapped by a towel. The logging company I worked for about twenty years ago had a prototype feller buncher by Koehring / Timberjack that had a 3000.00 plastic windshield. Ours was a circular saw head so we had no chain breakage but scratching from wiping mud from behind the grille guard reduced visibility to where we had to change it several times until we set up a king sized washer fluid reservoir and spray. You simply cannot rub with a rag and mud!

 

[Oregon Engineer]

Crofter,
You're right, polycarbonate is easily scratched. There are coatings that can be put on to reduce the scratches.

Begleytree,
Yes, polycarbonate can be degraded by many chemicals. The window manufacturers put out a list of chemicals not to use on the windows. That list must be followed.

Yes, the towel snap is very similar to the final dynamic action on a chain during a chain shot. If you like more on that whip cracking event, check out Alain Goriely's "whip wave" documents at the University of Arizona web site.

Test labs in Sweden and Finland have "chain shot" machines that can create a "chain shot" any time they want. The labs have measured the "chain shot" speed at over 750 miles/hour with the piece rotating at 400,000 rpm. They placed new 12mm (.472") thick windows directly in line with the chain shot path resulting a hole in the window. It is not until the window thickness is 15 mm (.590") that the windows prevent a .404 pitch chain from piercing the window. All this said, new 1/2" thick windows will get pierced by chain even without any chemical attack.

 

[ray benson]

Wow ,it seems a chain shot has the potential to be more damaging than a 45 cal. pistol shot at the window. How often should the windows be replaced due to aging?

 

[Oregon Engineer]

Ray Benson,
At one time GE had a maximum life for the windows before replacement. Now I heard (2nd hand info) they no longer require replacement due to UV degredation. However, if you own a machine then please ask the cab manufacturer for their requirements on window replacement. I know chain, bar, and sprockets very well. But regarding windows, I've had some exposure to information but I'm not an expert. The cab or window manufacturers need to be consulted.

Chain shot has appropriately been compared to rifle and pistol bullets in its ability to inflict great damage to machinery or people.

 

[woodshop]

When I worked for the pulpwood company, we looked at different ways of de-limbing the Virginia pine I layed down in fan shaped bunches with the Franklin feller buncher, and one way was a skidder-type of machine that had instead of a grapple, a huge spinning cylinder with chains attached almost like a giant wire brush or wire wheel, but with chains in stead of wire. You would lower this spinning drum with its slinging chains down and ride it down the trunks laying on the ground. The idea was the flaying chains would knock the limbs off the pines, so the ground guys didn't have to do it by hand with saws before the grapple skidders took it to the yard. It actually worked, and had an added benefit that it partially de-barked the logs too, but of course it didn't knock off the limbs facing down against the ground. We decided against that system though when one of the chains broke off of the spinning cylinder, and flung itself into the ground, buried itself several feet deep. Just imagining what a flying chain would do to an unlucky ground guy, or even the machine operator if it broke off was enough to kill that idea as too dangerous and we opted out. Don't remember what kind of windshield the thing had, I think it had a chain link though in front of the windshield.

Dave

 

[techdave]

Hi OE, thanks for the warning. just brainstorming, and I suppose the windshield people have thought of this, but perhaps a laminate windshield with air or some other fluid between the sheets. The concept is to spread the kinetic energy out over a greater area of the shield. Or Perhaps a first perforated sheet designed to brreak off in chunks, the chunk/chainpiece combo would approach subsequent layers with a greater frontal area.

If sensors can react to breakage fast enough, a shotshell type charge of sand or salt might be discharged at the chainhead to either destroy the chainbit or re-vector it into the ground or towards a more solid containment device.

Perhaps the best thing might be to sue a solid shield, and install a small camera and cctv monitior so operator can see the cutterbar?


Freaking just another way to buy the farm in the woods!!

 

[Oregon Engineer]

techdave,
Do a web search under bullet proof glass and you will see the laminated windshield concept in a marketed form. Several of the sites have quite a lot of information on the construction of the windshields which is a good education.