Author Topic: Throwing Chains  (Read 229 times)

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Offline Cut4fun .

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Throwing Chains
« on: April 17, 2014, 11:45:10 am »
Was trying to explain this stuff to a friend yesterday.  Figured others may be wondering about why they keep throwing chains.

Fixing Chain Derailures  Understanding Why Chains Throw

Pics in this Madsen's link  http://www.madsens1.com/bnc_derailure.htm

How To Reduce Chain Throwing On A Pro Saw

All pro saw users throw the chain from their saw periodically. Most of the time, a pro user knows exactly what happened. He touched a limb on the back side of a cut or touched another unforseen object with the tip of his bar. In an instant, the chain flys off its track.While derailures are usually caused by operator errors, it is true that some saws are more prone to throwing chains than others. If you have struggled with such a saw, the following information will help you understand why a chain derails and what you can do to reduce how often it occurs.
Chain Tension

Low chain tension is the most obvious cause of chain derailures. It is easy to understand how it affects chain stability. On today's pro saws, chain speed is faster than ever. A chain may travel around a guide bar more than ten times in a single second. At this speed, it doesn't take much to derail a chain.

So how tight should a chain be? Since almost all pro saws have sprocket tipped bars, you can run the chain fairly tight. When cold, the chain should be run tight enough so the chain's chassis is in contact with the bottom bar rails. When the chain warms up, it will stretch a little, so the chain's chassis should just sag beneath the bar rails in the center.

Retensioning (or tightening) should always be done when the saw is cold. If tensioned when the chain is warm, the chain may become too tight when it is cool. This, too, has obvious consequences, including causing bar tips to fail.

Obvoiusly, chain tension is an important factor in chain stability, but there are some other less obvious things that influence it. These facors include the size of the drive sprocket, wear on a bar's heel, and even the length of the saw chain. These also play a part in the stability of the chain on a pro saw. pro users are not as aware of these because most are hidden under the saw's side cover where it is hard for a user to see.
Large Drive Sprockets

Many pro saw users know that derailing or "throwing" a chain occurs more often when running large diameter sprockets. Most attribute this to the higher chain speeds a large diameter sprocket produces. This is true, but is not the whole story. Large diameter sprockets also increase the distance a chain travels out of the bar on its heel. The greater this distance is, the more unstable a chain is.
Worn Guide Bar Tail

The top bar is brand new and the bottom bar has had a lot of use. While the bottom bar is still usable, the heel profile is much thinner.
Worn Bar Heel

A worn bar heel can also play a part in derailing problems. The reason is, as the bar wears, its heel becomes more narrow. The narrower the heel is, the longer the distance the chain travels out of the bar rails.This means running a bar with a worn heel may cause a chain to be less stable and make it more prone to derail.

This is especially true when a worn bar heed is combined with large diameter drive sprocket. A 3/8 X 7 tooth sprocket is smaller than a 3/8 X 8 tooth sprocket. So running a large sprocket in combnation with a worn bar heel will creates conditions where the saw chain is less stable.
Long Saw Chain

Almost every pro saw user waits to remove a link from a stretched chain until the adjuster on their saw's powerhead runs out of travel. This is a mistake. Even if a chain can be properly tensioned, running a chain that is too long contributes to instability.
Sprocket To Bar Heel Distance

As a chain stretches and is retensioned, the distance between the bar's heel and the sprocket increases. In the photo on the left, running a stretched chain increases the distance between the red lines.

It is best to remove a link from the saw chain just as soon as a shortened chain will fit on the saw. By running a chain as short as possible, the heel of the bar will always be as close to the sprocket as possible. This minimizes the distance the chain travels out of the bar, and enhances chain stability. If you are running a slightly worn bar and/or a large drive sprocket, this is even more important. In the photo example, always keep the distance between the red lines to a minimum.
Our Advice

If you are having trouble throwing chains, before you switch to a smaller sprocket, first check to see if the chain can be shortened.

The combination of a large diameter sprocket, a worn bar heel, and a stretched saw chain will create conditions where chain derailures occure more easily.

Always run the bar's heel as close to the sprocket as possible.
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Offline 660magnum

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Re: Throwing Chains
« Reply #1 on: April 17, 2014, 12:08:43 pm »
I've noticed on old bars where the chain lays back on the bar after leaving the drive sprocket there becomes a notch in the rail. The same notch is on the nose of the bar behind the nose sprocket.

I consider it a problem when the nose sprocket becomes worn to the extent that the chain rides on the rails of the nose rather than the teeth of the nose sprocket.

When you finish a cut and seeing the chain drooping underneath the bar . . . This loose chain was on top of the bar at the heel area when the saw was in the cut.
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