Guide Bars (Part2)...

In the last post we started looking at how to maintain the guide bar; in this post we'll take a closer look at some of the problems that you may come across with a guide bar. More after the jump...

The guide bar is subjected to a lot of stresses, both physical and heat, and a poorly maintained chain will only exacerbate the problems and speed up wear.

Stresses

The importance of ensuring the bar is properly lubricated cannot be over-emphasised - the chain itself could be travelling around the bar at speeds in the region of 24m/s (but could be up to 28m/s).

In old money, that's speeds in the order of 55mph. Metal against metal. You can probably begin to see why that chain oil is so important.

The heat that is generated from any friction of the chain running against the guide bar, can cause the bar to overheat and turn 'blue' at the edges (called "blueing" for fairly obvious reasons). The overheating changes the properties of the guide bar and can cause it to become brittle, resulting in signs of damage on the bar rails.

It's clear then that the oil must be allowed to lubricate the bar and chain, and for this to happen it's important to keep the small oil hole clear and the bar groove free of debris.

Naturally, the chain tension will have a bearing on the amount of friction generated, and a chain that is set too tight can also lead to overheating.

Wear

A chain that is set to tight will increase friction and also wear (particularly on the bearings for the sprockets). A chain that is too loose will also hasten the distinctive wear on the bar under the nose sprocket (as well as where the chain enters the guide bar by the drive sprocket); you can detect this wear by looking just behind the nose sprocket - if the bar appears to be 'waisted' then it's probably on it's way out. As this only occurs on the underside of the bar, you can even out the wear pattern by turning the bar over when you carry out your maintenance.

A blunt chain will also damage the guide bar, but this time the wear is caused by operator usage; as the chain becomes blunt, the operator tends to push down on the saw in order to force the saw through the wood. Instead of the saw feeding itself in to the wood, the action of pushing down on the saw forces the chain to sit at an angle and this in turn will wear away the inside of the bar rails (on the underside of the bar).

There's a quick way of checking for this type of wear, where the inside of the bar grooves have become worn to such an extent that the bar should be changed. Looking at the photo (upper right) you can see that placing a straight edge touching the bottom of the guide bar and the cutter, should leave a definite gap between the upper edge of the guide bar and the straight edge. This is how it should be. Note, you have got to make sure that the edge is pushing up against the side plate of a cutter for this to work.

In this photo to the left, you can hopefully see that that gap is not there when checking out the wear. Because the bar and chain are worn, the chain moves over when placing the ruler up against the cutter and that gap we had, just disappears. This is a sure sign that the bar needs to be changed.

But that's not all - the raised rails on which the chain runs can be burred over (looks like a very sharp wire edge) and these must be removed with a flat file - ideally putting a slight champfer on the outer edge to slow down the burrs from reforming. Do not run your finger down the edges - those burrs are sharp!

The nose sprocket also suffers from a stressful life and you should check the following:

• Ensure the nose sprocket turns!


• If your saw is a Husqvarna or Partner you may want to check the end of the guide bar and see if it needs greasing. Other makers may also put grease points for the nose sprocket so check yours - you'll see the rivets that hold the sprocket in-situ and off to one side will be a small hole. The Stihl bars tend to be sealed for life and therefore do not need to be greased.


• The teeth also wear, and instead of becoming dull and they wear down, they actually become very pointed. The teeth should have a slightly rounded point on them - if it's a sharp, triangular point then those teeth are worn too far.

The image at right shows how the teeth have worn down to a sharp point on this guide bar.

That's about it for this post, although there are a couple more things about the guide bar that I might just throw in to another post soon. Keep checking back as we venture through maintaining our chainsaws.