Teaching Tension & Compression

I recently had a student that found it difficult to fully grasp the concept of tension and compression wood. This student understood that if a piece of timber was only held at one end, then the compression wood was underneath, and that's where the first cut would be made; but when it was balanced at either end, they just couldn't see it. What I needed was a way to help this student understand where the cut should be made - this gained particular significance as I'd been told that they had failed a previous attempt at CS30 on demonstrating knowledge of tension and compression! Find out my solution after the jump...

After we had been down the woodyard, practicing the crosscutting, I have to admit that I thought we'd got it sussed. But we hadn't - my fault really and now I needed to come up with something that would help this student. Walking back, I was running various ideas through my head when I had a moment of inspiration.

I needed something quick, simple and easy to understand. Perhaps something this student could take away with them to practice with.

The result can be seen in the picture at the top of this post - click on it to get it full size (A4). Here's how it works...

• Cut out around the edge of the rectangles, and then fold in half as it shows.


• After folding it, glue it together just to hold the halves in place.


• The elliptical pads represent where the wood is held up - so if you place the single pad touching on a table, or whatever, with the other end free, you'll see that the free end sags down (because there's nothing holding it up). Holding it this way, you can also see that the letter 'T' (tension) is uppermost and the number '2' (second cut) is showing. This tells you that the compression wood, which you would cut into first, is on the underside of the timber.


• Now turn the paper over and balance the two pads at either end across your hands. You can now see that the letter 'C' (compression) is on top, and the number '1' (first cut) is showing. This tells you that the first cut to make is the compression wood on top.

It's easier to do, than to explain ;-)

Anyway, if it's of use to anyone, then please feel free to download it and give it a go. If you're a trainer, then feel free to use it. As for my student? They passed with flying colours.

Stihl Quick Tension System...

Maintaining the correct chain tension is rather important - too loose and the chain may derail, too tight and the saw will effectively lose power. Many saws require the operator to carry around a combi-spanner to adjust the tension, but a number of smaller saws are available fitted with a quick tension system. In this article, we'll have a look at them and see how they compare after the jump...

I mentioned above a couple of issues regarding chain tension, but there's more; a chain that's too loose will also not cut as effectively as a correctly tensioned chain. Why? Because as the cutters come in to contact with the wood they sit back slightly, and that results in the cutting angles changing. A loose chain will also wear quicker, as well as increasing the wear on the guide bar. Fortunately you can easily recognise a loose chain - either because it's hanging down from the underside of the bar, or because it revolves around the bar when the engine is at tickover (with idle speed adjusted correctly).

A chain that's too tight will result in the chainsaw using up some of it's power to overcome the friction and that's what results in the lack of apparent power and unwillingness to rev freely. That increase in friction can lead to overheating too. It also puts the bearings of the nose sprocket and drive sprocket under undue stress.

So, convinced of the need to get the correct tension, the easiest and fastest method would seem appropriate; but different saws use different systems. The standard method, shown on the right, uses an adjuster screw that can be accessed through the side panel (although some saws have the adjuster screw at the front, facing forward, next to the guide bar - these are really awkward to use, especially with a combi-spanner). Before we can set the tension, those two nuts holding the panel in place must be loosened first as they're also clamping the bar in position.

Maintaining the tension in this manner requires the user to carry around a combi-spanner (with a flat end, not a Torx-head end). The operator first loosens off the two nuts, sets the tension using the flat screwdriver, and then tightens up the two nuts. Job done.

Nowadays, many users (particularly home-users) don't want the aggravation of having to have the combi-spanner to hand in order to adjust the chain, and manufacturers set about finding an even quicker way of correcting the chain tension with the least amount of fiddling about.

Enter stage left... the quick tension system. The example in the photo here is off of a Stihl MS250C, but it's similar to many other fast tensioners. The photo at the top of the post shows it with the side panel still fitted. You can see on that photo the large 'dial' on the side - this is not the tensioner, but it's a quick release for the side panel. The actual bit that the operator uses is on the top of the side panel - a small black wheel that links in to the cog fitted to the guide bar (that can be seen on the photo to the left).

To adjust this system, all you need do is flick the handle out of the quick release dial, back it off a fraction, then use the adjustment wheel to set the tension and tighten up on the side panel. It is quick and it doesn't require any tools. But is it any good?

Personally, I found it a bit awkward to get the right tension and it's not as finely adjustable as the good old screwdriver adjuster. Having said that, it does work, but I'm not sure I'd want one on a professional saw; for the home- / occasional- user however it's probably ideal. It also means that changing the guide bar is not quite as quick either, as that big cog is bolted to it and should be removed before swapping bars - the other thing that it means is that when it comes to your regular servicing and you turn the bar over to even out wear... you need to take the cog off and swap it over too.

So, for now at least, it's the old way of doing things that wins it for me; not just for setting the tension but also for ease of maintenance.

Creating Tension In A Video...

As promised in an earlier post, I thought I would try and create a short video about tensioning the chain, and here it is... after the jump...

Hopefully the following video will put across in a few short minutes what I was trying to get across on my earlier post.

Please, remember to wear gloves when you're handling the chain; lift the nose of the bar up when you adjust the tension; and tighten up those side plate nuts before you run the saw up!

Too Hot To Handle...

Just to finish off a couple of things about chain tensioning - things which many people forget to mention, but that have a huge impact on how loose, or tight, your chain is. Find out more after the jump...

If you have just adjusted your chain after carrying out some maintenance then your chain should be set correctly based on the information in the last post "Chain Tensioning...".

But what if you've just been cutting some timber and realised that the chain is a little loose? If you're not careful, this is where some of the problems can creep in.

Obviously, as you've been cutting a certain amount of heat is generated (that's one of those understatements - a lot of heat is generated!). As I'm sure you are aware, when something heats up it expands, then when it cools it contracts.

Now I'm pretty sure that you are ahead of me here - if you've been merrily cutting wood for the last ten minutes, that bar and chain will have heated up nicely and the chain will have expanded. That expansion makes the chain appear looser than it was when the saw was cold. And here is where the problems can occur.

If you were to adjust the chain tension whilst it was hot (quite apart from burning your hands whilst you held the bar nose up), you'd find that as it cooled down the chain would shrink and tighten up around the bar. The chain tension would now be way too tight.

To solve this problem, always let the chain and bar cool down first, before you adjust the tension - it's a good excuse to grab a cuppa while you wait.

Chain Tension...

If ever there was a subject about which produced lots of differing views, this has to be it. I've been told so many ways of how I should set the tension of a saw chain that I could write a book blog article about it.

Most of these ways are street-myth, complete rubbish or somewhere in between. Some have a certain amount of truth in them, whilst others belong in a Hans Christian Anderson tale.

So, for this post, we'll take a look at some of the ways of setting chain tension.

It should be easy really, just set the chain so that it's not too loose, and not too tight; somewhere between those two extremes is the correct tension. But where? Let's examine a few of the more popular methods of checking chain tension...

1. "You should be able to lift the top of the chain and see 1 / 2 / 3^* drive links." ^*Delete as necessary.

Nope. This just does not hold true - there are too many variables at play here to make this worth listening to. Your strength will have an obvious bearing on how much you can pull up on the chain; in fact, just how much force should you use when pulling up the chain? The length of the guide bar will have a huge bearing on how much you can pull up anyway.

2. "You should be able to fit a penny between the chain and bar."

No - complete rubbish! See above.

3. "You should be able to push the saw over the worktop and the chain should move round as the saw moves across the flat surface."

There's some truth in this one - but I have a major problem with it... how many nice flat surfaces are there out in the woods? However, this is one that I used to use to set the tension up in the workshop, and is similar to the next one...

4. "Grip the chain on the topside and underside of the bar; the chain should move round reasonably easily... now hold the chain on the topside of the bar and pull it towards you. The saw should move across the flat surface, but the chain should remain still."

This suffers from the same problem as the previous option - but for setting the tension in the workshop, it's pretty accurate. But no use in the field where flat, smooth surfaces are hard to come by.

So where does this leave us? Four different methods, two of which I would give some credence too (options 3 & 4), and two that don't deserve the time of day (options 1 & 2).

The simple answer is to set it in the workshop, get a feel for how tight it is and use that as your basis for future tensioning. It's a bit of a 'feel-thing' that you get with experience, but let's try to give you a better answer than "let your experience be your guide", as that's not particularly helpful if you are new to chainsaws!

First things first... take the chain brake off so the chain is free to move! You'd be amazed how many people forget that bit.

There are two very basic rules...

1. If you can see daylight between the underside of the guide bar and the chain, it's too loose.


2. If you can't move the chain around the bar, it's too tight.

Between these two extremes of tension is what we are after, so here's how to do it - in fact I might just make a video of this and post it up later...

1. Make sure the chain brake is off and that you are wearing suitable work gloves.


2. If necessary, slacken the nuts that are clamping the guide bar in place (or whatever system your saw uses to hold the side plate and bar on with). We don't need these really loose - just finger tight will do.


3. Slacken off the tension until the chain is obviously too loose.


4. Using one hand, hold the nose of the bar up.


5. Now, whilst the bar nose up, adjust the tension so that the chain just touches the underside of the bar. Adjust the tension just a little bit more, so that the chain fits snugly against the guide bar - no need to overdo it.


6. Whilst still holding the nose of the bar up, tighten up the side plate nuts / bolts / quick-release system.

That's it - you're done! There are a couple more points to note when tensioning your chain, but those will be mentioned in the next post.