loosen chain

Thermal expansion of the bar

I don't know what the exact number for steel would be but I expect the thermal expansion of the bar will be something like 0.0006" per foot of bar per 10°F.

A 36" bar would "grow" 0.018" or so with a 100°F temperature increase.

heimannm said:

I don't know what the exact number for steel would be but I expect the thermal expansion of the bar will be something like 0.0006" per foot of bar per 10°F.

A 36" bar would "grow" 0.018" or so with a 100°F temperature increase.

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The bar is not the culprit when a chain gets loose.

Most typically, when chain tension changes dramatically lubrication is the issue. (Except with long bars and a brand new un pre streched chain.)

romeo said:

On the bike saws I will set the chain on so tight that it will barely spin. Run it for 10 or 20 seconds and it will be hanging down 1/2" in the center of the bar. Then tighten it up again while its hot. When I am done with the run I pull the chain so I don't bend the crankshaft when it shrinks. This is very extreme for saw chain, most folks wont notice that much stretch.

I have seen saws tighten up when they get hot, it has to be the bar generating heat. The only saws I have seen do this were brand new BTW.

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Hey Romeo,
Tell us about the infamous "Blue Chain" you have.:hmm3grin2orange:

Andy

redprospector said:

Hey Romeo,
Tell us about the infamous "Blue Chain" you have.:hmm3grin2orange:

Andy

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Well it IS a very pretty shade of blue, the color that gunsmiths strive for. I will trade you for your rototiller chain

It didnt just turn the chain blue, it cooked the paint on the bar. That bar still has a nice smoky fade to it.

There was almost 12 seconds of cut time on that chain, had to be a record for chain trashing. The next bike saw will have NOOOOOOOOOOOOOO off switch for the oiler.

mine are usually loose at the end of the day...

Lakeside53 said:

mine are usually loose at the end of the day...

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Your what?

I mostly clean the saws, and have the bar etc off the saw, and clean it, after each use, so it happens automatically - after reinstalling, I leave it a bit slack until the next use is coming up.

Lakeside53 said:

mine are usually loose at the end of the day...

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That too!

Old thread but I was just reading this in the manual also so did a search. Then I reread the manual after reading this post and I now understand what it is tryin to say I think.
The manual says the chain at operating temp stretches and begins to sag so it then says to tighten the chain. After that it always says to slacken it back off after finishing work to keep from damaging the crank and bearngs.
Sooo im guessing if you dont have to tighten it after it warming up then you shouldnt have to worry about slacking it back off after.

Then on the next page it states After finishing work.... Slacken off the chain if you have retensioned it at operating temp.

Ok I feel better about that now lol.

i have been sawing for 20 plus years and have never slacked down a chain at the end of the day. seems that after cooling down they still roll freely and after warming back up the next morning they sit ready to cut with the proper tension. i have not ruined a crank from not slacking down. but it is feesable if you really got the chain hot and then tightened it that it would cause problems.

I've always slackened the chain before putting the saw up.

spacemule said:

I've always slackened the chain before putting the saw up.

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But you like to pull everyone elses's chain instead..........:biggrinbounce2:

heimannm said:

I don't know what the exact number for steel would be but I expect the thermal expansion of the bar will be something like 0.0006" per foot of bar per 10°F. A 36" bar would "grow" 0.018" or so with a 100°F temperature increase.

Click to expand...

Depending on which book you look at it can be anywhere from 0.0005 to closer to 0.0012" per foot of bar per 10F - lets just stick with your value of 0.0006 /ft/10F for now.

I have done some temp measurements using an infrared detector and here is what I found.

On a 65F day a well lubed freshly sharpened chain chain reached ~120F in softwood and 175 F in hardwood. If the chain is blunt, lube is poor/limited or a zillion other things the temps will be higher. So your 100F estimate is pretty good.

I have measured the chain reaching a steady temp under steady cutting in about one minute while the bar takes about 10 minutes to reach a steady temp.
This explains why the chain can appear to be very loose very quickly and why it should not be hard tightened (unless required), after just one minute of running as the still thermally expanding bar acting on an already thermally expanded chain will just stretch it even further.

After about a minute of running the bar will still be cool and not have expanded much but the chain will have expanded to
2 (chain is twice as long as bar) x 0.018" = 0.036" for a 3 ft bar and 0.06" for a 5 ft bar

This does not sound like much but if you use a simple pythagorean model to calculate chain droop (see "x" in the diagram= SQRT(30.06^2-30^3) for a 5ft bar) from the bar the droop is around 0.6" for a 18" bar, 1.1" for a 3 ft bar, and 1.9" for a 5" bar!


Doubling the temp difference will not double the droop but increase it to 2.7" for a 5 ft bar but it's still easy to see why a blunt chain contributes to why chains come off especially in that first few minutes after it goes blunt from hitting something etc.

Now something else to watch out for when adjusting.
The bar is typically 10-15 F cooler than the chain but the bar takes 10 minutes to cool down whereas the chain cools down in about a minute. This means if you stop the saw to adjust the tension you should do it asap or else the chain will shrink and you won't get the tension right.

It doesn't always happen but it sure is nice to see theory and practice line up reasonably well

i will usually have my chain to where i could pull up on it in the middle of the bar and have the rivets just barely come up out of the bar (the bottom of the rivet barely above the top of the bar) and i generally dont have to tighten it when it gets hot but if i ever do ill loosen it before i let it sit. in my experience if you run the saw with the chain overly tensioned it will start to "peel" the tip of the bar and if you dont notice it quick and do a bit of light filing youjr bar will be shot shortly after.