Do you reduce tension on BS blade when not in use?

[RonT]

I'm not going to directly disagree with Ron, or anyone else, on the need to detension; I honestly don't know. What I do know is that this is not just an advisory that's handed down from one woodworker to another, but rather something the manufacturers of the tools include in their manuals.

My Grizzly G0555 manual says:
"NOTICE
"All bandsaw blades will stretch. To reduce this stretching, remove the tension from the blade when not in use."

The Delta 14" bandsaw manual says:
"IMPORTANT: OVER-STRAINING IS A COMMON CAUSE OF BLADE BREAKAGE AND OTHER UNSATISFACTORY BLADE PERFORMANCE. RELEASE THE TENSION WHEN THE TOOL IS NOT IN USE."

Those are the only two manuals I've checked, but they essentially say the same thing. And I want to say that both Duginske and Bird advise that the blade be detensioned when the saw isn't being used, and I know for certain Scott Phillips once said to do it on an episode of "The American Woodshop." All this, and the few seconds it takes to do it, is reason enough for me to always detension my saw's blade. YMMV, etc.

Let’s analyze; OEM’s recommend detensioning of BS blades when not in use.

Let’s consider the difference between proper blade tension vs. over tension. Say you operate the saw all day long at the proper tension; is the blade damaged? No, so why would it be affected sitting overnight. Now consider over tensioning; taking the blade beyond its elastic limit, the point beyond which the metal cannot return to its original shape, yes it will fatigue and break in short order. Compare it to a rubber band; it can be stretched, held there and returned to rest with no problem. Now stretch it beyond the recommended point and it will break.

As for bearing fatigue, bearings are rated in millions of hours of cycle life. They are designed to carry a load and if properly selected will carry the pressure imposed by the blade tension all day and all night long, running or at rest.

As for taking the pressure off the tensioning spring; do you put your car up on blocks at night so your springs won’t weaken? Why do it to the saw!

The effect on tire deformation is the only reason to detension a blade. In time the tire will deform and cause the blade to run off hampering the performance of the saw. This is the core of OEM' recommendation to detension, it's a CYA. Change to a tire polymer with no memory (probably not available) and detensioning is a thing of the past.

PS: BSs in heavy duty machine shops applications run flawlessly for decades and no one is detensioning after use. I believe they have crowned metal wheels.

[jnesmith]

Let’s consider the difference between proper blade tension vs. over tension. Say you operate the saw all day long at the proper tension; is the blade damaged? No, so why would it be affected sitting overnight.

My understanding of the difference is simple. When the blade sits tensioned for long periods of time, it can develop some sort of "memory" that can lead to vibration. When it's running, the blade is constantly moving, so there is no "memory" effect. I think the arguement is strongest in situations where the saw sits idle for long periods of time. I may go weeks without using mine, for example.

[bigsteel15]

Can you believe that one of the latest WW mags actually paid someone to publish a tip saying he puts a spring clamp on his blade to remind him to tension the blade?

All you have to do is come here and get all the free ideas you ever wanted.

[RonT]

My understanding of the difference is simple. When the blade sits tensioned for long periods of time, it can develop some sort of "memory" that can lead to vibration. When it's running, the blade is constantly moving, so there is no "memory" effect. I think the arguement is strongest in situations where the saw sits idle for long periods of time. I may go weeks without using mine, for example.

Some of you probably wish I'd go away; I've beat this subject to death, this will be my last comment.

When the blade sits tensioned for long periods of time, it can develop some sort of "memory" that can lead to vibration. When it's running, the blade is constantly moving, so there is no "memory" effect.

For any given and constant force, in this case tension, the stretch is the same be it running or at rest, in use or overnight. True, overstress will lead to failure in which case the saw is improperly adjusted.

The OEM wants to make sure tires are not flattened which will lead to poor saw performance and reflect on their reputation.

[25]

Wouldn't there be more stress on the blade while it's running than while it's resting? I definitely can see that the tire flattening being the big reason to detension. I don't have one but will be detensioning when I get one .

[jnesmith]

Wouldn't there be more stress on the blade while it's running than while it's resting? I definitely can see that the tire flattening being the big reason to detension. I don't have one but will be detensioning when I get one .

I think while running, the blade heats up and stretches a bit. Then it cools and shrinks a bit while idle.

Having said all that, I don't detension.

[RickT]

RonT, We don't wish you'd go away. I think you've brought a very good, insightful, educated opinion to the discussion. I admit that I've had my Jet 14" bandsaw for about 2 years, and I have not detensioned the blade until I read this thread. I don't have a QR lever, so it's a little more work to detension and retension.
Thanks,
Rick

[LarryG]

Some of you probably wish I'd go away

A big +1 on what RickT wrote. One of the best things about this great forum is the the members know how to carry on an intelligent discussion, involving differing opinions and points of view, without the thread degenerating into a flame-fest. Which means that since people don't spend all their time digging in their heels and arching their backs, there's a good chance we might all actually learn something.

[Brian G]

I detension as soon as the wheels stop spinning after a cut. I figured it's easier to get in the habit to tension, use the saw, detension than it is to try and remember to detension at the end of the shop time. I also probably wouldn't be as diligent if I didn't have a QR lever.

Terry, I'm terribly disappointed.

Given your statements regarding the fondness you have for your bandsaw, I would expect more. I'd figure something like giving the blade a long soak in a warm oil bath, followed by a gentle rubdown with a chamois cloth, then a relaxing repose on a velvet lined peg. All, of course, beneath the warm glow of incandescent bulb while soft music plays in the background.



I release the tension in mine after I'm done with the cutting session.

[25]

I think while running, the blade heats up and stretches a bit. Then it cools and shrinks a bit while idle.

Having said all that, I don't detension.

Good point. Forgot about that. Still the blade would be stressed more from the cutting than would be relieved from heating up.

[RonT]

This might satisfy some curiousity around blade stretch, I got this off the net.

"When a metal is heated, its linear dimensions increase approximately in proportion to the temperature. Over moderate changes in temperature, the length of a metal sample changes by an amount

equation DL = Lo a (DT)

where the D (delta) indicates "change in." T is temperature, and L is length. The constant "a" is called the coefficient of thermal expansion. Lo is the initial length, before expansion."

The constant "a" value in this case is .0000078; if the blade goes from 70 Deg ambient to 185 Deg, which the temperature of a good size motor when it's been running steady, and I assume the blade is 144 inches long your lenght will change by 0.129 inches or a smidge over 1/8".

Plug in your own values, use the constant and see what happens.

[JimZ]

Yes -- I'm superstitious

I've read a half-dozen BS manuals that recommended it, even if mine doesn't.

Is it necessary ? Not sure, I figured it really can't hurt.

My safety mechanism is more paranoid. I have a hunting license pouch complete with big safety-pin.

I printed up a

DANGER
TENSION OFF

in red on my printer, and shoved it in the pouch.

My power switch came with two holes in it, that the license pouch clip fits neatly thru. So the reminder literally hangs over the start switch.

Even on my most addled moments, it's hard to miss that.

YMMV