Thread: Bandsaw tension

Originally Posted by Chrism3

Plus it will put a flat spot on the tyre.

Has anyone here ever either done or seen this first-hand?

The bandsaws at work were permanently under tension and would sometimes sit for weeks without being used and there was zero evidence of the wheel/tyre deforming in any way.

Honestly, a flat spot seems quite unlikely as the blade covers half the wheel so the force will be distributed over a significant area.

Not trying to rubbish anyone's ideas, I just want to see something more concrete than "in theory this happens"

I was just quoting Louis Iturra, reckoned to be one of the bandsaw gurus. However I suspect he is more interested in protecting the tension spring than the tyre.

I agree that for normal use the tyre flat spot theory approaches that of the flat earth theory.

To some extent it probably depends on the quality of the tyres.

The only time I have seen something like this was on a small cheap (12"?) BS from a deceased estate BS where the BS was left under (probably overly high) tension for many (at least 6) years after the owner had passed away. It was less of a spot than the band contact half of each tyre which seemed to have compressed and hardened under the band. As the wheels rotated the band then went from an under and over tensioned state as the high a low halves of each tyre presented themselves under the band. it was so bad we ended up replacing the tyres.

As far as tension on-off goes, for regular users, it depends on your environment. For example I don't recommend unloading the tension on BS at a mens shed because some ninny will forget to re-tension it and start the BS and ruin a band. In my shed the tension stays on all the time. If you are going away for a couple of years probably take the tension off.

It's very unlikely you will take the spring out of the spring even under maximum compression. To take the elasticity out of a spring it has to be deformed beyond it's elastic limit and this simply doesn't happen under compression on most springs.

BTW the relevant physics formula is Hooke's Law ,

a law stating that the strain in a solid is proportional to the applied stress within the elastic limit of that solid.

Most springs are designed to operate indefinitely under compression within their elastic limit.

Bob,
A quote from Iturra's catalog(ue):
"Primarily during maximum compression (no space between all coils) the elastic properties are exceeded and the spring no longer is capable of returning to its original condition upon release of the load. In engineering terms, the spring is said to have taken on a permanent set. This happens to all springs in varying degrees under maximum compression. Even our new yellow high tension spring decreases in length to some degree following maximum load. .........If you have ever compressed your JET spring to the maximum for even a short time, it probably is no longer giving you adequate tension pressures. During our testing, the spring is under load only long enough to record the blade tension pressure. One or two minutes at most,for each reading on the tension scale."

So it would seem to depend upon how much you normally have the spring compressed as to how likely it is to be damaged if left on for long periods.

Sorry to hijack my own thread!

I guess it all depends on the quality of the bandsaw.
I have never released the tension on a Bandsaw in over 55 years of using them.
I have never owned a Chinese Bandsaw.

Just recently I have retensioned the detensioned Bandsaw for the duty supervisor at the local mens shed when he couldn’t figure out why it was cutting funny.

I personally don’t have clearance to use this or other gear until they get their new members vetting organised.
H.

There was a method of calculating the correct bandsaw blade tension based on the amount of stretch in the blade. This stretch was measured with a "jig" that was clamped to the blade and the stretch between 2 points 100mm apart was measured with a precision dial indicator. Blade tension/stretch differs with bandsaw blade dimensions. A 6mm wide blade will require less tension than a 19mm wide blade. A 100mm blade seems to need the likes of a hydraulic jack to get it tight enough! The most common failures of over-tightened bandsaws are collapsed roller bearings and bent/cracked bandwheels. Sometimes the tension mechanism breaks.

Originally Posted by Old Hilly

There was a method of calculating the correct bandsaw blade tension based on the amount of stretch in the blade. This stretch was measured with a "jig" that was clamped to the blade and the stretch between 2 points 100mm apart was measured with a precision dial indicator. Blade tension/stretch differs with bandsaw blade dimensions. A 6mm wide blade will require less tension than a 19mm wide blade. A 100mm blade seems to need the likes of a hydraulic jack to get it tight enough! The most common failures of over-tightened bandsaws are collapsed roller bearings and bent/cracked bandwheels. Sometimes the tension mechanism breaks.

You can measure the stretch with a simple digital vernier, clamp the body and another clamp on the protruding bar to the blade and start winding.

Originally Posted by Chris Parks

You can measure the stretch with a simple digital vernier, clamp the body and another clamp on the protruding bar to the blade and start winding.

But you have to know the correct tension specification.

That value can only be provided by the band manufacturer.

Starrett values can be in excess of 20,000 psi. A number of years ago John White had an article in Fine Wood Working promoting 15,000 psi as a standard. He also demonstrated a jig that could help measure that value.
There are a number of bands that are marketed as low tension Swedish silicone steel. They typically use the flutter test.
How many psi is 0 flutter?
The values stamped or even cast on the saw are simply guides. Michael Fortune suggests starting at one mark lower than the blade width. And adjust from there. If you have a 1/2 inch start at 3/8 inch.

Originally Posted by Chrism3

Bob,
A quote from Iturra's catalog(ue):
"Primarily during maximum compression (no space between all coils) the elastic properties are exceeded and the spring no longer is capable of returning to its original condition upon release of the load. In engineering terms, the spring is said to have taken on a permanent set. This happens to all springs in varying degrees under maximum compression. Even our new yellow high tension spring decreases in length to some degree following maximum load. .........If you have ever compressed your JET spring to the maximum for even a short time, it probably is no longer giving you adequate tension pressures. During our testing, the spring is under load only long enough to record the blade tension pressure. One or two minutes at most, for each reading on the tension scale."

Thanks for that info even though I find it somewhat disturbing.

If the elastic properties of spring material are exceeded at max compression, especially for a a few minutes, then I call that poor engineering. If the design was cocked up and they had already heaps of machines in a warehouse, then adding a simple "stop" would prevent maximum compression from being reached.

Compression springs can change their springiness over time for a variety of reasons the most likely one being repeated compression cycling. I'm not talking about the couple of times a week that might happened on a BS but something like a car suspension or engine valve so repeated cycles per minute or even second.

It also depends on what is meant by "maximum compression". Are we talking about compression to the point where the spring is just fully closed or slightly over loaded, or exceeding the load by multiple factors. In the later case there is another material property called creep. If the top and bottom of something are squished hard enough it will ooze out the sides. This is more likely at high temperatures and over long periods, unlikely for a half decent spring in a woodies environment.

But all the tension in the world will not fix the problems caused by a blade that is dull, has teeth that are set wrong or uneven, has a buildup of pitch or where the guides are set wrong. If the saw is cutting "wrong" there are plenty of other things to look at as well as blade tension.

Originally Posted by Don Burch

But you have to know the correct tension specification.

That value can only be provided by the band manufacturer.

Starrett values can be in excess of 20,000 psi. A number of years ago John White had an article in Fine Wood Working promoting 15,000 psi as a standard. He also demonstrated a jig that could help measure that value.
There are a number of bands that are marketed as low tension Swedish silicone steel. They typically use the flutter test.
How many psi is 0 flutter?
The values stamped or even cast on the saw are simply guides. Michael Fortune suggests starting at one mark lower than the blade width. And adjust from there. If you have a 1/2 inch start at 3/8 inch.

Of course, I was presuming that information was available, this may help Bandsaw Blade Tension

Personally I have never worried too much, do it up until it is tight enough has always worked for me and my resaws are pretty good, FenceFurniture will atest to that.

The saw could be tensioned without a spring but what the spring does is allow the required tension to be finely tuned for different blades. Beyond that there is little need for a spring.