tension and band life

[arky217]

I have been reading old FF posts about setting the proper tension on a band using calipers. What I have read is to open the calipers to ~6", clamp to the loose band, zero calipers and tension band until the calipers read 0.005". I clamped the calipers on my band and tensioned to where I normally do and found I was way under-tensioned - I read 0.002". I am running WoodMaxx 1.25" , 0.045" thick bands on 19" wheels.

I don't get the band life I would like (maybe 2 to 3 sharpenings) and have always assumed it was because a 16HP engine saws slowly and so spends a lot of time in the log wearing the band out by getting cracks in the gullet. If I increase tension what can I expect? Advice needed.

Very interested in this method of setting tension.

I also read that at 5" every 0.001" on the calipers represented 6000 psi tension.
Well, I have a Norwood LM29 mill, running blades of 1.25" x 0.042" thick on 18.5" wheels.

Norwood uses a compression spring and their recommended method of setting
tension is to turn the handle 4 to 5 turns more after it contacts the spring.
I've always split the difference and turned the handle 4.5 turns past contacting the spring.
But I've always wondered if I was setting the tension too loose.

Well, I tried the caliper method and with the calipers open to 5", turning the handle
4.5 turns causes the caliper to read approximately ~ 0.0028".
If I turn the handle 5 turns, the caliper reads ~ 0.0032".
But that's all I got because at 5.25 turns, the spring bottoms out.

I don't suppose it would be good  to bottom out the spring because the spring is also supposed to accommodate shocks to the blade if, for example, a piece of bark got between the blade and wheel.

But I think that I will start setting the handle at 5 turns because unless the blade is very
sharp, I start to cut waves.

If the above formula is correct, then 5 turns should be approximately 20,000 psi tension.

Is that still not enough tension on the blade ?

[farmfromkansas]

Arky, it would be tempting to get a new spring from Cooks to try on your mill.  My mill is 15 years old, wonder if I need to order a new spring? It sets with no tension on the spring unless I am sawing. Was planning to saw a bunch of logs this winter, but instead I have to get my hernia fixed.

[mike_belben]

Try some bellville washers maybe?  Basically a tune-able spring. 

[kelLOGg]

I found a post from 2004 by Gelman (reply #9) https://forestryforum.com/board/index.php?topic=7923.msg107754#msg107754  that directly answers my question in my OP about setting the band via caliper reading to give the manufacturer's recommended tension. All his terms were not defined which confused me so I will reproduce it here for those like me who have mills without pressure gauges to set tension.

Stress (or tension in lbs/sq in) = E (modulus of elasticity in lbs/sq in) x strain (dimensionless)
We need the strain so we can set it with calipers, so
Strain=stress/E

In Cooks video he says 17,000 lbs/sq in is needed for a 1.25" band, and
E=30,000,000 lbs/sq in according to references I used, so

Strain = 17,000/30,000,000 = 0.000567

If the caliper is set to 6" before tensioning the band then it should be tensioned until it reads 6 x 0.000567 more or 6.0034.

This is where I will tension my band and see how it does and go from there. Hope this helps me and others.

[Ben Cut-wright]

I found a post from 2004 by Gelman (reply #9) https://forestryforum.com/board/index.php?topic=7923.msg107754#msg107754  that directly answers my question in my OP about setting the band via caliper reading to give the manufacturer's recommended tension. All his terms were not defined which confused me so I will reproduce it here for those like me who have mills without pressure gauges to set tension.

Stress (or tension in lbs/sq in) = E (modulus of elasticity in lbs/sq in) x strain (dimensionless)
We need the strain so we can set it with calipers, so
Strain=stress/E

In Cooks video he says 17,000 lbs/sq in is needed for a 1.25" band, and
E=30,000,000 lbs/sq in according to references I used, so

Strain = 17,000/30,000,000 = 0.000567

If the caliper is set to 6" before tensioning the band then it should be tensioned until it reads 6 x 0.000567 more or 6.0034.

This is where I will tension my band and see how it does and go from there. Hope this helps me and others.

Area of the band is calculated using the distance from the back of the band to the gullet of the tooth. Thickness and width are then calculated to determine area.  Different widths and thicknesses will have differing tension/per sq.in. specs.  Cooks most likely used proper calculations to determine their recommendations. This does not mean all bands should be tensioned to the same spec even if they have the same cross section.

A cheap weight scale attached between both band wheels can measure the force/stress being applied to the band by the wheels. Force can then be used in the calculation of strain on the band. Caliper methods are quite accurate, though, and directly correlate to strain.

You should see improvement from getting the band off the guide roller flanges, and the strain closer to specs. 

[mike_belben]

So 3 and a half thou stretch.  

[kelLOGg]

That's what i'm going to start with even though I'm running Kasco, not Cook bands. Kasco (Cutting  Edge) said to tension the same as I have been. Maybe some Kasco users will chime in with the psi they use. I can re-calculate if I have to.