Bandmill Blade Tension and Blade Temperature

      Bandsaw mil blades expand and contract with heating and cooling, which affects blade tension. Here is a discussion of how equipment should adjust to compensate. August 18, 2009

Question
On a band sawmill equipped with a hydraulic blade tensioner, will the blade heat up and expand during the cut and thereby lower the tension from beginning to end? I can see how a spring tensioning system would automatically adjust for the blade expansion, but a hydraulic system would just set the wheels at one constant distance from each other no? Or is the blade’s heat expansion during the cut insignificant and I am just over thinking the issue?

Forum Responses
(Sawing and Drying Forum)
From contributor J:
You are correct! A spring is a lot better than the hydraulic. I know of several of my saw mill buddies that have converted to the spring system for this very reason. There is one side note, the blade should not get too warm when cutting because if it does the blade is dull and should be changed. I like the spring system because if the pump or cylinder fails out if the field and you don't have all the parts to fix it, you’re in trouble. Simple is always better!



From Gene Wengert, forum technical advisor:
It is common to have a good supply of water (cold water) applied to a blade. This cools and also acts as a lubricant. If you do get heating, the heat would be more where the teeth are and this would cause that area to expand and then the blade would begin to wander in the cut.

Incidentally, a band saw is not flat but is curved (called tensioning). This tension will help the blade run straight when the centripetal forces develop as the band is spinning. Heat will unbalance the situation so that a band can fly off the wheels in the extreme cases.

I have seen dynamic hydraulic tensioning systems on most band mills (except for the very small ones), so if the blade were to change size, the hydraulics would automatically adjust the tension, just like a spring. Are you certain that the mill in question does not have a dynamic system? I do not know of anyone that has changed to spring tension if the hydraulic system was working properly.

In any case, if you do get heating, it is worth investigating why (dull blade, sawing too slowly, gullets not sharp, etc.); that is, heating should be prevented if at all possible.



From contributor A:
The new hydralic/airbag system on the WoodMizer LT70 is great. I installed a retro fit on the mill to the new style and love it. You get a good "shock" absorber from the air bag and if the sun shines on the unit the temp change does not increase the blade pressure. The blade always get warm in the cut and a small drop of pressure is not uncommon. Dull blades do heat up more and cutting dry wood will really heat it up. With a hydraulic system you can be at pressure and go to lunch and by the time you get back the blade has busted. So it is a good thing if you are not with it to back off the pressure. I have thought of hooking up an engine kill to a pressure sensor so when the blade broke or you forgot to pump up the thing the engine will not run. Need to get back on that.


From contributor G:
There should be no loss in tension in tension system that has the tension force applied at all times. The tension force could be delivered by a spring, air, liquid, or even weights. Any of these should be insensitive to environmental changes. There would be a problem if your mill manufacture uses a tension system that the energy to produce the force is switched on for only part of the time the tension is needed. Shame on them.


From the original questioner:
Thanks so much for all your input. I am building a band sawmill and am researching the best method for tensioning the blade. I do not have the resources to procure a specialized hydraulic tensioning system, and was thinking a simple hydraulic pump/piston would not be suitable alone. It sounds like the easiest and cheapest way to go is just a spring/screw setup. When using a spring for tension is there a need for damping vibrations or oscillations, or will simply compressing the spring to create tension suffice?


From contributor Y:
If you want to go the cheapest and best way just use trailer rim and tires for band wheels which will eliminate the problems of tensioning and having to put water on the blade.


From contributor U:
I built my own sawmill and devised what seems to be an effective blade tension system. Though I know the spring is commonly used, I had trouble conceiving how a spring system would work (finding the right spring to provide just the right tension all the time, every time etc.). I would call my system a simplified hydraulic pull. I used a small hydraulic cylinder - dual port, 9" retracted length, 1" dia. ram. I think longer cylinders could be made to work as well.

I elected to pull tension on my blade from the end as opposed to pushing tension from the center as most other hydraulic systems do. I left the cylinder fully retracted and filled it with oil through the front port. I then threaded a 5,000 psi gauge into that port. One end of the cylinder is attached to the sliding/tension wheel assembly. The other end has a 3/4" dia. threaded rod which passes through a fixed anchor point.

Essentially, by tightening a nut on the threaded rod, I pull the blade into tension and the cylinder simply acts like a link in a chain. The oil inside the cylinder is compressed and the amount of compression registers flawlessly as psi on the gauge. It's easy to adjust to the proper tension every time. The tension sweet spot for my mill is about 2200-2300 psi. It is easy to watch during use and if it changes slightly you just give the nut a slight turn to adjust.



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