LumberJocks Woodworking Forum banner

Air-tensioned bandsaw?

2215 Views 17 Replies 11 Participants Last post by  crank49
Hi there,

I'm looking to get some feedback on bandsaws tensioned with an air cylinder and a regulator vs. the traditional spring and screw mechanism on 99% of the bandsaws out there. I'm in the construction phase of a shop-made 20" bandsaw project, inspired by Matthias Wandel but the design is entirely my own. When I got to the point where I needed to come up with a design for the tensioning mechanism, I initially assumed that a screw and spring was the best option, but then I got stuck because I would have to buy a really heavy duty spring that could survive a lot of tension on a 3/4" or 1" blade, but which would be too stiff if I was using a 1/4" or 3/8" blade.

I realized that an air cylinder could apply a LOT of force (a 3-1/2" bore cylinder @ 100psi is about 962lbs), but had the added bonus of providing a cushioning action at the same time to absorb vibration. This would make the mechanism simpler overall, and air cylinders are incredibly cheap on ebay. In fact, even including fittings, valves, the regulator, and other items, I'm fairly certain it would be a price-competitive alternative to the spring and screw tensioning design. I'm trying to stay on a pretty tight budget but at the same time I'd like to be creative and innovative if possible.

I also like the idea of being able to de-tension the blade with the press of a button or the flick of a switch.

If I end up going with an air cylinder, I will probably use a pressure-actuated switch as a safety interlock to prevent the motor from starting if the blade is not tensioned, as well as to shut the motor off if the air supply dips below a certain threshold. I'm using a Square D variable frequency driver to run a 1hp 3-phase motor, which I can DC-injection brake to stop all motion in a matter of seconds, in an emergency like an air line bursting.

However, I've read a few articles or forum threads on other sites where a bunch of users are poo-pooing the idea of using an air cylinder to apply tension, saying things like "its a solution looking for a problem", "whats wrong with just cranking a handle?", and "not a single person i know has ever de-tensioned their blade when done working". These kinds of attitudes are making me doubt my idea.
See less See more
1 - 18 of 18 Posts
Never know till you try it. If there are doubts, leave room to modify it with a spring and such. Never hurts to be inventive.
Ordered two cylinders just now, new old stock (cheap), 1-1/2" bore, 145 PSI max… $15/ea. Even if they don't work for this project they will be fun to play with, I am sure. At 100 PSI, each cylinder can apply 235lbs of force and at 145 PSI it goes up to 341lbs. With two of the mounted in tandem to the wheel mounting mechanism, this should easily be able to tension even a 1" blade although I imagine that my saw frame (which is made of wood) will not enjoy that kind of abuse.
Give it a shot.
The bandresaw mill my grandpa had used a hydraulic tensioner. It was basicly a bottle jack with a pressure gauge and we used that to keep tension on the blade (1.25" wide). It worked pretty well you might think about that as an option.
It all sounds perfectly do-able to me.
I have had belt grinders that were tensioned with air cylinders.
The best small regulators I ever found for these types of applications were made by Wabco (Westinghouse Air Brake Company). They make a little regulator with a 270 degree dial that goes from no pressure to full pressure in that 3/4 turn of the knob. You could make your own dial with set points for each size blade right on it.
What mramseISU said. Using compressed air I would think the "cushioning" you speak of would heat up the compressed air and cause an increase in pressure. I think you would be chasing your tail trying to regulate the pressure as it heated up and cooled down. Go hydraulic. If your concerned with "cushioning;" incorporate a compression spring.
I'd like to hear and see how it works with pneumatic or hydraulic tensioning.
I de-tension my blade after every use!
It does sound complicated, but with proper design should work without a problem, Tracking and tension are both controlled pneumatically on most wide belt sanders, the concept is already in place. Connected to an air supply the near zero temperature fluctuation you will see with a well designed system will be controlled with the regulator.
Thanks for all the feedback. I had thought about hydraulic but that would involve buying additional parts that would add cost, complexity, and maintenance. I already have an air compressor (who doesn't?), and air is a lot 'cleaner' than hydraulic fluid. I do agree that hydraulic would probably provide better performance and smoother adjustment though.

Here is what I ordered:
There was a good discussion on this subject on another WW site,most thought it was a good idea but the cost of building it with a pneumatic cylinder was /is going to be way up there but imho ,if used parts can be found ,then may be is not going to be too expensive to build one.
Air cylinders do not heat up due to the pressure or "cushioning" they provide.
Air getting compressed heats up at the compressor, then gets cooled in the system's tank and piping system. Once the compressed air cools off before it goes to a cylinder it is not going to heat back up unless you do something to increase the pressure to a higher pressure and hold it there. Compression and decompression cycling will cancel each other out in terms of heat gain. In fact, dropping the pressure at the regulator will cool the compressed air substantially. Air flow going from 100 PSI down to 50 PSI through a regulator will have it's temperature reduced by 50%.
This is truly a great idea.I would highly recommend it to anyone who could make buy the materials to it work, and also have or able to buy the compressor etc.It if you already have the compressor,it would not be too costly IMHO and the results are very well worth the effort involved,In fact it would as I see it be a very enjoyable project. Have a bash at it your self I am sure I will later with help from my boys. Alistair
It sounds like a great idea.

Just one thing to think about.
When mounting a blade, you need to just get the blade tensioned so you can spin the wheels and get the blade to center.
Then you tension.

How will you accomplish this, where you just get the light tension?

I certainly see merit in this even as a retrofit for a regular band saw.
It's complicated. Never tried air tension on my band saw.
The parts for this are incredibly simple (I hope). I am still in the construction phase of the bandsaw frame but the 'carriage' that the upper wheel mounts to has a 16.75" x 16.75" open area to fit into. Because I have so much space to work with, it should be quite easy to fit and mount the components.

At this point it looks like I will need only:
  • Cylinders
  • Regulator
  • Hose and Fittings
  • Check valve
  • Regular valve

The check valve will allow air into the regulator, but prevent depressurization in the event of supply failure. A normally closed valve after the regulator would allow the air to be bled from the system in order to de-tension the blade. This could be made more complicated if necessary, but it should work as-is and it won't cost much.

EDIT: I forgot, a needle valve should also be added to reduce the flow of regulated air which will smooth out the operation of the cylinder. Otherwise, it will "slam" into the blade as soon as it's pressurized.
See less See more
If you use the regulator I suggested earlier you just dial down to 0 PSI to de-tension. Then turn the dial a few degrees to apply just a little tension for centering, etc. When ready to run, just turn it up to your set point. All this is less than one revolution of the knob. I have used these regulators to eliminate control valves and solenoids and flow controls.
I just looked it up and the Wabco regulator is now sold by Wilkerson. Google "Wilkerson Dial Air".
1 - 18 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.