Member
Stress is the force applied upon an object that tends to deform it. Stress is expressed in units of pressure ... like psi.Originally Posted by Andrew Pitonyak
Strain is the deformation caused by the stress. It is commonly expressed as a percent, signifying the percentage (of the unstressed dimension of the object) that the object stretches or compresses as a result of the stress.
Member
http://www.instron.us/wa/glossary/Tension-Test.aspx
This is probably a good description for the layman.
I did tests like these in engineering labs back in the day, and still have a scar on my left hand to prove it! (Mind you, the Instron is a safe piece of equipment - the injury was sustained when I was holding two pieces of a broken steel sample together near the break to get the elongation measurement, and someone bumped my elbow and cause me to shove a sharp piece of steel into my hand. Left a trail of blood all over the engineering building as we looked for the nearest first aid kit!)
daniel
Not all chemicals are bad. Without hydrogen or oxygen, for example, there would be no way to make water, a vital ingredient in beer.
Regardless of what you call the force acting on the blade, the reason for removing the tension (layman's terminology) on a band saw is to prevent the tires from taking a permanent set, it has nothing to do with the blade stretching.
Any piece of steel that is pulled with a force (tension or strain) such that the elastic limit of the material is not reached will return to its original length. Once you exceed the elastic limit of the material the blade will get longer. It also takes less force than the original elastic limit for it to get longer again. Therefore, unless you exert enough force on a band saw blade to exceed the elastic limit it will always return to the length that it started at when the force is removed. If you exceed the elastic limit of the blade it is a very short and slippery slope to failure, which is why you can break most small blades if you over tighten the tension (again layman's terminology) adjustment.
With that said, there may be some creep that occurs on thin members if the force is left applied for a long period of time, but the greater danger is having the elastomer in the tires take a set and creating an out of round condition on your wheels.
Lee Schierer
USNA '71
Go Navy!
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Member
Don't know. Don't want to know. Don't care. I just crank the handle before I use the saw and when I'm done at the end of the day.
Contributor
I agree with Lee. Removing the tension is more about relieving the load from the tires than anything else.
When I use my bandsaw, I increase tension by cranking it 2 turns. At the end of day when I'm done, turn it 2 turns to reduce the tension......layman's terms.
Ken
So much to learn, so little time.....
Member
The "elastic limit" of which you speak is called "yield strength," and even for mild steel, it's generally > 36ksi. For the steel in a band saw blade, it's probably > 70ksi.
You'll probably never even remotely approach either of these stress figures using the springs in a band saw tensioner. If a band saw blade breaks, it's probably due to metal fatigue at the weld or at a "hard spot," rather than because you exceeded the strength of the material.
I'm with Ken and Lee: The reason to relax a band saw blade is to prevent forming "flats" on the tires, which cause a "thumping" when the saw is run.
Member
In case anyone still cares...
Force - Externally applied to an object and is generally expressed in pounds (Imperial units). So moving the wheels of you bandsaw apply a force to the blade.
Stress - The force within an object and is dependent on its geometry (F/A). Generally expressed in terms of pounds per square inch (or similar). For example, you can apply a 500# force to a 1/4" blade and have a stress of 20,000 psi within the blade, while the same 500# force applied to a 1" blade may result in a stress of only 5,000 psi.
Strain - The elongation resulting from an object's stress, generally expressed in terms of length. This is dependent on the stress within the object and the material's properties (modulus of elasticity). For example, two blades that look identical, are the same size and have the same force applied to them will have the same stress, but if they have different material properties then one may have a higher/lower strain, or amount of deflection/stretch. This is really what we're most concerned with in regards to our bandsaw blades, but since most blade steels will have basically the same modulus of elasticity, then stress within the blade could easily be the comparison.
I know, more than you probably care to hear.
[OP]
Guest
I came up with the following conclusions:
1. The difference between the terms we use is mainly because of the level of language that each of us have. The “level” of the language that we possessed is greatly affected by factors like society and the field of expertise. Society includes the region you are located, culture, dialect and the technology around you. While expertise is achieved through thorough studies and through experience.
2. People view things differently. A layman may have said the word “tension” but what he really meant was “strain”. Or he might have been thinking that they have the same meaning. No matter how he say it, he’s a layman, a layman speaks the words of a layman. While engineers or machine designers view tension and strain as two different things.
Now, can we change this? I think no is the answer. Without differences, we aren’t humans.
The conversation brought communication issues and I think it’s probably my fault for bringing up this thread. But I have enjoyed the conversation though.
Last edited by James Heisbert; 08-16-2012 at 10:44 AM. Reason: checked some erros..
Member
One of the risks of technical knowledge is that it provides more ways to get annoyed, as terms that are important to distinguish in your field are used as ordinary words by those who don't want to make the same distinctions.
For instance "silicon" (the element) and "silicone" (a specific set of structures containing silicon, carbon, oxygen, and hydrogen, and sometimes other things) are not distinguished by most. To my ear they are as different as "curly maple" and "Chippendale highboy". Until now I have been able to refrain from pointing this out.
Friend of The Creek
Dear bob hertle, et al.
You are absolutely correct. Paul Smith and I do own sawmill supply companies. In addition Paul and I both have a long history, many decades each, of actually making saws run better.
The discussion under reference deals, in large part, with the fact that the language is highly imprecise. Your comment “I can assure you that the guys in that conversation do not have a clue as to what they are talking about!” is accurate from your viewpoint because we are speaking a different language. I call it “talking sawmill”. We work very hard to explain things in terms the other person understands.
Also we really try very hard not to argue with people or insult them. I am sure you are a very fine person but the opinion you expressed of me does not make me want to listen to you and certainly does not make me want to work with you.
Besides speaking a different language, we also live in a different world than you apparently do. We actively avoid telling anyone they are wrong. Instead we suggest what might be a better way. We believe that confrontation is a huge detriment to problem solving.
The average hobby woodworker is very intelligent, definitely upper income and generally very successful in life. These people are also very gracious and polite. We never correct them or tell them they are wrong. If they call and ask for a pointy thing to start holes we may suggest a gimlet and send them a picture but we do it in such a manner that we are working with them.
Aw heck, maybe you are just having a bad day. If you are going to IWF maybe we can meet and I will buy you lunch.
Tom Walz
I'm a Creeker, yes I m.
I fries my bacon in a wooden pan.
Member
OTOH, communication works better when we all agree to use the same word to refer to the same idea ... lest, when we tell someone to "turn right," they steer the car into an oncoming 18-wheeler.
[OP]
Guest
Yes, that should be our goal to solve communication issues. But that's almost unattainable because someone from another field view the term in a different dictionary. Can we decide that this should be the proper term to use and not theirs? No.
What's important is they get our message and we deliver it to them the proper way that it is most understandable.
When we tell someone to "turn right", they will steer the car to the right direction where they're safe.
Member
Lee described the difference between force, stress, and strain accurately. Being a mechanical engineer and in sales myself, I also see Tom's point in that you have to communicate with the customer and know what they are referring to.
I did not read the link, but I have always heard it referred to as blade tension. The tension is the only thing I can read directly off my bandsaw. It is a measure of the spring displacement. Sure, the tension (force) imparts a stress and strain, but I can't read that directly. If I change blades and set the scale to the same number, I have applied the same force, but the stress/ strain in the blade will be different.
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