Linebackers, Leashes, & Superman: A Look at Stress
- Published: November 30, 2003, By Timothy J. Walker TJWalker & Assoc. Inc.
Do you work well under pressure?
How you react is dependent on the level of pressure or stress exerted on you. How your product reacts to pressures or stresses also will depend on the stress level it feels.
Typically, we think about external loads as applied force with units of pounds (or newtons). However, to predict a material's response, all external loads must be converted to pressures and stresses in units of pounds per square inch (or pascals, 1 psi = 6.9 kPa).
Pressures and stresses, whether 10 or 10,000 psi, are not an intuitive variable to most people. We all may be able to imagine what a hit from a 250-lb linebacker would feel like, but what about 250 psi? Answer: That same linebacker standing on your big toe. Let's take a tour through increasing pressure events to improve our intuition.
Less than 1 psi — This is like a pat on the back or someone pulling your leg. In either case, the pounds applied are less than the cross-section area in square inches.
A tensioned web pulled over a cylinder generates low pressure. These pressures are so low that modest velocities of air can offset the tensioned web's pressure, such as on air turns or the top layer of a winding roll.
1-10 psi — Standing flat-footed, the floor will feel this pressure under your shoes. Gently pulling your dog away from the fire hydrant, the leash will feel this in tensile stress. Pressures and tensions in this range will begin to exert a cooperative force on something.
The pressure under a low-level winding or laminating nip will average 1-10 psi. This level of tension is created in an untensioned, hanging loop of web.
10-100 psi — Basketballs and car tires are at the low end of this range. Pulling your finger enough to crack your knuckle or hanging by one arm will create this level of average tensile stress.
Winding nips that significantly increase roll tightness or moderately high laminating or coating nips create this pressure in their footprint. The internal layer-to-layer pressure in paper rolls usually will be at the low end of this range, while films and low modulus materials will be at the high end.
100-1,000 psi — At this point you are deforming solids significantly, except the highest-modulus materials. One to three people hanging from a rope will create this level of tensile stress.
This is the typical range of most web handling tension. A paper calender or film embossing nip likely creates this level of footprint pressure. Exerting 100 PLI nip load over a 0.5-in. MD contact length will create 200 psi nip pressure.
1,000-10,000 psi — A compressed oxygen cylinder will hold 2,000 psi. Great safety precautions are taken in handling these tanks since, potentially, they are explosive. When you break off a sales tag, you create this level of tension in that small string of plastic.
To create this level of pressure, you are exerting a load over an increasingly small area. To average 5,000 psi in a nip would require 500 PLI over a 0.1-in. contact length, best achieved with a steel-on-steel nip. The low end of this range is high tension for papers and films.
Greater than 10,000 psi — These stresses require seriously high load and miniscule contact areas. You are crushing and breaking stuff. When you stomp on a beer can or use scissors, you are creating the level of stress to fracture and buckle solid materials.
Paper supercalenders greatly change the quality of paper using this solid deforming pressure (and temperature). All slitting or cutting methods are designed to create these stresses in the web, whether from the sharp edge of shear or razor blade or the tips of a crosscut knife.
We have to be from the planet Krypton to go any further. To better understand the stresses in a web's life, remember to think about area supporting an external force. The difference ranges from a pat on the back to Superman's handshake.
Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 404/373-3771; This email address is being protected from spambots. You need JavaScript enabled to view it.; tjwa.com.
