- August 01, 2010, By Timothy J. Walker Contributing Editor
If you want to compress something, you always have two choices. You can use either one of the following options:
Put it in a vise or C-clamp and slowly turn the screws until it compresses to the desired dimension (but let the force or pressure be whatever happens).
Pile weights over a given area to compress it (but not control the final dimension).
There is no right answer that fits every need for compression. You might choose to control the compression distance when you are looking to control the final dimension; when the spring constant of your system is consistent; or you want some tactile feedback to the compression that you can “feel” (such as the hand torque required to turn the screw).
You might choose pressure control when you know that your system spring constant will change over time; you want to avoid excessive loads; or you want to take the need for skill out of the repeatability of the compression.
Nipped rollers, used in many processes, rely on high pressure created by rolling a web between two pinching rollers. In nipping roller systems, you have the same choices:
- Control the nip pressure adjusting the indentation, gap, or footprint.
- Control the force pushing two rollers together, usually through a combination of roller weight and external load delivered by pneumatic or hydraulic cylinders.
In my June 2007 column “Under Pressure” (http://pffc-online.com/mag/paper_pressure_brighter_look/index.html), I reviewed the variables that determine the relationship between either rubber cover engagement or rubber roller nip load (in units of force per width) and pressure; however, I didn't make an argument as to which is better. Again, there is no one answer that fits all nip systems.
My first preference, whenever possible, is to design a system in which one of the two rollers is sufficiently large to avoid significant deflection and make the other roller nip from above using only its weight. The advantage that gravity loading has over end loading is that the force is applied uniformly across the nip's width just like your process probably needs. Any method — engagement or force loading — that relies on controlling a nip system by squeezing down on the roller's shaft or journal ends has to convert the outboard point loads into a uniform, evenly distributed central load. This will always mean deflection and the many problems associated with crossweb nip pressure variations.
Choose gap, engagement, or footprint if your goals are one of the following:
- You are looking for the least expensive design to push two rollers together.
- You want to control thickness, such as metering a coating or calendaring rubber or similar webs.
- You like that feeling the same resistance in turning the screw knobs on either side of your nipping roller let's you know that the left-right loading is uniform.
However, if there are other sources of resistance to screwing a nip shut, you may be fooling yourself that you have uniformity. The greatest argument against engagement control is that the same setting (such as three turns after contact or a micrometer-measured motion) may create different results over time when rubber coverings wear or harden. Also, engagement loaded nips will fluctuate more with diameter, product thickness, and roller eccentricity variations.
I will almost always recommend controlling nip load with force delivered by pneumatic cylinders. Load control creates a nip pressure largely independent of product thickness and roller covering variations. Sized right, they prevent overzealous operators from damaging your nip rollers. Cylinder pressure can be set from a computer hooked to an I/P (current to pressure) converter, allowing recipe control of your process.
There are still applications in which thumb screws are the right choice, but you can avoid unintentional web torture by setting your nip with either gravity or pneumatic load.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; email@example.com; www.webhandling.com.