- January 01, 2003, Michael Hebert, Perma-Flex Engineering
Minimal maintenance practices that resolve roll unbalance can add up to higher product quality, better equipment, performance, and a bigger bottom line.
More speed…it's what we all want from our process lines. More speed gives us more square feet of product, and more square feet of product means, hopefully, more profit. In the financial definition of balance, this is the credit side of the ledger.
Unfortunately, there can be a debit side. What happens if increased speeds create roll unbalance? Failing to balance rolls properly can result in some very dangerous scenarios. Among these are stresses within the roll components; excessive and very hazardous vibrations to other machinery parts; surging and varying roll speeds as in the case of idler rolls; and the wear and destruction of bearings.
There are two other developments that impact your operation more directly. One is the impairment of the quality of the product being produced; the second and more important is the ever-present potential for personnel injury.
Let's take a look at how we can help prevent all of the above.
Units of Measurement/Centrifugal Force
In general terms, the standard unit of unbalance measurement is based upon a weight and a distance. The amount of unbalance is the effect the weight would have, positioned at a given distance. For example, 1 “ounce-inch” is the effect produced by a weight of 1 oz positioned at a distance of 1 in. from the rotational axis.
Centrifugal force, the fundamental cause of unbalance in a rotating member, causes vibration. The centrifugal force increases proportionally with the square of the rpms at which the roll is rotating. The centrifugal force in pounds caused by a certain unbalance in “ounce-inches” is calculated as follows:
F = 1.75 (kRPM)2 × w × r
where F is centrifugal force in pounds; kRPM is thousands of revolutions per minute; w is weight of unbalance in ounces, and r equals the distance of the weight from the axis expressed in inches. Put in real terms: a 1 oz-in. unbalance rotating at 200 rpm is as follows:
F = 1.75 (0.2)2 (1) (1) or 0.07 lb.
Put the unbalance rotation at 2,000 rpm, and it calculates to 7.0 lb.
These examples show that for an identical unbalance weight of 1 oz-in., an increase of the rpms by 10x generates an increase in centrifugal force of 100x. Therefore, an increase in operating speed requires a decrease in allowable roll unbalance.
Types of Unbalance
Four types of roll unbalance exist: static, kinetic, dynamic, and whip.
Static unbalance is gravity at work. If a mounted roll on frictionless rollers has a heavy or weighted portion, the heavy or weighted portion would rotate to the bottom immediately (see Figure 1). This is corrected by simply adding weight to the light side of the roll or removing weight from the heavy side.
Industrial rolls should be statically balanced by the manufacturer as a minimum requirement. This usually is sufficient for operational speeds below 200 rpm. Above this speed, dynamic balancing is recommended.
Kinetic unbalance is static balance in rotation. As the roll spins, the static unbalance creates a centrifugal force causing deflection in the direction of the heavy side of the roll (see Figure 2).
From the equation above (F = 1.75 kRPM)2 x w x r), we understand centrifugal force increases proportionally with the square of the rpm of the spinning roll. Therefore, as roll speed increases, deflection also increases, causing still greater centrifugal force and resulting in greater deflection…ad infinitum.
For short-faced, relatively rigid rolls, kinetic unbalance is unlikely to occur, so the correction is the same as static unbalance. For longer, more flexible rolls, however, kinetic unbalance is corrected in the following fashion: Two weights will have the same effect as one heavier weight placed 180 deg opposite the original unbalanced weight (see Figure 3).
However, they are less likely to contribute to dynamic unbalance than the single weight. Also, the closer the correction weights are placed to 180 deg opposite the unbalance, the less likely the correction will cause dynamic unbalance.
Dynamic unbalance, in its strictest sense, is a couple or twist caused by two forces in two separate planes. If a roll were placed on frictionless rollers with each weight 180 deg opposite the other, no single point would roll to the bottom, because the roll would be balanced both statically and kinetically. If the same roll were rotated at an appreciable speed, each weight would cause its own centrifugal force in separate planes. This would cause an end-to-end rocking motion that would tend to twist the roll. In the purest sense, either dynamic or kinetic unbalance seldom occurs. What generally happens is a combination of the two.
After the roll is kinetically balanced (see Figure 4), the dynamic unbalance corrections can be performed by placing the proper correction weights in any two correction planes (see Figure 5). These weights and planes are determined with the aid of modern, sophisticated equipment.
Balance quality requirements have been specified with ISO-1940-1 and ANSI S2.19 for various types of rotors. These specifications recommend quality-grade G6.3 for more industrial roll applications. However, the more commonly specified grade is G2.5, due to requirements for higher quality product and increased operating speeds.
Because of today's trends to higher machine speeds and longer face lengths, whip unbalance can become a problem. This is the result of kinetic unbalance in the long, slender rolls at faster rotations. It is perhaps the most critical situation to deal with because it can produce a safety hazard to personnel as well as cause severe equipment damage.
Here's how it occurs: A long, slender roll may be balanced kinetically and dynamically for lower speeds and operate extremely well at these speeds. However, increase speed and the center section of the roll face may begin to throw out or “whip.” The deformity can cause dangerous vibrations that build quickly to a critical point.
When whip unbalance occurs, the roll must be corrected or balanced in three stages. The roll must be balanced kinetically and dynamically, and finally the whip must be measured with the roll rotating at its regular operating speed.
The unbalance can be corrected by moving the kinetic correction weights, the dynamic correction weights, or a combination of both. These movements must be made in such a fashion as to maintain proper kinetic and dynamic balance of the roll. If this fails to correct the problem, a separate correction must be made at or near the point of maximum whip.
It's easy to understand the requirements for whip balancing seldom are predictable before the fact. The process and material variables in roll manufacturing make it so. In a perfect world, if all manufacturing tolerances were closely held, and all materials and wall thicknesses were both homogenous and uniform, this unbalance might not be a factor.
To maintain a credit side to our financial ledger, minimal maintenance practices are required. A properly balanced roll running at its maximum operating speed truly is a precision item. The benefits include higher product quality, better equipment performance, and most importantly, greater personnel safety. Certainly the time and money spent to make these necessary corrections make sense.
Michael Hebert is general manager of Perma-Flex Engineering, Orange, MA. Mike received a B.S. in mechanical engineering from the Univ. of Massachusetts and a B.S. in architectural engineering from Wentworth Inst. During his 12-year tenure with Perma-Flex, he has been a production manager, sales engineer, and mechanical product engineer. Contact Mike at 800/828-8482; firstname.lastname@example.org.
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