ArticleVibration Analysis Of Press Rolls
by Norman F. Lopez
This is a well written and brief paper that introduces the reader to vibration analysis of paper machine press rolls and rolls in general. Four case histories are presented within this paper involving the following problems: roll unbalance, roll barring, bearing defects & felt seam impacting. The special techniques involved in collecting roll vibration data are presented including the use of low frequency vibration sensors and time synchronous averaging. For anyone wanting an introduction to vibration analysis of paper machine rolls, this paper is a good start.
“Some type of press has always been used in papermaking. Paper as we know it was first made in 105 AD by the Chinese. The basic process has not changed in 2,000 years. When paper is made by hand, the sheets are placed between felts. The stack, called a post, is placed in a hydraulic press and subjected several times to pressures of 100 or more tons to expel most of the water. This process is accomplished on modern paper machines with a series of rolls and felts. Their arrangement varies with the type of ma-chine and the grade of paper being made. Each arrangement has different characteristics. A Twinver press was used in the cases presented in this article.
Rolls and felts can cause vibration. Roll problems include barring, roll cover defects, misalignment, and rolling element bearing problems. Felts contribute sufficient vibration that most felt manufacturers have their own vibration service. Vibration problems affect overall press performance. A roll in a press nip (line contact between two rolls) often becomes barred or lopsided; in addition, the cover can become loose.
Rolls are routinely removed from service and checked on roll grinders equipped with electronic indicators positioned 180° apart. As the roll is turned slowly, the output is plotted in a roll profile plot on polar paper (Figure 1). Frequencies of interest are usually full orders, especially if the problem is unbalance, misalignment, barring, or a loose cover. Many press sections are equipped with on-line monitoring systems; the sensor is in the nip direction. A displacement probe is used with a trigger to obtain the operating speed of the roll. Data are conditioned and displayed in a roll profile plot of 360°. Not all of the press-section problems presented in this article were identified with data from on-line monitoring systems.
In one case the second V-nip roll was pressed to the center granite roll. Spectral data in the nip and non-nip directions contained high vibration levels at the operating speed of the roll. Data were taken with velocity transducers and a two-channel FFT analyzer.
Trigger marks at press rolls are very difficult to keep clean because of the dirty environment, and optical sensors usually do not produce good results. Steam and water spray tend to block the laser beam when laser sensors are used. One successful method is to use a strobe light with an out-put at the operating speed of the roll or felt.”
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