WO2000004228A1

WO2000004228A1 - Equipment for attenuation of vibrations in a paper machine environment

Info

Publication number: WO2000004228A1
Application number: PCT/FI1999/000618
Authority: WO
Inventors: Jorma Köliö; Jouko Karhunen; Esa Lehtovirta
Original assignee: Valmet Corporation
Priority date: 1998-07-17
Filing date: 1999-07-13
Publication date: 2000-01-27
Also published as: FI104209B; AU5042899A; FI981629A0; FI104209B1

Abstract

An apparatus for damping vibrations in a paper machine environment by means of a dynamic absorber. The apparatus comprises at least one vibration detector (6), by whose means the vibrations of a vibrating object are measured, and an amplifier device (7) which amplifies the measurement signal. The apparatus further comprises a data processing device (8) in which the measurement signal is analysed, and at least one regulation device (9) which is controlled by means of the data processing device (8) and by means of which regulation device (9) the dynamic absorber attached to the vibrating object is tuned to a desired problematic excitation frequency. The dynamic absorber comprises at least two additional weights (41, 42, 43) mounted on a common rod (3), whereby the dynamic absorber can be tuned by means of said additional weights to several problematic excitation frequencies.

Description

EQUIPMENT FOR ATTENUATION OF VIBRATIONS IN A PAPER MACHINE ENVIRONMENT

The invention relates to an apparatus for damping vibrations in a paper machine environment as defined in the preamble of claim 1.

In paper machines and in paper finishing devices, vibrations constitute a major problem that continues to become greater with constantly increasing speeds. There are several possible sources of vibration in paper machines, such as, rolls and cylinders, which comprise a very great mass revolving at a considerable speed. Attempts are being made to eliminate the vibrations of rolls by making them with high measurement precision, and in addition thereto they are also balanced.

However, present-day paper machines and paper finishing devices increasingly employ rolls provided with a soft coating, which rolls may form in operation a significant source of vibration. Such rolls are used, for example, in presses, calenders, coating machines, size presses, supercalenders and equivalent, where said roll provided with a soft coating forms a nip with another roll. A paper web and possibly a felt, wire or equivalent are passed through the nip. When in this kind of nip roll arrangement, a seam of the wire, felt or web, considerable impurities or something else causing a noticeable change in the thickness of the web travelling through the nip, passes/pass through the nip during running, the coating must yield elastically, with the result that the coating serves as a spring that excites vibration.

For example, in a size press and in a coating device of the size press type, a nip is defined by means of two rolls such that one nip roll is mounted by means of bearing housings directly on the frame structure of said device, while the opposite roll is mounted at its bearing housings on loading arms that are attached by means of articulated joints to the frame structure of the machine. In that case, the roll mounted on the loading arms in particular begins to vibrate, in which connection the coating of the soft-faced roll is deformed, with the result that vibration increases and the roll begins to resonate. In such a situation, one vibration frequency usually becomes clearly the most troublesome in its amplitude, in which case the situation may be taken care of by means of a dynamic absorber which is tuned to the problematic excitation frequency in question. The dynamic absorber is capable of damping a very narrow frequency band and the dynamic absorber has no effect on excitations which are outside said narrow frequency band.

In presses where a press nip may be formed between two hard-faced rolls or between a hard-faced and a soft-faced roll, the press nip and a press felt travelling through the press nip may induce problematic vibrations. The seam of the press felt and the water bound to the press felt may produce in the press felt a discontinuity that causes vibration of the press. In that case, in the press structure there are produced a vibration induced by the rolls at a higher frequency and a vibration induced by the press felt at a lower frequency. In such a situation, a dynamic absorber tuned only to one narrow frequency range damps only the problematic excitation frequency situated in said frequency range and other problematic frequencies remain undamped.

The applicant's FI patent application 971864 discloses a method and an apparatus for damping vibration in a paper machine or in a paper finishing device. The method uses a dynamic absorber which comprises an additional weight suspended from a vibrating object by means of a spring. In the method, the vibration frequencies of the vibrating system are measured constantly by means of one or more vibration detectors. The measurement signals of the vibration detector are amplified by means of an amplifier and fed into a vibration analyser which identifies a problematic excitation frequency and converts said problematic excitation frequency into a regulation signal. The regulation signal is fed into a regulation device which changes the spring constant of the spring of the dynamic absorber and/or the mass of the dynamic absorber in order to make the natural frequency of the dynamic absorber substantially equal to the problematic excitation frequency. US patent 4,420,371 discloses a vibration absorber for use in a vibrating headbox. The patent discloses a headbox provided with three separate vibration absorbers. The first vibration absorber fitted at the rear of the headbox damps vibrations in the machine direction, the second vibration absorber mounted on the headbox damps vertical vibrations, and the third vibration absorber mounted on the side of the headbox damps vibrations in the horizontal cross machine direction. The first vibration absorber damping the machine direction vibrations comprises three separate vibration absorbers mounted on a common base, each of said vibration absorbers being formed of a rod attached to said base at both ends thereof and of a weight connected to the middle of the rod, and each of said vibration absorbers being tuned to a different frequency by means of a weight of different magnitude. One vibration absorber intended for said machine direction vibration is tuned to the centre of problematic vibration frequencies, the second vibration absorber is tuned below the centre, and the third vibration absorber is tuned above the centre, in which connec- tion a larger range can be covered than by means of only one vibration absorber.

The object of the present invention is to provide an apparatus by means of which it is possible to damp several problematic vibrations in different frequency ranges by means of a dynamic absorber.

The principal characteristic features of the apparatus according to the invention are set forth in the characterizing clause of claim 1.

In the invention, vibration frequencies of a vibrating system are measured by means of one or more vibration detectors. The measurement signals of the vibration detector are amplified by means of an amplifier and fed into a vibration analyser which identifies the excitation frequencies which are problematic and converts said problematic excitation frequencies into regulation signals which control the dynamic absorber by means of a regulation device.

The invention allows several vibrations situated in different frequency bands to be damped. The apparatus in accordance with the invention is very simple in its construction and mode of implementation and it can be connected by simple operations to existing structures for the purpose of damping vibrations.

In the following, the invention will be described by way of example with reference to the figures in the accompanying drawing.

Figure 1 is a schematic view of one known absorber apparatus based on a dynamic absorber.

Figure 2 is a schematic view of vibration that occurs in several frequency bands.

Figure 3 is a schematic view of another known absorber apparatus based on a dynamic absorber.

Figure 4 is a schematic view of an absorber apparatus based on a dynamic absorber in accordance with the invention.

Figure 5 is a schematic view of the vibration form of the absorber in accordance with the invention.

In Fig. 1, a dynamic absorber is mounted on a vibrating object, i.e. in this case, on a bearing housing 2 of a roll 1. The dynamic absorber comprises a mass 4 suspended from the vibrating object 2 by means of a spring 3. A rod 3 rigidly mounted in the bearing housing 2 by means of attachment members 5 serves as the spring. The rod 3 is additionally provided with external threads, and it is mounted substantially in a horizontal plane in the bearing housing 2. A disc-shaped weight 4 serves as the mass and comprises a hole provided with internal threads, the weight 4 being disposed on the external threads of the rod 3 by means of said hole. The weight 4 can be displaced in the longitudinal direction of the rod 3 by rotating such that the distance a of the weight 4 from the point of attachment of the rod 4 in the bearing housing 2 can be regulated. Such a dynamic absorber has one degree of freedom and it can be dimensioned by the basic equation:

k/m = Ω²,

where k is the spring constant of the spring, i.e. of the rod 3 in this case, m is the mass of the weight 4, and Ω is the angular velocity of the vibrating object, i.e. of the bearing housing 2.

The effect of the dynamic absorber is based on the fact that the natural frequency of the absorber is tuned to be equal to a problematic excitation frequency. In the case of the figure, the spring constant of the rod 3 is inversely proportional to the power of three of the length of the rod 3, and thus the natural frequency of the absorber can be regulated by adjusting the distance a of the weight 4 from the point of attachment of the rod in the bearing housing 2. When the natural frequency of the absorber has been made equal to a problematic excitation frequency by changing the distance a, the bearing housing 2 ceases to vibrate and the weight 4 resting on the rod 3 begins to vibrate, respectively. In that connection, the dynamic absorber generates a force that is in an opposite phase and of equal magnitude to the excitation, whereby the vibration of the machine itself ceases.

In the arrangement of Fig. 1, the controllability of the dynamic absorber is provided such that the bearing housing 2 whose vibration is desired to be damped is provided with a vibration detector 6. The vibration detector 6 transmits a signal that is amplified by an amplifier 7 and passed further to a data processing device 8 serving as a vibration analyser, which filters and analyses the vibration frequencies and locates the problematic excitation frequencies among the frequencies. When the problematic excitation frequencies have been identified, the computer 8 computes the distance a such that the natural frequency of the dynamic absorber coincides with the problematic excitation frequency. After that, the data processing device 8 controls a regulation device 9 connected to the weight 4 of the dynamic absorber, which regulation device moves the weight 4 on the rod 3. The regulation device 9 is advantageously, for example, a stepping motor. The apparatus thus comprises a closed regulating circuit that constantly measures and analyses vibrations and, based on this, regulates the natural frequency of the dynamic absorber.

Fig. 2 is a schematic view of vibration that occurs, for example, in a press section. It is seen from the figure that there are several problematic vibration frequencies f _j , f₂,f₃- This may be due to the fact that the vibrating system comprises several devices which vibrate at different frequencies. For example, press rolls in a press section constitute one source of vibration, and the passing of discontinuities of a press felt through a press nip constitute another source of vibration. The vibration frequencies of these differ from one another, and thus there are created several problematic vibration frequencies in the system. For example, in size presses, rolls constitute a considerable vibration source, but the natural frequency of the entire vibrating system is, however, not necessarily equal to a multiple of the rotational frequency of the roll inducing the vibration (in most instances this is not the case).

The regulating circuit of the known apparatus based on a dynamic absorber shown in Fig. 3 corresponds to the regulating circuit shown in Fig. 1. The spring 3b of the dynamic absorber comprises a rod which is fitted and attached to a bearing housing 2 in a manner corresponding to the illustration of Fig. 1. The weight 4b of the dynamic absorber comprises a container and a liquid provided therein whose amount is regulated by means of a pump 21 and a valve 22. By pumping more liquid from a storage container 23 into the container forming the weight 4b, the mass of the weight 4b can be increased, and vice versa. The regulation device 9 thus controls, based on a regulation signal received by it, said pump 21 and valve 22 in order to change the amount of the liquid contained in the container associated with the weight 4 a In that case, it is a question of only regulating the mass 4b of the dynamic absorber. In the case of Fig. 3, it may be contemplated that the distance a of the weight 4b from the point of attachment of the rod 3b can also be regulated.

The absorber apparatus based on a dynamic absorber in accordance with the invention shown in Fig. 4 differs from the apparatus of Fig. 1 in respect of addi- tional weights. The apparatus in accordance with the invention comprises three additional weights ^^^A^ instead of one. The dynamic absorber provided with three additional weights has three degrees of freedom and three natural frequencies. The equations of motion of an undamped system of several degrees of freedom are of the form:

[M] {x} + [K] {x} = (P(t)}

where [M] is the mass matrix, {x} is the acceleration vector, [K] is the stiffness matrix, {x} is the displacement vector, and {P(t)} is the external force vector.

In the embodiment of Fig. 4, the masses of the additional weights ^IA AJ are kept constant, but the masses of the additional weights ^γA^A-i ^{are not} necessarily of equal magnitude. From the point of view of the accuracy of regulation, it may be advantageous that the masses of the additional weights

are selected such that the mass of the first additional weight 4_j closest to the point of attachment of a rod 3 is the largest, the mass of the second additional weight 4₂ is the second largest, and the mass of the third additional weight 4₃ is the smallest.

The dynamic vibration absorber shown in Fig. 4 is regulated in principle in the same fashion as the dynamic absorber shown in Fig. 1. Vibrations of a vibrating object are measured by means of at least one vibration detector 6, measurement signals are amplified by means of an amplifier 7, after which they are passed into a data processing device 8 for computation. The three most problematic excitation fre- quencies which are desired to be damped are determined in the data processing device 8 based on the results of measurement. After that, the data processing device 8 computes the distances a_{1 ;}a₂,a₃ of each additional weight 4_{l 5}4₂,4₃ from the point of attachment of the rod 3 such that the dynamic absorber is tuned to said problematic frequencies. After the distances a_{1 ;}a₂,a₃ have been determined, the data processing device 8 supplies corresponding control instructions to regulation devices 9ι,9₂,9₃ which adjust the additional weights so as to be at the computed distances aι,a₂,a₃. After the additional weights ^_lAiA^ have been adjusted so as to be at correct locations, each additional weight 4^₂,4₃ is locked in place, after which the dynamic absorber will vibrate at the desired three frequencies and damp the corresponding three problematic frequencies of the vibrating object.

Fig. 5 shows three natural vibration modes of the dynamic absorber shown in Fig. 4. Each figure shows an envelope curve of one extreme of the natural vibration mode. Fig. 5B shows a situation depicting the lowest natural frequency of the dynamic absorber. All the three additional weights 4_{1 ;}4₂,4₃ vibrate with the same phase, in which connection the vibration amplitude of the first additional weight 4_j is the smallest and the vibration amplitude of the third 4₃ additional weight is the largest. Fig. 5C shows a situation depicting the centre natural frequency of the dynamic absorber. The third additional weight 4₃ vibrates here with a different phase from those of the first 4_j and the second 4₂ additional weights. Fig. 5D in turn shows a situation depicting the highest natural frequency of the dynamic absorber where the second additional weight 4₂ vibrates with a different phase from those of the first 4_j and the third 4₃ additional weights.

By means of the simple dynamic absorber provided with three additional weights 4_{l s} 4₂,4₃ shown in Fig. 4 it is possible to damp three problematic vibration frequencies. By increasing the number of additional weights, the natural frequencies of the dynamic absorber are also increased.

The dynamic absorber shown in Fig. 3 may also be used in the invention, in which case a number of additional weights 4b formed of a liquid container are mounted on support of the rod 3b. In that case, it is possible to regulate the mass of each additional weight 4b instead of distances or both the mass of the additional weights 4b and the distance of the additional weights from the point of attachment of the rod 3b.

Above, the invention has been described by way of example with reference to the figures in the accompanying drawing. The invention is, however, not confined to relating only to the examples illustrated in the figures, but different embodiments of the invention may vary within the scope of the inventive idea defined in the accompanying claims.

Claims

1. An apparatus for damping vibrations in a paper machine environment by means of a dynamic absorber, which apparatus comprises at least one vibration detector (6), by whose means the vibrations of a vibrating object are measured, an amplifier device (7) which amplifies the measurement signal, a data processing device (8) in which the measurement signal is analysed, and at least one regulation device (9) which is controlled by means of the data processing device (8) and by means of which regulation device (9) the dynamic absorber attached to the vibrating object is tuned to a desired problematic excitation frequency, characterized in that the dynamic absorber comprises at least two additional weights (4_{1 ;}4₂,4₃) ^mounted on a common rod (3), whereby the dynamic absorber can be tuned by means of said additional weights to several problematic excitation frequencies.

2. An apparatus as claimed in claim 1, characterized in that the dynamic absorber comprises three additional weights (4╬╣,4₂,4₃) mounted on the common rod (3), whereby the dynamic absorber can be tuned by means of said additional weights to three problematic excitation frequencies.