GB2269880A - Damped detuning plate - Google Patents

Abstract

A damped detuning plate (26) is provided for location between the flanges (20, 22) of a vertically orientated motor) and the structural headpiece of a pump. The plate (26) comprises two flexible structural plate elements (28, 30) with an incompressible damping element (32) therebetween. Respective series of bolts (34, 36) are provided for separately mounting the plate (26) to the motor and pump. <IMAGE>

Description

DAMPED DETUNING PLATE The present invention relates to a damped detuning plate and in particular, but not exclusively, to damped detuning plates for use in vertically oriented motor driven pumpsets in which there is a requirement to limit self generated vibration levels within values specified in appropriate Machinery Vibration Guideline Standards.

Vertically orientated motor driven pumpsets are typically used to save space in a variety of applications where water is to be pumped in large quantities. The motor is usually supported on a structural headpiece or frame either integral with or connected to, by a series of other structural components, the pump casing. The motor shaft is coupled directly to a shaft connecting to the pump impellers. The complete structure is usually supported a floor level foundation formed by a network of steel or concrete reinforced beams. Vibration characteristics of the structure and foundation are important in relation to the vibration levels generated by the unbalance forces in the motor. If the operating speed is at resonance with a natural frequency in the structure, the unbalance forces can be amplified many times resulting in unacceptable levels of vibration.Typically magnification factors at resonance are quite high, around 25 to 50, as structures of this type possess only a small amount of inherent damping. This can have certain undesirable consequences: the bearings within the motors may be damaged; in the case of Slip Ring Motors the brush gear may vibrate causing sparking at the commutator and consequential damage; the windings within the rotor or stator may eventually become damaged or detached; and there may be transmission or vibration through the mechanical coupling which may cause problems within the pump. For these reasons it is desirable to be able to avoid resonance within the operating speed of the motor, or alternatively to introduce more damping to limit vibration magnification at resonance.

Finite element techniques are used extensively to model complex structures and predict natural frequencies of new designs. However, it is not always possible to be confident of the accuracy of these natural frequency calculations due to the difficulty of modelling bolted joints, flange to flange distortions and of establishing a reliable model of the support foundations. Quite often it is not until the pumpset is installed on the foundation at site that confirmation of natural frequencies is possible. By this time it is usually very difficult to engineer changes to the structure or foundation which will have any significant effect on the natural frequencies.

There are a number of possible approaches to solving such problems in situ. It may be possible to stiffen parts of the support structure or foundation by welding on additional metal components although this involves significant risks as deformation of components can occur and affect the alignment of the rotating parts. It is sometimes possible to install a dynamic vibration absorber, this usually requiring a mass of at least 20% of the motor mass to be effective, but this introduces additional natural frequencies which may still fall within the operating speed range of the motor. It may be possible to reduce the unbalance forces in the motor, and it is likely that a minor change in the magnitude or location of the forces will result in large changes in vibration, however neither this nor any of the other abovementioned steps guarantee a solution.

Accordingly it is one object of this invention to provide a simple and effective means to modify the modes of vibration of a vertically oriented motor for driving a pumpset and to increase the amount of damping available in these modes to ensure that resonance and resonance magnification is contained within bounds which result in an acceptable level of vibration at least within the operating speed of the pump.

According to the present invention there is provided a damped detuning plate for location between the flanges of a vertically oriented motor and the structural headpiece of a pump, the plate comprising two flexible structural plate elements with an incompressible damping element therebetween and being provided with securing means for separately mounting the plate to the motor and pump.

-In use, the plate elements bend in the direction of motor weight loading. The incompressibility of the damping element ensures that the plate elements act in series to provide an overall flexibility which, when combined with the mass/inertia of the motor, results in a reduction in the natural frequencies of the associated support structure. At the interface of each plate element with the damping element, the bending deflections of the plate result in movements in a direction normal to bending. In this direction the relative movement introduced between the plate elements and the damping element is governed by the frictional forces between the abutting surfaces. These forces combine to produce a resultant damping force which limits the magnification of the vibration response in the resulting modes of vibration.

For each particular application the plate will be configured to reduce the natural frequencies of the support structure such that they occur outside the pump operating speed. The overall flexibility of the plate determines such reduction and is controlled by various factors including plate thickness and, with the preferred annular plate elements, inner and outer plate element diameters. The plate elements must also meet structural requirements, to account for design parameters such as motor weight, maximum motor torque, upset or seismic loads, vibratory loadings, operating speeds and the control of alignment between the rotating shafts.The degree of damping provided by the plate is controlled by the frictional forces between the plate elements and the damping element, and the magnitude of such forces is related to the surface finish of the plate elements and the shear characteristics of the damping element.

Preferably, the structural plate elements are of metal, and the damping element is of a composite material. Most preferably, the damping element is a premium grade compressed fibre jointing compound comprising a blend of fibres with a nitrile rubber binder.

Preferably also, the elements are annular and of the same outer diameter as the motor flange and the motor support headpiece flange. Most preferably, spigots are provided on the plate elements which, together with the control of manufacturing concentricity, ensures the alignment between the motor and pump shafts.

Preferably also, the securing means are in the form of a series of bolts for securing the plate to the motor and a separate series of bolts for securing the plate to the headpiece or frame above the pump. Most preferably, the bolts pass through apertures in the elements and through short posts which serve to provide a clearance between the plate element and the respective adjacent motor or pump flange.

This and other aspects of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1 and 2 are views of a typical arrangement of a vertically oriented motor driven pumpset provided with a damped detuning plate in accordance with a preferred embodiment of the present invention; Figure 3 is an enlarged perspective view of the damped detuning plate of Figures 1 and 2; Figure 4 is a perspective view of an upper plate element and damping element of the damped detuning plate of Figures 1 and 2; and Figure 5 is a graph illustrating vibration levels of a motor driven pumpset before and after installation of the damped detuning plate of Figures 1 and 2.

Reference is first made to Figures 1 and 2 of the drawings which shows a pumpset 10 driven by a vertically oriented motor 12 via a shaft 13 and couplings 15 and provided with various bearings and seals.

Between the pump 10 and motor 12 are a vertical column 14 and a horizontal inlet 16 which incorporates a motor supporting headpiece 18 provided with a flange 20.

The motor 12 is provided with a corresponding flange 22 and a damped detuning plate 26 in accordance with a preferred embodiment of the present invention is located between the flanges 20,22.

The plate 26 is shown in more detail in Figures 3 and 4 of the drawings and comprises two structural annular metal plate elements 28, 30 and a relatively thin composite material damping element 32 held between the opposing faces of the elements 28, 30. The plate 26 is secured to the motor support flange 20 by a first series of bolts 34 and secured to the motor flange 22 by a second series of bolts 36. The plate elements 28, 30 are spaced from the adjacent respective flanges 22, 24 by short posts 38 through which the securing bolts 34, 36 pass.

The annular plate elements 28, 30 provide flexibility derived from the bending of the elements in the direction of motor weight loading. The thin material composite element 32 sandwiched between the plate elements 28, 30 is virtually incompressible in the direction of bending.

This ensures that the plate elements 28, 30 act in series to give an effective overall flexibility which, when combined with the mass/inertia of the motor 12, results in a reduction in the natural frequencies of the support structure 10, 14, 18. The required reduction depends upon the proximity of the natural frequencies to the pump operating speed. At the interface of each plate element 28, 30 with the damping element 32, the bending deflections of the plate elements 28, 30 result in movements in a direction normal to bending. In this direction the relative movement introduced between the plate elements 28, 30 and the material composite damping element 32 is governed by the frictional forces between the contacting surfaces. These forces combine to produce a damping force which limits the magnification of the vibration response in the resulting modes of vibration.

The overall flexibility of the plate elements 28, 30 is controlled by metal thickness, and inner and outer diameters. The magnitude of the friction force is controlled by the surface finish of the metal plate elements 28, 30 and the shear characteristics of the material composite damping element 32. In this particular example, the element 32 is a premium grade compressed fibre jointing compound which is formed of a blend of fibres with a nitrile rubber binder. Suitable material composites are Sil C4200 and Sil C-4400 from the Klinger (RTM) range as supplied by ARCO.

Maintenance of alignment between the pump and motor 10, 12 is achieved by careful control of concentricity of the plate elements 28, 30 during manufacture and the provision of spigots (not shown) on the plate elements.

Figure 5 of the drawings is a graph which illustrates the magnitude of motor vibration measured on the motor support structure illustrated in Figures 1 and 2 before and after the installation of the damped detuning plate 26. These results are also set out in Table 1, below.

TABLE 1 NATURAL FREQUENCIES AND MAGNIFICATION FACTORS BEFORE AND AFTER INSTALLATION OF A DAMPED DETUNING PLATE BEFORE AFTER Natural Natural Frequency Magnification Frequency Magnification (Cpm) Factor (Cpm) Factor MODE 1 1190 30 990 12 MODE 2 1280 25 1050 12 For both modes it will be noted that the natural frequency of the support structure is reduced from a level at or adjacent the operating speed range of the pump to significantly below the operating range, and further that the magnification factor is significantly reduced, such that the maximum magnitude of vibration, now only encounted on starting or stopping the pump 10, is considerably lower.

Hence it is evident that, through use of the present invention, it is possible'to alter the natural frequencies and damping of a motor support structure in a simple and effective manner and realise low vibration levels at the operating speed of the pump.

It will be clear to those of skill in the art that the above identified embodiment is merely exemplary of the present invention and the various modifications and improvements may be made thereto without departing from the scope of the invention.

Claims (7)

1. A damped detuning plate for location between the flanges of a vertically oriented motor and the structural headpiece of a pump, the plate comprising two flexible structural plate elements with an incompressible damping element therebetween and being provided with securing means for separately mounting the plate to the motor and pump.

2. The damped detuning plate of claim 1 in which the structural plate elements are of metal, and the damping element is of a composite material.

3. The damped detuning plate of claim 1 or 2 in which the elements are annular.

4. The damped detuning plate of claim 1, 2 or 3 in which spigots are provided on the plate elements to ensure alignment between the motor and pump shafts.

5. The damped detuning plate of any one of claims 1 to 4 in which the securing means are in the form of a series of bolts for securing the plate to the motor and a separate series of bolts for securing the plate to the headpiece or frame above the pump.

6. The damped detuning plate of claim 5 in which the bolts pass through apertures in the elements and through short posts which serve to provide a clearance between the plate element and the respective adjacent motor and pump flanges.

7. A method of reducing the frequency and magnitude of the natural frequencies of a vertically oriented motor driven pumpset, the method comprising: providing a plate comprising two flexible structural plate elements with an incompressible damping element therebetween; providing securing means for separately mounting the plate to the respective flanges of a vertically oriented motor and the structural headpiece of a pump; selecting the properties of the plate elements to provide the plate with an overall flexibility to reduce the natural frequencies of the structural headpiece such that said frequencies occur outside the pump operating speed; and selecting the surface finish of the plate elements and the shear characteristics of the damping element to provide a desired degree of damping of vibrations occurring at said frequencies.