Title of Invention

AN IMPROVED DEVICE USEFUL FOR EXTERNAL DAMPING IN HIGH SPEED ROTATIING MACHINARY

Abstract

An improved damping device useful for high speed rotating machinery which comprises an inner ring (3) housing a conventional bearing, supporting a rotating shaft vibration of which need to be damped, characterised in that the said inner ring (3) being housed inside an outer ring (4) to provide a clearance space (5), the said space (5) being provided with a plurality of foils (6), one end of the said foils (6) is fixed to the outer surface of the said inner ring (3) and the other end being fixed to inner surface of the said outer ring (4), the said outer ring (4) being provided with plurality of oil ports (7).

Full Text

This invention relates to an improved damping device useful for high speed rotating machinery. The device of the present invention particularly relates to a Multiple Multi Squeeze Film Damper useful for external damping in high speed rotating machinery mounted on rolling element bearings. A damping device is one which may contain part or parts, material of which may have inherent property of damping or an arrangement of part or parts combined functioning of which will provide damping action. The constituent part or parts used in a damping device may be of solid or fluid state and can be metallic or nonmetallic. The damping action prevents energy accumulation in machinery due to vibration that may become dangerous or unhealthy, if allowed. This approach is adopted and often incorporated in machinery designs to dissipate such energy accumulated. This increases the life of machinery and ensures its safe operation. The safety of the operator and installation are also ensured. Any high speed rotating machinery supported on rolling element bearings requires external damping devices to provide damping. This becomes extremely essential in order to alleviate the vibrations encountered in the rotors. High speed rotors, like gas turbine engine rotors develop vibrations due to residual unbalance or while crossing over the critical speeds or natural frequencies however small the unbalance may be. During the course of operations, the balancing grade of the rotors also deteriorate due to corrosions and erosions resulting in higher residual unbalance leading to higher vibration levels at all speeds which necessitates very high balancing grades at the beginning of the operation/installation stage, thus increasing the cost of fabrication. Hence, relaxations in balancing and tolerance band are desirable as it lowers the equipment cost. Therefore, external damping is required to be introduced at the supports to control the rotor vibrations. Also, bearing life is increased with the introduction of optimum damping at the bearing supports as the force transmitted through rolling element bearing comes down. The damping at the bearing supports is provided by several ways. The fluid film bearing has inherent damping just enough to contain the rotor vibrations. The rolling bearings comes with a little or no damping and hence, rotors supported on these bearings require external dampers. In external dampers, damping is obtained using elastomeric materials or through oil film. Damper with elastomeric materials has been found unreliable since even small changes in operating conditions such as temperature, load and speed affect the viscoelastic properties of the elastomers thereby altering the amount of damping provided. Oil film dampers have proved more reliable and hence have become more popular where temperatures encountered is relatively higher . Squeeze film dampers, which use oil as the damping medium are being extensively used in modern high speed gas turbine engines to attenuate engine vibration of the rotors without which vibration would be large since rotors in such cases are mounted on rolling element bearings. The schematic of a conventional squeeze film damper is shown in figure 1a & 1b of the drawings accompanying this specification. Conventional Squeeze Film Damper essentially consists of two circular rings, inner(3) and outer(4) placed one inside the other concentrically or eccentrically and oil is introduced in the radial clearance(5) provided in between them. In most of the cases, the same oil used for bearing cooling and lubrication purposes is pumped through this radial clearance to form squeeze film. The inner ring(3) houses the bearing(2) which in turn supports the rotating shaft(1). The outer ring(4) is fixed to the engine casing. The vibratory motion of the engine shaft(1) forces the oil in the clearance(5) between the two rings(3&4) to squeeze in between the rings alternately in different directions. The squeezing action provides damping due to viscous nature of the oil and absorbs the unwanted energy generated due to rotor vibrations. The oil takes thin film shapes between the rings as the clearance is of the order of micrometers. It has been observed and established that the damping obtained from the oil film is directly related to the ring width measured along the shaft axis and inversely related to nominal oil film thickness or radial clearance As the damping is directly dependent on ring geometry and film thickness, the maximum damping capacity offered by the conventional squeeze film damper is limited by the maximum value of the ring width and minimum possible oil film thickness that could be made available in the system under question. Maximum width of the ring is decided by the space constraints in the system and the minimum oil film thickness achievable is dictated by limitation in the fabrication techniques. Hence the maximum damping capacity obtainable in conventional squeeze film damper is limited to a particular maximum value. But, however increased damping capacity is preferable from rotor dynamics point of view. Keeping in view the limitation cited above, some modifications have been tried out in conventional squeeze film dampers in order to achieve higher damping potential and in this connection efforts are being made by several researchers from different countries on the ways and means of enhancing the damping capacity of such damper systems. One such modification, shown in figure 2, includes introduction of one single thin metallic foil(6) within the clearance space(5) so that multiple oil films which are continuous in nature could be obtained. The single metallic foil could be either single or multi wrap in nature. In literature, this configuration has been referred to as 'Multi Squeeze Film Dampers' (MSFD). Experimental efforts have shown that better damping potential could be obtained from multi squeeze film dampers as compared to conventional squeeze film dampers. Design of oil supply ports to flood the damper clearance becomes extremely difficult especially when the foil wrap is more than one. While information is available in plenty regarding several aspects of conventional squeeze film dampers (CSFD), the information on multi squeeze film dampers (MSFD) is very scanty and no researcher has tried the concept of obtaining multiple of multi Squeeze films. The damping capacity of conventional squeeze film damper is strongly dependent on the ring width and the squeeze film thickness, that is the radial clearance between the concentric rings and that damping is inversely proportional to the film thickness raised to the power of little over three. In most of the cases the ring width cannot be increased due to the space constraints as in modern gas turbine engines. Hence in order to obtain enhanced damping capacity, the required squeeze film thickness becomes extremely small leading to a very small tolerance band on the film thickness resulting in manufacturing difficulties. The present device allows for large tolerance bands on the overall radial clearance and in presence of multiple foils introduced, would provide higher damping as compared to conventional squeeze film damper / multi squeeze film damper having equivalent conventional squeeze film thickness. As for the multi squeeze film dampers are concerned there are two major drawbacks. The first one is that the oil film is continuous in nature leading to a reduction in the resistance offered by the damping medium to the vibratory motion as a result of the external force due to residual unbalance force. This is true whether the foil is single wrapped or multi-wrapped in nature. The second drawback is that, it is extremely difficult to design oil supply ports so that the oil film is sustained all through the squeeze film clearance since the film is continuous in nature. This is especially true if the single foil is multi-wrapped. The main objective of the present invention is to provide an improved device useful for external damping in high speed rotating machinery which obviates the drawbacks of the hitherto known devi ces. Another objective of the present invention is to provide multiples of multi oil films for enhanced damping in rotating machinery. Yet another objective of the present invention is to provide discontinuous multiple films which results in enhanced damping in rotating machinery. The device of the present invention aims at providing further improved damping as compared to the conventional squeeze film dampers or multi squeeze film dampers using the concept of introducing discontinuous multiple films with the help of plurality of metallic foils. Oil supply ports may be as many as the number of foils and may or may not be equispaced which will be much easier for positioning of the oil supply ports which are functionally more efficient unlike in MSFD. Two schematic diagrams of the embodiments of the improved device of the present invention are depicted in figures 3 and 4 of the drawings accompanying this specification. In figure 3, is shown the multiple multi squeeze film damper with two foils(6) introduced in the damper clearance(5) resulting in formation of two discontinuous oil films and in figure 4, is shown similar damper with three foils(6) introduced forming three discontinuous oil films. The introduction of foils result in multiple discontinuous oi1 fi1ms. Accordingly the present invention provides an improved/ device useful for high speed rotating machinery which comprises an inner ring(3) housing a conventional bearing supporting a rotating shaft, vibration of which need to be damped, /the said inner ring(3) being housed inside an outer ring(4) to provide a clearance space(5) the said space (5)being provided with a plurality of foils(6)one end of the said foils(6) is fixed to the outer surface of the inner ring(3) and the other end being fixed to inner surface of the^outer ring(4), the said outer ring(4) being provided with plurality of oil supply ports(7). In an embodiment of the present invention the plurality of foils fixed between the inner and outer rings may or may not be placed equidistantly. In another embodiment of the present invention the oil supply port may or may not be as many as number of foils and may or may not be equispaced. The introduction of one foil, single or multiwrapped results in radially multiple films which are circumferential1y continuous without any seperation. The number of radial multiple films depend on the number of wraps the single foil winds the inner ring. If the single foil is wrapped once, i.e., one full round around the inner ring connecting outer and inner rings, then two oil films are formed along the radial direction, which would be one continuous film along circumferential direction without any seperation. Likewise, if the single foil is wrapped thrice like a torsional or clock spring, there will be four oil films in radial direction, but the oil film is continuous along the circumferential direction. The introduction of plurality of foils results in multiple oil films, each film totally separated by the foil from the other in both radial and circumferential directions. Thus distinctly multiple discontinuous films are obtained unlike in MSFD. The film thickness of each oil film being much smaller (compared to the oil film thickness in conventional or multi-squeeze film damper) results in higher damping capacity. The formation of plurality of squeeze films which are distinctly multiple and discontinuous in nature in the damper because of introduction of plurality of foils is the main underlying principle in obtaining higher damping capacity. The present device allows for a large tolerance bands on the overall clearance. Oil supply ports(7) may or may not be positioned equispaced as the placement of foils are. The performance of a squeeze film damper can be defined quantitatively in terms of the damping capacity offered by it or the extent to which amplitude of vibration in the bearing plane would be controlled. The performance of the existing dampers and the present design have been obtained and compared in terms of attenuation of vibrations of a test rotor when the rotor was supported on these dampers. Vibrations of the rotor was monitored for a known induced unbalance excitation when the rotor was successively supported on conventional squeeze film damper, multi squeeze film damper and the present design i.e. multiple multi squeeze film damper respectively. The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the present invention. Examples Two typical configurations were tried out using the concept of introducing plurality of foils in order to obtain enhanced damping. a) In the first configuration, the nominal squeeze film thickness of the conventional squeeze film damper was kept at 0.450mm. Tests were conducted and data obtained in terms of amplitude of vibration. Multi squeeze film damper configuration was obtained by introducing a single foil and similar tests were conducted. The case of multiple discontinuous oil film was obtained by introducing two foils and vibration levels monitored. The relative vibration levels are shown in figure 5 from which it is very clear that the enhanced damping obtained through introduction of plurality of foils has attenuated the vibration levels. b) In the second configuration, the nominal squeeze film thickness of the conventional damper was kept at 1.00mm. Multi Squeeze Film Damper was obtained by introducing a single metallic foil. Multiple discintinuous oil films were obtained by introducing plurality of foils. Introduction of two foils and three foils were tried out. Relative vibration levels of conventional squeeze film damper, multi squeeze film damper and the configuration obtained through introduction of plurality of foils ( in this case two foils and three foils ) are shown in figure 6. It is very clear that larger attenuation of vibration levels have been obtained from the improved damping device. Advantages The main advantages of this invention is allowing of larger tolerance bands in machining of the squeeze film damper components in view of the large clearance that would be permitted as compared to film thickness in conventional damper and or multi squeeze film damper, still obtaining enhanced damping levels. In the conventional squeeze film dampers it is extremely difficult to vary the damping contribution which could only be done in a small range by way of monitoring the external oil supply pressure. In the present invension, this difficulty can be overcome by varying the number of foils within the squeeze film clearance so that the damping contribution from the squeeze film damper could be discretely varied. We Claim: 1. An improved damping device useful for high speed rotating machinery which comprises an inner ring (3) housing a conventional bearing, supporting a rotating shaft vibration of which need to be damped, characterised in that the said inner ring (3) being housed inside an outer ring (4) to provide a clearance space (5), the said space (5) being provided with a plurality of foils (6), one end of the said foils (6) is fixed to the outer surface of the said inner ring (3) and the other end being fixed to inner surface of the said outer ring (4), the said outer ring (4) being provided with plurality of oil ports (7). 2. An improved damping device as claimed in claims 1 wherein the said oil supply port same as number of foils and be equispaced. 3. An improved damping device useful for high speed rotating machinery substantially as herein described with reference to figure 3 to 6 of the drawings accompanying this specification.

Documents:

1872-del-1997-abstract.pdf

1872-del-1997-claims.pdf

1872-del-1997-complete specification granted.pdf

1872-del-1997-correspondence-others.pdf

1872-del-1997-description (complete).pdf

1872-del-1997-drawings.pdf

1872-del-1997-form-1.pdf

1872-del-1997-form-19.pdf

1872-del-1997-form-2.pdf