|Title of Invention||
AN IMPROVED SQUEEZE FILM DAMPER USEFUL FOR EXTERNAL DAMPING IN HIGH SPEED ROTATING MACHINERY
|Abstract||The present invention provides an improved squeeze Film Damper useful for external damping in high speed rotating machinery. The device of the present invention particularly provided a perforated Foils Oil Squeeze Film Damper useful for external damping in high speed rotating machinery mounted on rolling element bearing.|
|Full Text||The present invention relates to an improved Squeeze Film Damper useful for external damping in high speed rotating machinary. The device of the present invention particularly relates to a Perforated Foil Oil Squeeze Film Damper useful for external damping in high speed rotating machinary 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 non - metallic. The damping action prevents energy accumulation in machinary due to vibration that may become dangerous or unhealthy, if allowed. This approach is adopted and often incorporated in machinary designs to dissipate such energy accumulated. This increases the life of machinary and ensures its safe operation. The safety of the operator and installation are also ensured.
Any high speed rotating machinary supported on rolling element bearings requires external damping device 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
drrosions 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 excitation frequency affect the visco - elastic 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 temperature encountered is relatively higher.
Squeeze film dampers, which use oil as the damping medium are being extensively used in modern high speed gas turbine engine 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 la & lb of the drawings accompanying this specification Conventional Squeeze Film Damper essentially consists of 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 bjeaxing(2) which in turn supports the rotating shaft(l). The outer ring(4) is fixed to the engine casing. The vibratory motion of the engine shaft(l) 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 system.
One such modification, shown in figure 2a, 2b includes introduction of one or more thin metallic foils(6) within the clearance space (5) so that multiple oil films which are circumferentially continuous (as in fig. 2a) or discontinuous (both radially and circumferentially as in fig. 2b) in nature depending on whether single or multiple foils are used could be obtained. The metallic foils could be either single or multi wrap in nature. In literature, this configuration has been referred to as 'Multi
Squeeze Film Dampers' (MSFD) when single foil is used and 'Multiple Multi Squeeze Film Damper (MMSFD) when plurality of foils are used. Experimental efforts have shown that better damping potential could be obtained from the above introduction of foils as compared to the 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 which may lead to oil starvation in some pockets of the oil domain. While information is available in plenty regarding several aspects of conventional squeeze film damper (CSFD), the information on multi squeeze film dampers (MSFD), multiple multi squeeze film dampers (MMSFD) is very scanty.
The damping capacity of conventional squeeze film damper is strongly dependent on ring width and the squeeze film thickness, that is the radial clearance between the concentric rings 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. Although larger tolerance bands are allowed in MSFD,MMSFD, there are chances that oil may not flood the entire damper clearance especially when the foils are multi wrapped. Also since multiple films created is limited to the number of foils
which could be inserted in the clearance, damping obtainable from MMSFD reaches maximum value. The present device allows for large tolerance bands on the overall radial clearance as in MSFD and MMSFDand in presence of Perforated multiple foils introduced, would provide relatively higher damping as compared to conventional squeeze film damper / multi squeeze film damper and multiple 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 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 in case of MMSFD also if the foils are multi-wrapped.
In our patent application number 1872/Del/97 we have described a Multiple Multi Squeeze Film Damper wherein plurality of foils are introduced in the clearance space, thereby obtaining multiple discontinuous films. We have claimed that in the said patent application that the invention provides enhanced damping levels leading to decrease in vibration levels because of the
existance of multiple discontinuous films in the clearance space. We have claimed that the main disadvantage of having a single continuous as refered in the article Tribology Transactions,Vol 34, 1991, titled " Advanced Multi Squeeze Film Dampers for Rotor Vibration Control " has been improved by obtaining multiple discontinuous by way of providing plurality of foils. Although the above drawbacks are removed in multiple multi squeeze film dampers by way of obtaining discontinuous oil films, difficulties are faced when higher number of foils are to be introduced in the clearance space from the point of view of obtaining enhanced damping. Also the probability of some region in the damper clearance being starved of oil is not completely ruled out.
The main objective of the present invention is to provide an improved squeeze film damper useful for external damping in high speed rotating machinery which obviates the drawbacks of the hitherto known devices.
Another objective of the present invention is to provide multiples of multi oil films each of which is connected with the other through perforations for enhanced damping in rotating machinery.
Yet another objective of the present invention is to provide almost zero 'no oil region'.
The device of the present invention aims at providing further improvement as compared to the conventional squeeze film dampers or
multiple multi squeeze film dampers using the concept of introducing multiple perforations in the 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.
The schematic diagrams of the embodiment of the improved device of the present invention are depicted in figures 3a, 3b and 3c of the drawings accompanying this specification. In figure 3a, is shown the multiple multi squeeze film damper with two perforated foils (6) introduced in the damper clearance (5) resulting in formation of discontinuous oil films. The developed view of a typical perforated foil is shown in figure 3c of the drawings accompanying this specification. The introduction of perforations in the foils result in enhanced damping as compared to multiple multi squeeze film damper.
Accordingly the present invention provides an improved Squeeze Film Damper as shown in the figure 3b of the drawing accompanying the specification, useful for external damping in high speed rotating shaft, the said damper comprising an inner ring (3) to be mounted on a conventional bearing (2) that support a rotating shaft (1) of any machine , vibration of which need to be damped, the said inner ring (3) being housed inside an outer ring (4) having a clearance space (5), the said clearance space (5) being provided with a plurality of foils (6) , one end of the said foils (6) being fixed to the outer surface of the inner ring (3) and the other end being fixed to inner surface of the outer ring (4), said outer ring (4) being provided with plurality of oil supply ports (7) , wherein the said plurality of foils characterized with perforation in them .
In an embodiment of the present invention the plurality of perforated foils fixed between the inner and outer rings may or may not be placed eguidistantly.
In another embodiment of the present invention the oil supply ports may or may not be as many as number of foils and may or may not be eguispaced.
In yet another embodiment of the present invention the perforations may or may not be equispaced, may or may not be uniform in terms of distance between the perforations or diameter / area of the perforations.
In still another embodiment of the present invention the perforations may or may not be circular and may be of any shape.
In another embodiment of the present invention the number of perforations may or may not be same in each foil.
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 that of the present invention 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, multi squeeze film damper, multiple multi squeeze film damper and that of the present invention, i.e., perforated 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.
Three typical configuration were tried out using the concept of introducing single and plurality of perforated foils and various area of perforations in order to obtain enhanced damping.
In the first configuration, tests were conducted on conventional, multi squeeze film damper ( with one non perforated foil introduced in the clearance) and one Perforated Foil Squeeze Film damper ( with one perforated foil introduced in the clearance). The relevant results in terms of amplitude of vibration as a function of excitation frequency is shown in figure 4a of the drawings accompanying this specification. It is clear from the figure 4a that the enhanced damping is obtained when perforations are introduced in the foil inserted in the damper because amplitude of vibration has been reduced, at 2500 rpm amplitude is 40 micron as against 120 micron in case of non perforated foil used in same damper. The details of parametric values under
which the said study was conducted is shown in figure 4a accompanying this specification.
Example - 2
In the second configuration, tests were conducted on conventional, multiple multi squeeze film damper (with two non perforated foil introduced in the clearance) and Perforated foils Squeeze film Damper (with two perforated foils). The relevant results in terms of amplitude of vibration as a function of excitation frequency is shown in figure 4b of the drawings accompanying this specification. It is very clear that enhanced damping is obtained when perforations are introduced in the foils inserted in the damper indicating clearly effect of number of rows perforations also. At the speed 2500 rpm amplitude of vibration is 10 micron and 15 micron respectively in case of 3 rows and 5 rows of perforation as against 120 micron in ease non perforated foil is used in same damper.
Example - 3
In the third configuration, tests were conducted by varying area perforations in Perforated Foils Oil Squeeze Film Damper ( number of perforations are different). The relevant results in terms of amplitude of vibration as a function of perforated area is shown in figure 4c. of the drawings accompanying this specification. From this it is clear that enhanced damping is obtained when perforated area is varied in the foil inserted in the damper and that the perforated area should be optimized from maximum damping potential point of view.
The advantages of the present invention are,
The main advantage of this invention is in 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 although the tolerance bands are similar to that in multi squeeze film damper or multiple multi squeeze film damper, still obtaining enhanced damping levels compared to multiple multi squeeze film damper.
2. 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 invention, this difficulty can be overcome by varying the number of perforations in each foil and number of perforated foils within the squeeze film clearance so that the damping contribution from the squeeze film damper could be discretely varied.
1. An improved squeeze Film Damper as shown in the figure 3b of the drawing accompanying the specification, useful for external damping in high speed rotating shaft, the said damper comprising an inner ring (3) to be mounted on a conventional bearing (2) that support a rotating shaft (1) of any machine , vibration of which need to be damped, the said inner ring (3) being housed inside an outer ring (4) having a clearance space (5), the said clearance space (5) being provided with a plurality of foils (6) , one end of the said foils (6) being fixed to the outer surface of the inner ring (3) and the other end being fixed to inner surface of the outer ring (4), said outer ring (4) being provided with plurality of oil supply ports (7) , wherein the said plurality of foils characterized with perforation in them .
2. An improved Squeeze Film Damper useful for external damping in high speed rotating machinery substantially as herein described with reference to the drawings accompanying this specification.
|Indian Patent Application Number||539/DEL/1999|
|PG Journal Number||37/2010|
|Date of Filing||08-Apr-1999|
|Name of Patentee||COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG NEW DELHI-110001, INDIA|
|PCT International Classification Number||F16C 27/00|
|PCT International Application Number||N/A|
|PCT International Filing date|