Spring devices – Resilient shock or vibration absorber
Reexamination Certificate
2000-05-23
2002-08-13
Schwartz, Christopher P. (Department: 3613)
Spring devices
Resilient shock or vibration absorber
C267S141300, C267S293000
Reexamination Certificate
active
06431530
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates in general to the field of vibration isolation and in particular to a vibration isolator incorporating fluid and elastomeric elements to effectively eliminate the transmission of certain vibrational frequencies into structural components.
BACKGROUND OF THE INVENTION
For many years, effort has been directed toward the design of an apparatus for preventing the transmission of vibration from one vibrating body to another body. Such devices are useful in a variety of technical fields in which it is desirable to isolate the vibration of an oscillating or vibrating device, such as an engine, from the remainder of the structure. Typical vibration isolation and attenuation devices (“isolators”) employ various combinations of the mechanical system elements to adjust the frequency response characteristics of the overall system to achieve acceptable levels of vibration in the structures of interest in the system. One field in which these isolators find a great deal of use is in aircraft, wherein vibration isolation systems are utilized to isolate the fuselage or other portions of an aircraft from mechanical vibrations which are associated with the propulsion system and which are generated by the engine, transmission, propellers, rotors, or proprotors of the aircraft.
Vibration isolators are distinguishable from dampening devices although dampening devices are often erroneously referred to as isolators. As an illustration, a simple force equation for vibration is set forth as follows:
mx+cx+kx=F
A true vibration isolator utilizes acceleration of a fluid body (mx) to cancel the displacement of vibration (kx). In contrast, a dampening device is concerned with restricting flow of a fluid or other body and thus velocity (cx), and does not cancel vibration, but merely absorbs its energy.
Minimization of the length, weight and overall size of the isolation device is an important consideration in the design of an aircraft vibration isolation system. This minimization is particularly important in the design and manufacture of helicopters, which are required to hover against the dead weight of the craft and which are in many ways more constrained in their payload than fixed wing aircraft.
A marked improvement in the field of vibration isolation, particularly as applied to aircraft and helicopters, was disclosed in commonly assigned U.S. Pat. No. 4,236,607, entitled “Vibration Suppression System,” issued Dec. 2, 1980 to Halwes, et al., and which is incorporated herein by reference. This patent discloses a vibration isolator in which a dense, low-viscosity fluid is used as the “tuning” mass to counterbalance and cancel oscillating forces transmitted through the isolator. This isolator employs the principle that the acceleration of an oscillating mass is 180 degrees out of phase with its displacement to cancel the transmission of undesirable motion.
Halwes, et al. recognized that the inertial characteristics of a dense, low-viscosity fluid, combined with a hydraulic advantage resulting from a piston arrangement, could harness the out-of-phase acceleration to generate counter-balancing forces to attenuate or cancel vibration.
Although the Halwes device was a significant improvement in the art of vibration isolation, there remains in the field a continuing demand for improvements allowing for a reduction of the weight of such isolators without sacrificing the ability to attenuate or cancel vibration. Additionally, with the continuing emphasis on energy efficiency in transportation, there is a continuing demand for reduction in the weight of vibration isolators. At the same time, customers continue to demand more performance at a lower price, both in vehicles and in replacement parts, giving rise to a need for an isolator that can be manufactured at a lower cost.
SUMMARY OF THE INVENTION
The present invention disclosed herein comprises an improved vibration isolator designed to overcome many of the shortcomings inherent in prior devices. In many embodiments, the vibration isolator is smaller in scale than prior designs, facilitating more versatility with respect to design options. Additionally, many embodiments of the present vibration isolator weigh significantly less than prior designs. At the same time, many embodiments of the present invention can be manufactured at a significantly lower cost than prior isolators.
One embodiment of a vibration isolator of the present invention comprises an inner cylinder and one or more outer cylinders concentrically bonded together with elastomers to form two chambers that are joined by a tuning port. The elastomer serves both as the seal for the chamber and the compliant spring member in the isolator. The chambers and tuning port are filled with an inviscid fluid and pressurized to prevent cavitation.
One embodiment of an isolator according to the present invention incorporates a central elastomeric spherical bearing and two elastomeric tubeform bearings, one at each end. The dimensions of the tubeform bearings can vary according to the demands of a particular application, but the design must be sufficient to minimize elastomer bulging caused by oscillatory pressure in the device.
As the inner cylinder moves within the isolator, the volume of one chamber will increase as the other decreases. This change in volume creates a pressure differential between the chambers and a corresponding flow of the inviscid fluid from one chamber to another. In embodiments having a tuning port through the center of the inner cylinder, the movement of fluid will be in the opposite direction of movement of the inner cylinder. This movement of fluid causes an inertial force to be generated. Within a selected range of frequencies, this inertial force substantially or completely cancels out the elastomeric spring force in the isolator.
In order to stabilize internal fluid pressures, fluid and elastomer thermal expansion is accommodated in certain embodiments through the use of an integral volume compensator. The volume compensator alleviates the accumulation of excessive pressure and the risk of cavitation that would otherwise exist due to pressure changes caused by operation of the isolator across a broad range of temperatures. In certain embodiments, this compensator takes the form of an air spring filled with a gas such as nitrogen. In one embodiment, the air spring does not require a barrier between the gas and the fluid.
Additionally, this embodiment of the isolator communicates fluid pressure to the volume compensator via a small diameter orifice. The size of the orifice is such that the pressure pulses caused by oscillation of the inner cylinder do not pass into the volume compensator in any significant degree. With this design, the orifice acts as a fluid pressure filter, transmitting static pressure changes into the volume compensator while at the same time blocking pressure oscillations.
In one embodiment, damping within the system is minimized through the use of an elastomer having low damping characteristics, through the use of an inviscid fluid within the device, and through the selection of a hydraulic area ratio having a relatively low value.
The fluid used may vary from one embodiment to another, but it is desirable that the fluid have a low viscosity and be noncorrosive. Similarly, the elastomer used for the isolator bearings can vary, but it is desirable that the elastomer have a long fatigue life and exhibit low damping characteristics.
REFERENCES:
patent: 3769672 (1973-11-01), Eklund
patent: 3777672 (1973-12-01), Schneider
patent: 3782854 (1974-01-01), Rybicki
patent: 4236607 (1980-12-01), Halwes et al.
patent: 4651980 (1987-03-01), Morita et al.
patent: 4739962 (1988-04-01), Morita et al.
patent: 4811919 (1989-03-01), Jones
patent: 5174552 (1992-12-01), Hodgson et al.
patent: 5312093 (1994-05-01), Smith et al.
patent: 5374039 (1994-12-01), Schmidt et al.
patent: 5435531 (1995-07-01), Smith et al.
patent: 5439082 (1995-08-01), McKeown et al.
patent: 5439204 (1995-08-01), Yamazoe et al.
patent:
Braswell, Jr. James Lee
Ledbetter Timothy Kent
Smith Michael Reaugh
Stamps Frank Bradley
Bell Helicopter Textron Inc.
Emanuelson Kenneth T.
Gardere Wynne & Sewell LLP
Schwartz Christopher P.
Warren, Jr. Sanford E.
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