1. Field of the Invention
The present invention relates generally to vibration damping and isolation and more particularly to vibration damping and isolation for devices having extremely low vibration tolerances. Specifically, this invention relates to a compound diaphragm bellows which hermetically seals both ends of a rigid volume damper.
2. Discussion of the Related Art
Vibrations are often harmful to mechanical devices. Vibration control is important in practically any high precision structure containing or contacting moving parts. Take telescopes for example. Telescopes have stringent aiming requirements. Even the small vibrations caused by imperfections in the ball bearings or the race supporting the aiming system are highly undesirable.
Several vibration control systems have been developed. Two common methods of vibration control are viscoelastic damping and viscous fluid damping. Viscoelastic materials can provide damping for many applications; however, they exhibit cyclic wear and are excessively sensitive to temperature and other environmental conditions.
Viscous fluid dampers avoid some problems associated with viscoelastic dampers. Viscous fluid dampers rely on fluid shear forces to absorb vibrational energy. One such viscous damper is described in U.S. Pat. No. 4,760,966, which was issued to Davis on Aug. 2, 1988 and is assigned to Honeywell Inc.
The viscous damper described in U.S. Pat. No. 4,760,996 is essentially cylindrical. A shaft is disposed along the axis of the damper, and at either end of the shaft is attached a cylindrical hub with an outer radius greater than that of the shaft. A piston, having an aperture or bore slightly larger than the shaft, is coaxial with the shaft with the shaft passing through the piston bore. A flange extends radially from the piston and is adapted for connection to an external load. One of the hubs is also adaptable for connection to an external load. Two sets of cylindrical bellows connect the hubs to the flange extending radially from the piston. The bellows, hubs, and the piston form a first chamber and a second chamber.
When an axial force is applied to either the piston flange or the hubs, the piston moves relative to the shaft. This relative movement forces fluid from one chamber through the passageway between the piston and the shaft and into the other chamber. As the fluid passes through the narrow passageway between the shaft and the piston bore, viscous fluid shear occurs. To generate effective viscous fluid shear forces, the passageway between the piston bore and the shaft must be relatively narrow. Viscous fluid shear dissipates the energy conveyed into the system and thus dampens the vibration.
Not all forces applied to the damper are axial. Radial forces may enter the damper through the piston flange or the hubs. Unchecked, these radial forces might cause the shaft to contact the piston bore. Such contact is undesirable, creating friction and wear and perhaps engagement between the piston and shaft requiring more force to overcome than the damper is designed to handle. The bellows connecting the flange and the hubs prevent some radial movement of the piston relative to the shaft.
Radially stiff bellows are desirable to maintain the axial alignment of the damper members. While radial stiffness is desirable, axial stiffness is not. Relatively free axial piston movement is necessary for effective viscous fluid damping.
Although the damper disclosed in U.S. Pat. No. 4,760,996 provides many advantages, it did not solve all of the problems now appreciated. For example, a bellows resistant to distortions and deflections caused by relatively small radial forces is desirable. Further, it is recognized that the bellows disclosed in U.S. Pat. No. 4,760,996 tend to expand when fluid pressure within the damper exceeds ambient pressure. Such expansion robs the damper of efficiency.
On Dec. 15, 1992, a patent application entitled "Rigid Volume Viscous Damper," U.S. Ser. No. 07/991,025, was filed in the name of L. Porter Davis. The rigid volume viscous damper disclosed by Davis effectively eliminates the fluid pressure acting upon the bellows by encasing the piston between rigid bushings thereby providing advantages over the disclosure of U.S. Pat. No. 4,760,996. In one embodiment, Davis discloses a rigid chamber formed by two rigid bushings and a piston within a housing. A rod extends axially from both sides of the piston. Each rod passes through an aperture in the bushings and is thereafter connected to a diaphragm hub. A thin diaphragm connects each hub to a damper housing which encases the bushings. Each diaphragm hermetically seals the damper housing.
The bushings are connected to the damper housing which is in turn connected to a load. At least one diaphragm hub is connected to ground. An axial force acting upon the damper will cause the piston and rod assembly to move relative to the damper housing. The diaphragm connected to the damper housing is also connected to the rods extending axially from the piston. Therefore, the inner portion of the diaphragms moves with the piston relative to the damper housing while the outer portion of the diaphragms moves with the damper housing relative to the piston and rod assembly.
The relative axial movement of the piston forces fluid to move from a first compartment defined by the bushings and first piston face to the second compartment defined by the bushings and a second piston face. The fluid movement occurs through a passageway between the interior diameter of the bushings and the exterior diameter of the piston. As the fluid passes through this passageway, viscous fluid shear occurs which dissipates the energy brought into the damper.
U.S. patent application Ser. No. 07/991,025 discloses using two annular diaphragms, one at each end of the viscous fluid damper, which hermetically seal the damper housing. The diaphragms, made from thin sheets of titanium, are very effective in preventing radial movement of the piston relative to the rigid chamber. The titanium diaphragm is also compliant in the axial direction. However, the diaphragm's axial compliance decreases rapidly after a relatively short stroke. Axial compliance can be increased by increasing the radius of the diaphragm; however, in many applications available space is limited.
In an alternate embodiment, U.S. patent application Ser. No. 07/991,025 discloses a bellows replacing the diaphragm just described. The bellows connect either end of the rod and piston assembly to the viscous damper housing. These bellows permit greater axial deflection than the annular diaphragms described previously. Unfortunately, the bellows also permit greater radial deflection than the annular diaphragms.
The disclosures of U.S. Pat. No. 4,760,996 and those made in U.S. patent application Ser. No. 07/991,025 illustrate some of the problems confronting those in the damping art. High axial deflection in the bellows or diaphragms connecting the piston assembly to the housing permits a long stroke and thereby allows desirable damping of greater magnitude vibrations. It is also desirable for the diaphragms or bellows member to maintain radial stiffness. The previously known diaphragms or bellows with substantial radial stiffness have undesirable short strokes. Those diaphragms or bellows with a greater stroke have undesirable radial flexibility.