This invention relates generally to configurations used for forming statically pressurized gas bearings commonly utilized to facilitate precision translational and rotational motion in gas bearing slide and spindle assemblies, respectively. It is specifically directed toward solving the major design problem of applying statically pressurized gas bearings, that of achieving bearing stability in the direction orthogonal to the supported bearing surface.
Statically pressurized gas bearings are compensated by restricting the gas flow into each bearing. This can be accomplished with a flow restrictor located in, or a flow restrictor associated with, the gas inlet to the bearing so that the pressure of the gas in the bearing is reduced below the supply pressure. Gas pressure in the bearing is controlled by restricting flow out of the bearing, as determined by the bearing configuration and the inverse of the third power of the flying height of the bearing. Concomitantly, the gas pressure in the bearing is equal to the load supported by the bearing divided by the effective bearing area. Thus, the load supported by the bearing and the flying height of the bearing are related and mutually controlled in the manner of a closed loop servo system. The forward gain of this servo system is the ratio of the value of a small change in the supported load divided by the value of the related small change in the flying height and is commonly referred to as the spring stiffness of the bearing.
One of the methods commonly used to provide increased spring stiffness of the bearing is to increase the load supported by the bearing by incorporating a vacuum hold-down pocket within the overall bearing configuration. See U.S. Pat. No. 3,722,996 entitled OPTICAL PATTERN GENERATOR OR REPEATING PROJECTOR OR THE LIKE and issued Mar. 27, 1973, to Wayne L. Fox for an example of such a gas bearing. Such a design modification can have an effect on bearing stability as well. This effect can either be positive or negative depending on the other design characteristics of the gas bearing.
In a gas bearing, the spring stiffness of the bearing does not have a constant value with respect to a disturbing frequency. This is because of the compressibility of the gas in the bearing itself. The energy storage associated with this compressed gas, coupled with the gas flow resistances of the bearing, causes the spring stiffness of the bearing to vary in a complex manner, including both amplitude and phase variations. One component of the frequency-varying spring stiffness is out of phase with the characteristic damping coefficient of the bearing and effectively reduces bearing damping. Thus, it is quite possible for the algebraic sum of the characteristic damping coefficient and the out-of-phase component of the spring stiffness to become negative in character and cause the bearing to self oscillate.
Concomitantly, the frequency bandwidth of the amplitude of the spring stiffness of the bearing is limited by the amplitude variation referred to above. The volume of compressed gas in the bearing has the characteristics of a pneumatic capacitance. This capacitance coupled with the gas flow resistances of the bearing determines a time constant. The spring stiffness of the bearing rolls off in a square law fashion beyond the corner frequency determined by the time constant and the bearing is functionally inoperable beyond this corner frequency.
Gas bearing design is complicated by the fact that the corner-frequency-determining time constant and the algebraic sum of the desirable characteristic damping coefficient of the bearing and the undesirable out-of-phase component of the spring stiffness are both linearly related to the inverse of the third power of the flying height of the bearing. Thus, the simple "cut and try" methods often used in gas bearing design are inappropriate. What is needed is a more complete understanding of the myriad of factors that control gas bearing performance and a gas bearing design that optimizes these factors. This design should include an incoming flow restrictor and a pocket configuration ensuring optimal incoming and outgoing gas flow characteristics, respectively.
Accordingly, it is a principal object of this invention to provide a statically pressurized gas bearing having improved bearing stability characteristics.
Another object of this invention is to provide a statically pressurized gas bearing having increased operational frequency bandwidth.
Another object of this invention is to provide an improved incoming flow restrictor for a statically pressurized gas bearing.
Another object of this invention is to provide an improved bearing pocket configuration for a statically pressurized gas bearing.
Still another bearing of this invention is to provide an improved bearing pocket configuration for a statically pressurized gas bearing having a vacuum hold-down pocket feature.
These and other objects, which will become apparent from an inspection of the accompanying drawings and a reading of the associated description, are accomplished according to an illustrated preferred embodiment of the present invention by providing a statically pressurized gas bearing with a gas pocket shaped like a double cross formed as fine slots in the bearing surface with a minimum of resulting gas volume. Exhaust slots, centered between the arms of the double cross, are also formed in the bearing surface to minimize the outgoing flow resistance. Gas is supplied to the gas pocket from a source of pressurized gas through a hole and over an annular land of selected height, both of which are formed in the center of the double cross.
The aforementioned objects are also accomplished according to another illustrated preferred embodiment of the present invention by providing a statically pressurized gas bearing incorporating a vacuum hold-down pocket with multiple gas pockets formed as fine slots in the bearing surface with a minimum of resulting gas volume. The vacuum hold-down pocket is formed in the bearing surface and centered within the bearing surface area bounded by the slots forming the multiple gas pockets. Gas is supplied to each of the gas pockets from a source of pressurized gas though a hole, over an annular land and through an annular groove (disposed tangent to the slot forming the gas pocket), all of which are formed in the bearing surface opposite the vacuum hold-down pocket.