New Way Air Bearings Offers Bently Balance Bearings for Improved Rotor Balancing Operations

For both hard and soft balancing machines, mechanical, electrical and audible noise in balancing operations is reduced by supporting the rotor being balanced on air bearings. In “soft bearing” balance machines the unbalance force from a rotating rotor is transmitted though hard rollers, to a flexibly mounted yoke, and it is the resulting motion of the yoke that is measured and indicates the balance error. The “soft bearing” term refers to balance machine designs where displacement of the shaft is measured along an intentionally soft axis. But since the rotor is still supported in rotation by hard rollers, (Illustration 1) it is not just the rotor that is turning, there are other motions that cause rotor motions and mask out of balance force.

For instance, combined errors from runout of the four supporting rollers cause asynchronous motion errors of the supported rotor that often exceeds .001 of an inch. Alternatively, with the rotor supported on air bearings, (Illustration 2 and Photo 1), the roundness of the journal (often .0001 of an inch or better) is the only rotating reference surface and so the motion errors will be synchronous and averaged, so there will be less than one tenth of amplitude of the hard rollers’ motion errors. This results in a clean signal that is exactly the same each revolution, allowing for a finer resolution in balancing.

A rotor supported on rollers will also see some axial forces from misalignments of the rollers. Changes in this axial force, say from changes in unbalance force, are likely to be almost synchronous, but out of phase enough to obscure balance resolution. Air bearings impart zero axial force on the rotor, avoiding this error source. The radial friction is also near zero and so the heavy spot finds the low point of rotation when the air pressure is supplied to the air bearings. This improves the resolution in static balancing by an order of magnitude, because the static breakaway coefficient of friction of air bearings is 10 times lower than that of rolling elements for a typical roller stand, or horizontal-way static balancing set-up.

In high speed or “at speed” balancing, the stiffness and damping function of the support bearings becomes critical for good balancing (Photo 2). At least partly for this reason, large rotors are often balanced on their operational hydrodynamic oil bearings, although hydrodynamic oil bearings can make an oily mess of the rotor in vacuum environments. This is especially an issue for motors and generators with electrical windings, but oil is still a mess for any bladed roller. The air bearings eliminate the need to flood, contain and collect oil in vacuum, yet use so little air that vacuum pressure in the bunker is not affected.

Air bearings can be made to closely match the stiffness and damping of oil bearings. This is a challenge mainly because the stiffness and damping of oil bearings is dependent on rotor speed, and so will be different progressing though critical speeds. The stiffness and damping coefficient will also change based on the varying temperature of the oil. When looking for the axial unbalance distribution, and how it aligns with the mode shapes the rotor needs to traverse in operation, it is useful to support the rotor on adjustable stiffness balance bearings. Although speed has little effect on stiffness and damping in externally pressurized air bearings, input pressure is a first order effect, and can be used to adjust bearing stiffness at speed. Don Bently frequently advocated for varying the input pressure of externally pressurized support bearings as a means of dynamically tuning machines. This adjustability helps in getting the right stiffness and damping at different speeds, and in testing for sensitivities to changes in stiffness when confirming rotor dynamic models.

New Way provides “bolt-on” adaptive hardware for implementing air bearings into existing low speed balance machines (Illustration 3). The adaptive hardware, in turn, accommodates New Way’s gas bearing hardware, which is easily assembled to the adaptive hardware. In the image shown below, the balancing machine’s yoke is shown on the right hand side, and New Way’s adaptive hardware and gas bearings are shown on the left hand side. The adaptive hardware uses the same mounting features as the original hard rollers.

To further simplify air bearings for balance machine applications, New Way has designed the bearing pads to “snap on/snap off”, as shown in the image below (Illustration 4). This feature allows for several varying sizes of bearing pads to be easily switched out, allowing the balance machine operator to easily balance rotors with different journal sizes without having to adjust or plumb air into bearings (see video below).