Today, energy efficiency is more important than ever. When engineers consider how much energy their machines may use over its lifetime, it is clear; the machine’s efficiency plays a large role in operational sustainability. Friction losses are some of the lowest hanging fruit when trying to improve machine efficiency. Air Bearings are an old technology, but may offer new ways for engineers to dramatically reduce friction.
Most of the frictional losses in a machine are from their bearings. Plane sliding bearings, characterized by coulomb friction, with a coefficient of friction about 0.1, gave way to rolling element bearing technology with a coefficient of friction roughly 2 orders of magnitude less. Fluid film bearings are on the horizon though; bearings that float on a pressurized film of oil, water or air. These are characterized by viscous shear friction and can have coefficients of friction two or three times lower than rolling element bearings, around 0.00001.
Rolling element bearings have become ubiquitous. They have huge load capacities relative to their weight and size and have enabled all types of equipment. They freed motion in the 20th century. Still there are limits to how far they can be pushed. The reason for their low friction is the very low surface contact area. This becomes a disadvantage with high loads as the point or line contacts inherent in rolling bearings experience very high unit area loading, as there is theoretically no area. The main issue though is the faster you try to spin a rolling bearing, the more power it requires.
Let us consider the example of a paper converting line with 20 or more rollers, each with their own motor. Oftentimes here the motors are doing little work on the paper; the work is just keeping the rollers spinning and overcoming the roller bearing friction, which requires significant power to do so.
It is here that a fluid film bearing such as an air bearing, with a coefficient of friction two orders of magnitude less, could reduce the friction by almost 100%. Yes, the motors would do work to accelerate the roller, but once at speed, it takes essentially zero work to keep the rotor at speed. 2 orders of magnitude less friction reduces over 99% of the energy use.
It is recognized that in many applications the friction losses are a small percentage of the total work being done. But even in these cases, reducing the frictional losses can be significant. For instance a 50 MW centrifugal compressor in an LNG pipeline application may only have only three percent loss due to bearing friction, oil cooling, filtering and pumping. This still amounts to 1.5MW of power nearly 24 hours a day, 365 days a year, being lost to friction.
Gas bearings for a 50MW compressor are of a custom design, but there are a number of sources for standard, off the shelf air bearing products which can be integrated into machine designs quickly, allowing for rapid retrofitting with minimal downtime.
Standard air bushings allow for rotary and linear motion, fitted on standard size shafting and into standard size pillow blocks. Using O rings between the bushing and the pillow block provides compliance for self-alignment, easing tolerance requirements and providing damping. Air bearings may also be hard mounted. They are great for replacing plane and rolling element linear bearings, eliminating oil lubrication and in dusty environments the air pressure exiting the air bushing acts to self-clean the bearing face..
The classic flat pad air bearing provides frictionless motion in the XY plane but high stiffness in Z. They run on any flat surface; granite is a common option. Air bearings and granite enabled the birth of the coordinate measuring machine, as the stability of the granite and the accuracy of the air bearings allow coordinate measuring machines to be 10 times more accurate than conventional machine tools.
Radial air bearings constrain motion from an OD or an ID, and so are offered in concave or convex configurations. These bearings are game changers, able to eliminate center shafts and 360-degree steel races. Typically, radial rolling element bearings have an inner and outer race that goes 360° around the rotating body. These very hard races and the rolling elements they contain need to be fit with a high degree of precision. This has been the exclusive domain of bearing manufacturers. Because radial air bearings support the rotor from a small percentage of the circumference, the relative size between the rotor and the radius on the bearing is not very critical. This means you can buy the radial air bearings and machine bearing surfaces on the rotor yourself. The bearings mount on spherical ball gimbal seats so they are self-aligning to the rotor. Because the gimbals are on threaded studs, they are adjustable, so no precision is required on the stator to mount the bearings, just a threaded hole for the studs. This is in contrast to the 360-degree flat mounting surface with many taped holes required by a large roller bearing or slewing ring.
Rotor manufacturing is simplified also, as surfaces on the rotor can be used as an air bearing surface and supported directly by the radial bearings. The rotor may be aluminum, carbon fiber, glass or ceramic, any material that can be finished to a 16rms. This means no shrink fitting or bolting a steel bearing race to your rotor.
The modular radial air bearings also have the advantage of being kinematically correct. This is to say they are consistent with the exact constraint theory. Therefore, three radial air bearings could be used to constrain an axis of rotation for the same reason that a three-legged bar stool cannot rock. Modular flat air bearings from New Way can be used to constrain axial motion in the same way. It is also convenient that because the exact force paths are known through the bearings and into the structure, FEA analysis is simplified by neat, closed form equations.
Air bearings have a huge speed advantage over rolling element bearing, they are silent, light weight and are available in weeks instead of the years it may take for a custom rolling element slewing bearing.
There are linear guide air bearings and rotary table air bearings. They have the same functionality of their rolling element kin, but significantly better smoothness, precision and especially low friction. You can buy them with motors and encoders already integrated or buy just the air bearing and do the integration yourself. The Air Way linear guides pictured are especially convenient for engineers as they are purchased and bolted down just like conventional rolling element linear guides.
The advantages of air bearings go on and on, but remember it is the near zero friction motion, enabled by air bearings, that can move the needle on sustainability.
Author: Drew Devitt