This paper reviews the advantages of air bearings for machine tools. It presents bearings and assembly processes that are robust and economical. Results from straightness, stiffness and dynamic response measurements comparing similar rolling element and air bearing ram type guide assemblies taken at N.I.S.T. are presented. In summary, it is worthwhile to reconsider air bearings for some machine tool applications.
In broad terms there are three technical solutions to the problem of mechanical motion in machine tools:
Early machine tools were based almost entirely on plain bearings. Gradually machine tools have been revolutionized by rolling element bearings. First, plain journals and bushings were replaced with roller bearings; then, Acme lead screws were supplanted by the ball screw. Today linear guides are replacing slide ways as state of the art technology.
The revolution from sliding to rolling bearings has reduced friction by an order of magnitude, enabled higher speeds and greatly reduced backlash.
The evolution of machine tools is most likely to continue with the next wave of technology being fluid film bearings. This is because fluid film bearings have the great advantage of being non-contact bearings. Non-contact means no wear, no friction (save for fluid drag) and higher accuracies because surface imperfections are averaged out. The combination of these advantages will be important in the construction of truly sub-micron accurate machine tools for the average shop.
The fluid film bearings can be oil, water or air. Oil and water based bearings have excellent damping properties due to squeeze film damping.
This resistance to vibration is called dynamic stiffness and is an important factor in surface finish quality, tool life and in reducing subsurface damage when grinding ceramics. High dynamic stiffness is also important for controls to function properly on high performance machine tools.
Air based bearings have traditionally been thought to have a low damping function. This paper discusses types of air bearings and new assembly techniques that produce dramatically higher damping functions in air bearings.
Types of Air Bearings
As air escapes from the orifice it expands, and so its pressure drops as it flows across the face of the bearing resulting in variances of pressure in the air gap.
Porous Carbon Type
As air diffuses through the whole surface there are no high or low pressure areas resulting in a more uniform pressure in the air gap.
Air bearings function on a controlled film of pressurized air between two closely matched surfaces. There are two difficulties of producing air bearings: producing the closely matched surfaces and controlling the air flow through the bearing. A common approach is to control the air flow with precision orifices. These orifices are strategically placed on the bearing and often combined with grooves to distribute pressurized air evenly across the bearing face. The orifice is sized to meter the flow into the bearing. If the bearing face becomes scratched across a groove or near an orifice, the volume of air that escapes may be more than the orifice can supply, causing the bearing to crash. Under normal operation, as air expands from the orifice or groove, it loses pressure and creates pressure gradients in the air gap.
An ideal air bearing design would supply air pressure equally across the whole face of the bearing and automatically restrict and damp the air flow to the face at the same time. This can be achieved by diffusing the air through a porous bearing face. After considerable testing, porous carbon has been found to be one of the best materials for this purpose. Bearings employing porous carbon are more robust and hence, more suitable for production environments.
Some of the natural advantages of porous carbon air bearings include:
The torturous passageways of the carbon restricts and damps the supply of air to the bearing face. This makes the bearing highly resistant to “pneumatic hammer” and small variations in in supply pressure.
The full face supply of air results in a more uniform pressure profile in the gap. Smaller air gaps can be used without fear or damage to the guide way do to the safety factor of carbon.
The natural lubricity of the graphite in carbon eliminates the possibility of damage to the guide ways. The bearing carbon itself is tolerant to scratches.
Porous carbon bearings have no restrictors or orifices to tune or adjust and they cannot be clogged by debris.
The methods used to mount air bearings are critical to their performance. The most common mounting methods are swiveling and match grinding.
Ball mounts have the advantage of allowing the bearing to self-adjust to parallel with its running surface and when the balls are mounted on threaded rod, geometric alignment and preloading become easy. This is a very common and practical way of mounting air bearings. The swiveling ball does have disadvantages though. A ball mounted bearing with a total of .5″ cannot have a diameter of more than 1.75″ without experiencing deflection from flat under a 50 lb. load. This bending of about 50 micro inches will have significant effect on the pressure profile under the bearing. When the air gap is 150 micro inches. This results in loss of stiffness and stability.
Bearings can be made to have a higher total height but this is limited by space requirements. Also, with ball mounted bearings, care must be taken at the ball mount to see that this does not become of compliance. This adds cost and complications, often requiring matched sets. The worst problem is that the ball is mounted on a threaded rod which in turn mounts through a web or plate on the structure, which acts like a diaphragm. Several analysts have shown this to be a major source of compliance in CMM structures.
Match grinding of air bearings surfaces and mounting surfaces eliminates the above mentioned problems but comes with itsown problems. The main problem here is manufacturing difficulties. Maintaining flatness, squareness and appropriate clearances between components on the order of several microns requires excellent equipment, skilled operators and more than a little patience. Even under the best of conditions, this method is limited to producing components smaller than a meter in size.
The vacuum replication process (patent protected) is both a technically and economically effective method of fitting air bearings in precision assemblies. In this process, individual air bearings are plumbed to vacuum and placed on an accurate guide or ram. The stage or housing is then positioned over the bearings and aligned to a precise geometric alignment on jack screws or gage blocks. A small gap then exists between the back of the air bearings and the stage or housing. A replicant (engineering epoxy) is then injected into this gap. The vacuum is left on during the curing process. After cure the vacuum is switched for 60 psi air pressure. The air bearings then create a force that displaces the structure enough to produce a running clearance.
A high precision air bearing assembly as shown was developed as an alternative to a similar assembly employing rolling element bearings. Both assemblies were tested for straightness, static stiffness and dynamic response.
Stiffness was collected by fixing the shell to a machine frame and applying a load to the extended bearing rail. Applied force was measured with a load cell and displacement at the endpoint was measured with a capacitance probe, as shown.
The results of the static stiffness tests show that the air bearing system developed a stiffness of approximately 50 N/ym, compared to 6 N/micro meters for the spring loaded roller bearing system. The lower stiffness (12 N/micro meters) in the initial portion of the air bearing curve is due to compliance in the fastening of the bearing to the machine frame.
Test Results: Strightness of Linear Bearings
Straightness data on both bearing systems was collected using the same capacitance probe setup with a zerodur optical flat as a target. Both systems were using the bearing rails of the same dimension with 16 micro inches Ra surface finish. the results over 1 inch are shown in micro inches. Total peak to valley error motion of the air bearing was approximately 10 micro inches over 1 inch of travel. This is considerably better than the 100 micro inches peak to valley error produced by the roller bearing system.
Dynamic response data was collected by using an impulse hammer and a drive point accelerometer located at the same position as the load cell. Several hammers, accelerometers and frequency ranges were used, all with similar results. The results are shown below.
Generally, mechanical bearings (i.e. sliding contact and roller bearings) have much better damping performance than air bearings. However, the smooth response curve of the air bearing clearly shows that it is extremely well damped, even more so than the roller bearing system. The phase data (not shown) shows that the air bearing system has a single pole located between 1 KHz and 3KHz, which is obscured by the damping. The roller bearing system has a total of five poles between 500 Hz and 4 Hz. Examination of the coherence plot (also not shown) shows that data below 500 Hz is unreliable.
The superior damping of these bearings means improved accuracy of motion and better controllability. Credit for collecting and documenting this test data is given to Michael Chiu,a student of Dr. Slocum at M.I.T. The tests were performed at N.I.S.T. under Bradley N. Damzo.
Air bearings are a type of fluid film bearing that shows promise in new machine design. This is especially true of high speed dry diamond machining of aluminum; which could become very popular in the auto production industry. Air bearing provide smoother motion and higher accuracies while leaving ways dry – to avoid contamination of aluminum dust. Conveniently this leaves scrap clean of oil and coolant, minimizes space requirements and maximizes reuse value.
We have here, a technology that improves accuracy, resolution, smoothness and dynamic response performance over rolling element bearings, reduces manufacturing expenses and can be combined with the latest in dry diamond machining to make metal cutting a cleaner industry.