10-29-14: Ct0517
*** YELLOW STICKY FOR ET2 THREAD MARK - AIR SUPPLY ***
I had a quick read of your post. A few comments to clarify some misconceptions :
In an aerostatic compensated air bearing as used in the ET the bearing stiffness is a function of
Bearing surface area
Air Gap
Pressure
Compensation
Changes to any of these parameters will alter the "stiffness".
The air gap provides a restriction - on one side you have high air pressure and the other side is lower ( atmosphere ). Air will attempt to flow from the high to low pressure - the escaping air creates the air bearing.
Compensation is where you have a second restrictor between the air supply and air gap (bearing) - in this case this is the capillaries. The function of the capillaries is to
1. Distribute the air over the bearing surface in an optimum manner to get a stable air bearing.
2. Restrict the air flow to the bearing.
By restricting the air flow to the air bearing a reserve is created in the manifold. For example if a load on the bearing reduces the air gap at the bearing, flow is reduced. The reserve pressure that had been held back by the capillaries now allows for increased pressure in the gap, creating a restoring force that gives the air bearing stiffness.
As a general rule when compensation is used the pressure in the bearing is about 50% of the supply pressure.
Bruces comments regards to the quality of air and flow reflect that compensation has a greater impact on bearing stiffness than pressure.
Any restriction or damage to the capillaries or indeed scratches on the bearing surface could compromise the bearing stiffness to a significant degree. Hence your comments about maintenance are very pertinent.
The Air Bearing Spindle psi requirements are less than the manifold as only a portion of this air is required for it to float properly. Approximately 50%. That means on a base ET2 only 1.5 psi ! - if your manifold is restored and functional.
The "old" air escapes as per Bruce' design out the sides of the manifold.
Not really. As explained above a higher pressure is required in the manifold to provide a reserve air pressure using compensation. A leaky manifold would not be helpful. In my experience putting too much pressure through the ET2 pushes so much air out of the air gap between the spindle and manifold bushing the arm cannot reach the end of the record. At some point the rubber seals might leak but this has not been apparent.
Regarding bearing stiffness.
The main advantage of an air bearing compared to ball race or roller bearings as used in most tonearms is twofold -
1. Virtually no friction in the air bearing. In rolling bearings, the static coefficient of friction is higher than the dynamic coefficient of friction. In other words it takes more force to initiate motion than it does to maintain motion. In air bearings the static and dynamic coefficient of friction are the same. So the air bearing has a quicker response to changes required from the cartridge tracking.
2. Surface irregularities in the roller bearing surfaces mean that the rotational tracking is uneven compared to an air bearing.
Now in terms of conventional pivoted arms, most have roller bearings for both horizontal and vertical motion. Some "knife edge" tonearms such as the old SME's have knife edges for vertical motion, but still have roller bearings in the arm pillar for horizontal motion.
True unipivot tonearms have a simple point contact for both horizontal and vertical motion, so they are much closer to an air bearing than they are to a conventional gimbal arm using roller bearings. Because the unipivot has a point load, "bearing stiffness" is not an issue.
Coming back the ET some contributors have commented that with increased pressure the bearing tube arm is harder to pull out of kilter( and hence the comment the bearing is stiffer ). What actually happens is if you pull the air bearing tube perpendicular to the bearing you are closing the air gap on one side. The escaping air ( from high pressure manifold through the bearing to atmosphere ) will try to find the path of least resistance - which in this case will be the bigger gap. Basically the air bearing will collapse. Increasing the pressure will help.
