Any interest in an acrylic platter group buy, and a bearing?

[agent.5]

any updates?

[Nanook]

I've decided to go it alone and not rely on anyone else for anything else. We got a couple more samples via Ian Grant, and the factory could not follow my explicit instructions, (including using the proper materials with ASTM , DIN and SAE material numbers.

They completely pooched one of platter/bearing assemblies to the point that it may not be able to be salvaged at all.

I had a great discussion with one of the folks at Immedia (Spiral Groove) who assured me that finding competent manufacturers is one of the biggest hastles in high-end audio (and I've learned pretty quick.)

The quality control can occur at a local level.

To go with he "high end" maker required an investment of $15k-$20k, which is something I personally can't swing.

I am still working on this, but doing it all on my own. I know Ian and Rachel have been frustrated regarding the factory's inability to follow the instruction.

I thought I would calm completely down on this until I had a complete solution for the platters and bearings in hand. As soon as I have something in hand I will post, both here and perhaps a new thread as well.

[Mark Kelly]

Originally posted by Nanook
[ I've never looked into the load capabilities of ceramic balls and thrust plates

I have.

The usual failure mode is cracking of the thrust pad just below the contact surface of the ball (or above if the bearing is inverted) as this is where the maximal shear stress arises. This maximal shear stress can be taken as approximately half the mean contact pressure (the load divided by the contact area).

Because ceramics are stiff the contact area is small so the mean contact pressure is high. This is after all why they are used - the small contact area reduces the friction.

The trouble is that ceramics usually don't have very good shear strength. In ductile materials (metals) the maximal shear stress allowable is roughly half the stress required to stretch* the material by 0.2%. Ceramics won't do this for the simple reason that they break long before they reach 0.2% stretch.

The rule of thumb seems to be that for ceramics the shear strength and the tensile strength will be roughly equal but ceramics have woeful tensile strength, often about 0.1% of their elastic modulus.

The net result is that a ductile material like steel can support a contact pressure which is about ten times as large as a ceramic material of equivalent stiffness. This is why you'll often see a ceramic ball and a steel thrust plate - the contact area will be slightly larger and the steel is much tougher so the maximal supported load will be very much greater.

*Technically 0.2% plastic yield. This used to confuse me because for steel the 0.2% yield point happens to be very close to 2% of the elastic modulus.