Inverted bearing question

Re: Inverted bearing question

Postby Lcoblentz3 » Thu Oct 11, 2018 6:17 pm

Okay. Let me shed a bit more light here. I bought this ‘table as a demo. I have no idea what life it lead before I got it 2 years ago.

I have ordered a new ceramic ball, and a sapphire disc that will fit over the exiting PEEK disc. This seems to be a standard procedure for many rebuild outfits. If it doesn’t work out, it will go back to VPI for their experienced work.

Thank you for all of the input over here. There are some pretty sharp folks here.
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Re: Inverted bearing question

Postby nrw » Fri Oct 12, 2018 7:54 am

I have an HRX (with the heavy platter, 30lbs) that shows wear on the SS ball bearing and the PEEK plate. This table has been in use for approx. 11 years. I have purchased a new crystal plate and will replace the ball with ceramic once I can remove the old SS one and get the correct size, but I noticed in a previous post that the crystal plate could be installed directly on the PEEK plate. Is this an acceptable way to do this change? Does the dimple in the PEEK plate need to be filled with something? And then is it glued down?
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Re: Inverted bearing question

Postby Brf » Fri Oct 12, 2018 9:47 am

Peer Gynt wrote:
Your answers don’t lead me to your conclusions. The maintenance interval you describe would be considered unacceptable by most design engineers.

PEEK would not even be on my list of good choices for a thrust bearing. Neither would stainless steel.


Thermoplastic polymers have been used successfully as thrust plate in many high-end turntables. I am aware of PTFE/Teflon, PEEK, POM/Delrin, Nylatron, Vesconite and Vespel being used as thrust pads. As previously mentioned, thermoplastic polymers are known for their excellent damping properties and are often preferred over metal/carbide-based thrust disc whereby metal thrust plate interface can cause noise that is transmitted up the spindle due to the interface’s low damping factor.

I agree that an oil film bearing/thrust plate should last a very long time assuming an oil film between the bearing surfaces can be maintained. This is where IMHO, the VPI inverted bearing can be approved. On an inverted bearing and using an oil-impregnated grease, gravity will cause the lubrication to naturally flow away from the contact patch. I also own a J.C.Verdier La Platine table that also has an inverted bearing but employs an oil reservoir on top of the bearing that allows the oil to flow down the shaft over time. Yes, the Verdier needs periodic oil changes. In addition, other inverted bearing tables utilize a worm drive pattern cut into the bearing shaft to pump oil to the contact patch.

With respect to the “perceived” wear dimple on a PEEK thrust pad. This could be a result of cold flow due to the long-term exposure of pressure below the PEEK’s yield strength, but it may have stabilized, therefore, providing long-term stability (just my theory), therefore, we could be worrying over nothing.
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Re: Inverted bearing question

Postby soundwatts » Fri Oct 12, 2018 2:24 pm

Brf wrote:
Peer Gynt wrote:1. Does the Classic use a PEEK thrust plate?


Yes

Peer Gynt wrote:2. And how does a hard ball wear flat against PEEK?
No Lube? Transporting TT w/o removing platter? Dirt on ball?


PEEK and Stainless steel have a similar hardness, therefore, wear will manifest on both contact surfaces. The bearing has approximately 25,000 pounds per square inch of pressure, therefore, improper or inadequate lubrication, contamination, heat, etc will cause increased friction which will result in premature wear.

Peer Gynt wrote:Just curious. My sense is that PEEK might reduce noise when in good condition, but it seems like life issues would trump the early service potential noise advantage.


PEEK is easily machined, can take a high level polish, exhibits excellent wear resistance and has excellent damping properties which makes it an excellent choice for a bearing thrust plate. IMHO, due to the nature of the VPI bearing being inverted, the bearing needs more periodic cleaning and re-lubing than the specified once per year to ensure a long life expectancy.

Peer Gynt wrote:If the ball is really a ceramic, I guess wear debris would turn it into a mortar and pestle situation.


No, the current bearing ball is Stainless Steel. Some users replace the SS ball with a Silicon Nitride (Si3N4) ball which is non-porous, non-magnetic, non corrosive and much harder than SS. The non-porous nature of Si3N4 allows them to be manufactured to a higher grade than SS which allows them to operate with less lubrication and friction. Since Si3N4 is much harder than PEEK, the PEEK thrust plate will wear more than the ball

Peer Gynt wrote:How often does this bearing need to be replaced? And do current models have this characteristic too?


Most VPI tables use the same bearing material and configuration, therefore, wear should be consistent for all tables using the 20lb classic platter. Wear will also be dependent on usage and lubrication. To increase wear resistance, I've changed out the PEEK thrust pad for sapphire and the SS ball for Si3N4. My table's original bearing lasted 1.5 years, but my platter weighs around 30lbs.



@Brf

Is 1.5 years normal wear for the bearing? what is involved in changing it and how would you know it is worn
Last edited by soundwatts on Sat Oct 13, 2018 7:57 am, edited 1 time in total.
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Re: Inverted bearing question

Postby WntrMute » Fri Oct 12, 2018 7:13 pm

Too bad VPI transitioned away from the well style bearing. My TNT MK-3.5 has no wear after over a decade in service. Maybe we should search for a ball that is slightly softer than the PEEK thrust bearing as the ball is USUALLY user replaceable while the thrust pad is not. Or maybe a mag-Lev system to unload the bearing a smidge. Someone on this forum did that I believe.
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