The following is posted by myself in another Thread recently:
Most record pressing standards have a production tolerances that are producing a centre hole to groove eccentricity tolerance of 0.2 mm maximum.
Apply the 0.2mm to a playback radius of 100 mm (approximately the centre of the LP modulated groove area) equates to a wow of +/- 0.2 % peak ( 0.14 % RMS). As this is for most an people inaudible, it does seem like a fair tolerance for a standard production item.
Does a Premium production Item address the off-set to a Zero mm dimension?
Usually, when I refer to eccentricity of a rotation, I refer to the eccentric rotation of the LP, in conjunction, with the eccentric rotation of the Platter Spindle Bearing being the main concern.
What is an additional concern, is the condition of the TA in use, being able to maintain a True Axis when in use, decreasing any creep to the Spindle Axis / Pivot dimension.
Wear of the Tonearm Bearing has the potential to add to the movement at a critical mechanical interface, where maintenance of a dimension is a high importance.
Bearing condition, Quality of Bearing Parts along with Bearing Wear such as Brinelling, are if present able to add to the changes being made to what is ideally a fixed dimension.
What is the main concern is that a Typical Bearing design has proven the use of Captured Ball Race Bearings have a design that will produce energy that is able to be transferred to the Styli, along with other influences being restrictive to the freedom of movement.
Add to the typical design concerns a wear is present that is a detriment, where an excess of movement has manifested. As well as being an increased risk to producing energies that are being transferred to the Styli. The condition of wear if suspected of being present is also able to contribute to changing a critical dimension that is best maintained. Measures selected as an attempt to remove the influence are worthwhile as an investigation.
I will make claim there are TA's in use today that are between 40 - 50 Years since their being new models, that are used regularly with no knowledge of what has developed in the Bearing Assembly. It would be of interest to know who is using such a Tonearm Type and which Cart's are mounted on them.
At what time of usage does a bearing wear become a concern or bearing wear manifest?
In the modern day, there are Differing Materials selected to produce Captured Ball Race Bearings. The result being, Bearing Balls are available that are improved for being consistently Spherical, with improved dimensions between each Balls sizing.
Some Bearings as a result of Parts selected for the assembly, are offering Quality Control where very low micron dimensions are quoted as the contact between all contacts of balls in the assembly.
It is the levels of Polishing to the surfaces and how long these levels of Polish that can be maintained, that are one of the requirements to ensure low friction and substantially reduce the risk of stiction occurring.
When an individual does like the idea of renewing a Part such as a Captured Ball Race Bearing, it is not just the Model No for the replacement part that should be seen as the improvement.
What really matters is if the design has been produced to improve/decrease the likelihood of other concerns from older used items used for the same role still remaining.
It is also best to fully satisfy oneself that the individual being entrusted fully to carry out the exchanges, understands the mechanical need of the TA, and is not just adept at swapping out one Captured Ball Race Bearing Type for another.
It is also best, if the same individual fully comprehends how free a bearing can be, to be optimised for freedom oof movement when reassembled.
I have a adjustment device where one full turn is 1 micron, which then equates to an increased or decreased force on the freedom of movement.
It is the sort of control that only very very limited individuals should be let near. Certainly not a toy for myself to get an interest in.
From my end, I have a True Axis at the Platter Spindle, I have a True Axis at the Tonearm Pivot.
The interface between Platter Spindle / TA Pivot are a consistent unchanging dimension.
I have a Album in the mix that has a possible 0,2mm eccentricity.
There is only one concern I have for an unwanted influence on the Styli, but if the above calculation is correct, as I am sure the Record Industry has allowed for, then this is a non-concern as well.
I am yet to see any great difficulty in attaining such an attractive condition as a mechanical interface.
