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The Korf Blog

The inside story: our research,
development and opinions

7 April 2021
Production Tonearm Prototype — Vertical Bearings
We are sharing a little bit of the thinking and process that goes into building a brand new tonearm. Today's subject is vertical bearings.

There's a not entirely baseless opinion that the vertical bearings aren't as important as the horizontal ones. The movement they have to deal with is mostly caused by the LP's excentricity and is centered around 0.5 Hz. At these rates there's plenty of time to overcome bearing stiction.
We have shown that zero-stiction flexure bearings are great for dealing with warps, but do we really need anything advanced for the vertical axis? Shouldn't we just put a miniature ball bearing there and call it a day?

Yes and no. While a zero stiction, zero play flexure bearing would perhaps be an overkill, we need to keep in mind that some of the intense vertical movement caused by disc's warps does translate into the horizontal movement. It's not as big in amplitude, but still noticeable.

Another important role of the vertical bearing(s) is location and load bearing (excuse the pun). The entire weight of the moving arm assembly rests on them. They also need to keep the vertical axis, well, vertical.

Here, the choice must be made. Ball bearings with low starting torque often have quite a bit of play. This slack can result in the vertical axis rocking. If we use high precision bearings that keep the axis strictly vertical, we'll lose some of the starting torque (stiction) performance. What is more important?

The only way to answer this question is through experiments. Here is what we iterated through:
1) Normal Steel Bearing, Lubricated
This is our baseline. No mistracking or skips, no obvious negative influence on sonic performance.
2) Steel Bearing, C2 (tight) Internal Clearance, Lubricated
Occasional skips on warped LPs. No audible difference from (1)
3) Hybrid Bearing (steel balls, PEEK rings), C3 (larger) Internal Clearance, Dry
No no no no. This one is a non-starter. Doesn't locate the column properly, upper part of the arm is rocking back and forth. Sonically, predictably terrible.
4) Full Zirconia Bearings, Normal Internal Clearance, Dry
No mistracking or skips. Compared to the baseline, better 3D definition, better resolution, better dynamics. Perhaps slightly harsher sound.
Full Zirconia Bearings, Larger (C3) Internal Clearance, Dry
No mistracking or skips. Some undesirable movement of the tonearm. Sound similar to zirconia bearing with normal internal clearance, but with occasional sibilance.
5) Hybrid Bearing (Si3N4 balls, steel rings), Normal Internal Clearance, Dry
No mistracking or skips. Sonically, everything a full zirconia bearing offers, except harshness.
6) Full Si3N4 Bearing (not shown), Larger (C3) Internal Clearance, Dry
No mistracking or skips. It sounds like a full zirconia bearing, only more so. Alas, even more sibilance too.

Hybrid steel/ceramic bearing is a clear winner
To me, the winner is clear. Hybrid steel/ceramic bearing with normal internal clearance — not too tight, not too loose. It is also a bit more affordable compared to the full ceramic ones.

In a perfect world, I would probably try to source a full Si3N4 bearing with a normal internal clearance. Unfortunately such things cost close to 100 euro apiece (and we need 2 per tonearm).

Another major influence on both the stiction and the sonic performance of the bearings is lubrication. We have done the necessary experiments too, but for the time being it will remain our little secret.

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