Tonearm Bearing Arrangements

The majority of arms of a particular effective length are within a few percent of each other with regard to the position of the bearings relative to the stylus.

For many years now there have been a family of arms which have followed the gimbal or coincident bearing principle. Also known as cardanic bearings, many arms using this arrangement are little more than copies of each other. Some examples through the years are  Acos, Alphason, Breuer,  Connoisseur, Da Vinci, Dual, Grace, Helius, Pluto, Sumiko, Technics, Triplanar, Zeta, and many others.

The main characteristic is that there is a fixed, non-rotating mounting pillar, which is height adjustable, and which supports, in its basic form, a ring with two bearings holding a second ring also with two bearings, the axes of which pass through the arm tube axis. The centre point between each set of bearings is generally coincident, and each ring allows movement in a plane at 90 degrees to the other.

In other words, the arm tube axis intersects with the rotational axis of both bearings, its centre is coincident with the midpoints of both bearing pairs, and, additionally, the mid point of the bearing pairs are coincident.

Very many other arms, probably the majority, have a slightly different, but similar, arrangement, where the inner ring still holds the arm tube, but, instead of an outer ring, the bearings are displaced downwards into the pillar.

Therefore, to try and find a way of analysing the different arrangements,
1. The arm tube can intersect: no bearing axis, or one, or both (0,1,2).
2. The arm tube axis can be coincident with the midpoint of: no bearing pair, or one, or both (0,1,2).
3. The bearing pairs can have their axes: coincident or not coincident  (C or NC)

In this nomenclature, the typical arm arrangement mentioned above, typified by Breuer or Technics, would be called a 2/2/C arrangement.

In the case of the arms where the main pillar houses the bearings for lateral movement, the centre point between each set of bearings is displaced so that only one of these bearing pair mid points coincides with the arm tube, so,  a  2/1/C.

The Syrinx PU2 had the bearing midpoints and axes still coincident with the arm tube, though not with each other, so 2/2/N

The Triplanar has the armtube offset vertically, so only coincidence with one bearing axis - no midpoint. Bearings coincident, a 1/0/C

The Dynavector has arm axis coincident with one bearing, so a 1/1/N.

A unipivot, by definition, must have the bearings coincident, and most have the arm tube vertically offset from the pivot and aligned with it, and so are equivalent to 2/0/C, or 2/2/C if the tube is at pivot level.

The Odyssey RP1 has only the horizontal and vertical bearing axes coincident, a 0/0/C, and if I was re-designing it I would make it 0/0/N.

So is there a significance in all this? My view with the RP1 was that the job of the arm was to try and absorb as much energy from the cartridge as possible, while avoiding transmitting vibration in the opposite direction due to arm or turntable resonances. The offsets in the RP1, I felt, together with the disparate materials, were the key to this, and meant the arm was relatively non-resonant, despite not being as "chunky" and apparently less "rigid", as many other designs with more conventional arrangements, where all the vibrations were focussed at the one point where the bearings, the armtube, and the counterweight met.