VPI 2nd Pivot for 3D

Don - I know about everything..

Hi Raul,

I agree with your earlier statement that there will always be trade-offs.

My main reservations about gimballed bearings are “brinneling” & stiction.

40 years ago when I was making my earliest forays into “serious” hifi, “Flat-Earthers” would talk of how low output, low compliance cartridges (such as those popularly from Linn) could “wreck” the bearings of lesser tonearms i.e. those with poor(er) quality, loose bearings such as the old Rega “S” shape. For this reason one could never use such an MC in such an “inadequate” arm.

And so the era of the so-called “Super Arms” began (Zeta, Ittok, PU2, etc). I owned several of them. I am no longer a “Flat Earther” but brinneling hasn’t died, more “ignored”(optimistically) these days, especially when buying a S/H example 30 yrs old...

The Linn arms in particular tended to be run “dry” i.e. no bearing lubricant. This probably wouldn’t help very much, so, if poor reaction to warps is the (simple)Unipivot’s Achilles Heel, then brinneling could be defined as the gimballed tonearm’s Achilles Heel #1.

Achilles Heel #2 is probably, amongst other things, the result of the engineered solution to brinneling : forms of increased friction.

https://en.wikipedia.org/wiki/Brinelling

To me, Stiction sounds like an invented word rather than a real one but gimballed arms often have it. There can be moments when an arms ability to travel in either axis is hindered by a reluctance to move under light impulse. It was one of the reasons I switched to unipivots, because I could actually see it for myself in bearing tests. This can be attributable to long term bearing damage, deformation of the housing, contamination, drying out of lubricant (if applicable)etc.

When you have a situation where the LP is drilled off centre, the tonearm is suddenly presented with the problem of tracing/weaving from side-to-side in addition to handling the warp. Given the possible presence of stiction and the enormous forces mentioned in your article this will be compounded by inertia and, in some cases, fluid damping. The end result will be that, often, the arm doesn’t respond fast enough and the pressure on the groove lining is ramped up, multiplied to even higher levels as the locus of the arm switches from pressurising one groove wall to the other?

Surprisingly, our ears seem to be fairly immune to such effects, just as they can be with the forementioned bearing wobble. In fact, purists may pick up more on pitch errors due to non-concentricity than they might SQ differences resulting from variable pressure?

Forgot to mention what may be categorised as Achilles Heel #3.

When a warp occurs it usually emanates as a (lateral) angular undulation orientated from the LP centre to the outer rim which diminishes as we reach the inner groove. During this time a gimballed doesn't even operate in the same plane/angle as the warp, so it has lost its precise relationship and alignment with the groove. It's stable(!) but that's it!

(Note : this behaviour would include the arm that I currently use despite it being a unipivot)

In conclusion, Unipivots can offer more “certainty”, even if some of them do have an “Achilles heel”.

I’m always acutely aware of the need to avoid damaging gimballed arm bearings so wouldn’t even tighten a cartridge screw without stripping the arm completely from the T/T first (and if it possesses a captive cable, that too!) :^(

Despite doing it countless times in pursuit of perfection, whenever removing a gimballed tonearm I still resemble a munitions man defusing a bomb! :^D

Others may find it less nerve jangling but I don’t (not when I “paid an arm & a leg” for those bearings! ;^)

There is no question that many unipivots are much more facilitating and user-friendly in this respect. (Of course if one has interchangeable headshells then easier swapping but small prices to be paid on resonance & rigidity).

Finally I stress that I LIKE certain gimballed arms, I like certain gimballed arms with interchangeable headshells and I like certain unipivots. I also like some hybrid arms that don’t fall into these exact categories. I guess the upshot of all this is that there are no easy answers, just compromises, as you said.

Best regards & happy listening.

Bill.

 Bill / Moon   Very interesting post.  I remember owning a Helius arm...very high priced, but a miserable design..  It went back to England 4 or 5 times before I sold it.... it was gimballed and was supposed to be the second coming.  I had trouble with my ESL, Zeta, Rega, (gimbal) but never had any trouble with a Grado Labs or 3 different VPI's... (unipivot). 

Dear @moonglum : Achilees heel in fixed/gimball tonearm bearing designs?

Well, as in any tonearm design all depends on the design it self and the quality excecution level of that design.

In the fixed tonearm bearing design there is no inherent achilees heel as in all true unipivots. Gimball type tonearm bearing designs can comes with " mistakes "/bad design in that regards but what is happening down there and that you explain in wide way ( brinelling. ) is not inherent to those kind of design.

You can take for an example a vintage tonearm design as Technics where the designers decided to use four bearing points and use at each bearing points 5 ruby balls ( 20 in total. ) with a roundness tolerance of 0.5u that permits a bearing friction number ( in all directions. ) of less than 5mg. and it´s not only that but all the care Technics took for its production. A superlative product that even today is the " envy " for any tonearm manufacturer.
Brinelling there is almost non-existent. Of course that there are truly bad designs as the FR tonearms that in that regards is one of the worst examples you can find out but even this those stylus tip/groove huge forces makes that the tonearm moves!.

In the other side, the very tigth tolerances on today gimballed designs makes that you don’t have to worry about. In the past almost no one use ABEC 9 balls and today is a must and almost every one use it at the tonearm bearings.

Everything the same unipivots are in total disadvantage against giball/fixed bearing tonearms.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.

Is the addition of the second pivot restricted to the 3D arms and if not should the effect be of equal value?