The Korf Blog

The inside story: our research, development and opinions

12 December 2018
Flexure Bearings Revisited, Part I
This year, we have built a series of prototype tonearms around a simple shim type flexure bearing. While the sonics were quite interesting, reliability left a lot to be desired.

Rather than try and improve the handling performance of a simple shim, we have gone in a different direction. There are ready-made standardized flexure bearings, known also as flexure pivots, that are reasonably robust and, on paper, offer specifications that suit the tonearm requirements well. We have decided to build a prototype tonearm around one of those.

Here's a model 6010-800 double-ended flexure pivot from Riverhawk Company. It's about 8mm (0.3125") in diameter and 12.7 mm (0.5") long. The specified stiffness is 0.0036 pound-inch per degree. That's 0.0004 Newton-meter.

This means that, at a typical tonearm length, it would take about 0.18 gram force to deflect the bearing by 1°. Looks flexible enough for our purpose.

By the way, these flexures are quite expensive. The one we use is more than $100 US, and it's on the cheap side of things! Smaller and softer ones cost a lot more.

Flexure pivot up close

Tonearm central part with standardized double-ended flexure.
We have designed a simple central part around the 6010 type flexure pivot. Again, it's based on Ortofon AS-212 pillar and ancillaries (antiskating adjustment, arm clip etc). Arm tube and headshell are from Prototype 5.

Despite a well-known rule to only change one thing at a time, we've also used a different method to attach the arm tube and counterweight stub. Instead of our usual clamp, this time we used stopper screws. Will it result in lesser rigidity? We'll see .
This is what a CNC'd central part with an installed flexure pivot looks like. It feels a bit delicate in assembly, but is a lot sturdier than our single shim type flexures.

Assembled central part
And this is the complete tonearm.

In the next blog post, we'll mount the arm on our test turntable and take the vibration measurements. How will the complex flexure pivot fare compared to a single shim? Stay with us and you'll find out!

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