The Korf Blog

The inside story: our research, development and opinions

11 April 2918
A New Project
You might have noticed that the blog updates were a bit scarce lately. That is because we have a new exciting project going on. This post is an introduction, and I hope it will be the first in a series.
Flexure bearings are by their nature noiseless
Meet our next batch of tonearm prototypes.

We have identified the horizontal bearings as one of the major contributors to the performance of the tonearm. Vibrational energy from the cartridge excites the bearings, leading to often pronounced HF ringing. With lower quality ball bearings, this ringing can dominate the arm's vibrational profile.

Not only the usual types of bearings ring, but they also are a source of noise themselves. Unipivots and knife edges are best-known ways of minimizing movement noise, but there's one bearing type that is unjustly overlooked.

Flexure bearings have a lot of advantages. They are by their nature noiseless. They have zero stiction. They do not chatter. They can be designed so that their own frequency (1st mode of the resonance) would lie high above the threshold of hearing.

But they have their disadvantages too. Flexures can be fragile. Simple flexures flex in 2 axis rather too willingly, adding unwanted movement. Consumers are used to gimbal and knife edge bearings, and are apprehensive about flexures.

Still, I think the advantages might outweight the problems.
Flexure bearing closeup
Our first flexure prototype uses Ortofon AS-212 pillar and ancillaries (antiskating adjustment, arm clip etc), arm tube and headshell from Prototype 5, and custom machined central parts.

I have chosen a horizontal flexure configuration that is suboptimal in many respects. First, the whole weight of the arm and counterweights is deflecting the flexure. Second, in this position flexure is free to move in the azimuth plane. Why then?

This isn't a production prototype, but a test to expose the problems. Loading the flexure will give answers regarding its stability and longevity. Allowing for azimuth movement would let us measure it and select the material less prone to flexure in the "wrong" plane. Also, it's a bit simpler to achieve single point of vertical and horizontal rotation this way, thus removing one extra unknown from the measurements.
Flexure samples
The first material we are testing is carbon spring steel. Then it's stainless steel's turn, and then the brass spring. All are 0.1 mm thick. The effective length of the flexure is 4 mm, width is 3mm. In case of spring steel, the calculated deflection is 0.00039 Newton/meter/degree: a force of 0.39 Newton needs to be applied to deflect by 1 degree a millimeter-long bearing of this type.

We will start with the usual vibrometry. Then we'll dust off our laser angular displacement meter and measure the azimuth movement. This we'll repeat for all 3 flexures.

And, of course, the proof of the pudding is in the eating. We'll hear how the prototypes perform sonically. Stay with us, and you'll be the first to know!
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