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Knife Edge Bearing - Pros & Cons?

Posted: 04 Jun 2018 04:14
by watchnerd
The new TK series of Jelco tonearms (TK-950, TK-850) use knife-edge bearings, a departure from the ball bearings used in the SA-750 and its replacement, the TS-550.

As an owner of a Jelco SA-750D, I'm curious:

What are the pros & cons of knife edge bearing tonearms?

Re: Knife Edge Bearing - Pros & Cons?

Posted: 04 Jun 2018 04:57
by Spinner45
watchnerd wrote:The new TK series of Jelco tonearms (TK-950, TK-850) use knife-edge bearings, a departure from the ball bearings used in the SA-750 and its replacement, the TS-550.

As an owner of a Jelco SA-750D, I'm curious:

What are the pros & cons of knife edge bearing tonearms?
Depending on how they are designed, knife edge bearings can be fantastic.
Absolutely no "play" or tendency to vibrate.
It's nothing new of course, because the 1962ish Zenith "MicroTouch" belt drive record changer used them, and was able to track at unheard of low forces back then.

Re: Knife Edge Bearing - Pros & Cons?

Posted: 04 Jun 2018 13:03
by billshurv
con: easy to damage in transport.

Re: Knife Edge Bearing - Pros & Cons?

Posted: 04 Jun 2018 14:35
by watchnerd
Spinner45 wrote:
watchnerd wrote:The new TK series of Jelco tonearms (TK-950, TK-850) use knife-edge bearings, a departure from the ball bearings used in the SA-750 and its replacement, the TS-550.

As an owner of a Jelco SA-750D, I'm curious:

What are the pros & cons of knife edge bearing tonearms?
Depending on how they are designed, knife edge bearings can be fantastic.
Absolutely no "play" or tendency to vibrate.
It's nothing new of course, because the 1962ish Zenith "MicroTouch" belt drive record changer used them, and was able to track at unheard of low forces back then.
Why aren't they used more commonly?

Re: Knife Edge Bearing - Pros & Cons?

Posted: 04 Jun 2018 18:15
by analogaudio
In the hands of the careful user they can be fine however for the less careful the traditional closed bearing is more robust and less vulnerable to accidental disturbance.

A small amount of friction in the bearing can be a good thing, it provides some damping of the tonearm+cartridge subsonic resonance.

Re: Knife Edge Bearing - Pros & Cons?

Posted: 04 Jun 2018 21:57
by Spinner45
analogaudio wrote:In the hands of the careful user they can be fine however for the less careful the traditional closed bearing is more robust and less vulnerable to accidental disturbance.

A small amount of friction in the bearing can be a good thing
, it provides some damping of the tonearm+cartridge subsonic resonance.
Who said?
Damping via fluid is one thing, but mechanical (bearings) friction is a no-no.

Re: Knife Edge Bearing - Pros & Cons?

Posted: 05 Jun 2018 00:31
by nat
It seems to me that the argument for knife edge bearings is related to the argument for unipivots - compared to the force acting downward on a bearing as a result of the mass of the arm and cartridge, the forces, in whatever direction they are directed, of any and all resonances must be much smaller. So, despite not being trapped, unipivots and knife edge bearings are functionally tighter than gimballed bearings, where the bottom bearing must support a significant amount of the downward force, which then must loosen the vertical bearings which must have some play in order to minimize friction.
Every choice in audio has some cost - unipivots have woggly behavior during cuing, which many people cannot abide, knife edges can be damaged, and gimball bearings can be loose, or alternately, so tight they can be damaged, or have high friction.

Re: Knife Edge Bearing - Pros & Cons?

Posted: 05 Jun 2018 00:44
by edradour
I can live with a bit of cueing wobble - unipivot all the way for me!

Re: Knife Edge Bearing - Pros & Cons?

Posted: 05 Jun 2018 03:47
by watchnerd
analogaudio wrote:In the hands of the careful user they can be fine however for the less careful the traditional closed bearing is more robust and less vulnerable to accidental disturbance.

A small amount of friction in the bearing can be a good thing, it provides some damping of the tonearm+cartridge subsonic resonance.
Does this imply that if I have a suspended floor, suspended turntable, and cats....that knife edge is a less good choice than bearings?

Re: Knife Edge Bearing - Pros & Cons?

Posted: 05 Jun 2018 20:11
by analogaudio
Cats and turntables are sometimes an unhappy combination.

Suspended floors and turntables are not going to put the bearing under much stress.

It may be that some knife edge bearings include some sort of protective restraining mechanism however a simple unrestrained knife edge requires very careful handing.

Re: Knife Edge Bearing - Pros & Cons?

Posted: 05 Jun 2018 20:22
by analogaudio
Spinner45 wrote:
analogaudio wrote:In the hands of the careful user they can be fine however for the less careful the traditional closed bearing is more robust and less vulnerable to accidental disturbance.

A small amount of friction in the bearing can be a good thing
, it provides some damping of the tonearm+cartridge subsonic resonance.
Who said?
Damping via fluid is one thing, but mechanical (bearings) friction is a no-no.
Both fluid damping and bearing friction are forms of resistance to motion (drag) which the arm must do work to overcome. Energy is removed from the system and converted to (a very small amount of) heat. Provided that the energy removal method does not interfere with the function of the arm (a small amount of bearing friction do not interfere with arm motion) it makes no difference how the energy is removed.

The behavior of the arm+cart combination is an example of harmonic motion under the sub-heading driven damped oscillator. The damping quantity can be varied and the result is a change in the amplitude, the frequency and the rate of decay of the resonant motion of the arm.

Re: Knife Edge Bearing - Pros & Cons?

Posted: 05 Jun 2018 21:37
by Spinner45
analogaudio wrote:
Spinner45 wrote:
analogaudio wrote:In the hands of the careful user they can be fine however for the less careful the traditional closed bearing is more robust and less vulnerable to accidental disturbance.

A small amount of friction in the bearing can be a good thing
, it provides some damping of the tonearm+cartridge subsonic resonance.
Who said?
Damping via fluid is one thing, but mechanical (bearings) friction is a no-no.
Both fluid damping and bearing friction are forms of resistance to motion (drag) which the arm must do work to overcome. Energy is removed from the system and converted to (a very small amount of) heat. Provided that the energy removal method does not interfere with the function of the arm (a small amount of bearing friction do not interfere with arm motion) it makes no difference how the energy is removed.

The behavior of the arm+cart combination is an example of harmonic motion under the sub-heading driven damped oscillator. The damping quantity can be varied and the result is a change in the amplitude, the frequency and the rate of decay of the resonant motion of the arm.
Wha wha wha?..... HUH???
You're pulling my arm!
Do you write the "merits" usually advertised on those snake oil sites? :shock:

Re: Knife Edge Bearing - Pros & Cons?

Posted: 06 Jun 2018 02:09
by nat
Not sure why 'snake oil' needs to be introduced into the conversation. I'm not knowledgeable enough to evaluate all the description, but that doesn't mean it's BS. It may be right, or it may be wrong, but I think it's a good faith attempt to explain damping.

Re: Knife Edge Bearing - Pros & Cons?

Posted: 06 Jun 2018 05:58
by watchnerd
A practical question for the academy:

If given the choice between:

a) Keeping my current Jelco SA-750D gimbal tonearm, with the ability to tune damping with its oil reservoir


or

b) A new Jelco SA-850S, with theoretically superior knife edge, but no ability to tune

which would you pick and why?

Re: Knife Edge Bearing - Pros & Cons?

Posted: 06 Jun 2018 19:26
by analogaudio
Spinner, I disagree with your statement and explained the reason.

If there is a problem perhaps it is not with your arm being pulled or snake oil merits (whatever they are) but it may be with your understanding of the physics of tonearms. In an attempt to assist you I gave the categories for the behavior of an arm+cartridge, it is not difficult, and the topics are well documented online, however I cannot do for you the work of study.