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Old 17th August 2004, 04:06 PM   #1
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Default unipivot geometry

Hi all,

Few questions on unipivot geometry:
Tha shaft will be made from SS,any sugestions for cup materials?
Could I go with Delrin?
What would be the best tip geometry: very sharp or should it have a small radius?

Would appreciate any comments.
Attached is a drawing of my initial thoughts

Thanks
I.k.
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File Type: zip spindel bearing.zip (3.4 KB, 463 views)
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Old 18th August 2004, 04:50 PM   #2
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Remember that the entire mass of the arm will be on the unipivot, and as you make the bearing smaller (to reduce friction) the pressure increases proportionately. I used a ballpoint pen tip into PTFE.
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Old 19th August 2004, 07:10 PM   #3
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Thanks EC,

Any photos of the bearing/tonearm?

Regards

Izhak
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Old 19th August 2004, 07:28 PM   #4
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Yes, I posted a thread about it (with a couple of pictures) iin the analogue section.
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Old 24th August 2004, 01:15 PM   #5
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Thanks EC,

While I couldn't find your post with the pic, I did find a thread where you mentioned that the pivot point should be as close as possible to the record plan.
On other posts I've been reading that the pivot point should be in the center line of the arm tube. (that is arm radius + cartrige hight abouve the LP).

Could you elaborate on the best hight for the piivot point ?

Also, did you use any dumping fluid in your design ?

Thanks

Izhak
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Old 24th August 2004, 02:18 PM   #6
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For minimum wow as the arm rides up and down a warp, the pivot should be at record height. Additionally, if the pivot is at record height, any tendency for the arm to yaw merely reduces stereo crosstalk rather than changing the tracking force on either side of the record groove.

For a unipivot to be stable, the centre of gravity must be below the pivot point. That's why all unipivots have underslung counterweights. Some unipivots deliberately raise the pivot height in the mistaken belief that this improves stability. It doesn't, all it does is lengthen the pendulm, which lowers the resonant frequency. Because there are fewer oscillations per second, it is far harder to damp a low frequency oscillation. Thus, the ideal unipivot has its C of G just below the pivot.

Yes, my unipivot uses damping fluid. It's in the annular trough.

The picture is there, you just have to click to download it.
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Old 24th August 2004, 06:37 PM   #7
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Hey EC,

I've finely found you post with your arm.
Great looking arm.

You've posted a drawing of the headshell, any drawing of the pivot? anyone?

How do you implament the damping fluid in the horizontal plan
if you have a cup on top of a pin?


Thanks
Izhak
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Old 24th August 2004, 07:59 PM   #8
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Glad you like the arm.

I'm afraid I haven't done full drawings. When I did the drawings and explanation for the headshell, I realised that it would be a huge amount of work. Realistically, if you're capable of making it, you're capable of working out most of the details from the photograph.

The damping trough is the big 45mm diameter moat that the tapered pins either side of the bearing housing dip into.
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Old 24th August 2004, 09:04 PM   #9
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Hey,

This is a great solusion for the damping !!!

Did you try the arm with no damping fluid ?

I'm trying to find how important is the damping fluid?
It complicates building the arm & I'm trying to go as simple as possible.

Regards
Izhak
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Old 24th August 2004, 09:13 PM   #10
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You don't need much fluid, and it doesn't need to be very viscous (I use STP engine oil additive) but you do need some, otherwise the arm becomes very jittery.
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