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DIY Air Bearing Linear Arm
DIY Air Bearing Linear Arm
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Old 4th February 2015, 02:11 PM   #31
super10018 is offline super10018  United States
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At 40 psi, I couldn’t hear anything. Last night, I increased the air pressure to 50 psi. The time was about 11:45 pm. I guess the db level in my listening room was about 20. I put my ear about 6” away from the bearing. I could hear very low noise of air flow.
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Old 8th February 2015, 01:55 AM   #32
super10018 is offline super10018  United States
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I recently bought an Audio-Technica AT-LH18/0CC Headshell and modified it to fit my arm. AT-LH18 is made of special aerospace aluminum. It is hard. I also made an option so I can change different headshell in the future if I want to. It is easy to install certain kinds of cartridges now because I can take the headshell off the arm and install the cartridge first. The headshell is also ľ shorter than previous one. I usually donít like wood headshell because I think wooden headshell sounds slow and sloppy. I prefer fast and natural sounding. However, the aluminum body of 17D3 combined with AT-LH18 sounded too fast and edgy for me. In order to slow it down, I added a 2mm thick carbon fiber. It is the right combination for my taste now.

I am also experimenting eddy current damping now. Thanks for your input, Clavin! I looked Dennesen arm. First, I am not sure if their damping force is enough. 2ndly, damping force is not adjustab. For eddy current damping, I need to find a good balance between moving mass, adjustability and complexity. But for now, I canít find any better options than my damping device. It is simple, practical and user fully controllable. I proposely add 0.1 g more on the left side bucket so it will have slight heavier inward pulling force. I can tell the cartridge is running very smooth now.
Attached Images
File Type: jpg dennesen1.jpg (36.4 KB, 516 views)
File Type: jpg dennesen2.jpg (30.7 KB, 504 views)
File Type: jpg air-bearing14.jpg (302.3 KB, 495 views)
File Type: jpg air-bearing16.jpg (124.5 KB, 489 views)

Last edited by super10018; 8th February 2015 at 02:00 AM.
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Old 8th February 2015, 09:12 AM   #33
Max Headroom is offline Max Headroom  Australia
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Default Suggestions....

A neo disc shaped magnet dropped down a conductive metal tube falls slowly due to eddy current production.
Would a neo magnet mounted to the moving carriage and held in close proximity to the main cylindrical rail provide eddy current damping ?.

Dan.
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Old 8th February 2015, 09:35 AM   #34
Max Headroom is offline Max Headroom  Australia
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dennesen2.jpg
Ok, I just had a closer look at your photos and noticed this one.
Back when I was an apprentice, I made an eddy current clutch for a coil winder machine.
The form was quite similar to the diagram shown below, except both sides were aluminium discs, the driving side mounted with 6 magnets, and the driven disc was cut radially so as to form 6 segments.
The cuts went close to the centre, but not all the way.
The theory as explained to me at the time was that some electrical resistance in the driven disc is necessary for more ideal operation, and the radial cuts alter the induced current paths causing higher electrical resistance.
eddy current clutch.jpg
Perhaps your rectangular section damping rail would benefit by vertical cuts, making vertical fingers akin to a hair comb.

Dan.
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Old 9th February 2015, 12:41 AM   #35
super10018 is offline super10018  United States
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Quote:
Originally Posted by Max Headroom View Post
Attachment 464382
Ok, I just had a closer look at your photos and noticed this one.
Back when I was an apprentice, I made an eddy current clutch for a coil winder machine.
The form was quite similar to the diagram shown below, except both sides were aluminium discs, the driving side mounted with 6 magnets, and the driven disc was cut radially so as to form 6 segments.
The cuts went close to the centre, but not all the way.
The theory as explained to me at the time was that some electrical resistance in the driven disc is necessary for more ideal operation, and the radial cuts alter the induced current paths causing higher electrical resistance.
Attachment 464383
Perhaps your rectangular section damping rail would benefit by vertical cuts, making vertical fingers akin to a hair comb.

Dan.

Dan,

You must know about eddy current more than I do. Anyway, I found something new today. What I found means that eddy current is not a correct method to damp. Please read my next post. The picture you used is not mine. It is Dennesen arm.
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Old 9th February 2015, 02:26 AM   #36
super10018 is offline super10018  United States
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In my previous post, I quoted Michael Fremer's comment. He said:

The grooves of most LPs are not concentric because of pressing inconsistencies, which means the groove is constantly shifting the tonearm's mass back and forth. Because the stylus is at the end of a spring mechanism (the cantilever's suspension), if you use a cartridge with too high a compliance—too floppy a spring—you can have the tail wagging the dog, in that the motion of the groove spiral will move the cantilever from its central position. The risk is then that with a frictionless bearing the stylus might be slammed from one groove wall to the other.

His comment made me thinking. If the stylus is slammed from one groove wall to the other due to eccentric disk , what happens for a concentric disk? Is the stylus still centered or almost centered in the groove for a linear tracking arm?

To approve my point, I did a test today. Before I explain the test I did, I would like explain how my damping device works so everyone can understand my reasoning.

Please see Fig 1. In Fig 1, the pulling force F1 equals F2. The weights, W1 and W2, are same although the distance D1 and D2 are different. When the cartridge travels, the distance D1 and D2 change but the pulling force, F1 and F2 don’t change. Therefore, it damps unwanted lateral movements.

I used 1st track of side 2 of The Ultimate Analogue Test LP to do the test. This track is for adjusting anti-skating on a pivot arm. You may either listen or view the signal on an oscilloscope. If you hear buzz noise or see the distortion on an oscilloscope, it means anti-skating is not enough or too much. In other words, the stylus is not centered in groove because skating force. Usually, it is very hard to completely get rid of distortion at 12 db on a pivot arm.

I played this track on my air bearing linear tracking arm. I assumed that both channel should be same and no distortion exists because linear arm has no skating force. According Fremer, the distortion should move from one channel to another if the disk is eccentric. But for concentric disk, there should be no distortion at all because the force is equal for both groove walls and the stylus should be centered. Now let’s see the test result. Please see Fig 2.

In Fig 2, green line is left channel and red line is right channel. The distortion does exist in left channel even for a concentric disk(Test LP) under equal damping force for both channels. Is the distortion caused by skating force on a linear arm? I am not going to discuss this here. This is not my purpose here.

Then, I added more weight on the left side(left channel) of my damping device until the distortion disappeared. Please see Fig 3. The weight on left side is .7 g heavier than right side.

From this test, I can draw following conclusions.

1. Fremer is not correct. The stylus is always being slammed on the right side of groove wall for a concentric disk. On a eccentric disk, it may be still always slammed on the right side of groove wall.

2. Applying equal damping force is not correct damping method. The linear tracking arms on the market have either no damping or incorrect damping method.

3. My damping device is the only device which has correct damping method and adequate damping to keep the stylus centered in groove.

4. Both silicone oil damping and eddy current damping are not correct damping methods because they apply equal force on both sides laterally.

On my arm, I still keep right side damping force 1.6 g(the weight of small bucket) and left side 2.3 g just in order to keep it stable. In fact, it may be possible just to damp left side just as an anti-skating device on a pivot arm.
Attached Images
File Type: jpg 2015-02-08-19.06.14.jpg (58.9 KB, 461 views)
File Type: jpg Image-006.jpg (122.0 KB, 205 views)
File Type: jpg Image-004.jpg (134.7 KB, 142 views)

Last edited by super10018; 9th February 2015 at 02:32 AM.
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Old 9th February 2015, 04:32 AM   #37
Max Headroom is offline Max Headroom  Australia
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The issue is keeping the cantilever cantered in the cartridge magnetic circuit. This is why very high compliances are not suitable. Overly high compliance allows sled overshoot and lateral resonance causing too much lateral deviation of the cantilever...the tail wagging the dog condition.

Dan.
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Old 9th February 2015, 03:49 PM   #38
super10018 is offline super10018  United States
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My test shows the tail only wags the dog to right side.

Equal force damping may reduce inertia of bearing(It still needs to be approved), but it can’t keep the stylus centered in groove. Equal force damping means no damping at all. Kuzma’s damping device is not correctly implemented. So are the others.
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Old 9th February 2015, 06:44 PM   #39
pcb121055 is offline pcb121055  United States
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If I am not mistaken, damping will result from the friction in the axles of the pulleys and the friction associated with the string running in those pulleys. If it were not for that friction, the mechanism I see would only add to the lateral mass of the arm excepting that it is applied at some distance ahead of the linear bearing, forming a moment about that point and a consequent increase in the vibrational complexity of the assembly. It is interesting that a bias would be needed. It is not clear what the source of the need might be nor that it is necessarily common to other linear arms. If the other arms don't exhibit this need for bias, then a balanced approach to the damping mechanism might be all they need. Congratulations on finding a fix for yours. How much damping is offered by your device and can it still be beneficial at a lower level? i.e. a lower weight implemented on both sides while maintaining the necessary bias.
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Old 10th February 2015, 06:43 AM   #40
niffy is offline niffy  Europe
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Hi super

It seems strange to me that you need any bias with your arm or much additional damping. To get the tail wagging the dog you would have to have either a very high compliance cartridge or extremely heavy carriage. What is the mass, including cartridge, of the carriage? And what is your cartridge compliance?

The need for bias might be due to an uneven load presented by the air line or arm wires. Have you tried redressing either of these? I found with my ball race linear tracker that the arm wires puts about a half gram bias one way or the other if not set just right so even the slightest twist in your air lines could cause problems.

Niffy
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