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Old 19th May 2015, 11:16 PM   #1
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Join Date: May 2010
Default Air bearing tonearm problem

I built my 1st air bearing arm and love it so much that I decided to build 2nd one. The construction of 2nd arm is similar to Walker Audio’s. Please see attached image.

First air bearing arm link.

DIY Air Bearing Linear Arm

Although I still have a technical problem, I managed to play it. The sound from 2nd one is even better than 1st one. It is free of artifacts. The sound is so smooth and full 3D with the authority in bass. It even changes my impressions of some of my cartridges.

However, I still have a problem to solve.

I use two .5 inch Oav air bushings. The shaft is 11 inches long 2024 aerospace aluminum tubing. It weighs 30 grams.

The first construction I did is A in the attached image. But it bonus back and forth. I attached a video to show you that.

YouTube Video.

https://youtu.be/pEWht49wbT4

So, I tried the construction B. In construction B, I can play but there is still a tiny force to pull the shaft inwards at most outer groove. Once the needle passes certain point, there is a tiny force to push the shaft out. I had to raise the rear end of the arm a little higher to compensate the pushing force.

I have no idea what causes this. The shaft is pretty straight. Its tolerance in roundness and diameter is about .05 mm.If anyone had built similar kind of tonearm or you have the experience with air bushings, I need your advice how to solve this problem. Otherwise, I have to be very careful when I play the records. It also makes me feel uncomfortable to know there is tiny force is pulling the needle at beginning of playing.
Attached Images
File Type: jpg 2nd air bearing 1.jpg (552.8 KB, 148 views)
File Type: jpg 2nd air bearing 2.jpg (227.7 KB, 139 views)
File Type: jpg 2nd air bearing 3.jpg (411.0 KB, 139 views)
File Type: jpg air-bearing-problem.jpg (137.2 KB, 135 views)

Last edited by super10018; 19th May 2015 at 11:21 PM. Reason: Add link
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Old 20th May 2015, 08:48 AM   #2
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Location: Prescott, Arizona
Default Hello super10018

Quote:
Originally Posted by super10018 View Post
The shaft is pretty straight. Its tolerance in roundness and diameter is about .05 mm.
Watching the video, tells me that the shaft is not straight. It also rotates on its longitudinal axis as it oscillates back and forth. .05mm (.002") is not straight enough in my opinion. If the shaft is curved, it would act like a pendulum if set into motion. I would do the following:
Substitute a precision shaft of known straightness to see if it oscillates.
Take your existing tubular shaft and roll it on a clean granite surface plate to see how it behaves. Find a friendly neighborhood machine shop with a surface plate. High quality air bearings, being essentially frictionless, demand a precision shaft. If you visit the web page of the company that made your bearings, you should be able to find out what straightness and roundness tolerances they require.
There is one more possibility I just thought of:
Try rotating the tubular shaft in increments of, let's say 10 degrees, until you find a position where it won't oscillate. Good luck.

Sincerely,

Ralf

Last edited by Straight Tracker; 20th May 2015 at 08:50 AM. Reason: Missing comma
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Old 20th May 2015, 12:43 PM   #3
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Looking at the OAV website, it looks like the shaft should be +0.0000" / -0.0007"
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Old 20th May 2015, 02:34 PM   #4
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Ralf,

Thanks for replying and suggestions!

I had checked the straightness of the shaft on glass table top. I couldn’t tell and see too much of irregularity. The tubing rolled from one side to other side smoothly. But I think it is still inconclusive since I can’t detect small irregularity by just looking. I also tried one air bearing. The shaft was moving very well. However, it may not be ideal because the needle may be off center at most inner grooves. I do think I need to find a machine shop to process the shaft.

What amazes me is that the arm is almost like immunity from pop and click noise. The background is very quiet. I remember my 1st ball bearing arm. All the pops and clicks were amplified.

What also amazes me is that the music is so smoothly flowing out from same cartridges I used for different arm. I guess it is because this kind of air bearing construction has much large of air-shaft contact area. On my 1st air bearing arm, I use .5 inch air bearing while most of commercial arms use 3/4 inch or 20 mm air bearings. Their contact area is larger than mine. But one my 2nd arm, there are two bearings so the contact area is much larger.

It also reminds me the review I read.

Kuzma Stabi XL Turntable/ Air Line Arm, Walker Proscenium Black Diamond Record Player | The Absolute Sound

In the review, the author thinks Kuzma’s arm has more hifi elements while Walker’s sounding is more real. His conclusion is consist with my experience. My 1st air bearing arm is similar to Kozma’s but 2nd one is similar to Walker’s. On my 1st air bearing arm, the sound is not as smooth as 2nd one. High frequencies are slightly edgier than it on 2nd air bearing arm, esp. under higher air pressure.

I also have the feeling that the track ability of 2nd air bearing arm is better than 1st one although I don’t do any tests yet.

If I understand it correctly, Walker audio uses carbon fiber shaft. I may try it later on as well.

On my 2nd arm, I also add a silicone oil trough for damping. The oil I use is a mixture of 20 wt and 30 wt. and the paddle is just half sunk in the oil. So, the damping is very light. Although the damping is very light, it is needed for certain cartridges and LPs.

For 2nd air bearing arm, my small fridge compressor can’t keep up. I have to order a more powerful compressor. So, if anyone wants to do an air bearing arm which is similar to the construction of my 2nd arm. You should start with a more powerful compressor. I ordered a 1/3 hp fridge compressor.

Last edited by super10018; 20th May 2015 at 02:44 PM.
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Old 20th May 2015, 03:30 PM   #5
carolus is offline carolus  Belgium
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Location: Brussel
Quote:
Originally Posted by super10018 View Post
....my small fridge compressor canít keep up. I have to order a more powerful compressor. So, if anyone wants to do an air bearing arm which is similar to the construction of my 2nd arm. You should start with a more powerful compressor. I ordered a 1/3 hp fridge compressor.
One quid:
Is your fridge compressor "oil & water free" ?
It's a must (& quasi impossible) ...

My (first) experiences with my air bearing TT (& arm) and several type fridge compressors was not a success.

Fridge compressors ... not the good solution.
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Old 20th May 2015, 04:22 PM   #6
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If that is extruded tubing it will be anything but round. Since the mass of that part doesn't effect playback or performance I would look at getting some centerless ground tubing, possibly stainless steel. Air bearing clearance tolerances are very critical and having tubing that is out of round is going to cause all kinds of grief. Typical tolerances for extrusions are in the several thousands of an inch or more. Usually a lot more. You need tolerances held to 1/10,000 of an inch.
On the brighter side, your getting good performance on a sub-par bearing, just wait till you get a good one.

BillWojo
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Old 20th May 2015, 05:39 PM   #7
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Carolus,

No. It is not oil and water free. The compressor uses R-134a oil. Its oil may be pumped into the line from time to time. And, water may accumulate in the line under room temperature as well. My line is about 50 feet to the arm. It is long. This is why I use a 10 micron filter as 1st phase filter and .03 micron filter as 2nd phase filter on the compressor to keep oil out of the line, and another .03 micron filter just before the line going to the arm. I check the oil level drained from the filters from time to time to make sure there are enough oil in the compressor. If not, I add some oil. The more filters you add in the line, the slower of flow is. But it is ok for tonearm. Tonearm just needs right pressure. Flow is not so critical at all.

For smooth flowing of air, I use a 10 gallons air tank. The compressor has two pressure zones to keep the motor run time shorter and down time longer.

One thing to stop a lot of people using air bearing arm is air compressor noise. It is not a problem for me. I have a large closet in my listening room and put the compressor in the closet. I can’t hear any noise.
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Old 21st May 2015, 12:22 AM   #8
AVWERK is offline AVWERK  United States
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Centerless grinding and an accurate way to align both chambers.
Both are necessary and the second one will probably be more difficult than CL grinding the tube!

Regards
David
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Old 21st May 2015, 12:38 AM   #9
AVWERK is offline AVWERK  United States
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And after that, flip the tube around and reinsert , check that movement is the same either way.
Then you will know it won,t affect the diamond in any way across the record
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Old 21st May 2015, 02:23 AM   #10
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Air bearings are extremely precise parts. it seems like your shaft has slot at the end most likely that creates uneven distribution of mass which causes the weight force to push shafts ether directions.
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