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Old 13th November 2013, 11:56 PM   #1081
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Pdr,


Nice work, this was another idea I tried out, before and after settling on the 4 bearing as superior, but you will come to know it's pitfalls .



Colin
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Old 14th November 2013, 12:33 AM   #1082
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Default VTF - Stability & Reliability Researched

More research, with 5 photos to enhance the tedium. As I've noted previously the issue of VTF would seem to be more amenable to 'science' than 'art.' It is very clear that horizontal friction should be low, and that it is desired with this (these) designs that the vertical friction be somewhat higher.

I'd noted once I installed the Ortophon Super OM cartridge that the measured VTF with NO counterweight, during 5 sequential measurements clustered in the 1.37g to 1.41g range. Since there were going to be changes made to the Balsa Composite carriage I put further measurements off. When detailing the construction/weight issues last time I failed to note that the magnesium headshell had 3 coats of water-based polyurethane applied for corrosion resistance. No doubt a trivial addition of weight.

So, the wand has had about 1/2" cut off its end, the headshell has had its corners rounded for aesthetics, and the wand at the headshell end has been beveled to enhance cartridge clip clearance. In the full spirit of 'boy science' here's the photos explanations:

VTF 0 - SS pivot center to nominal stylus position set at 2 3/8" (hope that's the optimum?). The cutoff section of the wand at the left of the photo. Note the cartridge clips with solder blobs thereon. Careful viewers will note that there is no stylus seen - it got ripped out when setting up my customized Black Widow. "LPGear" replacement stylus puts the stylus at the exact end of the plastic 'nose'.

VTF 1 - Carriage balanced with a 1gm weight (venerable "Central Scientific Co. precision weights) at extreme end of wand. Total length of wand/headshell, end to 'stylus' is 7 7/8". Ortophon Super OM cartridge weight 4.6g.

VTF 2 - Calibrated 0-5g digital scale, resolution 0.01g, set at the balance point of cartridge 'stylus.'

VTF 3 - WITHOUT ANY COUNTERWEIGHT the carriage/cartridge VTF is an average of 2.12g, deviations -0.06g to +0.08g . 15 successive measurements were made, dropping the carriage to the scale from ~1/8" height after it had zeroed. Here is the sequence (in grams): 2.10, 2.13, 2.10, 2.17, 2.10, 2.15, 2.18, 2.17, 2.14, 2.05, 2.07, 2.15, 2.04, 2.08, 2.10. As you can see I took the photo at 2.09g. This was 'good' since I reread the Ortophon 'optimum' weight specification, which was 1.5g (maximum 1.75g).

VTF 4 - WITH A PRECISION 0.5G COUNTERWEIGHT 20 measurements were made of the VTF, counterweight positioned as seen in photo on a piece of dental floss. Average of 1.47g VTF, with deviations of -0.05g to +0.06g. Measurements made as above, in grams: 1.51, 1.42, 1.49, 1.44, 1.43, 1.53, 1.43, 1.50, 1.45, 1.43, 1.51, 1.46, 1.45, 1.44, 1.47, 1.53, 1.45, 1.48, 1.46, 1.49.

Obviously with a +10g cartridge weight the counterweight will have to be increased in mass. With the same digital scale, measuring the Denon DL-160 on the customized Black Widow, the measured weight never varies more than 0.01g.

IMO the stability and reliability of VTF using the test fixture (a not so polished SS rod) isn't all that bad. Once the cartridge was affixed the system, rod and carriage, became somewhat reasonable in regards VTF. I make no claim for the absolute weights measured, but clearly I put some effort into measuring a horizontally-balanced system.

VTF 5 - THE MONEY SHOT, with a calibrated digital scale (resolution 0.1g). The complete carriage with cartridge affixed and cartridge clips in place, no wiring. The 0.5g counterweight is not measured in this photo, but let's add it in:

22.2g as measured.
+0.5g counterweight
------
22.7g total weight, with the Ortophon Super OM cartridge mounted and 0.5g counterweight.

Sadly, my 3-mounting point custom Teres kit turntable is going to require a movable mounting system for the DIY Linear Tracker assembly (if I'm to be able to use all three anticipated tonearms). I started work on that swing arm today ... maybe I'll polish my (SS) rod a bit more in the meantime. Hope the measurements and 'boy science' add to the majesty of it all? At least we now have some reference to 'stable and reliable' VTF measurements - with a SS rod and polished bearing edges. Caveat Emptor!

John
Attached Images
File Type: jpg VTF 0.jpg (53.1 KB, 379 views)
File Type: jpg VTF 1.jpg (51.5 KB, 376 views)
File Type: jpg VTF 2.jpg (50.4 KB, 371 views)
File Type: jpg VTF 4.jpg (53.2 KB, 370 views)
File Type: jpg VTF 5.jpg (116.9 KB, 373 views)
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Old 14th November 2013, 12:40 AM   #1083
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Default Addendum - VTF Stability & Reliability

Sorry, I failed to upload the "VTF 3.jpg" photo showing the digital scale measuring the VTF without a counterweight.

John
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File Type: jpg VTF 3.jpg (50.6 KB, 224 views)
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Old 14th November 2013, 02:16 AM   #1084
Zene is offline Zene  United States
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Before I read John's book (just kidding) I'd like to mention why Colin's 4 bearing rig is winning the race (get it, race, sorry). Any time one edge of the outer race is riding on the same plane as the opposite edge you will effectively have a solid axle which means there can be a difference in the speed on one side of the bearing to the other. Example is a single bearing inside a glass tube, or between two rods. A four bearing setup on a rod is like an independent suspension. And this is only for lateral motion. I can't imagine what is happening once vertical movement is added.
Even RR box cars with its solid axle wheels are flanged with a small radius for the same reason as they must go around curves. The outside rail is longer than the inside. If not for that difference in diameters the scrubbing action would be enormous.

The ideal, to me, is to have the bearing ride on the flat part of the outer race on a smooth round surface, but is going to be difficult to do, they keep falling off. A straddled vee configuration may work, but is difficult to build. My only criticism on 4 wheel tracking is that only three wheels can be moving perfectly at any one time. A true sprung suspension may be in order. Colin (I think it was him) was right again with leaving the axles to inner race a tad bit loose.

Zene
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Old 14th November 2013, 03:17 AM   #1085
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Zene,

Not sure what you mean by "only three wheels can be moving perfectly at any one time." The felt-tip marks I put on the outer races of the four bearings of my linear tracker seem to move all-together in lockstep when rolling on the rod. I'd been worried over the sage advice that I'd create divots and/or flats in the SS rod by polishing it. That didn't seem to happen (yet).

After I learned that I'd bought the 'wrong' ceramic bearings ($32) based upon earlier 'conjectures', I vowed to try to be 'clear and complete' in what I write, even if violated brevity protocol here - there's been a lot of revealed secrets in the past 100 or so pages. One sure has to pay 'reading dues' in these threads.

Then again, as a retired academic researcher type 'operational definitions' has become a sort of religious doctrine. Hey - remember the satellite that failed due to the use of english versus metric units (or vice versa, what was it)? Clarity requires rigor, and sadly, maybe too much text/photos. I'm glad this isn't a Twitter feed. Truly, I've been quite troubled by the 'just listen' VTF setting advice. I'm comforted by the reasonable 'slop' I find with setting VTF in my creation. It appears that with care in construction one can set VTF using a digital scale with some hope of getting in the ballpark.

Ad Astra,
John

John
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Old 14th November 2013, 03:57 AM   #1086
Zene is offline Zene  United States
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John ... did read your long post, good stuff. Will print that one.
I look at most things micro rather than macro. A 1mm ball traveling the same rate of a record travels about 1/72 revolution per second (math critics welcome). That's not the same as rolling a bearing mount along a tube with your finger and checking turnover rate. Do I think the three wheel analogy is critical, certainly not, but getting to the nit picking analogies is certainly worth looking at, even for a second or two. If they have some validity they should be examined further, if not they can be disguarded permanently unless they come up again with other changes.
My favourite micro scenario is to be shrunk down and ride the stylus in the groove. Should be a trip.

Zene
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Old 14th November 2013, 09:07 AM   #1087
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Zene,
The bearing are far from being perfectly rigid, so as long as they are not grossly misaligned all four will make contact. secondly, being that they contact the tube at an angle, they center themselves and distribute the load to each pretty much equally. Again assuming that a they are not grossly misaligned.
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Old 14th November 2013, 07:51 PM   #1088
Fitero is offline Fitero  Canada
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It's obvious to me that Colin has obsessed over this design quite a bit.
That being said, I am very grateful for his effort, and will be building it just as he proscribed. I'll just obsess over making mine as purdy as I can...
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Old 14th November 2013, 09:50 PM   #1089
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my guess and it is only that is that with four bearings each bearing is leveraged on an edge and what this does is to set tension to the internal rave assembly. The race does not have the ability to wobble in this configuration and stiffness in the assembly is what Bo called for and the four bearing system delivers that better. This is to me the same reason why rim drive sound better than belt drive turntables. With a rim drive the bearing assembly is biased into position. A belt drive TT can be made to function in the exact same fashion if the table is slightly tilted. I think the this same type of bias system can also be accomplished in a two bearing arm assembly. Just my take on this and I could very well be wrong but I hope that this may provide possible food for thought. Best regards Moray James.
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Old 15th November 2013, 04:31 AM   #1090
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Default Hello Moray

[QUOTE=moray james;3704030]my guess and it is only that is that with four bearings each bearing is leveraged on an edge[QUOTE]

I was initially intrigued by the two-bearing concept because of lower weight and cost etc.
After making a drawing of the two concepts, I tend to take the side of the four-bearing concept.
The top figure in the drawing shows the vertical pivot to be above the track rods which is not a good idea considering the fact that the vertical pivot is already too high.
The lower figure in the drawing shows the vertical pivot to be at the center of the track rod which is better.
I also showed the bearings in the lower figure in an exaggerated tilted position to illustrate how the bearings are preloaded in that design.
The only thing I would change in the four-bearing design is that, I would mount the track rods in one bearing at each end, which someone else suggested in this thread. If the track rods were glass tubes, the added inertia would be of no consequence.
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File Type: pdf SINGLE_VERSUS_DOUBLE_BEARING.pdf (12.0 KB, 138 views)

Last edited by Straight Tracker; 15th November 2013 at 04:37 AM.
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