Tonearm pivots

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Hi, the bearings in the RB250 and 300 are actually very *low* grade, a fairly cheap, unbranded shielded. flanged radial miniature ball race of ABEC 3 spec. If that sounds a bit mean, they are actually the right choice for the job.

You can get very high-spec direct replacements (eg SKF abec 5 or 7) for just a few pounds from suppliers like RS components, but there's no point in replacing them with anything better using the standard mounting method because the way that the bearings are mounted and set up needs the loose tolerance of ABEC 3. To use a finer spec means using a different mounting method. Even then, I found that ABEC5 was a better for this application than ABEC7 which are very closely toleranced, need very precise alignment and still need maybe 50 hours bench running in to be free enough for this kind of use. The standards were set for using the bearings at their rated loads rather than just waggling around with a few gramms of tonearm. From memory, I think the Rega bearings are rated at something like 70kg at 50,000rpm!

I've been doing bearing replacements on Regas:
http://www.william-reed.net/audio/
and I've recently developed a method of making bearing carriers that can be user set-up and adjusted to *very* fine limits, like well under 0.0001", without using ultra-high-precision machining. This is letting me use very high-precision ceramic bearings on composite carbon fibre shafts. Haven't finished it yet, but as it's taking so long it had better work! That bit's not on the site yet, but I'll try to get photos up soon.

regards, Jeff
 
Hi, I had started out ball races because i have been modifying Rega arms and that's what they used. I have just started to use small (10mm OD) ceramic ball bearings that are probably too expensive for most manufacturers at about 20 Euro a pair, but OK for a one-off, especially when you look at the price of a bit of tonearm wire!

I'm not sure about jewell bearings, they are tiny and very light which could be a major advantage but my own view has been that the bearing should be capable of transmitting energy from the arm to the mounting rather than isolating it. This might be a factor in the difference between the sound of pivoted and unipivot arms. I do know there are a lot of different kinds of jewell bearings but I don't know where to buy them in the UK.

If it will be useful, I can look up the RS part number for a bearing that is a straight swop for the standard RB250 ones - I just don't have it to hand at the moment.

regards, Jeff
 
Hi, I'll look up the part number tonight as I saved the packets they came in.

The paint comes off the Rega arm very easily with ordinary paint stripper, I used "Nitromors" brand. This is much quicker than sanding or scraping it off.

Personally, I wouldn't recommend removing the paint unless you intend to either re-paint or laquer the arm tube at the end of your modifications or if you are adding some kind of internal resonance control like cross bulkheads or the foam filling that Michel use. This is because the bare arm tube is very resonant and the rather thick coat of paint actually helps to damp it. A bare, unmodified armtube rings like a bell!

regards, jeff
 
Hi, here's the specification for a bearing that is a direct replacement for the ones in the RB250 etc.

Outside diameter: 13mm (15mm over flanges)
Inside diameter: 6mm
width: 5mm
ABEC 5, replacing ABEC3

The part number from RS components
www.rswww.co.uk
is 540-306, but this is a very common component. They are about 6 Euro a pair.

As you can see, this is quite a large and heavy bearing for the use we're putting it to, so if you are doing any serious modification it is worth using the smallest you can find. A 10mm OD bearing reduces the mass by about 50%.
Because these things are really meant to run at quite high speeds with a load, I think it is worth running them in before you mount them in the arm as they improve in smoothness and friction considerably.

I use a really cheap little DC motor salvaged from a toy car, with a sleeve on its shaft to suit the bearings and the bearings secured from rotation with masking tape. I then just run them for as many hours as I have patience, occasionally oiling with light machine oil.

regards, Jeff
PS: Health warning: The Rega arms aren't meant to be dismantled. If you disturb the bearings, you risk wrecking the arm!
 
Hi,
My understanding is that you can take an RB300 armtube off the yoke without disturbing the bearings, but definitely not on an RB250. The two arm yokes are different.

On the RB250 the bearings are a press-fit in the arm tube and there is a threaded brass stub bearing carrier in each side of the arm yoke. On each RB250 I have examined, these brass stubs are fitted with a *lot* of locktite and are *very* difficult to remove. You usually damage the screwdriver slot doing this, which is OK if you are throwing this part away. You are also left with a messy thread in the yoke.

see:
http://www.william-reed.net/audio/images/regabearing_web.jpg

At the factory, the bearings are assembled in a jig and it would be very difficult to get the right side load without it.

On both arms, the vertical bearings are press-fit into the base. They are loaded by finely adjusting the nut on the brass spindle. Again, this nut is secured with locktite and it's adjustment is fairly critical.

regards, Jeff
 
Hi Jeff,

I don't know if you are already aware of this but Loctite can be 'destroyed' with regard to its 'adhesive effect', simply by heating it.

Clearly it will depend on the parts concerned, but getting the parts not much hotter than boiling point has worked for me in some other engineering projects, and the heat did no damage to the parts themselves.

If this application might be of any use in dissembling these 'otherwise difficult' tone-arms, it would be worthwhile contacting Loctite to see what temps are needed to destroy their effectiveness.

I hope this helps.

Regards,
 
Hi Jeff,

You are welcome, and I guessed that maybe you weren't aware of this from your remarks over struggling here.

See also dice 45's comments on Epoxy in the penultimate para. of post #102 of "dc motor speed controller", where he has just stated 160 Deg. C will be enough to allow dissembly.

Different grades of Loctite *could* have different temp requirements here, which is why I suggested contacting Loctite for precise info.
IIRC merely 120ish Deg. C was sufficient to destroy the integrity of the toughest (Engineering Grade of Loctite), and I doubt that in production any tone-arm manufacturers would use such tough stuff as this. :xeye:

Regards,
 
Hi Jeff,

Another thought which crosses my mind is related to your comment on running in new bearings.

Whilst I haven't (yet!) tried this for myself, I have no doubt that the following will work well and save a lot of time here, as I have adopted the idea in other similar situations.

Simply replace the oil in the bearing with some abrasive metal polish like Brasso, and run this for a while.

My guess is that you will need to watch how long you do this for, so as not to go too far and loosen the bearing up too much, but a good clue as to what is happening is to look at the colour of the 'polishing lubricant'.

It will depend on the hardness of the balls & cage, but as the metal wears away in the polishing process, it will start to look darker until it is quite black eventually, with most metals.

Finally, a good swill out with some cleaner (white spirit, meths, or any petroleum-based or cellulose-based thinners, works well here) and then re-lubricate with the chosen oil, and you are away.:)
 
Hi Moray,

You are quite right here, and this is another method which I have found works quite well if there are substantial 'exposed' edges for the solvent to work on.

However, in these cases of pick-up arm bearings (at least those I have been involved with) usually the machining is very good and close-tolerance to begin with, and this generally leaves only a thousandth or so of an inch (at most) of exposed 'glue line' for the solvent to attack when it is soaking. Allied to that, as probably the bearing width will be say 1/8" to 3/16" wide, in my experience it will take a very long time (if ever) for this to fully loosen the entire bearing assy.

I have even tried Loctite's own "Detach" which is Heterocyclic Ketone, and expressly intended to dissolve cyano acrylate, but I have not had any better succes with this in these situations.

Of course, although they are both anaerobic adhesives, the makeup of cyano and 'Loctite' used in engineering will be rather different, and this may have some bearing (no pun intended!) on the issue.

Careful heating does work well, provided the assembled components can take the required heat without any damage, and I was pleasantly surprised to discover just how little heat is needed, even for the strongest grades of Loctite.

Regards, :)
 
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