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What material for tonearm counterweight insert?

S

starbuck

pfm Member
I am looking for the name of a suitable material to use in the mounting hole of a metal counterweight, for where it contacts around the end stub on a tonearm. It needs to have enough friction to keep the counterweight in place once balance is found but be relatively easy to slide in place on the rear of the arm. Something like the material used in counterweights on arms from Hadcock or Moerch,to name a couple, I'm sure other tonearm makers use something similar. Looks like plastic of some sort but I just need the name of the right material so I can search for something of the correct inner and outer diameter.

Thanks for any suggestions.
 
The 3 that I have seen were O rings and I very, very strongly suspect that bespoke polymer O rings were not used.
To get O rings to work requires very careful sizing of stub and groove in the c/w to get standard, probably nitrile, O rings to do what you want.

Tricky - Roksan may have spec'd everything correctly, except for tolerance, as some of the Nima c/w's migrate very slowly - in the case of the one here, towards the pivot.

Forget designing the opposite way around - buying/finding something polymeric to fit what you have. Unless you have very deep pockets.

Just checked - loads of design information online from O ring manufacturers.
 
Thanks. O rings could work though I am re-purposing a different manufacturer's counterweight which doesn't have inner grooves to locate them into, so they would ideally need to be secured inside the counterweight hole to prevent them from just sliding out as I moved the weight into place. Alternatively I could position the o rings onto the end stub of the arm in an approximate position and then slide the weight onto them, it mightn't be perfect but could work okay. Tracking weight is adjusted with a smaller secondary weight(which already has the correct sized insert) so once the larger weight is in situ it wouldn't be going anywhere. I have a box of various sizes of o rings here, I will have a play with some to see if any suit.
 
Grooves in the stub rather than c/w? Identical principle but requiring reasonably precise prediction about where the c/w will be used.

It all rather depends on what you are using for what.

Grub screw and nylon or hard rubber insert in front of the screw?
 
For an MM or MC? Some say solidly coupled better for MC (AO?) - my 774 has a solid coupled counterweight, and I’ve never felt need to change it.
 
On a Roksan Nima (which has a crap c/w ) I once used nylon tube inserted into the counterweight bore , used a grub screw in the counterweight to "nip" the nylon and keep the counterweight secure . It worked well , decouples the counterweight without the problems rubber o rings bring .
 
I suggest that you engineer a means of ensuring that the grub screw does not bear directly on the arm. A ball bearing in the bottom of the drilling will do this.
 
Thanks for the replies. The counterweight I want to try and re-purpose (from a MKII Decca International) has a thread already drilled underneath for a grub screw, I would need to identify thread size as the hole is presently empty. I am guessing imperial given the age of the Decca arm but will check a few small metric screws to see if any fit without forcing/cross-threading. Grub screw plus nylon insert seems like a possible combination, nipped up just enough prior to installation to keep it snug, it would also mean there was no chance of the screw doing any damage to the tonearm end stub. This is just an experiment and I want it to be reversible so if it doesn't work, the various parts can be moved on unharmed.
 
Thread could be BA. If you are careful, you could use a nylon grubscrew (cut-down nylon setscrew). Easily drilled out if you are too kack-handed and strip the slot in the top.
 
I'm not familiar with your Decca, the Rega and Hadcock counterweights have o-rings inside the counterweight, seems to work well without any slippage or movement, the asymmetric Isokinetic aftermarket counterweight for a Rega has a little red possibly nylon or Teflon sleeve inside the counterweight, an Allen head grub screw is then tightened from the top securing the counterweight in its correct position. Seems to work well too.
I like @stevec67 idea of using a small ball bearing as a contact point for the grub screw. The threads in the Decca may be BSF or BA ?
 
Darren L said:
I'm not familiar with your Decca, the Rega and Hadcock counterweights have o-rings inside the counterweight, seems to work well without any slippage or movement, the asymmetric Isokinetic aftermarket counterweight for a Rega has a little red possibly nylon or Teflon sleeve inside the counterweight, an Allen head grub screw is then tightened from the top securing the counterweight in its correct position. Seems to work well too.
I like @stevec67 idea of using a small ball bearing as a contact point for the grub screw. The threads in the Decca may be BSF or BA ?
Click to expand...
My guess is that the threads will be BA, this was a standard on electrical material. Interestingly, some metric screws will fit BA because BA used a metric thread pitch. Bizarre, I know. This is why an M6 nut will go on a 0 BA screw but not the other way round. Both have a pitch of 1.0mm, the 0 BA screw is a *little* smaller. You can feel the M6 nut has some play in it until you nip it up. You may find that other BA (0 BA, 2BA, 4BA are favoured sizes, you won't find odd numbers very often) sizes share pitch and diameter with an M value.
 
stevec67 said:
My guess is that the threads will be BA, this was a standard on electrical material. Interestingly, some metric screws will fit BA because BA used a metric thread pitch. Bizarre, I know. This is why an M6 nut will go on a 0 BA screw but not the other way round. Both have a pitch of 1.0mm, the 0 BA screw is a *little* smaller. You can feel the M6 nut has some play in it until you nip it up. You may find that other BA (0 BA, 2BA, 4BA are favoured sizes, you won't find odd numbers very often) sizes share pitch and diameter with an M value.
Click to expand...

Yes, I can across a very wide variety of nuts, bolts, screws and fastners whilst building the Norton, as it was built from a variety of parts produced over a 70 year period, there was just about an example of every different type of thread used, UNF & UNC for the American parts (wheels) , Whitworth, British Standard & BA on the old British parts, and metric on some of the European 'aftermarket' parts, it wasn't too long until I needed a wide variety of thread pitch gauges, taps and dies and a lot of sockets & wrenches.
 
Darren L said:
Yes, I can across a very wide variety of nuts, bolts, screws and fastners whilst building the Norton, as it was built from a variety of parts produced over a 70 year period, there was just about an example of every different type of thread used, UNF & UNC for the American parts (wheels) , Whitworth, British Standard & BA on the old British parts, and metric on some of the European 'aftermarket' parts, it wasn't too long until I needed a wide variety of thread pitch gauges, taps and dies and a lot of sockets & wrenches.
Click to expand...
Sounds like hours of fun. I used to encounter UNF and metric together when I had the Caterham, and 80s Metros etc were the same. Early Fiestas had a few hangover UNF bits too, iirc, before iy all statred coming from Valencia with sensible fastenings. I have a couple of *very old* bits with Whit and BA, but I can't remember the last time I saw a BSF in use in the wild.

Every so often when I want to bait a food factory engineer I propose buying an Urschel food slicer, made in America with UNF fastenings. The explosions are quite memorable. Sadly even Urschel are wise to this and now make metric variants for Europe and ROW.
 
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