Kalani wrote:

Ixzed23 wrote:

Never mind folks!

I took the belts out of my 1964 SCM Electra 120, made in Canada (it might be different than thebones made in the USA).

The front belt is actually 311 mm long but I would settle for 310 mm while the rear one (with the silver mark) is 320 mm long. Measured by walking the outside flat side on a measuring tape.

The rear belt has a pretty bad set that has not gotten better since October 2021, when I became its new owner.

Daniel

Aloha Daniel,
In one of my earlier posts, I have a picture of using two 315 mm belts per the links and sources given me on this thread.  I have tried them on a number of Electra 120's, also 250's.  There are some variances between machines that small adjustments in motor positions and other adjustments can't cover.  On some Electra 120's, two 315 mm belts were just too tight to get on.  On others, they were mountable but too tight for good motor bearing life.  And, in others, they fit just fine.  This is where the greater "stretchy" tolerance of the various sizes of "O" ring type belts, also on this thread, come in handy.  And, even with the "O" ring belts, sometimes a combination of 2 different sizes works better than identical size belts.  The rear belt has to clear the body to carriage trim plate without being too tight, and yet with enough flex to allow the center pulley to slide up and down on its pivot arm.  Same for the front belt to the drive bar.
If two 315 mm V belts work for you, I have the serial numbers and sources upthread, then great.

Have you tried the "O" rings yet instead of pursuing V belts?
k

Forgot to include the link to the 315 Polyflex belts.
https://www.bdiexpress.com/us/en/Mechanical-Power-Transmission/Belt-Drives/Drive-Belts-V-Belts/3M315/p/1105312

thetypewriterman wrote:

The green PU round belting may not be the solution... 

There are limitations, for sure.  One is the min diameter of the pulley. For 3 mm diam belt, the pulley should be 30 mm diam or bigger, according to specs. 

Ideally, the belt should be pliable but not stretchy. V-belts are still best. 

Thanks for sharing your experience.

M. Höhne wrote:

Ixzed23 wrote:

Thanks Jim.

Two belts of 315 mm would work. I assume it would cause the typebar to hit the platen faster than if the rear belt was 320 mm long.... snip ...
Daniel

Why would you assume that??

Because a shorter belt will affect the variable diameter center pulley by increasing its rotational speed.

Ixzed23 wrote:

Hi Kalani,

Thanks. Yes, I read the complete conversation of this post from page 1.

I am really puzzled that the same belt size is not used on all the variations of SCM series 6 and 7 electrics. Hence my initial question.

You're right. I don't know why the actual applications are different.
From the early SCM Electra 120's platforms with manual carriage returns to the power carriage returns that include the Coronet Electric 10's, Super 12's, 220's, 250's (Secretarials), and various models with different casings and names but same (innards), there are variances in the assembly and maybe materials tolerances such that, for example, the same 315 mm belts are just too tight to either install at all or too tight on the motor and clutch.
I've ordered the next size up to try out varying the belt sizing, or using that one size larger on both belts.
I'm also trying various sizes of the "cog belts" which are 2-4's and 3-4's on the cogged belt SCM later model electrics with either ribbon spools or the ribbon cartridges. It costs from 4-9 dollars per belt plus 7-10 dollars shipping.  I count the cogs and micrometer measure everything on the belts, but still, it's a slow process.  There's not much info about sources of these belts so I hope it will benefit others in the long run.
Also, the later SCM's with single cog belt power/motor drives have tapered wheel belt tensioners or guides on some models and not on others.  I have to see how this effects the belt lengths as well.
So far, the main drive single long cog belts are available, but not the 3mm ? cartridge drive side belt. If others here know other belt maker/distributors to search for those sizes, please let me know. (thanks!).
Anyway, I hope this is helpful.  Others here are the "typewriter gurus" and "experts", not me. We're getting good help here.
k

Yes!

I'm sure in the recesses of some antique store somewhere, you could find an o-ring travelling salesman's display case for these. That'd make the perfect home for these.