Sunday, November 19, 2023

Typewriter 'whack a mole' getting the carrier return/tab/escapement cords set up


The cords were removed and I had to replace the drum/mainspring system with parts from the donor typewriter, documented in an earlier blog post. This left me with the tab drum sitting on the front of the shaft but not fastened down and the mainspring installed but not wound up. I hadn't put the two screws into the CR drum to fasten it on the shaft either, and neither cord was attached. 


The front of the shaft where the tab drum sits has a flattened area and the tab drum has a setscrew that will lock the drum on the shaft at that flat spot. I moved the drum so the gear on the drum edge would engage with the mating gears on the operational shaft, tight enough to meet the backlash adjust of no more than .006". The setscrew was tightened to lock the drum in this position. 


The mainspring was installed on the rear of the shaft and I could replace the mounting bracket for the motor start capacitor which otherwise blocks access to the spring. The procedure for reattaching cords first puts about five turns on the mainspring which would be the force remaining when the carrier is fully rightward. This is a minimum sufficient to power the spacing and tab movements of the carrier even at the right margin. 

Then when the CR drum pulls the carrier to the left margin it will wind additional turns on the mainspring. In normal operation, for a return operation the motor turns the shaft through the tab drum to wind cord on the CR drum, let cord off the tab drum, and wind power into the mainspring. All this energy in the mainspring is available to pull the carrier rightward whenever the escapement teeth disengage, either to space one column over or to fly to the next tab stop or all the way to the right margin. The power for rightward movement of the carrier comes from energy stored up in the mainspring. 


My next move was to attach the return cord to the CR drum, route it around its three pulleys and attach the other end to the bottom of the carrier which was sitting at the right end of the machine. When that was done I would manually turn the operational shaft to draw cord onto the CR drum, wind the mainspring and pull the carrier to the left side of the machine. 

When it reached the left side, I would then attach the tab cord to the tab drum, getting at least one wind on it while at the left side of the machine, route the cord over two pulleys and attach it to the right side of the carrier. Turning the CR drum on its shaft (since the screws were loosened to let it turn freely on the shaft), while turning the tab drum in the other direction, I could get the lengths of cord approximately correct and lock down the CR drum screws. 

Pulling the tab cord over the spring loaded pulley on the right side of the machine will indicate whether the tension on the cords is correct or not, by the position of the pulley against a fixed mark. Slightly loosening the CR drum screws and turning that while holding the tab drum/mainspring from turning, while differentially loosen or tighten the cords after which I can lock down the CR drum. 

Unfortunately, this is where the Selectric began to play whack-a-mole with me. That is the game where as you try to knock down a mole popping out of one hole, others pop up from different holes, such that you never have all the moles down at the same time. The theory sounds simple, per the manual.

Lets summarize the number of items that need to be controlled or manipulated for this all to work out:

  • CR cord attached to bottom of carrier
  • CR cord positioned around two pulleys on left side of machine, in the groove on each pulley
  • CR cord positioned over small pulley in rear of machine on post, in groove
  • small pulley does not jump or fall off post
  • CR cord wrapped smoothly around CR drum

  • CR drum turned and held to tighten or loosen CR cord
  • Tab cord attached to right side of carrier
  • Tab cord fit round the spring loaded pulley on the right side of the machine
  • tab cord fitted over small pulley on post above tab drum
  • small pulley does not jump or fall off post
  • tab cord wrapped smoothly around tab drum
  • Tab drum and mainspring turned to tighten the tab cord
  • CR drum turned on shaft to tighten CR cord without releasing tab cord
  • Two screws set 90 degrees apart on CR drum must be tightened down while cords in tension
I no sooner get the CR cord properly wound on the drum when the cord pops off 1 to 3 of its pulleys. I get the cord back on the pulleys and the end falls off the bottom of the carrier. 

The issue is the number of hands which need to be involved to keep all of the elements in their proper positions - one hand for the CR drum and one for the tab drum. Other hands so the cord tension doesn't slacken so much that it detaches from the carrier or loses the even wind on the drums. Other hands to tighten the screws on the CR drum. Eyes and hands to ensure that the two small pulleys don't jump off. Eyes and hands to make sure the cords run in the grooves on two small pulleys and three big ones, plus in two drums. 

I put in a good two hours until the last time that the CR cord fell off the bottom of the carrier. When next I get into the shop, with a packed Thanksgiving week in the way, I will ever so patiently reinstall it, thread the cord on all the grooves, fix up the drum wind and then try to get the tab cord started on the tab drum. 


  1. A couple of things came to mind as I read this. Questions really.
    1. Could you use a bit masking tape to secure the CR cord to the bottom of the carrier so it can't fall off? Failing that, I think I'd use some temporary glue, like maybe model airplane cement.
    2. If the shafts are long enough for the two small pulleys, is there room to put a small binder clip on the shaft so the pulley can't fall off? Or perhaps use a small circlip for the same purpose even though there's no groove for it?

    1. Both are excellent suggestions!

      Might put masking tape or hot glue to hold the cord where it comes out from under the carrier on the left, rather than underneath, but the important thing would be to maintain very slight tension so that it doesn't back off.

      I would need something smaller even than a binder clip but it needn't be too heavy duty. As you said, perhaps a circlip of suitable diameter

    2. Glad that was helpful! I was just thinking about how to reduce the number of hands you needed at once. :-)
      It later occurred to me that you might instead temporarily glue a small washer to the end of the shafts the small pulleys run on with the internal dimension of the washer a bit smaller that the diameter of the shaft.

    3. After I am back home from my Thanksgiving trip with extended family I will get back to this and make use of the tips to ease the task.