TENSION IN THE COMBINED TAB AND CARRIER RETURN CORDS
These cords hook onto the carrier of the typewriter and run in opposite directions, providing the force to move the carrier left or right. The reverse direction around pulleys on the sides of the machine and then are wound around drums. These drums are on the same shaft hooked to the mainspring.
They hook onto attachment points on the carrier and at the start of spiral grooves on each drum. As the mainspring shaft rotates, one drum winds in cord while the other drum plays out the same amount. Turning one direction, the carrier is pulled to the left by winding on the carrier return drum while cord is released by the tab drum. The other direction winds the tab cord on its drum while the carrier return drum releases some of the carrier return cord.
One of the pulleys on the side of the machine is spring loaded, in order to maintain a target tension on the cords. A scribed line on that assembly marks the ideal position of the spring loaded pulley when the cords are at the proper tension.
To keep the cord from popping off its drum, each drum must have about one wind of cord when the carrier is at its extreme position - left side of the machine for the tab drum and right side for the carrier return drum. With the cords attached to the drums with the desired one wind of cord on one and the other drum wrapped as many turns as needed to keep that cord tight, the tab cord should fit around the spring loaded pulley and sit at the scribed line.
MY ATTACHMENT LEAVES THE TAB CORD TOO SHORT
The process of installing the cords puts the carrier return cord the carrier, routes it around pulleys on the left side of the machine and hooks it to the attachment point of the CR drum. Latching in a carrier return operation causes the CR drum to wind up the cord as you manually crank the machine, pulling the carrier to the left side of the machine.
Each spiral of the drum pulls the cord by a specific amount (pi times the diameter of the bottom of the spiral groove on the drum) until carrier reaches the left margin (side) of the machine. When the carrier is at the far right margin/side of the machine, there would still be a bit of cord wound around the CR drum thus the cord won't pop off.
Next the tab cord is hooked to the attachment point of the tab drum, wound around it once and then routed around the side pulleys until it is hooked to the right side of the carrier. Each turn of the tab drum pulls the cords the same amount, pi times the diameter of the groove bottom on the drum spiral.
What this means is that the attachment point position of the two drums determines the start of the cord and then each rotation of a drum pulls up pi times the groove diameter of cord. The attachment point position on the drum can be anywhere in a 360 degree rotation, thus the angle where the attachment point sits when the cord is attached can shorten the cord by up to pi*D.
If the drum's spiral groove bottom is one inch in diameter, to simplify the math, then the cord length is from 0 up to 3.1415" shorter, determined by the angle of that attachment point. This is important since the cords don't have a fixed net length, it is determined by the attachment point angles.
When I installed the two cords per the various installation instructions, I end up with the tab cord net length too short by about 3/4 of an inch or it is too long when hooked to the tab drum attachment point without any turns wound on the drum as the carrier is all the way on the left side of the typewriter.
I can compensate for the overly long cord by tying knots but the lack of any winds on the tab drum results in the cord popping off during a carrier return operation. Thus I need one wind of the cord on the drum, producing the deficit of about 3/4 inch.
READ MANY IBM ADJUSTMENT MANUALS BUT ONLY ONE MENTIONED THIS KEY TIP
I have several manuals describing the adjustment procedures for the I/O Selectric (and regular Selectric). Every one, including the documents that came with the IBM 1130 system, skip this key adjustment. When they talk about adjusting cord tension/length they only cite the ability to tie knots in the cord to shorten it. I then found one manual online with this figure that offered a hint.
This does not show the two screws for adjustment, nor mention rotation of the drum. The right hand transport pulley is the one that I call the spring loaded pulley. In retrospect, I see that this comment means intends the mechanic to loosen the two screws on the CR drum, turn it to set the position of the 'transport pulley', then tighten them up.
Setting a new rotary angle for the CR drum effectively can shorten that cord by up to 3.1415" but the target is to shorten it by the missing 3/4" which simultaneously moves the attachment point of the tab drum enough to have one wind of cord on the drum with sufficient space to fit onto the spring loaded pulley.
CAUTION ON THE TAB DRUM WITH ITS ONE SETSCREW
The front drum (tab drum) is affixed to the shaft with a single small setscrew. The documents all warn that this setscrew fits onto a flattened section of the shaft thus you can't turn the tab drum to adjust the cord length. It only fits at one rotary position relative to the shaft.
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| Tab drum |
Otherwise this would be the obvious pulley to try to adjust, as it is the tab cord whose length over the spring loaded pulley is the subject of adjustment. Due to its construction, however, that drum cannot be rotated around the shaft. Instead we have to adjust the CR drum, which changes where the tab drum stops when the carrier is at the full left position.
TWIN SCREWS ON THE CARRIER DRUM ALLOW IT TO BE ADJUSTED WITH WRENCHES
The rear (carrier return) drum has two screws placed at ninety degree positions around the shaft, with nut heads to make it easy to turn with a wrench or nutdriver. These anchor the drum to the shaft and thus don't depend on flat spots as the tab drum does.
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| Different model drum |
I couldn't find a good picture of the CR drum, which looks very much like the tab drum above but has the nut head screws I mentioned. This picture from a different Selectric model has setscrews instead of the nut head type but you can see the same basic adjustment scheme with two screws at ninety degree angles.
That is what the terse comment in the IBM manual above refers to when it says to 'adjust carrier return drum until . . . right hand transport pulley is clearing side frame . . .' as we want to cause the tab drum attach point to have moved.
THE MODIFIED PROCEDURE THAT I NEED TO IMPLEMENT
The mainspring can remain wound up, under tension, because the gears and clutches at the front of the machine hold the tab drum from movement when carrier return is activated, thus I can loosen the carrier return drum without the spring spinning around.
Loosening the two screws with a wrench, I can spin the carrier return drum on its shaft, letting out a bit of cord until the tab cord side would be in the approximate position to sit at the scribed line of the spring loaded pulley that we wish it to attain. Once tightened up, I can pull the tab cord over the spring loaded pulley and verify the cord tension.
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