LINC, LINCtape, DECtape

LINCtape drive and tapes

A few years ago, I picked up this DECtape-like drive. In Aug 2009 I realized it's a LINCtape drive, probably produced in 1966 for a LINC computer of the period. Another Web page describes the LINCtape drive and work I've done on it. This page describes DECtapes I obtained in Aug 2011 branded "Astrotype" from Doug Jones; and Scotch tapes from Sept 2011 from David Gesswein. Doug says it's hard to mount and dismount his Astrotape reels; that lead me to make precision measurements of my LINCtape hubs and various tapes.

In 2011 I acquired some DECtape motors, Alnico brand, with hubs. I've not measured or analyzed them yet (2015) but may do so.

- Herb Johnson

Astrotype tapes

[Astrotype tape reel] On the left is a photo of one Astrotype tape as received. It's in a blue-tinted plastic container. Beneath is apparently the content directory of the tape, written in the mid-1970's. The dimensions of the tape and reel are discussed below.

[Astrotype tape] On this image to the right of the physical tape, you can see it's about the width of a US penny. The tape is 3/4" wide. This end has had a lot of handling, so part of the darker-brown coating is debraded. You can see the ligher brown is the Mylar? tape base material. I'll add more information about tape and coating when it's available.

[Astrotype tape on drive] In the photo on the left, the tape is mounted on the LINCtape drive. The 2.5-inch diameter inner ridge, and the slight ridge on the "back" of the reel, make contact with the hub. No doubt, these provide the physical reference points for aligning the tape on the hub.

A close look at the hub on the right, shows a "top hat" kind of profile, with a spring wrapped around the circumference of the hub to also secure the tape. The front/top edge of the hub is beveled. See the sketch above for a profile of the hub. A closer look at the hub under the spring, shows a narrow bit of dried-out rubber under the spring. Doug Jones confirmed his 8/L Astrotype system drives use the same hubs.

Doug's Astrotype PDP-8/L system

Doug described his system to me in Aug 2011: "I have a BE01 DECtape controller, and 2 TU55 DECtape drives. This material from Chicago was originally an Astrotype typesetting system. The BE01 DecTape controller is an OEM version of the TC01, and the TU55 drives were OEM versions, with Astrotype labels instead of the usual DEC paint job." For more on Doug Jones' PDP-8 collection, check his pages at this Web link. For more on the Astrotype product, see Doug's Astrotype notes on this Web page.

LINCtapes and DECtapes

In Sept 2011, David Gesswein was kind enough to send me two tapes - one each Scotch brand LINCtape and DECtape - and two empty reels. Here's a photo of the boxes which have descriptive labels. The "LINK" boxed tape lable says "84-9800-4161-4, 481 3/4 x 150 R42 LINK". The other box presumably DECtape says "481-3/4-150 R42, 84-9800-4161-4" and "plastic reel 3/4 x 150' (19.05mm X 45m)". Both describe a 3/4-inch wide tape which is 150 feet long.

The tapes have slightly different coatings: this photo shows the tapes side by side. The DECtape on the left has a brownish coating; the LINCtape on the right is darker. (The boxes under the tapes are probably reversed.)

I have more references to David Gesswein's PDP-8 site and about his work with me on my LINCtape drive, on my LINCtape drive Web page. Thanks again to David for these tapes and empty reels.

Tape reels and hubs - "fit"

Doug says he needs "a sledgehammer...and crowbar" to move tapes on and off his TU55 tape drive hubs. I found some variation of fit of his tapes, and David's tapes, on my LINCtape drive hubs. So I made a series of measurements of hubs and reels, to determine why this is, and to consider what can be done about it. (Note to self - I also need to add dimensions from the LINC documents of the hubs.)

[Astrotype dimensions] On the left is a dimensioned sketch of one of Doug's Astrotape tape reels, and my #1 LINCtape drive hub. The reel is 3.8 inches in diameter, with a inner hub ridge of 2.50 inches diameter. The 3/4-inch (.750") tape is wrapped around an approximately .778 inch inside-width reel (with slight taper or warpage). The outer width of the reel varies from .935 to .945 inches. The sketch shows some dimensions of the ridges on the back and inside of the hub. The apparant signifigance of the ridges are discussed below.

[LINCtape DECtape dimensions] On the right is a dimensioned sketch of one of David's empty DECtape/LINCtape reels. The reel is 3.88 inches in diameter, with a inner hub ridge of 2.50 inches diameter, inside a larger unridged diameter of about 2.640 inches. The .750 inch tape is wrapped around an approximately .8 inch inside-width reel of 2.795 diameter. The outer width of the reel varies from .960 inches closest to the hub (excluding the ridge of 4-6 mills height) to a few thousands smaller at the edge of the reel. (The ridge is hard to measure directly because of its shape and location.)

LINCtape hubs: If I number the leftmost hub as #1, my micrometer measurement for each hub are as follows.
#1 - 2.489 to 2.490; #2 - 2.490 to 2.491; #3 - 2.490 to 2.491; #4 2.491
..if I also measure the diameter of the metal spring band around the hub, I get roughly these numbers:
#1 - 2.502 to 2.504; #2 - 2.503 to 2.505; #3 - 2.503 to 2.504; #4 2.503 to 2.505

However, these don't indicate variations in roughness or any bumps in the aluminum hub; more error for the springs; and they are otherwise subject to some few thousandths error of measurement. But in fact, my slightly smaller #1 drive hub imakes for a smoother "fit" for one of the Astrotape reels, and David's two LINCtape/DECtape reels, than the other three which require some larger effort to mount the tape in proper position (seated on the base of the hub. This suggests that only a few thousandths difference in smoothness or diameter (either on the reel or on the hub) can have a notible effect on mounting effort. (Note: I visually confirmed the Astrotape reel is smaller in edge-to-edge diameter than the DECtape/LINCtape reel.)

In June 2015, I was asked to make precise measurements of the hub spring and groove. I made more measurements of the left-hand hub #1, or as better described the left hub for tape head #2769.

The hub measures close to 2.45 inches in diameter, just below the flat taper at the top. The top measures about 2.27 inches diameter. The base of the hub measures about 2.49 inches in diameter - that may be a slight taper or just variation in production. The height of the hub from lip to top is about .560 inches. At the base, there's a 1/16" thick lip which has a hub diameter of 2.84 inches. Below the top of the hub, there's a groove for the spring of about .117-.124 inches deep, and .129-.131 inches wide. (Missing measurement: height of groove relative to top or bottom.)

The spring when uncoiled, is 6-7/16 inches end to end, including the diameter of the single loops, which is about .136". The spring body length is 6-3/16". The spring diameteris .125 to .127 inches (most likely .125 or 1/8"). Underneath the spring was two loops of thin rubber band that was much thinner than the groove. (Bob Vines asked me for spring measurements; he plans to get stainless-steel springs from McMaster-Carr. HOwever custom orders appear to require quantity 50. - Herb)

PS: In 2013 I got this note from Maury Pepper, one of the LINC developers: "Regarding tape reels and how they fit on the hubs: I can tell you that it was routine to have one or more thin rubber bands under the spring. As they aged, it would be necessary to replace them. The technicians at CSL did an amazing job of keeping the LINCs reliable. As a user, I did not appreciate how much craft went into it."

Doug Jones measurements, TU55 Astrotype, solid "hartman" hub

In June 2015 Doug Jones made measurements of a tape hub. "I went down in the basement with a machinist's ruler and a magnifying glass, picked a TU55 at random (the one with a hub at the right height) and set to work.". This is another solid-metal hub. "The body of the hub is a cylinder 2.5" in diameter, with a flange at the back that is 1/8" larger and 1/16" thick. The cylinder is 1/2" long, with an additional 1/16" protrusion on the front that is tapered at 45 degrees. The groove is 1/8" wide and is centered 3/8" forward of the flange." Here's Doug's sketch.

Later he added: "I pulled the hubcap off of the Astrotype TU55 hub and measured it, same methodology as for my humb measurements. The cap is injection-moulded ABS or PVC. Can't say which. The pebble texture is much finer than the pebble texture on the H960 cabinet covers. The hubcap is 2.25" diameter, and all parts of it are 1/16" thick. There is a 45 degree chamfer that takes off 1/16" around the edge of the face. The back of the hubcap is hollow, with a cylindrical rim that protrudes 1/16" from the back of the face, making the front surface of the hubcap sit 1/8" forward from the face of the aluminum hub."

"There is a central lug 1/4" diameter that fits into the 1/4" screw hole on the front of the aluminum hub body (the screw head is recessed 1/16" from the face of the hub, and the lug (which is hollow, with a 1/16" wall) protrudes 1/8" from the back surface of the hubcap, which is to say, it protrudes 1/16" into the screw hole."

But note! I have two different styles of Hartman hub. The hubs on the Astrotype drive have the central screw hole, glue or perhaps double-stick tape residue, and I have one hubcap that was very loose when I picked it off for measurement.

The TU55 drives I have do not have the central hole, and they do not have glue or tape residue that might have held on a hubcap. Instead, the outer 1/4 inch of the flat face of the hub is turned with no visible tool marks and a matte texture, while the inner part of the face, all the way to the center, is turned with a very fine texture so it is almost a circular diffraction grating, clearly intended as a decorative finish. I strongly suspect that these two hubs never ever had hubcaps. As I said earlier, each of my TU55 and Astrotype drives has one starfish hub and one Hartman hub." - Doug Jones Here's Doug's updated sketch.

One of Doug's Astrotype drives is pictured at this link.

Comparing to my measurements: His hub flange is different in diameter, but he may not account for any taper to the cylinder. Otherwise his measures seem similar to mine. He includes the vertical distance from the flange to the center of the groove. Doug could not measure the groove depth due to decayed O-ring material. - Herb

Herb Johnson
New Jersey, USA
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