LINC tape drive - belt replacement

This page Last updated July 14 2011, updated Jan 2 2015. Cppyright 2015 Herb Johnson To email me or to order, see see my ordering Web page for my email addresses.


I discuss my LINCtape drive on this linked Web page. I acquired my drive in 2009 but only did a bit of work at the time. Later, I was contacted by Digibarn, who hosts a restored LINC system, and I encouraged distribution of their LINC documents. See my Web page for details and photos of my drive.

On this page, I discuss drive belt replacement during June-July 2011, as a result of contacts from David Gesswein and Jack Rubin. David acquired a similar drive, a Computer Operations Inc CO-600 LINC tape drive, and wanted information from me about belting. This led to our discussion as below. Thanks to David Gesswein for identifying a source for the belts in this note. Thanks also to Jack Rubin, for supplying both David and I with images of original LINC documents. Details of Jack's interests are in this note and in my LINCtape Web page referenced above.

- Herb Johnson, July 2011

Alternative belt suppliers, with issues

I looked for some toothed belts with Google, and came across some belts availiable from Robot Marketplace, one of many dealers of small mechanical parts for hobby robot builders. I thought to check some of the "robot kit" sites, and that's one of them.

They sell Gates brand timing belts, Gates is a major producer of belts. Gates have a "PowerGrip Timing Belt series 9257". The MXL model has a tooth pitch of .080". (1/12 inch is .08333) and 1/4" wide. The site offers a 100-toothed belt that's JUST a little short - 8.0" - and a 110 tooth belt that's 8.8 inches. The belts are under $5 each. But....they only ship UPS and it's $16 minimum for shipping! Meanwhile, I found tha many of these timing belts are METRIC and come in 2mm (.0787 inches) teeth widths. That's pretty close, 1/12 inch is 2.12mm. But a 100-tooth belt in inches is longer than a 100-tooth belt in mm.

As an expedient, I turned to a local source: sewing machine repair parts. Sewing machines use a variety of belts but those parts are identified by brand, model, and function - not by length, tooth, etc. However I found a cooperative local "sew and vac" retailer and repair shop, who allowed me to root through an assortment of belts with a ruler. At $5 each, I obtained belts of the "correct" tooth pitch (12 per inch) and various widths. Those allowed me to verify that about 12 to the inch and something over 1/4" was correct. If I were desperate, I'd try to "splice" one of these belts - an unlikely course of action.

Some of these belts retain their manufacturer's information. One I bought was actually a GATES, Powergrip GT, "412 30 26 01" and "2MR 428 5" and "3041DS". By measurement and by Gates information, this belt measures 7/32" (5mm) wide, 12 tooth/inch (2mm tooth), 12 inches (500mm?) long. Another belt was TSubaki brand 320P2M6. A P2M6 is a PX belt type, 2mm pitch, 6mm wide. It measures 12 tooth/inch (2mm) 6mm wide, 420 mm long.

Either of these belts appeared to "fit" the gearing; the teeth are flexible rubber. Both were the incorrect lengths. But as David noted later, "12 per inch vs .0816 is about 1 tooth off in 3 inches." The belt's total length is about 8 inches; the largest gear engages maybe four inches of belt.

Better data and a source

But in the course of our June 2011 discussions of belts for each of our LINC-tape-like drives, Jack Rubin provided a copy of the earliest LINC documents to both of us. (Details on my LINC Web page.) David then determined an appropriate size for my LINCtape drive, as follows: "The LINC document package states the belt is Rusco B1096 1/4" x 8.323" 40 D.P. Looking on the web 40 D.P is 40 diametric pitch which is .0816" tooth spacing or 12.255 per inch. The pitch is well within my measurement error, the length probably not. At 8.323 thats 102 teeth. 8" is 98 teeth....The modern standard is .08" pitch ([Gates] MXL) which is easy to find but it is not compatable." (I later tried to count teeth on that belt - I counted 101 teeth, once. 8.25 X 12 = 99) David had also noted his belts "measured 8" with 12.333 tooth spacing."

David then found an Amazon seller "Small Parts", who also sells other kinds of "industrial and scientific" metals and small mechanical hardware. They offered from close-out stock the following belts: "Timing Belt Urethane/Polyester, Single-Sided, 0.0816" Pitch, 1/4" Wide x 8.3232" Long, 102 Teeth".

I had previously measured my decayed belt as 8-1/4" long, 1/4" wide, and has 12 teeth per inch. I concurred with David that the belt he found was a likely fit for my drive, and possibly a fit for his slightly different drive. I ordered the entire remaining stock (11 belts) at a remarkable price of under $2 each. Adding some nylon screws to the order raised the total for Amazon's free shipping. They arrived about a week after ordering. My results are below.

David believes he can get suitable belts from Stock Drive Products -- Sterling INstruments. But I'll send him one of my belts and my sewing-machine belts, and he can see how they work.

Disassembly and fit

On July 11th 2011, I've cleaned up the gearing in preparation for fitting the belt. I used some Xacto-type knife tools to scrape away most of the crumbled rubber of the old belt.

To test the tooth fit, I laid a portion of the belt around the large driven gear. It felt OK, it seemed all the teeth were aligned.

Whille looking at the mechanism, I saw that the large-gear bearing to the back plate was not even a "snug" fit. So the bearing was held in place only by the pressure of a rubber compression fitting at the tape hub. By unscrewing and moving the back plate, I was able to expose the bearing and move it out from the plate.

With continued unscrewing and back-movement of the rear plate, I was able to slide the belt around the bearing and shaft, without disassembling the mechanism. This was a relief! Still, it tooks a bit of time to re-align the plate to the hexgonal posts which hold the rear plate. These posts are screwed into the back of the front plate of the drive; slightly loosening them at that end allowed me to re-align the posts to fit the rear plate.

With the belt "inside" the circumference of the large gear, It was only a slight stretch of the belt to wrap it around both gears. The belt fits with a resonable tension. Part of the tension was due to remaining debris on the gears. As I rotated the gears, the teeth of the belt abraided some of that debris. Ultimately, I'll have to clean the teeth with some solvent. But this "fit" confirmed that these belts were correct for this drive.

In addition, if you look closely at the series of photos above, you can see that the drive motors are screwed through the rear plate through SLOTTED holes, not circular holes. This suggests the motors could be adjusted to control tension on the belts. I had not noticed this before.

- Herb Johnson July 11 2011

Another belt source, 2014

In Dec 2014 I was contacted by Charles Lasner, who recommended as follows: " Stock Drive (I actually live not that far from them in NY). I can recommend the stuff they cut [for my LINC drives] as working great. I don't remember any specifics, but the ones that work are actually orange in color and have the right characteristics for nice pliability and long life, instead of the old stuff usually just hanging on by threads if that much." Charles is a long-time contributor and developer in the PDP-8 and LINC world.

Herb Johnson
New Jersey, USA
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