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IBM 729 Clutch Brushes

Table of Contents:
    - Feb 23, 2016 - from Ignacio Menendez
    - Feb 24, 2016 - from Ignacio Menendez
    - Feb 24, 2016 - from Stan Paddock
    - Feb 25, 2016 - from Dave Bennet
    - Mar 01, 2016 - from Marc Verdiell, (Happy News)
    - Mar 02, 2016 - from Stan Paddock, (from status report)
    - Mar 04, 2016 - from Marc Verdiell, (Manufacturing Trial)
    - Mar 10, 2016 - from Dave Bennet, (a quote)
    - Mar 10, 2016 - from Ignacio Menendez, (permission to order, using money pot)
    - April 06, 2016 - from Marc Verdiell, (Brush Flex Wire)
    - April 11, 2016 - the Brushes Arrive - the Adventure Starts
    - April 13, 2016 - the Adventure seems successful :-)) - Now to replicate it six more times
    - April 21, 2016 - New clutch brushes, and manufacture - 1959 IBM 729 Vacuum Tape Drive
          - video, 11 minutes, by from Marc Verdiell

Feb 23, 2016 - from Ignacio Menendez

Marc volunteered his services to machine down our needed carbon proofs, and he suggest that to do about 100 needed pieces, perhaps it would make more sense to have a CMC capable business do it for us, based on the working piece from Mark.

I enclose further below the emails that Marc and I had on this subject.

Also please reference 3 figures below, for exploded view of Bakelite holder with carbons, drawing of proposed carbon brush, and location where these are installed within the 729 tape drive.

So, bottom line, it appears to me that the most viable way to make these carbons is :

  1. determine the type of carbon material needed
  2. establish the correct dimensions for the brush
  3. a) machine a .250 round stock bar to the .189 diameter and length TBD
    b) turn around machined piece from above and trim the ,250 diameter portion to the length of .100
  4. make hole in the .250 end for insertion of contact rod (dimensions TBD)
  5. use contact epoxy to secure rod to carbon piece
  6. solder contact wire to rod now secured in carbon (item 99 in figure below)
  7. insert free end of wire through spring (item 100 in figure)
  8. solder free end of wire to contact retaining piece(s) (items 101 and 102 in figure)
  9. install in 729 and test

(Please excuse the poor drawing below, where I attempt to show the carbon brush)

------------------------ 3 figures follow -------------------------------------

Feb 24, 2016 - from Ignacio Menendez
Held meeting after lunch, and continued activity in CE room.

It was determined by Ron Crane that the material that is available with the square shaft contacts with the wire should be OK for our 729's at CHM

On the matter of the dimensions, Glen found in the CE room two additional brush blocks, one of them with 'brand new', unused carbons.

Measured these, and the dimension that we needed, the length of the .187 diameter part, the two new ones, are of different lengths ! measures .375, while the other .335. I believe we should that the longer length, to make the test carbons, to see if at this length they can be inserted in the double holder between the front two clutches, where the amount of space is limited. If unable to install these, we can always cut down up to perhaps .040 and see how long it can be, and still fit.

I enclose pic. below, with the new, previously unknown length, and we verified that the other dimensions are exactly right.

See updated required drawing here

On the matter of the processes available to make our own carbon brushes, it was determined by the group to make in order of less to more difficult, to try them in this order :

First and easiest. From available brush stock, square .250 carbons, use Drill press to make them .187 all the way, then cut to the .450 new total total length, then epoxy on flat plastic washer at the end where the wire comes out, the washer is available with .250 (1/4") outside diameter, and .187 (3/16) internal hole diameter. The cutting down on the drill press will be done with cutting hollow tube jig, which Stan offered to obtain, and make the first pair of test carbons for use in our 729's, possibly in our CE room.

The second, medium difficulty, also involves getting brush stock of .250 with attached wire, and take two passes, with two different cutting hollow tube jigs, one 1/4" all the way through, then, the other 3/16" cut, stopping just .100 short to leave the desired larger diameter head, on the wire end.

The third, is the previously described process, of using a lathe to make the required cuts and at the end, tap a hole, and attach wire with the conducting epoxy, as Marc suggested. This was thought by all to be perhaps the most precise, but also the more difficult way, because of having to deal with adding the contact wire afterwards.

We will try these processes until found the easier one that works properly.

The nice drawings provided by Stan, slightly modified by me, to incorporate the latest thinking of what would be easier to accomplish.


Feb 24, 2016 - from Stan Paddock
Iggy has become an expert on our IBM 729 tape drives.
This is because the only IBM 3850 Mass Storage Systems the museum has is in a wear house on the other side of town and he is not allowed to touch it.

Each IBM 729 tape drive has six magnetic clutches to control the movement of the supply and take up reels.
Each clutch has a pair of electrical brushes to activate the clutch.
We have had problems with these brushes and IBM will not send us new brushes.

The whole team (near and far) has discussed how to obtain new brushes.
This has been the source of 107 e-mails and a number of meetings.
Today at the CHM, the subject was brought up and various options were discussed.
Iggy is the chairperson of the project and he feels a good solution has been found.
Allen Palmer sent an e-mail saying he had purchased some brushes to machine down to fit in the 729.
Should be in a box someplace.
Glenn Lea searched through out the work shop and found the box!!! (Way to go Glenn)
Not only did he find the raw brushes in the box, he found a virgin IBM 729 brush block with the new brushes in place.
We now know the correct size of a new brush.
All we have to do is make a hundred or so.

Feb 25, 2016 - from Dave Bennet
Now that there is a clear picture of what is needed, there may be an easier approach to get there. There is a supplier, Helwig Carbon, which offers to press carbon brushes in any configuration. It might be a good idea to request a quote on 100 pieces, if that is the final number. If the quote comes back as something reasonable, it would surely be a lot easier than machining them all. If you Google "electric motor brushes," their site will come right up, along with others, of course. There may well be other similar sources.

Also, if the decision is to machine them, drill them, and insert a piece of wire in the hole with conductive epoxy, it has been my observation that the wire used on motor brushes is rather special, being copper and extremely flexible, This wire is made from many strands of extremely fine copper wire. I'm sure that this wire is gettable, but the flexibility is obviously very important. I don't think commercially made motor brushes are made this way, seemingly being pressed around the wire. Not that conductive epoxy is a bad idea...

Dave Bennet

Mar 01, 2016 - from Marc Verdiell, (Happy News)

     The round brushes I had ordered arrived today.

     I was able to turn them to the requested size no problem, including the 0.1Ē lip. Just a bit slow because I canít hold them by much, so I have to take little itsy bitsy cuts. I could do 10 a night about.

     They are something like $3.50 a pop. I made two that we can try tomorrow.

     Original on top, turned to size and shape on the bottom.


Mar 02, 2016 - from Stan Paddock, (from status report)
Marc installed in a mounting block his two new machined carbon contacts, then installed in the German tape #3 right reel shaft, rearmost mag clutch, tested, and ran perfect ! WAY TO GO MARC !

Mark had to shorten the .186 diameter part to .330 in order for mounting block to fit in the space between its bracket and the contact rings.... Attached is the latest revision from Marc on the brush.

Updated 729 brush drawing

Mark took 8 brush holders with him. to machine and install 16 more carbons in his shop.

Also, we have pending the estimate for 100 ready made from a brush shop, that Dave Bennet is contacting for us.

Stan obtained and brought to the CHM a very neat miniature camera & led light, that mount at the end of a flex shaft, and displays via USB on the laptop screen....
With this tool we can make an easier and correct installation of the set of brushes that go on the two front clutches that face each other. THANKS STAN !

Mar 04, 2016 - from Marc Verdiell, (Manufacturing Trial)
The pure graphite rods have arrived (preferred material for slip rings). As expected, itís the cheapest (by far) and fastest way to make them. Made 8 brushes in short order. But now I have to solve the next step, attaching the wire (the big problem right now is sourcing the wire). More to follow.

I tried to machine the square brushes, and as expected this is the worst idea: it chips or goes out of chuck alignment way too easily, and they are too far from final size to make it a time efficient process.

I tried to machine some more round brushes at a pace that would be suitable for 100 parts production. I failed twice on just one step, bringing them to length quickly. Either could not hold them tight enough, or I would break the small holding tail. If I can find a good process for that step, it could be a good middle way. Iíll order a few more for trial.


Mar 10, 2016 - from Dave Bennet, (a quote)
Attention: Dave Bennet
Part #: TBD
Description: H580 Qty USD ea Total $
Size: X .25 X .38 Style: Q1 100 $4.42 $442.00
Grade: H580 Stock: N
Lead time ARO and not including transit time: 3-4 Days
Terms and Conditions: Sincerely,
Quotation is valid for 30 days. Helwig Carbon Products, Inc.
Customer Service Department
Min. Release
Account #: 000001
Quote #: Q-306490
Date: 03/10/2016
Helwig Carbon Products, Inc. has been family owned and 
operated since 1928. Our high-quality carbon brushes are 
proudly manufactured entirely in the U.S.A.

Mar 16, 2016 - from Ignacio Menendez, (permission to order, using money pot)
Dave, I talked to Robert.

Robert told me that you can order the brushes, the amount is fine.

Just get the bill and write a note to him for the reason of the purchase, then give them to him, and he will get it to The CHM CFO, to get your money reimbursed.


After some delivery delays in UPS and on our end, the new brushes are now in the 1401 Work Room.

April 06, 2016 - from Marc Verdiell, (Brush Flex Wire)
Kind of moot now since we are getting the ďproĒ brushes thanks to Dave [Bennet], but I canít resist to show you the spool of new IBM spec super flexible cable I just got! Turns out that the company I work for, Samtec, has a wiring plant in Oregon that makes very high end micro-coax, which is truly a wonder of modern cable engineering. I explained my predicament to an engineer over there, send them a sample of the original wire, which they reverse engineered. Then they manufactured a new one from scratch replicating the original cable structure exactly. And voila, new old spec ultra-flexible wire. It has been upgraded to silver coated copper while we were at it. Feels exactly like the original, but looks way fancier. I will try to finish a few brushes if I have time in the next few days. We better send a warm thanks to that engineer and maybe even an SMS card souvenir. Or a framed picture of Iggy with his signature on it ;-) .
This is the side of the reel.
and the silver plated strands of really thin wire.

April 11, 2016 - the Brushes Arrive - the Adventure Starts
The adventure got off to a shaky start. The brushes did/didn't arrive in California, and did/didn't get temporarily lost, and the manufacturer did/didn't make a new set ...

However, Aurora Tucker carried The Box from the Shustek center in Fremont to the Guest Desk in CHM and hence to The 1401 Team :-))
Let the Adventure Begin :-))

Iggy Menendez opens the box which has had so many adventures. Included is the drawing we sent, to help remind us what we ordered ;-))
About Dave Bennet, who did the drawing - Dave works on the IBM RAMAC team, and the RAMAC has two electromagnetic clutches just like ours to operate the RAMAC arm. And the Packing List :-))
And the mandatory warning - Iggy and I agreed we should have died horrible deaths many years ago. All the above for 100 of these little guys

A quick review of the IBM 729 electromagnetic clutches. Basically, there is a magnetic powder between the clutch plates, and if the electro magnet is energized (electric current driven through the copper windings) the loose powder becomes solid, coupling the two plates of the clutch.

When the "German" 1401 came from Germany, there were many discussions of how to bring the tape drives up to reliable operating condition. It was decided to "rebuild" the electromagnetic clutches, in part because the working parts are hidden and of unknown condition and wear. Some of this "rebuild" is described here

To get the electric current into the rotating clutch, carbon brushes are used. This is quite similar to the use of carbon brushes in generators and motors. The brushes slowly wear (get shorter) with usage, and they are usually easy to check visually for sufficient length until the next periodic inspection.
Fortunately, for the "rest of us" these brushes are usually out in plain sight, so we can check for sufficient reserve length.

Unfortunately !! This easy visual inspection feature was not included in the design and manufacture of the IBM 729 Tape Drives :-((

The entire brush, spring, and flexible copper wire are hidden in a block of opaque plastic material (seems to be "Bakelite"). And to compound the problem, two of the three of the Bakelite brush blocks are rather inaccessible between two of the clutches :-((
The previous is best case, this is a worse case. Two clutches are 13/16 inches apart. Two brush blocks are in that limited width, with needed clearance and some length for the brushes to protrude to make contact with the slip rings.
Iggy says "[This is] the new style brush block, which is very easy to install from the outside of the tape drive...."
This is a pair of brush blocks with bracket that fits into the 13/16 inch gap between the two clutches closest to the reel of tape.
Iggy says "[This is] the 'old' style, where both brush blocks are mounted in a non-adjustable bracket, which is installed from the bottom, then rotated to a point away from the side where you took the picture in #1 pic. above, making it very difficult to secure, attach wires and cap, etc.
A worn brush with spring and conductive bracket. Brushes that are worn too short arc with its copper slip ring. Arcing causes electrical noise, and pitting of the copper slip ring, making faster wear on these and also faster wear on replacement brushes :-((
The new brush has a long wire pig tail makes for easier handling and soldering. The excess is cut off after soldering. Note the flanges that control the brush in the brush block.
Iggy is holding a ?Bakelite? brush holder or "block". Note the flange in each brush hole to control the extension of the brush. If this was not there, a worn brush could pop out the hole and get into mischief :-((
Here Iggy is working with a new brush. Any slip and the brush and spring pop out and onto the floor :-((
Here an almost restored brush block. The last step is to solder the copper "pig tail" to the brass holder_conductor. Our nice little electronic soldering "iron" had a nice small heated tip for fine soldering, much to wimpy for heating that brass bracket for proper solder flow. Someone will bring their robust soldering "iron" on Wednesday.
"The proof is in the pudding". Can we get the bracket, brush blocks, with springy brushes into the 13/16 inch gap and get them all lined up on the slip rings ??
Here is my attempt to get the brushes into the gap without snagging on anything and not slipping into the center opening of the clutch assembly.
This is difficult under ideal conditions on a work bench.
And with the IBM cards as shown, you cannot see to align the brushes with the slip rings. We will need transparent material of about 0.007 inch thickness so we can see and constrain the brushes.
Who ever succeeds in doing this with out removing the clutches and shaft from the 729 unit will deserve a "Hero Medal".

April 13, 2016 - the Adventure seems successful :-)) - Now to replicate it six more times
Report by Iggy Menendez, photos by Stan Paddock
Marc [Verdiell] brought in 5 holders with his lathe formed carbon brushes, which we will try soon...
also Stan [Paddock] reworked a couple of additional holders with the new purchased brushes,
with the aid of Frank's [King] 'cattle prod' soldering iron (good and large enough to brand horses !).

Also Dave Bennet came by to ensure that we had received the brushes that he obtained from a vendor (to our specs), and were working OK. We assured Dave that these were fine, and thanked him for his effort in procuring these brushes in such timely fashion.

If you wish, please take a read at my status report attached below, and you can see what it took to get the brush bracket into its position in the 729, and the pain in the neck difficult connection to the wires that Marc labeled "a piece of cake"

Status Report
Sent: Wednesday, April 13, 2016 6:14 PM
Subject: CHM Tape LONG status

German 1401 System
Marc completed soldering the remaining 2 carbon brushes to the holder in the dual holder (pain in the neck) bracket.....

Went to install the bracket on tape #1, and did as follow, with the total of 6 'CE' hands, simultaneously:

  1. Loosened the clutches' drive motors and moved the 4 "V" belts out of the way to make room for the next step
  2. With the aid of two IBM Cards, carefully formed and positioned, the brush bracket with holders and brushes, was inserted between the front and middle clutches, on the left shaft. This was necessary due to the interference of protruding brush tips and clutch contact rings, so the cards compressed the carbons on its tensioning springs.
  3. With much difficulty the bracket was secured on the frame casting, aligned, and tightened.
  4. Now came the difficult task of connecting the two wires and arc suppression caps on both brush holders, with very short wires from an unmovable cable harness. Marc continued to excel himself and had this done in less time that it would take someone to tie his shoe laces....
    Then Marc exclaimed "a piece of cake!" (as my jaw hit the floor in astonishment!).
  5. We repositioned the tape unit to its normal resting place, and after struggling with the "small cables," Glen finally got them all connected.
  6. Powered up and tested load/unload/high-speed-rewind, all OK, and NO SPARKS now. (Marc even made a video of this successful process)
  7. Stan tested the drive with the program that was registering thousands of errors, now running 'error free!'....
    We presume that the previous sparking problem with the old, too short brushes was causing the myriad of read errors.
High-fives, kudos, accolades, and applause for all involved in this 3 hour surgical procedure!!

Connecticut 1401 System
In the process of our normal checkout of the tape drives, we noticed that the infamous tape #3 was not loading the tape correctly, it would stop with the tape just beginning to go into the left vac. column, and loaded good on the right col. AND HUNG !

After some analysis and (empirical processes (read: trial of this, that and the other 'usual suspects,') we finally noticed that the left shaft front 'stop' clutch carbons were barely making contact (a repeat of what we had just finished fixing on tape#1 in German Sys.)

Adjusted the brush holder in this new style bracket to make better contact tension ('new bracket allows individual brush block adjustment). Tried loading the tape again, and now it would load OK on the left col., but would not load on the right col.

After adjustment of both stop capstans. Load/unload works OK.

At time of demo, all SEVEN drives working OK (#3 drive in Conn.Sys ok only for demo as Docents know).

(BTW, visual inspection gave us the count of FIVE new style brush (adjustable) brackets, and two 'old style pain in the neck' ones.

Glen, Marc, Stan, and Iggy.