PDP-11/44 gas struts

Alan Frisbie Flash688 at flying-disk.com
Mon Jan 3 19:17:10 CST 2022

Earlier, I wrote:

 > 30-pound struts are not strong enough.  They improved things a bit,
 > but it still takes a lot of effort to raise the box.  I have ordered a
 > pair of 50-pound struts and will post an update when they arrive (next
 > week).

The pair of 50-pound struts arrived today, and in my opinion they are
just barely adequate.  If I was trying for perfection, I would use the
60-pound ones, but they currently have a 6 to 7 week lead time.  I can
easily live with these.

In summary, what I have learned about replacement gas struts for the
DEC PDP-11/44 in a 40" high cabinet:

1. The specifications are: 15.24" extended, 9.77" compressed, stroke
length 5.47", threaded ball studs, extension force 50 or 60 (preferred)

2. A good replacement is the McMaster-Carr 4138T55 gas strut in either
the 50 or 60 pound force version.  The price as of January 2022 is
$20.29 each.  <https://www.mcmaster.com/catalog/127/1377>

3. The threaded ball ends on the 4138T55 are 5/16"-18 threads, while the
original DEC ones are 5/16"-24.  They are easy to swap without
unscrewing them from the cabinet, and this is the method that DEC
recommends and describes on pages 5-5 to 5-7 of the PDP-11/44 System
User's Guide, EK-11044-UG.  You simply unsnap a retaining clip (no tool
needed), pop the strut off the ball end (a screwdriver might help),
and reverse the process with the new strut.  A piece of 4"x4" lumber
does a good job of holding the box up while doing this (stick it
under the rear of the cabinet when it is in the raised position).

EK-11044-UG is available at:
(In my earlier posting I gave the wrong DEC P/N for the manual.)

4. The 4138T55 struts that McMaster-Carr sells are manufactured by Suspa
Parts <https://www.suspaparts.com/>.  The 30-pounds struts are their 
part number C16-24186, and the 50-pound struts are C16-24188.   I
suspect that the P/N for the 60-pound struts would be C16-24189, but
cannot confirm that.  Their price is $32.00 each, so it is less
expensive to buy them from McMaster.

5. Suspa's design guide recommends mounting the struts with the rod end
down, the opposite of how DEC mounted them.  I have verified that they
can easily be mounted with the rod down as long as you connect the
upper (body) end first.  That is the way I have them now in my system.

I hope that this helps someone who might need to replace the gas struts
on their PDP-11/44 system or something similar.  It has been an
interesting learning experience.

Alan Frisbie

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