Originally posted in January 2017

I was lucky enough to obtain a Tektronix 202-2 Scope-Mobile Cart.

Upon examining it, I found that the scope tray was loose and the tilt mechanism did not function properly.  The cause for this was broken trunnion mounts on both sides of the cart as follows:

Three of the four bosses were fractured with the fracture surface indicating tensile overload.  I suspect that the bolts were over-tightened and the bosses fractured. Over-tightening loaded the bosses in tension by compressing the aluminum box frame member.  Unfortunately this type of polymer (some type of hard thermoplastic, possibly reinforced) is not good in tension.

Other mobile carts I have seen use cast aluminum for these trunnion mounts, which is a better choice for strength and durability.  The cart I have for my 556, a 205-2 (triple wide), uses cast aluminum trunnion mounts.

Broken Bosses Closeup. The fracture surface shows brittle tensile overload:

I did try to glue the bosses back into place using cyanoacrylate (super glue).  However I believe this polymer is a low surface energy material, so the super glue did not hold under low load testing.  Thinking about the problem, I decided that similar repeat failures would be likely if the bosses were loaded in tension or subjected to sudden high impact loads (like placing a scope on the tray or bumping a loaded cart).  An additional problem in a cold climate and my semi-heated lab is the glass transition temperature of this unknown hard polymer.  It may be high enough that the trunnion mounts become very brittle at cool winter ambient temperatures. Depending upon the polymer it may already be in the glassy state.  Thus,  I concluded that the bosses needed to be loaded in compression in order to make the trunnion mounts reliable.

The original design used 5/16″ button head bolts which screwed into the bosses and attached the aluminum box frame member to the polymer trunnion mount:

It is obvious that over-tightening these bolts put the bosses into tension which caused them to fail.  The gap between the member and the boss “spring loads” the system and puts additional tensile stress on the boss.  While an aluminum casting could easily handle these stresses, the polymer obviously could not.

I glued the fractured bosses into place using a low surface energy epoxy to use them as a drilling guide.  Then I drilled completely through the trunnion mounts with a drill press, using the bosses as a guide.  The original boss was tapped for a 5/16″ thread, so a 1/4″ bit was easy to center in the drill proess, but did not damage the original threads:

I used 6mm bolts, nylon lock nuts and washers/spacers to clamp the box frame member and place the bosses into compression.  The washers prevented spring loading of the boss, while the nylon lock nuts allowed me to choose a holding torque and be certain that the nuts will not loosen.  This prevented tensile failures and took advantage of the compression strength of the polymer:

The holes in the trunnion mount were countersunk to fit the flat head socket bolts:

The other trunnion mount was repaired in the same way. This included modifying the one intact boss to accept a through bolt.

Next, felt lining was added to the plug-in storage compartment:

Finally, the cart was loaded up for testing:

This concludes my repair of a Tektronix 202-2 Scope-Mobile Cart.  In summary: Three fractured trunnion mount bosses rendered the cart useless (and unsafe) under load. The fractures occurred due to tensile overload of the polymer material; essentially a materials selection problem.  My solution was to place the bosses into compression by putting a 6mm countersunk bolt through the trunnion mount concentric with the existing hole in the boss.  By using appropriate sized washers between the top of the boss and the frame member I ensured that the boss is placed into compression and the frame member is securely clamped.  No tensile loading means no tensile fracture was possible.

I hope this information is useful in repairing similarly damaged carts.

Update November 2025: The cart, as repaired, is still in service some 8 years on.