Tektronix Scope-Mobile Cart Repair Part 2
Tektronix Scope-Mobile Cart Repair Part 2
This post continues my repair of the Tektronix Scope-Mobile Cart and trunnion mounts. Here is Part 1, which details the problem and my proposed solution.
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:
Trunnion to member overview-1
Trunnion to member closeup-2
Trunnion to member closeup with original fastener-3
Trunnion to member closeup with original fastener and cover in position-4
It is obvious that overtightening 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.
What I did was to glue the fractured bosses into place using a low surface energy epoxy and use them as a drilling guide (see Part 1). 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, but did not damage the original threads:
Holes drilled for new bolts
Holes drilled for bolts
Holes drilled for bolts closeup
I will be using 6mm bolts, nylon locknuts and washers/spacers to clamp the box frame member and place the bosses into compression. The spacers will preclude spring load on the boss, while the nylon locknuts will allow me to choose a holding torque and be certain that the nuts will not loosen. This should prevent any future tensile failures and take advantage of the compression strength of the polymer:
Trunnion to member closeup with new bolt and washers in place-5
Trunnion to member closeup with new bolt and washers in place (2)-6
Trunnion to member closeup with new fastener, washers and cover in position-7
The next step will be to countersink the outside surface of the trunnion mount in order to accommodate flat head bolts for a flush installation. While the look is not completely stock, the modification is structurally much superior to the original construction and visually not objectionable. All in all, I would rather have a strong and useable cart with a scope on it over one that sits in pieces in a box.
To be completed in Part 3.
Shaun M.
