Plug the CF tubes to provide load spread

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William Bristow


On the mechanical systems that I am familiar with, where hollow CF tubes are used, fixings points are reinforced by either gluing into external ferrules which are then clamped or bolted, or an internal solid plug is glued inside the tube at the fixing points that then allow for external clamping or through bolting.

Unsupported or unreinforced CF tubing is too brittle to be secured by just single or double grub screws pressing against the outside of the very ends of the tubes in a hexagonal holder.

For CF tubes in a truss OTA either reinforcement methods would be suitable though an internal glued plug of brass or aluminium is easier to build as it requires no changes to the outer holder and being hidden doesn’t need to be cosmetically perfect.

For light loading applications plugging the tubes at the clamping points with epoxy resin mixed with chopped glass fibre, or CF, extending a couple of centimetres either side of the grub-screw locations would work. If this method is used a breather hole ~0.5mm diameter, some distance along the tube from the plugs is necessary to prevent air pressure changes affecting the tube geometries once the plugs have cured and the instrument is in use.

Plugging with an epoxy CF/GF filler sounds easy but in practice is quite difficult to do successfully and in a production environment requires careful measurement of the volume of filler and spinning the filled tube on a centrifuge to ensure the filler plug sets level in the tube ends and without trapped air voids. Solid machined plugs glued into the tube ends are easier to mass produce.

How about a short piece of hard-wood doweling, epoxy glued into the ends of the tubes at the clamping points? Should be easy enough to turn to size with a bit of DIY tooling.