January 26, 2014

Technical Post - carbon fiber snatch block

How I made a carbon fiber snatch block because I wanted a snatch block but manufactured ones are heavy and expensive.

- make a mold shape with an angled step. The step height should be equal to the 1 washer thickness + 1/2 the sheave thickness.  (the sheave will have a s.s. washer on the inside between the two halves.)  You can see the angled step shape with the shadow on it.

- cover the mold with mylar packing tape - best mold release ever
- mark the mold with the size of finished piece - about 2x sheave diameter for the width and 4x sheave diameter for the length
- cut your pieces of carbon fiber cloth and unidirectional cloth. Use 1 layer of fiberglass on the inside layer to prevent corrosion where the s.s. washer would otherwise rub on the carbon fiber.  The small pieces of carbon fiber are to add thickness where the sheave axle goes through.  The twill is the top layer for looks

I'm not going to spell out the laminate schedule I used.  If you've used carbon fiber before you will know how it's a bit harder to wet out for an amateur and I don't want to give everybody the impression that they will make a suitable block the first time using carbon.  Composite properties depend very much on the skill of the laminator. If you are a so-so laminator the strength of the part might be as low as 1/3 the strength of a good laminator.  For such a safety related part, I don't feel like being responsible for the design for others to copy.

- wet out the fabric with epoxy resin, cover with clear plastic and squeegee hard.

- release the part from the mold (gentle tug will get it off)

- cut part into 2 halves, round the corners
- drill a hole for the axle and for the soft shackle. Soft shackle hole has beveled edges with a countersink bit to remove sharp corners.  Soft shackle is 5/32" Amsteel blue which should have a breaking strength > 4000 lbs.
- assemble and test

- total cost less than $15 (though I had the roller sheave).  If you don't have a suitable sheave a fixed plastic sheave with 1/2" bronze bushing would be suitable and costs about $15.  Bigger axle spreads the load better and reduces bearing stress on the carbon.

How it looks in use

On a budget and can't afford a marine snatch block? Check out a rock climbing rescue pulley.  You'll need a carbiner but quite cheap and strong.