Saturday, January 7, 2012

Cabin fever and new designs

The lakes might be frozen here for the next few months, but at least the holidays have given me some time to kick around some ideas I might be able to try in the spring.  I get to wait and see how much is in the piggy bank once I'm ready.  I've also been able to watch what others have been up to.  Rick and Mike have both tried new ideas and have had positive results.

I don't expect to do any major work on the hull.  My focus is improving the gearbox stiffness and maximum torque and increasing the drive leg durability with a stronger chain.  I have some new designs worked out in CAD, and I'll think it over for a few weeks before doing anything in case I change my mind.

My original gearbox used a 4" x 6" rectangular aluminum extrusion.  This was oriented so the top and bottom were open and had cover plates.  Under load, the only features to prevent the sides from flexing out were the cover plate fasteners, and clearly this wasn't sufficient.  The new design uses a 4" x 4" extrusion with the fore and aft sides open.  The aft side with the output shaft will have an extruded section welded on as a cover plate and and also serving as a mounting area for the shaft bearings.

The second CAD image shows a cross section with the top cut away.  This shows two additional machine screws positioned as close to the gear interface as possible to provide additional reinforcement against flexing.  The final forward side is covered with a third piece of extrusion sized to nest inside the main housing.  This cover has a cutout for the large gear and is screwed to the housing near the crank shaft spindle.  This should effectively create reinforcing ribs on the side opposite the output shaft.  The crank shaft is also slightly larger diameter and is available as a ground high precision rod, so that I can more easily achieve a tight clearance with the gear.

I am working on new designs for the drive leg as well, although this is not as critical.  I considered various options of using a double chain loop or upsizing a ANSI 35 chain.  However, potentially the easiest option to increase durability is to create a sealed assembly so the chain can be plain steel rather than stainless steel.  The stainless steel chain has a significantly lower strength, but I had chosen it to simplify the design and because the miles I put on the boat shouldn't wear it out anytime soon even if it runs in water not oil.  Production drive legs such as the Wavewalker, use a mineral oil bath to lubricate parts and help prevent water inflow.

The new leg design uses two standard bicycle bottom brackets with machined end caps to house shaft bearings.  I can add o-ring grooves and o-rings to the shafts to seal against the bearing ID.  The top and bottom each have standard tubing welded at right angles and these nest similar as before.  In addition, I can now order steel chain in a continuous riveted loop (no connector links) because the bottom bracket shells are large enough to slide the chain loop on over the sprocket.  The current leg was too tight to allow this and required a connector link.  The continuous loop is rated at approx. 170 lb. working load and this should be more than sufficient.

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