Thursday, July 2, 2015

Will it work

Each hour I put into the redesign, calculations, and analysis of the gear train for the pneumatic ankle-foot orthosis, a nagging thought in the back of my mind grows a little louder -- "what if it doesn't work?"

The form of the former gear system has been reduced and encased (in design anyway.)  And analysis of gear strength seems promising.  The new compact gear design utilizes a compound gear to double the tangential load from that seen in the previous design.  Unfortunately, the gears currently in use and those readily available for purchase are not strong enough to take this load.  We've realized that if we want to implement this design, we'll have to cut the gears ourselves via electric discharge machining out of a stronger material.  AR500 alloy (Yield Strength 187 ksi) seems the most likely candidate unless I am able to source a less brittle more resilient steel (a maraging steel for example.)  A maraging steel might even give us a higher yield strength than AR500, however the suppliers I've been calling all have minimum order requirements making the use of this material cost prohibitive.  AR500 is available to consumers in small quantities in the form of shooting targets. About $30 worth of shooting targets should give us enough material to cut enough gears and possibly a few to spare.

Reading up on spur gears, I discovered the Lewis equation -- an approximation of the bending stress on the root fillet of a gear tooth based on tangential load, diametral pitch, and a geometry factor, Y, specific to a gear's pressure angle and the number of teeth.  This geometry factor takes into account the shared load between teeth during operation.  Based on the Lewis equation, our gears should give us a factor of safety of about 1.6.  However, finite element analysis has been showing us we don't have quite so much wiggle room -- though our root fillet stresses lie below 187 ksi.  This conflict between the Lewis equation and FEA has been frustrating, but we've finally achieved a gear design which satisfies both forms of analysis.

The design and analysis and re-design and re-analysis has composed the majority of my work since the last posting.  Constructing one set of gears in Creo, only to discover one of them will not work -- and if one does not work, all of them must be changed in order to match pressure angles and diametral pitches.  Gear ratio of the entire system must be preserved throughout this process as well.

I have enjoyed learning about the nuances and intricacies of spur gears that I never even knew existed -- modern spur gear standards vs. the many, many outdated standards -- and the advantages of certain designs, over others in terms of strength, efficiency, noise, and precision.  Fascinating!

I really hope this thing works!

My apologies for the dryness of this post.
But we wouldn't want it to get too soggy either.  Would we?


No comments:

Post a Comment