Tuesday, June 17, 2014

Failing: Learn From Your Mistakes

Usually I talk about what has gone right.  Seldom do we like to dwell on what has gone wrong.  But in this case, I fought a good fight and so don't feel too badly about acknowledging when it was time to call it quits with a repair job that just didn't want to be.

It all started when I reassembled a mower deck after obliterating the spindle housings after hitting a fist size rock.  In that case, I was fortunate enough to find ereplacementparts.com, and was able to get replacement spindle housings for less than the cost of a new deck.  A week later, I had my mower back in operation but I failed to consult a parts diagram, and as I now know, omitted an important spacer.  With the spacer between the top of the spindle housing and the bearing missing, the pulley rode directly on top of the spindle housing, creating a lot of friction and heat.  Eventually the, the somewhat toothed hole in the center of the pulley wore smooth, and then as it loosened, oblong until it was so worn it had no more ability to hold the mower belt in tension and it failed with much noise.

I was able to purchase a replacement pulley for $13, and was back in business.  That is, until late last week when yet again, I discovered the same pulley wearing out. Fortunately, I caught it before it was bad, but it was definitely spinning free.  Exasperated, I pulled things apart and discovered that the spindle itself had worn to the point where there were no more ridged teeth to engage the pulley.  So, I got back on the parts site and ordered a new spindle.  While doing so, referencing the part diagram for the part number, I noticed the spacer.  It all clicked in a moment of realization - the spacer was the problem.  Without it, the spindle and pulley could not hope to function properly for long.

This is where the fail really starts rolling though.  So, first lesson learned: consult a parts diagram and make sure you have everything when you reassemble something.  Not being one to give up easily, and wanting to mow my lawn while waiting on new parts, I devised a way to get the spindle and the pulley to lock up and work as intended.  All I needed was a little machine work and a suitable washer to turn down as a temporary replacement spacer.

As you can see below, with a little drilling and filing, I created an index slot in the pulley, and a hole in the spindle to receive a set screw which I imagined would engage with the slot.

Notion courtesy of: "507 Mechanical Movements"
In theory, this was a sound mechanical principle.  I made a couple of miscalculations, however.  First, my set screw was just a happy find in the parts bin.  I had no idea what the strength of the material was in terms of hardness or even composition.  It might have been mild steel from the ease with which it failed.  Second, I drilled and tapped the hole in the spindle, but the spindle was hollow, and the wall thin, and getting my hand held tapper to cut threads correctly was a challenge and it didn't go so well.  As a result, the screw jammed in the threads and turned itself in half with the torque of the vice-grip pliers I was using to insert it.

SO close, but not working out.  I reasoned at this point that if I had spacers, I could make it work, so I drove all the way to the hardware store hoping to get a replacement pulley and maybe find a lucky washer that would match the spacer dimensions.  I found one with the right ID (inside diameter), but not the right OD.  So I thought - no problem, I have a metal lathe, I'll just put the washer on the outside of the chuck, turn it down and have a spacer.

Unfortunately, my bench lathe is a 7" made for larger, heavy duty work.  I haven't a chuck small enough.  My machinist mentor later asked me: "Did you try putting the washer on a bolt and..."  "DOH!"  Yes, I could have mounted it to a bolt with a couple of nuts and put the bolt in the chuck.  Duh.  But, by the time I learned that, it was 24 hours past the time I humbled myself, gave up on trying to repair my mower with make-do parts, and wandered down the street to borrow my neighbors riding mower.  The grass looks great.  His new John Deere all-wheel steering mower was a beast and got the job done with nary a complaint. I might have to upgrade... some day.

I strive to learn from my mistakes so that I might not repeat them, and that I might learn to be more thorough, and careful, in the future.  I hope this little story helps someone else avoid similar pitfalls!

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