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Troubleshooting Article Archive:  May 2007      
Got Clearance?
When we finish a repair job in my shop, we make four checks before letting it go out the door.  We check hydraulic oil level in the reservoir; we check for proper operation; for leaks; and for clearance of moving parts.  The practice of checking for clearance to make sure moving parts don't hit other parts is an important part of the troubleshooting process, and one that requires care and finesse.

A brand new repo-type tow truck was driven into my shop in January with damage to the small hydraulic hoses on the jaw cylinders.  The owner had new hoses installed, and within a week the hoses became scraped, sometimes pulled in two, and the hose ends bent.  Several repair people, including the manufacturer, had looked at the jaw hoses and no one could see any clearance issue where the jaw hoses might be hitting a physical object.

I manually moved the wheel-lift crossbar all the way from 90 degrees on the passenger's side to 90 degrees on the driver's side while watching the jaw hoses.  The jaw hoses did not come close to hitting anything.  Then I returned the crossbar to center and asked the driver to open and close the jaws while I watched the jaw hoses.  The jaw hoses had plenty of clearance to all parts throughout the entire arc of the jaws' opening and closing.

Then I decided to move the crossbar to 45 degrees and asked the driver to open the jaws.  In the special case when the crossbar was at 45 degrees and, at the same time, you cracked the jaws about 30 percent, the end of the wheel-lift extension tube pinched the jaw hoses (see photo for area of interference).  Because the interference happened only in this special case near the middle of the travel of two separate hydraulic functions when operated simultaneously, and not in any other case, a quick and cursory check of clearance would fail to turn up any problem.

The management consulting firm McKinsey & Company has a principle you can use when troubleshooting clearance problems like this one.  McKinsey people call the principle MECE, which stands for "mutually exclusive, collectively exhaustive."  When evaluating possible problems, your list should not contain overlapping or duplicate elements, hence "mutually exclusive;" and your list should systematically exhaust every possibility, without leaving out any possibility, thus "collectively exhaustive."  Consider the full range of movement of the crossbar, by itself, for example.  Consider the full range of jaw cylinder movement by itself.  Then also consider the entirety of the combination of the two arcs of movement when done at the same time.  The MECE principle makes your checking procedure as organized and thorough as it can be.  You can also apply the principle to any troubleshooting activity that involves making a list of items for any purpose.