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