Model 6730, 6735, 6738, 6741 Mechanical Combination Lock
Troubleshooting/Dialing Diagnostics
wheeL sLiPPage
This is a very rare condition on current production S&G locks,
and is more often associated with other brands. S&G employs
a double-locking wheel in its premium locks, using a total of
28 gear teeth to lock the wheel center to the wheel case.
When slippage occurs, it most often involves the last wheel (#3
in a three-wheel lock). The last wheel is between the drive cam
and the other wheels. It takes the brunt of any abuse delivered
via the drive cam. If the dial is moved in a snapping motion,
the drive cam literally slams the last wheel into motion. The
impact drives the wheel center into motion while the laws of
physics tell us the mass of the wheel case wants to stay at
rest. Sometimes it will do so, if only for a fraction of a second;
long enough to change its relationship with the wheel center
by a few numbers.
Another situation which can cause wheel slippage is setting
the last number of the combination on or between the lock’s
two contact points – in the “Forbidden Zone.” This causes
a condition known as a “lock-in.” The combination can be
entered to retract the bolt. However, when it’s time to extend
the bolt the drive cam attempts to move the last wheel before
the fence is lifted out of the wheel’s gate. It’s like isometric
exercise; two forces are working against each other, and no
movement is taking place. Stress is created between the last
wheel’s center and its case, and one way for this stress to be
relieved is for the wheel to slip a few numbers, until the fence
can be raised far enough to release the wheel.
The end result is a new number on the last wheel. Some simple
and efficient dialing will allow us to open the lock, however.
Let’s use our sample combination of 20 • 60 • 40. After dialing
four times left to 20, dial three times right to 60, then two
times left to 62. Remember, the “left two times to 62” is only
two numbers more than a single revolution. Every time you
pass a number, no matter how close it is to your starting point,
it’s counted as one turn to that number.
Now dial right to the contact area and oscillate the dial between
the contact points a few times. If the wheels are almost aligned
well enough to allow the fence to drop into the gates, the vibration
may move the wheels enough to allow the lock to open.
If the lever doesn’t drop, dial left directly to 64, then back
to the contact area and oscillate. The only wheel you have
moved is the last one, and only by two numbers. You should
repeat this procedure, adding 2 to the last wheel number,
until the lock opens or until you have worked your way all the
way around the dial. The procedure only takes a few min-
utes. Dialing to overcome a slipped wheel in another position
requires more time (and dialing), but is accomplished the
same way. Keep two of the wheels set on their original num-
bers while trying every other number on the remaining wheel.
the unLockeD wheeL
The problem of one or more
unlocked wheels is almost always
due to combination changing by
an untrained person, and arises
from the fact that a severely worn
change key can be turned before
it is completely removed from
the lock. The combination changing procedure may have gone
smoothly right up until the time the key is withdrawn from the
lock. However, as the key is being pulled out of the lock, the
operator attempts
to rotate it. As long as the change key is passing through all
three wheels (in a three-wheel lock), the flange on the key
contacts the change key stops in the lock cover, preventing
key rotation. If either the key or the change keyhole in the lock
cover is sufficiently worn, the key can rotate once the flange
clears the cover. This will result in the #1 wheel being locked
on its combination number, but wheels #2 and #3 unlocked.
In most cases, you will find that only the #3 wheel is unlocked,
but there is a distinct possibility the #2 wheel has suffered the
same fate.
Diagnosing an unlocked wheel is difficult. A good quality safe
amplifier will help. If the torque adjuster is properly set, wheel
overrun should be eliminated. In other words, if you move the
dial very rapidly, then stop, the wheels stop rotating as soon
as you stop moving the dial. However, if your amplifier picks
up the sound of wheel movement (a very soft scraping sound)
after the dial stops, an unlocked wheel is a very real possibility.
If the symptom coincides with an admission of do-it-yourself
combination changing, the probability of this condition
increases even more. Inspection of the operator’s change key or
an admission that the operator has had the cover off of the lock
body could confirm your diagnosis.
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© Copyright 2006, Sargent & Greenleaf, Inc.
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