Last month
I saw a hydraulic hose unscrew itself. The wrecker bed it was
installed on was brand new, and the customer's complaint was that the hose
leaked oil no matter how often he tightened it. To see the leak with
my own eyes, I tightened the hose and operated the equipment. When
oil pressure filled the hose, the hose end immediately unscrewed itself
from the fitting it was attached to, as if by an invisible hand.
The hose unscrewed itself because
the builder's final tightening of the hose ends left the hose twisted
instead of straight. When oil pressure filled the hose, the hose
tried to straighten itself out, thus unscrewing the end. Because
hydraulics operate at high pressure, the force trying to straighten the
hose is substantial enough to unscrew even a hose end that was put on very
tightly.
Most hydraulic hose comes with a
colored stripe, colored lettering, or an embossed linear marking of some
kind repeated periodically along the length of the hose. Hose
specialists call that the "lay line." When installing
hydraulic hose, watch the lay line to make sure the hose is not twisted.
On hoses that have a flare-nut
end, hose installers commonly twist the hose in the last few moments of
tightening. When the flare nut's almost done up, and you twist the
flare nut another quarter-turn to make it tight, the whole hose tends to
twist that last quarter-turn along with the flare nut, leaving the hose
tight and twisted. Avoid this hose twist by using two
wrenches: one on the main body of the hose end, and one on the flare
nut. Use the wrench on the main body of the hose end to hold the
hose still while using the wrench to tighten the flare nut.
Wrong way