Reactives
forces can be dangerous! In any chain
saw, the
powerful force used to cut
wood can be reversed
(and
work against the
operator).
If the rotating chain is suddenly stopped by
contact with
any solid object like a log or branch or is
pinched, the
reac-
tive
forces
may occur
instantly. These reactive forces
may
result in loss of control which
may, in
turn, cause
serious
or fatal injury. An understanding of the causes of
these re-
active forces may help
you avoid loss of control.
The
most common
reactive forces
are
kickback,
pushback,
pull-in.
Kickback:
Kickback occurs when the upper quadrant
of the bar nose
contacts
a
solid object
or
is pinched
(ill. 11). The reaction
of the cutting
force of the chain causes a
rotational force
on the chain saw in the direction opposite to
the
chain mo-
vement.
This
may
fling
the bar
up
and back in an uncon-
trolled arc mainly in the plane of the
bar. Under some cut-
ting circumstances
the bar moves towards the operator,
who
may
suffer
servere or fatal injury. Kickback
may occur
when
the nose of the guide bar is pinched unexpectedly,
unintentionally contacts solid
material in the wood, (ill. 12)
or is incorrectly used to begin a plunge or boring cut. It
may
also
occur
during limbing.
The
greater the force of the kickback reaction, the more
difficult it becomes for the operator to control the saw.
ANSI B
175.1-1985 chain saw kickback standard
Many factors influence the occurrence and force
of the
kickback reaction. These include chain speed, the speed
at
which
the bar and chain contact the object, the angle
of
contact,
the condition of the chain
and other factors.
The type of
bar and saw chain you use is an important fac-
tor in the
occurrence and force of the
kickback reaction.
Some STIHL bar and chain types are
designed to reduce
kickback forces. STIHL
recommends the
use of reduced
kickback bars and low kickback chains.
Section 4.12 of ANSI standard
B 175.1-1985,
sets
certain
performance and design criteria related to chain saw kick-
back. 11
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