Stihl MS 391 Instruction Manual page 16

English
stopped by contact with any solid object
such as a log or branch or is pinched, the
reactive forces may occur instantly.
These reactive forces may result in loss
of control, which, in turn, may cause
serious or fatal injury. An understanding
of the causes of these reactive forces
may help you avoid the element of
surprise and loss of control. Surprise
contributes to accidents.
The most common reactive forces are:
kickback,
–
pushback,
–
pull-in.
–
WARNING
Kickback:
Kickback may occur
when the moving saw
chain near the upper
quadrant of the bar nose
contacts a solid object or
is pinched.
When this occurs, the energy driving the
saw chain can create a force that moves
the chain saw in a direction opposite to
the saw chain movement at the point
where the saw chain is slowed or
stopped. This may fling the bar up and
back in a lightning fast reaction mainly in
the plane of the bar and can cause
severe or fatal injury to the operator.
14
Kickback may occur, for example, when
the saw chain near the upper quadrant
of the bar nose contacts the wood or is
pinched during limbing or when it is
incorrectly used to begin a plunge or
boring cut.
The greater the force of the kickback
reaction, the more difficult it becomes for
the operator to control the chain saw.
Many factors influence the occurrence
and force of the kickback reaction.
These include saw chain speed, the
speed at which the bar and saw chain
contact the object, the angle of contact,
the condition of the saw chain and other
factors.
The type of bar and saw chain you use
is an important factor in the occurrence
and force of the kickback reaction. Some
STIHL bar and saw chain types are
designed to reduce kickback forces.
STIHL recommends the use of reduced
kickback bars and low kickback chains.
Chain Saw Kickback Standard
The following standard apply with
respect to kickback:
§ 5.11 of ANSI/OPEI B175.1-2012
–
This standard, in the following referred
to as "the chain saw kickback standard"
sets certain performance and design
criteria related to chain saw kickback.
To comply with the chain saw kickback
standard:
a) Chain saws with a displacement of
less than 3.8 cubic inches (62 cm³)
must, in their original condition,
–
meet a 45° computer derived
kickback angle when equipped with
certain cutting attachments,
and must be equipped with at least
–
two devices to reduce the risk of
kickback injury, such as a chain
brake, low kickback saw chain,
reduced kickback bar, etc.
b) Chain saws with a displacement of
3.8 cubic inches (62 cm³) and
above
must be equipped with at least one
–
device designed to reduce the risk
of kickback injury, such as a chain
brake, low kickback saw chain,
reduced kickback bar, etc.
The computer derived angles for chain
saws below 3.8 cubic inches (62 cm³)
displacement are measured by applying
a computer program to test results from
a kickback test machine.
MS 311, MS 391