Stihl MS 171 Manual page 23

7. Chain Brake
7.1 Checking Operation
The chain brake is one of the most
important safety devices on the
chainsaw. Its efficiency is measured
in terms of the chain braking time,
i.e. the time that elapses between
activating the brake and the saw
chain coming to a complete
standstill.
Contamination (with chain oil, chips,
fine particles of abrasion, etc.) and
smoothing of the friction surfaces of
the brake band and clutch drum
impair the coefficient of friction.
This, in turn, reduces the frictional
forces and thus prolongs the
braking time. A fatigued or stretched
brake spring has the same negative
effect.
– Start the engine
– With the chain brake activated
(locked), open the throttle wide
for a brief period (max. 3
seconds) – the chain must not
rotate.
– With the chain brake released,
open the throttle wide and
activate the brake manually – the
chain must come to an abrupt
stop.
The braking time is in order if
deceleration of the saw chain is
imperceptible to the eye.
If the chain brake does not operate
properly, refer to troubleshooting,
b 4.2.
22
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7.2 Removing and Installing
the Brake Band
1
– Troubleshooting, b 4.2
– Remove the chain sprocket
cover, bar and chain, b 5
: Disengage the chain brake by
pulling the hand guard (1)
towards the front handle – the
brake band is loose.
– Remove the clutch drum, b 6.1
– Push the hand guard (1) towards
the guide bar – the brake band is
tensioned.
To reduce the risk of injury, do not
activate the chain brake while the
chain brake cover is open.
1
: Use a screwdriver to ease the
cover (1) away at the points
shown (arrows).
– Remove and inspect the cover
(1), service or replace if
necessary.
1
: Pry the brake band (1) out of its
seat (arrow).
1
: Swing the brake band (1)
outwards.
Do not over-stretch the brake band.
1
: Turn the brake band (1) slightly to
one side and disconnect it from
the brake lever (arrow).
MS 171, MS 181, MS 211