Technical Information · issue 50 of 1994
Service: Servicing STIHL Quickstop chain brakes
IssueTI 50/1994
Full text
Page 1 of 4
Technical Information
50.94
Servicing STIHL Quickstop Chain Brakes
1. Basic function
1.1 Brake 1.2 Control mechanism
In certain circumstances the chain brake can be STIHL Quickstop chain brakes can be activated
one of the important safety devices on the chain manually by pushing the front hand guard forward.
saw. Its efficiency is expressed primarily in terms The chain brake on most new model STIHL chain
of the braking time achieved. This is the period saws can also be activated by inertia. If kickback
that elapses between activating the brake and the forces are sufficiently great, the front hand guard
chain coming to a complete standstill. The shorter
the braking time, the more efficient the system and
will tend to move forward relative to the rest of the
power head. For inertia activated chain brakes, the
the greater the protection offered against injury shape, mass and mounting of the hand guard are
from contact with the rotating saw chain. calibrated to the forces and friction acting in the
lever mechanism.
A high degree of chain brake efficiency with a
suitably short braking time (= high braking power) Under the prevailing conditions (confined space,
has to be balanced against high mechanical and high forces to be controlled, acceleration of low
thermal loads on the components involved in the mass, narrow tolerance for activating moment) the
braking process. High frictional and centrifugal for- mechanism reacts extremely sensitively. Even
ces occur when moving masses (crankshaft, very slight changes over the design specification
flywheel, saw chain) are quickly braked to a stand- (mass of hand guard, lever friction) can result in
still. For this reason the brake band and clutch the activating point being displaced. Late activa-
drum in particular are subject to an increased rate tion of the chain brake could mean a reduction of
of wear. safety,
while premature activation is a nuisance which
The function and efficiency of the brake are chiefly detrimentally affects the utility of the saw.
dependent on the brake band and clutch drum.
The frictional forces between these two compo-
nents and the brake band’s contact pressure are 1.3. Conclusions
the parameters which can change during operation
and influence the braking time. High efficiency can To preserve the safety function of the chain
only be achieved if all component parts are proper- brake it is necessary to service it and carry out
ly maintained. preventive maintenance at regular intervals.
The activating and braking mechanisms must
The coefficient of friction deteriorates as a result not be modified in any way.
of contamination (particularly with chain lubricant,
wood chips, dust particles, etc.) and smoothing of
the friction surfaces on the brake band and clutch
drum. These factores reduce the frictional forces
and thus extend the braking time. The same
negative effect is produced by a fatigued or stret-
ched brake spring. Page 2 of 4
Technical Information 50.94 - Page 2
2. Maintenance / Repair
2.1 General
The intervals for inspection, servicing and repair of
chain brakes depend on the degree and frequency
of use.
The following servicing intervals should be
maintained for the different user groups:
• Professional full-time users:
at least every three months
• Semi-pros, farmers, construction workers:
at least every six months
• Hobby and occasional users:
at least yearly
The inspection, servicing and repairs must be
performed by trained and authorized personnel.
Removal and installation of the brake mechanism
should be carried out as described in the service
manual for the chain saw powerhead concerned.
Operational reliability of the chain brake can be
achieved only if original STIHL replacement parts
are used as per the latest parts list.
2.2 Activating mechanism
The activating lever mechanism should be inspec-
ted after removing the sprocket cover from the
crankcase.
If the levers appear to be relatively clean and in
good shape, it is normally sufficient to clean all
parts with a brush and parts cleaner, lubricate the
pivot points with resin-free oil and coat friction
faces of the activating mechanism with STIHL
multipurpose grease or, even better, with molyb-
denum grease (e.g. Molykote).
If the levers are heavily soiled and covered in resin,
it is necessary to disassemble the entire mecha-
nism and wash it in parts cleaner, cold cleaner
(alkaline cleaner), resin solvent or similar solution.
Worn and damaged parts must be replaced, inclu-
ding the bearing pins.
000T1002
Before installation, clean the housing recess
and coat all pivot points and friction faces of the
activating mechanism with STIHL multipurpose
grease or, even better, with molybdenum grease Lubricate with oil
(e.g. Molykote). Lubricate with grease Page 3 of 4
Technical Information 50.94 - Page 3
In order to attain proper operation of the activating
mechanism the front hand guard must be in an
undemaged condition, be completely clean and
move freely.
2.3 Brake
Remove the clutch drum (chain sprocket) and
brake band and clean in parts cleaner, cold
cleaner (alkaline cleaner), resin solvent or similar
solution.
If thera are signs of subtantial wear on the inside
or outside circumference of the brake band,
replace it. The remaining thickness of the brake
band must not be less than 0.6 mm (0.024") at any
point.
If there are clear wear marks on the inside circum-
ference of the clutch drum, check the remaining < 0,6 mm
wall thickness. If it is less than about 80% of the (0.024")
000TI005 L
original wall thickness, fit a new clutch drum.
If the brake band and clutch drum can be used
again, the friction faces of both parts (inside of
brake band, outside of clutch drum) must be
suitably cleaned and roughened. Use emery cloth
or paper with a grain size approx. 120 µm for this
purpose. The entire contact surface must then be
roughened and have a uniform appearance.
Before installation, clean the recess in the crank-
case. Lightly coat the brake band with chain lubri-
cant, preferably STIHL Bioplus, to help prevent
corrosion and "grabbing" of the brake during the
first few applications (which could damage the
brake mechanism).
The windings of the brake spring must butt
against one another in the relaxed condition. If this
is not the case, fit a new spring.
100 %
< 80 %
000TI001 Page 4 of 4
Technical Information 50.94 - Page 4
3. Functional check
After installing the chain brake and reassembling
the saw, start the engine.
• With the chain brake engaged, open the thrott-
le fully for no more than 3 seconds - the chain
must not rotate.
• With the chain brake disengaged, open the
throttle fully and activate the brake manually -
the chain must come to a sudden stop.
Rule of thumb: The eye should not be able to
register the deceleration of the chain as it
comes to a standstill.
Note: Precise measurement of braking time neces-
sitates considerable technical facilities and is,
therefore, not practical for service shops. If the
chain brake is not functioning properly after full
maintenance, contact your STIHL distributor.
Last Technical Information on guide bars: 23.94
Waiblingen, January 10, 1995 Andreas Stihl
D1/MTK Technical Documentation Extracted from the manufacturer's 2012 service catalogue. Most of these PDFs embed fonts with a shifted character map, so the text above was recovered rather than copied — read it as a faithful but imperfect transcription, and treat the page image above each block as the authority where the two disagree — the figures exist only there, because they are vector drawings that text extraction cannot keep.