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Husqvarna power cutters
Workshop manual
Supplement for models 371K and 3120K
101 90 71-26
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CONTENTS
Workshop manual – power cutters
Supplement for models 371K and 3120K
Contents
Filter system .................................................... 3
Carburettor ....................................................... 5
Clutch ............................................................... 6
Ignition system ................................................ 6
Cutting head .................................................... 7
Tools ................................................................. 9
Cutting equipment ......................................... 10
Cutting discs .................................................. 16
Wet-cutting with diamond discs .................. 19
This supplement only covers the tasks that are specific to the abovenamed models. For complete information on service procedures we
recommend that this supplement is studied in conjunction with
workshop manuals 101 88 55-26 and 101 89 83-26.
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FILTER SYSTEM
Dust
The cutting of stone and concrete produces small particles of dust that must not be
allowed to get into the engine under any circumstances. The design of the air filter
and its maintenance are by far the most important factors that affect the service life
of a power cutter. Designing a good air filter is a question of getting the right
balance between effective filtration and long intervals between services.
The development of more effective filters has improved air filtration, but for power
cutters the service interval has remained impractically short. Tool hire companies
often have problems due to service work that is not carried out by the user during
the hire period, or due to the cost of regular service visits that have to be made to
the work site.
Dust consists of very fine particles, usually so small that we cannot see a single
particle with the naked eye, but in large quantities it appears as a dust cloud. The
stone and concrete dust particles that are often produced during cutting operations
cause the greatest damage to the sliding or rotating components of an engine. If it
gets into the engine this dust combines with oil to produce the perfect grinding paste,
resulting in rapid wear to pistons, piston rings, cylinder walls and engine bearings.
We usually measure dust particles in microns, µm (1 µm = 0.001 mm) or thousandths of a millimetre, and the particles that are trapped by the filter system are
generally in the size range 50 µm down to 5 µm. (It takes about 2 minutes for a 10
µm stone particle to fall one metre in stationary air).
One physical property that is vital to the effectiveness of Husqvarna Air Injection
Filtration is the way that the size of dust particles affects their behaviour in a stream
of air:
A small particle is affected more by a moving air stream than a large particle.
The reason for this is that small particles have a larger surface area in relation to
their mass. A small particle is therefore deflected more easily by moving air, while a
larger particle is affected more by centrifugal force or gravity.
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FILTER SYSTEM
Husqvarna Air Injection is a filter system that efficiently cleans the engine intake
air in three separate stages that use different methods of filtering.
The obvious practical benefit of Air Injection is that the service interval is considerably longer than with previous systems.
1. Centrifugal cleaning is the first stage of filtering the intake air on the 371K/
3120K Air injection. Centrifugal cleaning was originally only used on larger machines in dusty environments, e.g. on contracting machinery (cyclone air cleaning).
The fan blades on the flywheel supply cooling air to the cylinder while also
forming the active component of the centrifugal cleaning system for the engine
intake air. An intake nozzle is located close to the fan blades.
Centrifugal force prevents large particles from following the bent air stream into
the nozzle. Instead they are thrown past the nozzle. Only very small dust particles
are able to follow the diverted air stream towards the engine air intake. Tests show
that up to 80% of all dust is separated out by centrifugal air cleaning.
2. The foam plastic air filter is the next separation stage in cleaning the air. The filter
covers the whole area of the engine cover and thanks to this design has a total filter
area of 3.5 dm2 (371K), 3.7 dm2 (3120K). The filter base plate spreads the air over
the entire filter area so that the filter is equally effective across its whole area.
The filter is soaked with oil and has three layers with different pore sizes.
The route the air takes through filter is like a labyrinth. Dust particles that come
into contact with the filter material do not bounce off the surface, but are instead
caught by the sticky oil. A dry foam plastic filter is much less effective at filtering
air than one soaked with oil.
An oiled foam plastic filter is by far the most effective filter for stone dust, since
the entire volume of the filter is used as a dust trap, not just the surface. The total
dust-capturing area of the filter is therefore extremely large. The foam filter
absorbs around 95% of the total volume of dust that passes through the centrifugal cleaning stage. The foam filter can be washed clean and must be soaked in
clean oil during servicing.
3. The paper filter takes care of the small amount of dust that escapes through
the foam filter, mostly by chance. Only a very small amount of very fine dust
particles reaches the paper filter. The filter’s dense, mesh-like structure of
cellulose fibres traps these particles. The paper filter also acts as a protective
filter during a filter service. The paper filter must be replaced at each service.
Husqvarna air filter oil
Use Husqvarna’s pine needle oil, which is made from renewable ingredients and
is completely biodegradable, for oiling foam plastic filters. This oil can be washed
off with soap and water. It collects and traps dust particles very effectively in the
filter, leading to a cleaner engine and less wear. The oil is thixotropic, which gives
it an actively sticky surface, and it does not contain solvents or ingredients that
dry out.
Soak the filter thoroughly and squeeze out.
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CARBURETTOR
Carburettor with built-in filter compensation
The carburettor has a built-in filter compensation mechanism that ensures the
machine gets the right fuel/air mixture almost regardless of how badly the filters
are clogged. The results of this design are:
• consistently high engine output
• better filter economy, long service intervals
• lower fuel consumption
• less air pollution
Thermally insulated, vibration-damped carburettor
Every engine produces some vibration. The carburettor contains a number of
moving parts, such as the inlet needle with its control lever and diaphragm, and
the choke and throttle shafts. Their operation is affected by vibration, and the
greater the mass (weight) of the parts, the greater the effect. (Lightweight parts
can accommodate fluctuating vibration more easily than heavy parts).
The moving parts of the carburettor are larger in large machines, and the 371K
and 3120K Air Injection therefore have a vibration-damping element between the
cylinder and carburettor.
The element between the cylinder and carburettor also acts as a thermal insulator
and prevents vapour locks forming in the carburettor.
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CLUTCH
Self-lubricating clutch bearing
The clutch bearing fitted to the 371K and 3120K Air Injection is lubricated automatically, which is an old Husqvarna speciality. A channel through the crankshaft has a
vent under the clutch bearing. The positive pressure that is developed inside the
crankcase is transferred to the clutch bearing. This prevents external dust particles
from getting inside the bearing and lubricates the bearing at the same time.
NOTE!
The channel must be plugged before pressure-testing the crankcase.
IGNITION SYSTEM
The ignition system is fully sealed and has no moving parts. It is resistant to damp
and dirt. It is designed so that there is no need to adjust the timing.
The Husqvarna 371K and 3120K Air Injection have a rev limiter built into the
electronic unit which limits engine revs to 9,750 rpm.
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CUTTING HEAD
Belt drive
As with most other Husqvarna power cutters, the cutting disc on the 371K/3120K Air
Injection is driven by a drive belt running between the pulley on the crankcase and
the pulley on the drive shaft. As well as being simple, this type of transmission is
also lightweight.
This design also has a safety feature. If the cutting disc stops suddenly the drive
belt will slip and allow the engine to slow down more gently.
Sealed transmission cover
Large machines, such as the 371K and 3120K Air Injection, are frequently used for
wet-cutting. If water gets on the belt it acts as a lubricant and can lead to slipping.
These machines are therefore fitted with a sealed cover that prevents water from
reaching the belt. It also prevents dust from reaching the belt, which reduces wear.
In order to provide cooling for the transmission there are cooling vents in the cover
near the clutch (3120K).
Semi-automatic belt tensioning
There are almost as many different ideas about what is the correct belt tension as
there are power cutter operators. Husqvarna’s semi-automatic belt tensioning
system is designed to solve this problem. A standard spring ensures the correct
tensioning force.
Under controlled laboratory conditions Husqvarna has determined the ideal belt
tension to ensure maximum belt life and ensure the right torque before the belt slips
on the pulley.
However it is still necessary to adjust the belt tension occasionally to compensate
for belt wear and stretching. This involves just three quick steps.
Drive belt
The drive belt on a power cutter is subjected to harsh, uneven wear and sometimes
high transient loads. The drive belt is also bent round a relatively tight radius over
the pulleys, which places special demands on the belt. The original belts fitted to
Husqvarna power cutters may look like any other belt, but they have been specially
designed for this purpose. The belt tensioning force that is set by the tensioning
spring has been chosen to suit original Husqvarna belts.
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CUTTING HEAD
Reversible arm
The cutting arm can be reversed on the machine. This makes it possible to cut right
up against a wall or floor as close as the disc guard allows, i.e. around 20 mm from
the cutting disc.
Naturally some of ergonomic benefits of the machine are sacrificed by reversing the
cutting arm, but for certain jobs the ability to reverse the arm can be essential in
order to do the job.
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TOOLS
Drift for ball-bearing
Drift
Allen key NV 4 M
Combination tool NV 13/19
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CUTTING EQUIPMENT
NOTE!
The cutting disc is driven by a drive belt that is correctly tensioned throughout its life by a powerful spring, on
condition that the belt tensioning screw is correctly adjusted.
The belt is subjected to hard, uneven loading. The pulleys also have relatively small radii, which places special
quality requirements on the belt. Always replace the belt with an original Husqvarna belt that has been thoroughly
tested to meet these special requirements.
When servicing or repairing cutting equipment it is important to visually inspect the guard for cracks and wear, and
ensure that the guard locking mechanism works in all positions.
Correct all faults as soon as you discover them to eliminate any risk of danger to the user!
Removal
Do as follows!
Models 371K, 3120K
Lock the cutting disc by inserting a
suitable drift through the hole in the
cutting arm. Undo and remove the bolt,
flange washer and cutting disc.
Remove the cutting disc.
Release the tension in the belt and
remove the front belt guard and cutting
arm.
Undo the nuts (A).
Unscrew the belt tensioning screw (B)
to release the belt tension.
Remove the front belt guard and the
cutting arm.
Remove the rear belt guard and drive
belt.
Remove the nuts (C) and screw (D)
(371K). Then lift off the guard and drive
belt.
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CUTTING EQUIPMENT
Remove the pulley from the cutting arm.
Lock the pulley using a suitable drift
and unscrew the bolt that holds the
pulley. Lift off the pulley and the washer
that is between the pulley and the ballbearing.
Drive out the shaft with the aid of a drift
and hammer.
Place a suitable socket underneath the
cutting arm and drive out the shaft
using a drift and hammer.
Lift off the spacer and flange washer.
NOTE!
If it is only necessary to remove
the spacer and flange washer,
this can be done by inserting
two screwdrivers between the
flange washer and the disc
guard and prising upwards.
Remove the bolts, cover plate,
the friction washers and the
disc guard.
NOTE!
If it is only necessary to remove
the spacer and flange washer,
this can be done by inserting
two screwdrivers between the
flange washer and the disc
guard and prising upwards.
Then prise off the flange washer
and spacer.
Remove the bolts, cover plate, friction
washers and disc guard.
Remove the three bolts, cover plate,
plastic washer, rubber washer and disc
guard.
Remove the ball-bearings from their
housing in the cutting arm.
Heat the cutting arm to around 150ºC
using a hot air gun and remove the
bearings. If necessary use a suitable
drift (505 38 17-09) and hammer to tap
them out.
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CUTTING EQUIPMENT
Refitting
Do as follows!
Models 371K, 3120K
Models 371K, 3120K
Clean and inspect the various parts.
Clean and inspect the various parts.
Heat up the cutting arm and fit new
bearings. Do not forget the spacer ring
between the ball-bearings!
Replace the cutting arm mounting bolts
if the threads are damaged. Tap out the
bolts with a hammer.
Fit new mounting bolts if necessary.
Heat the cutting arm to around 150ºC
with a hot air gun and fit new ballbearings.
Don’t forget the spacer ring between
the bearings and make sure that they
are pressed in as far as they will go.
Fit new mounting bolts, if required,
while the cutting arm is still hot.
Spread a little grease on the indicator
washer and place it on the disc guard.
Spread a little grease on the indicator
washer (A) and place it on the disc
guard.
Turn over the disc guard while holding
the other parts in place.
Now fit the rubber washer (C) tight to
the disc guard, followed by the indicator
washer (D) and cover plate (E).
Reassemble the cutting arm and disc
guard.
Reassemble the cutting arm and disc
guard. Tighten the three bolts and
check that the disc guard can rotate.
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CUTTING EQUIPMENT
Drive the shaft in through the bearings
until it projects out 5.5 mm on the other
side.
Fit the pulley (with the reinforcing
washer inwards), not forgetting the
spacer ring over the ball-bearing.
Drive in the shaft using a plastic mallet
so that it goes through the ball-bearings
and projects about 5.5 mm beyond the
other side. Use a sleeve or socket that
supports the inner race of the ballbearing.
Fit the pulley. Place the spacer ring on
the ball-bearing, followed by the pulley,
with its welded reinforcing washer
facing in towards the cutting arm.
Tighten the bolt.
Tighten the bolt.
Fit the spacer sleeves and flange
washer as illustrated.
Fit the spacer sleeves and flange
washer as illustrated.
Use a hot air gun to heat the spacer
sleeve separately to around 150ºC so
that it fits on the shaft more easily. Fit
the spacer sleeve with the bevelled end
towards the cutting arm.
Press it down using a suitable drift.
Fit a new drive belt around the clutch
drum and fit the rear belt guard.
Fit a new drive belt around the clutch
drum.
Fit the rear belt guard. Do not forget the
screw on the underside of the guard (on
371K).
Screw out the adjuster screw to reduce
the belt tension.
Screw out the adjuster screw.
!
WARNING!
Wear protective glasses and gloves when fitting the spring.
The spring is very powerful and could cause serious injury if it
jumps out.
Release the belt guard.
Release the belt guard by removing the
(2) screws, adjuster plate, spring and
square nut.
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CUTTING EQUIPMENT
Clean, refit and adjust.
Clean and replace damaged or worn
parts of the belt tensioning mechanism
before fitting.
Lift the cutting arm into position and fit
the front belt guard and belt tensioning
mechanism.
Lift the cutting arm into position. Place
the front belt guard and belt tensioning
mechanism in position.
Tension the drive belt and tighten the
nuts.
Partially tighten the nuts.
Tension the drive belt by screwing in
the belt tensioning screw (A) until the
nut (B) is in line with the arrow (C).
Then tighten the nuts.
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CUTTING EQUIPMENT
Inspect the drive belt for wear and damage before fitting.
Fig. A. Normal appearance of a belt after extended use.
Fig. B. Replace the belt if there are cracks running through the thickness of the belt
after a short time in use, even if there is minimal wear to the sides.
Fig. C. Worn, frayed edges on a belt are the result of incorrect alignment with the
pulleys or poorly tightened nuts on the cutting arm.
Fig. D. Uneven wear caused by belt being too loose or engine idle speed being too
high.
Fig. E. Tears in belt and damaged reinforcement caused by poor alignment, oil on
pulleys or substandard belt. Use Husqvarna original belts.
Fit a new belt around the clutch drum
and fit the cutting arm in its rearmost
position.
Work the drive belt over the front pulley
and tension the belt.
Tighten the cutting arm nuts and fit the
belt guards.
Fit a new drive belt around the clutch
drum.
Fit the cutting arm and fit the nuts
loosely.
Work the drive belt over the front pulley.
Tension the drive belt by screwing in
the belt tensioning screw (A) until the
nut (B) is in line with the arrow (C).
Then tighten the nuts.
Check that the pulley does not rub on the front belt guard. If it does, fit the cutting
disc and tighten the centre bolt hard. This should move the drive shaft and pulley
sufficiently to prevent the pulley touching the guard.
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CUTTING DISCS
Rating plate
All Husqvarna power cutters are fitted with a plate on the guard that provides
valuable information when selecting cutting discs for a machine. The measurements
that the operator should be familiar with are as follows:
Disc diameter
The disc diameter is given in mm or inches. Husqvarna 371K Air Injection can be
used with the following cutting discs: 12 " (300 mm), 14 " (350 mm) or 16 " (400 mm).
Husqvarna 3120K Air Injection can be used with the following cutting discs: 14 " (350
mm) or 16 " (400 mm).
Centre diameter
The size of the mounting hole in the cutting disc must match the diameter of the
bushing on the drive shaft exactly. There are various standards depending on the
country the machines are sold in. Common standard diameters are 20.0, 22.2, 25.4
(1 " ) and 30.5 mm. Replaceable bushings are available for Husqvarna power cutters
for all these diameters.
Speed
All cutting discs are marked with a speed that is usually measured in revolutions per
minute (rpm). This is the maximum speed of rotation of the disc. The maximum
speed of a disc is determined by the disc manufacturer. The rating plate on the
power cutter gives a different speed that is determined by the machine manufacturer and is known as the rated speed. The rated speed is the speed given on the
rating plate and it determines what speed rating a disc must have in order to be
used with the power cutter. A cutting disc must be rated at the same speed or a
higher speed than that given on the machine’s rating plate.
The maximum speed of the machine is a measurement that does not concern the
machine operator and is not even stated on the machine, but is used for inspection
purposes by the workshops. The maximum speed is the highest permissible speed
that the drive shaft may reach with no load on the disc and the engine at full throttle.
According to a European standard the maximum speed must not exceed the rated
speed on the machine’s rating plate by more than 10%. For example, a machine
with a speed rating of 5,100 rpm must never exceed a maximum speed of 5,100 x
1.1 = 5,610 rpm.
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CUTTING DISCS
Peripheral speed
Peripheral speed is measured in metres per second and is directly proportional to
the rotational speed. The definition of peripheral speed is the speed at which a
given point on the edge of the blade travels through the air. It is also the speed that,
for example, a given diamond particle will have in relation to the point of contact
during cutting. Another way of visualising peripheral speed it to imagine the power
cutter as a vehicle, and the cutting disc as its driving wheel. A rotational speed of
5,100 rpm with a 300 mm cutting disc would give the following speed:
Circumference of disc: 0.3 m x = 0.94 m. Rotational speed of disc: 5,100 rpm =
85 rps. Circumference x rotational speed = peripheral speed. So the peripheral
speed = 0.94 m x 85 rps = 80 m/s.
Expressed in terms of a vehicle travelling down a road, this speed is roughly 290 km/hr!
Husqvarna cutting discs are designed for a peripheral speed of 100 m/s. A European colour code prescribes a green label for a maximum peripheral speed of 100
m/s, while a red label indicates a maximum speed of 80 m/s.
Types of disc
Hand-held power cutters require cutting discs that are approved for use with handheld power cutters. It is not permitted to use blades intended for bench-mounted
power cutters, for example. The speed marking must be equal to or higher than that
given on the machine’s rating plate. Husqvarna cutting discs are naturally designed
for hand-held cutting.
•
Diamond blades are a type of cutting disc that is becoming increasingly
common for cutting stone, concrete, asphalt and similar materials. The main
advantage of a diamond blade is that it maintains practically the same cutting
depth throughout its working life and has a high cutting speed. If used correctly it
is a very economical blade. Husqvarna sells diamond blades for professional
and occasional users. The range includes several grades, which should be
chosen to suit the material to be cut. Diamond blades are available for both wet
and dry-cutting.
•
Abrasive discs, which were originally the commonest type of disc for cutting all
materials, are now used mainly for cutting metal. Husqvarna sells abrasive discs
for cutting steel and concrete, and its concrete discs are also suitable for cutting
soft steel, such as reinforcing bar and structural steel.
•
Husqvarna Rescue is a special blade with carbide bits. It can only be used by
trained personnel in rescue work.
Replaceable disc bushings
Husqvarna power cutters can be fitted with a centre bushing to fit various standard
disc sizes. Replaceable bushings are available for 20.0 mm, 22.2 mm, 25.4 mm (1 " )
and 30.5 mm hole diameters. The Husqvarna 371K/3120K Air Injection is supplied
as standard with various centre bushings depending on the country of sale.
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CUTTING DISCS
Which type of disc should I use?
Hard steel
Stainless
steel
Soft steel
+ other
metals
Granite
Hard brick
Pre-cast
concrete
Hard
concrete
General
concrete
Concrete
pipe
HDC 410
HDC 500
HDA 900
Steel
Concrete
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Abrasive
building
block
Soft brick
Green
concrete
Asphalt
WET-CUTTING WITH DIAMOND DISCS
Wet-cutting practically eliminates
cutting dust, while also cooling the
cutting disc, which extends its working
life.
The disc guard has been prepared to
accept Husqvarna’s wet-cutting kit.
The wet-cutting kit has been redesigned and can be fitted very quickly.
The nozzles are easily fitted to the disc
guard and secured in place by tightening a single screw from the outside. A
tap mounted on the handle allows the
water flow to the cutting disc to be
controlled. The supply hose is now
fitted with a standard Gardena coupling.
Husqvarna Power Cutter
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