|NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Small engine technology
1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.
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Internal Combustion Engines
If you have some idea of how your automobile engine operates - or a Model T
Ford for that matter - then you know the basic operating principles of your
small engine as well. In fact, your Craftsman Eager 1 has a lot more in common
with a Model T than a Honda Accord. However, strip off, the electronics,
pollution control devices, and engine powered accessories, and the basic
mechanical construction is very similar, though the lawn mower engine is
not manufactured to quite the same tolerances and with the same quality
materials as an automobile engine.
Nearly all small engines up to 20 HP or so are single cylinder affairs - one
piston, one spark plug, no distributor, forced air cooled - about as simple
and straightforward as it gets.
If you have never been under the hood of your automobile, then the description
in the following sections may be of some help.
The next chapter: "Engine Diagrams" provides an explanation of each of the
4 strokes of a 4 stroke engine. However, if you cannot get the hang of my
fabulous ASCII graphics, check out the following site:
The How Stuff Works Web
site has some really nice introductory material (with graphics) on a variety
of topics relating to technology in the modern world. Of relevance to this
document is an article on "How Car Engines Work" which is really mostly
about the basic principles but WITH some real animated graphics!
Unless otherwise noted, most of the descriptions and procedures in this
document apply to both 4 stroke and 2 stroke engines. However, there are
fundamental differences in the proper fuel and oil that is used with each
The small 4 stroke engine has a separate oil sump just like the engine in
an automobile. Therefore, gasoline and oil are separate. Oil changes are
WARNING: a new lawn mower or other piece of yard equipment will very likely
be shipped without oil or just a minimal oil fill. Check it first and add
oil if necessary. Running an engine without oil for a few minutes can cause
serious - or terminal - damage. Even if your mower was assembled by the
store where you bought it, don't assume they filled it with oil and tried
The 2 stroke engine requires that the gas and special oil be mixed prior to
use in specific proportions. Leave out the oil - or get your gas cans mixed
up - and you will quickly ruin a 2 stroke engine due to lack of lubrication if
plain gas is used by mistake. Clearly label the gas cans for each type and
instruct anyone using them in the proper fueling technique.
Portable tools like chain saws, weed whackers, and backpack type leaf blowers
use 2 stroke engines as these need to operate in a variety of positions.
Stationary or wheel-about equipment including most lawn mowers, rototillers,
shredders, backup electric generators, and large blower/vacs, use 4 stroke
Another distinction is that engines smaller than about 2 horsepower are
generally 2 stroke while those larger than 2 horsepower are generally 4
stroke but there are exceptions. Lawnboy lawn mowers tend to have 2 stroke
engines and there are some types of equipment with very small 4 stroke
engines. Of course, if your engine has a cap marked 'oil' then it is a
Larger pieces of yard equipment like riding mowers and lawn tractors use
4 stroke engines that are really very similar in most respects to their
smaller cousins - much more so than to the engine in your automobile, for
example. Similar servicing procedures apply. In fact, if you read the
respective chapters in any of the engine repair books for engines (listed
in the section: References) under 5 horsepower and
those between 5 and about 20 horsepower, the only significant differences
will be in the size of the various engine parts!
You may be surprised at the large number of individual parts which comprise
the engine even on a $100 mower. The following description is for a typical
single cylinder 4 stroke engine as would be found on most rotary mowers,
rototillers, shredders, backup electric generators, larger snow throwers and
leaf blowers, and even modest size riding mowers and lawn tractors:
These are internal combustion engines which means that the burning of the
fuel-air mixture itself powers the engine. External combustion engines use
the heat from combustion to expand or boil a working fluid as in a steam
engine. Other examples of internal combustion engines are the rotary
Wankel engine and gas turbines (jet engines).
- Fuel tank and fuel line. This may be on the side, above, or below the
engine intake. A fuel pump may be present - either as part of the
carburetor or a separate unit if it is not gravity fed. There may
be a fuel cutoff valve and fuel filter and/or filter screen) as well.
- Shroud or blower housing. Since these engines are all forced air cooled,
(no radiator, water jacket, or thermostat) the proper air flow is extremely
important as air is much less efficient than water at removing the
substantial heat energy generated by a running engine. This piece of
sheet metal precisely guides the airflow. The engine should not be run
for any length of time with the shroud or blower housing removed.
- Oil filler pipe. On many engines, this is just a hole near the base fitted
with a screw-in plug. On fancier models, there is an actual extension tube
with a dipstick usually fitting into that hole in the base and sealed with
- Starter. Like an automobile engine, some means must be provided to
supply an initial rotation.
- Pull or recoil starter. This is by far the most common on small pieces of
yard equipment. A self retracting cord wrapped around a one-way clutch
acts on the crankshaft directly. Less common variations include wind-up
starters and a simple rope wrapped around a drum.
- Electric starter. A small motor - usually powered from a wall socket -
drives the edge of the flywheel via a self releasing gear. On larger
equipment, as in an automobile, an on-board battery provides the
electricity. The battery is recharged by an alternator while the engine
- Air filter. The air used by the engine should be free of dust and larger
particles. Most yard equipment will include one of several kinds of air
filters - paper, foam, or oil bath. However, engines used exclusively in
equipment used in low dust environments like snow throwers and outboards
may lack an air filter. Types of air filters include:
- Paper air filter. This is similar to the filter in a vacuum cleaner.
The pleated paper element must be replaced when clogged.
- Foam air filter. This uses a piece of foam rubber lightly coated with
engine oil and can be washed in detergent or soap and water when dirty
and recoated though replacement will be required if it is badly
- Oil bath filter. This contains an oil reservoir and filter element.
These can be drained and cleaned. Refill to the 'full' mark with the same
type of oil as used in the engine crankcase. The oil bath air filter is much
less common than the paper or foam type.
- Carburetor. The function of the carburetor is to provide a precise
air-fuel mixture to the engine under a variety of operating conditions
- Starting. A higher ratio of fuel to air is needed when starting and/or
the engine is cold.
- Idle. Just enough flow is maintained to keep the engine from stalling.
- High. Full flow is provided with engine speed limited by feedback from
- Load pickup/acceleration. The proper air-fuel ratio and amount of fuel
must be maintained when the equipment is placed under load or encounters
an increased load and when switching from IDLE to HIGH, for example.
Several functions are incorporated in a carburetor:
- Fuel control - float, diaphragm, suction. A variety of means are provided
to maintain a nearly constant supply of fuel despite varying levels of
gas in the fuel tank.
Craftsman and Lawnboy carburetors use a float similar to the system used
in automobile engine carburetors (when they still used this technology).
A hollow float maintains a constant level of fuel in a reservoir called a
float bowl by acting against an inlet needle valve. The fuel tank will
usually be above the level of a float carburetor though a fuel pump could
also be used. See the section: Basic operation of a
float type carburetor for a more detailed description and diagram.
Where equipment may be operated in any position, a diaphragm carburetor
is used. A flexible disk (diaphragm) with atmospheric pressure on one
side and intake pressure on the other is used to maintain the proper
air-fuel ratio (this is a gross simplification!).
Briggs & Stratton engines usually have a suction lift carburetor with
a built-in fuel pump. Some simple ones don't even have a fuel pump but
depend on suction alone to pull gas from the tank. In both cases, the
fuel tank is usually below the carburetor. If there is no fuel pump, the
fuel tank should generally not be less than 1/3 full for proper operation.
- Venturi. If you look into the air intake of a carburetor, you will see
a narrowing of the passageway. This is called a venturi and results in
higher velocity air flow and lower pressure at the center of the narrow
section. Just as when you blow across a soda straw submerged in a liquid,
or use a bug or lawn sprayer, this lower pressure will suck up the liquid
and inject it into the moving air stream. In the care of gasoline,
evaporation is almost immediate.
- Throttle. A plate that can rotate to close or open the outlet from the
carburetor to the engine intake pipe controls the flow of the air-fuel
mixture to the engine. When fully closed, the engine will stop. When
slightly open, the engine will idle. When fully open, maximum power
is developed. To limit rotation speed with little or no load and to
maintain proper speed under load, a governor acts on the throttle plate
to keep it at the proper setting under all normal operating conditions.
- Choke. A plate that can rotate to partially close off the air supply
is placed in the air intake side of the carburetor. This is usually
set manually when starting, particularly in cold weather, to make the
mixture richer - more fuel with respect to air - as needed. The
carburetors most newer equipment have automatic chokes or other methods
of accomplishing the same objective.
- Flywheel. The flywheel in a small engine serves several functions:
Provides some or all of the inertia so the engine can coast through the
3 non-power strokes of the full cycle (4 stroke). However, with rotary
mowers in particular, the blade provides some of the inertia due to the
use of a lower mass (lighter, so this is not all bad!) flywheel.
- Provides the electrical energy for the most common magneto based ignition
system. A powerful magnet mounted on the edge of the flywheel passes by
a coil (and possibly other stuff) once per rotation. This is part of the
ignition system and either provides the high voltage for the spark
directly or charges a capacitor which is then discharged (by a solid
state switch triggered by the flywheel magnet rotating past a sensor)
into an ignition coil at the proper time to generate the spark.
- Includes the fan blades which provide most of the essential cooling
- Links to the starting mechanism. For recoil or other manual starters,
the starting device attaches to the center of the flywheel. For electric
start, the flywheel will have a large gear along the outside that is
engaged by the starter motor gear (with an overrunning clutch).
- Engine/blade brake. Releasing the dead-man bar engages a brake pad
that is usually applied to the outside of the flywheel.
A soft metal key locks the flywheel to the crankshaft. This is designed
to shear (break) to protect expensive engine parts should the engine stop
abruptly (due to blade of a rotary mower striking an obstruction, for
example). In this case, the flywheel inertia will attempt to keep it
moving and it is better to shear the key than to fracture the flywheel
- Blade adapter. This fastens the two ended cutting blade to the crankshaft.
A key part (no pun intended, well almost) is the locking key. This may
be a half circle (called a Woodruff key), rectangle, or part of the blade
mount bracket itself. Its purpose, like the flywheel key, is to break
cleanly should the blade strike an obstruction thus protecting - hopefully -
the expensive engine parts from damage. However, see the section:
Why you really don't want to attempt to move an immovable
object for the reasons this isn't always successful and using your mower
to trim rocks and curbs really should be avoided. Therefore, it is essential
that the key be made of the proper material - a soft metal. Never replace a
broken key with one made of steel 'because it is stronger'.
For other types of equipment, various adapters are used to couple the
driven device to the engine crankshaft. These may include gear reducers or
multispeed transmissions, belt or chain drives, or a direct connection to
a blower, vacuum, or pump impeller, or electric generator or alternator.
As with the blade adapter, a soft metal key or shear pin will likely be
used for protection.
- Spark Plug. The ignition source for all small gasoline engines is a high
voltage spark. There is nothing magical about a spark plug - it is just
an insulated electrode with a specific size gap poking into the combustion
chamber. However, damage or fouling (gumming up with carbon or other
combustion residue) is one of the most common causes of a hard or impossible
to start engine. Not all spark plugs are created equal despite their simple
function so the recommended replacement type should always be used. In
addition, most automotive spark plugs have a longer reach - more threads -
and will likely bash the piston at TDC.
- Magneto ignition. These used to use mechanical contact points - controlled
by a cam on the crankshaft to determine the timing. Nowadays, most are
fully electronic and require no maintenance.
- Point type. The magnet mounted on the flywheel induces a current in
a coil with pole pieces mounted in close proximity to the flywheel.
When this current is near its peak, a set of contacts operated by a
cam on the crankshaft - the breaker points - open interrupting the current
and diverting it to a condenser (capacitor). The current drops abruptly
and induces a high voltage in another coil wound on the same core. This
coil is connected to the spark plug terminal. It will be 10 kilovolts
or more to jump the spark plug gap even under the more difficult conditions
during maximum compression. The exact time - angle of the crankshaft - at
which the points open is the critical setting. This is usually adjusted
by moving the piston to a specific dimension below Top Dead Center (TDC)
or aligning timing marks on the flywheel and crankcase and setting the
points to just open at this location.
- Electronic type. These can be designed in several different ways but all
must emulate the behavior of the point type ignition replacing just the
magneto coil/pole piece assembly and without requiring alterations to the
flywheel. See the section: What is inside a small
engine electronic ignition module?.
- Engine block. The casting which includes the cylinder, crankcase, intake
and exhaust ports and valve guides and seats. The other major part of
the engine structure is the crankcase cover/oil sump.
- Cylinder. This is the, well, cylindrical component in which the piston
moves. Most small engines are made of either aluminum (most common these
days) or cast iron. In most cases, the aluminum type will have a cast
iron liner for long life where the piston actually slides. With regular
oil changes, there will be virtually no wear.
- Cylinder head (or just head). The cap which encloses one end of the
cylinder. On most small engines, the only component mounted in the head
is the spark plug which screws into a threaded hole. However, on the most
modern (and expensive) small engines (as well as all automobile engines),
the valves will also be mounted above the piston in the cylinder head.
Fins on the head increase its surface area - absolutely necessary for
these air cooled engines to keep their cool
- Crankcase. This encloses the lower working parts of the engine and
also contains the oil supply. The flywheel/magneto end main bearing is
mounted in one end of the crankcase. The crankcase cover/oil sump
seals the crankcase and also includes the PTO/blade end main bearing.
Also see the section: Bearings and bushings.
- Valves. These look like metal mushrooms and seat against metal rings
mounted in the engine block casting. They are spring loaded in the closed
position, slide in valve guides (bored holes), and operated by valve lifters
from cams on the camshaft.
- Intake valve. This valve is opened only during the intake stroke. It
is subjected to less stress than the exhaust valve as it is cooled by the
fresh air-fuel mixture. The intake valve may be stamped with an I.
- Exhaust valve. This valve is opened only during the exhaust stroke. It
is subjected to more stress than the intake valve since the hot exhaust
gases pass out through its opening. The exhaust valve may be marked
with an E.
Note that in most small engines, the valves are not mounted above the
cylinder (in the head) as they are in automobile engines. Rather, they
are mounted on the side of the cylinder in the engine block. This is
done for reasons of cost requiring fewer parts and simpler construction.
The cylinder head is then just a casting with cooling fins and a threaded
hole for the spark plug. The disadvantage of the side valve engine is
lower efficiency and higher pollution. The newest (and currently most
expensive) lawn mowers and other yard equipment are incorporating the
better overhead valve technology. Eventually, environmental protection
laws may force this on all small engine operated devices. However, both
types can apparently be made to meet the emissions standards mandated
for all new mowers sold after September 1996.
- Valve lifters (tappets). These also look like metal mushrooms with the
wide part riding on the cams of the camshaft. Their ends press on the
ends of the valve stems to open them during the appropriate part of the
- Compression release. This mechanism reduces the compression somewhat at
low rpms (to make starting easier) by opening the exhaust valve slightly
early. A weight disengages the compression release once the engine
- Piston. The piston is a cylindrical close ended plug that slides in
the cylinder and is acted upon by the hot gases during combustion.
In small engines, it is usually made from an aluminum alloy. Power
is obtained by the piston driving the off center crank pin journal on
the crankshaft via the connecting rod.
- Piston rings. Springy split rings of cast iron (usually) which fit in
grooves or slots around the piston. When the piston with rings is installed
in the cylinder, the rings expand and tightly contact the cylinder wall
forming a remarkably good seal.
- Piston pin (or wrist pin). The piston is attached to the connecting
rod via a polished steel pin (actually a tube) which fits closely in
a bearing in the connecting rod.
- Connecting rod (or just rod). A rod with bearings at both ends. Small
4 stroke engine rods are usually made of an aluminum alloy and have no
special bearing inserts at either end. 2 stroke rods will have needle
bearings due to the type of lubrication available.
- Crankshaft. The main rotating part of the engine. See the diagram in
the section: Crankshaft anatomy.
- Main bearings. The crankshaft is supported at each end by the main
bearings. Most inexpensive 4 stroke engines use plain bearings without
inserts - just the finely polished steel crankshaft surface rotating
in equally finely finished surfaces of the engine block casting. 2 stroke
engines may use needle bearings in these positions. Some engines use
- Crank pin journal. The offset surface where the connecting rod bearing
rotates. The is the bearing with the most stress as the full force of
each power stroke is applied here. It is the location where damage will
likely show up first due to contaminated oil or lack of oil.
- Counterbalance weights. These two massive weights are part of the
crankshaft casting opposite the crank pin journal. They help to
reduce the vibration of the engine by partially counteracting the forces
of the piston and connecting rod as they move back and forth. Larger
engines may have additional rotating balancer shafts which can be
designed to do a better job of minimizing vibration than just the
counterbalance weights on the crankshaft.
- Crank gear or timing gear. A small gear mounted on the crankshaft which
rotate the camshaft at half the crankshaft speed to control valve timing.
The crank gear has half as many teeth as the camgear.
- A precision taper is located at the magneto/flywheel end for mounting
the flywheel and starter components.
- A cam (possibly removable) may be present under the flywheel location
for operating the breaker points on a point type ignition system.
- An auxiliary PTO gear may be present. This is a worm gear or other
mechanism for providing a secondary, usually lower speed, higher torque,
power output. A typical speed reduction ratio is 8:1. This may be used
to power a front wheel drive sprocket, for example.
- Camshaft. This includes the two cams which operate the valves and runs at
1/2 the speed of the crankshaft. In addition, it may have a compression
release mechanism to reduce starting effort and in the case of many Tecumseh
vertical crankshaft engines, is part of the oil pump as well.
- Oil/lubrication device. Several types of mechanisms are used to distribute
engine oil to all the critical parts in a small engine:
- The piston rings (oil ring mainly) scrape oil off of the cylinder walls
which passes through holes in the oil ring grooves of the piston to
lubricate the piston pin at the top of connecting rod.
All other important parts are lubricated typically by one of the following
- Oil dipper. An extension to the connecting rod dips into the oil sump on
each rotation of the crankshaft and provides lubrication by literally
just splashing oil onto where it is needed. (Horizontal crankshaft
- Oil slinger. A pinwheel like affair rotated off of the crankshaft or
camshaft flings oil onto where it is needed. (Horizontal or vertical
- Oil pump. A camshaft driven pump distributes oil to the important parts
via various passages in the camshaft, engine block, and possibly the
crankshaft and connecting rod as well. (Horizontal or vertical crankshaft
- Governor. This provides the feedback to the carburetor throttle plate to
regulate the engine speed (usually for anything but idle speed). Thus,
the speed is maintained relatively constant despite changes in load and
a engine temperature
- Air vane governor. The air flow provided by the flywheel/blower passes
by a plate which is coupled to the carburetor throttle plate acting
against a spring force.
- Flyweight governor. A spinning gear assembly with a pair of weights
is driven by the camshaft. As the centrifugal force of the weights
increases, they move a sleeve which presses against a lever whose shaft
passes through the crankcase. This lever then operates the carburetor
throttle plate against a spring force.
With both types, increasing the spring force will increase the engine speed.
- Breather. Since the crankcase is otherwise sealed with the piston moving
up and down or in and out, some means is required to maintain a negative
pressure or else oil might be forced out various places like the bearings
an oil seals. The breather is a leaky one way valve which easily passes
air out but is mostly blocked in the other direction.
- Muffler. The muffler mounts on the exhaust port and serves two important
- Noise reduction. As its name implies, the muffler muffles or reduces
the loud noise resulting from the explosions of the air-fuel mixture.
- Spark/flame arresting. The internal baffles of the muffler prevent most
sparks or flame from existing the engine even if it backfires.
- Oil seals. These are spring loaded rubber rings which prevent the escape
of oil through the main bearings to the outside.
The type of engine in your lawn mower or automobile operates on what is
called the 'Otto' cycle (if you care). A complete 'cycle' is needed to
supply one power impulse to the output shaft. All engines must provide
the following regardless of whether they are 2 stroke or 4 stroke, rotary,
or turbines (though turbines or jet engines operate in a continuous rather
than pulsed manner):
For the following, refer to the section: The four strokes
of a four stroke engine in living ASCII art.
- Intake stroke. Air is mixed with fuel (gasoline for piston engines) and
drawn into the combustion engine.
4 stroke: The air-fuel mixture is sucked into the cylinder through the
open intake valve as the piston moves downward on the intake stroke.
- Compression stroke. The air-fuel mixture is squeezed into a smaller space.
This heats it to some extent and prepares it to be burnt. (Note: in a
diesel engine, this heating alone causes the mixture to ignite and there
is no spark needed). Compression ratios for small engines are typically
low compared to automobile engines.
4 stroke: Both valves are closed. The piston moves upwards thus reducing
the space above it and compressing the air-fuel mixture.
- Power stroke. The compressed air-fuel mixture is ignited at a precise
time by the ignition system (spark).
4 stroke: Both valves are closed. The heat produced by the rapidly
burning gases to expand and drive the piston downward and because it
is connected to the crankshaft, drives the load as well.
- Exhaust exhaust. The burnt combustion products are driven out of the
cylinder. These consist of carbon dioxide, carbon monoxide, water vapor,
oxides of nitrogen, some unburnt hydrocarbons, and numerous other mostly
4 stroke: The exhaust valve is open. The piston moves upward and pushes
the exhaust gasses out through the muffler.
A relatively massive flywheel attached to the crankshaft provides the inertia
to allow the engine to coast through the non-power strokes (1-3). However,
this is not always enough by itself - the blade on a rotary lawn mower is often
required as well and a rotary mower may not start easily if at all without the
blade in place and tightened securely.
Note that the terms '2 cycle' or '4 cycle' are often used incorrectly when
what is meant is 2 stroke or 4 stroke. The cycle is the entire sequence of
events including intake, compression, power, and exhaust. The complete cycle
for a 4 stroke engine is two complete revolutions of the crankshaft. The
complete cycle for a 2 stroke engine is one rotation of the crankshaft. This
means that a 2 stroke engine produces a power stroke on every rotation
of the crankshaft while a 4 stroke engine does this only on every other one.
Thus, a 2 stroke engine will be more powerful than a similar size 4 stroke
engine. However, on the down side, 2 stroke engines tend to be less efficient
in fuel utilization and pollute much more than 4 stroke engines.
The shafts of rotating parts normally are mounted in such a way that friction
is minimized - to the extent needed for the application. A bearing is any such
joint with more specific terms used to describe the typical types found in
lawn mowers - or small motors, automobile engines, or 100 MW turbines.
The bearings to be concerned with in a lawn mower or small engine are:
- Main bearings - supports the crankshaft at the blade/PTO end and at the
- Crank pin journal - links the bottom of the connecting rod to the rotating
crankshaft. This single location is most critical as it is subject to
the largest forces from the piston.
- Piston pin journal - links the piston to the connecting rod and is subject
to the full pounding of the piston but only rotates back and forth through
a small angle.
- Gear reduction - additional bearings may be used inside a gearbox. For
example, a snow thrower or rototiller require lower speed high torque
outputs to drive the auger and tines respectively.
- Auxiliary power takeoff - a lower speed output for front wheel drive or
other secondary purpose.
- Wheels - although this is a must less critical application, higher quality
bearings will make for easier and smoother operation and longer life.
A variety of bearing types are available. For most inexpensive rotary lawn
mowers, plain bearings are most popular due to their simplicity and low cost.
- Plain bearings consist of an outer sleeve called a bushing in which a
polished shaft rotates. The bushing may be made of a metal like brass
or bronze or a plastic material like teflon. In the case of many small
engines, the aluminum alloy casting of the crankcase and cover are simply
then bored and reamed to a fine finish to accept the ground and polished
shaft. There is no actual bushing. However, if these bearings ever need
repair, the hole(s) in the casting would need to be enlarged and a bushing
would then be pressed into place and finish reamed.
The shaft of a small engine is made of steel though other materials may be
found depending on the particular needs. Where a metal bushing is used,
there must be a means provided for lubrication. In a small engine, this is
provided by the engine oil via a variety of means. For example, a typical
Tecumseh lawn mower engine lubricates the blade end/PTO main bearing by
direct immersion in the oil sump, the magneto/flywheel main bearing via
machined passage fed from the camgear driven oil pump, and the crank pin
journal by oil sprayed or dripped from above.
Under normal conditions, a plain bearing wears only during start and stop
cycles. While the shaft is rotating at any reasonable speed, there is
absolutely no metal to metal contact and thus no wear. With a properly
designed and maintained bearing of this type, a very thin oil film entirely
supports the shaft - thus the importance of regular oil changes to assure
clean oil with effective lubricating properties.
To put this into perspective, the clearance between the connecting rod and
crank pin journal of a typical small engine crank pin journal may be
0.0005 inches or less - one half of one thousandth of an inch. This is
roughly 1/10th the thickness of a sheet of computer paper! The same engine
may be considered worn to its limit when this clearance increases to .0015
inches. Your automobile engine's crankshaft is entirely supported by these
types of bearings as well - and the tolerances are even finer.
If oil is lost even for a short time, serious metal-to-metal sliding contact
results in significant wear and possible destruction of one or both bearing
surfaces. Particles in dirty oil can get trapped and embedded, usually in
the softer outer bushing resulting in scoring and additional wear. It is
your responsibility to assure that your engine oil is maintained at the
proper level and changed regularly. Any warranty is not likely to cover
failure due to negligence.
- Frictionless bearings are usually of the ball, roller, or needle variety.
An inner ring called a race rotates supported by a series of balls, rollers,
or elongated rollers called needles inside an outer race. An optional
'cage' positions the rotating objects uniformly around the periphery.
There is virtually no friction even at stand-still with these bearings.
However, rolling metal to metal contact is maintained at all speeds so
they are not quite as wear free as a properly maintained and constantly
rotating plain bearing. Loss of lubrication, while not as immediately
destructive as with a plain bearing, will still result in accelerated
wear and premature failure.
- Ball bearings may be found in the main bearings of yard equipment where
significant sideways loads are encountered. Lubrication is provided by
engine oil, gear oil in a separate gearbox, or by being packed with
grease either as a maintenance item or for the life of the bearing at
- Roller bearings may be used on larger engines where significant loads
of all types are encountered. Since the contact between rollers is
a line rather than a point as it is with balls, they can support much
greater loads. Lubrication needs are similar to those of ball bearings.
- Needle bearings are found in 2 stroke engines and may be used for the
main bearings as well as the crank pin journal. The 'needles' are really
just long rollers - a ratio of 5:1 to 10:1 is typical of length to width -
and there is no cage to position the needles, they are packed in around
the shaft as many as will fit. Due to the type of lubrication - oil mixed
with the gasoline - plain bearings are generally not suitable for this
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