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Chain & Sprocket Systems and Maintenance


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Chain and sprocket drives
of one form or another have
been driving machines and
conveying materials for over
a thousand years. The chain
manufacturing industry has developed
numerous types and sizes of drive
chains. Although it is a mature form
of power transmission, there are still
many industrial applications for which
drive chain is suited. It is economical,
resistant to shock loads, easy to install,
has the ability to transmit high torque,
operates in hostile environments, and
is efficient. Chain and sprocket drives
are highly engineered and manufactured
within close tolerances.

Chain Function
Chain can be grouped into two functions:
material handling and power
transmission. Some chains are used
for both purposes. If the chain is transferring
or conveying raw material or
finished products, it is considered a
material handling chain. If the chain’s
primary purpose is transferring power
from one shaft to another, it is for power
transmission. In some applications,
special attachments—for moving materials—are
part of a power transmitting
chain’s construction.
The chain drive system consists of a
driving sprocket, one or more driven
sprockets, and a loop of chain. A
sprocket is a wheel that on the outside
diameter has evenly spaced, uniformly
shaped teeth, which provide positive
engagement with the chain. Because
the chain is hinged at every link, it is
able to wrap around the sprockets’
teeth, connecting the driver and driven
units.
Power and torque are transmitted
from the drive sprocket mounted
on the shaft of the prime mover, and
then connected to one or more driven
sprockets by the chain. As the chain
drive system operates, every link in
the chain undergoes “cycle loading.”

The working or tight side of the chain
is under full tension, while the slack
side is under minimum tension. The
sprockets, turning on their respective
shafts and connected by a chain,
deliver the power or transfer the load
to accomplish work.
During operation, a phenomenon
known as “chordal action” occurs in a
chain drive system resulting from the
fact that chain link is a straight-line
segment trying to follow the circular
path of the sprocket pitch diameter.
Because the line of approach of the
chain is not tangent to the pitch circle,
it is lifted to the top of the circle and
then dropped down. There is a surge
of force in the chain caused by the
change in speed as it makes this rise
and fall. An increase in chain velocity
can aggravate this event, resulting
in vibration and pulse loading in the
system. Excessively worn parts may
amplify this effect.
Standard roller chain is the most
common type of drive chain used in
industry. ANSI & ISO maintain standards
for design, dimensions, and
interchangeability. Pitch, distance
from center of pin-to-pin, roller diameters,
width, and load ratings are
defined. Standard roller chain is made
up of alternating roller links and pin
links. Each link is constructed by placing
rollers onto bushings and pressing
roller link plates onto the bushings.
The side plates into which two pins are
pressed, hold the rollers and bushings
in place. The assembly is secured by
the press fit between the pins and the
plates (often accompanied with a rivet
on the end of the pins), spring clips, or
cotter pins through holes in the end of
the pins.
Chain & Sprocket Alignment
Proper alignment within the recommended
tolerances prevents premature
component wear. It also reduces the loads that are applied to the supporting
shafts and bearings.
A simple straight edge, wire, or laser
system can be used to align the sprockets.
Sprocket shafts should be parallel.
The offset of sprocket faces should be
<.0625"/foot of shaft centerline distance.
Angularity should be Axial run-out (wobble) of sprockets
should be <.001"/inch of diameter.
Chain and Sprocket Inspection
Check the condition of the drive components.
The articulation of chain
as it enters and leaves the sprockets
causes the pins and bushings to wear,
and the chain will gradually elongate.
This is sometimes referred to as “chain
stretch.” The wear of a chain may be
minimized by proper lubrication and
maintenance procedures. Some wear
is inevitable and normal. Measure the
chain and if elongation is greater than
2% (.24" in one foot), replace the entire
chain. Operating beyond the 2% elongation
maximum recommendation will cause the sprockets not to engage
properly and may cause damage to
the other components in the system.
A simple go-no-go gauge set with machined
steps (see Figure #1) is a great
tool for determining when to change
the chain. Do not join a new section of
chain to a worn section because it may
not operate correctly. Check the side
plates for wear due to misalignment
or cracks from fatigue. Make sure all
chain joints are free to articulate and
not stiff or frozen. Inspect carefully for
signs of corrosion.
Sprocket Wear
Sprockets should be closely examined
for wear. Loose or wobbly sprockets on
the shaft can indicate a worn bore, key,
or shaft, and this improper fit could
result in catastrophic failure. Light interference
fits or the use of a tapered
bushing system are recommended for
most normally loaded applications.
Wear on one side of the sprocket plate
usually indicates misalignment. Wear on the working faces of the sprocket
teeth may indicate a problem of inadequate
lubrication. Watch for scratches,
galls, grooves, or visible changes
in the tooth form such as hooked or
shark-fin shapes. Replace all worn and
broken parts. Running a new chain on
old sprockets will result in early failure
of the chain. Worn sprockets damage
associated equipment and will create
vibration in the system.
The use of indicator-type sprockets in
recent years is a major breakthrough in
determining the point when a sprocket
should be replaced. (See Figure #2.)
By using a sprocket with strategically
placed wear pins on the thrust faces
of the tooth, the maintenance technician
has a visual warning of excessive
wear. It allows for the scheduling of
maintenance rather than run-to-failure.
(These sprockets are available in a
wide range of sizes and types.)
Without a wear indicator, it was anybody’s
guess as to what was acceptable.
When a chain would jump, the
usual course of action was to treat the
symptoms by shortening the number
of links, and not deal with the cause,
worn sprockets. This may keep the
chain from coming off but increases
the load on the bearings and doesn’t
eliminate vibration.
Lubrication
Roller chain is a series of interconnected
plain bearings that requires lubrication
to resist wear, cushion impact, dissipate
heat, flush away contaminants,
hinder corrosion, and provide a film
between chain & sprocket contact surfaces.
There are special lubricants formulated
for use with chain drives that
are designed for sliding friction and don’t leave a varnish or gummy deposits.
Heavy oils and greases that are too
stiff will not flow to the needed areas.
The important point is for the lubricant
to be clean and applied properly,
based on the application. There are
four basic methods to apply oil to the
chain drive. Manual/ spray can, drip or
brush, static oil bath, continuous circulating
oil system that is filtered. The
application, speed, and horse-power
determine the best method to use.
For operations wherein conveyed
materials must be free from contact
with oil, lube-free chains are an option.
This type of chain does not require
additional lubrication, as it uses oilimpregnated
sintered bushings. The
use of this chain decreases maintenance
costs and reduces product contamination,
which is ideal for food and
beverage applications and operations
where lubrication is not possible.
Chain Tension
Proper tension for drive chain is extremely
important. When chain is
too tight, the additional load results
in excessive wear on the chain joints
and sprockets. Extreme tension also
imposes additional loads on the bearings
and shaft. When chain is too slack,
vibration, noise, wear, shock loading
occurs on the system. If the chain is
excessively loose, it may jump off the
sprockets. A good rule of thumb is a ?”
of slack or mid-span movement for every
10" of drive center distance. If the
drive orientation is vertical, then the
allowable amount is one-half of that
recommendation.

Conclusion
The sum of parts is equal to the whole
when it comes to a roller chain drive
system. Worn parts that are not replaced
correctly in a timely manner
will cause a ‘chain-reaction’ resulting
in ancillary damage to connected
components, increased labor, higher
maintenance costs, more unscheduled
downtime, and potentially an inferior
manufactured product. Check your
chain & sprocket drives often.

2024-07-11