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On track-type machines,
the undercarriage is the single most expensive item, soaking up
an approximate 50% of the total costs attributed to crawler maintenance.
Yet it’s been said that some contractors treat the undercarriage
like an old shoe—they don’t think about it until they’ve
worn a hole in the bottom.
Consider that when the
job is all done, a win or a loss is often decided by your operating
costs per hour. So with today’s tight profit margins, it may
be time to overhaul your maintenance strategy rather than your undercarriage.
Ongoing improvements to an undercarriage maintenance program are
clearly a payoff to the bottom line—whether you own one machine
or a fleet of crawlers.
Undercarriage Overview
Undercarriage wear cannot be eliminated but it can be controlled
and cost-effectively managed. Yes, you may take advantage of the
latest innovations in undercarriage design and metallurgy, and you
may always use quality replacement parts. Certainly these initiatives
play a large part in minimizing downtime. But they do not replace
a solid understanding of the causes of wear (some that you can influence,
and some that you cannot), and a commitment to the operational and
maintenance practices that increase the working life of the undercarriage.
In a nutshell, the undercarriage
comprises the track assembly and its frame. Track components include
shoes, pads, bushings, links, and pin and track seals; while the
track frame assembly includes the frame itself, track rollers, carrier
rollers, idlers, sprockets, recoil springs, track guides, track
guards, and a hydraulic track adjuster. All parts of the undercarriage
are designed and manufactured to work and wear as a system. So as
soon as one of the components starts to wear, it will eventually
have some effect on the rest of the system.
Wear rates differ greatly
depending upon the job profile and one’s approach to it. Working
conditions—such as the lay of the land, material abrasiveness,
and moisture content—cannot be changed. But owners and operators
can minimize their affects on wear by applying best practices in
track configuration and operational methods, and by staying on top
of maintenance and repair tasks.
Start with Fuel Consumption
Data
A good maintenance strategy begins with recording the fuel
consumption of each machine. Commonly, contractors log fuel consumed
on a total job-by-job basis rather than noting the actual cost of
fuel per individual machine—regardless of the fact that consumption
varies with different applications and operating conditions. “Fuel
consumption will tell you how hard a machine is working for a living.
The hour meter doesn’t mean as much,” says Richard Schaefer,
senior product manager for undercarriage at Komatsu. “Machines
must be looked at individually. You may have put 1,000 hours on
each of two machines, but one machine may be pushing wood chips
while the other may be tackling shot rock—and consuming more
fuel. Maintenance needs to be adjusted to the particular machine,”
he says, adding that contractors need the “right” preventive
maintenance program. “Different oil change intervals may be
needed per machine. One size does not fit all,” says Schaefer.
Simple Strategies
Count
Undercarriage experts stress two integral, yet simple strategies
in ensuring maximized undercarriage wear life— maintaining
proper track tension (or sag) per application and cleaning the undercarriage
frequently.
Correct track adjustment,
which significantly affects external bushing life, is the most important
controllable factor in undercarriage wear. Tight tracks can increase
wear up to 50% as they magnify the load and put more wear on the
link and sprocket tooth contact. Increased wear also occurs at the
track-link to idler contact point and track-link to roller contact
points. Also, a tight track requires more horsepower and more fuel
to do the job. Track tension should be checked at least once a shift—more
often if the materials change, or become wet and begin packing.
Track adjustment procedures vary between elevated sprocket and conventional
sprocket designs, so check your owner’s manual.
“We promote the
concept of measuring and monitoring your track sag, rather than
track tension. We want contractors to envision a normal amount of
sag, getting them to know that sag is normal and is key to getting
your best undercarriage life,” says Brett Errthum, John Deere’s
product manager for crawlers.
Track manufacturers suggest
that the correct sag for all mini-excavator rubber track units is
1 inch (plus or minus .25 inch). Again, check your owner’s
manual for recommended track sag. As an example, on rubber tracked
crawlers in the 80-horsepower range, .25 inch of track sag results
in 5,600 pounds of track chain tension; while the same machine at
suggested track sag results in just 800 pounds of track chain tension
when measured at the track adjuster. To adjust track tension, a
track link must be centered over the carrier roller. Put a straight
edge over the track from the carrier roller to the idler wheel,
and measure the sag at the lowest point.
Next, frequent undercarriage
cleaning is imperative to eliminate packed soil and debris around
components, a condition that causes increased loads on undercarriage
components and higher wear rates. For example, packing prevents
the proper engagement between mating components such as sprocket
teeth and track links.
“If you choose a
crawler with a track frame that’s easier to clean, you’ll
clean it more often. There should be enough open space between track
frames and the main frames to be able to shove a spade all the way
through, pushing the compacted material out easily,” says Errthum.
Importance of Correct
Shoe Width
Using the appropriate shoe width per application extends undercarriage
life. As a general rule of thumb, the shoe should be as narrow as
possible while still giving the proper flotation and traction. The
softer the soil condition, the wider the shoe should be. On the
other hand, wide shoes on hard surfaces will put an increased load
on the track link system and can affect link retention in the rubber
track.
“Shoe width can
affect such widely ranging items as track seal and lubricant integrity
to link cracking, roller flange wear, and bushing wear rate,”
says Jose Westin, undercarriage marketing manager for Caterpillar.
“Wider shoes do not improve wear life. Cracking, bending, and
hardware loosening increases as the shoe width increases. Consequently,
wear rates also increase on link rail sides, rollers and idler flanges,
and pins and bushings. Shoes that are too wide in a high-impact
application or on steep terrain can cause pins and bushings to loosen
in the link bore. This becomes more evident with high single grouser
shoes. Loss of pin and bushing retention prevents successful turn
and replacement maintenance,” says Westin, who submitted the
following chart to illustrate how the percentage of bending stress
rises with increased shoe width.
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Key Component Considerations
In addition to choosing the correct shoe, there are other key
component decisions to make if you wish to extend undercarriage
life. Pay close attention to lubrication systems, sprocket types,
rock guards, and derailing guides.
“From those with
one unit to those with a fleet, everyone should consider an automatic
lubrication system. It removes the human element,” says Reece
Norwood, product manager for Kobelco.
Undercarriage experts
report that, for the most part, contractors are switching to sealed
and lubricated pins and bushings, so there is very little wear within
the pin-and-bushing joint when maintained properly. Engineers report
that the critical indicator of when to turn pins and bushings is
the ratio of external bushing wear to chain link wear.
As to sprocket type,
two designs are available—either a one-piece design or a segmented
sprocket. Carefully monitor the sprocket’s wear as it is an
indicator of the internal condition of the pin and bushing assembly.
In most cases, sprocket replacement is necessary whenever the mating
surface of the bushing is changed. Unlike the one-piece sprocket,
a segmented sprocket can be replaced without breaking the track,
a big savings in both time and money.
Rock guards will protect
the track from material getting between the undercarriage components.
But guards will also retain material between the undercarriage components
once it does get in. So use guards in drier, rocky conditions, but
not in operating conditions where severe material packing exists.
“Make sure your
unit comes factory standard with derailing guides, which are metal
shoes that are on the side of your tracks. Derailing guides ensure
that the tracks stay under the machine. A derailing situation can
really chew up your bottom rollers,” says Walter Reeves, excavator
product manager for Volvo.
A Team Approach
Most manufacturers of track-type equipment agree that a team
approach is the best way to deliver extended undercarriage wear
life. In other words, the manufacturer, dealer, and end user all
play a key role in getting the most performance at the lowest cost
per hour.
“It’s important
that the manufacturer and its distributor provide the best possible
machine, replacement parts, and service. And, it’s equally
important that the end user should follow the manufacturer’s
recommendations and perform the maintenance,” says Norwood.
“Contractors, especially those with smaller fleets, may find
that it’s less expensive to go with a distributor’s maintenance
agreement. They share the overhead costs with other contractors
when they take advantage of distributor service and maintenance
contracts,” he says.
Based upon your machine’s
application and the amount and type of wear, a qualified distributor
can develop an undercarriage management program that includes regular
undercarriage inspections and timely replacement and repair. Most
programs use some sort of ultrasonic measurement tool to make undercarriage
inspections quick, easy, and accurate, particularly in hard-to-measure
locations. Sound waves are used to precisely measure the thickness
of undercarriage components. Based on this data, and other critical
factors, the service technician can make the appropriate recommendations
for your machines, applications, and operating conditions.
With a regular preventive
maintenance inspection program, small problems are found before
becoming major repairs. Then repairs can be scheduled at times convenient
to your workload, thus increasing the machine’s availability.
And, a well-maintained machine operates better and longer between
major repairs.
“Some contractors
really do treat their undercarriage like that old pair of shoes,”
says Reeves, product manager for Volvo. “They don’t react
until their feet are getting wet,” he says.
Construction-industry
writer Carol Wasson owns JCL Marketing & Communications Inc.
GEC
- May/June 2005
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