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.