Managing the undercarriage on a mid-size crawler tractor is a complicated task. There are a daunting array of combinations possible, between the different shoe types and widths combined with different chains and guarding options. There are also issues of monitoring track tension and alignment and measuring the undercarriage to determine proper maintenance intervals.
Because the amount of money you can charge to move a yard of dirt is roughly equivalent to what it was in the early 1980s, undercarriage management is a task that cannot be ignored. “Depending upon the environment, the undercarriage can be the most costly system to maintain on the crawler tractor,” says Dick Schaeffer, Komatsu America. “Depending upon what it is doing, about 40% to 60% of the maintenance costs of the machine are going to center around the undercarriage, and 40% to 50% of the initial purchase cost of the machine will be replaced in the undercarriage. So if you have a $100,000 machine, during its life cycle it is going to consume $50,000 worth of undercarriage.”
But you can control undercarriage cost through many variables. “While wear is inevitable, it is controllable and it is our job to advise the end user how he can extend his undercarriage life,” says Schaeffer.
Improper tension kills
Keeping your tracks properly adjusted is a good place to start. “Track tension is the single most important and yet the most controllable maintenance item on the crawler tractor,” says Schaeffer. “It is reported that for every extra inch of track tension, you add 30,000 psi of tension to the bushing/sprocket relationship.”
“You can sacrifice two-thirds of your undercarriage life just by having the tracks too tight,” adds Tom Neeley, undercarriage commercial manager, Caterpillar.
But track tension constantly changes with the soil conditions. Packing can quickly turn a properly tensioned track into one that is severely tight.
“Track tension should be monitored in actual working conditions,” says David Koester at Berco of America. “Track tension increases if the sprocket and chain become packed with mud, snow or other materials. The track tension decreases as the packing becomes less.” Normal wearing of the track chain may also create pitch extension, which loosens the track. “The track should be adjusted accordingly. The reality is that very few people are good at that.”
Matching the conditions is critical when making the adjustments. “Don’t put the tractor on a clean slab, wash it off and adjust it,” advises Neeley.
“Get out there in the mud and gunk and run it a little bit, then adjust it. You reach a point where you can have proper track adjustment even in a packing environment.”
The weather may dictate that you make several track adjustments throughout the day. “Sometimes people will say it rained this morning,” says Neeley. “It is just really a pain to go and adjust it. Well, we like to say it will adjust itself. You will wear away iron in a rapid fashion. Adjust it if you really want to control your undercarriage cost.”
Schaeffer was previously an engineer for a large contractor and has witnessed the benefits of frequent track adjustments. “I have been on projects where there was a lube truck that just drove around and inspected machines because the operating conditions were that bad,” he explains. “They would pump them out and start running them a little bit loose so they would have room to pack. Then, as they operated throughout the day, they would stop and make sure the track tension was proper.”
If you have to choose between tight and loose, opt for a little bit loose, Schaeffer advises. You not only reduce chain life when it is too tight, but it puts a strain on other components, like the front idler and carrier rollers.
But too loose also causes its own set of problems. “We really emphasize the tight track, but sometimes a loose track is a real issue as well from the standpoint of slapping,” says Neeley. “On an elevated machine without a carrier roller, you might have the track sitting down on the frame, which is not good.”
Shoes play crucial role
There are many factors that influence shoe selection. For example, wide shoes are intended for flotation, but they also provide additional track on the ground for improved blade control in some applications.
“Any choice of track shoe features is a compromise,” explains Koester. “Selecting the correct track shoe for a specific application has a significant effect on machine productivity. It is common that the selection of track shoes is based only on machine productivity, while the effects on potential undercarriage life are ignored. Productivity may be increased, but the life of the undercarriage may be reduced.”
Using wide shoes serves as a perfect example. “Our research shows that for every 2 in. the shoe width is increased, the stress on the chain is increased 20% to 25%,” says Neeley. This varies by chain size. “But if shoe width is increased by 6 in., it puts about 60% more bending stresses in that chain. A tractor that is working in high-impact [conditions] with wide shoes may sacrifice two-thirds of its undercarriage life.”
“The general rule of thumb when you are looking at track shoes is you want to use the narrowest pad that maintains adequate flotation,” says Schaeffer. “The table of the shoe should be fully resting on the ground. The grouser should be fully penetrated into the ground without material coming up over the top of the shoe. You want the full weight of the crawler tractor resting on the table of the shoe, not on the tips of the grouser.”
What dictates your choice is the environment that the tractor is most likely going to be operated in.
The environment also effects shoe style. “Include shoes with open centers when running in materials that are extrudable, such as mud or sand,” says Koester. “Use extreme service shoes in areas where abrasive soils are the norm and where high-impact conditions may cause structural damage to the shoes.”
Determining when to change shoes is another issue that deserves close attention. You need to look at more than grouser condition when evaluating shoe life. “Customers will sometimes judge shoe life strictly on the remaining life on the grouser,” says Neeley. But if you’re working in a highly abrasive environment, it’s possible the grouser still has life remaining, while the leading and trailing edge of the shoe exhibit significant wear.
This allows sand and abrasives to enter the system and decrease undercarriage life. “So you should not only look at the grouser wear, but you should also consider the amount of wear on the plate of the shoe, particularly the leading and the trailing edge,” he adds.
To guard or not to guard
Full or partial roller guards serve a couple of purposes — as guides and to keep non-extrudable materials out. The downside is they can trap extrudable materials in the undercarriage. “The disadvantage of using roller guards in packing materials, such as mud or sand, is that they usually will not keep this material out but in fact, tend to keep the material in,” says Koester. This, in turn, accelerates wear.
Therefore, roller guards should only be used when absolutely necessary. “Track guards should be used only if there is a need to keep rocks, tree limbs or other non-extrudable material from entering the roller areas,” advises Koester.
In some instances, partial roller guards can be useful. “End guiding guards may be used with partial length guides to help guide track chains as they enter and leave the sprockets and idlers,” says Koester. “Center guiding guards may be used in the middle of the track to help maintain track alignment while allowing for packing material to fall free.”
It really depends on the application. “Some guiding guards, if you are on a side slope, can be beneficial,” adds Neeley.
Replacement and inspection tips
When it comes time for track replacement, a few simple tips can translate to longer life. This is not the time to get cheap — the sprocket and chains should always be replaced together. “People will skip that, but they are asking for fast wear,” says Koester. “You have a mismatch of pitch at that point.”
“One of the rules is never put an old surface against new. Always change the sprockets when you change the face of the bushing, whether that is through turning the bushing or putting a new bushing into an existing chain,” says Schaeffer.
Also check the alignment. “The alignment is often overlooked when changing the undercarriage,” says Schaeffer. “If you look under a crawler tractor and see something shiny that shouldn’t be shiny, it’s an indication of alignment.” Left unchecked, this can cause problems elsewhere.
Finally, turn to your dealer to help select the most appropriate undercarriage components. Caterpillar lists over 300 part numbers just for shoes, according to Neeley. It’s important that you consult an expert to pick an undercarriage that offers the best balance between cost and productivity.
Both Caterpillar and Komatsu also offer computer software that helps you maximize the return on your undercarriage investment. Seeking this type of expertise can mean the difference between profit and loss in today’s competitive earthmoving industry.