Bulldozers are tough machines, and owners and operators can extend their machines’ working lives by making critical decisions about their undercarriage. The decisions begin before a new machine is ordered from the dealership.
“One type of undercarriage does not fit all applications,” warned Tim Nenne, a Caterpillar undercarriage market professional. Nenne began working with drivetrain and undercarriage issues at the Caterpillar Tech Center almost 25 years ago. “Now, we’re offering more undercarriage than ever before.”
Same, But Different
Why offer undercarriage variations? It’s true that in the rumbling world of tracked equipment, the machines have some things in common. The continuous steel or rubber tracks that move dozers, excavators, mobile cranes, track loaders of all sizes, and other machinery all have sprockets that drive linked plates with raised grousers for traction. The chained shoes cycle across idlers and rollers and under road wheels … around and around and around.
But there are differences in the machines, too. How a piece of equipment is used varies widely and undercarriage reflects that. Consider dozers and excavators, two of the mainstay pieces of equipment in any earthmoving contractor’s yard: They have greatly different undercarriage requirements. A dozer and excavator can be operated side by side in a pit or a site preparation project and their undercarriage experiences much different stresses.
Dozers have to be moving across a job site to be productive. Sitting still, a dozer is just a lump of iron. Until its blade starts pushing or its ripper ripping, it might as well be a big boulder. However, an excavator can rumble up to a dig site, the tracks on the machine stop moving and subsequently not experience a full revolution for an extended time. Yet while the tracks are mostly still, the machine is busy digging a hole with its scoop, or swinging over and dropping a concrete box into a hole using its boom, or loading trucks from a replenishing pile of material.
“An excavator undercarriage is mostly just a platform,” Nenne explained, so the life rollers and link assemblies on excavators can differ greatly from those on dozers. On the other hand, undercarriage on track loaders more or less replicates the technology on, say, a D6, because the two machines’ function is more-or-less the same.
How Is It Used?
So the first critical undercarriage decision for a contractor buying a piece of tracked equipment is to know how it will be used.
“The big thing is this: Choose the undercarriage that best meets all of your needs,” Nenne said. “Consider the machine type, the machine’s application, the material it will be working in, even your financial situation. Choose the undercarriage that will provide the best performance.
“It always comes back to choosing the right undercarriage,” he said. “You save major dollars in the end if you choose correctly. It eventually will come back and cost you more if you don’t.”
Nenne added that the most common error a contractor makes is not fully specifying the weight of the machine to be carried on an undercarriage. It might be a 25-ton dozer as a base model, but one blade or another can alter the weight, as can the addition of a ripper or a winch. The total machine weight is the operative number in choosing the right undercarriage set-up.
The next most important consideration for selecting an undercarriage is the conditions in which it will be operated.
“It is so important to understand the application of a machine,” Nenne said. “What type of material will it be running in? What level of abrasion will there be? What are your hours of use for the machine? How long do you plan to keep it? All of that determines which undercarriage to choose.”
That’s why Caterpillar developed a new General Duty line of undercarriage, which Nenne described as the “Volkswagen” line, an allusion to the economical but well-engineered VW “Bug” the German manufacturer introduced to the United States in the 1960s. General Duty is the lowest-cost undercarriage option and will meet the needs of contractors in general purpose, moderate-use dozer situations.
Cat Heavy Duty Undercarriage targets most Cat customers and is for machines that are operated many hours a day, every day — the high-production machines found on many work sites. Cat SystemOne Undercarriage is the third option and also is aimed at high-use machines. However, SystemOne incorporates rotating bushings and extended-life sprockets, idlers and rollers. The system reduces maintenance costs and lengthens the operating life of various components.
So from day one a buyer makes numerous set-up decisions that will affect the bottom line. Example: A dozer to be operated in forestry work, crawling over stumps and such, needs a heftier undercarriage than one loading some gravel now and then in an equipment yard. Another example: The type of soil underneath the operating machine and the degree of slope at a work site can determine whether to use standard track, wide track, or low ground compaction shoes. And so on.
In the end, there are consequences from having the wrong undercarriage on a machine. Nenne said the consequences can become evident in pretty short order.
“If it’s a matter of a shoe width error, you might notice that in the first hour when there’s a buildup of packed material in the undercarriage.”
Other misjudgments may not show up for a month or more as parts begin to break or become dysfunctional under stress.
Operator, Maintenance Errors
Yet even when a tracked machine has an appropriate undercarriage, problems still can develop if an operator fails to follow best-practices in the driver’s seat. Nenne said the most frequent failing of this type is habitual use of a big tracked machine for nonessential travel.
“Don’t use a tractor as a pickup truck,” he said. “They are very expensive to move. For example, don’t drive the machine across a job site to get it fueled. Drive the fuel truck to it. It is not the number of hours that wears out an undercarriage, it is the number of miles or kilometers. Limit the miles, limit the amount of movement.”
The kind of movement also can exact a toll on an undercarriage. Nenne recommended that operators limit high-speed reverse movement, or limit traveling in reverse in general. While SystemOne handles high-speed reverse better than other systems, additional bushing wear always occurs when a machine is traveling backward on its tracks. Another tip: Turning a dozer in the same direction all the time causes uneven wear on undercarriage components, so operators should try to alternate their turns.
“An operator has the ability to make or break an undercarriage,” Nenne said. He said most operator abuse or misuse of a tracked machine occurs on rubber-tracked mini-loaders, which are apt to be run by less-trained operators. But owners of huge dozers also can incur extra operation and maintenance costs when jockeys operate the machines inefficiently.
Field service personnel or shop mechanics have a checklist of inspections for an undercarriage, and Nenne believes track tension is checkpoint number one. If the links are too tight, the system wears more quickly and more fuel is expended. Proper tension depends on the application and working conditions.
“Checking the track tension is the single biggest thing you can do,” Nenne said. “You can’t control an application of a machine or operating conditions, but you can control track tension.”
He said operators have the first responsibility to keep the track properly tensed. If working in dust in the morning turns to working in sticky conditions after a rain shower, an adjustment in the tension may be required to optimize performance.
“When conditions change, you have to think about doing an adjustment. It’s probably a half-hour job, but the cost of not doing it over the period of operation can be quite high.”
Components of the undercarriage wear differently under different conditions. In a high-impact environment of hard rock, grousers will wear out quicker. If lots of sand — particularly wet sand — is running through the undercarriage, bushing wear is accelerated. In working in especially packable soils or refuse, material can get wedged into undercarriage areas and cause components to engage improperly and increase wear.
Nenne said the simple act of cleaning out debris from the undercarriage at the end of a work day will help keep down operating costs. He adds that engineering attempts to seal off critical undercarriage components from such debris haven’t worked. “A lot of people have tried to keep material away from parts, but if you do too much of that, it keeps debris in instead of keeping it out. You can overprotect.”
The rule of thumb is that an undercarriage will absorb half of the cost of operating a tracked machine. Nenne was asked if perhaps that ratio of expense didn’t shift somewhat after the Tier IV regime increased the cost of engines and emission controls.
“I would say that it hasn’t changed,” he said. “Tier IV is an upfront cost for a buyer, but when you look at operating costs, the undercarriage is still going to come in at 50 percent.”
When a contractor opts to purchase a used dozer or other tracked machine, the condition of its undercarriage becomes a critical issue. Nenne said it is possible to correctly determine the working condition of an undercarriage by inspecting for dry joints and other indicators of wearing and aging. Knowing the conditions in which the machine operated — sand or rock versus loamy soil — also can signal how much life the undercarriage still has in it.
Nenne has no predictions about a future mechanical system that will replace the track system, which dates back more than a century in its modern form. But the Cat undercarriage specialist said the manufacturer is producing undercarriages for some foreign markets that have rubber-coated and sound-deadening components, which suggests the familiar clanking of a passing dozer might become a thing of the past.
“That undercarriage is more expensive and harder to produce, but sound reduction in some parts of the world is important to customers so it has become a big part of our thinking at this time.”
For contractors focused on keeping down costs, holding down noise might be a lesser concern.
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