In a world where the impact of heavy equipment at a job site can be subject to environmental scrutiny and regulation, the choice of equipment to mitigate that impact becomes critical. Pick the wrong machine and you might create liability for yourself.

By Lynn Merrill

What Shoes to Wear
Doing the Maintenance Shuffle
Treading in the Real World
The Forces of Machine Versus Nature
Tracking Softly

You just got a call for an excavation job that sounds like a piece of cake. Dig a trench for a replacement pipeline. No problem, you think as you hang up the phone. Sounds like a simple job for the ol’ backhoe. There’s just one problem you discover when you reach the job site: The line goes through a community park, and the superintendent has made it clear that tearing up his turf with wheel ruts is not acceptable. Suddenly you’ve got to literally tread softly if you’re going to get the job and leave the customer happy.

In a world where the impact of heavy equipment at a job site can be subject to environmental scrutiny and regulation, the choice of equipment to mitigate that impact becomes critical. Whether that equipment uses tracks or wheels to move can make a difference in soil compaction, equipment stability, and mobility—all of which translate to the bottom line. Pick the wrong machine and you might create liability for yourself.

What Shoes to Wear

Deciding whether wheeled or tracked equipment is appropriate for the job depends on the specific needs of the project and the physical conditions present at the job site. "It’s a pretty simple thing," states Kirk Kessler, marketing manager for material handling at John Deere in Moline, IL. "If you’ve got a situation in which you’re either working on grades that are too steep for a rubber-tired machine to negotiate or your ground conditions are so poor that you don’t have the tractive effort and get stuck too easy with the rubber-tired machine, you use tracks. If you’re concerned about ground compaction—you don’t want to compact the ground too much—then switch from tires to tracks."

Wheeled machines are most appropriate for environments where damage to pavement or maneuverability may be factors, says Thomas Kirchner, product development and support manager for mini excavators and mini skid-steer loaders with the Gehl Company in Lebanon, PA. "If you’re doing city work where preserving the roads is important and where four-wheel drive is good enough traction, that would be an application for wheels." Wheeled machines need a large turning radius to negotiate, however, which can cause problems in narrow urban environments. The speed at which the equipment moves over the job site can be an additional consideration. "Wheeled machines are faster," observes Kirchner. "Our 6.5-ton wheeled machine has two speeds, and in the high speed it has a 12.5-mile-per-hour top speed. The same weight-class machine with tracks only has a 3-mile-an-hour speed in high gear. If you need a machine that can get from one spot to another quickly and you’re in a situation where four-wheel drive is acceptable, then I would go with a wheeled machine."

Wheeled machines are more predominant in Europe and Japan than they are in the United States, says George Lumpkins, product development manager for Kobelco American Corporate in Stafford, TX. "You’ve got a much older infrastructure in Japan and Europe than you do in the US. In Europe, particularly around the German area, somewhere around 65 to 70 percent of all machines sold are on wheels. For that reason, most of the wheeled vehicles come from Europe. In the United States, it’s still a very small percentage. Most of our job sites are much larger, and most of the excavators are working more off-road than they are in Europe. The other thing more prevalent in Europe and Japan than the US is that wheeled machines are licensable for highways, and in most states here you can’t do that."

Wheeled equipment only requires selection of the appropriate tire for the application. A tire’s tread design can be used to increase or decrease tractive effort. "You want to know what your ground conditions are," Kessler points out. "If you’re in mud a lot or you’re in regular dirt conditions, you want a traction tire that has smaller, more aggressive lugs. If you’re working in hard surfaces or rock surfaces, then you want a rock tire because it offers longer life, more wear, and protection against cuts and abrasive-type situations." In tracked equipment, there are actually three different tread configurations available. These include steel tracks, rubber tracks or a hybrid in which pads of either rubber or polyurethane are fitted onto each plate of the track. The type of application determines the selection of the appropriate track (see the "Tracking Softly" sidebar).

Doing the Maintenance Shuffle

Tracked machines tend to be more expensive than their wheeled counterparts, reports Kessler. "I can give you a good rule of thumb that we’ve used forever when it comes to tracks. When you’re thinking about a machine with tracks, it cost about 20 percent of the overall initial investment of the machine. When you’re purchasing a machine with tracks, about 20 percent of your dollars are tracks, so it’s a bigger chunk of the overall cost of the machine. You’ve got a bigger investment to begin with. Then the tracks are going to represent approximately 50 percent of your maintenance cost through the lift of the machine, so you’ve got a lot of maintenance involved there."

Tracked equipment requires additional facilities and training in order to properly maintain them—things not likely to be sitting in the average contractor’s garage. "When you’re working with a track machine, you do have to have quite a bit of specialized equipment," notes Kessler. "One of the big ticket items is the chain that you normally have to rebuild and/or replace, and it takes a track press to get things like that done." Other specialized equipment includes welding and milling machines that allow you to rebuild the track assembly, including the track, sprockets, idlers, and rollers. Because of this investment, a contractor who is contemplating adding tracked equipment to his operation should take into consideration the availability of maintenance facilities within their operating area. "Unless he’s a big operation with a lot of tracked equipment, it’s easily outsourced. You go to an independent in-house repairer of tracks or go to the dealer where you bought the piece of equipment from if they have the facility to do the track work," Kessler says.

Wheeled equipment has its own set of costs and maintenance concerns in the form of tire usage and engine wear. "A wheel loader does have a higher maintenance cost because of tire usage and/or damage," states Art Hine, product manager for Komatsu America International Company in Vernon Hills, IL. "The wheel loader’s operating life may be less than an excavator. For example, the engine rpm varies constantly, from a low idle to a high rpm in a matter of seconds, because of the duty cycle of the wheel loader. On the excavator, the engine is used more like a generator system whereby the engine stays constant so you don’t get the variance on the engine rpm on a crawler excavator. Therefore, the life expectancy of the engine in an excavator will be longer than the wheel loader, depending on the application."

Maximizing tire life on a wheeled loader is similar to the steps a contractor should go through for over-the-road equipment. "The key thing is proper inflation of the tires," Kessler emphasizes. "They need to watch them to see whether there are any cuts or deterioration, checking on the amount of wear and how much tread they have left. A good, accurate tire gauge used daily is how you pretty much maintain a tire. Other considerations are to keep the area you’re running in flat and clean, try not to spin your tires too much when you’re running, and operate at the correct speeds because different tires get heavier ratings. Keep the speed down, otherwise the tires tend to get hot. A lot of that is common sense."

Treading in the Real World

Operating each type of machine is as simple as pulling a lever or turning a wheel. "The drive mechanisms are different," Kirchner points out. "On tracked machine you have drive levers: the left lever will control the left track and the right lever will control the right track. Steering a wheeled machine is done with a steering wheel. They have different hydraulic plumbing to accommodate both types of steering." Control mechanisms are moving toward standardization, according to Hine. "A few manufacturers in North America have adopted a joystick control standard [from] the Power Crane and Shoveler Association. If you jump from the major manufacturers’ [equipment], the joysticks are the same."

While a wheeled tractor is designed to go almost anywhere, tracked equipment is primarily designed to go forward. "Excess operation in reverse would tend to increase the amount of wear that you’d get per hour," Kessler notes. He also suggests balancing the work the machine does so that wear is evenly distributed on the machine. "Try to work both sides of the machine so that you’re turning the same amount one direction or the other. Don’t favor one side when you’re running through the cycle, otherwise you’ll have a lot more wear on one track."

There is no one-size-fits-all solution when it comes to using tracks versus using wheels. Rather, each type of equipment is designed to accomplish a particular type of work at a job site. While both a hammer and a sledge are designed to strike objects, you wouldn’t use a sledge to build a house or a claw hammer to break up rocks. "I certainly would not say that the excavator is better than the wheel loader or wheel loader is better than an excavator," says Hine. "They complement each other. I believe that the end user is right. If he or she chooses a wheel loader or a crawler excavator, whatever that person chooses is correct. We can certainly suggest the best product for their job site, but only they know what they would like to do with it beyond this particular job they’re working on."

The measures of an excavator’s ability to perform work boil down to three numbers in a specification: bucket force, stick force, and horsepower. Each number provides insight into the machine’s ability to pull material out of a trench and move it away. According to George Lumpkins, product development manager for Kobelco American Corporate in Stafford, TX, bucket force measures the power of the bucket to scoop material out of a trench, while stick force measures the power of the excavator’s arm. The combination of the two is used to calculate the machine’s breakout force. "There are regulations on how you calculate breakout force," he says. "Stick force is always less than bucket force. For example, if you’re handling trench boxes, more stick force is desirable because you’re dragging it toward you and you have more power to pull that trench box. If you were going to be digging rock or trying to carry a larger bucket, bucket force is very important. All manufacturers have more bucket force than arm force so that you can then curl it around and pop the rock up. There’s only so much you can do going straight into a rock. You reach a point where you’ve got to break it upward. If you’re in the areas where you run into rock or you want to carry large buckets, you’ve got to have lots of breakout power." Horsepower is the other measure of a machine’s productivity. "Machines within one class have a lot of different horsepower ratings," notes Lumpkins. "In a nutshell, horsepower does not give you more breakout force. If you are able to put all the horsepower into the hydraulic system, what it gives you is speed. Speed is not defined as how fast it’ll swing around in the air or curl a bucket in or out in the air. Speed as I’m defining it is how fast it will come through the material, how fast it’ll lift the boom out of the ground, how fast can it get over and dump into the dump truck and get back. As a general rule, the more horsepower, the faster the machine will be under load. And of course, the faster the machine is on your job site, the less money you’re going to spend on the job. A lot of people say if you have more horsepower, you have more digging force. That is not true. Horsepower only gives you speed in productivity. It does not change digging force. It does not change swing torque. Horsepower is defined as force [multiplied by] movement."

GX
March / April, 2000