Equipment Type

The Long and Short of Excavator Design

Our in-the-dirt comparison of Doosan’s reduced-tail-swing and conventional-tail-swing models had a few surprises

June 01, 2011

Everyone agrees that reduced-tail-swing excavators are great for working in confined spaces, such as taking up just one lane of traffic (not two) on road jobs, or in situations that place the machine against a wall, fence or other obstruction. But some excavator buyers question if reduced-tail-swing models are as capable as their conventional-tail-swing counterparts. “Okay, I can work next to a wall with a reduced-tail-swing machine, but won’t I have to compromise overall digging and lifting capability to gain that benefit? Does it make sense to pay more for a less-capable machine just to work in tight spaces?”

Good questions. Construction Equipment editors had the same thoughts, so we welcomed the opportunity to work with Doosan to compare the relative performance characteristics of the two excavator designs. The comparison involved running the reduced-tail-swing DX235LCR side-by-side with the conventional-tail-swing DX225LC. Doosan considers the recently introduced DX235LCR to be the DX225LC’s reduced-tail-swing counterpart.

You might be asking if a legitimate “apples-to-apples” comparison can really be made between two such models, given that their designs differ so significantly. The DX235LCR, after all, has just 3 inches of overhang, compared with 4 feet 1 inch for the DX225LC when both are using 31.5-inch shoes. Plus, the DX235LCR is heavier by nearly 5,000 pounds.

But, that said, when both are fitted with their standard boom (18 feet 8 inches), standard digging arm (9 feet 6 inches), and identical buckets (as were the test machines), digging envelopes are similar, with maximum digging depths within 2 inches of each other.

Both also use the same 5.9-liter Doosan DL06 six-cylinder diesel, although net horsepower is higher for the DX235LCR (166 versus 148). Both also use the same components in their implement-hydraulic system, but the DX235LCR does produce slightly more hydraulic flow (116 versus 110 gpm) and slightly more main-relief pressure in (5,263 versus 4,978 psi).

It was a judgment call, but we think we came as close as possible to an apples-to-apples comparison from a specification standpoint. Chad Ellis, Doosan’s product manager, made the point that reduced-tail-swing machines in most excavator manufacturers’ lines tend to be more “power dense” than their conventional counterparts—that is, to some degree, these models pack more horsepower and more hydraulic capability into a much smaller upper-structure package that has significantly more mass.

On site

The on-site comparison included typical excavator applications—trenching, truck loading and lifting—and we also made a swing-speed comparison to determine if the dynamics of the two designs might affect that aspect of operation. In addition, fuel-consumption data collected during trenching allowed calculating the fuel efficiency of both machines.

Overall results were interesting. We were left asking whether the comparative numbers collected during testing might apply—in a general way, at least—to the product lines of other excavator manufacturers. We’re of the opinion they would.

We convened during the second week of April at the Doosan Real Operation Center (ROC) located just outside Tucson, Ariz. The “we” included Ellis, CE editors, and others from Doosan, including Shane Reardon, product trainer; Aaron Kleingartner, segment application marketing manager; and Bruce Kim, product support specialist, who installed auxiliary fuel tanks on the two machines and checked that idle speeds and main-relief pressures were correct.

The two most important participants in the DX235LCR/DX225LC evaluation, however, were James Thomas and Byron “Corky” Turner, both expert operators and both with their own excavating/grading businesses in the Atlanta, Ga., area. Although each had many hours in an excavator seat, neither had operated a large reduced-tail-swing machine.

Weather was as close to perfect as it probably gets in Tucson, mid 70F, cloudless skies and low humidity. The material in which the machines worked was heavy, hard, desert soil: sand and gravel mixed with clay and patches of caliche, the later being hard deposits formed by the cementing action of calcium carbonate. Estimated weight was 2,800 pounds per cubic yard.

Test units

As noted, both machines used the same 5.9-liter Doosan engine, an inline-six-cylinder, Tier-3-compliant model using a Bosch common-rail fuel-injection system and an internal exhaust-gas-recirculation system, which employs a slight valve-opening overlap during the exhaust/intake cycle. Doosan also makes its own hydraulic components—pumps, valves and motors (both swing and drive)—and, said Ellis, Parker hoses and fittings are used throughout to facilitate replacement.

Both machines have three power modes—Economy, Standard and Power—and two working modes—“Digging” for general excavation, loading and lifting and “Trenching” for swing priority.

As noted, booms, digging arms and buckets were identical for the machines. Buckets were 1-cubic-yard Geith models (Geith is a Doosan company), 36 inches wide with teeth and side cutters.

The machines use identical undercarriages with the same track length (14 feet 7 inches), and both test units were fitted with 31.5-inch (800-mm) shoes. The undercarriage features heavy-duty links and rollers, as well as a one-piece recoil mechanism designed to maintain consistent track-chain tension. Both upper frames use D-section construction, giving them added strength, said Ellis, compared to C-channel-type construction.

Operating weight was 48,722 pounds for the DX225LC and 53,572 pounds for the DX235LCR; the DX235LCR’s added weight resides almost entirely in its heavier counterweight. Cab size and appointments are essentially the same for both models, which were both equipped with new ROPS-certified cabs.

Truck loading

During the truck-loading comparison, the first test event, the machines were set in their Power and Digging modes. Both operators used each machine to seven-pass load a tandem-axle dump truck multiple times. The machines dug from a loose-material pile, swung 180 degrees, and dumped over the end of the truck.

We timed the operators as they worked, then weighed the truck on a near-by truck scale after each seven-pass cycle to determine payload. Using this data, we calculated an hourly production rate for each machine. The DX225LC turned in a production advantage of around 6.5 percent.

The data allowed us to calculate an average per-bucket load (bucket-fill) and an average cycle speed for each machine. Comparing these figures, we determined that the DX225LC’s production advantage resulted from the combined effect of loading an average of 3.5 percent more material per bucket and by cycling some 3.0 percent faster.

After the truck-loading test, we solicited the operators’ impressions and comments about the relative performance of the machines. Both agreed that loading reach and height were no problem for either machine, that both machines were essentially equal in digging power, that visibility was equal, and that the DX235LCR might have a slight edge in its return-to-dig motion. Both were convinced, however, that the DX225LC felt more flat-footed than its reduced-tail-swing counterpart.

Trenching

During the trenching comparison, each machine worked for approximately an hour, digging a bucket-width wide to a nominal depth of 8 feet. Between them, the two excavators opened nearly 550 linear feet of trench. The hour’s working time for each machine was divided between the operators. The machines worked in their Power and Trenching modes.

We timed each operator on each machine and counted total cycles for each run. Before each run, we weighed the auxiliary fuel tank by lifting it slightly and inserting a small electronic scale beneath. At the conclusion of each run, we again weighed the tank to determine fuel used. This gave two fuel-consumption numbers for each machine, allowing us to calculate an hourly fuel-usage average.

We measured the trenches by taking depth and width measurements at 3-foot increments, then calculating an average depth and width to use with measured length to determine the approximate volume of material excavated.

The results were unexpected, compared with those from the truck-loading exercise. The DX235LCR out-produced the DX225LC by about 12 percent. We determined that the reduced-tail-swing machine’s bucket-fill was about 16 percent more on average than that for the DX225LC. But the DX235LCR, as in the truck-loading test, cycled a bit slower—by about 4 percent this time—thus accounting for the net production advantage of 12 percent.

Even more unexpected was the DX235LCR’s fuel efficiency (material moved per gallon of fuel). The reduced-tail-swing machine proved some 10 percent more fuel-efficient than the DX225LC.

Our sit-down with the operators after the trenching exercise seemed to uncover a possible explanation for the disparity between the trenching and truck-loading numbers. Their comments made it apparent that they began to feel more comfortable with the reduced-tail-swing machine during trenching, and that during the truck-loading test, their expectation that the DX235LCR would be less capable resulted in their working it more cautiously.

“You get used to running it [the DX235LCR],” said Thomas, “and it runs better than the 225. I still think the 225 is just a bit more stable, but there’s absolutely no problem with the 235. I think the 235 also was smoother on the swing; yesterday I’d probably have told you the 225 was smoother. You get used to the 235—I don’t know what it is—I just changed my opinion.”

Turner had similar comments: “When I was swinging around to the truck in the first test, I knew the 235 wasn’t going to tip, but you have the perception that it’s a smaller machine than the 225 and needs to be handled easier. But the longer you run it, the more you realize that it’s responsive and stable. The 235 has a slightly different feel when you’re operating, but you soon get comfortable with it and work it just as you would a conventional machine.”

For what it’s worth, after watching these two machines dig for an hour, we’d say that one was as flat-footed as the other—just based on looking for track lift. It’s our opinion that when the operators began to use the DX235LCR to its potential, the machine’s edge in hydraulic power began to pay off in more efficient bucket loading.

But how do you account for the reduced-tail-swing machine’s fuel-efficiency advantage, given that both use the same engine and that the DX235LCR has more horsepower? Part of the explanation might be that the reduced-tail-swing machine, with its slightly more hydraulic capacity, digs somewhat easier and labors less in tough digging. Someone in the group advanced the theory that perhaps the compact mass of the DX235LCR takes less power to swing than the elongated upper of the conventional machine, resulting in less fuel burned per cycle. Interesting thought; could be some logic in that.

Lifting

For the lifting test, we first chained together old mining tires, loaded them on a trailer behind a big pickup, then ran truck, trailer and load across the truck scale to determine how much the machines would be lifting: 11,500 pounds.

With Thomas operating both machines in the lift comparison, he increased the lift radius until the rear roller just began to lift from the track chain (for over-front lifting) and until the rollers just began to lift from the chain when lifting over the side. At those points, we measured the radius. The DX225LC out-lifted the DX235LCR by an average 9 percent over the front and by an average 3 percent over the side.

“Surprisingly, they were pretty much the same,” said Thomas. “The 235’s rollers lifted just a bit sooner than the 225, but performance was nearly equal between the two until that point. You’d think that 235 would have been much less capable than the 225, but it wasn’t.”

Give and take

Taking a long view of the information collected during the comparison and considering comments from the operators, a number of conclusions seem to surface. Perhaps most revealing was the observation that reduced-tail-swing machines must prove themselves to the operator. Only an extremely open-minded person could look at the two machines parked tail-to-tail and not conclude that the conventional machine would be more capable.

Yet, test data for the two Doosan machines indicate otherwise. The reduced-tail-swing machine’s production advantage when trenching was a surprise, as was its fuel-efficiency advantage. True, the conventional machine was a better lifter, but only marginally so. And if an operation involved intense truck loading, the conventional machine might be a more productive choice, but again, only marginally so.

We can only speak for the Doosan models we had on site, but in this instance, the decision between a conventional-tail-swing machine and a reduced-tail-swing machine would not hinge on a choice between capability versus compact size. Rather, the decision would be simply whether the compact dimensions of the reduced-tail-swing machine are worth an approximate 10-percent premium in purchase price.

 
 

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