Ever since they were introduced in North America a decade or so ago, bobtail excavators have been gaining pop-ularity. The reason is simple: A shorter tail swing lets them work in areas where standard excavators with prominent counterweights can’t.
Available in compact up to midsize machines, this design minimizes the amount of counterweight and, in some cases, the front corners of the cab that protrude beyond the tracks when swinging the upper structure. And there is little if any loss of power digging performance or lifting capacity, manufacturers say.
Case describes its CX225 minimum-radius excavator as having “the performance of a 20-metric-ton machine with the work envelope of a 7-metric-ton, standard-configured machine.”
A shorter tail swing makes it easier to place spoil in confined areas and reduces the chance of striking a wall, tree or other object when rotating. That can pay off when installing septic systems in cramped residential yards.
Comparing swings
You get an idea of how an excavator with a smaller work envelope might fit your operations by comparing three Caterpillar midsize excavators. The 21.5-metric ton 320D L has a standard tail swing radius of 9 feet. The tail swing of the 23.7-metric 320D LRR has been reduced so that the back end of the machine doesn’t extend beyond the length of the tracks. This reduced radius tail swing of 6 feet, 7 inches is 2 feet, 5 inches less than the standard design.
The compact radius of the 24-metric-ton 321D LCR model is designed to rotate with little if any of its counterweight extending beyond its tracks. It has a 5-foot, 6-inch tail swing radius, 3 feet, 6 inches shorter than the 320D L.
“The additional 5,000 pounds of counterweight on the compact radius 321D LCR and 6,000 pounds on the reduced-radius 320D LRR gives these machines better over-the-front lift and far superior over-the-side stability when compared with more traditional machine designs,” reports John Walker, manager of Caterpil-lar’s hydraulic excavators for the heavy construction market.
Performance features
To further minimize its working envelope, while maximizing performance, the boom tower of the compact-radius 321D LCR is positioned farther back in the upper frame compared to the standard 320D L. “The design difference of the compact-radius model reduces the front swing radius, when the boom is pulled to the full up position for superior operation in confined spaces,” Walker notes.
It also greatly improves the lift capabilities of the machine in close. This model demonstrates a 21-percent advantage in lifting capacity in key areas over the front compared with the standard model. The longer stroke of the boom cylinders reduces the front swing radius when the boom is pulled up and improves lift and stability for easier, safer and more productive operation in limited spaces.
“You can place a dump truck or trailer right next to the tracks of the 321D LCR and load it,” Walker says. “With a more traditional design, with the boom tower and boom cylinders positioned farther out over the swing bearing, trucks may need to be positioned 10 to15 feet away from the center line of the excavator to simply get the stick and bucket in the right position to load a hauler.”
With its counterweight positioned closer to the center line, the compact-radius excavator requires less swing torque, especially with heavy payloads in the bucket. The result is quicker starts and stops and faster cycle times.
The reduced-radius 320D LRR, with its slightly cantilevered tail and heavier counterweight, can demonstrate as much as a 10 percent over-the-side lift advantage over the compact-radius 321D LCR at ground level and above points on the lift charts and a 20 percent over-the-side advantage over the 320D L at similar locations.
“If you’re loading trucks with larger buckets or using heavier attachments and couplers, the reduced radius machine offers better static stability,” Walker says.
Compact choices
Among compact excavators with a shorter radius tail swing are those made by Bobcat. For its 3.5- to 5-ton machines, the company offers conventional tail-swing models, in which the tail of the upper structure extends beyond the tracks when rotating; and zero-house-swing models, in which both the rear end and the right and left front corners of the house remain within the width of the machine.
In many applications, zero-house-swing models provide more freedom of placement than a conventional design. Consider, the midsize conventional tail-swing Bobcat 331 and the similar size zero-house-swing model 430.
“The 331 can slue only to the point where the tail begins to encroach on an object, which is no more than about 90 degrees,” says Tom Connor, excavator product specialist. “However, the 430 can slue as much as 320 degrees in either direction without restriction for many more spoil-placement choices.”
Ironically, Connor notes, the conventional tail-swing design can be a plus when site access space is limited. Typically, as engineers compress the upper structure to reduce tail swing, they also expand the width of the undercarriage to accommodate the various components. As a result, a Bobcat zero-house-swing excavator is about 10 inches wider than similar-sized conventional models. “For contractors facing tight access on jobsites that extra width isn’t a viable option,” Connor says.
It’s worth looking into these machines if you work in settings constrained by buildings, trees, and other site obstacles. And in the onsite industry, who doesn’t?
Greg Northcutt is a freelance writer based in Port Orchard, Wash. He can be reached by e-mailing this publication at editor@onsiteinstaller.com.
