They go back and forth, jump up and down and hammer asphalt. That’s a pretty simplistic explanation of smooth, tandem-drum vibratory compactors in the >8-to 10-metric-ton class. But although there are differing philosophies among manufacturers regarding basic machine design and how to best transmit impact energy into asphalt, the primary application for machines in this class is compaction work on roads and highways. According to Dale Starry, compaction product manager, Ingersoll-Rand, two-thirds of all compactors in this class currently work in road compaction applications and more than three-fourths of these machines are being used for compaction of hot-mix asphalt materials.
“This is a good universal compactor size because the tonnage range allows them to work on both residential and highway paving,” notes Steve Wilson, manager, marketing services/product manager, Bomag Americas. Because of the physical size and weight of these machines, Wilson says they are extremely flexible for big and small compaction jobs, yet still compact enough to be easily transported in a residential area. “They work well on city streets because they’re not so heavy or large that they can’t maneuver around within those narrow areas (like cul-de-sacs and parking lots),” he adds. “At the same time, they have enough size and they’re fast enough to handle highway-class paving. So these models are a good, economic choice for contractors wanting to do different types of compaction jobs.” Compactors in this weight class (and typical drum size) can also be used for many airport compaction jobs.
Double-drum vibratory compactors have two vibrating drums. Each drum applies a specific level of compaction energy to a material surface. This energy, Starry explains, is capable of changing material density by eliminating air voids within the material mass. “The effective depth to which compaction is achieved is generally believed to be limited only by the drum amplitude,” he says. “There is also added benefit – somewhat difficult to quantify – of having a ‘trailing’ vibrating drum pass over material immediately following the leading drum coverage. Some force overlap is experienced, depending upon material characteristics and rolling speed.” Because of this double compaction effort, Starry says a double-drum vibratory compactor will always enhance productivity compared to a single-drum vibratory compactor.
Dave Brown, product manager, Sakai America, is more blunt: “Double-drum compactors double your productivity,” he says. “Single-drum vibratory asphalt rollers are basically obsolete. The rubber tires of a single-drum roller simply get in the way without delivering any real benefit to the compaction process. Compactors with two vibrating drums will always achieve required material density in fewer passes than compactors with one vibrating drum.”
Asphalt only, please
According to Starry, contractors historically purchased compaction equipment in the 8-to-10-tons class to meet minimum operating weight requirements of state transportation agencies. “The minimum operating weight specification was originally developed when steel-wheel static rollers were the only compactors available,” he explains. “As vibratory rollers became more popular, it was a simple specification carry-over that required them to also satisfy this minimum operating weight requirement. Double-drum vibratory compactors in this weight class offer much greater performance and versatility than static rollers of the same weight.” Starry says this increased versatility is reflected by the “flip-flop” in contractor compaction preferences in the past 30 years. In 1970, he notes, 90 percent of all machines sold in this class were static, steel-wheel rollers. In 2000, 90 percent of the compactors sold in the >8-to-10-metric-ton class were vibratory rollers.
Today, Starry notes, double-drum vibratory compactors weighing more than 8 metric tons can effectively and efficiently compact granular soils and unbound aggregate materials. “In fact,” he says, “they can often achieve required density for the material in fewer rolling passes than required when single-drum vibratory compactors are used.”
In general, Starry recommends using smooth, double-drum vibratory compactors on projects where granular soils or unbound aggregate materials are placed in lifts (uncompacted layers) no greater than 8 inches in thickness. “For double-drum models with only two drum amplitudes, I recommend compacting with high amplitude,” he says. “On models with three or more amplitudes I recommend use of the higher amplitude selections. The choice of amplitude selection is generally made on the basis of actual amplitude for the vibrating drums.
Ingersoll-Rand’s experience has shown that amplitude selections of five or higher are needed for effective compaction of granular or mixed soils in lifts up to 6 inches thick. The lift thickness reduction during the compaction process is typically between 20 to 25 percent of the loose lift thickness. Therefore a lift of 8 inches of loose thickness will be 6 inches following compaction.”
Brown says machines in this class should never be used in soil compaction applications. “Absolutely not!” he stresses. “A single rock in the soil can ding the smooth drum, which in turn will ding the asphalt that you’re trying to make smooth. A second problem is that these machines compact at 4,020 vibrations per minute, while soil is compacted most effectively between 1,650 and 2,200 vpm. So you’re not really accomplishing your goals by using a double-drum asphalt machine for soil compaction work.”
But, warnings aside, contractors are still likely to use these machines in soil compaction applications. When they do, Bomag’s Wilson says it’s typically in soil and rock-base site preparation work. Smart contractors, he says, will limit their newer and better machines strictly to asphalt so they don’t do any damage to the rolling surface. And they’ll dedicate some of their older machines to do something other than asphalt compaction so that if they do put a ding in the drum it won’t cause a blemish in the surface when they do finish rolling on asphalt.
Perimeter frame compactors feature a rigid-box design with a suspended drum. The drum supports the frame, which supports the operator’s platform, engine and other components.
Design changes blend European and American compaction philosophies
There are two specific types of tandem rollers available in North America, and accordingly there are two specific design philosophies behind each one. Perimeter frame type compactors have been a mainstay in North America for more than a century. “Even before steam-powered rollers were invented, when contractors were using pull-behind sheep’s foot rollers, compactors were built with the frame around a center pivoting axis to hold the axle in place while the drum was being pulled,” explains Bruce Monical, marketing manager, Hamm. “That’s the way blacksmiths hammered these machines together in the very beginning,” he says. “And this style of designing around the drum was – and is – a good, solid system and remains in use to this day.”
“In essence, a perimeter frame machine is a rigid box-design with a drum suspended within,” says Wilson. “Basically the drum is supporting the frame and it, in turn, supports the components, the operator’s platform, engine and everything else.”
In Europe, though, compact design criteria evolved more rapidly than it did in the United States. “This was because mix designs changed over there more due to the Autobahns and Autostradas – high-speed freeways with no speed limits,” Monical notes. “To meet these different application demands, European manufacturers came up with vertical leg design models.”
Vertical leg design machines feature an open-frame design. The highlight of this layout is bolt leg supports that allow the drum to carry the balance of the frame weight suspended off of it. “The real advantage to this configuration is visibility,” Monical says. “Because components don’t have to be wrapped around the drum, an operator sits slightly higher on the machine and has a clear field of vision ahead and below. The other advantage is that the open-frame design allows designers to place components inside the machine’s frame and out of harm’s way, instead of having them mounted externally. So there’s less risk of component damage because components like water tanks are protected from impacts.”
“Leg design machines, in most cases, are shorter in length and have less lateral overhang than a perimeter frame machine,” Wilson adds. He says these features allow the leg design models to work up closer against obstructions in the front and rear of the machine. “Additionally, a vertical leg design’s pitch-out in the leg structure is high enough that in 90 percent of applications it’s above any curbing line obstructions – even guardrails in some cases,” Wilson adds. So the operator can place the drum and work right up against that obstruction or obstacle.” The end result of these features, according to Wilson, is they significantly minimize the amount of finish work that has to be done by laborers once the machine is done compacting, saving contractors both time and money on paving jobs.
Generally speaking, perimeter frame machines have a little more front and rear overhang than leg design models and the lateral overhang from the edge of the drum to the edge of the frame is a little more as well, from 2 to 5 inches, depending on the model. “So perimeter frame models aren’t as adept at getting in close and maneuvering when you’re doing finish work,” Wilson says.
But that doesn’t mean they don’t have advantages. Perimeter frame machines characteristically seem to have a little bit lower profile, and the operator’s station is generally lower to the ground as far as the operator’s position. So the perception among many contractors is that a perimeter frame unit has a lower center of gravity and is more stable than a vertical leg model. “That’s not really the case,” Monical insists. “The engine is positioned in the middle of the frame on a leg design compactor. So it’s situated low to the ground. This concentration of weight down low gives these machines a center of gravity and stability factor equal to perimeter frame models. At the same time, perimeter frame models, thanks to their lower overall machine profile, do have a tendency to put a little more non-vibratory static weight on the drum, which allows the machine a slight increase in its total applied force – being a function of vibratory centrifugal force plus the weight of the machine.”
One knock against leg design machines when they first appeared in North America, according to Wilson, was that they didn’t have the same performance standards that contractors expected from their traditionally styled compactors. “Those concerns no longer apply,” he says. “Today’s leg design compactor has the same engine horsepower, high centrifugal force, ability to travel with vibration, travel speeds and impact frequencies that contractors expect.”