Technology: Asphalt paving and milling
| July 09, 2007 |
Aside from a few high dollar sport and luxury cars, most automobiles roll down the road with the same basic technology.
Not so with highway-class asphalt paving and milling machines. While there are similarities in the basic designs, almost every manufacturer comes to market with one or more innovative technologies.
So, when evaluating these machines it’s important that you don’t just compare common specs – horsepower, speeds, tonnage, etc. You also need to evaluate how well each manufacturer’s distinctive technology can help achieve a better end result. Since manufacturers are continually improving their product lines we can’t say for certain that what’s discussed below is completely unique to any single manufacturer, but in each case we’ve attempted to pick a leader in a certain technology to discuss its attributes.
For the past 50 years speed and tonnage powered the profits behind asphalt paving. The more miles you paved, the faster you made money. But in the past few years DOTs have started to base bonuses on pavement rideability, says Brodie Hutchins, general manager, Vogele America. And speed and quality don’t always go together. This is especially true with some of the newer pavement mixes such as stone matrix asphalt or SMA.
Slow speed/high quality paving and SMA both come from the European paving practices, Hutchins says. “So it makes sense the tools to put it in come from Europe, too.” Vogele, like most European market pavers, puts a tamping bar ahead of the screed, which is rarely found in the states. And Vogele goes it a step further by putting what it calls a pressure bar behind the screed. Both the tamper bar and Vogele’s pressure bar substantially increase the mat’s density before it sees the first roller.
“The more you roll, the more you sacrifice rideability, so we’re shifting some of the breakdown process to the screed and away from the roller,” Hutchins says. “If you put the mat down with a high-density screed, you gain 10 points or more in density. Then once you get the rollers on it, you have the same amount of time to get three or four points of density as you used to have to get 13 or 14 points.”
The pressure bar or bars (some models have two) push down on the mix until they meet a specified resistance. “It kneads the asphalt, and it’s adjustable. It’s not going to break the stone,” Hutchins says.
The tradeoff is that the tamper bar pavers go slower: 30 to 40 feet a minute on average, Hutchins says. But in addition to creating a higher quality mat, the tempo of this design is more in line with the pace of continuous paving. “A lot of crews go 100 feet and then stop and then do another 100 feet and stop,” Hutchins says. But if you do the math backwards and start with how much mix you can produce and deliver, then your paving speed becomes less of an artificial goal and more of a precise calculation. You’ll get longer pulls, fewer interruptions and more consistency.
User friendly interface
When Ingersoll Rand (now Volvo Road Machinery) introduced its milling machines, the MW-500 and MT-2000, last fall it gave its customers something rare in any field, an intuitive, easy-to-understand user interface – in other words, a control screen that looks like what you’re working on and talks to you in a language you understand, not codes.
“By improving the communication between the man and the machine, it enables the operator to become more efficient,” says Patrick Wakefield, marketing manager for milling. “It allows them to learn the controls more quickly and understand where those controls are.”
The main control panel is laid out as if you were looking at the machine from the top down, Wakefield says. The four leg controls are in the four corners of the panels. The drum controls are in the center. The side skirt control buttons flank the drum area.
Gauge information is displayed on a 6-by-8-inch full color screen panel. The panel is what Wakefield calls “trans-reflexive,” which means it can be read in even the brightest sunlight. Real time track position indicators show up as small gray rectangles on the screen and show you by their position if the tracks are straight or turned. When the drum is spinning, its screen icon turns red.
When it comes to diagnostics, rather than give you a series of codes or numbers the screen tells you in plain English what the issue is. And this feature is not just in English, but any one of eight languages you choose. “From a troubleshooting point of view it reduces downtime when you can quickly and easily identify an issue rather than have to pull out a laptop and diagnose,” Wakefield says.
Changing cutter drums on milling machines can take up to two days, but several years ago Wirtgen came up with what it calls the flexible cutter system that cuts the drum change time about in half.
“There are a lot of 10-foot shoulders done on the interstates,” says Jeff Wiley, vice president sales and marketing for Wirtgen. “So if you’re starting with a 7-foot drum, you can add additional cutter drums and with the extensions cut a shoulder that’s exactly 10 feet wide in one pass,” he says. “When you buy a machine, you’re not stuck with one width.”
The company also recently introduced a similar system it calls FCS Light. “This gives you the capability of changing the cutter pattern from a standard 5⁄8-inch spacing down to /-inch spacing in less than three or four hours,” Wiley comments. “This fine milling is becoming increasingly popular with state DOTs. If you’re milling shallow, you can open it back up to traffic and it won’t interfere or create vibration for cars or motorcycles. It doesn’t have the tendency to guide you from one side to the other like a rougher texture pattern would.”
Fine texture milling also gives you a smaller recycled asphalt product or RAP. In most cases this means the RAP can skip the crushing stage back at the plant and go directly into screening, speeding turnaround and cutting costs.
Hardwired for fail-safe performance
“The one thing we hear time and again from our milling and paving customers is “reliability,” says John Irvine, vice president of sales for Roadtec. “When you have 20 or more trucks on the road hauling asphalt, plus your crew and the DOT people, it might be costing you $200 to $300 a minute when you’re not paving.
“We don’t make it overly complicated,” Irvine continues. “We hardwire everything and try to give the customer a hardwired electrical route with duplicate wires in the harness, so if one wire burns out they can jump to a spare wire in the harness. We try to use interchangeable solenoid and coils, so if one function goes down you can switch out.”
Roadtec uses the hardwired design rather than what’s known as a CAN-Bus system, which shoots multiple signals across the same wire and uses a computer at the end to sort out the signals. “We can’t eliminate the use of computers 100 percent,” Irvine says, “but we make it as simple as possible. It costs more to produce a machine this way but the uptime reliability is worth it.”
Dual purpose design gathers no moss
It profits no one to buy a machine and park it 80 percent of the time. So when Terex Roadbuilding designed their CR66RM machine they configured the rear of the machine to accept either a conveyor (making it a material transfer vehicle) or a screed to make it a paver.
The design evolved out of the Cedarapids Remix paver which uses a pair of lengthwise counter rotating augers (rather than a conveyor with slats) in the hopper to deliver the mix to the screed, says William Rieken, paver application specialist for Terex Roadbuilding. This aggressive remix design reblends asphalt mixes that may have started to segregate, either by temperature or by aggregate size, delivering a more consistent quality mat under the screed.
From that design it was not hard to justify putting a conveyor attachment on the rear to give the Remix paver the capability of functioning as a material transfer vehicle.
“If you look at all the solutions for continuous, non-contact paving, whether it’s a windrow machine, a material transfer vehicle or a buggy, they’re a pain to load, park and transport,” Rieken says. “The CR66RM loads, moves and parks like a paver. It’s a lot friendlier to deal with.”
Even more important is equipment utilization. If the contractor isn’t working a job that requires non-contact paving, a traditional material transfer vehicle might sit idle. “With the dual purpose machine, the conveyor can be dropped off and a screed attached within about a half a day,” Rieken says. “Then you can use it every day.”
Self cleaning machine
The Blaw-Knox Blaw-Kote release agent spray is familiar to paving contractors who need an environmentally friendly way to keep asphalt from sticking to their machines. But with the introduction of the Volvo (formerly Ingersoll Rand) PF-6110 track driven and PF-6160 and PF-6170 wheeled versions of this machine, the Blaw Kote spray system has now been integrated into the machine. Push a button on the operator’s console and you’re covered.
“There’s plumbing and a nozzle built into the machine for each track,” says Scott Wiley, marketing manager for large paving at Volvo Road Machinery. “If an operator is traveling to his next job he can press the button and coat the tracks while he’s traveling.”
Another unique feature to the Volvo Road Machinery pavers is the feed system. “The auger conveyor system is controlled by four sonic sensors, one on each of the conveyors and one on each of the augers,” Wiley says. “It does away with the need for flow gates. It takes away the need for the operator to be finessing the material flow if he changes his paving parameters. If you’re paving 12 feet wide and want to take the extension further out, the sensor will see that there isn’t any material at the auger and speed up the auger to bring material out the end gate automatically.”
Also from Vogele America:
- Integrated fume extraction routes asphalt fumes into the exhaust port and funnels both out a stack to the rear of the screed creating a better operator environment and improving visibility
- No daily maintenance checks: a panel in the operator’s station warns you if fluids are low or of other maintenance issues eliminating the need for the daily walkaround
- Two cylinders on the tracks for self tensioning
- Plows in front of the tracks sweep aside spillage and debris
- Four independent monitors measure head of material – paddles on the conveyors and sonic sensors on the augers
- Purpose-built storage spaces for things like shovels, ice chests, and levels
Also from Volvo Road machinery milling machines:
- Three milling speeds, selectable from the control panel, adjust without having to stop and make mechanical changes
- Walk-in service compartment gives standing room access to all engine and hydraulic maintenance points
- Active obstacle detection in the front and rear of each track is designed to stop the machine if somebody or something moves into the path of the tracks
Also from Wirtgen milling machines:
- Variable speed temperature controlled fan runs off hydraulic system
- Internal gradation grid holds pressure on pavement to prevent slabbing
- Solenoid valves lighted for quick troubleshooting
- Infinitely variable speed track motors
- Totalizer adds up how much material conveyor delivers to truck each day
Also from Roadtec:
- The new RX-400 cold planer offers a rear-mounted drum that enables you to start cuts in front of curbs, barrier walls, buildings and other obstacles.
- Caterpillar ACERT engines on the pavers together with sound suppression kits lower noise levels by 4 decibels.
- Milling machine moldboards angled in 10 degrees get teeth closer to the moldboard for less slabbing and more finely processed RAP. It also carries less material inside cutter housing which reduces wear on the drum and housing.
- Independently driven conveyors, a narrow auger drive and a push-pull flight design to help eliminate centerline segregation on asphalt pavers
- New variable cutting systems (VCS) and segmented drums make changing milling machine cutting widths easier.
Also on the CR66RM:
- Spreading auger drive mounted outboard, rather than in the center, eliminates center chain case and reduces centerline segregation when operating as a paver.
- Large diameter, 12-inch augers turn slower for less wear.
- Total surge capacity is up to 43 tons for the CR662RM MTV and paver with hopper insert.
- Due to the low ground contact pressure of the track drive system it can be used on all paving lifts.
Also on the Volvo Road Machinery pavers:
- Hydraulic cylinder maintains proper tension and adjustment on conveyor chains.
- Speed limiter allows you set working speed and have the machine come back to exactly that speed after a pause to create consistent results.
- Self-priming conveyor and feed systems maintain the same head of material for different pulls.
- Hydrostatic, direct-traction drive system eliminates 70 percent of mechanical drivetrain components.