Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/662812
April 2016 | EquipmentWorld.com 54 asphalt pavements. "The free-floating screed was in- troduced in the 1930s," says Wayne Jones, senior regional engineer for The Asphalt Institute. "Free-floating means it will lay a uniform depth so long as all the forces affecting that screed are in balance." The free-floating screed is not held to grade (the thickness of the mat) by the mechanical- or hydraulics-controlled set position of the paver. Instead, the free-floating screed is held to grade by five forces in balance or equilibrium, including: • The force exerted by the tractor to pull the screed. • The weight of the screed. • The force created by the head of material in front of the screed. • The frictional resistance of the material as it flows under the screed plate, and… • The upward force induced by the paving material. "When these forces are in balance, the screed is held to grade with an equilibrium angle of attack," says Laikram "Nars" Narsingh, product manager for Vögele asphalt pavers. "This angle of attack ensures that the trailing edge of the screed plate does the final screed compaction and sets the surface texture. Any changes in these forces will result in a change in the equilibrium angle of attack of the screed, hence the grade, resulting in roughness and mat blemish. So, it's important to ensure maximum stability of the five forces to optimize screed per- formance." The head of the material provides a critical force. "The head of the material is the mix near the augers that the screed must plow through and level," Jones says. "If there's too much asphalt, the screed is forced to rise; the correct amount, the screed remains level; too little, the head diminishes and the screed will drop." This can be achieved by maintain- ing the right ratio of feed by adjust- ing feed gate heights in conjunction with conveyor speed, he says. "We recommend 20 to 40 revolutions per minute for auger speed," Jones adds. "Too high or too low, and mat streaks will appear. If the speed is too fast, you can literally throw aggregate out of the area in advance of the screed, causing ag- gregate mix segregation." The amount of mix at the screed is gauged by feed sensors, which can include sonic or infrared sen- sors. "They need be pointed, as much as possible, 90 degrees to the face of the live pile of material," Jones adds. "Otherwise, if it points at a dead spot, it never will send the right amount of material." Paver propel speeds also impact screed performance. The faster the paver-tractor, the more the screed tends to drop, Jones said. "Be as constant as possible," he says. "Make sure the feeders match the paving speed, and if speed chang- es, that the feeders adjust." Two tow arms attach the screed to tractor tow points on both sides of the tractor at its center of gravity. The arms are lifted hydraulically, and the screed pivots up and down around the tow arm. The tow points represent points of pull, so the trac- tor begins the leveling process as it moves forward, staying level despite any pavement roughness. "The height of the tow point relates to the height of the screed," Jones says. "A 1-inch change in the height of the tow point equates to a 1/8-inch change back at the screed." 21st Century screeds Though the principle of the free-floating screed has remained unchanged since its inception, screed mat quality has improved for a number of reasons, including im- proved paver technologies, like use of a controller area network (CAN- bus), a vehicle systems connection standard that allows microcon- trollers and devices to communicate with each other in applications without a host computer. "Improved control technology, such as a CANbus with digital display, non-contact sensors and improved hydraulic controllability, has allowed better manipulation of the five forces to ensure stability," Narsingh says. road science | continued An easily adjustable width extending screed is ideal for commercial or suburban paving. Volvo Construction Equipment