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Compactor type engineering vehicle

A road roller (sometimes called a roller compactor, or just roller) is a compactor type engineering vehicle used to compact soil, gravel, concrete, or asphalt in the construction of roads and foundations.

The first road rollers were horse drawn, and were probably borrowed farm implements (see Roller). Since the effectiveness of a roller depends to a large extent on its weight, self powered vehicles replaced horse drawn rollers from the mid 19th century. The first such vehicles were steam rollers. Single cylinder steam rollers were generally used for base compaction and run with high engine revs with low gearing to promote bounce and vibration from the crankshaft through to the rolls in much the same way as a vibrating roller. The double cylinder or compound steam rollers became popular from around 1910 onwards and were used mainly for the rolling of hot laid surfaces due to their smoother running engines, but both cylinder types are capable of rolling the finished surface. Steam rollers were often dedicated to a task by their gearing as the slower engines were for base compaction whereas the higher geared models were often referred to as "chip chasers" which followed the hot tar and chip laying machines. Some road companies in the US used steamrollers through the 1950s. In the UK some remained in service until the early 1970s. As internal combustion engines improved during the 20th century, kerosene , gasoline (petrol), and diesel powered rollers gradually replaced their steam powered counterparts. The first internal combustion powered road rollers were similar to the steam rollers they replaced. They used similar mechanisms to transmit power from the engine to the wheels, typically large, exposed spur gears. Some users disliked them in their infancy, as the engines of the era were typically difficult to start, particularly the kerosene powered ones. Virtually all road rollers in use today use diesel power.

Road rollers use the weight of the vehicle to compress the surface being rolled (static) or use mechanical advantage (vibrating). Initial compaction of the substrate on a road project is done using a padfoot or "sheep's foot" drum roller, which achieves higher compaction density due to the pads having less surface area. On large freeways, a four wheel compactor with padfoot drum and a blade, such as a Caterpillar 815/825 series machine, would be used due to its high weight, speed, and the powerful pushing force to spread bulk material. On regional roads, a smaller single padfoot drum machine may be used. The next machine is usually a single smooth drum compactor that compacts the high spots down until the soil is smooth. This is usually done in combination with a motor grader to obtain a level surface. Sometimes at this stage a pneumatic tyre roller is used. These rollers feature two rows (front and back) of pneumatic tyres that overlap, and the flexibility of the tyres provides a kneading action that seals the surface and with some vertical movement of the wheels, enables the roller to operate effectively on uneven ground. Once the soil base is flat the pad drum compactor is no longer used on the road surface. The next course (road base) is compacted using a smooth single drum, smooth tandem roller, or pneumatic tyre roller in combination with a grader and a water truck to achieve the desired flat surface with the correct moisture content for optimum compaction. Once the road base is compacted, the smooth single drum compactor is no longer used on the road surface (there is an exception if the single drum has special flat wide base tyres on the machine). The final wear course of asphalt concrete (known as asphalt or blacktop in North America, or macadam in England) is laid using a paver and compacted using a tandem smooth drum roller, a three point roller or a pneumatic tyre roller. Three point rollers on asphalt were once common and are still used, but tandem vibrating rollers are the usual choice now. The pneumatic tyre roller's kneading action is the final roller to seal the surface. Rollers are also used in landfill compaction. Such compactors typically have padfoot drums, and do not achieve a smooth surface. The pads aid in compression, due to the smaller area contacting the ground.

The roller can be a simple drum with a handle that is operated by one person and weighs or as large as a ride on road roller weighing and costing more than US$150,000. A landfill unit may weigh .

Drums are available in widths ranging from .

Tyre rollers are available in widths ranging up to , with between 7 and 11 wheels (e. g. 3 wheels at front, 4 at back): 7 and 8 wheel types are normally used in Europe and Africa; 9 and 11 in America; and any type in Asia. Very heavy tyre rollers are used to compact soil.

On some machines, the drums may be filled with water on site to achieve the desired weight. When empty, the lighter machine is easier and cheaper to transport between work sites. On pneumatic tyre rollers the body may be ballasted with water or sand, or for extra compaction wet sand is used. Modern tyre rollers may be filled with steel ballast, which gives a more even balance for better compaction. Additional compaction may be achieved by vibrating the roller drums, allowing a small, light machine to perform as well as a much heavier one. Vibration is typically produced by a free spinning hydrostatic motor inside the drum to whose shaft an eccentric weight has been attached. Some rollers have a second weight that can be rotated relative to the main weight, to adjust the vibration amplitude and thus the compacting force. Water lubrication may be provided to the drum surface from on board "sprinkler tanks" to prevent hot asphalt sticking to the drum. Hydraulic transmissions permit greater design flexibility. While early examples used direct mechanical drives, hydraulics reduce the number of moving parts exposed to contamination and allows the drum to be driven, providing extra traction on inclines. Human propelled rollers may only have a single roller drum. Self propelled rollers may have two drums, mounted one in front of the other (format known as "duplex"), or three rolls, or just one, with the back rollers replaced with treaded pneumatic tyres for increased traction.