A road milling machine is a heavy-duty construction machine designed for layer-by-layer removal of asphalt and concrete pavements. The core working element is a horizontally rotating milling drum equipped with replaceable carbide-tipped chisels or pick tools that fracture and pulverize the bound material. The loosened material is collected by a conveyor belt system and deposited onto trailing dump trucks or stockpiles.
Operating Principle and Drum Configuration
The milling drum rotates at 100–250 rpm depending on material hardness and desired finish quality. Drum widths typically range from 0.5 m to 2.5 m, with cutting depths adjustable from 10 mm to 350 mm in single passes. Horizontal positioning relative to the road surface is controlled by electronic grade-control systems, often using laser or GNSS sensors to maintain design elevation tolerances of ±10 mm. The cutting speed of the drum can be independently regulated from the forward travel speed (0.5–4.5 m/min) to optimize cutting efficiency and surface finish.
Material Conveying and Discharge
The conveyor belt transports milled material from the drum to either side or rear discharge. Belt widths typically match the milling drum (0.5–2.5 m) and operate at speeds of 2–8 m/s. Material flow rates on full-width cuts can reach 150–300 t/h, depending on milling depth and pavement composition. Discharge height is typically 2–4 m above ground level to allow direct loading into haul vehicles without secondary handling.
Machine Specifications and Power Requirements
Milling machines are classified by working width and engine power. Common categories include 0.5–1.0 m machines (30–60 kW), 1.0–1.5 m machines (60–100 kW), and 1.5–2.5 m machines (100–180 kW). Engine power can reach 250+ kW for large units. Hydraulic systems typically operate at 250–280 bar working pressure. Total machine mass ranges from 8 t for compact units to 25+ t for full-size models, with ground contact pressure around 0.8–1.2 MPa on standard oscillating tracks.
Material Recycling Capability
Modern machines produce reclaimed asphalt pavement (RAP) with grain-size distribution controllable by milling speed and drum configuration. Cold-milled RAP typically contains 85–95 % particles ≤ 50 mm when produced at depths of 40–80 mm. Material is reusable directly in wearing-course recycling plants or as-is in unbound base layers, reducing disposal costs and environmental impact.
Surface Finish and Texture
The final texture of the milled surface depends on drum design and cutting speed. Close-spaced tool geometry produces a finer surface finish (macrotexture depth 1–2 mm) suitable for bonding new asphalt. Wider tool spacing creates a coarser profile (2–4 mm depth) used for drainage improvement or grip enhancement before chip sealing.
Operational Constraints
Milling machines are most efficient on pavements 40–150 mm thick. Very thin or very thick sections require multiple passes or alternative methods. Concrete pavements require carbide tools rated for compressive strengths up to 60 MPa. Ambient temperature affects hydraulic performance; operation below −10 °C or above +40 °C typically requires system adjustments. Noise levels during operation reach 85–95 dB(A) at 1 m distance.