Asphalt Milling Machine

Overview

An asphalt milling machine, also called a cold planer or road planer, removes the worn top layers of an asphalt pavement so a fresh surface can be laid. It does the work cold, without heating the road, by dragging a fast-spinning drum of carbide bits across the surface and grinding the asphalt into small granules. The machine sets its own cutting depth, picks up the broken material, and loads it straight into a haul truck running ahead of it, so a single pass leaves a clean, textured floor ready for the next layer.

A large cold planer is built around ten subsystems: the Diesel Power Unit that supplies power, the Milling Drum that cuts, the Drum Drive that turns it, the Conveyor System that removes the millings, four Crawler Track crawlers for propulsion, the Hydraulic System that move everything, four Height-Control Leg columns that set depth, the Water Spray Dust Suppression for dust and cooling, the Operator Station operator station, and the Electrical and Control control that ties them together.

Power unit

The Diesel Power Unit centres on a six-cylinder Diesel Engine of around 450 kW. Unlike a truck engine, almost none of its output goes to driving wheels; the engine instead spins the drum belt directly and runs the hydraulic pump drive that powers the tracks, conveyors, and cylinders. The engine breathes through a heavy-duty Air Intake Filter, because milling throws up fine grit, and it is fed by a common-rail Fuel System with a high pressure High-Pressure Fuel Pump and six Fuel Injector injectors. The Engine Cooling pack carries an oversized Coolant Radiator and a reversible Cooling Fan that blows the core clear of caked dust. Emissions are handled by the Exhaust and Aftertreatment line, where a Diesel Particulate Filter traps soot and an SCR Catalyst catalyst, dosed from the DEF Tank, cuts NOx to meet Stage V and Tier 4 Final limits.

Cutting drum and drive

The Milling Drum is the heart of the machine. Its Drum Body, roughly 1 m across and 2 m wide, carries 160 Pick Holder holders welded in a spiral pattern, each gripping a conical Cutting Pick. The picks are tungsten carbide tipped and free to rotate in their holders so they wear to a sharp point on all sides. At normal cutting they grind asphalt at a line spacing near 15 mm, which sets how coarse or fine the milled texture comes out. Inside the housing, Throwing Paddle paddles and a Kickback Bar sweep the broken material toward the conveyor. Power reaches the drum through the Drum Drive: a set of multi-rib Drive Belt belts held by a Belt Tensioner, an Engagement Clutch for a soft start under load, and a planetary Drum Gearbox that turns belt speed into the high torque the cut demands.

Material removal

Milled asphalt is hot and abrasive, so two conveyors move it clear quickly. The Primary Collecting Conveyor sits under the drum and scoops the millings off the floor with its cleated Conveyor Belt, running on Belt Idler idlers and driven by a hydraulic Conveyor Drive. It hands the load to the long Discharge Conveyor boom, which throws it forward into a truck. The Discharge Slew Mechanism mechanism, built on a Slew Bearing and a pair of Hydraulic Cylinder cylinders, swings the boom side to side and lifts it so the operator can aim the stream into a moving bed.

Propulsion and depth control

The machine rides on four Crawler Track crawlers, each with a Track Chain of polyurethane Track Pad pads that grip without scarring the road, a Drive Sprocket driven by a hydrostatic Track Drive Motor, and a tensioning Idler Wheel. Each track bolts to a Height-Control Leg column. The four Lift Cylinder cylinders inside those columns raise and lower the frame to set milling depth from zero to about 330 mm, and a Height Sensor on each reports its extension. All of this is fed by the Hydraulic System, where variable Hydraulic Pump pumps and Control Valve Block valve blocks meter oil from the Oil Reservoir reservoir, cooled by an Oil Cooler.

Dust, control, and accuracy

The Water Spray Dust Suppression sprays the cut through 20 Spray Nozzle nozzles fed from a 3000 litre Water Tank, cooling the picks and knocking down dust. From the Operator Station, the operator drives with two Travel Joystick joysticks and watches the cut over the Side Plate plates and the spring Moldboard. The Electrical and Control system runs it through a Machine Controller, and the Grade and Slope Control system reads a Grade Sensor at each side and a Slope Sensor to hold a level, accurate cut so the new surface can be laid to grade.

In use

On a typical job the machine works at 0 to 35 m/min, the speed falling as depth and pavement hardness rise. Pick wear is the main running cost, since a worn Cutting Pick cuts slower and burns more fuel, so crews check and rotate or replace picks through the shift. When a section is done the operator raises the frame on the Height-Control Leg columns, retracts the Side Plate plates, folds the Discharge Conveyor for transport, and tracks the machine to the next stretch. The combination of cold cutting, on-board loading, and grade control lets one machine resurface a lane to depth in a single controlled pass.