The inspection methods, procedures and precautions for asphalt milling machines

Asphalt milling machines are indispensable specialized equipment in highway maintenance, municipal road renovation, bridge pavement removal, airport runway repair and other projects. They perform precise cold milling operations on asphalt or concrete road surfaces through the high-speed rotating milling rotor, achieving the recycling and reuse of old pavement materials. The core value lies in the milling accuracy, operation efficiency and continuous construction reliability. Faults such as wear of the milling rotor tools, aging of the conveying belt, and drift of the depth control system will directly result in uneven milling surfaces, a sharp drop in construction efficiency and even project delays. In scenarios such as new machine delivery and acceptance, second-hand equipment transactions, lease return and lease inspection, and major repair factory certification, a comprehensive inspection covering the milling rotor, conveying system, traveling system, depth control and power transmission hydraulic system is crucial for accurately determining the equipment's operational capability and remaining economic lifespan. 
I. Overview of Inspection Methods
Based on the structural form of the asphalt milling machine (wheel type/track type), the milling width grade (half-width/ full-width), the conveying method (front conveying/ rear conveying), and the client's requirements, the inspection usually employs the following methods in combination: 
Static full inspection: The vehicle is left stationary. The entire vehicle frame, milling rotor assembly, conveying belt system, traveling tracks/tires, driver's cabin/controls console, power transmission assembly, hydraulic system, electrical control system, water spraying dust suppression system, etc. are inspected item by item through visual inspection, manual touch, and precise measurement with measuring tools. 
Dynamic operation test: On a safe site, start the engine and verify the functions of the milling rotor's no-load operation, the operation of the conveying belt, movement and steering, lifting and lowering of the milling drum, actions of the side panels/hood, and sprinkling of water and mist. 
Verification of milling accuracy (if conditions permit): Conduct short-distance actual milling on the designated test section, and use a 2m ruler or laser level to verify the lateral flatness of the milling surface and the consistency of the longitudinal milling depth. 
Document and label traceability: Verify the consistency between the entire machine label, the host frame number (PIN code), the engine label, the milling rotor label, the hydraulic main pump label and the certificate. 
The third-party inspection is carried out based on the "static full inspection + dynamic operation test + functional verification" package. For high-value transactions or disputes over accuracy, it is strongly recommended to add the trial milling accuracy test and the measurement of rotor tool wear depth. 
II. Inspection Process
1. Data Collection and Preparation for Inspection 
The client provides a list of the machines to be inspected, specifying the manufacturer, model, rated power, milling width, maximum milling depth, travel form (tracked/wheeled), feed direction, working hours, applicable standards (such as GB/T 25642, JT/T 970, etc.). The inspector prepares: long tape measure/laser distance meter, 2m straight ruler/feeler gauge (for flatness inspection), vernier caliper, steel plate thickness gauge, hardness tester (for auxiliary detection of cutter seat wear), infrared thermometer, hydraulic pressure gauge set, noise meter, oil sample bottle, etc. 
2. Confirmation of Venue and Safety Conditions 
The inspection site should be a solid and flat hard surface with sufficient area for the full-scale operation and movement of the milling machine. If a trial milling is to be conducted, the client should coordinate with a suitable road surface and confirm that there are no obstructions (such as pipelines, reinforcing bars, etc.) beneath the surface. The test area should be isolated with warning signs, and non-inspection personnel are prohibited from entering. Inspectors should wear safety helmets, reflective vests, protective shoes, and safety goggles (during tool inspection and trial milling). 
3. Identity and Label Verification 
Host rack number (PIN code): Make an imprint and compare it with the entire machine's nameplate and factory documents to ensure there has been no grinding or tampering. 
Machine nameplate: Verify the manufacturer, model, factory serial number, manufacturing date, working weight, rated power, milling width, etc. 
Engine nameplate: Verify the model, serial number, power, and emission stage. 
Machining rotor nameplate: Records rotor model, milling width, number of cutter heads, and factory serial number. 
Main hydraulic component nameplates: Model and serial number records of the walking pump/motor, milling drum drive pump/motor, and conveying pump/motor. Any discrepancies should be immediately upgraded to a serious non-conformance item. 
4. Inspection of Racks and Structural Components 
Chassis frame: From the milling bin area to the rear engine compartment, use a strong flashlight to inspect closely. Key areas: milling bin frame, rotor bearing seat mounting base, front steering column / rear drive axle connection area, material transfer machine mounting base. Check for weld cracks, base material deformation, and repair weld marks. 
Machining of the silo cover shell: Inspection of the wear condition of the wear-resistant liners around the silo - If the liners are worn through, it will expose the structure of the silo and cause wear. The hinges and locking devices of the silo cover are in good condition. 
Side plates/sliding shoes: There is no leakage in the up-and-down oil cylinders of the left and right side plates. The measurement of the wear of the wear-resistant base plate at the bottom of the side plates' sliding shoes - the wear of the sliding shoes directly affects the benchmark accuracy of the milling depth. 
Weight counterweight: Fixed bolts are securely fastened, and the appearance is intact. 
5. Inspection of the rotor assembly by milling - Core working device 
This is the most valuable and technologically advanced assembly of the milling machine. 
Rotor appearance: After milling the rear door of the hopper or opening the hopper cover, check if there are any weld cracks on the surface of the rotor drum. Examine each knife holder (knife clamp) visually - for any detachment, fracture, or deformation. Check for any cracks at the welding root of the knife holder and the rotor drum. 
Cutting tools (tool heads): The tool heads of the milling machine are the most consumable and easily damaged parts. Using a vernier caliper or a dedicated template, measure the remaining height of the hard alloy heads of multiple representative tool heads and compare them with the specifications of new parts. When the wear of the tool head exceeds the limit (usually when the alloy head is worn down by 2/3 or the steel body begins to wear), it is necessary to replace the tool head. The count of missing or abnormally broken tool heads - if there are too many missing ones, it indicates poor daily maintenance or that the machine has been impacted by foreign objects such as steel bars. 
Rotational flexibility of the cutting tools: Use specialized tools or manually rotate each cutting head to ensure that the cutting head can freely rotate within the tool holder. If the cutting head gets stuck, it will cause uneven wear and a significant increase in milling resistance. 
Rotor bearing housing: Both end bearing housing shells are free from cracks, the grease injection pipeline is in good condition, and during the operation of the bearings, monitoring is conducted (executed during the dynamic test). 
Grinding drum drive: The condition of the coupling between the hydraulic motor/减速器 and the rotor. The reducer housing shows no oil leakage and there is no abnormal noise during operation. 
6. Inspection of the feeding system - The "vein" of continuous operation 
Primary conveying belt (aggregate belt): Located at the bottom of the milling bin. Check the surface wear, edge tears, and integrity of the vulcanized joints of the belt through the observation port or by removing the cover plate. Also check the wear condition of the scrapers/stopper plates. 
Secondary conveying belt (discharge belt): Overall appearance inspection. Belt tension - the deflection at the middle of the belt when pressed should be within the specified range. Belt deviation - observe whether the belt runs centrally during no-load operation. Severe deviation will lead to increased wear on the edges of the belt and structural fatigue. 
Conveyor belt idler rollers: Check one by one whether the rollers rotate flexibly, whether there is any abnormal noise from the bearings (listen during dynamic testing), and whether the surface of the rollers is uniformly worn. Idlers that are stuck and do not rotate will cause the bottom of the conveyor belt to wear out rapidly. 
Discharge port: Inspection of the wear-resistant liner at the discharge port shows no wear. The function of the discharge angle adjustment cylinder is normal. 
Conveyor system folding/swinging mechanism: Some milling machines' conveyors have the function of hydraulic folding or left-right swinging, and the operation has been verified. 
7. Inspection of the walking system 
Track-type walking system 
Track plates: Inspection of wear and damage on rubber track plates/steel track plates. Record any detachment or loss of rubber blocks. 
Chain track / Drive wheel / Guide wheel / Bearing wheel: Chain track pitch measurement (method is the same as for bulldozers). Wear on the drive wheel surface. The tensioning device of the guide wheel functions normally. The bearing wheel rotates flexibly and there is no oil leakage. 
Wheeled walking system 
Tires: Inspection of worn-out solid tires or pneumatic tires, no cuts on the tire sidewalls. Wheel bolts are securely fastened. 
Drive axle for walking / Steering axle: The housing shows no oil leakage. Inspection of the steering cylinder and tie rod ball joint clearance. 
Travel performance: The milling machine usually has low-speed operation mode for work and high-speed mode for moving, and the functions of each gear need to be verified through dynamic testing. The synchronization and response sensitivity of left and right steering. 
8. Depth and leveling control system inspection - Ensuring the accuracy of milling and grinding 
This is the core control system that distinguishes the milling machine from ordinary earthmoving machinery. 
Side guard plate/slipper system: Independent hydraulic lifting control function for the left and right side plates. The wear condition of the slipper directly affects the zero reference point of the milling depth. 
Depth control sensors: The appearance of the wired type/ultrasonic type/laser type depth sensors is intact, and the cable connections are tight. The sensor brackets show no deformation or looseness. 
Automatic leveling system: The controller self-check passed without any fault codes. Set a certain depth, manually raise and lower the milling drum, and observe the real-time changes and responses of the depth display instrument. 
Slope sensor (if equipped): The slope control system self-check passed, and the sensor is installed firmly. 
Floating control of the rear tailgate: The floating function of the rear tailgate (scraping plate) - it should automatically align with the milled road surface after milling, and scrape the remaining material into the conveying belt. Inspection of the tailgate lifting cylinder and linkage mechanism. 
9. Inspection of Power Transmission System 
Engine: Cold and hot start performance, stable idle speed, sensitive response to sudden acceleration, normal smoke color. Power output stability at full load (milling operation speed). External leakage checks. High-pressure common rail, turbocharged intercooled system status. Inspection of air filter and pre-filter (extremely dusty working environment for milling machines). 
Power transmission between the engine and the milling drum: Usually, it is driven by a hydraulic closed-loop circuit or a mechanical belt drive (for small machines). For hydraulic drive type, check the hydraulic pipelines; for mechanical drive type, inspect the wear, tension, and condition of the transmission belt and pulleys. 
Auxiliary engine (if any): Some large milling machines are equipped with an independent auxiliary engine to drive the feeding system. It should be inspected according to the same standards. 
10. Hydraulic System Inspection 
Hydraulic oil tank: The liquid level is within the scale range, and the respirator is clean. The oil color is normal, without emulsification or discoloration. If possible, take samples. 
Each hydraulic circuit: 
Grinding drum drive circuit (high-pressure closed system): There is no leakage in the pump and motor housings, and the pipelines are free from aging. 
Walking drive circuit: Status of the pump and walking motor. 
Auxiliary circuits (conveyor belt, side panel lifting, rear tailgate, etc.): All control valve groups and hydraulic cylinders show no leakage and operate normally. 
Piping and joints: Check the aging and cracking of all hoses on the entire machine. Remove and inspect the hydraulic hoses and cables that pass through the internal part of the milling chamber. Due to being exposed to high temperatures, high vibrations, and stone impacts for a long time, this area is prone to failures. It is essential to thoroughly check whether the protective covers are damaged and whether the outer rubber of the hoses has worn out. 
11. Water sprinkling for dust suppression and electrical system inspection 
Sprinkler system: The appearance of the water tank shows no damage or leakage. The water pump is operating normally and the water pressure meets the standard. The water spray from each nozzle (inside the milling bin and at the discharge port) is in a misting state - the number of blocked nozzles must be recorded. The waterway filter is clean. 
Electrical system: The functions of the control console instruments are normal, and the emergency shutdown button is effective. All lighting and warning lights are in good condition. 
Video surveillance (if equipped): The rearview/feeding monitoring camera images are clear. 
12. Dynamic operation testing and safety device verification 
On the safety site, after starting the engine and reaching the operating temperature, proceed as follows: 
No-load operation: Raise the idle speed to the operating speed, engage the milling rotor, and perform no-load operation. Listen for any abnormal noises from the rotor bearings and the hydraulic pump/motor. Check if the milling rotor speed reaches the nominal value. 
Conveyor belt operation: The first and second conveyor belts have been started respectively. The operation is smooth without deviation. Each idler roller rotates flexibly without any abnormal noise. 
Walking test: Switch between forward and backward gears, and perform steering operations. When the crawler milling machine turns on the spot, there should be no abnormal resistance or abnormal noise. 
Functionality and operation verification: All functions such as the lifting/lowering of the milling drum, the elevation of the side panels, the floating of the rear tailgate, and the folding/swinging of the feeding machine are fully executed. 
Safety devices: Emergency shutdown button (located on the control console and throughout the machine body) will cause the engine to immediately shut down upon being pressed. Anti-reverse device for the feeding belt. Safety interlock switch for the maintenance door of the milling bin (if present) - when opening the door, the milling rotor should be unable to start or automatically shut down. 
13. Precision test for trial milling (optional, strongly recommended) 
Under permitted conditions, conduct actual milling over a short distance (10 - 20 meters) to verify: 
Consistency of milling depth: At the starting point, midpoint, and end point of the test milling section, measure the actual milling depth using a depth gauge or level instrument and compare it with the set depth. The deviation at multiple points should be within the manufacturer's accuracy range (typically ±3mm). 
Horizontal flatness: Use a 2m ruler to cross-connect the milling surface horizontally, and measure the maximum gap between the ruler and the milling surface with a feeler gauge. Record the maximum value as the horizontal flatness indicator. 
Roughening texture: The texture of the machined surface should be uniform and continuous upon visual inspection, without obvious tool chipping or steps. The horizontal joints should be smooth. 
Feed material efficiency: Observe whether the crushed material is smoothly discharged from the first and second conveyor belts, and check for any blockages or spillage. 
After the trial milling, immediately recheck the wear of the cutting tools and the condition of the feeding belt. 
14. Oil Sample Collection and Analysis (Suggested) 
Extract samples of engine oil, hydraulic oil, and the oil in the milling drum drive circuit (if it is an independent circuit) for testing. 
The milling drum drive circuit is subjected to extremely high impact loads. The abnormal increase in metal elements in the oil indicates damage to the pump or motor internals. 
Excessive contamination of the hydraulic oil may damage the precision proportional valve (used in the automatic leveling system). 
15. Report Output 
Summarize all inspection items, measurement data (such as tool wear, shoe wear, milling surface flatness, belt condition, etc.), dynamic tests and trial milling records, as well as oil analysis reports. Mark the qualified items, observation items, general defects and serious defects. Form a "Third-Party Asphalt Milling Machine Inspection Report" that includes photos with PIN codes, photos of tool wear measurements, photos of trial milling surface flatness measurements, and detailed special photos of the conveying belt condition. Deliver this report to the client. 
III. Important Notes
Grinding rotor - The single component with the highest value for the entire machine 
The tool assembly of the milling rotor (including the cutter head, cutter seat, and the welded cutter seat) is the most costly component of the milling machine and is also the best indicator for assessing the usage intensity and maintenance level of the equipment. The inspector must: 
Count the number of missing shanks, and measure the remaining height of the alloy heads of at least 20 representative shanks. 
Check for any fractures or uneven wear on the knife holder - a fractured knife holder usually indicates that it has been subjected to severe foreign object impact or improper operation (such as turning without lifting the rotor). 
The high rate of knife heads getting stuck indicates that the daily lubrication and maintenance are not adequate, which will result in rough textures on the milling surface and a significant increase in fuel consumption. 
The cost of replacing the rotor bearing housing is high. During no-load operation, use a stethoscope or audio detection instrument to listen for any periodic abnormal sounds from the bearings - any "rumbling" or "clattering" noises must be flagged and a further inspection recommended. 
Conveying belt system - A high-risk area for shutdown failures 
Once the conveying belt breaks at the working surface, the repair process will be time-consuming and costly. During the inspection, particular attention should be paid to the depth of wear on the belt edges, the quality of the joint vulcanization, and whether there are any penetrating damages. 
The jammed idler rollers that do not rotate are the main cause of abnormal wear of the belt. During inspection, the belt must be running. Listen to each idler roller one by one. Any idler roller that does not rotate should be classified as "must be replaced". 
The belt deviates - Observe it in both no-load and load conditions (such as during trial milling). The reasons for the deviation could be misalignment of the idler rollers, uneven belt tension, or deformation of the belt itself. Each issue needs to be investigated separately. 
Slide shoes and side plates - The "reference plane" for the accuracy of milling depth 
The depth control of the milling machine relies on the bottom surface of the side plate sliding shoe as the mechanical reference. Once the wear of the wear-resistant bottom plate at the bottom of the sliding shoe is uneven or deformed, it will cause inconsistent milling depths on the left and right, and result in steps on the milling surface. During inspection, a straightedge is placed tightly against the bottom surface of the sliding shoe to check the flatness and the amount of wear. 
The internal leakage of the side panel lifting cylinder can cause the side panel to gradually sink during the milling process, thereby changing the milling depth. The pressure holding test can be conducted in a static state: raise the side panel to a certain height, mark the position, and leave it stationary for 5 minutes to observe the sinking amount. 
Hydraulic System - Hazards in High-Temperature and High-Dust Environments 
The hydraulic system of the milling machine operates at full load continuously, resulting in extremely high oil temperatures. The hydraulic hoses age at a faster rate than those of ordinary construction machinery. The hoses inside the milling bin are exposed to the harsh conditions of stone impact and high-temperature radiation. The inspectors must enter the milling bin (confirming that the engine has been turned off and locked), and conduct a thorough inspection of each hose, recording those with damaged protective covers but intact outer rubber as If the outer rubber is worn through and the steel wire layer is exposed, it will be immediately classified as a mandatory replacement item. 
The dust removal and water spraying system should not be overlooked. 
Clogging of the nozzles is the most common maintenance oversight for the milling machine. Check the water output of each nozzle one by one. If the number of clogged nozzles exceeds a certain proportion of the total (such as 20%), it indicates that the daily maintenance is not adequate and it should be recorded in the report. Insufficient water supply or failure of the sprinkler system will lead to excessive dust in the milling operation, violating environmental regulations and endangering the health of the operators. 
Trial milling - the most direct test and the most convincing one 
If conditions permit, conducting a 10-20 meter trial milling is the most effective way to verify the actual operational capabilities of the milling machine. One milling surface can simultaneously be used to test: 
Milling depth accuracy (measured with a depth gauge) 
Lateral flatness (measured with a 2-meter ruler) 
Tool status (milling texture) 
Feed system function 
Engine power adequacy 
After the trial milling, immediately recheck the cutting tools, belts and hydraulic temperatures. Problems that could not be detected during the no-load test can be identified. The photos of the flatness measurement of the trial-milled surface are attached in the third-party inspection report, which is highly persuasive to the client. 
Key points of the photo archive 
The PIN code, the engraved nameplate of the milling rotor, the measurement of the alloy head wear of the cutter head (the reading is displayed by a caliper), the status of the cutter base, the measurement of the wear on the bottom surface of the sliding shoe, the special shot of the edge wear of the conveying belt, the measurement process of the flatness of the test milling surface (with a 2m straight ruler and a feeler gauge for reading in the same frame), the status of the idler rollers of the conveying belt, and the special shot of the wear on the hydraulic hose, all of these need to be independently and clearly archived.