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The road milling machine is one of the main types of machinery used for the maintenance of asphalt pavements; it is primarily employed to remove problems such as bulges, oil waves, ripples, and ruts from the asphalt concrete surface layers on highways, urban roads, and similar surfaces. Using a road milling machine to mill away the damaged old pavement layer and then laying a new layer is the most economical modern method of maintenance. It is widely used in the maintenance of urban municipal roads and highways due to its high efficiency, simple construction process, easy control over the milling depth, convenient and flexible operation, good mobility, and the ability to recycle the milled old material directly. I. Classification and Application Areas Based on the milling method, planers can be divided into cold milling type and hot milling type. The cold milling configuration has a high power level; the cutting tools wear out quickly. The particle size of the cut material is uniform, and a water spraying system can be installed for watering. It is widely used, and the products are now available in various series ； The hot-milling type has a more complex structure due to the addition of a heating device, and is generally used for road surface regeneration tasks. Based on the rotation direction of the milling rotor, it can be divided into forward milling and backward milling types. When the rotation direction of the rotor is the same as that of the wheels while the paver moves, it is called forward milling; otherwise, it is called reverse milling. Based on their structural features, they are divided into wheeled and tracked types. It has good mobility on wheels and is easy to relocate, making it particularly suitable for small and medium-sized roadwork tasks ； Caterpillar-type machines are mostly large planers with a milling width of 2000 mm or more, equipped with old material recycling systems, and suitable for large-scale road surface renewal projects. Based on the position of the milling rotor, it can be divided into rear-mounted, mid-mounted, and coaxial with the rear axle types. The rear suspension type means that the milling rotor is suspended at the rear of the rear axle ； The mid-mounted type means that the rotor is milled between the front and rear axles ； The rear axle coaxial type features the milling rotor arranged coaxially with the rear axle. Based on the working width of the milling rotor, they can be divided into three types: small, medium, and large. The milling width of small planers ranges from 300 to 800 mm. Mechanical transmission is commonly used for the milling rotor, and these planers are mainly suitable for road repair projects with a construction area of less than 100 m2 ； The milling width of medium-sized planers ranges from 1000 to 2000 mm, and the drive mechanism for the milling rotor is usually hydraulic ； The milling width of large milling machines is over 2000 mm; they are generally used in combination with other machinery to form a complete set of equipment for road surface regeneration and repair. The drive system for their milling rotors is usually hydraulic. Based on the transmission method, they are divided into mechanical and hydraulic types. Mechanical systems are reliable in operation, easy to maintain, have high transmission efficiency, and low manufacturing costs. However, they have a complex structure, are not convenient to operate, have lower work efficiency, and provide less traction; they are suitable for small-scale road maintenance tasks that involve shallow cutting ； Hydraulic systems are compact in structure, easy to operate, highly mobile, and offer strong traction; however, they have high manufacturing costs and are difficult to maintain. They are suitable for medium- to large-scale road maintenance tasks that involve deep cutting. II. Application Features 1. Using a planer to mill the road surface allows for rapid and effective treatment of surface defects, helping to keep the surface smooth. 2. The resurfacing of roads using the milling process allows the original level elevation of the pavement to be maintained. The milling process allows the damaged road surface to be removed, with new material filling in the resulting space; after compaction, this new surface reaches the same level as the original one, thus maintaining the original elevation of the road surface. This prevents the road traffic passing over viaducts or interchanges from exerting impact forces on the bridge structure, and also ensures that the elevation of the bridges remains unchanged. 3. Ensure a good bond between the old and new pavement materials, thereby extending their service life. The milling process enables the sides and bottom of the filler pockets to be made even and of uniform depth, creating a toothed geometric surface that facilitates the bonding of old and new materials, thereby **increasing the service life of the renewed road surface**. 4. It facilitates the recycling of old pavement materials. Since the cutting depth can be controlled, the material milled out is not only clean but also in the form of regular small particles; it can be used to cover the site or stored for reuse without any further crushing process. **This reduces construction costs and also constitutes an environmental protection measure. III. Main Structure and Working Principle A typical milling machine consists of an engine, frame, milling rotor, milling depth adjustment mechanism, hydraulic components, aggregate conveying system, steering system, and braking system. The milling rotor is the main working component of a paver; it consists of a milling rotor shaft, cutter seats, and cutting heads, and it comes into direct contact with the road surface. It performs milling by means of its high-speed rotating cutters. The planer is equipped with an automatic leveling device; the rotor side cover is used as the milling reference surface, and the two positioning hydraulic cylinders are controlled to maintain a constant specified milling depth ； Its hydraulic system is used to drive the rotation of the milling rotor, the movement of the entire machine, and the operation of auxiliary devices. It is generally a multi-pump, independent closed-loop hydraulic system that operates without interference among the pumps, offering high reliability ； Some planers are equipped with tilt adjusters as needed, to control the tilt of the rotor ； Typically, large milling machines are equipped with a material collection system consisting of conveyor belts and collectors, which gathers the material produced by milling and transports it to a mobile carrier. The height of the conveying arm can be adjusted, and it can swing left and right to adjust the unloading position. When the specifications and models of milling machines vary, their structure and layout differ slightly, but their basic working principles remain the same or similar. Power transmission path of the planer: engine → hydraulic pump → hydraulic motor, hydraulic cylinder → working device. IV. Development Level and Trends 1. Domestic Level and Development Direction The production of planers in China started relatively late, with research and development beginning in the early 1980s. There are two main types of such machines: one is a simple cold planer created by installing a milling device on agricultural wheeled tractors; it has a simple structure and is suitable for milling asphalt pavements of moderate strength or lower ； The second type is the self-propelled cold milling machine, which has seen some development in the past two years. The performance of the main models available today is fully sufficient to meet the maintenance requirements of high-grade highways and municipal roads in China. Their favorable cost-performance ratio gives them a significant competitive advantage over imported products from abroad; however, there are still substantial gaps in terms of variety, technical standards, and auxiliary components. At present, domestic models draw more on the technology and experience of European planers, employing international-standard components for their power, hydraulic, and control systems; it can be said that their system configuration has reached an internationally advanced level. From the perspective of intellectual property, some domestic products such as the CM2000 produced by Shaanxi Construction Machinery Co., Ltd., as well as the CM1900 and CM1200 manufactured by Xi’an Hongda Traffic Technology Co., Ltd., not only possess complete intellectual property rights, but also have reached international advanced levels in terms of the design and manufacturing processes of their overall systems, control systems, and software. In future market competition, China’s planer manufacturers should leverage the following advantages: ① Take advantage of China’s membership in the WTO to adopt an internationalized design approach, strengthen technical cooperation on the global stage, keep up with international development levels in terms of technology, and strive for a larger share of the high-end product market ； ②Leverage local advantages to provide users with comprehensive services, from complete machine technology to spare parts supply, in a timely manner ； ③By taking advantage of the low labor costs in our country, we can reduce the cost of the entire product, gain a price advantage, attract more mid-to-low-end consumers, and expand the market ； ④Pay attention to developing proprietary technologies and product lines tailored to the characteristics of China’s roads, to serve road maintenance projects and promote technological progress in the country’s road transportation sector. 2. Overseas Levels and Development Trends: The development and practical use of road milling machines abroad have a long history, during which extensive experience has been accumulated. This has given rise to a European style represented by the products of German company Wittgen, and a North American style represented by the products of American companies such as RoadTech, CMI, and Caterpillar. Their working principles and processes are the same, and their engine capacities are roughly equivalent. The difference lies in the fact that the milling machines used in Europe employ a four-track propulsion system; they have a compact and sophisticated design, and make more use of electronic control technologies, particularly modern digital electronic network control systems ； In North America, all planers use a three-crawler propulsion system, featuring a rugged design and greater durability. The technology of road milling machines abroad has reached a high level, and it can be summarized by the following characteristics. ①The chassis of advanced and rational planing machines is primarily composed of a fully rigid frame and a four-wheel driving system, with hydrostatic transmission being the main method for both driving and steering. ②It fully utilizes the optimal milling power. The automatic hydraulic power regulator on the planer can control the feed speed of the milling rotor based on the hardness of the road surface material and the milling depth; this allows for automatic adjustment of both the rotational speed of the milling rotor and the moving speed of the planer, ensuring that the planer is always operating at its maximum capacity without experiencing any overload conditions. ③New planers with increased engine power and the same milling width have greater power, resulting in improved production efficiency. ④The new type of milling machine features a large milling depth, with each milling pass exceeding 300 mm, which makes it possible to mill the entire thickness of the pavement surface. ⑤Most milling rotors with good performance in cold planers have the milling head fixed to several semi-circular tiles; the milling width of the planer is adjusted by determining the number of tiles installed on the rotor. ⑥By simply loading the milled material onto the paver and attaching an aggregate conveying device at its rear, rapid collection can be achieved, and the milled material can be loaded into transport vehicles. The unloading height is adjusted via a hydraulic mechanism, and the conveyor belt can swing left and right by 40°~50°, thereby enabling loading at the roadside. ⑦It makes extensive use of advanced technologies such as all-wheel drive technology and mechatronic control technology, intelligent fault diagnosis and maintenance systems, precise automatic leveling systems, safety self-protection systems, and automatic power distribution systems. ⑧The large-capacity container water tanks and diesel tanks ensure that refilling with fuel and cooling water is required less frequently during machine operation, resulting in a shorter downtime. ⑨The engine and its peripheral components are designed in a modular fashion – the hydraulic pump, hydraulic valves, and cooling system are all mounted on the same chassis, and all solenoid valves are installed on a single distribution valve, which facilitates adjustment, inspection, and maintenance. V. Development Prospects With the rapid progress of road transportation, particularly through the rapid and healthy development during the Eighth and Ninth Five-Year Plans, significant improvements have been made in China’s road infrastructure. By the end of 2002, the total length of roads had reached 1.76 million km, of which 25,200 km were expressways and 240,000 km were high-grade roads of class two or higher. To maintain the safety, comfort, and speed of road traffic, carrying out timely, effective, and high-quality maintenance of these roads will be a key focus in future daily operations. With the development of China’s economy and the enhancement of its overall national strength, trends such as the urbanization of towns and villages, the expansion of small and medium-sized cities, and the transformation of large and medium-sized cities into garden-like areas are becoming increasingly evident. As a result, there are more and more projects for the renovation and expansion of urban roads. Under such circumstances, road maintenance work has become increasingly demanding, placing higher requirements on the quality of such maintenance tasks; modern mechanical methods for maintenance are thus essential. With favorable market prospects and strong demand from the market, an era of rapid development for road maintenance equipment has arrived. As one of the essential devices for the mechanized maintenance of roads, asphalt pavement milling machines will see an increasing demand in the domestic market. In fact, the mechanized maintenance method that uses milling machines as the main equipment has been adopted on many highways across the country, becoming a standard maintenance approach under the current circumstances. Large-scale road milling machines have also become one of the highway maintenance machines that need to be developed prioritarily in China during the 10th Five-Year Plan period. According to statistics on the development of roads in our country, there will be an increasing demand for various modern mechanical equipment for road maintenance in the future. With the arrival of the major maintenance period for highways, and as the road transportation industry and urban management agencies gain an understanding of modern maintenance methods, the demand for pavement milling machines will increase year by year, at a rapid pace. Although imported products from abroad have advanced performance, they are expensive and come with inconvenient maintenance services; therefore, it can be expected that domestically produced asphalt pavement planers will have broad market prospects. Domestic manufacturers should seize this opportunity to develop a complete range of domestic asphalt pavement planers as soon as possible, and continuously improve the performance and reliability of these products, in order to provide users with cost-effective, practical, and high-quality planers.