Product Introduction and Development Trend Analysis of Road Milling Machines

The road milling machine is one of the main types of machinery for asphalt pavement maintenance and construction. It is mainly used to remove lumps, oil waves, net patterns, ruts and other impurities from the asphalt concrete surface layer of highways, urban roads, etc.

Milling the damaged old pavement layer with a pavement milling machine and then laying a new surface layer is the most economical modern maintenance method. Due to its high working efficiency, simple construction process, easy control of milling depth, convenient and flexible operation, good mobility, and the direct recycling of milled old materials, it is widely used in the maintenance projects of urban municipal roads and expressways.

I. Classification and Application Scenarios

According to the milling form, milling machines can be divided into two types: cold milling type and hot milling type. Cold milling type configuration has a relatively large power, the tool wears out quickly, the particle size of the cutting material is uniform, and a water spraying device can be set up to spray water. It is widely used and the products have formed a series. The hot milling type has a more complex structure due to the addition of a heating device and is generally used for road regeneration operations.

According to the rotation direction of the milling rotor, it can be divided into two types: climb milling and reverse milling. When the rotation direction of the rotor is the same as that of the wheels when the milling machine is moving, it is a climb milling type; otherwise, it is a reverse milling type.

According to structural features, it is divided into two types: wheeled type and tracked type. Wheeled vehicles have good mobility and are easy to transfer, making them particularly suitable for medium and small-sized road operations. Crawler-type milling machines are mostly large milling machines with a milling width of over 2000mm, equipped with old material recycling devices, and are suitable for large-scale road surface regeneration projects.

According to the position of the milled rotor, it can be classified into rear-suspended type, mid-suspended type and coaxial type with the rear axle. The rear suspension type means that the milled rotor is suspended at the rear end of the rear axle. The mid-suspension type means that the rotor is milled between the front and rear axles. The rear axle coaxial type means that the milled rotor is arranged coaxially with the rear axle.

According to the operation width of milling the rotor, it can be classified into three types: small, medium and large. The milling width of a small milling machine is 300 to 800mm. The transmission mode of the milling rotor is mostly mechanical, and it is mainly suitable for road maintenance projects with a construction area of less than 100 square meters. The milling width of medium-sized milling machines is 1000 to 2000mm, and the transmission method of the milling rotor is mostly hydraulic. The milling width of large milling machines is over 2000mm. They are generally used in conjunction with other machinery to form complete sets of equipment for road regeneration and repair. The transmission mode of their milling rotors is mostly hydraulic.

It is classified into two types according to the transmission mode: mechanical and hydraulic. Mechanical operation is reliable, easy to maintain, has high transmission efficiency and low manufacturing cost. However, it has a complex structure, is not easy to operate, has relatively low working efficiency and small traction force. It is suitable for small-scale road maintenance operations with shallow cutting. The hydraulic type has a compact structure, is easy to operate, highly maneuverable and flexible, and has a relatively large tractive force. However, it has a high manufacturing cost and is difficult to maintain. It is suitable for medium and large-scale road maintenance operations with deep cutting.

Ii. Application Features

1. Using a milling machine to mill the road surface can quickly and effectively deal with road surface diseases and keep the road surface smooth.

2. The renovation project of the road can maintain the original horizontal elevation of the road surface by using milling technology. The milling process can remove the damaged road surface, fill the original space with new materials, and after compaction, it is at the same height as the original road surface, maintaining the original horizontal elevation of the road surface. This ensures that the road surface load passing through elevated Bridges or overpass culverts does not cause impact loads on the bridge body, and the passing elevation of the bridge and culvert remains unchanged.

3. Ensure a good combination of new and old pavement materials to increase their service life. The adoption of milling technology can make the sides and bottom of the filler pit neat and the depth uniform, forming a toothed geometric surface that is easy for new and old materials to combine, thereby greatly increasing the service life of the new pavement after renovation.

4. It is conducive to the recycling and reuse of old pavement materials. Because the cutting depth can be controlled, the milled material is not only clean but also in the form of regular small particles. It can be recycled on-site or in a fixed material yard without further crushing and processing, which greatly reduces the construction cost and is also an environmental protection measure.

Iii. Main Structure and Working Principle

A general milling machine is composed of an engine, frame, milling rotor, milling depth adjustment device, hydraulic components, aggregate conveying device, steering system and braking system, etc.

The milling rotor is the main working component of the milling machine. It is composed of the milling rotor shaft, the cutter seat and the cutter head, etc. It is in direct contact with the road surface and achieves the purpose of milling through the high-speed rotating milling cutter. The milling machine is equipped with an automatic leveling device. The milling rotor side cover is used as the milling reference surface to control two positioning hydraulic cylinders, keeping the given milling depth constant.

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, etc. Generally, it is a closed hydraulic system with multiple independent pumps. During operation, they do not interfere with each other and have high reliability.

Some milling machines are equipped with tilt adjusters as needed to control the tilt of the rotor. Generally, large milling machines are equipped with a material collection and conveying device composed of a conveyor belt and a collector. It can collect the milled bulk materials and convey them to the randomly moving transport vehicles. 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 structures and layouts may also differ slightly, but their basic working principles are the same or similar. The power transmission route of the milling machine: engine → hydraulic pump → hydraulic motor and hydraulic cylinder → working device. Iv. Development Level and Trends

1. Domestic level and development direction

The production of milling machines in China started relatively late, with research and development only beginning in the early 1980s. There are mainly two structural forms: one is to install a milling device on agricultural wheeled tractors to form a simple cold milling machine, which has a simple structure and is suitable for milling asphalt pavements of medium strength or below. The second type is self-propelled cold milling machines. In the past two years, they have made certain progress. The performance of the main models can fully meet the maintenance requirements of domestic high-grade highways and municipal roads. Compared with imported products from abroad, they have a good performance-to-price ratio and a significant competitive advantage. However, there are still considerable gaps in terms of variety and specifications, technical levels, and supporting parts.

At present, domestic models have drawn more on the technology and experience of European milling machines. They have adopted international matching in power, hydraulic and control systems. It can be said that the system configuration has reached the international advanced level. From the perspective of intellectual property rights, some domestic products, such as the CM2000 produced by Shaanxi Construction Machinery Co., Ltd. and the CM1900 and CM1200 produced by Xi 'an Hongda Transportation Technology Co., LTD., not only possess complete intellectual property rights, but also have reached the international advanced level in terms of the design and manufacturing process of the overall machine system, control system and its software.

In the future market competition, China's milling machine manufacturers should leverage the following advantages: ① Take advantage of the favorable condition that China has joined the WTO, establish an international design concept, strengthen international technological cooperation, keep pace with the international development level in technology, and strive for a larger high-end product market; Leverage the advantages of localization to provide users with timely and all-round services ranging from complete machine technology to parts supply. ③ By taking advantage of the low labor cost in our country, we can reduce the cost of the entire machine, leverage the price advantage, attract more mid-to-low-end users, and expand the market. ④ Pay attention to the research and development of proprietary technologies and product series that are in line with the characteristics of China's roads, serve highway maintenance projects, and promote the technological progress of China's highway transportation industry.

2. International levels and development trends

The development and engineering application of road milling machines abroad have a long history, accumulating rich experience and forming a European style represented by the products of Wirtgen Company in Germany and a North American style represented by the products of RoadTech Company, CMI Company and Caterpillar Company in the United States. Their working principles and processes are the same, and the installed capacity of the engines is basically the same. The difference lies in that the milling machines in Europe adopt a four-track walking mode, have a compact and exquisite external structure, and make more use of electronic control technology, especially the current digital electronic network control technology. In North America, milling machines all adopt a three-track walking method, featuring a rugged design and being more robust.

The technology of road milling machines abroad has reached a relatively high level, which can be summarized into the following characteristics.

① Advanced and reasonable chassis structure: The chassis of the milling machine is mainly composed of a fully rigid frame and a four-wheel traveling structure. The driving and steering methods are mainly hydrostatic transmission.

② Fully utilize the optimal milling power. The automatic hydraulic power regulator on the milling machine can control the feed speed of the milling rotor based on the hardness of the road surface material and the milling depth. It can automatically adjust the rotational speed of the milling rotor and the traveling speed of the milling machine, ensuring that the milling machine is always in a state of maximum power utilization and will not experience overload operation.

③ As the engine power increases, the power of the new type of milling machine with the same milling width is getting larger and larger, and the production efficiency is improving.

④ Large milling depth: The new type of milling machine can achieve a milling depth of over 300mm at a time, making it possible to mill the entire thickness of the pavement on the driving lane.

⑤ For most cold milling machines with well-performing milling rotors, the milling cutter head is fixed on several semi-circular discs, and the milling width of the milling machine is adjusted by the number of discs installed on the rotor.

⑥ Simple loading of milling materials: By hanging a material collection and conveying device at the rear of the milling machine, rapid material collection can be completed, and the milling materials can be loaded onto the transport vehicle. The unloading height can be adjusted through the hydraulic mechanism, and the conveyor belt can swing left and right by 40° to 50°, thus achieving roadside loading.

⑦. A large number of advanced technologies are adopted, 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.

⑧ The large-capacity container water tank and diesel tank have larger volumes. When the machine is working, the intervals between fuel and cooling water refilling are longer, and the standby time is shorter.

⑨ The modular design of the engine and its peripheral components - the hydraulic pump, hydraulic valves and cooling system - are all installed on the same base frame, and all solenoid valves are assembled on the same distribution valve, which is easy to adjust, inspect and maintain.

V. Development Prospects

With the rapid development of the highway transportation industry, especially during the "Eighth Five-Year Plan" and "Ninth Five-Year Plan" periods of rapid and healthy growth, the total amount of highway infrastructure in China has achieved a huge breakthrough. By the end of 2002, the total mileage of highways had reached 1.76 million kilometers, among which the mileage of expressways was 25,200 kilometers and that of high-grade highways of grade two and above was 240,000 kilometers. To ensure the safety, comfort and speed of road traffic, timely, effective and high-quality maintenance of them will be the focus of daily work in the future.

With the development of China's economy and the enhancement of its comprehensive national strength, the trends of urbanization in towns and townships, large-scale development in small and medium-sized cities, and gardenization in large and medium-sized cities are becoming increasingly evident, and the number of urban road expansion and renovation projects is also on the rise. Under such circumstances, highway maintenance work has become increasingly arduous, and higher requirements have been put forward for the quality of highway maintenance operations. Modern mechanical maintenance operation methods are imperative.

With a promising market outlook and strong demand, the era of great development for road maintenance equipment has arrived. As one of the indispensable devices for road mechanical maintenance, the demand for asphalt pavement milling machines in the domestic market will continue to increase.

In fact, the mechanized maintenance operation mode with milling machines as the main equipment has been adopted on many highways across the country and has become a standard maintenance mode under the current situation. Large-scale pavement milling machines have also become one of the high-grade highway maintenance machinery that China needs to focus on developing during the "Tenth Five-Year Plan" period.

According to the statistics of China's highway development, the demand for various modern mechanical equipment in highway maintenance will continue to increase in the future. With the arrival of the major overhaul period of expressways and the recognition of modern maintenance methods by the highway transportation industry and urban management departments, the demand for road milling machines will increase year by year and at a very fast pace.

Although imported products from abroad have advanced performance, they are expensive and inconvenient to maintain. Therefore, it can be expected that domestic asphalt pavement milling machines will have a relatively broad market prospect.

Domestic manufacturing enterprises should seize the opportunity to quickly form a complete series of domestic asphalt pavement milling machines, and continuously improve the performance and reliability of the products, providing users with economical, practical and high-quality milling machines.