As the single-machine length, conveying capacity, and lifting height of belt conveyor systems increase, the required strength of the conveyor belt becomes increasingly higher. To further reduce the strength requirements of the conveyor belt or the specifications of the drive unit, intermediate drives must be arranged on the conveyor line. Existing drive methods include tire drive, linear motor drive, roller drive, intermediate roller drive, and line friction drive.

This paper briefly introduces the research progress of linear motor drive, roller drive, and tire drive, focusing on the development of line friction drive belt conveyor systems, providing a reference for the selection and design of future multi-drive belt conveyors.

1 Drive Methods of Belt Conveyors

1.1 Tire Drive

The main function of tires is to transmit driving force, braking force, and steering force between the vehicle and the road surface, to achieve the functions of vehicle driving, braking, and steering. The tire drive unit consists of Four tires form a drive unit, as shown in Figure 1. Two pressurizing tires are pressed on the left and right sides above the conveyor belt, and the other two drive tires are on the left and right sides below the conveyor belt, pressing the conveyor belt upwards. Rubber-coated flat rollers are used under the return branch of the conveyor belt to press it against the drive tires. Due to the tight pressure of the tires on the conveyor belt, both the conveyor belt and the tires will deform, forming friction between them. The rotation of the drive tires generates torque, causing friction in the contact area, driving the conveyor belt to move. To improve the contact of the conveyor belt, the edge of the conveyor belt needs to enhance the steel wire rope stiffness.

Figure1 Tire Drive Unit

1. Pressurizing tire; 2. Drive tire; 3. Transmission gear; 4. Return roller; 5. Return conveyor belt; 6. Conveyor belt edge with enhanced stiffness steel wire rope.

The main problems of tire drive are: the contact area between the tire and the conveyor belt occupies a certain bandwidth, reducing the carrying width of the conveyor belt, and the tire needs sufficient adhesion to transmit the driving force, requiring sufficient pressure, which limits the driving force.

1.2 Linear Motor Drive

The working principle of a linear motor is the same as that of a rotary motor. One side, evolved from the stator, is called the primary, and the other side, evolved from the rotor, is called the secondary. The maglev train is the most typical example of the practical application of linear motors.

Hannover University of Technology IFA established a 5 m long linear motor belt conveyor test bench. The linear motor can achieve a completely contactless way of transmitting force, directly providing linear motion without rotating transmission components. Currently, it is only considered for application in light-duty belt conveyors. This drive has the characteristics of compact structure and reduced conveyor belt tension. Linear motor drive is a research hotspot internationally, and its practical application in general, long-distance belt conveyors is expected in the future. A prototype of a light-duty conveyor belt with direct linear drive is shown in Figure 2.

Figure2 Prototype of Light-Duty Conveyor Belt with Direct Linear Drive

1. Conveyor belt; 2. Direct linear drive; 3. Deflecting roller; 4. Control device.

1.3 Roller Drive

Roller drive is a drive method proposed by Polig Heckel Bleichert (PHB) company, which changes the centralized roller drive to a drive distributed on the rollers, similar to how a train distributes the drive of the locomotive to all the axles. The drive roller is an intermediate drive device that transmits the motor torque to the conveyor belt through the roller body. To simplify the drive unit, a direct drive method of a synchronous motor is proposed. The advantages of this drive method are small traction force and small starting power; when the drive components are stuck or frozen, the conveyor belt tension is reduced, and a medium-strength conveyor belt can be used. The disadvantages are that there is waste in the distributed power allocation; a large number of drive electric rollers not only increase investment but also increase management complexity. Therefore, this drive method only remains in the research stage and has not been put into practical application.

Hannover University ITA uses roller drive units to study the driving force allocation problem of this type of conveyor using discrete optimization methods to reduce energy consumption. The roller drive unit is shown in Figure 3.

Figure3 Roller Drive Unit

1.4 Intermediate Roller Drive

Although the concept of intermediate roller assist drive appeared earlier, itdid not become widely adopted until the mid-1980s. The unloading roller assist drive system can extend the length of the conveyor in the original conveyor system without installing a new conveyor belt. This drive method requires dynamic control of the input power to achieve power balance among the drives, and requires the use of viscous clutches, frequency converters, speed-regulating fluid couplings, etc., which can achieve regenerative braking control of the conveyor. Some companies have already serialized this type of drive, and its layout is shown in Figure 4.

Figure4 Layout of Unloading Roller Assist Drive

1. First drive roller; 2. Force sensor; 3. Second drive roller.

The advantages of this drive are that it can reduce the conveyor belt tension and effectively reduce the strength grade of the conveyor belt; the“unloading” transfer on the conveyor belt is not as difficult as the traditional transfer between two conveyors, because they are always in the same direction, and the unloading belt speed and the receiving conveyor belt speed are always the same; scattering and sweeping are minimal.

The disadvantages of this drive are that a well-functioning system needs tension control, and due to the unloading roller unloading onto the same conveyor belt, there is a certain energy loss due to the material drop height difference, especially when conveying highly abrasive materials, it will increase the wear and impact of the conveyor belt, leading to a shorter lifespan of the conveyor belt; when conveying highly viscous materials, it may cause blockage at the transfer point, requiring careful handling.

Intermediate roller assist drive is the most widely used, replacing other intermediate drive methods in most applications, including the line friction drive method discussed later.

1.5 Line Friction Assist Drive

1.5.1 Development Status Abroad

The linear friction drive belt conveyor is developed based on the general belt conveyor. In order to reduce the tension of the conveyor belt, several drive belts are set along the conveyor line as the carrying and traction mechanism. The drive belt and the carrying belt contact the friction belt to transfer the driving force on the drive belt to the carrying conveyor belt. Its transmission mechanism is friction transmission driving force, and the drive belt has sufficient length, generally called“Linear friction drive”. The system composed of the carrying conveyor belt and other components is called the “main machine”, and the drive belt system can also be regarded as a separate conveyor, called the “auxiliary machine”. Linear friction drive cancels the intermediate transfer of materials, which can reduce the degradation of materials, avoid the problem of material spillage during transfer, and can increase the conveying capacity of the conveyor or extend the system without changing the original system's drive and conveyor belt specifications. The linear friction drive belt conveyor is shown in Figure 5.

Figure5 Linear Friction Intermediate Drive

1. Friction drive device; 2. Carrying belt; 3. Drive belt; 4. Roller.

The linear friction method was first proposed by the Soviet Union in 1920, and was studied by Dowty-McKee in the UK, and Germany conducted experimental research on it in the 1970s (Krupp). In 1973, Germany's first TT drive was used in

Fortuna open-pit mine, with a total drive power of 9 × 430 kW, and the TT driver is 3 × 430 kW. HESE developed this product in 1975, with a total application of more than 150 units. HESE initiated bankruptcy proceedings in April 2014, and VOITH took over its conveyor technology business in 2015. Prosper Hanier Belt NW2 (2004) is a typical linear friction drive system of HESE company. The main drive power of this system is 3 × 500 kW; 3 TT drives are set, with a power of 3 × 500 kW; PVG2500 conveyor belt is used. If the traditional solution of end drive unit is used, the conveyor belt to be used is ST6300, and the diameter of the drive roller is 1600 mm.

In 2009, HESE provided the No. 3 main shaft linear friction belt conveyor for Huainan Mining Group Xin Zhuangzi Mine, and transformed the original conveyor of the mine to improve the conveying capacity. The configuration and layout of the main and auxiliary machines are shown in Figure 6. The total length of the main machine is 1440 m, with three-machine drive, the drive method is motor + CST, the drive power is 3 × 560 kW, the bandwidth is 1200 mm, the strength is ST3150, the drive roller diameter is 1430 mm, the transmission device uses CST 630 ks, the belt speed is 4 m/s, and the tension is adjusted by tail weight; the auxiliary machine length is 200 m, with single-machine drive, the drive method is motor + CST, the drive power is 1 × 560 kW, the bandwidth is 1200 mm, the strength is ST1250, the drive roller diameter is 1430 mm, the belt speed is 4 m/s, and the tension is adjusted by tail hydraulic tension. The head of the auxiliary machine is 580 m away from the head of the main machine. The conveying capacity of the main conveyor is increased from 800 t/h to 1200 t/h, an increase of 50%, while reducing the total tension and elongation of the conveying system.

Figure6 Xin Zhuangzi Mine No. 3 Main Shaft Linear Friction Belt Conveyor

1. Main machine drive roller; 2. Main machine; 3. Main machine conveyor belt; 4. Auxiliary machine drive roller; 5. Auxiliary machine; 6. Auxiliary machine conveyor belt; 7. Auxiliary machine tail roller; 8. Main machine tail roller.

The All-Union Institute of Lifting and Transportation Machinery adopted the modular design concept and manufactured the first industrial prototype of linear friction drive in the Soviet Union. This equipment was installed in the open-air yard of a building materials joint venture sand and gravel field and underwent comprehensive tests. The company cooperated with Takelaf to develop multiple linear friction belt conveyor systems.

(1) Verify the working performance of the multi-drive belt conveyor of the friction drive device under different working conditions (no-load, full-load, intermittent loading) and in different seasons (spring, summer, autumn, winter).

(2) Verify the structural design of each component of the conveyor and the possibility of mass production of the conveyor according to this design.

(3) Study the traction performance and the collaborative work of each intermediate drive device.

(4) Study different control systems of the conveyor's electric drive device and select the optimal scheme.

1.5.2 Development Status Domestically

Shanghai Lifting and Transportation Plant has conducted a series of studies on the linear friction drive belt conveyor model and given the possible layout methods of the linear friction drive belt conveyor, as shown in Figure7. Among them, Figures 7(b) and 7(c) have been put into practical application. Usually, linear friction drive is used as an auxiliary means. The situation in Figure 7(a) is not a practically applicable method, and the method in Figure 7(d) has been proved to be inapplicable, because this scheme is difficult to guarantee the reasonable allocation of the drive belt in the upper and lower branches.

Figure7 Possible Layout Methods of Linear Friction Drive Belt Conveyor

1. Drive belt; 2. Carrying belt; 3. Drive auxiliary machine; 4. Head roller; 5. Booster roller.

Friction drive belt conveyors were first used domestically in 1974 in Shanghai Port Zone 7. This conveyor has a bandwidth of 1000 mm, a length of 410 m, an open-air horizontal layout, can operate forward and reverse, and a belt speed of 3.15 m/s. It is used with the QD3025 bucket wheel machine, and the coal conveying capacity is 1000 t/h. It consists of a head (or tail) drive roller and 3 15 m long linear friction drives installed in the middle. The total installed power of the equipment is 7 × 30 kW drive, and the heads of the 3 linear friction drives are respectively located at 100, 200, and 300 m from the tail (110, 210, and 310 m from the head). Since the beginning of 1978, after more than 2 years of production and use, it has withstood the test of -10 to +38 ℃ temperature and strong winds, heavy rain, and heavy snow, with an annual coal transportation of nearly 2 million tons and a usage time of more than 4000 h.

After the 1990s, more than ten linear friction conveying systems have been put into operation in China, mainly used for extension and increasing the tonnage.

2 Comparative Analysis of Multi-Drive Belt Conveyors

Various multi-drive belt conveyor methods are compared in terms of application quantity, reliability, expansion and extension applicability, development potential, and investment, with the results shown in the table1. Due to its low reliability, tire intermediate drive has no new applications. Roller drive still has the possibility of application. The linear motor drive method has development potential due to its application in maglev trains, and further development is needed. The discharge roller drive is the mainstream of the current multi-drive belt conveyor system. Although the line friction assist drive method has a high investment cost, it has advantages in system expansion and extension and is widely used.

Table1 Comparison of Multi-Drive Belt Conveyors

3 Conclusion

(1) The intermediate roller drive method is the preferred method for intermediate drives, but it may increase wear on the conveyor belt and material degradation.

(2) Although the line friction drive method is more expensive, it can reduce conveyor belt wear and material degradation, making it an option for multi-drive belt conveyors.

(3) Although there are currently few application examples of linear motor drive methods, with technological advancements, it will become an optional method for multi-drive belt conveyors.