The structure and features of the power shift transmission for vibratory rollers

Abstract

The working principle and control mode of the power shift transmission were analyzed, and the application, connection methods and usage precautions of the power shift transmission on the vibrating roller were introduced.

Key words

The working principle and application of the power shift transmission for road rollers

Hydraulic transmission vibratory rollers, due to their features of stepless speed change (within a certain speed range) and easy operation, are showing a trend of replacing mechanical transmission rollers. However, due to the poor quality of domestic hydraulic walking pumps and motors, while the prices of imported related pumps and motors are relatively high, the prices of hydraulic-driven rollers are relatively high. However, due to limited purchasing power, many domestic users have restricted the promotion and application of fully hydraulic-driven vibratory rollers. How to resolve the contradiction between ease of operation and price? Adopting a power shift transmission is a relatively good choice.

The structure and working principle of the power shift transmission

A power shift transmission is generally composed of a hydraulic torque converter and an integral box-type multi-speed power shift gearbox. It can drive both the front and rear axles and can be equipped with a locking clutch. Some transmissions can also be equipped with a one-way clutch on the guide wheel as required. According to the requirements of different operating procedures for construction machinery, boxes with different speed gears such as front three-to-three, front four-to-three, and front six-to-three can be selected. Due to the presence of several multi-plate wet clutches controlled by hydraulic pressure in the transmission, they can engage and disengage under load, thereby achieving power shifting. Its characteristic is that each transmission shaft is arranged in parallel, and the gears in the transmission are all in constant meshing transmission. Compared with planetary gear transmissions, it has the characteristics of large gear module and large load capacity on a single tooth surface. The power shift transmission is generally of a parallel shaft (fixed shaft) structure. The multi-plate friction clutch controlled by hydraulic pressure can engage and disengage under load, that is, it can shift gears without cutting off the power. All transmission gears are supported by rolling bearings, and the gears are in permanent meshing transmission with each other. The three-speed transmission is equipped with five multi-plate wet friction clutches, while the four-speed to six-speed transmission is also equipped with six multi-plate wet friction clutches. When shifting power, the clutch friction plates of the corresponding gear are pressed tightly by the piston pushed by the axial oil pressure, achieving the power engagement of that gear. During gear shifting, the clutch friction plates at this position return the piston under the action of the reset spring, disengaging the power of that gear.

2. Types and working principles of control systems

According to different control principles, it can be divided into two types of control methods: mechanical-hydraulic control valves and electro-hydraulic control valves.

2.1 Principle of Power Shifting under hydraulic Control

Inside the transmission, there is a gear pump, which is used to supply oil to the torque converter and the control valve. The gear pump is directly driven by the engine through the power take-off shaft. When shifting power, the shift (directional) control valve (one for the front and one for the rear) is adjusted. The oil passes through the oil suction filter (coarse filter) and the rotary filter (fine filter) in the oil circuit, and then the working pressure is limited by the control pressure valve (main pressure regulating valve), and then enters the control valve through the pressure control valve. The pressure oil through the control valve is directly connected to the clutch, pushing the corresponding piston to act and completing the power shift. The main function of the pressure control valve is to adjust the pressure-increasing characteristics of the clutch cylinder at the moment of gear shifting, that is, to instantly reduce the oil pressure during gear shifting and then restore it to the normal value after the gear shifting is completed. This can reduce the shock of gear shifting and improve the reliability and stability of gear shifting. The control pressure valve, while limiting the maximum oil pressure, sends the overflowing oil to the torque controller and the lubricating oil circuit.

2.2 Electro-Hydraulic Control Principle

The oil circuit of electro-hydraulic control is similar to that of mechanical-hydraulic control, except that four electromagnetic valves are used to replace two shift (directional) valves. When shifting power gears, the gear selector is manually operated to control the various solenoid valves connected to the selector, and the control valves on the transmission are manipulated to achieve the connection and disconnection of the control oil circuit, thus completing the power gear shifting operation. Due to the simplicity, convenience of electro-hydraulic control and the ease of cable connection and installation, most domestic power shift rollers currently adopt this control method.

2.3 Hydraulic circuit of vibratory roller with power shift transmission

In a closed hydraulic system, the pressure oil flowing out of the hydraulic pump does not return to the oil tank through multiple valves. The system pressure is regulated by the main relief valve, and the magnitude of the pressure varies with the variation of the load. Therefore, the closed circuit is more advanced than the open circuit. Compared with open circuits, the main advantages are reflected in small flow loss and low heat generation. When the machine operates slightly or semi-, the performance difference becomes even more obvious. Therefore, for vibratory rollers with a power shift transmission, it is advisable to use a closed hydraulic circuit composed of a plunger pump and a motor. The main oil pump and the rotary oil pump (this pump is mostly a gear pump) are installed on the diesel engine. The main oil pump is used to supply oil to the vibration system. The hydraulic oil is cooled by the vibration valve and the vibration motor and then returns to the main oil pump. The steering pump supplies oil to the steering system. The oil enters the steering hydraulic cylinder through the steering valve, is cooled by the cooler, and then flows back to the hydraulic oil tank. To achieve lubrication of the transmission, a lubrication pump can be installed in series before the main hydraulic pump. Meanwhile, to ensure the working quality and reliability of the hydraulic system and enhance the excitation force of vibration compaction, imported parts should be used for pumps and motors as much as possible. Available brands include SAO, Rexroth, Linde, Eaton, etc. Considering various factors such as the overall price of the machine and market demand, in order to reduce the cost of mechanical transmission rollers, the rollers equipped with power shift transmissions produced by the major domestic roller manufacturers at present all adopt the rear-wheel single-drive type. Therefore, the transmission should also be of the single-drive type.

2.4 Connection method between the transmission and the diesel engine

For construction machinery such as motor graders, garbage compactors and loaders that operate under non-uniform conditions, due to the large variations in load, hydraulic torque converters must be used to meet the requirement that the output torque automatically changes with the load. When the above-mentioned equipment adopts a power shift transmission that integrates the torque converter and the transmission, the connection between the torque converter and the engine can be a direct connection, that is, by using the diaphragm to connect with the flywheel housing. If there are special installation requirements, a separated connection can also be adopted, that is, the torque converter and the engine can be connected by flanges and universal joints. For some construction machinery such as road rollers that are basically in uniform motion, since the requirement for the variation of power torque output is not high. Therefore, a torque converter can be omitted, and a power shift transmission can be directly adopted. This way, while meeting the power shift requirements of the equipment, the procurement cost of the transmission can be effectively reduced. From the perspective of practical application, for the connection between the transmission and the diesel engine, the author suggests that the connection structure as shown in Figure 1 should be considered as much as possible. Diesel engine 1 is first connected to the clutch assembly 2, and then connected to the power transmission 5 through an elastic coupling 4. After the transmission 5 changes speed, the power is output through the power output shaft end 6. After passing through the drive shaft, the power is transmitted to the drive axle to achieve the rear wheel drive of the roller.

1- Diesel engine; 2- Clutch 3- Clutch housing; 4- Elastic coupling; 5- Power Shift transmission 6- Power output shaft end;

This connection method can replace the original ordinary transmission with a power shift gearbox on the basis of the original ordinary mechanical transmission roller without making major changes to the frame structure, thereby improving the generalization and serialization level of the roller, which is conducive to reducing design costs, shortening the design cycle, and enhancing the competitiveness of enterprises.

2.5 Connection between the transmission and the drive axle

The connection method between the power shift transmission and the drive axle is basically similar to that of a common transmission. Its connection structure is shown in Figure 2. The output shaft end 6 of the power transmission is connected to the drive axle 1 equipped with a NO-SPIN differential (self-locking anti-slip differential) through the transmission shaft assembly 5 and the connecting flanges 2 and 4. The drive axle drives the two rubber wheels to realize the rear-wheel drive of the roller and the differential speed during steering.

1- Drive axle assembly; 2- Connect to Flan; 3- Drive shaft assembly; 4- Connect flange; 5- Transmission power output shaft

The adoption of a drive axle with a NO-SPIN differential can not only automatically achieve unequal torque distribution between the left and right wheels to fully utilize the traction force of the equipment, but also significantly improve the off-road performance and economy of the vibratory roller. Therefore, this type of drive axle should be given priority. The main manufacturers include Jiangxi Fenyi, Chengdu Engineering, Qinghai Gear, Shandong Feicheng, etc.

2.6 Precautions during Operation

When the roller is in operation and the gear controller is used for gear shifting, the operation should be carried out gradually and successively, and gear skipping operation is not allowed. In addition, some transmissions can only be reversed in first gear, so full attention must be paid when driving. In addition, during driving, when coasting downhill, the corresponding gear should be operated to ensure that the engine speed does not fall below 1200r/min, so as to meet the lubrication requirements of all parts of the transmission.

The power transmission assembly generally uses No. 8 hydraulic transmission oil. The first oil addition should be appropriate. When checking the oil level, first let the engine idle. When the oil temperature is 80℃, the oil level should reach the mark on the dipstick. When the oil temperature is 40℃, the oil level should drop to the mark below. The transmission oil must be changed after the first 100 hours of operation. Thereafter, the oil should be changed every 1,000 hours of operation, and the oil filter should be replaced at the same time as the oil change.

When the engine is turned off and the main unit is towed, the transmission requires that the towing speed should not exceed 10Km/h and the towing distance should not exceed 10Km to prevent damage to the transmission.

At present, domestic products that have adopted power shift transmissions include models such as XCMG's XSM220 and Luojian's LSS220D. Due to their features of low cost, easy operation and reliable quality, these products have become the new favorites in the market for mechanized drive rollers and are favored by an increasing number of users.