2.1 calculation of steering resistance torque
In the matching design of hydraulic steering system, the steering resistance moment should be calculated first. Generally, the steering resistance moment is divided into in-situ steering resistance moment and driving steering resistance moment. Considering that the in-situ steering resistance moment is 2 ~ 3 times of the driving steering resistance moment, the steering system of the self loading mixer shall be designed to meet the requirements of in-situ steering torque. The factors that affect the steering resistance moment of automobile include the load of automobile front axle, the friction coefficient between tire and ground, tire pressure and so on. Currently commonly used empirical calculation formula:
Where: MR1 - steering resistance moment, n · m;
f ——轮胎与地面摩擦系数，取 0.7；
F - friction coefficient between tire and ground, taken as 0.7;
G1 -- axle load of vehicle steering axle, N;
P -- tire pressure of vehicle tire, kPa.
自上料搅拌车单轴轴荷 75000N，正常工作胎压550kPa，代入数值可得 Mr1= 6462N·m。
Since the maximum single axle load of the charging mixer is 75000n and the normal working tire pressure is 550kpa, the value can be substituted to get MR1 = 6462n · M.
2.2 steering cylinder selection and verification
转向系统设计需要考虑两种情况，种情况，转向油缸随车桥确定，且受结构限制不能随意改变，此刻应计算转向桥载荷，通过调整整机质心，使转向桥载荷在合适的范围内，同时对转向油缸进行校核计算。第二种情况，转向桥所受载荷已确定，需要对转向油缸进行选型设计，使之符合转向要求。目前自上料搅拌车选用林德10t 车桥，油缸性能参数已经确定，先需要对油缸进行校核，参数见表 1。
Two situations need to be considered in the design of the steering system. In the first case, the steering cylinder is determined with the axle and cannot be changed arbitrarily due to structural constraints. At this time, the steering axle load should be calculated, the steering axle load should be within the appropriate range by adjusting the mass center of the whole machine, and the steering cylinder should be checked and calculated at the same time. In the second case, the load on the steering axle has been determined, and the steering cylinder needs to be selected and designed to meet the Steering requirements. At present, Linde 10t axle is selected for the self loading mixer, and the performance parameters of the oil cylinder have been determined. First, the oil cylinder needs to be checked, and the parameters are shown in Table 1.
The maximum thrust of steering cylinder is
By substituting the value, Fmax = 76302n can be obtained
自上料搅拌车车桥转向油缸的转向作用力臂随转向角的关系曲线如图3 所示。车桥转向角为 32°，由图可知更小力臂 Rmin＝125mm。
The relationship curve between the steering action arm of the axle steering cylinder of the self loading mixer and the steering angle is shown in Fig. 3. The maximum steering angle of the axle is 32 °. It can be seen from the figure that the minimum force arm Rmin = 125mm.
根据转矩公式：Fmax =Mr2 / (Rmin η)
According to the torque formula: Fmax = MR2 / (Rmin) η)
Where: Rmin - minimum force arm, mm;
η—— Steering mechanical efficiency, taken as 0.8;
MR2 -- steering torque, n · M.
By substituting the value, we can get MR2 = 7630.2n · M
According to the calculation results, MR2 ＞ MR1, it meets the use requirements.
2.3 determination of displacement of steering gear
When the steering wheel moves from one limit position to another, the number of turns n of the steering wheel should not only meet the driving conditions and prevent the poor stability of the vehicle body caused by excessive steering of the wheel, but also meet the requirements of less work cycles and improve the convenience of operation. In general design, n = 3 ~ 5 cycles.
Full stroke oil volume of steering cylinder: v = SA
Where: s -- piston stroke, mm;
A - oil pressure action area, mm2.
Substitute the value to obtain v = 858ml
先选取 n=4，则转向器排量为V/n=858/4=214，对照转向器厂家样本，选用排量为 200mL/r 型号的转向器。
If n = 4 is selected first, the displacement of the steering gear is v / N = 858 / 4 = 214. Compared with the sample of the steering gear manufacturer, the steering gear with a displacement of 200ml / R is selected.
2.4 determination of gear pump displacement
Steering system flow: q = QN
Where: Q -- displacement of steering gear, ml / R;
N -- steering wheel speed, taken as 0.8r/s.
Substitute the value to get q = 160ml / s
The displacement of the gear pump is analyzed according to the idle state of the engine
Where: Nd -- engine idle speed, taken as 750r / min;
η—— Steering pump efficiency, taken as 0.95.
Substitute the value to get QC = 13.5ml/r
The displacement of gear pump is 16ml / R, which meets the use requirements.
Full hydraulic four-wheel steering system has the characteristics of convenient layout and flexible steering. At present, it is widely used in wheel excavator. Starting from the principle of hydraulic steering, this paper introduces in detail the application of four-wheel hydraulic steering in self loading mixer truck, checks the strength and displacement of steering cylinder through calculation and analysis, and determines the displacement of steering gear and gear pump, so as to provide a reference for the design of hydraulic steering system.
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