Application of Closed Hydrostatic Double Track Drive in Crawler Transporter

At present, there are more and more crawler transport vehicles using closed hydrostatic double-track drive in China, especially in crawler bulldozers, crawler tractors, and transport-type crawler transport vehicles. The following advantages: It is beneficial to isolate the impact of the ground on the engine; it can meet the higher traveling speed of the vehicle; it can realize stepless speed change to meet the requirements of users for different operating speeds; start stable and meet the needs of special working conditions; eliminate complex machinery The transmission system reduces the weight of the whole machine; the hydraulic components are highly versatile, and the main engine is easy to be modified. On high-power, large-scale dual-track drive vehicles, the advantages of using closed system hydrostatic drive are more obvious. Although the hydraulic system requires high pressure and large flow, hydraulic components, hydraulic accessories, oil tanks and open hydraulic systems of the same power In comparison, the volume is small, which is conducive to the layout of the host. The following is some information summarized by the author based on years of experience in hydrostatic transmission design for reference by colleagues.

1 System composition

At present, under normal circumstances, the hydrostatic double-track drive of a closed system is composed of a hydraulic or electronically controlled variable pump and a fixed motor. In order to meet the requirements of different operating speeds and traction forces, a stepped variable motor is sometimes required. Figure 1 shows the basic structure of a closed-system hydrostatic double-track drive transmission system, which is composed of pilot handles, pumps, motors, reducers and other components.

2 Matching of pilot handle and variable pump

设计当中，泵的排量—压力 （电压）控制曲线以及先导油路或者电路调压曲线的选配非常关键 。全液压双轨驱动车辆驱动油泵的压力控制曲线死区稍大些较好，可减少先导手柄不正常晃动引起泵的误动作，利于提高整机的直线行驶性；斜率越大越好，有利于降低转向灵敏度，控制泵在小排量时转向， 降低转向速度，转向时，需克服的惯性阻力也小，不易导致发动机熄火 。一般情况下， 先导手柄的调压曲线两种。在作配套设计时， 先导手柄的调压曲线越过死区时起始压力 （电压 ）点 要低于泵死区排量对应的最大压力 （电压 ）（f点 ）， 利于主机起步平稳， 可 0公里起步。先导供油压力 （供电电压 ）要高于泵最大排量对应的压力 （电压 ）， 可最大限度控制油泵排量的无级变化， 一般情况下， 液控时 ,供油压力要高于泵最大排量对应的压力 5 ×105Pa左右 ,利于减少由于手柄在高速行车过程中抖动造成主机车速不稳 。控制油泵排量无级变化对应的手柄行程越大越好 ,如图 2所示， 控制曲线Ⅲ与控制曲线 Ⅱ相比， 使用控制曲线 Ⅲ利于扩大控制油泵排量无级变化对应的手柄行程（nj段 >mi段），但受手柄操作行程限制。比例曲线段斜率越小越好 ,曲线Ⅲ与曲线Ⅱ相比 ,在同样的操作行程Δs下 ,曲线Ⅱ的油泵排量的变化大于曲线Ⅲ （Δq2 <Δq3 ），对于驾驶员操作来说， 使用控制曲线Ⅱ， 操作起来就过于灵敏，使用控制曲线Ⅲ， 要比控制曲线Ⅱ操纵手感要好。另外，在采用控制曲线Ⅲ时，比例段最大压力 （电压）（c点 ）应当接近于泵的排量—压力 （电压）控制曲线对应的最大控制压力（电压）点（h点）。为改善主机的转向性能 ,提高主机的机动性能 ,减少小半径转向时导致的转向阻力过大 , 耗用发动机功率过多 ,造成发动机熄火，可对油泵的排量作一些限制 。通常的办法是在转向时， 对先导控制手柄的供油压力 （供电电压 ）作一些限制 ,使先导手柄的供油压力（供电电压 ）低于泵的最大排量对应的控制压力 （电压 ），供油压力（电压 ）的限定值可根据主机的整机参数，在保证发动机不熄火的前提下设定 。

3 Determination of system pressure

For closed-type hydrostatic dual-track drive vehicles, under normal circumstances, when the steering resistance is overcome, the corresponding pressure is the largest, and this pressure can be used as the set pressure of the system. However, in determining the system pressure, in addition to the requirements of the overcome steering resistance for the system, it is also necessary to comprehensively consider the current hydraulic components and hydraulic accessories can achieve performance indicators, as well as factors such as the operating conditions and cost of the main engine.

4 Confirmation of the motor

The use of two-speed motors is necessary to meet the requirements of vehicle speed and maximum traction, and to improve the efficiency of the hydraulic system. In addition, high-speed motors and low-speed high-torque motors should also be used differently. High-speed motors can be small in size and can be built-in. Or the scheme integrated with the reducer, the best way is to hide them in the track, which will help improve the driving performance of the main engine. The low-speed and high-torque motor has a large volume. To improve the passability of the whole machine, the motor should be set higher than the center of the driving wheel. Keep away from the ground.

5 Determination of oil pump

The oil pump should use a higher calculated speed as much as possible, preferably close to the rated speed of the oil pump, so that even if the engine is in an idling supply condition, the circuit system composed of the oil pump and the motor has a higher transmission efficiency. To determine the maximum displacement of the oil pump, it is usually appropriate to use the first gear with the smaller displacement of the two-speed motor to determine the displacement of the pump. It can reduce the supporting power and help improve the transmission efficiency of the system.

6 Special attention should be paid to the following issues in the selection of hydraulic accessories for closed hydraulic systems: 1) The suction pipe should be as short as possible, with a large inner diameter, so that it is not easy to deflate, and only a thicker suction filter can be added to the suction line. If the space is arranged Allowed, try to use a high-level hydraulic oil tank to make the oil level of the hydraulic oil tank higher than the oil suction port of the oil pump to improve the oil suction condition of the oil pump. When connecting the oil suction pipe, pay special attention to the leakage of the oil pipe, otherwise it will cause the vehicle to shake , Poor straight-line travel, pump, motor damage and other failures. 2) The inner diameter of the drain pipeline of the pump and motor should be as large as possible, and a separate DC return tank is required. No oil filter is added to the drain pipeline to minimize the oil drain resistance to prevent the pump and motor shaft seals from being washed away. The hydraulic system and the connected transmission components string oil, causing the hydraulic oil to be lost and contaminated, and even more serious system failures.