Created on:2021-06-29 14:52

Tractor hydraulic failure -- output flow reduced

Tractor hydraulic failure -- output flow reduced

(1) Analysis and inspection of problems

During the bench test of hydraulic output power of Dongfanghong 1004 / 1204 High-power Tractor, it is found that the no-load output flow is far less than the design value. The designed no-load flow rate is 45 L / min and 50 L / min, but the measured result is only about 20 L / min, and there are many bubbles in the output oil. At this time, the lifting time of the hydraulic lifter is 2.6s, and no abnormal phenomenon is found, which indicates that the flow of the input lifting hydraulic cylinder meets the design requirements. So, why is the no-load hydraulic output flow far less than the design value? For this purpose, check and test according to the following steps.

① Check the sealing condition of the external oil inlet pipe and joint, replace the sealing element and re test, the output flow is still about 20L / min.

② The replacement of a new filter element did not solve the problem.

③ The hydraulic pump and multi way valve were replaced, and it was suspected that the multi way valve control mechanism was not in place. The problem was not solved by removing the control mechanism and directly operating it by hand.

④ The flow meter of the test bed is recalibrated, and there is no problem with the flow meter.

⑤ As no problem was found in the above inspection, we had to doubt the original system design, that is, whether the structure of the oil return pipe in the rear transmission case inserted into the oil suction pipe joint hindered the oil suction. This structure is shown in Figure a.

The upper end 2 of the oil return pipe is inserted into the lifter housing as the upper cover 1 of the transmission case, and the lower end 5 of the oil return pipe is inserted into the rear oil suction pipe joint 6 through the hole at the bottom of the transmission case, and is supported and positioned by the steps in the joint. There is an oil inlet groove 4 at the lower end of the oil return pipe. The oil in the transmission box can enter the rear oil suction pipe through the oil inlet groove 4 to the hydraulic pump.

It can be seen from figure a that the outer diameter of oil return pipe and the inner diameter of oil suction pipe joint are both 28mm, which are clearance fit. The oil groove 4 and the joint 6 form a 4mm wide semi-circular oil groove with an area of about 200mm2. During the hydraulic output test, all the oil entering the hydraulic pump passes through the semi-circular oil groove. Through calculation, the liquid flow velocity through the oil tank is 3.6 m / S (Dongfanghong 1004) and 5.6 M / S (Dongfanghong 1204), which is obviously much higher than the generally recommended oil absorption velocity of 1 ~ 2 m / s, resulting in relatively large local resistance loss. Therefore, air is sucked in from the upper part of the return pipe inserted into the lifter, which leads to the reduction of the output flow of the hydraulic pump and the generation of bubbles.

In the hydraulic suspension system, the output oil circuit of the hydraulic pump is connected in series. The output oil first enters the hydraulic valve, and then enters the lifter. The return oil of the lifter enters the rear oil suction pipe joint through the oil return pipe.

When the lifting test is carried out, the hydraulic pump not only sucks the oil in the transmission box and the return oil of the lifter through the semi annular oil groove, but also through the return oil pipe, so the oil passing area is large and there is no suction phenomenon. Therefore, when the lifting test is carried out, the lifting time meets the design requirements.

During the hydraulic output test, the return oil flows directly into the transmission case, there is no return oil in the lifter, there is no oil flow in the return pipe, there is no oil flow in the back suction pipe joint, and all the oil entering the suction pipe of the hydraulic pump flows through the semi annular oil groove. Because of the small oil area, the suction pipe enters the air, and the output flow of the hydraulic pump decreases. This explains the reason why the output flow of hydraulic pump is enough and the output flow of hydraulic output test is greatly reduced.

(2) Improved scheme

As shown in Figure B, the oil inlet groove 4 is moved upward for a certain distance, so that the oil passage area of the oil groove reaches 1245mm2, and the flow rate through the oil groove is 0.6m/s (Dongfanghong 1004) and 0.9m/s (Dongfanghong 1204), which meets the requirements of oil absorption flow rate. The results show that the maximum no-load flow is 44.14l/min (Dongfanghong 1004) and 51l / min (Dongfanghong 1204), and the output power is enough, which fully meets the design requirements.

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