Keywords

1 Introduction

As the key basic parts of the equipment manufacturing industry, the hydraulic pump is known for its high reliability and long lifespan, among them, especially its reliability plays a decisive role in the reliability of host products. It is necessary to conduct the reliability test to evaluate, improve, and enhance the reliability of the hydraulic pump.

At present, hydraulic reliability life tests in developed countries (such as Sundstron in the United States [1], Nippon Steel [2], UK National Laboratory, Korea Institute of Metallurgy and Mechanical Engineering, French Center for Mechanical Industry Research (ETIT), etc.) usually adopt longer cycle full life tests.

There is a lager gap between the whole life test method of foreign countries and the condition of our country, and the accelerated experimental method is favored. Among them, accelerated degradation test has absolute advantages in solving the evaluation problem of high reliability and long life [3]. Hydraulic pump is a typical system with degraded performance, and performance degradation data actually contain a large amount of life information. The performance reliability analysis of plunger pump was carried out with plunger clearance and outlet valve opening as performance criteria, and it was pointed out that the flow characteristics of plunger pump were affected by the degradation of component design parameters [4]. The oil return flow rate was used as a sensitive signal to reflect the wear condition of the aircraft hydraulic pump, and its wear condition was predicted [5]. Beijing Aeronautical Engineering and Technology Research Center has well predicted the remaining life of a hydraulic pump by analyzing and studying the failure mode and degradation parameters of a certain type of hydraulic pump [6]. The Second Artillery Institute of Engineering used the trained SRELM model to extrapolate the vibration signal of the hydraulic pump and predict the health status of the hydraulic pump [7]. On the basis of comprehensive consideration of numerous parameters (vibration, pressure, flow, temperature, oil information, etc.) affecting the life of hydraulic pump and their correlation, vibration signals are used to predict the life of hydraulic pump [8].

Based on the current hydraulic pump reliability accelerated degradation test method, on the premise of shortening the test cycle and reducing the test energy consumption, a new type of parallel energy-saving reliability test method of multiple hydraulic pumps is proposed in this paper, the parallel energy-saving accelerated test bench is built, and the in-depth theoretical and practical research work is carried out.

2 Hydraulic Pump Reliability Test

2.1 Selection of Test Conditions

Load pressure, medium temperature, higher speed and medium pollution will affect the service life of hydraulic pump, according to the principle of the choice of test conditions, and combining with the use of the hydraulic pump and field test conditions, choose hydraulic pump load pressure and rotation speed as the accelerated stress, control the medium temperature and cleanness of the oil in the process of test in the test within the prescribed scope of the standard. The reliability test object of this paper is A4VS series axial piston pump produced by a domestic enterprise. The rated pressure of this series of products is 35 MPa, and the peak pressure is 40 MPa. The four selected samples cover the open-loop hydraulic pump with a displacement of 250 ml/r, the closed-loop hydraulic pump with a displacement of 250 ml/r, the open-loop hydraulic pump with a displacement of 125 ml/r, and the closed-loop hydraulic pump with a displacement of 125 ml/r. Failure detection and determination.

In this paper, volumetric efficiency is selected as the main performance monitoring parameter. In addition, outlet pressure pulsation and sealing performance of dynamic seals are also tested during the test. The failure criteria are shown in Table 1.

Table 1 Standard for failure detection and judgment of axial piston pump
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Table 2 30 MPa pressure condition hydraulic pump life parameters
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Table 3 20 MPa pressure condition hydraulic pump life parameters
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2.2 Parallel Reliability Testing System and Scheme Design

According to the reliability life requirements of hydraulic pumps, a multi machine parallel energy-saving reliability testing device for hydraulic pumps has been developed. The hydraulic system schematic diagram is shown in Fig. 1. Compared with the conventional hydraulic pump reliability testing device, this testing device and testing plan adopt a parallel design concept. Four A4V series domestic hydraulic piston pumps are tested simultaneously using a dual axis extension motor, with a sample size of four in a single test.

Fig. 1
A schematic diagram of the parallel reliability test bench for axial piston pump. It features transformers labeled T 1, T 2, T 3, T 4, and T 5, points labeled P 1, P 2, and P 3, numerical values, and various electrical symbols.

Working principle of parallel reliability test bench for axial piston pump

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In the test sample, the first hydraulic pump is an open circuit hydraulic pump with a displacement of 250 ml/r, the second hydraulic pump is a closed circuit hydraulic pump with a displacement of 125 ml/r, the third hydraulic pump is an open circuit hydraulic pump with a displacement of 125 ml/r, and the fourth hydraulic pump is a closed circuit hydraulic pump with a displacement of 250 ml/r. The experimental subjects are both large displacement and medium displacement hydraulic pumps, which are mainstream products of hydraulic axial piston pumps and include two forms of hydraulic pumps applied to open and closed circuits.

The experiment adopts the step-by-step stress acceleration method, with pressure as the acceleration factor. The test is conducted for 1030 h at a stress of 35 MPa in the first stage, and 846 h at a stress of 41 MPa in the second stage. Using the volumetric efficiency of the tested plunger pump as the performance degradation parameter and failure criterion, the final life data in terms of time was obtained.

Because axial piston motors and axial piston pumps are structurally reversible, the 2nd and 4th closed hydraulic piston pumps can be used as hydraulic motors. One end of the dual axis extension motor is connected to the No. 1 hydraulic pump through a torque tachometer, and it is dragged to continue rotating. The other end of the dual axis extension motor is connected to the No. 4 hydraulic pump; The pressure oil output from the 1st hydraulic pump drives the 2nd hydraulic pump. The 2nd hydraulic pump is connected to the torque tachometer as a hydraulic motor, and the torque tachometer is then connected to the 3rd hydraulic pump; The second hydraulic pump is used to drive the third hydraulic pump to rotate, the pressure oil output by the third hydraulic pump is used to drive the fourth hydraulic pump, and the fourth hydraulic pump serves as a hydraulic motor to drive the motor.

In the experimental system, while the No. 4 hydraulic pump is the subject, it completes the power recovery of the hydraulic system and uses the recovered energy to drive the motor to rotate. On the one hand, it can reduce the input power of the motor and reduce electrical energy consumption; On the other hand, due to the extremely small flow rate passing through the loading valve during high-pressure operation, the heat generation of the hydraulic system is greatly reduced, which reduces the requirements for the heat dissipation system and also reduces the energy consumption of the heat dissipation system.

The total input power of the system is calculated from the collected input torque and speed of pump 1. The recovered power is calculated from the collected output flow and pressure of pump 4. The lost power can be obtained by subtracting the recovered power from the total input power of the system. After matching the flow of the whole test with the speed of the tested pump, adjust the displacement of No. 1 pump to V = 165 ml/r, the displacement of No. 4 pump to V = 111.6 ml/r, and the motor speed to n = 1500 r/min, where the total efficiency of No. 4 pump is n = 0.907. When the test pressure is 35MPa, the measured input torque T = 849.3Nm, and the power distribution diagram of the test is shown in Fig. 2.

Fig. 2
A line graph of power versus time. The curves for total system input power, motor recover power, and loss power have fluctuating trends. Total system input power (240, 147) has the highest estimated value.

Total input power, recovered power, and lost power of the system at 35 MPa

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3 Hydraulic Pump Reliability Test Data Processing

Combined with the time conversion equations, and substituting the failure data into \(t_{j} (b)\), the estimated value of shape parameter \(\hat{m} = {2}.0{138}\), the estimated value of characteristic life \(\hat{\eta } = {4657}.{9}\), the average life of 4127.5 h, and the median life of 3882.8 h can be calculated.

By putting the estimated values of the parameters into Equation, the probability density function of the hydraulic pump can be obtained as:

$$f\left( t \right) = \frac{{2.0138}}{{4657.9}}{\left( {\frac{t}{{4657.9}}} \right)^{1.0138}}{e^{ - {{\left( {\frac{t}{{4657.9}}} \right)}^{2.0138}}}} $$
(1)

The probability density function curve is shown in Fig. 3b.

Fig. 3
Two line graphs plot life test on the horizontal axis. In the graph on the left, the curves for F and R have increasing and decreasing trends, respectively. In the graph on the right, the curves for f and r are bell-shaped and increasing, respectively.

35 MPa reliability index function curve

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Putting the estimated values of the parameters into Eq. (2) and setting γ = 0, the distribution function of the hydraulic pump life can be written as:

$$ F(t) = 1 - e^{{ - (\frac{t}{4657.9})^{2.0138} }} $$
(2)

The life distribution function curve is described in Fig. 3a.

Putting the estimated values of the parameters into Eq. (3) and setting γ = 0, the reliability function of the hydraulic pump can be obtained as:

$$ R(t) = e^{{ - (\frac{t}{4657.9})^{2.0138} }} $$
(3)

The reliability function curve is shown in Fig. 3a.

Putting the estimated values of the parameters into Eq. (4) and setting γ = 0, the failure rate function of the hydraulic pump can be obtained as:

$$ \lambda (t) = \frac{2.0138}{{4657.9^{2.0138} }}t^{1.0138} $$
(4)

The failure rate function curve is shown in Fig. 3b.

Calculate the average life of hydraulic pump:

$$ E(t) = \int\limits_{0}^{\infty } {tf(t){\text{d}}t} = \hat{\eta }\Gamma (1 + \frac{1}{{\hat{m}}}) = {4127}.{\text{5h}} $$
(5)

In addition, according to the acceleration model equation, the reliability index of the hydraulic pump under the pressure condition of 30 and 20 MPa can be obtained, as shown in Tables 2 and 3. The reliability function, distribution function, probability density function and failure rate function curves are shown in Figs. 4 and 5.

Fig. 4
Two line graphs plot life time on the horizontal axis. In the graph on the left, the curves for F and R have increasing and decreasing trends, respectively. In the graph on the right, the curves for f and r are bell-shaped and increasing, respectively.

30 MPa reliability index function curve

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Fig. 5
Two line graphs plot life time on the horizontal axis. In the graph on the left, the curves for F and R have increasing and decreasing trends, respectively. In the graph on the right, the curves for f and r are bell-shaped and increasing, respectively.

20 MPa reliability index function curve

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4 Conclusion

  1. (1)

    The reliability test bench designed based on the concept of parallel energy-saving has a higher degree of automation compared to general reliability test benches, with more types and quantities of single test samples, significantly reducing test time and system heating. The energy-saving effect is significant.

  2. (2)

    The volume efficiency, which is closer to the failure physical results, is selected as the performance degradation evaluation standard of hydraulic pump. Based on the inverse moment estimation method, the optimal modeling analysis of the volume efficiency performance degradation of hydraulic pump is carried out, and the acceleration model of hydraulic pump reliability test is calculated, and the reliability evaluation method under different stress conditions is proposed. The reliability index of the hydraulic pump under 20 and 30 MPa stress is obtained.