拖拉机动力输出轴动态转矩载荷谱编制与验证

拖拉机动力输出轴(power take-off, PTO)室内台架耐久性试验中转矩加载主要以静态加载或静态逐级加载方式为主,这种加载方式输出的载荷与PTO实际作业中承受的载荷动态特性不同,导致试验中PTO的失效形式及测试寿命与实际作业中存在差异。针对此问题,该文对多工况下PTO动态转矩载荷谱编制方法进行研究,测取了沙土、黏土2种土壤类型下旋耕、驱动耙典型作业的PTO转矩,利用希尔伯特黄算法对转矩进行预处理,采用四点循环雨流计数法对转矩均幅值进行频次统计,利用散点矩阵统计方法分析4种作业工况均值、幅值相关性及均值、幅值分布类型。基于Miner法则及轴类20CrMnTi修正S-N曲线,在损伤概率50%、相对误差5%、置信水平95%的统计条件下,确定了转矩样本容量。利用时域外推方法对PTO转矩数据进行外推。以阈值区间内峰谷值均值差的方差为指标,检验了外推过程中选取的阈值造成峰谷值均值波动的可能性。对比外推1次后载荷时间历程与原始载荷时间历程变化趋势。在时域频次外推因子为240、转矩累计频次达到106、平均加权4类转矩时,得到PTO动态转矩载荷谱。利用编制的动态载荷谱,驱动拖拉机传动系动态加载试验台对PTO进行台架耐久性试验,试验结果显示,PTO发生失效的时长为642 h,失效形式为PTO轴头花键磨损,失效时间、失效形式与实际作业一致,验证了动态转矩载荷谱的有效性。

Compilation and verification of dynamic torque load spectrum of tractor power take-off

Abstract: The power take-off (PTO) of the tractor is the key power transmission component for the tractor unit when used in the rotary tillage and driven harrow operations. It is subject to the random loads of different characteristics from the farm implements, and is the main loading part of the bench test for the tractor drive train. Torque load plays a decisive role in the service life of the PTO. The torque load spectrum is an important basis for the strength design and durability test verification of the PTO. In the indoor durability bench test of the tractor PTO, the torque loading is mainly based on static loading or static step-by-step loading. The dynamic characteristics of PTO load output by this mode is different from that of PTO load in actual field operation, which leads to the difference both in the failure mode and the test life of the PTO between the bench test and the actual field operation. Aiming at this problem, in this paper, the compiling method of the PTO dynamic torque load spectrum under multiple working conditions was studied. Firstly, the PTO torque in the rotary tillage and driven harrow operations under two soil types of sand and clay was measured. Secondly, the test data were preprocessed by the Hilbert Huang algorithm, and the torque frequency was distributed and statistically analyzed by the rain flow counting method. The variance matrix statistics were used to test the torque mean value and amplitude value correlation of four working conditions while the max correction decision coefficient was 0.009 99. The torque mean value followed a normal distribution and the amplitude value obeyed the Weibull distribution. Based on Miner''s law and 20 CrMnTi modified S-N curve, the sample capacity of the torque data was determined under statistical conditions of 50% damage probability, 5% relative error and 95% confidence level. The PTO torque data was extrapolated using the time domain extrapolation method. The variance of the mean value of the peak-to-valley value in the threshold interval was taken as an index to check whether the threshold value selected in the extrapolation process caused the mean value of the peak-to-valley value to fluctuate drastically or not. Then, the trend of load time history and time history of original load after extrapolation was compared. The PTO integrated dynamic torque load spectrum was obtained when the time domain frequency extrapolation factor was 240, the torque cumulative frequency reached 106, and the average weighted four types of torque. The mean value and amplitude value two-parameter rain flow counts were performed on the PTO torque. The counting results showed that the PTO torque mean value, amplitude value and cycle number were synchronously extrapolated. Compared with the original data rain flow counting result, the mean value and amplitude value distribution law had similarity. Finally, the dynamic load spectrum compiled was used to drive the dynamic loading test bench to test the durability of the PTO. The engine torque was simulated by the motor in the driving unit while the PTO dynamic torque load spectrum was simulated by the motor in the PTO loading unit. During the test, a sudden drop of the torque measurement value in the PTO loading unit occurred at the moment of 642 h as the PTO shaft spline was worn and could not effectively transmit power. The failure time of PTO shaft spline wear in tractor actual field operation was 644.5 h. The test results showed that the failure time of the PTO was 642 h, and the failure mode was PTO shaft spline wear. Both the failure time and failure mode were consistent with the actual field operation, which verified the effectiveness of the integrated dynamic torque load spectrum.