Research on Service Life Prediction Model of Thrust Needle Bearing

Transcription

1 TELKOMNIKA, Vol.10, No.7, November 2012, pp. 1621~1628 e-issn: X accredited by DGHE (DIKTI), Decree No: 51/Dikti/Kep/ Research on Service Life Prediction Model of Thrust Needle Bearing Hu Zhanqi*, Qi Xiaowen, Fan Bingli, Li Wei Mechanical engineering of Yahnshan university, Qinhuangdao, Hebei province of China *corresponding author, Abstract Needle roller thrust bear is small in size and of high ability in load bearing, therefore it is widely used in fields of aviation and automobile etc. But the relation between their service life and pre-tightening torque is not very clear, so the using design of the bear depends mainly on experience of engineer, because of lack of references. In the paper, the theoretical analysis on relation between torque and load is done, special wearing test instrument is developed and wearing test of thrust needle bear is conducted. Based on the results of the test, mathematical model of relation between the losing amount of pre-tightening torque and the pre-tightening torque is built, based on which use of the bear in engineering will be more reasonable, and their pre-tightening torque will be given more accurately. Key words: needle roller thrust bear, service life, prediction model, pre-tightening torque Copyright 2012 Universitas Ahmad Dahlan. All rights reserved. 1. Instruction Needle roller thrust bears (Figure1) is a new type of thrust bear, they can bears greater axial and radial load simultaneously and have characters of small in size, light in weight, and long in service life. Therefore they are widely used in such area of aircraft and airship that load is great and having strict requirement in size and weight [1,2]. But because history of the bears is not very long, and references about them are not very easy to get, selection of the bears in engineering is based mainly on experience of engineer. Because these experiences are gotten mainly from ball roller bears, they are not very appropriate to needle roller bear [3,4]. Qiao Huxiang etc. make some analysis to ball roller bears component with finite element method [5], result of which can be referenced in research of bears services life and failure, but can not be used to predict the services life of needle roller thrust bear. Li Zhenzong and Brigitte etc. make some researches on roller bears and failure mechanism [6,7], propose the failure mechanism model of roller bears, which is still not the prediction model and can not be used in engineering. Because the researches until now are almost all done with ball rolling bears which are different from needle roller thrust bears, the results are therefore cannot be predict the service life of needle roller thrust bear and can not be used in engineering. In some important cases, tests must be done in order to predict the service life of needle roller thrust bears accurately. The service life test is usually high cost and time consumed. In this paper, authors built service life prediction model of needle roller thrust bears, and give the quantitative relation between the axial load and the service life, by using statistics and based on service life test of a needle roller thrust bear. The result the research can be used in selecting of the bears in engineering and in other similar researches about the needle roller thrust bears. 2. Wear life equation of needle rolling thrust bears The service life of bears is generally Wipur distribution [8,9], which probability density function is: φ(t, P) = 1 exp( ct a P b ) (1) Received July 23, 2012; Revised September 29, 2012; Accepted October 10, 2012

2 1622 e-issn: X Equ.1 shows that failure probability of bears is Ф under the load P, after service time t. a,b, and c in the Equ.1 are constants related to structure and material of bears. When Ф is given a value, (tapb) is constant,which is take as Ka, that is: a b a t P = K or a b t = KP (2) If service time t is taken as characteristic value, Equ.2 is therefore estimation value of wear life under the failure probabilityф. The equation shows that estimation value of wear life is related to the load upon the bear. Generally speaking, the more the load upon the gear, the shorter the estimation value of wear life of the bear is. Equ.2 can be called wearing equation of bears. This equation usually is used in fast wearing test of bears, in order to get service life test results of bears with shorter time. It is difficult to measure wearing amount of bear directly in wearing test of needle roller thrust bears. But the wearing amount is generally proportional to change of clearance of the bear, and the change of clearance is also proportional to the change of pre-tightening torque. Among elastic range of bear material, the change of the three factors is linear relation. Therefore change of pre-tightening torque can be taken as the character value of wear of bears which is easy to be measured. Wear life equation of needle roller thrust bear can be thus expressed as: n T = kp (3) In the equation, ΔT is the change of pre-tightening torque of bears before and after test, k and n is constants determined by material and structure of bears, which has relation to the lubrication condition in the test. Equ.3 is similar with Lunderberg-Palmgren equation of load and fatigue life [10] of bears. It can be understand to be special form of Lunderberg-Palmgren equation in condition of needle roller thrust bear. By using the relation shown in equ.3, measurement of wear amount of bears can be turned to the measurement of the change of pretightening torque, and pre-tightening torque is very easy to be measured in test with torque wrench. force sensor Testing Bearings 传感器 testing bearing torque sensor Figure 1. needle roller thrust bears 伺服电机 10~100rpm servo motor Figure 2. wear test instrument for needle roller thrust bear 3. Wearing testing instrument of thrust bears TELKOMNIKA Vol. 10, No. 7, November 2012 :

3 TELKOMNIKA e-issn: X Fixture of bears According to the analysis of condition between wear amount and torque change during wearing of needle roller thrust bear, a test instrument for dynamic load performance of the bear is designed (Figure2). When testing, the thrust bear is fixed on core shaft and turns with core shaft under the driving of servo motor through synchronous toothed belt, the torque is measured with torque meter during turning. A round nut is used to apply axis pre-tightened force to the bear, which is used with anti loose washer preventing the nut from loose during turning. The testing instrument is used on servo press, which applies axis load on the bear. The axis load is measured with force meter and is returned to the hydraulic servo system, therefore closed loop control is realized. Because wear of bear during test is reflected to the change of clearance of bear system after test, and the change of clearance is again reflected to the change of pre-tightening torque of round nut, therefore pre-tightening torque of round nut before and after test are recorded as the parameter measuring wear amount of bears. 3.2 Measurement and data processing system Measurement and data processing software system is developed for testing of the bears. Its function structure is shown in Figure 3. Data acquisition card PCI-1716 is main component of the system. Through an interface card, load, temperature, and torque information are gotten, from testing machine. Pressure close loop control is realized with upper position computer through the acquisition card. Some auxiliary functions are also performed with upper position computer, such as display of test status, save of test data, and imputing control parameters of system. Interface of control system is shown in Figure 4. Left part of the interface is parameter display and inputting region, control parameter of test are input and display in the region. Middle region is curves of pressure and torque. Right region is function selecting region, and testing functions are selected in the region with soft buttons. Figure 3. Function structure of testing system Figure 4. Interface of control system 4. Test research on wear life of needle roller thrust bears 4.1 Calibration of pre-tightening torque In testing instrument, axis pre-tightening force is exerted through rotating pre-tightening nut. Relation between axis force and torque rotating nut is: d 2 M = Qtg( α + ϕ2) 2 Here: d2 - middle radius of thread, α - helix angle of thread, φ2 - friction angle of thread couple, Q - axis force corresponding to thread torque. (4) Research on Service Life Prediction Model of Thrust Needle Bearing (Hu Zhanqi)

4 1624 e-issn: X According to the size of core shaft, middle radius of thread is 22.5mm,helix angle of thread is 1.215o, friction angle of thread couple is 6.6o. Therefore relation between the axis force and the torque rotating nut is: M= Q When the pre-tightening force changes between 5~60% rated dynamic load of the bears, changes of pre-tightening torque is M=(1~12.3)N-m. Pre-tightening force of thrust bear is generally given with axis force, but in engineering, the axis force is applied by means of torque, therefore relation between the axis force and the pretightening torque have be built before test conducted. The calibration of pre-tightening torque is explained in this part. Principle of calibration is shown in Figure 5. In the calibration, axis force is applied to the bear through a lever, which amplify function let us can apply greater force using smaller weight. Turning round nut, number of corresponding torque will be show on the meter of torque wrench, which is proportional to the pre-tightening force. When pre-tightening force of bears changes from 5% to 60% of rated load, corresponding torques are listed in Table 1. load force test bearing torque wrench Figure 5. Sketch map of torque calibration instrument Table 1 pre-tightening force and pre-tightening toque Bearing number pre tightening force ratio Pre- tightening force/n Pre-tightening torque/nm 1-3# 5% # 10% # 20% # 30% # 40% # 50% # 60% Test process After preparation has been finished, test will be conducted with following steps: Applying pre-tightening torque: According to the torque numbers in table 1, pre-tightening torque is applied to 21 test bears. In order to improve the reliability of test results, each three bears are in a group, same amount of pre-tightening torque is applied on three bears in a group. When pre-tightening torque is applied, we have to insure that tighten nut under the pure torque as possible. Process of applying pre-tightening torque is shown in Figure 6. (1) Turning test: Turning test condition in national standards is that under the action of axis load of 19000N, bear is turned revolutions, in rpm. After the test, change of pretightened torque of the round nut is measured, and friction factor of the bear is also monitored during test. In this research test, axis load is applied with hydraulic servo loading system, amount of the load is measured inline, precision of measure is 5%. Torque is also measured inline with torque meter, precision of measure is 1%. Rotation speed is 90rpm. Situation of test is shown in Figure 7. (2) Measurement of residual torque: After turning test, bears are removed from test machine, and residual pre-tightened torque is measured. Method of measure is the same as that in Figure 6. Measure results of 21 test bears is show in table 2. Factor of friction in Figure 2 is defined as the ratio of friction torque and axis load, both of which are measured inline. TELKOMNIKA Vol. 10, No. 7, November 2012 :

5 TELKOMNIKA e-issn: X 1625 Figure 6. Applying pre-tightening torque on thrust bears Figure 7. Wear test of n eedle roller thrust bears Table 2 Data of wear test No. of Pr-tightened torque/nm Friction factor pre-load test bear before test after test minimum maximum 1# # 5% # # # 10% # # # 20% # # # 30% # # # 40% # # # 50% # # # 60% # Analysis of test data Testing results of pre-tightening torque before and after test for 21 needle roller thrust bears is in Figure 6. We can see from the data: (1) Pre-tightening torque after test is smaller than that of before test; (2) Along with increasing, the change of pre-tightening torque after test increases in exponential form. Results of test reflect the character of the wear amount and the torque of the bear. The more the wear amount of the bears becomes, the greater the clearance of the bears becomes, and the smaller the pre-tightening torque becomes. But the relation of the change is not linear, because the relation between the wear amount and the load if not linear. Using the test data in Table 2, linear regression is conduct for parameter k and n in Equ 3. Result is k=0.2, n=3.3. Therefore in test condition of Table 2, wear model of the bears is: 1.2 T = 31.32P (5) Research on Service Life Prediction Model of Thrust Needle Bearing (Hu Zhanqi)

6 1626 e-issn: X pre-tightening torque(n-m) gear number "bfore test" "after test" Figure 6. change of pre-tightening torque before and after test loss of torque(m-m) pre-tightening force(p/c) "measure value" "calculating value" Figure 7. Compare of measure value and calculation value of change of pre-tightening torque w e a r f a c t o r bear number minimumu wear factor maximum wear factor Figure 8. Maximum and minimum friction factor TELKOMNIKA Vol. 10, No. 7, November 2012 :

7 TELKOMNIKA e-issn: X 1627 Changes of pre-tightening torque are compared in Figure 7, of measure number and calculation number according Eq. 4. The average error is within 10%. Therefore this model can be used as a basis in design of pre-tightening torque of needle roller thrust bears. Change of maximum and minimum friction factor along with increase of pre-tightening torque is shown in Figure 8. The curve shows that the friction has a little decrease along with the increase of the pre-tightening torque, and become stable after the torque over some limit. This is because the micro roughness of contact surface becomes stable after the torque over some limit. Average friction factor is within Recommendation value of pre-tightening force After pre-tightening force is exerted on thrust bears and deformation coordination takes place, the force on core shaft is the same as the force on bear base and balance status of force system is achieved. When the balanced bear system is affected by outer force, balance status is destructed, the force on the bear facing to the force direction is increased, and the force on the bear backing to the force direction is decreased. In order to guarantee the stable working of the bear system, the thrust bear should be in non-gap status in both the two direction. Engineering method is to make sure that under influence of the axis load smaller pre-tightening axis force should be 25% more than outer load. According to the principle, the relation between the pre-tightening torque and the outer load is: Q p 1 Cm = ( + 4 C + C b m here: Qp - axis pre-tightening force, Cm - rigidity of bear base, determined with test, Cb - rigidity of core shaft, determined with test, F - outer load. )F (6) Value of axis pre-tightening force from Eq.(6) has great relation with structure of thread system. For example, rigidity of bear base Cm is related to the rigidity of needle roller thrust bear itself, the rigidity of bear base, and the rigidity of bear symmetric to the tested bear. Rigidity of core shaft Cb is related to the material of shaft, the size of shaft, and the structure of shaft. Eq.(6) is only the qualitative relation between the pre-tightening force and the outer load of the bear system. Aim of the paper is to identify the torque coefficient of the model in special condition. According to the theoretical analysis and testing result, in order to guarantee 25% residual pre-tightening force under the outer load, recommendation value of pre-tightening force should be 20~40%of outer load. 5. Conclusions (1) Loss amount of pre-tightening torque of needle roller thrust bears will be greater as the pretightening increases. Too great pre-tightening torque will make the serves life of the bears become shorter, and too small pre-tightening torque will lead bears system to vibration. Therefore the bears system should be pre-tightened properly, before using. (2) Relation between the decrease amount of pre-tightening torque and the pre-tightening 1.2 torque itself is exponential relationship, and T = 31.32P. This relationship can be used to determine the number of pre-tightening torque of needle roller thrust bears. (3) Torque friction factor of the bears will be decrease slightly along with increase of the pretightening torque, average friction is among References [1] Li Yaohua, Liu Jingyu, Ma Jian, Yu Qiang. A Simplfied Voltage Vector Selection Strategy for Direct Torque Control. TELKOMNIKA, 2011;.9(3): [2] Gayuh Titis Perman, Maman Abdururohman, Mohanmmad Khairudin,Mohammad Lutfi. Automated Navigation System based on Weapon-Target Assignment. TELKOMNIKA, 2011;.9(3): [3] Li Zhenzong, Zhang Qiping, Xu Yin, and Mao Mingming. Analysis to failure mechanism and service life of rolling bears. Equipments machine, 2012; 1:10 Research on Service Life Prediction Model of Thrust Needle Bearing (Hu Zhanqi)

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