1,MC pulley failure form and reason analysis　

　　MC nylon material becomes polyamide chemically and consists of covalent and molecular bonds, i.e. intra-molecular bonded by covalent bonds and inter-molecular bonded by molecular bonds. This structure of the material has a variety of advantages such as light weight, wear resistance, corrosion resistance, insulation, etc. It is a very widely used engineering plastic [1].　

　　The MC nylon pulley applied to the shield door of Tianjin Metro Line 2 will have the following two forms of failure after a period of time: (1) wear on the outer edge of the pulley; (2) clearance between the inner ring of the pulley and the bearing.

The reasons for the above two forms of failure, the following analysis is done.　

　　(1) The door body is not correct, and the position of the pulley will be incorrect during operation, which will cause the outer edge to wear, and the force of the inner side of the pulley and the bearing will appear in different directions of space stress.　

　　(2) track is not straight or track surface is not flat, causing wear on the outside.　

　　(3) When the door opens and closes, the sliding door moves, the sliding wheel is subjected to cyclic load for a long time, resulting in fatigue deformation, the inner wheel of the pulley is deformed and a gap is generated.　

　　(4) door at rest, the pulley has been bearing the weight of the sliding door, a long time to bear the fixed load, resulting in creep deformation.　

　　(5) There is a hardness difference between the bearing and the pulley, and the long time extrusion action will produce deformation and cause failure [2].　

　　2 MC pulley life calculation process　

　　MC nylon pulley is a polymer structure of engineering materials, in the actual working operation, by the temperature as well as the role of the load, the molecular structure of irreversible deformation, which eventually leads to the destruction of the material [3].　

　　(1) Considered in terms of temperature: with the change of temperature within the environment, the following relationship exists between the physical properties of the equipment components and the time of failure, expressed as a function of　

　　F (P) = Kτ (1)　

　　where P is the physical and mechanical property value; K is the reaction rate constant; τ is the aging time.　

　　If the material is determined, then the value P of physical parameters of this material is determined, and the guaranteed values of tensile and bending are set above 80%, then the relationship between the critical time and K constant is　

　　τ=F(P)/K (2)　

　　The K constant and the temperature T satisfy the following relationship.　

　　K=Ae(- E/RT) (3)　

　　where E is the activation energy; R is the ideal gas constant; A and e are constants. Taking the logarithm of the above two formulas mathematically and processing the deformation, we get　

　　lnτ = E/(2.303RT) C (4)　

　　In the above obtained equation, C is a constant. According to the above equation, it is known that there is a similar positive relationship between critical time and temperature . Continuing with the deformation of the above equation, we obtain.　

　　lnτ=a b/T (5)　

　　According to the theory of numerical analysis, the constants a and b in the above equation are determined, and the critical life at the service temperature can be calculated.　

　　Tianjin metro line 2 is basically an underground station, due to the role of the shield door and ring control, the temperature at which the pulley is located is relatively stable throughout the year, measured by taking the average value of 25 °, after checking the table, we can get a = -2.117, b = 2220, bring t = 25 ° into (5), we can get τ = 25.4 years. Take the safety factor of 0.6, and get the safety value of 20.3 years.　

　　(2) load on the fatigue life analysis: the above projection for the consideration of the temperature of the pulley life calculation, and in actual use, the pulley will also be subject to the role of load, its principle is: polymer molecular structure under the action of alternating load produced irreversible evolution and deformation of the molecular structure, mechanical workers on the role of molecular chain, produced rotation and distortion, the formation of the silver pattern and shear band silver pattern, foreshadowing fatigue, with The accumulation of a large number of alternating cycle loading, the silver pattern gradually expanded, forming a crack, and sharply widened, and eventually led to the fracture of the material damage.　

　　In this life calculation, the life analysis is carried out under the conditions of ideal environment, i.e. the track is flat and the door body position is also flat.　

　　First consider the impact of load frequency on the life: each sliding door has four pulleys, each pulley share a quarter of the door weight, after checking the information that a sliding door weight is 80kg, the gravity of a door can be obtained: 80 × 9.8 = 784 N.　

　　Then share the gravity on each pulley as: 784 ÷ 4 = 196 N.　

　　The width of the sliding door is 1m, that is, each time the door is opened and closed for 1m, and then measure the diameter of the pulley is 0.057m, can be calculated as its perimeter: 0.057 × 3.14 = 0.179m.　

　　Then the sliding door opens once, the number of turns the pulley needs to go can be derived: 1 ÷ 0.179 = 5.6 turns.　

　　According to the data given by the Department of Traffic Management, the number of runs on one side of a month is 4032, which can be derived from the number of runs per day: 4032 ÷ 30 = 134.　

　　every morning the station will test the screen door about 10 times, so the total number of sliding door movements per day is: 134 10 = 144 times.　

　　sliding door switch once, the pulley to go 11.2 turns, a day sliding door has 144 switch cycle, so the total number of pulley laps a day: 144 × 5.6 = 806.4 turns.　

　　Each lap of the pulley, we must be subject to a cycle of force, so that we can get its force frequency: 806.4 ÷ (24 × 3600) = 0.0093 Hz.　

　　After checking the data, 0.0093 Hz this frequency corresponds to the number of cycles close to infinity, indicating that the frequency of the load is very low, here do not need to consider.　

　　(3) again consider the impact of the pressure on the life: after analysis, the contact between the pulley and the track for the surface contact, roughly estimated its area: 0.001.1 × 0.001.1 = 1.21 × 10-6m2　

　　According to the pressure metric: P = F / S = 196 ÷ 1.21 × 10-6 = 161 × 106 = 161MPa　

　　After checking the table, the number of cycles corresponding to 161MPa is 0.24×106; according to the monthly cycle number 4032 times, the number of cycles in a year can be obtained: 4032×12=48384 times　

　　Then we can get this pressure corresponding to the life of the pulley: 0.24 × 106 ÷ 48384 = 4.9 years