Design the MC nylon sleeve


Cast nylon sleeve allowable working condition is caused by many factors, such as different formulations and molding the obtained material physical and mechanical properties, sleeve design, assembly methods, load characteristics, running speed, lubrication, shaft set working environment (temperature, humidity, the influence of impurity), the work of the friction conditions, the material of the grinding performance.  Therefore, in the design of the hub, should be considered in all aspects.
  1 sleeve design and the PV value is calculated
  Sleeve design, the most important thing is to ensure the sleeve material to its full performance under the premise of working conditions to master the limit.  Generally, we use the factor of generating heat - the load per unit area of P and the surface speed of the product V PV value to determine the limits.  PV value is too large, easily lead bushing temperature rise.  Heavy at low speed, the use of nylon bushings prominent advantages.
  PV value is calculated as follows:
  V = лdn/60 (1)
  The formula: V - surface speed, unit: m / s
  d - shaft diameter, unit: m
  n - sleeve speed unit: r / min
  P = W / (dl) (2)
  Where: P - the load per unit area, which added to the total load on the sleeve and the sleeve projected area ratio, unit: Pa
  W - sleeve suffered total load, unit: N
  d - shaft diameter, unit: m
  l - sleeve length, unit: m

  2. PV value is used
  2.1 PV limit PVa
  The ambient temperature is 24 ℃ and the specific lubrication, continuous operation of the PV value called PV limit, with the PVa to represent.  24 ℃ in the non-state and non-continuous operation, the PVa is the basic value, with the temperature correction coefficient and running time correction factor to be amended.  Table 1 PVa of some engineering plastics.
 


 Table 1 PVa of some engineering plastics (Unit: MPa.m / s) 

 Type of material   Non-lubricated   Periodic lubrication 
 MCPA   0.11   0.57 
 Nylon 66   0.095   0.40 
 PTFE   0.035   0.045 
 Polyacetal   0.092   0.36 
 
 

 Note: Continuous to oil, the lubricant varieties and a slight change in dosage, PV value will change a lot.
 


 2.2 Temperature correction coefficient T
  The ambient temperature and does not include the operation of the friction
  Heat the sleeve temperature.  Ambient temperature is high, the shaft
  Sets of heat and feeble, carrying capacity decreases.  Figure 1 provides
  Temperature and the temperature correction coefficient T between.

  Running time correction factor C 2.3
  Sleeve only works intermittently, at this time does not stop working to create friction heat, but the heat is still in progress.  At this point, the temperature rise is gradually declining sleeve.  If the operating time is shorter than the stop time, the heat rising is low, and the PV value is higher.  If the running time of over 10 minutes, it should be treated as a continuous working condition.  Table 2 shows the operating state intermittent operation time and the time between the correction factor C.  In the figure, the curve 1X, 2X, 3X ...... means stop time and running time.

  2.4 Maximum permissible PV value

 PV = PVa.TC (3)


 The formula: PVa - Richard from Table 1, MPa.m / s
 T - temperature correction coefficient, from Table 1 Richard
 C - time correction coefficient, from Table 2 Richard
 When the ambient temperature is 24 ℃ (often referred to as ambient temperature) in the case, T value is 1.0; when continuous operation, C value is 1.0.  At this point, using the values ??in Table 1 can be regarded as the maximum allowable PV value.  If continuous operation, PV value <0.1, we can use non-lubricated, but preferably coated with dry grease during assembly.  If the PV value> = O.1, you must give periodic lubrication.  Under dry oil lubrication in the case, PV value limit may reach 1.0.  To ensure that the problem does not appear to use and effective work, should control the maximum allowable PV value at 0.22-0.24; in a circulating oil or sleeve in the oil used, the maximum allowable PV value can be increased to 2.5 or even 2.5 more, because this situation heat in good condition.

 Table 2 The loading P, sliding friction linear velocity V, PV value and the friction coefficient
 

 

 Materials   Stress PMPa   Allowable speed V m / s   PVa value MPa.m / s   Coefficient of friction   Wear steel shaft (not quenched, lubricated) mg / (cm2.km) 
 No lubrication   Lubricating 
 Bearing alloy   20   50   10   0.280   0.005   0/005 
 Bronze   10   3   15   0.120   0.01   0.03 
 MCPA   10   3   10   0.156   0.0185   Not found 
 Graphite cast nylon +5%   10   4   12   0.080   0.015   Not found 
 Graphite cast nylon +30%   8   10   15   0.025   0.008   Not found 
 Plastic wood   6   3.5   8   0.160   0.02   Not found 
 
 

 In the process of using nylon sleeve, sometimes showing creep phenomenon, or "cold."  When the load exceeds the PV limit one of the reasons for this phenomenon.
 

 3 sleeve with the gap
  3.1 bushings amount of interference <br /> sleeve amount of interference in practice can be obtained by pressing the empirical formula:
  h = 0.002D (4)
  h - sleeve amount of interference, mm
  D - stem diameter, mm
  Sleeve outer diameter, wall thickness increased accordingly, fit tension is also large, the amount of interference desirable smaller; otherwise, then take bigger.  Large aspect ratio bushings, some take a small amount of interference, otherwise take a little bigger.
  Cast nylon thermal expansion coefficient of approximately 10 times larger than steel, so at runtime sleeve caused by frictional heat expansion deformation of the reduced diameter, with the gap affecting the friction surface.

  3.2 bush clearance
  A = A 1 + A 2 + A 3 + A 4 (5)
  A - bush clearance, mm
  A 1 ---- basic gap, mm, the value found from Figure 3,
  A 2 ---- a sleeve wall thickness caused by the change of the gap, mm,
  A 2 = Kt K: sleeve wall thickness variations caused by temperature correction coefficient, is found in Figure 4
  T: sleeve wall thickness, mm
  A 3 ---- press fit through the gap caused by changes in the amount of interference, mm, as shown in Figure 5
  A 4 ---- sleeve wall thickness due to changes caused by moisture, i.e., when the relative humidity is 100% or in the water, use it to correct the gap due to the reduction in moisture absorption, A 4 = 0.055t
  Or use the simple formula: A = (0.005 - 0.006) d 0 (6)
  d 0 is the diameter of the assembled sleeve, mm

  3.3  sleeve wall thickness thermal conductivity due MCPA much smaller than the metal, so do the bushings, the wall thickness should be as thin as some of the.  If you use a slower sliding friction, the load is not great, but the operation is subject to a great impact, and you should use thick bush.  If the sleeve thin wall, lack of rigidity, combined with the use of the metal jacket.  Sleeve wall thickness selection, with the South nylon our technical department.

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