Machinery's Handbook, 31st Edition
STRUCTURAL ANALYSIS OF PLASTICS 581 Press Fits: Press fits are used widely in assembly work for speed and convenience, although they sometimes are unsatisfactory with thermoplastics parts. Common applica tions are to a plastics hub or boss accepting a plastics or metal shaft or pin. Forcing the pin into the hole expands the hub, creating a tensile or hoop stress. If the interference is too great, very high strain and stress develop, and the plastics part may a) fail immediately by developing a crack parallel to the hub axis to relieve the stress, a typical hoop stress failure; b) survive assembly but fail prematurely due to creep rupture caused by the high induced-stress levels; or c) undergo stress relaxation sufficient to re - duce the stress to a level that can be sustained, which can result in slippage during end use. For a typical press fit, the allowable design stress depends on the particular plastics material, temperature, and other environmental considerations. Hoop stress equations for such a design make use of a geometry factor g : (21) where d s = diameter of the pin to be inserted and d o = outside diameter of the boss. When both the shaft and the hub are of the same or essentially the same materials, the hoop stress s given the diametral interference i = d s − d i is (22) and the allowable interference i a given the permissible design stress s a is (23) When the shaft is metal and the hub is plastic, the hoop stress given i is obtained from (24) and the allowable interference i a given the permissible design stress for plastic’s s a is (25) where E p = modulus of elasticity of plastics and ν p = Poisson’s ratio for plastics. Pipe Threads: Pipe threads on plastics pipes and other parts used in plastics plumbing and pneumatic assemblies typically require only hand tight assembly to effect a good seat, especially if a compatible sealant tape or compound is used. Assembling a tapered male pipe thread into a mating female thread in a plastics part is analogous to driving a cone into a round hole and may result in a split boss. Sometimes straight threads and an O-ring seal can avoid the need for pipe threads. When pipe threads must be used, torque control is essential. When mating metal to plastics pipe threads, the threaded plastics component should be the male member, so that the plastics are in compression. If torque can be controlled during assembly, fluoroplastics tape should be used on female plastics pipe threads. If torque cannot be controlled, an external hoop ring, either pressed on or molded in, should be considered. Flats should not be designed into plastics parts for assembly purposes because they can encourage overtightening. If some provision for improved gripping must be made, wings or a textured surface may be used. An approximate formula for the hoop stress s produced in a plastics boss with internal pipe threads is (26) where T = torque in in-lb, t = wall thickness of the plastics boss in inches, d = pipe outside diameter in inches, and L = length of thread engagement in inches. tdL T 3 σ= γ = 1 + ( d s ⁄ d o ) 2 1 − ( d s ⁄ d o ) 2 d i E 1 s p σ γ γ = + i d E a s p a σ = 1 γ γ + d i E s p p + ^ h γ ν γ σ = i d E a s a = ν p p σ γ + γ
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