Press-fit assemblies

Horger undertook rotating-beam fatigue tests of press-fitted assemblies using specimens as large as 305 mm diameter shafts. 

Self-fastening joint designs generally produce very high stresses in the plastic part during the assembly operation. With brittle plastics, such as thermosets, press-fit assembly may cause the plastic to crack if conditions are not carefully controlled. In addition, certain plastics, especially thermoplastics, are subject to cold flow under stress. Under continued stress, which is the fundamental adhesive that holds self-fastened parts together, the plastic may relax, causing the joint to fad. 

Polystyrene modified polyphenylene oxide can be joined with almost all techniques described in this chapter. Snap-fit and press-fit assemblies can be easily made with this material. Maximum strain limit of 8 percent is commonly used in the fiexing member of PPO parts. Metal screw and bolts are commonly used to assemble PPO parts or for attaching various components. 

Press fitting produces very high stresses in the plastic parts. With brittle plastics, such as thermosets, press-fit assembly may cause the plastic to crack if conditions are not carefully controlled. 

Press-fit assemblies are extensively used in high-performance applications. PAI press-fit suspension bushings are used in race cars [20]. Drill bushings are press-fit assembled [21] solderless connectors are press-fit into printed circuitboards [22] compliant press-fit connectors are assembled with bi-spring press-fit contacts [23] security magnet sensors with crush-proof plastic casings are press-fit installed on doors, windows, and cabinets [24] and General Electric Home Security sensors are press-fit assembled [25]. 

The maximum allowable diametral interference between a shaft and a hub for press-fitting assembly is a function of part geometry, stress, modulus of elasticity of shaft and hub, and Poisson s ratio for shaft and hub materials. When the shaft and hub are made of the same material, the moduli of elasticity are the same and Poisson s ratio is the same for the shaft and hub, simplifying the equation for maximum diametral interference. Calculating diametral interference is simplified when the shaft is made of high-modulus material. 

Optimum insertion force is a high-value property for press-fit assembly. It is determined by the following equation [6] ... 

In some applications, torsion is applied to a press-fit assembly. Transmitted torque M is calculated by [6]... 

Anderson Power Products, Press-Fit Assembled Printed Circuitboards Datasheets, Sterling, Mass., USA. 

Positronic Industries, Inc., Press-Fit Assembled Security Sensors Datasheets, Springfield, Mo., USA. 

General Electric, Press-Fit Assembled Home Security Sensors hterature, Fairfield, Conn., USA. 

All component leads attach to the PCB by being inserted into holes that pass through the PCB.The components may be secured by wave soldering or by pressing into holes that result in an interference fit (press fit). Assembly involves a component placement operation followed by a wave-soldering operation. This method is still the workhorse of the low-cost consumer electronics industry. 

Instead, the press-fit pin is anchored solely by the mechanical interference between connector lead and the PTH barrel of the PWB. Oxides on the surface of the press-fit pin and mating PTH barrel are broken loose by the high frictional forces associated with the press-fit assembly operation, so nobility of materials, oxide build-ups, and material shelf-life are less critical considerations than in soldering. If conditions are proper in terms of sizing and materials, the press-fit process results in a gas-tight contact between the connector pin and the FTH barrel wall. 

For best results, a method that allows real-time measurement and control of the pressing process is preferred and is available on today s commercial presses.There are four commonly used pressing routines, but only three take advantage of force-sensing, distance-sensing, or real-time feedback control of the connector press. Connector complexity and equipment availability dictate press-fit assembly methodology. 

Press-fit assemblies are generally designed to accommodate at least two connector replacement cycles. The two cycles are predicated on (1) the normal force exerted by the press-pin s compliant section on the PWB s plated through-hole barrel wall, and (2) the quality and condition of the PWB s plated through-hole barrel surface after subsequent press-fit connector removals and replacements. 

Associated with press fit assembly methods are others such as molded-in inserts usually used to develop good holding power between the insert and the molded plastic. 

In recent years, adhesive-like formulations have found increasing use as retaining compounds to augment the strength of, or even to replace, press-fit assemblies. 

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