Press-fitting

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. 

Temperature variations during operation cause total interference changes with dimensional changes of the shaft and hub. Total interference at a given temperature above room temperature is determined with the following equation [6]  

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

Pc = contact surface pressure between hub and shaft, MPa (psi) P out = pull-out force, N (lb) 

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Snap-Fit and Press-FitJoints. Snap-fit joints offer the advantage that the strength of the joint does not diminish with time because of creep. Press-fit joints are simple and inexpensive, but lose hoi ding power. Creep and stress relaxation reduce the effective interference, as do temperature variations, particularly with materials with different thermal expansions. 

Porous P/M products can be sinter bonded to soHd metals. They can also be welded, brazed, or soldered. Filling the voids with flux or molten metal has to be avoided. P/M porous products can be machined, but blocking of the porous passages has to be avoided. Press fitting and epoxy bonding are commonly used. 

The materials used in a total joint replacement ate designed to enable the joint to function normally. The artificial components ate generally composed of a metal piece that fits closely into bone tissue. The metals ate varied and include stainless steel or alloys of cobalt, chrome, and titanium. The plastic material used in implants is a polyethylene that is extremely durable and wear-resistant. Also, a bone cement, a methacrylate, is often used to anchor the artificial joint materials into the bone. Cementiess joint replacements have mote tecentiy been developed. In these replacements, the prosthesis and the bone ate made to fit together without the need for bone cement. The implants ate press-fit into the bone. 

The next advance in total hip arthroplasty came with the development of various porous surface treatments which allow bone tissue to grow into the metal porous coating on the femoral stem of the hip implant and on the acetabular component of the total joint replacement. These developments arose because of patients who were not able to tolerate cemented implants because of allergies to the cement, methylmethacrylate. More youthflil patients are better served by a press-fit implant as well. Figure 12 shows the difference between textured and beaded surface-treated orthopedic prostheses. 

Hydroxyapatite (HA) coating on the surface of the hip stem and the acetabular cup is the most recent advancement in artificial hip joint implant technology. This substance is a form of calcium phosphate, which is sprayed onto the hip implant. It is a material found in combination with calcium carbonate in bone tissue, and bones can easily adapt to it. When bone tissue does grow into HA, the tissue then fixes the hip joint implant permanently in position. These HA coatings are only used in press-fit, noncemented implants. 

In a typical screw assembly, the flights are fabricated, then welded to a pipe that has bushings press-fitted or welded into each end to provide reinforcing for the conveyor couplings. There are two types of flights heUcoid and sectional. 

Fig. 2. Press-Fit pias of A, soHd B, crescent or C, spHt-beam (compHant) design, are forced iato through-holes of priated circuit boards where they...

Figure 18 shows a typical line of compression presses fitted with vacuum chambers. This type of press can be purchased with platen sizes in the range 355 x 355 mm to 710 x 710 mm. The choice of press size is dependent on product size, product design, and volume. 

Brass water fittings are normally produced from two-phase brass by hot pressing. Unfortunately this material is vulnerable to dezincification in certain water areas. In areas where the hot pressed fittings are vulnerable. 

A typical anode for practical use would be in the order of 25 to 48 mm in diameter, with hard platinum alloy pins of 0-50 mm diameter by 10 mm length, spaced every 150 to 300 mm and progressively positioned around the circumferenceThe pins are a press fit into holes in the lead or lead alloy (approximately 01 mm diametric interference) and lie flush with the surface. The lead is peened around the pins to improve the mechanical and electrical contact. 

When two metals in intimate contact are subjected to vibration, a dark powder forms at the areas of contact. The effect is referred to as fretting corrosion though it is due to wear rather than true corrosive attack. The galling effect between nickel and steel ensures good resistance to fretting corrosion and lubricated nickel against steel is a very satisfactory combination used widely in industry for components assembled by press-fitting. 

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

Many designs incorporate the phenomenon of stress-relaxation. For example, in many products, when plastics are assembled they are placed into a permanently deflected condition, as for instance press fits, bolted assemblies, and some plastic springs. In time, with the strain kept constant the stress level will decrease, from the same internal molecular movement that produces creep. This gradual decay in stress at a constant strain (stress-relaxation) becomes important in applications such as preloaded bolts and springs where there is concern for retaining the load. The amount of relaxation can be measured by applying a fixed strain to a sample and then measuring the load with time. 

Frequently, a product becomes loaded when it is subjected to a defined deflection. The actual load then is a result of the structural reaction of the product to the applied strain. Unlike directly applied loads, strain-induced loads are dependent on the modulus of elasticity and, with TPs, will generally decrease in magnitude over time. Many assembly and thermal stresses could be the result of strain-induced loads. They include metal insert press fits in the plastic and clamping or screw attachments. 

A common use is with a plastic hub or boss accepting either a plastic or metal insert. The press fit operation tends to expand the hub creating a tensile or hoop stress. If the interference is too great, a very high strain and stress will develop. The plastic product will (1) fail immediately by developing a crack parallel to the axis of the hub to relieve the stress, a typical hoop stress failure, (2) survive assembly but fail prematurely when the product is in use for a variety of... 

A simpler, although less accurate, method of evaluating these press fits is to assume that the shaft will not deform when pressed into the plastic. This is reasonably accurate when a metal shaft is used in a plastic hub. The hoop strain developed that is reasonably accurate in the hub is then given by the equation ... 

A second approach is to carefully machine the rod and hole to obtain a good press fit. The copper is soft and difficult to machine, but it can be done. The carbon is hard and difficult to machine, but it also can be done. It is a lot of work and it is difficult to push a piece of copper into a closely fitting hole 110 cm deep in a piece of carbon. 

A sludge is filtered in a plate and frame press fitted with 25 mm frames. For the first 600 s the slurry pump runs at maximum capacity. During this period the pressure difference rises to 500 kN/m2 and a quarter of the total filtrate is obtained. The filtration takes a further 3600 s to a complete at constant pressure and 900 s is required for emptying and resetting the press. 

When an aqueous slurry is filtered in a plate and frame press, fitted with two 50 mm thick frames each 150 mm square at a pressure difference of 350 kN/m2, the frames are filled in 3600 s. The liquid in the slurry has the same density as water. 

Press-fitting the working electrode in ptfe or PCTFE or even melt-fitting the working electrode in glass does not overcome the problem after prolonged use and electrode exchange will be the only remedy. 

All SERS experiments were conducted with a polycrystalline silver working electrode prepared by press-fitting a 6 mm diameter cylinder of silver into one end of a 0.375 inch diameter Teflon rod through which a 6 nm diameter concentric hole had been drilled. Electrical contact was made via a copper wire soldered to the silver. The geometric area of the silver disk was 0.28 cm2. 

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