A press fit is an assembly in which one part is inserted tightly into a hole in another part. The inserted part is typically 0.001 to 0.002 inch larger than the mating hole. The assembly stays in place through friction and the force of the two parts pushing against each other. In most cases, the press fit is strong enough to stand on its own. In others, the joint is augmented by an additional assembly method, such as adhesive bonding or brazing.
Many parts are assembled with press fits, including bushings, bearings, pins, studs, rotors, gears, pulleys, shaft collars and gland seals. In the automotive industry, press fits are used to assemble valve seats, fuel injectors, cylinder sleeves, muffler baffles, transmission components and impellers for water pumps.
Most press-fit parts are round, but they can also be oval, square, rectangular or triangular. The parts can even have a keyway. The part to be inserted can be solid, like a pin, or hollow, like a bushing. Either way, putting a taper on the inserted part and a chamfer on the mating part will help the parts go together smoothly.
Press-fit parts can be metal or plastic, and similar or dissimilar materials. However, if dissimilar materials are used, engineers should choose materials with similar coefficients of thermal expansion, but different hardness ratings.
Choosing materials for press fits, particularly plastics, often involves trade-offs. For example, polycarbonate isn’t recommended for shaft hubs, because it doesn’t tolerate excessive hoop stress. More flexible plastics, such as nylon, ABS and thermoplastic polyurethane, can better tolerate hoop stress, but may also exhibit more stress relaxation over time.
Brittle materials, such as die-cast metals, are not good choices for press-fit assembly, particularly for thin-walled parts. If the hub has thin walls, consider a material that is somewhat pliable.
Tolerances for press-fit parts are more a function of the product and the materials than the assembly method. For example, tolerances will become tighter as material hardness increases. For critical assemblies, like a pawl or lever in a lock, parts will have tighter tolerances than, say, a stud that’s being pressed into the leg of an office chair.
The roundness and the surface finish of the parts can also affect the quality of a press-fit assembly. For example, no part is perfectly round. In most cases, however, that’s OK, because any voids between the parts will be filled in during assembly.
For press-fit assembly, straight-acting presses are preferred over toggle presses. The amount of force required to press parts together depends on the hardness, slipperiness and surface finish of the materials; the size, thickness and geometry of the parts; and the amount of interference between them, that is, the difference in size between the inserted part and the hole. Various formulas can help engineers estimate press force to start, but testing is required to pin it down.
Controlling both the speed and smooth advance of the ram are important, particularly for delicate assemblies.
Force and distance monitoring can enhance the accuracy of press-fit assembly significantly. Engineers can set upper and lower limits for the pressing force. If the force falls above or below the set figure, an alarm is signaled. For example, if the press is pushing a part over a 1-inch distance, engineers will want to know where the most interference occurred along that 1 inch distance. If it’s high at the beginning and then it suddenly drops off, that could mean the part has a taper to it. Ideally, the force-distance curve should be a straight line.
Often, press monitoring can identify part defects that aren’t visibly apparent. For example, a part that hasn’t been heat treated is indistinguishable from a part that has. But, if the untreated part is used in the assembly, the press will immediately identify the defective part because it will produce an abnormal force-distance curve.
Lubricating the parts prior to assembly can prevent galling and ensure a smooth, clean installation. However, lubrication is rarely necessary.
Anaerobic adhesive is another material that can be applied to the parts to augment press-fit assembly. The adhesive will completely seal the joint, prevent corrosion, distribute stress more evenly, and produce a stronger, more rigid assembly. The adhesive also allows engineers to loosen the tolerance requirements for the parts and reduce the bulkiness of the parts, which is needed to generate the pressure that holds the parts together.
If assemblers do use adhesive to complement press-fit assembly, they should remember that the adhesive will significantly increase the amount of force needed to assemble the parts. As a result, assemblers may need to increase the amount of force applied by the press or increase the speed of the ram.