Bushings, bearings, pins and other parts are often assembled with an interference fit.

Back in the day, Larry E. Stockline, president of Promess Inc. (Brighton, MI), was a machinist. As he set up a lathe to cut a shoulder into a shaft, he'd wonder, "Why is that feature necessary?"

The answer, of course, was that the shoulder served as a stop for whatever part would eventually be pressed onto the shaft. But, if the location of the shoulder was off a bit either way, and the assembly didn't function correctly, well, c'est la vie. That's just how the process worked.

These days, the process of pressing two parts together to make an assembly is much more sophisticated. Presses can be equipped with sensors that provide feedback on the assembly process. "Today, you can press the part to a very precise position," says Stockline. "It doesn't matter if there's a shoulder or not. In fact, press monitoring technology can often eliminate the need for a shoulder."

Also known as a force fit or interference fit, 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.

"If the components are correctly designed, press-fitting is the least expensive assembly method, as opposed to gluing, welding or fastening," says Daniel P. Baumann, president of Schmidt Technology (Cranberry Township, PA).

Part Design Issues

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, similar or dissimilar. However, if dissimilar materials are used, engineers should choose materials with similar coefficients of thermal expansion, but different hardness ratings.

"We've done press fits with extremely hard metals, and we've done them with very soft plastics. It all depends on the application," says Charles A. Rupprecht, executive vice president of BalTec Corp. (Canonsburg, PA).

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," advises Martin Frischknecht, vice president of engineering at Schmidt.

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, you're going to see tighter tolerances than you'd see with a stud that's being pressed into the leg of an office chair," says Rupprecht.

The roundness and the surface finish of the parts can also affect the quality of a press-fit assembly. "No part is perfectly round," says Stockline. "In most cases, that's OK, because you will fill any voids during assembly. You're making the part round."

Equipment Needs

Pneumatic, hydropneumatic, electric and even manual presses can be used to assemble press fits, but straight-acting presses are preferred over toggle presses. "The most common units our customers use are straight-acting pneumatic presses ranging in size from 4 to 56 kilonewtons," says Rupprecht.

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 (see sidebar), but the results are just that-estimates.

"It amazes me how many engineers design an interference fit, but then don't know how much force it will take to press the parts together. They usually underestimate it," says Baumann. "The force is very difficult to calculate. That's why we advise our customers not to rely on calculations, but to do some testing."

Tooling for press-fit assembly must accurately and consistently align the parts. However, that doesn't necessarily mean the parts should be held rigidly. It depends on their geometry.

"In some cases, the part can center itself," notes Frischknecht. "In that case, we may build some compliance into the tooling. You want the part to line up well enough so that it can find the hole it's being pressed into. You don't want to fight the component."

Controlling both the speed and smooth advance of the ram are important, particularly for delicate assemblies. "If the ram has a jerky motion, that may put added stress on the material," says Frischknecht.

Force monitoring can enhance the accuracy of press-fit assembly significantly. A force sensor can be installed between the cylinder and the ram. The sensor can measure the force produced during the entire assembly cycle, or an adjustable limit switch can be used to set a specific position at which the force is reported. Through the press controller, 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.

A distance sensor can be added to the force sensor so that the press controller can display curves of force over distance or force over time. Measuring both force and distance during the pressing operation can give assemblers valuable information about the quality of the parts and the assembly.

"If you're pushing something over a 1-inch distance, you want to know where along that 1 inch was the most interference," explains Stockline. "If it's high at the beginning and then it suddenly drops off, it could mean that the part has a taper to it. Ideally, the [force over distance] curve should be a nice straight line. But if there's some distortion in the parts, that curve won't look so picturesque."

Often, press monitoring can identify part defects that aren't visibly apparent, says Stockline. 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 over distance curve.

Lubricating the parts prior to assembly can prevent galling and ensure a smooth, clean installation. However, Rupprecht says lubrication is rarely necessary. "If it's a very hard part, or if you're worried about marring, you may consider lubrication," he says. "A lot of the need for lubrication can be eliminated through careful design of the upper press tooling, which will assist in inserting the part correctly."

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.

Despite those benefits, however, adhesive is rarely used with press-fit assembly, says Rupprecht. "Most of the time, the parts are designed for a pure press fit," he says.

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.

Do the Math

If the press fit involves inserting a round pin or shaft into a round collar or hole, and if both parts are made of the same material, the following formulas can be used to estimate the pressures and stresses related to the assembly.

The radial pressure, P, in pounds per square inch, between the pin and the collar can be determined by:

P = [E * I * (Dc2 - Dp2)] / (Dp * Dc2)

where E = the modulus of elasticity of the material, in pounds per square inch; I = the initial difference between the ID of the collar and the diameter of the pin, in inches; Dc = the OD of the collar, in inches; and Dp is the OD of the pin, in inches.

The maximum effective stress, S, in pounds per square inch, occurs in the collar at its ID and can be calculated as:

S = (2 * P * Dc2) / (Dc2 - Dp2)

The force, F, in pounds, required to push the pin back out of the collar can be approximated by:

F = P * L * π * Di * Cf

where L = the length of engagement between the pin and the collar, in inches; Di = the ID of the collar; and Cf = the material's coefficient of friction.

The breakaway torque, T, in inch-pounds, of the assembly can be calculated by:

T = (P * L * π * Di2 * Cf) / 2

These formulas were adapted from the following sources:

  • Groover, Mikell P., Fundamentals of Modern Manufacturing, Prentice Hall Inc., 1996.
  • Toback, Alex, et. al., Cylindrical Assemblies With Adhesives, Henkel Loctite Corp., 1985.