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Adapting to many situations, holding parts in perfect alignment, easily introduced but forming a lasting bond—I could either be describing press fits or your favorite pair of jeans. Press fits have so many great features, you might wonder why they don’t replace all other connections.
The truth is, press fits aren’t for every application—just like skinny jeans, there are pros and cons to interference fits (unlike skinny jeans, press fits will never go out of style). So, what are the key aspects of press fits, and are they right for your application?
Looking at interference fit applications, the easiest start is the definite no: Never use press fits in plastics. Why? Two words: cold creep.
Press fits rely on constant stress and friction. In steel, if you press an oversized pin into a hole, they’ll stay together indefinitely. But plastic will flow under constant strain, eventually causing the stress—and thus the friction—to disappear. Like denim at a black tie reception, it’s never a good idea.
With that negativity out of the way, let’s look at the proper use of press fits. As mentioned, the assembly method relies on having two parts trying to occupy the same space. But how much interference is right?
Again, think about fitting jeans: A little tight is good; too tight and you risk an impossible fit, or looking like the Biebs. (A note for “Beliebers”: You can skip to the end of the article. I have nothing to teach you.)
Unlike jeans, we can calculate exactly how tight the interference fit should be. For ease of calculation, we’ll use dowel pins (other press fits require exorbitant machining). Use common tables to determine interference; though deviation is possible, these are a good baseline. But how do you know if the interference is enough to hold the parts together in your design?
We’ll get into the formula in a moment, but let’s start with a mental model. When the pin is pressed into the hole, the pin presses radially outward, trying to regain its original diameter, while the hole presses radially inward, also trying to regain its original diameter (aren’t we all?). The stress of these two parts pressing against each other gives us a normal force which, with the friction coefficient, allows us to calculate the resulting hold.
For a practical example, look at a steel dowel pin pressed into a steel plate at a nominal half-inch diameter, one inch deep. (Why nominal? Because the pin is slightly larger, and the hole slightly smaller, thus nominal—a half-inch in name only.) A standard series half-inch pin is 0.5002 inches in diameter—two ten-thousandths oversized. With a suggested minimum hole size of 0.4995 inches, we end up with 0.0007 inches of diametrical interference. That may seem small, but as you’re about to see, that’s actually quite a lot.
What is the pressure between the parts? We can calculate that using:
Where P is pressure, r is the nominal radius , E is Young’s modulus (sub h = hole; sub p = pin), v is Poisson’s Ratio, and δ is the radial interference (half the diametrical interference).
And once we have the pressure, we calculate the area and use the coefficient of friction between the two parts to determine the axial holding force as:
F = μ . pmax (source)
In our example of a half-inch pin, using 210 GPa for the Young’s modulus, 0.292 for the Poisson’s ratio, and 0.30 for the friction coefficient, the resulting axial force is about 45kN—a little more than the weight of a Ford F350 (incidentally, a great place to wear your favorite jeans). Amazing, right?
In comparison, a half-inch bolt can hold more than twice that amount. However, with a bolt, you can drill a hole with a diameter tolerance of 0.020 inches. With the press fit, if your hole is 0.0007 inches too big, you won’t have any interference at all, so tolerances become extremely important. Which takes us to our next area of discussion…
Small interference results in enormous force. And the axial holding force is not just keeping your parts together—it’s also the force required for assembly. You’ll need to be very careful when specifying press fits or risk breaking the hydraulic press. This tight machining tolerance is also one of the primary reasons you should avoid press fits for common industrial assembly—it’s not DFM/DFA friendly.
You can use the calculator here to find the minimum holding force and maximum assembly force with different diametrical tolerances on the holes. However, the diameter isn’t the only tolerance to consider: Pins often come in pairs, so consider the distance between the pins, too.
Second rule of press fits: never more than two pins per assembly operation. Even better, use only one interference fit and align the parts with a slip-fit second pin. If you must use two press-fit pins, be certain to use GD&T tolerancing, with the first hole as the datum for the second hole, to minimize error between the two features.
Another salient rule of nature: Everything shrinks in the cold, but not all materials shrink at the same rate. This is important when designing press fits—only use like materials if the parts will experience temperature variation.
Let’s say you’ve used a one-inch nominal aluminum pin in a hole on a 410 stainless steel part, with 0.0007 inches of diametrical interference. How cold can it get before the shrinkage completely negates the interference?
Looking at the coefficients of linear expansion in the two parts, we can see that for every degree Fahrenheit the parts cool, the aluminum will shrink by about 0.0000125 in/in, while the steel will shrink by less than half that amount, 0.0000055 in/in. If the parts are assembled at 75 degrees, and then taken to minus 25 degrees, you’ll lose all the holding power of the press fit. So use materials with similar thermal expansion when designing press fits.
The seeming strength of press fits—that they both locate and join—is also a weakness. Instead of independently calculating the necessary accuracy and the necessary strength, these two functions are entangled, creating a machinist’s nightmare of dimensioning to the ten-thousandth (or to the micron, for those across the pond). Though press fits have their limitations, don’t despair: Modern design has many alternatives.
For the traditional use of press fits, where the pins both join and accurately locate the parts, better designs use slip-fit dowel pins for self-locating and bolts to join the parts. In plastics, use locating pins for alignment and snap fits for assembly. These are just a couple quick examples; there are a nearly unlimited options for mechanical assembly.
While press fits are right for some designs—machined parts of similar materials with tight tolerances requiring close-tolerance alignment—using an interference fit may not be right for your assembly; just like while they’re perfect for a date, your jeans aren’t right for that upcoming awards dinner. Try on some alternatives, and if your old-fashioned boss asks why you didn’t use press fits, feel free to send him this article. To test out your design, download our free press fit calculator here and then also check out our 3D printing and CNC services!