![]() ![]() You can use the calculator here to find the minimum holding force and maximum assembly force with different diametrical tolerances on the holes. 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’ll need to be very careful when specifying press fits or risk breaking the hydraulic press. And the axial holding force is not just keeping your parts together-it’s also the force required for assembly. Small interference results in enormous force. Which takes us to our next area of discussion… Tolerances and Alignment Restrictions 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. However, with a bolt, you can drill a hole with a diameter tolerance of 0.020 inches. In comparison, a half-inch bolt can hold more than twice that amount. 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). 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).Īnd 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: What is the pressure between the parts? We can calculate that using: That may seem small, but as you’re about to see, that’s actually quite a lot. With a suggested minimum hole size of 0.4995 inches, we end up with 0.0007 inches of diametrical interference. (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. 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.įor a practical example, look at a steel dowel pin pressed into a steel plate at a nominal half-inch diameter, one inch deep. 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?). We’ll get into the formula in a moment, but let’s start with a mental model. But how do you know if the interference is enough to hold the parts together in your design? Use common tables to determine interference though deviation is possible, these are a good baseline. ![]() For ease of calculation, we’ll use dowel pins (other press fits require exorbitant machining). Unlike jeans, we can calculate exactly how tight the interference fit should be. (A note for “Beliebers”: You can skip to the end of the article. ![]() But how much interference is right?Īgain, think about fitting jeans: A little tight is good too tight and you risk an impossible fit, or looking like the Biebs. As mentioned, the assembly method relies on having two parts trying to occupy the same space. With that negativity out of the way, let’s look at the proper use of press fits. Like denim at a black tie reception, it’s never a good idea. But plastic will flow under constant strain, eventually causing the stress-and thus the friction-to disappear. ![]() In steel, if you press an oversized pin into a hole, they’ll stay together indefinitely. Press fits rely on constant stress and friction. Looking at interference fit applications, the easiest start is the definite no: Never use press fits in plastics. So, what are the key aspects of press fits, and are they right for your application? No Press Fits in Plastic 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). Press fits have so many great features, you might wonder why they don’t replace all other connections. 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. ![]()
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