Time to read: 5 min
适应许多情况，将零件保持完美的对齐方式，轻松引入但形成持久的联系 - 我可以描述媒体合适或您喜欢的一条牛仔裤。Press Fits具有许多出色的功能，您可能会想知道为什么他们不取代所有其他连接。
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?
没有按塑料安装
查看干扰拟合应用程序，最简单的开始是确定的否：切勿在塑料中使用按下拟合。为什么？两个词：冷蠕变。
压力拟合依赖于恒定的压力和摩擦。在钢铁中，如果您将一个超大的销钉压入一个洞中，它们将无限期地保持在一起。但是塑料会在恒定应变下流动，最终导致应力（因此）消失。就像牛仔布在黑色领带接待处一样，这绝不是一个好主意。
Calculating Force in Interference Fits
否则，让我们看一下正确使用媒体。如前所述，装配方法依赖于试图占据相同空间的两个部分。但是多少干扰是正确的？
同样，考虑一下安装牛仔裤：有点紧好；太紧了，您可能会冒着不可能的合身，或者看起来像Biebs。（“信徒”的注释：您可以跳到文章的结尾。我没有什么可以教你的。）
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?
我们将稍后进入公式，但让我们从心理模型开始。当销钉被压入孔中时，销钉向外压，试图重新恢复其原始直径，而孔则向内压，也试图重新恢复其原始直径（不是所有人吗？）。这两个部分相互压力的压力使我们具有正常的力，借助摩擦系数，我们可以计算结果。
在一个实际的例子中，请查看被压入一个的钢销钉钢板在一个名义英寸直径,一寸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（资源)
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…
Tolerances and Alignment Restrictions
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 notDFM/DFA friendly.
您可以在这里使用计算器在孔上找到具有不同截止性公差的最小固定力和最大组装力。但是，直径并不是要考虑的唯一容忍度：销钉经常成对，因此也要考虑销钉之间的距离。
Download our free Press Fit Calculator here!
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 useGD＆T耐受性, with the first hole as the datum for the second hole, to minimize error between the two features.
像材料和热限制
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.
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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?
看着线性扩展的系数在这两个部分中，我们可以看到，在各种程度上，零件很酷，CNC铝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.
过度和替代关节
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 forself-locatingand bolts to join the parts. Inplastics，使用定位引脚进行对齐和快照for assembly. These are just a couple quick examples; there are a nearly unlimited options for机械组件.
您应该在设计中使用按拟合吗？
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 ourfree press fit calculator here然后还查看我们的3D打印andCNC machining services!
