Press Fit Calculator and Tolerances [ Free Software ]
Press Fits, also known as friction fit or interference fit are very common. It involves two parts which are pressed together using the force of press of some kind. Friction holds the parts together, and if the metal of the outer part has shrunk onto the inner part, that friction goes way up. Pipe fittings also generate these fits becauses they’re tapered, so the inner thread swells up against the outer thread as the joint is tightened.
Typical examples of Press fitting include holding bearings on shafts or stacking metal plates with shafts as supports.
Ever wonder what press fit tolerances are or where to get a press fit calculator to make it easy for you to machine press fits? Perhaps you need this information for a press fit bearing, for example.
Look no further–we offer a free press fit calculator. In fact, it figures out all sorts of ISO tolerances such as Loose Fit, Sliding Fit, Slight Interference Fit, Force Fit, and more. It looks like this:
If you already have G-Wizard, you can find it under “Geometry” and then “Fits/Tols”, which is short for Fits and Tolerances.
To find the tolerances, you can start from either a hole diameter or a shaft diameter. It’s easier to make a shaft of a particular size on a lathe than a hole on mill (at least for many machinists without digging out a boring head), so I prefer to go from the hole basis.
For example, let’s suppose we want to know the press fit tolerances for a 1″ diameter hole. You’d set that up in G-Wizard like this:
In practice, you’d enter the measured diameter of the hole which you can determine with a telescoping gauge or dial bore gauge.
As you can see, the hole can range from a minimum of 1″ to a maximum of 1.0008″. You’ll need to hold a fairly tight tolerance to hit that range, which why I suggest just measuring the hole and holding the tolerance on the shaft if you can. For a press fit bearing, you have that luxury for the shaft but not for the bearing pocket.
The shaft outer diameter can range from 1.0009″ to 1.0015″. So, we’re going to have at least 0.0001″ (if the hole is 1.0008″ and the shaft outer diameter is 1.0009″) of material interfering with the fit. That’s why it’ll have to be pressed together.
Using this fit calculator, we’ll have you press fitting to pro quality tolerances in no time!
No Press? How About a Shrink Fit With Hot and Cold Temperatures?
It’ll take a press to force the two parts together, or will it? It just so happens that another free calculator that’s built into G-Wizard is a thermal expansion calculator. I wonder what temperature difference is needed to expand the hole enough to clear the 1″ shaft if the shaft is the worst case 1.0015″?
In other words, we need to expand the hole, which is treated as a ring in G-Wizard, to 1.0016″ or more. Here’s that problem all set up for you in G-Wizard:
Note that I selected a ring, I’m using a standard reference temperature, and my result tells me the change in the radius. Since I want to hit a diameter of 1.0016″, I need a change in radius of 0.0008″. That’s pretty easy. Assuming I measured the original hole diameter at the reference temp (put in whatever temp you measured it at if not), I need to heat it up to 180 degrees and I should be able to carefully slide the shaft in.
I can make things even easier by putting the shaft in the freezer so it shrinks.
Doing this kind of shrink fit or interference fit in the shop is easy if the parts are small enough to use a fridge and an oven to heat and/or freeze them. I do this all the time to put a drill chuck onto an arbor before using it.
Fits and Tolerances are Standards-Based
Wonder how these numbers and calculations came to be? Like so many things, there are standards governing the exact press fit tolerances allowable. In this case they are ISO 286, ANSI B4.2, and DIN 7172. These standards tell Engineers how to choose tolerances when the way parts fit together is important. The whole topic is called “Fits and Tolerances”, and there are various kinds of fits such as Sliding Fit, Press Fit, Force Fit, Free Running Fit, and so on. The standards ensure nobody has to guess what is meant by a term like “Press Fit” so that parts are consistently made and their performance can be relied on.
Fits and Tolerances involve a fair bit of math, so the best way to deal with them is not as a Chart, but with a special Fits and Tolerances Calculator. G-Wizard includes a free ISO Tolerances Calculator that makes a convenient reference when you’re doing Fits and Tolerances work.
You access it from the “Geometry” tab. If you have G-Wizard Calculator handy, bring it up now so you can play with it as you’re reading. If not, take a minute to sign up for the free trial, we’ll wait for you.
The next thing is, “What exactly are Fits and Tolerances and why would I need a calculator for them?”
I thought you’d never ask!
Basically, whenever we’re designing a system with a shaft and a hole, Fits and Tolerances are a set of standards that define how large they should be relative to one another and what their tolerances should be for them to perform in the way you need them to. There are a series of different types of “Fit” you might want to consider. G-Wizard’s Fit Calculator offers the following types of Fits:
– Loose fit: Used where the most clearance is needed on things like latches or parts that may corrode
– Free running: Shafts rotate easily without noticeable clearance or gap in the hole
– Easy running: The part slides easily
– Sliding fit: Precise clearances for guiding and centering
– Close clearance: Getting tighter
– Location clearance: Tight, but still without interference, used for precision location
– Slight interference: Assembly will take a rubber mallet
– Transition: Fit will depend on tolerances and could fall to either side
– Press fit: Cold pressing will be needed for assembly
– Medium fit: Cold pressing with large forces will be needed
– Force fit: Assembly will require different temperatures as well as pressing forces
As you can see, quite a range of applications are covered. An interference fit is one where the inner piece is larger than the hole it fits into. This begins at “Slight Interference”, which requires a rubber mallet, and the effort required as well as the holding power of the fit increases all the way to “Force Fit”.
One of the great things the Industrial Revolution and subsequent manufacturing sciences has brought us is various standards that tell exactly what to do. A number of people had to do a great many experiments to arrive at what would work and what wouldn’t for these various types of fits. G-Wizard Calculator encapsulates the ISO 256, ANSI B4.2, and DIN 7172 standards, which are the most widely used.
So These Calculators are Free?
Yes indeed. We do charge for G-Wizard Calculator, but it’s only for the Feeds and Speeds Calculations. All of the many other useful calculators are yours for free when you sign up for the Free Trial. When the trial ends, Feeds and Speeds will stop working (unless you purchase), but you can go right on using all the free calculators like Fits and Tolerances or the Thermal Expansion Calculator. Pretty cool eh?
Sign up here:
When to Use Press and Interference Fits in Your Designs
Avoid in Plastics (Cold Creep)
First, try to avoid Interference fits for plastic parts. The trouble is they won’t last due to Cold Creep. If constant force is applied to plastic, it will flow until it has deformed enough that you no longer have a press fit. At that point the components come apart and you (or your customers) are left unhappy.
Check the Forces Involved
Using a temperature-based shrink fit reduces this problem, but with a press you still need to keep in mind assembly forces. They can be very large. This makes these fits not the greatest thing for ease of assembly. Plus, you’re introducing relatively tight machining tolerances to create the parts to start with.
Interference fits seem simple, but you want to make sure that this simple design isn’t making assembly too hard for your part.
Use One Press Fit and One Sliding Fit Alignment Pin
Here’s another important tip–use only one press fit pin and make the second pin a sliding fit for alignment only. Trying to line up two interference fits is again painful for assembly.
Keep to the Same Materials
Press Fitting two different materials together can be problematic because they have different rates of thermal expansion. If your parts have to deal with much temperature variation at all, you’ll want them to be made of the same material so the rate of thermal expansion is similar.
Use GD&T to Specify Your Tolerances
If your design uses Press Fit Tolerances, you should specify them using GD&T where possible.
The Ideal Case for Press Fits
The ideal case is when the materials for both parts are going to be the same and both parts are made to close tolerances and require close tolerance alignment. If that’s not the case, you should consider alternatives.
Press Fit Alternatives
For metals, align with sliding fit dowel pins and lock it down with bolts. For plastics, you can still use locating pins with a sliding fit and snap together construction for assembly.
More Info and Definitions
These are some terms you may see used when discussing fits and calculating their tolerances.
Diametrical Interference simply states the interference in diameters rather than physical units. To calculate it, divide the amount of interference by the diameter of the hole or shaft and you’ll have the diametrical interference.
In simple term’s, Hooke’s law says that the force required to extend or compress a spring by some distance scales linearly with that distance.
An Interference Fit is any fit where the shaft outer diameter is physically larger than the hole, such as press fitting. The larger the interference, the more force is required to assemble the two parts. The Interference Fit is the only fit that can hold without additional fasteners of some kind.
Simply put, Poisson’s Ratio tells you how much a piece of material will bulge out as you compress the ends. Obviously Possion’s ratio may come up in press fit applications.
The value of Poisson’s Ratio will vary from one material to the next. Interestingly, steel and aluminum are pretty similar with a Poisson’s Ratio for aluminum of 0.32 and for steel it is 0.27 to 0.30.
The Young’s Modulus of a material tells us how easily it can be stretched or deformed as we apply stress (compression) or strain (pulling apart) to it.
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Recently updated on September 24th, 2022 at 12:15 pm
Bob is responsible for the development and implementation of the popular G-Wizard CNC Software. Bob is also the founder of CNCCookbook, the largest CNC-related blog on the Internet.