Ball Bearing Fitting Series: Basics of Fit Charts

I recently finished a decent-sized project where I had to define several ball bearing fits for a prototype — all with different fitting arrangements. I have done this so many times, I normally don’t think much about it. But this time I was training a young engineer and kind of had a new-found respect for the number of steps that go into getting that “perfect fit” the first time. Now, don’t get me wrong; when the new parts arrive for the first prototype build, I’m pacing outside the lab doors, wringing my hands — just like everyone else. It usually turns out ok. In this series, we will start by understanding the basic fit charts, then move into understanding the application, then on to calculating residual clearance after mounting, and finally, determine shaft and housing dimensions.

Let’s begin with the shaft and housing fit chart that you can find in most bearing catalogs. An image search of “shaft and housing fit charts” will also pull up a few dozen versions for you as well. I like this one from Koyo because it shows the tolerance lines (single plane mean () diameter deviation) and clearly defines the clearance, transition and interference fit zones. The bottom half of the chart is the shaft fit section and the top half is the housing fits. Notice the shaft fit letters are in lower case in contrast to the upper case letters in the housing section. The vertical location of the blocks is meant to represent the position of the bearing relative to the shaft. Notice on the left, the f6 fit is a very loose fit, so the block is dropped away from the lowest tolerance line. This indicates that an f6 shaft would never interfere with the bearing. In contrast, the p6 fit on the other end would always be in interference with the shaft. The vertical size of the block is meant to represent the size of the tolerance range which is proportional to the standard international tolerance grade (IT). Notice the 5 and 6 blocks are very short, while the 7 blocks are long. The 5 and 6 blocks are very tight precision, while the 7 blocks have a larger tolerance. The same rules apply to the housing side in that things are just represented a little differently because a clearance housing is larger than the bearing outer diameter, while a clearance shaft is smaller than the bearing inner diameter. To summarize this chart, we start with the fit letter designation (lower is looser), followed by the IT grade (larger number = larger tolerance).

Now let’s have a look at an actual IT chart — usually tucked away in the back of most bearing catalogs and readily available online. This is a great, underutilized resource and can be used for anything — not just bearings. It is very handy to know what tolerances you can hold for various parts of your system, particularly for shaft and housing fits. For instance, a typical, turned housing bore will usually fall around an IT7 or IT8, while the more precise ground shaft might be an IT6 or IT7. Most bearing catalogs recommend tighter tolerances than we are going to hit in reality.

Now let’s review a housing fit chart. If you haven’t been through this exercise before, these charts do not mean much on their own. Right now, we are just learning how to read them. As we saw on Chart 1, the lower letters are looser. All of the tolerances listed on this chart are in microns µm. In the E column you will notice the tolerances are all loose fits as indicated by the positive bilateral tolerances (+) and become tighter fits and you move along the chart to the right.

What you are going to do here is take your mean housing dimension and add those dimensions to it. For a 100 mm housing with an E6 tolerance, your housing is going to be 100.072 – 100.094. If your housing department tells you to take a hike, they are holding 50 µm, just jump back to your IT chart, look across the 100 mm line until you see something close to 50 µm. In this case, that is an IT8. So from Table 3 you need a column that ends in 8. Of course these don’t have to be an exact match; we are eventually going to deviate. Just to get started, it looks like an H8 might be a good match.

Now onto our shaft table. The same rules apply here; just keep in mind you are working on a different diameter (an easy mistake to make). Ok — lets say we have a 60 mm shaft and your shaft people say they can hold a 20 µm on a shaft without too much trouble. Back to our IT chart. This time in the 60mm line we look for 20 µm and find that value pretty close to an IT6 tolerance. Let’s say we need light interference fit this time. We need our column to end in 6, so it looks like k6 might be a good starting point. Again applying the tolerances to our mean shaft diameter of 60 mm, we have a shaft dimension of 60.002 – 60.021. In reality, am I going to call that 60-60.02? Yes, but we are just learning for now.

Stay tuned. Next week we will look up our application and determine what fits we are going to need.