ENGINEERING TOLERANCES - FITS

Audio: Engineering tolerances – fits

In my previous article, Engineering tolerances – Introduction, we explained what tolerances are and the reasons for size variation. We also described different types of tolerances and showed you how to define them on drawing. As we move through our series of articles regarding the dimensioning on the drawing, in this article, we are going to look into fits.

In this article, we will learn what fits are, basic terminology, and how to select a proper fit. As a mechanical design engineer, at one point in your career, you will have to define the right fit between different components. I am sure that this encounter will happen sooner rather than later. Understanding the fits and how to define them is a powerful tool for every mechanical design engineer, so make sure that you spend enough time understanding this topic properly.

Introduction

Imagine that it is Sunday afternoon, you are driving a car on an empty road, and the weather is perfect., You are close to your destination, and suddenly, you have a flat tire. There is a safe place for you to stop: “I am so lucky,” you said to yourself. You remember that you have a spare tire in your trunk that came with the car. You do all the safety precautions and take out the spare tire and necessary tools.

You remove the flat tire and take the spare one. You align the holes from the spare tire to the lug bolts, and two of the lug bolts are not going through the holes. You think that you probably did something wrong and rotate the spare tire, but still, two of the lug bolts are not going through the holes. Now you are confused, you are scratching your head and then you finally realize: IT DOES NOT FIT! Two holes are too small to fit on the lug bolts.

Now you must call roadside assistance, but you don’t have any service on your phone. You go significantly far away from the car, and finally, you manage to get roadside assistance on the phone. They say that you will have to wait for one hour and that your insurance is not covering the cost. It starts raining. You are running back to your car and see that you forgot to close the sunroof on your car. Did I forget to mention that you slipped and fell?

I bet you would like to meet the engineer who designed that spare tire rim.

What are engineering fits?

Okay, I admit, maybe I went a bit too far with the previous example, but this story has a point. And the point of the story is equally important for fits like for any other design work that you will do. Your everyday work impacts other people (customers), however insignificant design details seem to you. This example is precisely the reason why the rules for fits (and other design elements) were defined: to prevent this from actually happening.

When designing different products, mechanical design engineers design different components and join them in assemblies. These assemblies can have different functions, and depending on the function, components will have a different relationship between them. For example, sometimes, we want components to slide on each other, or slightly pressed in each other, or to be pressed in each other preventing the components from disassembling. These relationships are defined, and we call them fits.

The fits are defined by ISO 286-1:2010 and ISO 286-2:2010/COR 1:2013.

According to ISO 286-1:2010, the fit is defined as a relationship between an external feature of size and an internal feature of size (the hole and shaft of the same type) that are to be assembled.

In other words, the relationship between two joining components is defined in such a way that the interchangeability between two joining components is assured. Precise manufacturing is the basis of interchangeability; the components are manufactured in the precise and repeatable way that the components can be taken randomly and assembled as intended.

The terms “hole” and “shaft” are used in ISO standard, but they are not only referring to the cylindrical fit but also to parallel fit surfaces of components (for example, the width of the slot).

Basic terminology

Shaft

Nominal size – dimension of the feature defined on the drawing. This dimension represents the dimension of the feature with the “ideal” form (marking with the letter N is no longer in use, but for easier understanding, we will use it).
Actual size – is determined by measuring the defined dimension after it has been manufactured.
Upper limit of size (ULS) – N + es – is the larger permitted size of the two limit sizes.
Lower limit of size (LLS) – N + ei – is the smaller permitted size of the two limit sizes.
Upper limit deviation (es) – is defined as upper limit minus the nominal size.
Lower limit deviation – (ei) – is defined as lower limit minus the nominal size.
Fundamental tolerance (IT) – the difference between the upper limit deviation and lower limit deviation. Tolerance is an absolute value.

Hole

Nominal size – dimension of the feature defined on the drawing. This dimension represents the dimension of the feature with the “ideal” form (marking with the letter N is no longer in use, but for easier understanding, we will use it).
Actual size – is determined by measuring the defined dimension after it has been manufactured.
Upper limit of size (ULS) – N+ES – is the larger permitted size of the two limit sizes.
Lower limit of size (LLS) – N+EI – is the smaller permitted size of the two limit sizes.
Upper limit deviation (ES) – is defined as upper limit minus the nominal size.
Lower limit deviation (EI) – is defined as lower limit minus the nominal size.
Fundamental tolerance (IT) – the difference between the upper limit deviation and lower limit deviation. Tolerance is an absolute value.

As already mentioned, the terms “hole” and “shaft” are used in ISO standards, but they are not only referring to the cylindrical fit but also to parallel fit surfaces of components (for example, the width of the slot).

The concept of the shaft and the hole terminology is simple. Basically, if you have a component that is fitting in something is considered to be shaft. If you have a component that something is fitting in that component, that is considered to be a hole. For example, when you are trying to fit into the ring on your finger. In this case, your finger is a shaft, and the inner surface of the ring is considered to be a hole.

There are three general categories of fits:

Clearance fit,
Interference fit,
Transition fit.

Clearance fit

When it is required for the shaft to rotate or slide freely within the hole, the clearance fit is used.

When the clearance fit is defined, the actual size of the shaft should be equal to or smaller than the actual size of the hole. That means that the clearance between the shaft and the hole must be equal to or greater than zero. Or in other words, the difference between the lower limit of the hole size and the upper limit of shaft size is equal to or greater than zero (LLS hole – ULS shaft ≥ 0).

The minimum clearance will occur when the shaft is manufactured at the upper limit of size and when the hole is created with the EI = 0. In that case, minimum clearance is c_min = es.

The maximum clearance will occur when the shaft actual size is manufactured on the lower limit of size and the hole is manufactured at the upper limit of size. In that case, the maximum clearance is c_max = ES + ei.

The actual clearance will be somewhere in the range between the c_min and c_max.

Interference fit

When it is required for the shaft to be securely held within the hole, the interference fit is used. To disassemble the components, high force is required, and the components could get damaged.

When the interference fit is defined, the actual size of the shaft should be equal to or greater than the actual size of the hole. That means that the interference between the shaft and the hole must be equal to or greater than zero. Or in other words, the difference between the lower limit of shaft size and the upper limit of the hole size is equal to or greater than zero (LLS shaft – ULS hole ≥ 0).

The minimum interference will occur when the shaft’s actual size is manufactured on the lower limit of size, and the hole is manufactured at the upper limit of size. In that case, the minimum interference is i_min = ei – ES.

The maximum interference will occur when the shaft is manufactured at the upper limit of size, and when the hole is created with the EI = 0. In that case, the maximum interference is i_max = es.

The actual interference will be somewhere in the range between the i_min and i_max.

Transition fit

When it is required for the shaft to be securely held within the hole but that the components still can be disassembled without damage, the transition fit is used.

The transition fit is located between the clearance fit and the interference fit. When the components are assembled with the defined transition fit, the transition fit can become the interference fit or the clearance fit. The values of transition fit are around the nominal value.

The maximum clearance will occur when the shaft’s actual size is manufactured on the lower limit of size, and the hole is manufactured at the upper limit of size. In that case, the maximum clearance is c_max = ES – ei.

The maximum interference will occur when the shaft is manufactured at the upper limit of size and when the hole is created with the EI = 0. In that case, the maximum interference is i_max = es.

Tolerance class

Usually, fit tolerances are marked with the use of the tolerance class. The tolerance class consists of the fundamental deviation identifier followed by the standard tolerance grade number

For example: Toleranced size for a hole 80 H7 and a shaft 80 m6:

80 is the nominal size in millimeters;
H is the fundamental deviation identifier for a hole;
m is the fundamental deviation identifier for a shaft;
7 is the standard tolerance grade number.

Fundamental tolerance

The fundamental tolerance is the limit deviation that defines the placement of the tolerance interval in relation to the nominal size. It is identified with the letters, e.g., A, H,h,m…

The upper case letters are used for holes (A, B, H, etc.), and the lower case letters are used for shafts (a,b,h, etc.)

The sign + is used when the tolerance limit identified by the fundamental deviation is above nominal size, and

the sign − is used when the tolerance limit identified by the fundamental deviation is below nominal size.

Tolerance class

The standard tolerance IT is any tolerance belonging to the ISO code system for tolerances on linear sizes. In the ISO code system for tolerances on linear sizes, the standard tolerance grade identifier consists of IT followed by a number.

The values of the fundamental tolerance, upper limit deviation (ES, es), and lower limit deviation (EI, ei) can be calculated, but the expert that were working on the ISO standard already did it for us. In ISO 286-1:2010 and ISO 286-2:2010/COR 1:2013 you can find equations, clarifications, tolerance tables, recommendations, and basically everything you need to learn and apply the ISO fit systems.

The tolerance tables can be found online or in the different technical guides, handbooks etc. Most of the time, we cannot find all the values in these tables as we can in ISO 286-2, but instead, we can find the tables with the selection of the tolerance classes.

Source: Bosch Rexroth AG.

Let us look now in one random example for nominal size 100mm. For the hole we will take tolerance class M6 and for the shaft we will take g7:

100M6g7

For hole: EI: -6 µm

ES: -28 µm

For shaft: ei: -12 µm

es: – 47 µm

Using upper limit and lower limit deviations for the hole and shaft, we can calculate what type of fit this is. We can do it manually, or we can find an online calculator. Personally, I am using: Calculation of fits according ISO 286 (2010).

For our random fit, we have defined the transition fit:

I find this tool extremely powerful for checking my fits. I would advise you to always double-check features like this. Creating a proper fit requires fine and precise manufacturing, and it could affect the price of your part (higher manufacturing cost).

Selecting proper fits

ISO experts went even further than just calculating the tolerance classes. In ISO 286-1:2010, they advise how to choose the proper fit between the parts and what type of tolerance classes we should use to define the proper fits. The first step in selecting the proper fit is choosing either the hole-basis or the shaft-basis system.

Hole-basis fit system

The hole-basis fit system is defined in the way that the hole fundamental tolerance is always defined as “H.” That means that the lower deviation of the hole is always zero. We are “matching” the tolerance of the shaft to get the required fit.

The hole-basis fit for the economic reasons should be chosen for general use.

Shaft-basis fit system

The shaft-basis fit system is defined in the way that the shaft fundamental tolerance is always defined as “h”. That means that the upper deviation of the shaft is always zero. We are “matching” the tolerance class of the hole to get the required fit.

The shaft-basis fit system is useful when a few different parts with holes are mounted on one shaft.

Preferred fits

Entry of fit tolerances on the engineering drawing

In the picture below, you can see examples of how we can mark the fit tolerances on the engineering drawing. The first two rows are related to the marking of the hole and shaft on the individual component drawing, and the last two rows show how we can mark the fit when the hole and the shaft are on the same drawing.

Closing words

Defining proper fits on the components is essential for every mechanical design engineer. If you are like me, the best way to learn is actually to see, feel, and assemble components yourself. I would advise you to go through the product drawings in your company and find the components with specified fits between them.

Go to your production area and try to assemble them yourself. See the surface specification on the drawing and feel it under your fingers. Get the feeling about the pressure required to assemble the components. Ask the manufacturing workers for their insights etc. The better you understand the implications of your design on manufacturing, the better design engineer you will be.

Now you have an excellent overview of the fits. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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