Engineering Fits; Explanation, Standards and Types

Engineering Fits; Explanation, Standards and Types

Types of engineering fits.

In mechanical design, we are using fits extensively. According to the purpose of machinery, designers use different types of engineering fits. Here, you can find detailed information about mechanical fits. You can find this information here;

  • What is fits? What are the applications of engineering fits?
  • What are the types of fits?
  • What are the tolerances and limits of fits?

What is Engineering Fits?

Engineering fits are the mechanical structures that transmit loads between fit parts. In a fit, there is a round hole and a round part that fit into this hole. There are various types of applications of engineering fits. Because of these applications, there are different configurations of fits.

The tolerance values of engineering fits are very important. According to the application, the tolerances of mating parts should be very accurate. With the changing tolerances, the type of applications changes.

In general, designers use engineering fits in the design of sliding bearings. If we want to see a tight combination between mating parts, we need to adjust tolerances according to this.

In fit configurations, there are the largest allowable clearance and the minimum allowable clearance between the mating parts. Between these two values, the tightness and looseness of fits are adjusted. We call the difference between these values allowance.

What are the Types of Engineering Fits?

In general, there are three types of fits we are using in mechanical design. These three engineering fit types are;

  • Clearance Fit,
  • Interference Fit,
  • Transition Fit,

Clearance Fits

As you understand from its name that the clearance fit is the type of fits where there is a clearance between the mating parts. There is a space between the mating parts which allows a level of looseness. The allowance value in the clearance fits is always positive.

The general clearance fits are; H11c8, H9d10i H9e9, and H7g6.

Also, there are different types of clearance fits;

  • Loose running fits: In the clearance fit family, this loose running fit has the largest clearance. In these fits, the biggest problem is the contamination between the mating parts.
  • Free running fits: The clearance between the mating parts is somewhat lower than the loose running fits. These fits are suitable for applications where there are temperature fluctuations and very high-speed working of the fit.
  • Close running fits: We are using the close running fits in the applications where moderate accuracies are important compared with loose and free running fits. Designers are using close running fits in applications where moderate running speeds and application pressures are required.
  • Sliding fits: The clearance between mating parts is very small in sliding fits. In mechanical systems where high accuracy is a problem, these types of clearance fits are used by engineers.
  • Locational clearance fit: Designers use this clearance fit type where they need very high tolerances and accuracies. In general, they can turn with the proper lubrication. Designers are using locational clearance fits for the shaft assemblies.

Interference Fits

In the interference fits, the diameter of the internal shaft is bigger than the hole. So for the assembly of these mating parts, there are special applications required. Around these applications, they are heating the holes to fit the shaft inside it. After the cooling of the hole, they obtain very tight interference fits. In engineering fits, interference fits are also called press fits or friction fits.

The general interference fits are; H7p6 and H7s6.

In the mechanical design, we use interference fits in the applications where we need a power transmission. Because the rotation of the shaft is not allowed either freely or with lubrication. Also, there are different types of interference fits.

  • Press fits: This is the type of fit where we need the loosest interference fit. The positive difference between the shaft and the hole in the smallest for this type of engineering fit. In the application of bearing attachments, designers use press fits generally.
  • Driving fits: Driving fits is the type of fit where we need moderate interference between the mating parts. So, we are using driving fits in the mounting of gears and shafts.
  • Forced fits: This is the type of fit where the tightest fits are required. An example of forced fits is the shaft and gear assemblies for power transmission.

Transition Fits

This class of engineering fits has moderate fit tightnesses between the clearance fits and the interference fits. In general, there are two forms of transition fits.

The general transition fits are;

  • Similar Fit: This is the type of transition fit where small clearances or very small interferences are obtained. In the assemblies of hubs, we use similar fits generally.
  • Fixed Fits: Unlike similar fits, there is a requirement for small force application for transition fits.

Tolerances of Engineering Fits

Tolerance is one of the most important things in engineering and mechanical design. In the design of mechanical fits, we are using different standards such as ISO and ANSI. According to these standards, let’s check the general classifications and presentations of fits.

Available clearance values for engineering fits.
Image Source: Wikipedia.

While we are establishing engineering fits, we are using this standard table that shows the general tolerances of fits.

You can see the engineering fit types in the table above. From right to left, the table shows the clearance fit, transition fits, and interference fits scales. Below these scales, there are some letters and numbers.

The meanings of these letters and numbers; ‘H’ means the holes and the lowercase letters such as c, d, e, and f… means the shafts.

There is the main application diameter list on the left side of the chart. From here, you can see which size your shaft and hole fit the application that falls into. So, you can match this size with the fit type that you want to obtain. Finally, you can find the applicable tolerances from the chart by this match.

Mechanical Fit Example; Transition Fit That Has 130mm Diameter

In this example, you will understand how to use this chart to find the applicable tolerances for the defined fit. We will find the available tolerances for a transition fit that has 130mm diameter

First of all, you need to select an application diameter of the engineering fit. For example, you want to design a transition fit that has a main diameter of 130mm. On the left side of the chart, you can see the basic size columns and ranges. The 130 is between the 120-140mm range. If you match this range with the transition fits, you will find out that you can obtain these fits; H7h6, H7k6, and H7n6.

So, you can find the tolerance deviations from this table like this;

  • H7h6; The upper tolerance of the hole is 40μm and the lower tolerance value is 0. The upper tolerance value of the shaft is 25μm and the lower tolerance value of the shaft is 0.
  • H7k6: The only difference between this fit from the H7h6 is that the upper tolerance value of the shaft is 28μm and the lower tolerance value is 3μm.
  • H7n6: This is the available transition fit which has the upper tolerance value of the shaft 52μm and a lower tolerance value of it is 27μm.

According to the level of tightness, you can select one of these available tolerance values for a transition fit that has a 130mm diameter.

Another Mechanical Fit Example: A 55mm Very Tight Interference Fit

For example, we would like to obtain a very tight interference fit for the power transmission through shafts. So, the engineering fit must withstand these loads. We need to take a look at the chart.

According to the mechanical fit table above, the available interference fits are H7p6 and H7s6. If we match the diameter span with these fits, we can see that;

  • H7p6: The upper tolerance value for the hole is 30μm and the lower tolerance value is 0μm. For the shaft, the upper tolerance value is 51μm and the lower tolerance value is 32μm.
  • H7s6: The only difference is the shaft tolerance values which are 72μm as the upper value and 53 as the lower value.

So, we need a tighter mechanical fit, we can select the H7s6 one for our application.

A Mechanical Fit Example for a Plain Bearing Application

For plain bearings, the fit between the hole and the shaft end must allow comfortable rotational movement. And there must be a clearance between the shaft and the hole to fill here with sufficient lubrication. So, we need to design a clearance fit.

Say our diameter is 35mm. So the available clearance fit applications are; H11c11, H9d10, H9e9, H8f7, and H7g6. If of take a look at the available tolerances of these clearance fits in 35mm, we have;

  • H11c11: The upper tolerance for the hole is 100μm and the lower tolerance is 0. And the upper tolerance for the shaft is 240μm and the lower tolerance is 120μm.
  • H9d10: For this clearance fit, the upper tolerance of the hole is 62μm and the lower tolerance is 0μm. And the upper tolerance for the shaft is 180μm and the lower tolerance is 180μm.
  • H9e9: The only difference between this clearance fit from the H9d10 is; that the upper tolerance of the shaft is 112μm and the lower tolerance is 50μm.
  • H8f7: Also for this clearance fit, the upper tolerance of the hole is 39μm and the lower tolerance is 0μm. And the upper tolerance for the shaft is 50μm and the lower tolerance is 25μm.
  • H7g6: The top tolerance of the hole is 25μm and the down tolerance is 0μm. It is also the same with the shaft.

So it will be very good to select the biggest allowance. The H11c11 one has the biggest allowance value which is 240μm. We can use this clearance value for the lubrication of the plain bearing system.

Conclusion

So, engineering fits are very important in the design of hole and shaft systems. Hole and shafts systems are generally used in various mechanical applications. Thus, we need to select the proper mechanical fit for our application.

There are standard tables according to the different standards and we need to use these standards while selecting the proper values. Different tolerances for fits can be obtained with different methods such as turning operations. 

The types of fits change according to the applications and tolerances and the designations of the engineering fit change. Thus, we need to properly select the required one for our application.

Finally, these are the general aspects of engineering fits.

So, do not forget to leave your comments and questions below about the mechanical fits.

Your precious feedbacks are very important to us.

FAQs About Mechanical Fits

What is the meaning of engineering fit? 

Engineering fits define the clearance between the hole and shaft systems in mechanical design. A proper application of engineering fits is required to obtain successful designs.

Why are the fits are important in engineering? 

Because they are important for most mechanical applications. Designers can easily define which fit tolerances they can use in different types of hole and shaft applications.

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