Types of Fits: How to Choose the Right One in Engineering

Fits are a crucial aspect of engineering, and they help to join parts together so that they can assemble and hold the load of various components. If you are planning to have new components manufactured and need to know about different types of fits and how they work, the guide below has it all. So, let's read on.

What is an Engineering Fit?

Engineering fits are the parts that help with mechanical assembly; the two parts that mate are joined together. These fits are used for either temporary joints or permanent ones. The fit represents the mechanical clearance of the physical contact between the components that mate.

The parts can handle loads only if they are properly assembled. Otherwise, they cannot handle the load.

Basis of Fits: Hole and Shaft System

The hole and shaft basis systems offer a famous standard for engineering fits. It has two types of variants: shaft basis and hole basis. These two have the base component, which has a fixed dimension. The other component is altered in size so that it can achieve the required fit.

Among both of these systems, the hole basis system is the most popular one, and it is even more convenient for controlling the shaft diameter when compared to the hole. The shaft basis is also significant in the high-speed rotating shaft, the hole size is altered so that the required fit is achieved.

3 Main Types of Fits

There are three main types of fits, and each type has its subcategories; all three types of fit are discussed below.

Type 1: Interference Fit

Interference fits are such a fit that has a high frictional force that can hold the mating surfaces tightly together. The negative clearance of the interference fit gives the tightness of the mating components, which keeps them pressed against each other.

Interference fit is also created by shrink fitting; one of the components is heated or cooled so that it either contacts or expands, and a negative clearance is achieved. It compresses the components together tightly. The clearance in the interference fit ranges between -0.001mm and -0.042mm.

Press Fit

Press fit is a type of variant that has the least negative clearance and is suitable for medium-strength joints.

Driving Fit

Driving fit has medium interference, which can help carry load and needs hot or cold pressing for assembling the components with force.

Forced Fit

Forced fits are the engineering fits that need pressing in hot or cold form; these are permanent fits. They need careful tolerancing so that the parts do not break.

Type 2: Clearance Fits

Clearance fits have a positive allowance and leave no gap between the surfaces that are assembled together. The parts in a clearance fit do have some freedom, and an example of a pin and frame can be considered. It allows components to move independently and also stay locked in their positions. The range of clearance for these engineering fits is between 0.025mm and 0.089mm.

Loose Running Fit

In a loose running fit, the clearance is at a range higher than the one above, and the components can easily slide and rotate.

Free Running Fit

Free running fit is the same as loose running; the components can maintain high speed, and the joint can have thermal expansion. However, the location accuracy in this type of fit is low.

Close Running Fit

Close running fits have improved position accuracy, and the components can move at high speeds and temperatures.

Sliding Fit

The sliding joints have a high-accuracy engineering fit, which has a minimum clearance so that the degree of freedom is restricted and only sliding is allowed.

Locational Clearance Fit

The location fits offer high precision and help locate the parts accurately for mating. The clearance in this type of fit is very low, allowing smooth movement with lubrication.

Type 3: Transition Fits

Transition fit is a combination of both clearance and interference fit. Based on the application requirement, it can have either limited clearance or interference.

In case of negative interference, the load capacity is less, and in case of a clearance, the play is not much.

Transition fit offers highly precise locating components for assembling operations, and the relative movements are restricted in this case. The mechanical clearance or interference in these gifts is between +0.023mm and -0.018mm.

Similar Fit

A similar fit leaves a very minimal clearance, and the rubber mallet is used to obtain the assembly.

Fixed Fit

A fixed fit leaves a small interference, and by using light force, assembly is achieved. The pulleys and gears fall under the category of fixed fit.

Push Fit

It requires a negligible amount of force to assemble the parts.

Wringing Fit

This type of fit is used to replace the parts without difficulty.

How to Achieve Dimensional Tolerances for Fits?

In order to achieve the dimensional tolerance for fits, some of the ways are provided below.

CNC Precision Machining

CNC machining offers high accuracy with a tolerance of +/- 0.001 mm, and if you choose the right tools, you can produce highly accurate components.

Rettifica

Rettifica is a method that offers ultra-precision machining and has an accuracy of +/- 0.25 microns.

Alesatura

Alesatura is a specialized application that helps with hole making, and the holes are popular for mating components in engineering fits; hence, reaming is suitable. It is a highly precise method that helps to remove the material within the workpiece and create holes with tight tolerance.

How to Choose a Suitable Fit for Your Project

Some of the important factors that help in choosing a suitable fit for your project are as follows:

Applicazione

The application requirements help you choose the right fit. You have to analyze the way components interact when assembled, whether they need to rotate or not, and what function they are meant to perform.

Budget

The budget and functionality requirements must strike a balance. When it comes to creating a tight tolerance, you need to choose an accurate manufacturing process that is in your budget and also offers you the right fit.

Tolleranza

Tolerance for every application varies, and hence, choosing the fit tolerance that suits your component application is crucial. You need to analyze whether the component rotates completely or not, and how much free play is required.

Conclusione

When manufacturing components that mate with one another, the consideration of the right tolerance and the right type of fit is crucial. As you have read the guide above, you must have learned everything about it. So, get in touch with us at DEK, and we will design highly precise components with the right fit for you.