Engineering Fit

Engineering Fit Overview: How to Choose a Right Fit in Engineering

“Engineering fit” is a technical concept within the realm of mechanical construction. It pertains to the precise alignment and integration of two interfacing components. This concept entails ensuring that the dimensions and the clearances between these components are accurately calibrated. The components may be designed with a slight gap (clearance fit), a press-fit (interference fit), or a compromise between the two (transition fit), based on the functional demands and specifications of the assembly. The objective is to achieve an optimal balance between excessive play and excessive tightness, ensuring that the components interact efficiently, with the appropriate level of friction, mobility, and stability.

What is Engineering Fit?

As a part of geometric dimensioning and tolerancing, engineering fits play a crucial role during the design phase of a part or assembly. The term “fit” in engineering refers to the clearance between two mating parts. The size of this clearance dictates if the parts can move or rotate independently at one end of the range, or if they are temporarily or permanently joined at the other end. The concept of engineering fits is often represented by a “shaft and hole” pairing, but it’s not strictly confined to round components.

Generally, fits can be categorized into three types: clearance, location or transition, and interference. The choice of fit is typically made during the design process, based on the requirements of the mating parts – whether they need to be precisely positioned, allowed to freely slide or rotate, easily separated, or resist separation. The cost is a significant consideration when choosing a fit, as precision fits are costlier to manufacture and tighter fits are more expensive to assemble.

The importance of engineering fits in manufacturing and design processes cannot be overstated, as they establish the clearance of the mating parts according to the size specifications. Selecting the appropriate fit facilitates the easy rotation of the shaft through the hole. Thus, the fit is essential to guarantee that parts are correctly assembled and operate as intended.

Hole and Shaft Basis System for Engineering Fit

Engineering fit can be specified either as a shaft-basis or a hole-basis. It depends on which part’s size is controlled to establish the fit. In a hole-basis system, the hole’s size is kept constant, and the shaft’s diameter is adjusted to determine the fit. In contrast, in a shaft-basis system, the shaft size is fixed while the hole size varies to determine the fit.

Engineers often prefer the hole-basis system for its simplicity. With the hole size fixed, the type of fit is dictated by the upper and lower deviation values of the shaft. The precision in drilling is somewhat limited, given that the tools are available in specific dimensions. In addition, CNC turning services can produce shafts with precise dimensions, making it more convenient to attain the desired fit.

The ISO system designates tolerance ranges of fits using an alpha-numeric code, with upper-case letters indicating the hole tolerance and lower-case letters indicating the shaft tolerance. For instance, in H7/h6 (a widely used fit), H7 denotes the tolerance range for the hole, and h6 denotes the tolerance range for the shaft. These codes enable machinists or engineers to rapidly determine the maximum and minimum size limits for either the hole or shaft. The possible range of clearance or interference can be calculated by subtracting the smallest shaft diameter from the largest hole diameter, and the largest shaft diameter from the smallest hole diameter.

Hole and shaft basis

Three Typical Types of Fit

There are three typical types of engineering fit – clearance, interference, and transition fits. Each allows a different degree of looseness or tightness between components.

Clearance Fits

Clearance fits provide play or clearance between the shaft and hole. This allows the shaft to rotate or slide freely inside the hole. With clearance fits, the hole size is always larger than the shaft size. When the shaft has the minimum diameter while the hole has its maximum diameter, resulting in maximum clearance. Conversely, when the shaft diameter is maxed and the hole diameter is minimum, there is a scenario of minimum clearance.

The clearance between parts allows for lubrication and accounts for temperature changes. Fasteners may be required to locate parts. Common clearance fits include loose running fits and sliding fits. These are used in applications like rotating machinery shafts, sleeves, bushings, and oil-lubricated parts.

types of fits

Clearance fits can be divided into five categories:

Loose Running Fit

A loose running fit is a clearance fit with the largest clearance. It can be used in applications where accuracy is not paramount, such as pivots, latches, and parts affected by corrosion, heat, or contamination. In this type of fit, the hole basis has H11/c11, H11/a11, and H11/d11, and the shaft basis has C11/h11, A11/h11, D11/h11.

Free Running Fit

Free running fit suits where no special requirements apply to the accuracy of matching parts. It leaves room when involving high running speeds, large temperature variations, or heavy journal pressures. It can be found in applications where maintaining a film of oil lubrication is important. The hole basis of this fits covers H9/d9, H9/c9, H9/d10, and shaft basis includes D9/h9, D9/h8, D10/h9.

Close Running Fit

A close running fit is suitable for small clearances with moderate requirements for accuracy. It can withstand moderate running speeds and journal pressures. Its example uses include shafts, spindles, sliding rods, and machine tools. The example of hole basis has H8/f7, H9/f8, H7/f7 and shaft basis has F8/h6, F8/h7.

Sliding Fit

Sliding fit leaves minimal clearances for high accuracy requirements and can be easily assembled. The parts will turn and slide freely. Therefore, this fit can be used in guiding shafts, sliding gears, crankshaft journals, automobile assemblies, clutch discs, slide valves, parts of machine tools, etc. The hole basis includes H7/g6 and H8/g7. The shaft basis covers G7/h6.

Location Fit

Location fit has very close clearances for precise accuracy requirements. The parts can be assembled without force and will turn and slide when lubricated. Its uses include precise guiding of shafts, roller guides, and so on. Its hole basis is H7/h6 and shaft basis is H7/h6.

Interference Fits

Interference fits, also known as pressed fits or friction fits, can produce a permanent joint between two tightfitting mating parts. The shaft size is larger than the hole size. During assembly, the shaft must be forced or pressed into the hole. Parts can be joined using a tap from a hammer or forced together using a hydraulic press. Sometimes heating of the hole and freezing of the shaft help increase/decrease the hole and shaft sizes respectively to make for an easier process.

The pressure contact provides improved load capacity and bearing support. Interference fits are inherently self-locating and resist relative motion between parts. They can transmit torque well. Interference fits see use in parts like gears and pulleys on shafts, machine component joining, and fasteners like retaining rings. The friction from tightness also prevents fretting wear.

There are three sub-categories of interference fits:

Press Fit

Press fit has light interference and can be assembled or disassembled with cold pressing. The uses in engineering cover hubs, bearings, bushings, retainers, etc. In press fit, the hole basis has H7/p6, and the shaft basis has P7/h6. 

Driving Fit

The driving fit provides medium interference and needs to be assembled with hot pressing or cold pressing with large forces. This fit has a more prominent interference fit than press fit and perfect for permanent mounting of gears, shafts, and bushes (the tightest possible with cast iron). The hole basis has H7/s6. The shaft basis has S7/h6.

Forced Fit

Forced fit has high interference shrink fit. It requires a large temperature differential of parts to assemble. That’s to say, its assembly requires heating the part with a hole and freezing the shaft to force the mating parts together. It should be noted that the permanent coupling of gears and shafts cannot be disassembled without risking destruction. Generally, forced fit can be used for shafts, gears, etc. The example of fit includes H7/u6 for hole basis and U7/h6 for shaft basis.

Transition Fits

Transition fits provide a tight sliding or locating clearance. They are between clearance and interference fits. A small amount of clearance allows sliding but parts still fit snugly together. This fit requires mild force to assemble or disassemble.

The hole size can be equal to or slightly smaller than the shaft size. Therefore, transition fits include two potential scenarios. The shaft could be fractionally larger than the hole, necessitating a certain amount of force to achieve the fit. At the other end of the spectrum, there could be a clearance fit that allows for a small room of motion.

Engineers and machinists also call transition fits slip or push-fit. Transition fits guide parts into the right position. They locate components but don’t fully restrict axial movement. Common uses include bushings, locating pins, and lightly loaded joints that require precision alignment. Minimal clearance also prevents fretting corrosion.

Transition fits include two types: 

Similar Fit

A similar fit leaves negligible clearance or interference fit. This fit can be assembled or disassembled by using a rubber mallet and its application includes hubs, gears, pulleys, bushes, bearings, etc. The hole basis covers H7/k6, and the shaft basis covers K7/h6.

Fixed Fit

Fixed fit creates negligible clearance or leaves a small interference fit which can be assembled or disassembled with light pressing force. It can be used for plugs, driven bushes, armatures on shafts, etc. Fixed fit has H7/n6 for hole basis and N7/h6 for shaft basis.

Useful Tolerances Chart for Shafts and Holes in Engineering Fit

The following chart describes the common tolerance sizes ranging from 0 to 120 mm for shafts and holes and their respective useful tolerances. 

Tolerance chart for shaft and hole of fit

As mentioned before, the potential range of clearance or interference can be found by subtracting the smallest shaft diameter from the largest hole, and the largest shaft from the smallest hole. According to the chart above, we will provide an example of how to calculate the potential range of clearance or interference in clearance, interference, and transition fits.

1. In interference fits, using an H7/p6 press fit on a 50mm diameter:

H7 (hole) tolerance range = +0.000 mm to +0.025 mm

p6 (shaft) tolerance range = +0.042 mm to +0.026 mm

Then, potential interference will be between −0.001 mm and −0.042 mm.

2. In clearance fits, using an H8/f7 close-running fit on a 50 mm diameter:

H8 (hole) tolerance range = +0.000 mm to +0.039 

f7 (shaft) tolerance range = −0.050 mm to −0.025 mm

Then, potential clearance will be between +0.025 mm and +0.089 mm.

3. In transition fits, using an H7/k6 similar fit on a 50 mm diameter:

H7 (hole) tolerance range = +0.000 mm to +0.025 mm

k6 (shaft) tolerance range = +0.002 mm to +0.018 mm

Then, potential clearance/interference will be between +0.023 mm and −0.018 mm.

How to Achieve Dimensional Tolerances for Fits?

Dimensional tolerance plays a vital role in ensuring precise engineering fits. Nevertheless, producing the mating parts within the necessary dimensional tolerances is a task that requires considerable skill. Generally, the parts should be manufactured within tolerance limits set by GD&T. 

Dimensional tolerances for fits can be achieved by the following methods:

The first step is to understand the function of the assembly and the role the fit plays in it. This will determine the type of fit required: clearance, interference, or transition. Next, the hole and shaft sizes are determined based on the fit type and the basis system (hole or shaft). The upper and lower limits of the sizes are calculated using the standard ISO tolerance grades. Finally, the manufacturing process is chosen to achieve these tolerances. This could involve various methods like turning, milling, grinding, or even additive manufacturing.

CNC Precision Machining: Modern CNC machining can achieve precise tolerances of +/- 0.001mm. By utilizing the proper cutting tools and fixturing, machinists can fabricate parts to the exacting dimensions required for engineering fits. The high accuracy capabilities of CNC machines enable the production of mating parts that conform to even the tightest tolerance classes specified for fits.

Reaming: Reaming is a specialized hole-making process commonly used for engineering fits. Holes are a frequent feature that must meet tight tolerances for fit. Reaming removes just a small amount of material, making it valuable for fine-tuning drilled holes to precise dimensions. By post-processing holes to improve accuracy, reaming enables holes to meet the tight tolerance requirements for interference or clearance fits. Since holes are integral to engineering fits, reaming is an important method for achieving the hole precision needed.

Grinding: Grinding is an ultra-precise manufacturing method capable of accuracy up to +/- 0.25 microns. For demanding applications such as a forced interference fit, tolerances within this range are typical. The extreme precision of grinding makes it well-suited for producing components requiring tight dimensional control, like those used in engineering fits. With its ability to hold tolerances under one micron, grinding is an ideal process when minute clearances or interferences are needed.

Fits on engineering drawing

How to Choose the Right Fit for Your Projects?

Choosing the right fit for your projects can often feel like a daunting task. However, it doesn’t have to be. Here are a few pointers to guide you:

Understand the Function: The function of the assembly is the most important factor. If the assembly requires free motion, a clearance fit might be appropriate. If rigidity and alignment are crucial, an interference fit could be the way to go.

Consider the Manufacturing Capabilities: Not all fits are easy to manufacture. Interference fits, for example, require high precision and can be costly. It’s essential to consider the manufacturing capabilities and constraints before deciding on a fit.

Evaluate the Material Properties: The material properties of the components can greatly influence the choice of fit. For instance, materials with high thermal expansion coefficients might not be suitable for interference fits, as they could expand and cause problems.

Cost: Employing fits with tighter tolerances will inevitably increase the cost compared to standard ones. Hence, it’s crucial to thoughtfully consider your choices. The ideal approach would be to select a fit that provides the right tolerance for performing its functions, concurrently minimizing product development expenses.

Summary

Manufacturing often involves creating mating parts that need to be integrated to fulfill a common purpose or execute one or more tasks. The interaction between these parts is of paramount importance, influencing the overall performance of an assembly. The term “fit” describes the relationship between two mating parts, referring to the degree of tightness or looseness when they are joined. Selecting the appropriate fit facilitates the easy rotation of the shaft through the hole. Thus, the fit is essential to guarantee that parts are correctly assembled and operate as intended.

Reference

Engineering fit – From Wikipedia

FAQs

The main distinction between a press fit and a slip fit is that a press fit dimensions result in interference between the mating parts, while a slip fit dimensions yield clearance.  

Considerations include loading, movement, alignment, assembly method, lubrication, wear, temperature, vibration, and cost. The intended relationship between parts guides fit selection.

Interference fits create tight contact for improved strength and is used for gears, pulleys, retaining rings, press fits, and components requiring precise location.

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