Introduction
In engineering, tolerance is like the binding glue for putting everything together. In assemblies, nuts and bolts must have the exact tolerance to slide onto each other. This shows the importance of tolerance for projects with hundreds of parts.
There are many types of tolerance grades and systems you can use for your assemblies and press-fit tolerance is one specifically for parts that require slight force to mount or join.
Don’t confuse tolerance and press fits. Press fits are a type of fit where one part is slightly larger than the other and tolerance is the limit of deviation from a dimension for a shaft or hole.
Learn about press fits and tolerances for your project in less than 8 minutes. This guide covers the way basic principles of tolerancing work for different types of fits.
What are Press Fits?
Press fits, also known as interference fits, join two components together using friction. A press fit or interference fit occurs when a ring or hole in a housing is tight on the shaft. This requires force to insert the shaft into the hole and the friction allows for a snug and secure fit.
Interference fits need the right tolerance so components can be machined and cut without any challenges or misalignment when assembling them. This is specifically used when designing bearings and shafts that move together.
There are many other fits, but the most common are clearance fits, which have a minimal gap that separates the two components in the assembly and allows for free movement.
Unlike clearance fits, interference fits are used for permanent joining. Removing the two parts can damage either the hole or the shaft. This is why press fit tolerance is important!
Key Factors Influencing Tolerance In Press Fits
The most common example of press fit is in injection-molded plastic pieces. Lego sets commonly use an interference fit, and millions of plastic injection-molded pieces require the exact tolerance for a snug fit.
Many factors can influence an interference fit even if the correct dimensions are given. Let’s consider some of the most important factors in press fit tolerances.
Material Properties
All materials have a unique hardness, elasticity, and thermal coefficient. Designing an interference fit requires the known effect of thermal expansion after assembly or during use. A material with a high thermal expansion expands more when heated.
This can affect press fit tolerance if the parts are exposed to the sun or are intended for use in high-temperature environments.
Elasticity is the property affecting the deformation of a material. Press fit requires force to fasten two parts together. A material with a high elasticity can deform under force, rendering your part useless if this force equals the force needed for the interference fit. Usually, elastic deformation is a concern when tolerancing two parts of different materials.
Surface Finish
Interference fit secure two parts without additional fasteners. This means the joining parts have their surfaces in contact. Often tight tolerances in design fait at the press fit assembly because of a surface finish process previously not accounted for. Polishing a surface can result in a loose fit and a rough finish might not allow interference fit at all.
Assembly Type
Some press fits and interference fits are assembled using shrinking (see shrink fit guide here). Shrinking involves lowering the temperature of the piece to decrease its diameter for example a shaft’s diameter and passing it through a hole.
When the shaft returns to its normal operating temperature it expands, securing a fit. This requires a different interference amount and also allows for tighter tolerances compared to standard press fits.
How To Give Tolerance In a Press Fit?
Giving tolerance in a press fit is often a confusing task. Engineering fits use a hole basis system or a shaft basis system to systematically define tolerances. Clearance fit or transition fit also utilizes hole and shaft basis systems.
To understand press fit tolerances we will analyze the tolerance chart and hole basis system.
Hole basis system: this system fixes the size of the hole and all the variations in dimensions are laid on the shaft.
Shaft basis system: this system fixes the diameter of the shaft and the fitting hole’s dimensions are altered to facilitate the shaft.
Step By Step Guide On Press Fit Tolerances
Press fits generally rely on hydraulic press to press the mating parts together. The standard press fit tolerance procedure involves the following steps.
1. Nominal dimensions
Find out the nominal dimensions of your mating parts. The nominal dimensions are the standard dimensions but are not necessarily accurate. For example, a 15 mm hole can be close to 15 mm but not exactly 15.000 mm.
2. Choose a standard of measurement
Select a standard of measurement like ISO, British or American National Standard Running and Sliding Fits to follow a tolerance chart.
3. Use the tolerance chart for your fit
In engineering fits, a tolerance chart acts as a guide to specify the type of hole tolerance and show how much interference a fit will have.
The simplest version below shows a hole on top and a shaft at the bottom. In a clearance fit, the two will always be separate and have clearance. However, in press fits and interference fits, there will always be a negative overlap.
In interference fits the hole is smaller than the shaft. In this example, we will go through the ANSI tolerance chart. You can find many types of functions like RC or LN but for most press fits, FN or Force Interference is used.
For this example, our nominal dimensions are in the range of 0.24 – 0.40 inches. Next, you select the class of fit, in this case, FN 4. As you move towards higher force fits, the nature of force required for the fit increases and is often used for permanent fits.
Structural joints and permanent couplings use FN 4 and this is a strong interference fit.
The tolerance table shows the interference in a thousandth of an inch. The first column and third row shows
Minimum interference: 0.6
Maximum interference: 1.6
The minimum tolerance for this class of interference fit is the nominal size of the hole and the maximum tolerance value is 0.6 thousandths of an inch.
For the shaft, the diameter can be 0.0012 inches to 0.0016 inches more than the nominal shaft diameter.
Assumiung the hole diameter is 0.3”, the hole tolerance range will be 0.3000” to 0.3016”.
For the shaft, the minimum shaft diameter can be 0.3012 inches and the maximum can be 0.3016 inches.
For even tighter fits, FN 5 tolerance range should be used.
Tolerance Press Fit Chart
A tolerance fit chart is a standard for dimensioning mating parts. There are many charts under various international standards. The most common standards are ISO and ANSI standards.
Engineering fits worldwide use the ISO IT Grade tolerance chart.
This chart shows the clearance fis, transition fits and interference fits. In this chart press fit classes are H7/s6, H7/p6 and other overlapping combinations.
Calculating Force in Interference Fits
When using interference fits, the shaft is pressed into a hole for permanent mounting. When cold pressing or shrinking the mating parts, there is a normal force as the components return to their normal temperature.
Failure of components may arise when this force exceeds the limit of the material or when the shaft walls or cylindrical walls of the hole experience force.
To calculate the force in interference fits, you can use a simple formula
Where Ei is the shaft’s young’s modulus
vi is the poisson’s ratio of the shaft
d0 is the hole’s outer diameter
di is the shaft’s internal diameter
d is the nominal diameter.
P=dE0(d02+d2d02-d2+v0)+dEi(d2+di2d2-di2-vi)
The minimum force to mate or remove the parts is given by
f= pmax A
This force is also called the axial holding force.
Where A is the area of contact between the shaft and the hub and is the friction between them.
Best Practices For Tolerance In Press Fit Design
Press fit design require careful consideration to ensure the fit of components is ideal and there is no failure. There are many ways to validate your engineering fit especially for rotating components involving friction fits.
Start with Materials
It is not possible to design press fits with materials having a small stiffness or materials with a large thermal expansion coefficient. Softer materials tend to deform permanently and press fits require elastic deformation when mating.
It is best practice to use materials with similar thermal expansion and strength. Even using aluminum with steel in a press fit can be dangerous because of their behavior to large thermal variton.
Use Tolerance Chart
When designing a press fit use an IT tolerance chart or any other standard chart to understand the level of interference of your press fit.
Test Your Design
Before mass producing parts, it is ideal to test all your press fits. You can use a 3D printed assembly, a simulation software or prototype before manufacturing.
Best Practices For Tolerance In Press Fit Design
There are many ways to test your tolerances before printing your shop floor drawings. This involves validation tests at the prototype stage and calculations.
Force Calculations
You can always validate your tolerance by finding out the normal force after selecting interference fit combinations from the ANSI table.
Thermal Expansion Calculations
Thermal properties of materials are fixed. Their melting points, heat absorption rate and coefficients can be used to find out the expansion under different operating temperatures.
There are many instruments you can use to test the engineering fit.
CMM
Coordinate measure machine allows for precise calculation of diameters and sharp angles. This is useful and accurate as tolerances are often thousandth of an inch. This highly accurate probe can accurately check the dimensions at the prototype stage.
Surface Analysis
When finalizing your press fit designs, pay careful consideration to the final surface finish. Many time clearance fit no longer have a free running space and components do not slide freely. This is mainly because surface coating processes like paint or electroplating can compromise the tolerance.
For interference fits this is even more crucial and there should be a margin that allows for any post process surface finishes.
Common Applications of Press Fits
Shafts and Hub
Most press fits are designed between shaft and hubs. This involves selecting the best radial interference for your application.
Bearings
Bearings commonly use tolerance ranges for permanent coupling. This is a delicate process because bearings are produced in standard sizes and shaft must be size down with high accuracy for best fitting.
Couplings
Couplings are used to join two shafts. A press fit in couplings bounds the two rods and prevents any relative movement for efficient transmission of rotary motion.
Structural Components
In crankshaft assemblies, press fits allow for a permanent and strong joint. There’s no slip and this maintains syncronized gears. Other structural components include joining wheel hubs to axles.
Learn More About Press Fit And Get Expert Opinion on Your Project
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