Press Fit and Slip Fit: What’s the Difference?

Engineering fits play a vital role in manufacturing components that require assembling together. When it comes to considering the different types of fits, the press fit and slip fit are the two most common ones.

The guide below explores press fit and slip fit, so let's learn about them.

What is a Press Fit?

Press fit is also called interference fit, which helps components to fit tightly with one another through friction. These parts consist of intentional interferences between components that have to mate, which causes added friction.

Press Fit Example

As an example of press fit, consider that a shaft is machined with a larger diameter compared to the inner diameter of the bearing. A hydraulic press is used to force the bearing in the shaft. Such a fit is called a press fit, and it keeps the shaft and bearing connected.

How to Calculate Press Fit Force?

Press fit can be calculated using a formula. It represents the force that is required for a part to be inserted into another to create a perfect fit.

This force is dependent on many things, like the amount of interference, material properties like modulus of elasticity, and length of engagement. Moreover, the coefficient of friction between surfaces is also crucial.

The formula for calculating the press fit force considers surface area, energy force, and elasticity. The formula is as follows:

Each symbol is provided in the table below:

How is Press Fit Achieved?

Appropriate assembling is a very important task that needs proper attention and skills. As there is interference between components due to the dimensions of press fit, the assembling process is not very simple.

There are two ways you can assemble the mating parts, and these are as follows:

Force

The parts of the press fit are kept in front of one another, and it has one fixed part and one moveable. The force of assembling is calculated using the formula given above and it is applied on the movable part. It forces the moveable part into the fixed one, and a press fit is achieved. This method is dependent on the raw force, and it needs at least one part to have a chamfer.

Thermal Expansion/Contraction

The hub in this method is heated to its points of thermal expansion which makes it expand enough to slide on the shaft. In other cases, it is sometimes called to a température where it contracts efficiently to slide in the hub. Once the components are assembled, they return to their room temperatures.

What is a Slip Fit?

Slip fit involves assembling components that are free to move and are not locked like a press fit. However, the movement is restricted. You can take an example of a door hinge where the hinge rotates followed by its pin, but only in a given range.

The main aim behind keeping them moving is to give some gap between components allowing them to slide over one another.

Slip Fit Example

An example of a slip-fit is a bolt that slides through the hole in a metal plate and is fixed with a nut on the other side. Another example is a piston, which moves along the axis of the cylinder freely, but due to slip fit, its movement is restricted to two lateral motions.

How is Slip Fit Achieved?

Slip fit between the mating components can be achieved in the following two ways.

Force

The slip fits require applying minimal interference, which is applying some force. The components are positioned properly, and the force is applied using a machine.

Using Hand

Slip fits slide into one another without any external force in many cases. However, in some cases, they are forced with a hand rather than a machine.

Standards for Press Fits and Slip Fits

In many applications, the tolerances of press fits and slip fits are very critical and are governed by standards. The section below discusses each type of fit and relevant standards.

Press Fits

Press fits are designed to help create a secure connection between the shaft and its hole. There are different types of press fits, like light press fits, which are meant for ease of assembly. The medium press fit offers moderate interference, and the heavy press fit for permanent joints. Standards that govern these press fits consist of ISO 286, ANSI B4.2, and DIN 7157.

Slip Fits

Slip fit is also known as clearance fit; it can easily allow one part to slide on another and have a controlled movement. These fits are suitable for such applications which require disassembling and have rotational movements. These fits consist of either loose fits or rotational fits. The standards governing slip fits are ISO 286 and ANSI B4.1.

Difference Between Press Fit and Slip Fit

Press fit and slip fit differ in any way, and some of these differences are discussed below:

Interference/Clearance

In press fit, the mating parts need interference in dimensions. Whereas, in the slip fit they create a clearance.

Degrees of Freedom

The degree of freedom is also a difference between press fit and slip fit. In press fit, the mating parts are locked, and they cannot produce any motion. Whereas, in the slip fit, the parts are not completely locked and they can move in a controlled range.

Déformation mécanique

Mechanical deformation is also a difference between the two types of fits. In press fit, the mating parts pass through a deformation; mostly, it is elastic deformation, but in certain cases, it may be plastic deformation.

In slip fit, the components do not deform as there is a positive clearance. Hence, the surface wear is there but no deformity.

Assembly & Disassembly

In press fits, assembling and disassembling is very challenging as it needs a lot of force. It also has various challenges in controlling parameters like temperature, force, heating, and cooling. Hence, the chance of damage also prevails.

Slip fits are easy to assemble, and this can be done by hand very quickly and risk-free.

Manufacturability

Press fits need precise manufacturing so that the required interference is achieved. Even the slightest change in the dimensions can lead to instability and failures.

Slip fits are flexible in terms of manufacturing tolerances and hence, they do not need very tight tolerances.

Applications

Press fits are suitable for such connections which are permanent and rigid. They can be opted for bushings and bearings followed by some structural parts.

Slip fits are very easy to assemble and disassemble; hence are suitable for components like housings, pivots, and hinges.

Conclusion

After going through the detailed guide above, you must have learned everything about the press fit and slip fit.

If you are looking forward to having your components manufactured and keeping these fits in them, we at DEK can cater to you. We offer high quality and precise manufacturing of components keeping the assembling context as our top priority.