Understanding Slip Fit and Press Fit in a Mechanical Assembly

Understanding engineering limits and fits are a staple for every engineer in order to create well functioning mechanical assemblies. In this article we are focusing on slip fit vs press fit. When fitting a bearing, bush, seal or any propitiatory part most suppliers are extremely helpful and supply fitting dimensions dictating the recommended fit for their products… ALWAYS FOLLOW THESE INSTRUCTIONS!

However, in some cases, if this information is not supplied or you are in control of the interface there are a few common limit and fit tolerances that will get you through the majority of mechanical design cases.

Press Fit vs Slip Fit

In this article we are going to focus on slip fit vs press fit. What are the key differences, the advantages and disadvantages of both and some practical examples.

Press Fit assemblies are rigidly fixed
Slip Fit assemblies have degrees of freedom

Slip Fit

Slip fits or sometimes referred to as clearance fits come in 6 sub-categories: loose running, free running, close running, sliding, close clearance and locational clearance. For all these fit types, the shaft is sized smaller than the hole size.

Most general engineering applications use hole bias fit in which the larger part of the tolerance is allocated to the hole. This is because it is harder to achieve an accurate hole

A Slip fit assembly tolerance will allow for a defined clearance gap between components allowing for controlled degrees of freedom within the assembly interface. This is typically an axial and a rotational freedom, imagine a door hinge or a piston cylinder for example.

Using the examples in the table above, a 25 mm nominal diameter sliding fit hole and shaft arrangement, H7/g6 fit gives a minimum clearance of 0.007 mm and a max clearance of 0.041 mm.

Slip fit tolerances are more generous than that of a press fit tolerance, this makes them easier and subsequently cheaper to manufacture than press fit tolerances.

Slip fit components can usually be assembled by hand without the need of fitting tools with little risk to component deformation however they may experience some form of surface wear due to the sliding and or rotation action over time.

Download our essential Limits and Fit Table here

Press Fit

Press fits, sometimes referred to as transition or interference fits come in 5 sub-categories: similar fit, fixed fit, press fit, driving fit and forced fit. This interface can be achieved in two ways, depending on if the shaft or the hole is taken as the referenced dimension.

If the hole is sized smaller than the shaft = Shaft Bias
If the shaft is sized larger than the hole = Hole Bias

Most general engineering applications use hole bias fit in which the larger part of the tolerance is allocated to the hole. This is because it is harder to achieve an accurate hole

Press fit tolerances are sized so that there is intentional interference between the components causing friction between the mating surfaces allowing for a ridge and permanent connection.

Using the examples in the table above, a 25 mm nominal diameter press fit hole and shaft arrangement, H7/p6 fit gives a min interference of 0.001 mm and a max interference of 0.035 mm.

Press fit tolerances are much tighter than that of a slip fit tolerance due to precision required to achieve the necessary interfaces. Small changes in press fit tolerances can lead to increased pressure levels and subsequent failures upon assembly or during use.

The two most common ways of achieving a press fit assembly are:

force

This method speaks for itself. Typically, one of the components is held in position, the two components are positioned together and a force is applied until the mobile component is pressed into the fixed component.

To help achieve this, a lead in chamfer (1mm x 20deg) on the fixed component is highly recommended. Fitting tools can also help ease assembly depending on access or the amount of force required to achieve the type of press fit required.

Thermal Expansion or Contraction

This method is more commonly used for applications requiring very tight or precise fitments. In thermal expansion the hole side of the component is heated until it thermally expands to allow the shaft to slide through. In thermal contraction, the shaft is cooled to a temperature where is shrinks enough to slide through the hole.

Once the components are fit, the assembly is allowed to return to room temperature, and therefore return to designed dimensions, creating the interference fit.

In both cases, press fit components will experience some form of mechanical deformation in order to assemble.