PressFit Force / Force Required for Assembly
Press fit assembly is one of the traditional methods to secure two parts. Presses are used which combine the two parts together with large amounts of force. The presses are usually hydraulic, although there exist small hand operated presses which work with mechanical means (e.g. jackscrew). Usually the edges of the mated parts are chamfered thus providing a smoother and more efficient operation of the process.
Interference fit is widely used in a wide variety of applications for its high ability to transmit axial force or torque. The performance of interference fit changes during duty operation and strongly depends on stress concentrations and environmental conditions.
Pressfit is prefered through other means of fastening because is economical and does not require complicated tooling.
The amount of force required for press-fit is a function of parameters such as:
- friction coefficient
- Area of surface in contact
- Interference fit value
- Surface roughness
- Environmental conditions
In addition, a designer must have in mind the purpose of the press-fit or in other terms what is the part’s function in the assembly. The later is of paramount importance for a robust and reliable connection.
Calculating press fit force
Formulas and empirical equations exist for basic and simplified geometries. For the first stage of analysis the designer can use these simplified equations to gain a first impression on the force required for assembly. A simplified calculator can be found on the link below:
Figure below gives an illustration of the tools GUI (source: abc-tribology.com).
Cautious is required when someone applies simplified equations for determining press-fit force for real case and thus more complex problems. I have witnessed situations, where engineers had used simplified formulas for calculating press-fit forces that were significantly far beyond the real case scenario. In addition, purchasing decisions of large hydraulic press machines were made based on this false estimation with significant costs and critical delays for the whole product development process. At the end, a simple FEA study was able to predict the real force required for assembly with significant accuracy.
Small Case Study
A small case study is presented below which consists of a ring and a shaft that are fitted together.
Initially, the ring (yellow part) is at the starting position of the assembly process. The press machine applies a force on the shaft until the final assembly configuration is achieved.
The following diagram gives the force versus time (pseudo-time) of the assembly process. The force required for the assembly is given at the last step. For this example the approximated force is around 7 kN.