What is the Difference Between Press Fit And Slip Fit in Machining?
Mechanical engineering tolerances are essential for the performance of devices and machines. The concept of tolerances ensures that parts not only fit as expected, but also perform to their optimum levels. When the variation is either too much or too little, damage could occur, even to catastrophic levels.
When it comes to the mating of two engineering pieces in machining, the concepts of press fit and slip fit are often in the conversation. What is the difference between the two types of fit? That’s what this article is about. Understanding the difference will help you ensure that your parts perform as expected. Join us as we briefly expound on the two concepts.
What Is A Press Fit?
Before explaining what a press fit is, it might be worthwhile to revisit the fit topic briefly. When assembling two engineering parts, there is a way these parts relate due to size differences. That relation refers to the fit. In other words, a fit is the level of looseness or tightness of the resulting joint.
In regards to the fit concept, there are two important terms to remember: Interference and clearance.
Clearance is the difference between the size of the shaft and the size of the hole before assembly. This value may be either maximum or minimum. The hole has to be larger than the shaft.
There are instances where the shaft is larger than the hole. In other words, the difference of size between the hole and the shaft, before assembly, is negative. That difference is called interference. Similarly, interference can be minimum or maximum.
The concept of interference brings what is called interference fit, or press fit. You may also call it friction fit. Some little pressure is needed to assemble one part to the other one. Sometimes, you need to slightly hammer the two components for mating to occur.
Since friction is at work in this type of fit, no adhesive or fastener is needed. This fit is common in engineering devices that need to minimize vibrations or carry heavy loads. Given the level of force needed to establish a press fit, disengaging the components needs significant force. Often, the process of separating the parts leads to damage.
Example Of Press Fit in tight tolerances
Let’s consider a bearing and a shaft. The intention is to press fit the bearing onto the shaft. First, the steel shaft is machined to accurate dimensions, but its diameter is slightly larger than the bearing’s inner diameter. This will create a press fit.
The ball bearing is cleaned nicely to get rid of any contaminants on its surface. After placing the bearing in position on the shaft, a hydraulic press is engaged to force the bearing onto the shaft. However, this force is controlled.
This process creates an interference fit or press fit, which keeps the bearing and shaft tightly connected together. Note that this kind of connection does not require more fastening. It is reliable as it is.
How do Calculate Press Fit Force?
There are mechanical engineering formulas for determining the forces in press fit. The pressure at the interface between the parts has a specific formula. There is also a formula for calculating the force for assembling and disassembling the force. This force can also be referred to as the axial holding force.
How Is A Press Fit?
Achieving an effective press fit requires a lot of attention and prowess. Assembly of components in press fit situations can be challenging. The use of force and thermal expansion/contraction are the two most prominent way to achieve press fit.
Using Force
The two mating parts are placed alongside each other. A predetermined force is applied to one of the parts. A chamfer is necessary for this fit that uses raw force.
Using Thermal Expansion/Contraction:
Where the press fit is extremely tight thermal expansion and contraction are more applicable. It could be thermal expansion of the material with the hole. It could also be the contraction of the shaft – shrink fitting.
What Is A Slip Fit?
Slip fit tolerance is the difference of press fit tolerance. In this fit, there is need for freedom of movement. Some level of play between the mating parts is required. You will find this type of fit where the different parts need to glide in and out of each other.
Different Types of Slip Fit
Where the press fit is extremely tight thermal expansion and contraction are more applicable. It could be thermal expansion of the material with the hole. It could also be the contraction of the shaft – shrink fitting.
Running fit
For this kind of slip fit, the accuracy of the combination is not that essential. It is characterized by big clearances. The speeds and temperatures in running fit are not constant.
Easy Slide
For this one, the clearance between the parts is significantly smaller. It is best used where there is irregular motion. A good example is in pistons.
Loose Running Fit
Accuracy is not critical in this fit, but the clearance is large. It is ideal for parts that rotate fast. Pivots are examples of loose running fit.
Slide Fit
There is no visible clearance between the moving parts. It is the fit of choice in car assembly, where accuracy and precision are paramount.
Location Clearance Fit
This one needs precision so the clearance is very small. Lubrication may be added to help the mating parts to glide and turn smoothly. Roller guides belong to this category.
Example of Slip Fit Tolerance
A perfect example of slip fit is where a bolt slides through a hole and is attached to a nut on the other side. This type of slip fit would start with the machining of a hole size slightly larger than the bolt. The hole is in a metal plate. The bolt is also prepared according to the dimensions – its diameter should be smaller than the hole diameter.
Because there is a clearance fit, the bolt slips through the hole in the plate. The other end of the bolt is secured using a nut. Once the nut has been secured using a suitable tool, a slip fit is achieved. The assembly and disassembly of this type of fit is simple.
How Is Slip Fit Achieved?
Slip fit can be achieved between different parts in machining if there is a small clearance. Considering that the parts can slide together, either force or manual manipulation can apply. Mechanical force is the most popular force source when trying to achieve the slip fit. You could also go manual and use the hands to achieve the slip fit.
Below is an overview of both ways of achieving slip fit:
Using Force
As we have said, this involves the application of force, usually hydraulic press means. This approach is preferable where the parts being merged are sizeable, so the use of manual manipulation may not work. The consistency and preciseness of the the force applied is advantageous because one is able to achieve the intended fit easily.
By Hand
Alternatively, you can use the manual approach to achieve the slip fit. This method works perfectly where the parts are smaller and lighter. As long as the person doing it is skilled, the fit achieved here can be flawless.
Press Fits Vs. Slip Fits, What are Differences?
Press fit and slip fit are fundamentally different. From this, several other differences emerge related to the functionality, relevance, and other aspects. Here is a rundown of the major differences between the two types of fit in machining:
Interference Fits/Clearance fits
An interference fit has a positive interference between the shaft and the hole. A clearance fit has a positive clearance between the shaft and the hole. This is the most notable difference between press fits and slip fits. Interference fits are related to press fit while clearance fits are related to slip fit.
Degrees of FreedomDegrees of Freedom
The degrees of freedom in the mechanical engineering context for the two fits are different. For press fit, the only degree of freedom available is in the axial direction. With the rotational movement restrained, this fit provides the required resistance and stability.
For the clearance fits, there is more degree of freedom. Because the hole and shaft connect more loosely, there is more movement and flexibility. This can equally be advantageous in some instances.
Mechanical Deformation
There is also a difference in the amount of mechanical deformation subjected to the mating parts. For press fit, mechanical deformation of the parts can be substantial. This deformation is typically elastic deformation. If the press fit is extremely tight, plastic deformation could also occur.
Components in slip fit scenarios do not undergo mechanical deformation. There is neither plastic nor elastic deformation. The only result of this interaction may be a bit of surface wear over time.
Tighter Tolerances for Press Fit compared to Slip Fit
In press fit, the tolerance is tighter. That means that the room for deviation from the design parameters is very limited. That may not be the case for slip fit, where the tolerance is not as tight.
Assembly & Disassembly
The assembly and disassembly process is also different for the two types of fit. Because of the resulting rigidity, press fits usually have difficult assembly and disassembly processes. The lack of relative movement is a challenge. It is different for slip fit because these have relative movement – rotational and sliding motion.
Manufacturability
The tolerance level comes into play when manufacturing for the two fits. It is easier to manufacture for slip fit than it is for press fit. Press fit has limited room for dimensions adjustments. There is more flexibility in the manufacture of slip fits. This is not say that accuracy is disregarded. Manufacturing tolerances for slip fit still need to ensure smooth motion between the mating parts.
Applications
Given all the above differences, there are bound to be different applications for press fit and slip fit. The former finds application in tight and often permanent connections. Examples of these are bushings and bearings. For slip fit, uses include piston cylinders and pivots. Here, easy assembly and disassembly is a a priority.
Conclusion
Press fit and clearance fit are major types of fits in machining. Each of these fits has its pros and cons. In this article, we have differentiated between the two fit tolerances.
The article also looked at how you can achieve both press fit and slip fit. The importance of a good fit has been reiterated through this discussion.
Aria Manufacturing encourages clients to provide as much details as possible on tolerances in the engineering drawings. Other that, we always prevent measurement problems by performing due diligence on every project.
So, are you looking for an accurate fit for your part? Consult Aria Manufacturing for your next project.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.