Differences Between Press Fit and Slide Fit in Manufacturing

Technical adjustments are an important aspect in producing well-functioning mechanical assemblies. Therefore, designers must understand the nuances between the different types of designed adjustments.

This article provides a detailed explanation of the differences between slide tuning and interference tuning to clarify differences that often cause confusion.

What is a pressure fit?

An interference fit, also called an interference fit, is a type of engineered fit in which mating components are held tightly together by friction. The parts present intentional interference between the contact surfaces, which is the cause of this excessive friction.

To provide more clarity, let's look at standard axle and hub tuning. When a hub needs to fit tightly onto a shaft to prevent lateral and rotational movement, its opening is intentionally kept slightly smaller than the outer diameter of the shaft. This is called positive interference play or negative play.

Press fit is communicated to the manufacturing department via technical drawings, which must then be maintained by machine operators. A practical example of this is the mounting of bearings on shafts, where shaft machining is critical in terms of press fit dimensions.

Example of pressure adjustment

Practical examples of press fit and slide fit as well as technical calculations help differentiate between slide fit and press fit. After determining that the design requires an interference fit, the engineer must first select an appropriate tolerance class and base system depending on the assembly application.

The base system comes with either a hole base or a wave base system. The hole base system uses the hole size as a reference (fixed) and suggests the corresponding technical tolerance range for the shaft. With the wave-based system, however, the opposite occurs.

Additionally, the tolerance class depends on how tight the fit needs to be. The following table shows different tolerance classes for different technical fits for both the hole and the shaft base.

Let's go back to the bearing shaft example mentioned earlier. The following figure shows that the bearing inner ring requires an interference fit with the shaft. In this case, the hole base system is applicable because the bearing size is standardized and the shaft size is customizable.

Suppose the engineer chooses an interference drive fit (H7/u6) and assumes the bearing outer diameter is 25 mm. Then the interference fit tolerance in the hole should be +48/-35 μm according to the ISO 286 shaft base tolerance table.

How to calculate pressure adjustment force?

We can go a step further and perform engineering calculations to determine the force required to assemble/disassemble the components. In addition, we can also determine the final assembly tightening torque, which must be known under difficult loading conditions.

Pressure on the contact surface

The following formula applies to calculate the pressure on the contact surface:

The terms and their respective values ​​for our bearing box are:

For simplicity, we assume that both the bearing outer ring and the shaft are made of steel. For some of the other unknown dimensions we also use arbitrary values. Furthermore, we assume that the radial interference is 10 μm, which is within the interference adjustment tolerance in the table.

If you plug all the values ​​into the formula, you get a pressure of 11.98 MPa. Assuming that the contact surfaces are completely smooth (which is obviously not possible), this pressure acts uniformly across the entire contact surface. Therefore, this value is applicable to the Pmax value that we will use in the next calculation.

Axial holding force

To calculate the axial holding force (assembly/disassembly force), we can use the following formula:

As before, you can find an explanation of the values ​​in the table below. The contact area is calculated assuming the bearing has a width of 10 mm.

The axial holding force for this configuration is 564.6 N. This is the force that the manufacturer should use as a reference when mounting/dismounting the bearing on the shaft. Furthermore, the axial load on the bearing must also not exceed this value to avoid displacement of the inner ring.

How is pressure fit achieved?

Correctly assembling a pressure adjustment is a task that requires skill and attention. Because there is interference between mating parts due to the dimensions of the interference fit, assembly is not as easy as, for example, tightening a nut on a bolt.

In general, there are two common methods for obtaining an interference fit.

1. Strength : The pieces are positioned in front of each other, with one part fixed and the other movable. The assembly force calculated in the previous section is applied to the moving part, forcing it onto/into the fixed part, thus achieving an interference fit.

This method uses brute force and generally requires one of the parts to have a chamfer (usually between 10° and 30°) on one of the mating corners to facilitate assembly.

2. Thermal expansion/contraction : The other method, which is more suitable for very tight or precise pressure adjustments, is to use heat. This method either heats the hub to the point where it thermally expands enough to slide easily onto the shaft, or cools the shaft to a temperature where it contracts enough to slide into the hub. The latter method is also known as shrink fit.

Once the assembly is in position, the components will gradually return to room temperature. The resulting thermal expansion/contraction creates the interference fit.

What is a slider adjustment?

Let's continue with this comparison between slide fit and press fit. The components of the sliding fit assembly are able to move freely relative to each other, unlike interference fits where the components are locked. However, the range of motion is still quite limited. Imagine a door hinge where the hinge and its pin rotate freely, but only within a defined range of motion.

Mechanically speaking, the idea is to leave a small space/gap between the mating surfaces so they can slide easily over each other. Another name for clearance is negative interference when setting the slip fit tolerance.

From a technical standpoint, sliding adjustments are relatively easy to evaluate. Since there is no excessive friction in the mating zone due to the lack of elastic deformation, pressure and force are easier to calculate.

Slider adjustment example

Dimensions for sliding fits can be obtained directly from a tolerance table. We can refer to our bearing example in the press fit section. The outer ring of the bearing has a clearance fit in the housing.

In this case, the shaft base system is applicable because the bearing ring size is constant while the hole in the housing/hub is adjustable.

Assume the designers chose a slip fit (H7/g6) to properly position the bearing in the hub while maintaining freedom of movement and assembly. As before, we determine the slip fit tolerance from a standard table.

Assuming the bearing outer ring has a nominal diameter of 30 mm, this results in a slip fit tolerance range of +21/-0 μm according to ISO 286. Although this seems like a very small value, it makes a big difference in work efficiency and the useful life of the assembly.

How is slide adjustment achieved?

Just like slide fit calculations, slide fit assembly is also more convenient than press fits. The general methods for mounting a slide fit are:

1. Force : For sliding adjustments with small play or interference, the traditional method of applying mechanical force is applicable. As with press fits, parts are placed in the correct position and force is applied by an appropriate machine.

2. By hand : Since most sliding adjustments are interference-free, mating components simply slide past each other without any external force being applied. In these cases, manufacturers perform the assembly manually.

Differences between press fit and slide fit

Let's summarize this entire discussion and summarize it in a few tips. The differences between push-fit and press-fit are not that big, but they make a big difference in how the assembly works.

Interference/free space

The obvious difference between these two engineering fits is that interference fits create interference between mating parts, while slip fits create clearance.

In other words, with an interference fit, the hole is smaller than the shaft, which means you have to use force to assemble it. With sliding adjustments, the hole is larger than the shaft and slides easily over it.

Degrees of freedom

Another important difference between the two lies in the degrees of freedom of movement of the corresponding components. In press fit, the corresponding parts are rigidly connected to each other, preventing any type of movement in any direction.

With sliding adjustments, on the other hand, there is relative movement between the components. For example, in a piston-cylinder system, the piston moves freely along the axis of the cylinder. However, the sliding adjustment still restricts the other two lateral movements.

Mechanical deformation

Mechanical deformation is another difference between slide fit and interference fit. Generally, in an interference fit, mating parts undergo physical deformation at mating surfaces. In most cases this is elastic deformation, but plastic deformation can also occur in very tight fits or in plastic materials with low creep resistance.

In contrast, slide fit components do not deform due to positive play in the slide fit tolerance. There may be some surface wear over time due to sliding action, but that is about it.

Assembly and disassembly

Due to the force required and thermal expansion/contraction, assembly and disassembly of pressure adjustments are more demanding.

Manufacturers need to control many other parameters, such as temperatures, heating/cooling application points, strength, and impact resistance of materials. Furthermore, there is also a risk of assembly components being damaged during the process. For example, high or misdirected forces during assembly can sometimes damage bearings.

On the other hand, plug-in connections are easier to assemble. Assembly is usually possible manually, which is quick, easy and less risky.

Manufacturing capacity

Pressure adjustments may require more precise manufacturing to obtain the required interference. As we saw in the Calculations section, press fit tolerance is critical to a successful press fit. Small changes to press fit dimensions can result in unsustainable pressure levels and subsequent failures.

Slip fits offer more flexibility in manufacturing tolerances, making them a little easier to achieve without the need for extremely tight tolerances. However, manufacturing precision is still a fundamental requirement. The entire interface must have the correct tolerance to allow smooth sliding motion. Otherwise, the parts may become blocked, misaligned or very loose.

Forms

A key point in the push-fit versus press-fit discussion is the different applications. As expected, both have very different areas of application.

Press fits are suitable for rigid, permanent connections with minimal or no relative movement between parts. Examples include bearings, bushings, and certain structural components.

Sliding adjustments are preferable when designers require ease of assembly and disassembly or some degree of movement between parts. Slip fit applications include housings, hinges, pivots and piston cylinder systems.

Quick view: Diagram of differences between press fit and slide fit

Concluding

Technical adjustments are one of the most important concepts in design and manufacturing. This discussion on slide fit vs press fit highlights several aspects including their usage, technical analysis, and comparison. Of course, choosing the correct technical fit and producing it within tolerance is crucial for a functional mechanical assembly.

Common questions

What standards are used to determine interference fit and sliding fit dimensions?

ISO 286 and ANSI B4.1 are the two most recognized standards for technical fit tolerances.

Are transition adjustments also considered sliding adjustments?

Both transition fits and clearance fits generally fall into the sliding fit category because they share similar properties. However, there is no specific dividing line.

Is heating/cooling always necessary for press connections during assembly?

Thermal expansion/contraction is not always necessary with press fit. For low-interference pressure fits, applying force at room temperature is often sufficient to successfully assemble mating components.