The Fit Between the Bearing and the Shaft: How to Choose Perfect Match

What is a “Fit”? A Simple Analogy

Imagine you are putting a lid on a jar:

• If the lid is much larger than the jar’s opening, it will be loose and wobbly. This is a Clearance Fit.
• If the lid is slightly smaller than the opening, you need to press it in firmly. It will be very secure. This is an Interference Fit.
• If the lid and the opening are very close in size, it might fit easily or require a little push. This is a Transition Fit.

A bearing fit describes the size relationship between the bearing’s bore (the hole) and the shaft’s outer diameter. The choice of fit depends on the machine’s working conditions.

First, Understand “Tolerance”

In the real world, no part can be made to an exact size. A shaft labeled as 50 mm might actually be 50.01 mm or 49.99 mm. This allowed range of variation is called “Tolerance”. Because of tolerance, we need to define fits scientifically. This ensures that the connection between the shaft and bearing achieves the desired result within these variations.

Three Common Types of Fits

We typically choose from three types of fits based on rotation and load conditions:

Clearance Fit – The Flexible Partner

• What it is: The bearing’s bore is always larger than the shaft’s diameter. After assembly, there is a gap between them.
• Advantages: It is easy to install and remove. The precision requirements for the shaft and housing are lower.
• When to use it:
  • It is often used for the non-rotating ring. For example, if the inner ring rotates with the shaft, the outer ring and the housing will usually have a clearance fit for easy assembly.
  • It is also used when the shaft needs to slide axially or expand due to heat.

Interference Fit – The Tight Hug

• What it is: The bearing’s bore is smaller than the shaft’s diameter. Assembly requires force (press-fitting) or temperature difference (heating the bearing or cooling the shaft).
• Advantages: The tight fit creates high pressure. This securely fastens the two parts together. It prevents the ring from slipping on the shaft during operation (a problem called “creep”), which can cause serious wear.
• When to use it:
  • This is the most common fit for a rotating ring. If the inner ring rotates, its fit with the shaft must be an interference fit to transfer torque and prevent wear.

Transition Fit – The Balanced Choice

• What it is: This fit is between a clearance and an interference fit. Depending on the actual tolerances, the final assembly might have a small clearance or a small interference.

• Advantages: It provides good centering accuracy without needing the high assembly force of an interference fit.

• When to use it:

• • It is suitable for applications that need high precision but do not have heavy loads or high speeds. It’s a compromise between positioning accuracy and ease of assembly.

Other Factors to Consider When Choosing a Fit

Besides the basic rules of rotation, engineers also consider these factors:

• Load Magnitude and Type: Heavier loads, or shock and vibration, require a tighter interference fit to prevent loosening.
• Operating Temperature: During operation, temperatures rise. This causes materials to expand. The shaft often gets hotter than the housing, which makes the fit tighter. This change must be calculated in the design.
• Material of Shaft and Housing: Steel and aluminum expand at different rates. If a bearing is mounted in an aluminum housing, the housing will expand more than the bearing’s outer ring when hot. This can loosen the fit.
• Ease of Assembly and Disassembly: For equipment that needs frequent maintenance, a looser fit might be chosen.

Conclusion: The Right Fit is the Best Fit

Choosing a fit for a bearing and shaft is like matching a key to a lock. Too loose, and it will wobble. Too tight, and it will jam. Only by considering all factors—load, temperature, speed—can you select the perfect fit. This allows the bearing, our “magician of motion,” to reach its full potential and provide long, reliable service for your equipment.