Bearing clearance: everything you need to know

The choice of a suitable bearing is of utmost importance for your machine to function properly. Choosing the right bearing starts with determining the type of bearing you require. For example, there is a distinction between plain bearings and bearings with roller elements.

Bearings with roller elements are suitable for larger loads. Because they have a rolling function, they cause less friction and slide. After the type of bearing, you choose the bearing with the right load direction, depending on the type of application. In the end you choose the correct C code for the bearing, which indicates the degree of internal bearing clearance.

The choice of a correct C code is crucial for the functioning of the bearing and your machine. During operation, the internal bearing clearance must be as minimal as possible. Therefore, all factors affecting bearing play must be taken into account beforehand.

C codes for degree of bearing clearance

The C code of bearings at the rear follows the complete coding. Here you will find all the characteristics of the bearings in terms of material, lubrication and therefore also the degree of internal bearing clearance. These are the 5 most common C codes for internal bearing play: C1, C2, C3, C4 and C5.

If a bearing has a C code, it offers extra internal clearance. C1 offers the least extra clearance, C5 offers the most extra clearance. The extra margin C3 offer is considered 'normal'. Bearings with C3 are in fact the most commonly used.

The radial play grades for non-assembled bearings are defined in ISO standards:

• C2 - Less clearance than normal
• C0 of CN - The standard for applications where the normally recommended fits, and under normal operating conditions, leave the correct clearance. The C0 code is not indicated in the bearing.
• C3 - Clearance greater than C0
• C4 - Clearance greater than C2
• C5 - Clearance greater than C4

Bearing clearance and bearing lifespan

We advise you to calculate the bearing clearance so that you choose the correct bearing. If the bearing play is not optimal, this will affect the function and lifespan of the bearing.

Your choice can determine whether there is an internal problem with the bearing. This is often the case in high-temperature electric motors. Due to the high temperature, the rings in bearings expand even more (than expected) and therefore these bearings require a higher degree of internal bearing clearance.

What is internal bearing clearance?

In a nutshell, internal bearing play is the freedom of movement of the two rings of the bearing. It is the distance over which one ring can move in relation to the other. This clearance can be measured for a bearing that has not yet been installed by moving the rings in the opposite direction.

Because the rings have play in two directions, there are also two types of bearing play:
Radial bearing play, measured perpendicular to the centre axis
Axial bearing play, measured along the centre axis

With both radial and axial bearing play, we measure the total distance of the play in the axis: from one extreme position to the other extreme position. The bearing is not under load during this measurement.

Important: for a realistic value of the internal bearing clearance, you must load the bearing. There is a difference between the measured value before load (the geometric bearing clearance) and the actual value of the bearing clearance in operation (the theoretical bearing clearance).

As soon as the machine is running and the bearing is loaded, the inner rings can be stretched and the outer rings pressed together. This results in elastic deformation, or elastic deformation. This affects the internal bearing play.

In addition, thermal expansion can also affect the internal bearing clearance. The size of the bearing play decreases with these deformations.

What is the maximum theoretical bearing clearance?

The actual value of the internal bearing clearance should be as minimal as possible (a few microns) or even 0 under load and therefore after assembly. Incidentally, the correct degree of bearing play also depends on these factors:

• the application of the bearing
• the temperature in the machine and the bearing
• bearing shaft and casing applications

Because of these influencing factors it takes precision work to choose the right C code of the bearing. You must first calculate the theoretical bearing clearance correctly.

Calculating the internal bearing clearance

You can calculate the internal bearing clearance before mounting the bearing. This is done by assigning a certain load to the bearing. This load causes elastic deformation and thus also changes the internal bearing clearance. In bearings with rolling elements, the elastic deformation is minimal.

The rolling function reduces friction in rolling bearings. As a result, the inner rings stretch less and the outer rings compress less. The theoretical bearing clearance and the measured bearing clearance are therefore virtually the same for rolling bearings.

In total, we distinguish 5 types of internal bearing clearance. All these types can be used for calculation. All types take into account other factors that influence bearing clearance. Such as heat, load and elastic deformation.

Measured internal bearing clearance (Δ1)

This is the measured bearing clearance at a certain load. This includes the elastic deformation (δfo) caused by the load.

With this formula, you can calculate this form of internal bearing clearance: Δ1 = Δ0 + δfo

Theoretical internal bearing clearance (Δ0)

This is the radial internal bearing play, measured without load. This value does not include the elastic deformation.

With this formula, you can calculate this form of internal bearing clearance: Δ0 = Δ1 + δfo

Ball bearings have a significant elastic deformation (δfo), but for rolling bearings this is nihil or 0, so then the formula is: Δ0 = Δ1.

Remaining internal bearing play (Δf)

This is the bearing clearance after assembly before the machine is in service. The elastic deformation is therefore not applicable here. However, the bearing play can decrease because the ring expands or is compressed. (δf).

With this formula, you can calculate this form of internal bearing clearance: Δf = Δ0 + δf

Effective internal bearing clearance (Δ)

This is the bearing clearance created in the machine by the operating temperature. The elastic deformation due to load is not included. In other words, this value of bearing clearance applies when you only take into account the changes due to the fitting of the bearing (δf) and the temperature difference between the inner and outer rings (δt). The value of the base load of the bearing applies only when the effective bearing clearance is 0.

With this formula, you can calculate this form of internal bearing clearance: Δ = Δf - δt = Δ0 - (δf+δt)

Operational bearing play (Δf)

This is the actual bearing clearance of a mounted and loaded bearing in an operational machine. The effect of the elastic deformation (δf) is included as well as the influencing factors of fit and temperature. In general, the operational bearing clearance is not used in the calculation (Δf).

This is the actual bearing clearance of a mounted and loaded bearing in an operational machine. The effect of the elastic deformation (δf) is included as well as the influencing factors of fit and temperature. In general, the operational bearing clearance is not used in the calculation.

With this formula, you can calculate this form of internal bearing clearance.: Δf = Δ + δf

The most important bearing clearance to calculate

What type of bearing play do you actually need to calculate to find out which C code you need? That is the 'effective internal bearing clearance'. This value of the bearing clearance takes into account the changes due to the fit of the bearing and the temperature differences that influence the bearing clearance. The elastic deformation due to load is not taken into account, because it only affects the bearing play if the effective bearing play is 0.

The actual bearing clearance also indicates how long the bearing will last. In theory, a bearing with an effective bearing clearance of a very small negative number has the longest service lifespan. Why does this value have to be negative? Because this degree of bearing clearance at load - when the machine starts operating - transforms. The value becomes positive, but just. And that is the ideal bearing play. How negative the value of the effective bearing play should be, depends on the degree of the load that the bearing will have to deal with.

Despite all the formulas for calculating bearing clearance, it is virtually impossible to find the perfect bearing clearance for each bearing beforehand. You should take into account the theoretical bearing clearance for an effective bearing clearance value of 0 or a slightly negative number.

Would you like to know when the effective bearing clearance is as optimal as possible? Then you should calculate these two values as accurately as possible:
1. The correct degree of reduction of bearing clearance by expanding and/or pressing the ring in the bearing (δf).
2. The correct degree of bearing clearance change due to the temperature difference between the inner and outer ring of the bearing (δt).

What influences the internal bearing clearance?

Earlier we indicated that the internal bearing clearance means the same as the freedom of movement of the rings of the bearing. However, there are several factors that influence this freedom of movement.

• The fitting of the inner ring: The ring is always slightly smaller than its axis. Therefore, the inner ring will expand in operation and the outer ring will contract.
• The fitting of the outer ring: If the outer ring carries a static load (and the inner ring rotates), a pressure force is generated on the inner ring. The outer ring either presses on the inner ring or the 'free space'. If, on the other hand, the inner ring carries a static load (and the outer ring rotates), the same effect occurs as when the inner ring has a compression fit: the inner ring expands and the outer ring contracts.
• The temperature difference between the inner and outer ring: When the bearing is loaded, a temperature rise always occurs throughout the bearing. The temperature of the rolling elements also changes. This temperature change is very difficult to assess and measure. Therefore, it is assumed that the rolling elements get the same temperature as the inner ring. These temperature differences influence the expansion of the rings.

To what extent do these factors influence the degree of internal bearing play? This can be calculated with this formula:
δt = α Δt De

Explanation:

δt: Reduction of radial bearing clearance due to temperature difference between inner and outer ring (in mm)
α: The coefficient of linear thermal expansion for bearing steel = 12,5 • 10-6 (1/⁰C)
Δt: The temperature difference between the inner and outer ring (in ⁰C)
De: Outer ring channel diameter (in mm)

For ball bearings: De = (4D + d)

For rolling bearings: De = (3D + d)

Why is the correct C code of your bearings so important?

The C code of the bearing indicates how much internal clearance the bearing gets in the machine. Your choice is crucial to prevent your bearings from having too much internal clearance.

The size of the internal bearing clearance largely determines the operational performance of the bearings. It affects the durability, the degree of vibration, the amount of noise generated in the machine, and the heat released from the machine. Therefore, calculate the effective internal bearing clearance to be able to choose the correct C coding for your bearings.

These are a number of specific applications with their optimal internal bearing clearance:

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