All about the internal bearing clearance

In order to guarantee the correct functioning of your machine, it is essential to choose the correct bearing. To do this, you need to start by determining the type of bearing you need. There are plain bearings and rolling element bearings.

Rolling element bearings are suitable for larger loads. Due to their rolling property, they cause less friction and slippage. After selecting the appropriate bearing type, you need to choose the permissible load direction, depending on the intended application. And to finish? Finally, you will select the appropriate class C for the bearing.

This class corresponds to the size of the internal bearing clearance. The class C chosen will be decisive for the operation of the bearing and of your machine. During operation, the internal bearing clearance must be minimal. You must therefore first take into account all the factors likely to influence the bearing clearance.

Bearing clearance dimension classes C

Class C bearings are shown as a suffix of the full bearing code. This code summarizes all the characteristics of bearings in terms of material of construction, lubrication, and, as noted above, internal bearing clearance. The following 5 classes C correspond to the most common internal bearing sets: C1, C2, C3, C4, and C5.

Class C is assigned to a bearing as soon as it has additional internal clearance. C1 bearings have the least additional play. On the contrary, C5 bearings offer the most additional play. C3 bearings, for their part, offer additional play considered “normal”. These C3 bearings are the most commonly used.

The radial clearance classes of bearings before assembly are determined by ISO standards:

  • C2 – Less than normal clearance
  • C0 of CN – Standard standard for applications where adequate clearance is maintained within recommended normal tolerances and under normal operating conditions.

Class C0 does not appear on the bearing.

  • C3 – Clearance greater than C0
  • C4 – Clearance greater than C2
  • C5 – Clearance greater than C4

Bearing clearance and life

We recommend calculating the bearing clearance in order to select the correct bearing. Indeed, a bearing clearance that is not perfectly adapted would affect the operation and the service life of the bearing. Your choice can be decisive in avoiding the appearance of problems inside the bearing.

This is often the case with electric motors that reach a high operating temperature. Due to this high temperature, the bearing rings expand even more (than expected); this is the reason why these bearings require a greater internal clearance.

What is internal bearing clearance?

To put it simply, the internal bearing clearance corresponds to the freedom of movement of the two rings which are in the bearing. This is the distance one ring can move relative to the other ring. It is possible to measure this clearance on a bearing that has not yet been installed by moving the rings in the opposite direction.

The rings have played in two different directions, which is why two types of bearing play are mentioned: radial play and axial bearing play.

Radial bearing clearance is measured perpendicular to the central axis.

The axial bearing clearance is measured parallel to the central axis.

In the case of both radial and axial bearing clearance, the total distance of the clearance in the axis is measured, from one end to the other. This measurement must be carried out when the bearing is not subjected to any load.

Important: In order to obtain a realistic value of the internal bearing clearance, it is necessary to subject the bearing to a load. Indeed, the value measured before subjecting the bearing to a load (geometric bearing clearance) differs from the actual clearance value of the bearing in operation (theoretical bearing clearance).

As soon as the machine starts to operate and a load is applied to the bearing, the inner rings can expand and the outer rings can be compressed. Elastic deformation occurs. This affects the internal bearing clearance. Thermal expansion can also affect bearing play. These deformations cause a reduction in the bearing clearance.

What is the authorized dimension of the theoretical bearing clearance?

The actual value of the internal bearing clearance subjected to a load (therefore once fitted) must be as small as possible (a few microns) or even zero. The correct bearing clearance also depends on the following factors:

  • intended use of the bearing
  • the internal temperature of the machine and the bearing
  • the axis and the bearing housing

Because of these determining factors, choosing the right Class C is a real job of precision. This is because you have to start by calculating the correct theoretical bearing clearance.

Calculation of internal bearing clearance

You can calculate the internal bearing clearance before mounting the bearing. To do this, you must subject the bearing to a specific load. Indeed, this load causes an elastic deformation and therefore modifies the internal clearance of the bearing.

The elastic deformation of rolling element bearings remains minimal. Due to their rolling property, roller bearings cause less friction. As a result, the inner rings expand less and the outer rings also compress less. The theoretical bearing clearance and the measured bearing clearance are therefore almost identical on roller bearings.

There are a total of 5 types of internal bearing sets. All of these types can be calculated. For each type, the account will be taken of different factors which affect the bearing clearance: for example, heat, load, and elastic deformation.

Internal bearing clearance measured (Δ1)

This is the measured bearing clearance when a specified load is applied. The elastic deformation (δfo), caused by the load, is taken into account in this game.

The following formula is used to calculate this form of internal bearing clearance: Δ1 = Δ0 + δfo

Theoretical internal bearing clearance (Δ0)

This is the radial internal bearing clearance, measured when no load is applied. This value, therefore, does not take into account the elastic strain.

The following formula can be used to calculate this form of internal bearing clearance: Δ0 = Δ1 + δfo

Ball bearings have significant elastic deformation (δfo), but this is zero (0) for roller bearings. The corresponding formula is, therefore: Δ0 = Δ1.

Residual internal bearing clearance (Δf)

This is the bearing clearance once fitted before the machine is started. There is therefore no elastic deformation in this case. However, the bearing clearance may decrease due to the expansion or compression of the ring (δf).

The following formula is used to calculate this form of internal bearing clearance: Δf = Δ0 + δf

Effective internal bearing clearance (Δ)

This is the bearing play that occurs in the machine due to the operating temperature. It does not take into account the elastic deformation caused by the application of the load.

In other words, this bearing clearance value applies when one takes into account only the variations caused by the adjustment of the bearing (δf) and by the temperature difference between the inner and outer rings (δt). The basic bearing load value applies only when the effective bearing clearance is zero.

The following formula is used to calculate this form of internal bearing clearance: Δ = Δf – δt = Δ0 – (δf + δt)

Operational bearing clearance (Δf)

This is the actual clearance of a bearing mounted and subjected to a load on a running machine. It takes into account the effect of elastic deformation (δf) and the determining factors of adjustment and temperature. As a rule, the operational bearing clearance is not used for the calculation.

However, the following formula can be used to calculate this type of internal bearing clearance: Δf = Δ + δf

The most important bearing clearance to calculate

What type of bearing clearance is it really important to calculate in determining which class C should be used? This is the “effective internal bearing clearance”. In fact, this bearing clearance value takes into account the variations caused by the adjustment of the bearing and the temperature differences, which influence the bearing clearance. It does not take into account the elastic deformation caused by the load, because this has an influence only in the event of zero effective bearing clearance.

The actual bearing clearance gives an indication of the life of the bearing. In theory, bearings with a negative effective clearance value very close to zero have the longest service life.

Why does this value have to be just negative? Because this type of bearing clearance changes when a load is applied, that is, when the machine is turned on. The value then just becomes positive. This is the ideal bearing play. The negative value of the effective bearing clearance varies depending on the load applied to the bearing.

Despite all the formulas for calculating bearing clearances, it is almost impossible to determine in advance the ideal clearance for each bearing. You must also take into account the theoretical bearing clearance for a zero or slightly negative effective bearing clearance value.

Do you still want to know when the effective rolling clearance value is the most optimal? You should then calculate the following two values as accurately as possible:

  1. The correct degree of decrease in bearing clearance caused by expansion and/or compression of the ring in the bearing (δf).
  2. The correct degree of variation in bearing clearance caused by the temperature difference between the inner and outer bearing end rings (δf).

What influences the internal bearing clearance?

We mentioned earlier that the internal bearing clearance corresponds to the freedom of movement of the bearing rings. A number of factors influence this freedom of movement. It’s the case:

  • From the fit of the inner ring: The ring is always slightly smaller than its axis. This is the reason why the inner ring expands and the outer ring contracts when the machine is running.
  • From the fit of the outer ring: When the outer ring is supporting a static load (and the inner ring is rotating), a compressive force is created on the inner ring. As a result, the outer ring presses on the inner ring or expands into the “free zone”. On the contrary, when the inner ring is supporting a static load (and the outer ring is rotating), the same effect as when the inner ring is pressed occurs the inner ring expands, and the outer ring contracts.
  • The temperature difference between the inner ring and the outer ring: When the bearing is loaded, the temperature always increases in the entire bearing. This also changes the temperature of the rolling elements. It is very difficult to measure and estimate this temperature variation. It is therefore assumed that the temperature of the rolling elements is identical to that of the inner ring. These temperature differences influence the expansion of the rings.

To what extent do these factors influence the size of the internal bearing clearance? You can calculate this using the following formula:

δt = α Δt De

Explanation:

δt: Decrease in radial bearing clearance due to the temperature difference between the inner ring and the outer ring (in mm)

α: Coefficient linear thermal expansion of bearing steel = 12.5 • 10-6 (1 / ⁰C)

Δt: Temperature difference between the inner ring and the outer ring (in ⁰C)

From: Diameter of the outer ring channel (in mm)

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

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

Why is it so important to know the correct Class C for your bearings?

Bearing class C indicates the size of the internal bearing clearance in the machine. It is essential to make the right choice in order to avoid excessive internal bearing play.

Indeed, the size of the internal bearing clearance largely determines the operational performance of the bearings. It influences the durability, the degree of vibration, the operating noise of the machine as well as the heat released by it. Therefore, you need to calculate the effective internal bearing clearance in order to be able to select the correct Class C for your bearings.

 

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