Bearing Preload

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What Is Bearing Preload?

Bearing preload is related to the issue of clearance and can be critical for the proper functioning of your bearing. There are multiple techniques and ways of looking at the preload process, and we will examine them in depth here. A bearing needs to be fitted with a shaft and housing and will have some level of clearance, meaning there will be some give between the different parts of the ball bearing. To remove this internal clearance and create an interference fit, a preload is necessary. The preload essentially describes the process wherein a permenant thrust load is applied to the bearing, essentially using force to push the bearing so that it is secure in the groove and has no axial clearance.

Purpose for Preload

The reason we preload is to eliminate that “give” in both the radial and axial directions for the bearing that exists even after the bearing is mounted on a shaft radially, where such give is highly undesirable. For example, when a ball bearing is used in a motor, it has "Zero" radial clearance when an axial load is applied. If there is any radial clearance, vibration and noise of the balls will occur, and the stiffness of the ball bearing will be very low. This force that is applied in the axial direction is known as preload. The bearing preload process gets rid of all the unwanted clearances, creating high stiffness and reducing noise and vibration. An added benefit is that reducing the clearances can help control the rotational accuracy of the bearing and reduce runout. The preload also helps prevent the balls in the bearing from skidding on the races. An optimum preload should be individvally specified for each ball bearing size. If the Preload is applied excessively excess heat can be generated in the bearings, with the result that Bearing Fatigue Life will be short and will increase raceway noise as well. Bearing starting and running torque will also be high by necessity, to overcome the tightness in the bearing. This also increases the power demands of the system. If the applied Preload is insufficient, fretting corrosion can occur. This happens as a result of vibration causing the balls to resonate and abrade on the raceways. Therefore, obtaining the correct Preload is very important.

Optimum Preload

Optimum Preload is normally recommended after calculating the optimum operating surface stress at the contact ellipse. The contact ellipse is the area of contact between the ball and raceway that occurs as a result of elastic deformation of both parts under load.

Regarding the figure, the contact ellipse area (S) between the ball and raceway is formulated as S = πab
(a: the major axis of the contact ellipse area, b: the minor axis of the contact ellipse area). Operating surface stress (P) is given by Q/S, where Q = Ball load or load on the raceway (Perpendicular to the area of contact), and S = Surface area of the contact ellipse. Generally, the unit is shown "MPa" (Kgf / mm2). The aim for the surface stress is below. The following is one of the guidelines for noise life. If the noise life requirement is over 10,000 hours, the Preload can be calculated based on an optimum surface contact stress that does not exceed 800 MPa {80 Kgf/mm2}.
Bearing Preload

For general applications with a noise life requirement between 5,000 and 10,000 hours, the optimum Preload can be calculated using a contact ellipse stress that does not exceed 1000 MPa {100 Kgf/mm2}. For stiffness critical applications requiring an operating noise life of less than 5,000 hours, a surface stress of less than 1500 MPa {150 Kgf/mm2} should be used. A way of looking at the Preload from the Basic Dynamic Load Rating (Cr)
Over 10,000 hours noise life requirement: 0.5/100 - 1/100·Cr
5,000 - 10,000 hours noise life requirement: 1/100 - 1.5/100·Cr
Less than 5,000 hours noise life requirement: 1.5/100 - 2/100·Cr
If a surface stress of 2700 MPa {270 Kgf/mm2} is applied to a high carbon chromium bearing, permanent raceway and ball deformation will occur. It is possible that stresses below 2700 MPa {270 Kgf/mm2} will result in no permanent raceway or ball deformation, but we would recommend to use a maximum safe operating stress of 1600 MPa {160 Kgf/mm2}.

Preload and Stiffness

What Is Stiffness?

The stiffness of a bearing is a reference to its resistance to deflection. Deflection is the tendency of a structural element to displace in response to a load placed on it. Since the bearing is designed to carry loads, a high stiffness, meaning less displacement when load is applied, is generally desirable. Different preload methods may have different consequences when it comes to bearing stiffness.

Preload Methods

There are two basic methods of Preloading: Solid Preload and Spring Preload. Solid Preload can be obtained by mechanically locking all of the rings in postion while under an axial load. The advantages of this type of design are that the components remain simple and the stiffness is high. The disadvantage is high variation in Preload under temperature variation, and that the Preload can reduce with wear. Spring Preload can be applied using a coil spring or a spring wave washer, etc. An advantage of Spring Preload is that it maintains consistent Preload with temperature variation.The disadvantages are that the designs are more complex and normally have lower stiffnesses. Typically, a spring preload is the first choice, as, although the design may be more complex, spring bearing preloads are simpler to execute, and, as mentioned, do not tend to be disturbed by temperature variation, which means they will not be affected by thermal expansion of the bearings due to applied heat. The spring preloading method can also compensate for small misalignments during preloading and for bearing wear.

More on Spring Preloads

A bearing can be preloaded through the spring method using a number of different types of springs. Coil springs are most common, but wave springs, finger springs, and Belleville springs are also used. The way spring loading works is that during bearing assembly, the coil spring (or whichever type of spring is used) is compressed against the outer race of the bearing. What follows are diagrams of both the solid preload and spring preload (or Constant Pressure Preload) methods of reducing bearing clearance. This should give you a better idea of which method of preloading is more appropriate for your ball bearings.
Solid Preload
Solid Preload
Spring Preload
Spring Preload
The Preload can be applied in two directions, Duplex face to face (DF) and Duplex back to back (DB).When considering stiffiness, DB is stiffer under moment loads than DF.
Duplex face to face(DF)
Duplex face to face (DF)
Duplex back to back (DB)
Duplex back to back (DB)

Angular Contact Ball Bearing Preloads

There are unique methods for preloading angular contact ball bearings, which are designed to handle both radial and axial loads. For single row angular contact ball bearings, axial preload is achieved through displacing the bearing rings relative to each other axially, a distance that corresponds to the desired preload force. If you are spring preloading an angular contact ball bearing set, you will need the spring on one side of the bearing set to displace the outer rings of both bearings in the axial direction.

More on Bearing Preloads

If you have any additional questions about preloads, such as how to determine the best preload method to use, whether you need to preload at all for your particular applications, how preloading will affect the type of ball bearings you buy, why ball bearings are built with clearances requiring preloads at all, or any other questions, NMB will do its best to answer them. If you can't find the answer here on the site, feel free to contact the qualified expert ball bearing engineers at NMB at any time. Present us with your question, and we'll find you the answer. Your ball bearing applications are important to us, and we want to make sure that you get exactly the products you need and know exactly how to use them.

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