Preload Information of Radial Bearings

Lubricant selection is of critical importance for the performance of a ball bearing. It affects Life, Torque, Speed, Noise, Grease migration, Outgassing, Temperature effects and rust prevention.

The two basic types of lubricants available are oil and grease.

Oil Lubricants

• Applications that require extremely low torque or narrow range of torque variation, are suited to use oil as a lubricant.

Grease Lubricants

• Greases are comprised of a base oil and a thickener. The life obtained from greases is considerably greater than that of just oil. This is because of a continuous mechanism of lubrication by base oil separation coming from the construction of the thickener.
  
  
• Calculating grease life and performance in sealed-for-life ball bearings includes many complicated factors, such that grease life calculation formulas in the industry at this time will likely show different results than actual life.
  
  
• In recent years, using plastic material around the bearing has become more common, so the proper selection of lubricant is even more critical to prevent compatibility issues.
  
  
• Miniature and instrument ball bearings are typically filled with a 30% (±5%) by volume fill quantity.
  
  

Purpose

When a radial 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. An optimum preload should be individually specified for each ball bearing size. If the Preload is applied excessively, Bearing Fatigue Life will be short and will increase raceway noise as well. Bearing starting and running torque will also be high. 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 = pab

(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}.

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

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 (or Constant Pressure 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.

Radial Bearings :
Deep Groove Ball Bearings | Self Aligning Ball Bearings | Angular Contact Ball Bearings | Cylindrical Roller Bearings | Needle Roller Bearings | Spherical Roller Bearings | Taper Roller Bearings