3 minutes to understand the bearings of the spindle system

The spindle bearing is a key component of the machine tool, which directly affects the rotation accuracy, speed, rigidity, temperature rise, noise and other parameters of the machine tool, thereby affecting the processing quality of the workpiece.

As you can see, there are many factors that determine the final decision. The spindle that is expected to have the highest speed will not have the greatest stiffness, and the spindle with the highest stiffness will not be able to operate at high speeds without sacrificing bearing life.

Angular Contact Ball Bearings vs. Tapered Roller Bearings

Angular contact bearings are the most commonly used spindle design today for very high speeds. This is due to the fact that angular contact ball bearings provide the precision, load carrying capacity, and speed required for metal cutting spindles. In some cases, tapered roller bearings are used. In most cases, tapered roller bearings are used due to their higher load capacity and greater stiffness than ball bearings. However, tapered roller bearings do not allow the high speeds required by many spindles.

Angular contact ball bearings utilize a number of precision balls mounted to a precision steel race. They are designed to provide both axial and radial load carrying capacity when properly preloaded.

An important concept to consider is the maximum speed that the bearing and, ultimately, the spindle motor will be able to achieve. In theory, this is determined by considering the bearing type, lubrication method, preload, load, etc. In practice, datum points are used. In practice, a reference point is used, known as the dN number. The dN number is obtained by multiplying the bearing bore diameter by the rotational speed (RPM). For high-speed spindles with ball bearings, the dN value may be as high as 1500,000.

The manufacturing specifications for angular contact ball bearings include a contact angle. The contact angle is the nominal angle between the line of contact of the balls against the race and the plane through the center of the balls (see Figure 3). (See Fig. 3) The contact angle determines the ratio of axial loads, with radial loads being the main advantage. Normally, contact angles of 12 º, 15 º, and 25 º are available. The lower the contact angle, the higher the radial load carrying capacity, and the higher the contact angle, the higher the axial load carrying capacity. For this reason, bearings with a contact angle of 25º are used for spindles primarily used for drilling, and bearings with a contact angle of 15º are used for spindles primarily used for grinding.

Contact Angle

All precision bearings are manufactured to tolerance standards. The most commonly used standard in the USA is the ABEC standard (American Bearing Engineers Council). This standardization has been accepted by the American National Standards Institute (ANSI) and basically conforms to the equivalent standards of the International Organization for Standardization (ISO). The ABEC standard defines tolerances for the main bearing dimensions and characteristics. They are divided into mounting dimensions (bore, I.D.) and bearing geometry. The accuracy ratings range from a low of ABEC 1 to a high of ABEC 9, which describes a high accuracy bearing suitable for use in high speed spindles. Typically, spindle bearings are manufactured with an ABEC 9 geometry to provide minimum runout and rotational accuracy. The bores, outside diameters, and widths are in accordance with ABEC 7, allowing for more rational assembly and mounting.

Angular contact ball bearings are available with a choice of preload sizes, usually specified as light, medium, or heavy. Lightly preloaded bearings are designed to allow maximum speed and less stiffness. Heavy preloads allow lower speeds but higher stiffness. Light preloaded bearings are typically used for very high speed applications where the cutting load is also light and maximum speed is required.

In order to provide the required load carrying capacity for the spindle of a metal cutting machine, several angular contact ball bearings are used together. In this way, the bearings share the loads seen and increase the overall stiffness of the spindle. Depending on the required characteristics, the bearings can be stacked in several ways. Angular contact ball bearings must be preloaded to provide both axial and radial capacity. A simple method is to divide the bearings into groups of two or three so that the preload is correctly applied to the bearings. This is possible by using duplex bearings, which are manufactured for this purpose. The inner or outer bearing rings are grounded so that when clamped together, the bearings will have the proper preload.