Spindle bearings – precision, high-speed and hybrid

What are spindle bearings and what are they used for?

A spindle bearing is a precision angular contact ball bearing manufactured to very tight dimensional and running tolerances. Its task is to guide the machine spindle accurately, rigidly and with minimal heat generation, even at very high rotational speeds.
Spindle bearings differ from conventional angular contact ball bearings mainly in their precision class, smaller contact angle and cage design. These properties make them suitable for applications where bearing accuracy directly affects machining quality.
Spindle bearings are mainly used:

• in the main spindles of lathes, milling machines and grinding machines,
• in electro-spindles and motor spindles,
• in machining centres and CNC machines,
• in high-speed drilling and milling heads,
• in measuring instruments and precision drives,
• in other bearing positions requiring high running accuracy.

Main properties of spindle bearings

Spindle bearings combine properties that are less important in general industrial bearings.
Key properties include:

• high precision class, most commonly P4,
• low ring runout,
• suitability for very high rotational speeds,
• high rigidity of the bearing arrangement with correct preload,
• contact angle of 15° or 25°, matched to speed or load,
• cages made of lightweight materials that reduce friction and heat generation,
• designs for paired or set mounting with a specified preload,
• hybrid designs with ceramic balls for the highest speeds.

Spindle bearings are sensitive to contamination, incorrect mounting and inadequate lubrication. They therefore require a clean working environment and precisely machined bearing seats.

Types of spindle bearings in the M&M Intercom range

We have divided the range into three groups that differ in design and intended application.

Group	Characteristic	Application
Standard spindle bearings	steel balls, series 70, 719 and 72	general machine tool spindles
High-speed spindle bearings	smaller balls and adapted internal geometry	high-speed spindles and electro-spindles
Hybrid spindle bearings	ceramic balls and steel rings	highest speeds, lower heat generation

Most products are in the standard spindle bearings group.

Standard spindle bearings

Standard spindle bearings have steel rings and steel balls. The range mainly includes bearings of dimension series 70, 719 and 72, for example B 7008 C, B 71908 E or 7208 C.
For FAG bearings, the designation B indicates the standard spindle bearing design. Versions sealed on both sides are also available, designated with the suffix 2RSD, which are factory-greased.
The dimension series differ in the bearing cross-section:

• 719 series – thin-section design with a small cross-section and a larger number of balls,
• 70 series – medium design, most commonly used in spindles,
• 72 series – larger cross-section and higher load capacity.

High-speed spindle bearings

High-speed spindle bearings have an adapted internal geometry and smaller balls. This reduces centrifugal forces and heat generation at high rotational speeds.
For FAG bearings, these designs are designated with the prefix HS or HSS, for example HS 7014 E or HSS 71908 C. They are mainly used in high-speed spindles and electro-spindles where the standard design is not sufficient.

Hybrid spindle bearings

Hybrid bearings have steel rings and ceramic balls. Ceramic balls are lighter and harder than steel balls, so they generate lower centrifugal forces and less heat.
Hybrid spindle bearings are suitable for:

• the highest rotational speeds,
• longer relubrication intervals,
• bearing positions where low heat generation is important,
• electro-spindles where electric current can pass through the bearing.

For FAG bearings, hybrid designs are designated with the prefix HC, for example HC 7008 C or HC 71908 E. Other manufacturers designate hybrid designs with different suffixes.

Contact angle of spindle bearings

The contact angle is one of the most important selection criteria for spindle bearings.

• A 15° contact angle enables higher rotational speeds and greater radial rigidity. For FAG bearings, it is designated with the letter C; for Nachi bearings, with C or CYU.
• A 25° contact angle enables higher axial load capacity and axial rigidity. For FAG bearings, it is designated with the letter E; for Nachi bearings, with AC or ACYU.

The choice of angle depends on the ratio between radial and axial load and on the required spindle speed.

Preload and arrangement of spindle bearings

Spindle bearings are almost always mounted in pairs or sets with a predefined preload. Preload eliminates internal clearance and increases the rigidity of the bearing arrangement.
The preload is indicated in the bearing suffix:

• UL or GL – light preload, for higher rotational speeds,
• UM or GM – medium preload, for greater rigidity.

Bearings can be mounted in an O, X or tandem arrangement. The range also includes bearing sets, such as versions with the suffixes DUL, DBL or QBC, which are factory-prepared for a specific arrangement.
Excessive preload causes heat generation and a shorter service life. Insufficient preload reduces the rigidity and accuracy of the spindle.

Precision class P4

Most spindle bearings in our range are manufactured to precision class P4 or P4S. This class specifies very small deviations in dimensions and ring runout.
When replacing a spindle bearing, it is important to keep the same precision class as the original bearing. A bearing of a lower class can impair machining accuracy and cause vibration.

Mounting spindle bearings

Mounting spindle bearings requires a clean environment, precise tools and attention to the markings on the bearings. With paired or set mounting, the correct sequence and orientation of the bearings must be observed.
Bearings are often heated to make mounting on the shaft easier. When doing so, follow the manufacturer's instructions on the maximum permissible temperature. Suitable tools can be found in the bearing tools category, and bearing locknuts are available for axial fixing.

Lubricating spindle bearings

Spindle bearings are lubricated with grease or with oil in the form of oil mist or an oil–air system. Grease lubrication is simpler, while oil lubrication enables higher rotational speeds.
With grease lubrication, the correct quantity is important. Too much grease causes heat generation, too little causes premature wear. After mounting and lubrication, bearings generally require a run-in procedure at gradually increasing speed.
Suitable greases are available in the greases and pastes category.

How to choose the right spindle bearing?

When selecting a spindle bearing, first determine the dimensions and dimension series, then the design that suits the rotational speed and load of the spindle.
Important selection data include:

• bore diameter, outside diameter and bearing width,
• dimension series 70, 719 or 72,
• contact angle of 15° or 25°,
• standard, high-speed or hybrid design,
• precision class,
• preload and bearing arrangement,
• open or sealed design,
• lubrication method and maximum rotational speed.

When replacing a bearing, the most reliable information is the complete designation of the existing bearing, including the suffixes for contact angle, cage, precision class and preload. If the designation is illegible, measure the bearing and check the machine manufacturer's data.
Where the bearing position supports a ball screw, ball screw support bearings are more suitable. For general applications with combined loads, angular contact ball bearings are suitable.

Spindle bearings at M&M Intercom

At M&M Intercom, we offer 1,600+ spindle bearings for machine tools, electro-spindles and precision drives. The range includes standard spindle bearings, high-speed bearings and hybrid bearings with ceramic balls, in various dimension series, contact angles and preloads.
Bearings from FAG INA, Nachi and other manufacturers are available. For mounting and maintenance, we also offer bearing accessories and bearing tools.
If you need help selecting or replacing a spindle bearing, send us the designation of the existing bearing or the spindle data. We will help you choose the right design.

 

Frequently asked questions about spindle bearings

1. What is a spindle bearing?
A spindle bearing is a precision angular contact ball bearing designed for machine tool spindles and other bearing positions requiring high running accuracy, rigidity and rotational speed.
2. What is the difference between a spindle bearing and a conventional angular contact bearing?
A spindle bearing is manufactured to a higher precision class and generally has a smaller contact angle and a cage adapted to high speeds. A conventional angular contact bearing is intended mainly for general industrial applications.
3. What do the letters C and E mean in a spindle bearing designation?
For FAG bearings, the letter C indicates a 15° contact angle and the letter E a 25° contact angle. Bearings with a 15° angle are better suited to higher speeds, while bearings with a 25° angle are better suited to higher axial loads.
4. What do the suffixes UL, UM, GL and GM mean?
The suffixes indicate the preload of bearings in paired mounting. UL and GL denote light preload, while UM and GM denote medium preload.
5. What is a hybrid spindle bearing?
A hybrid spindle bearing has steel rings and ceramic balls. Ceramic balls are lighter, so the bearing achieves higher rotational speeds and generates less heat.
6. What does precision class P4 mean?
P4 is a high precision class that specifies very small dimensional and running deviations of the bearing. Most spindle bearings are manufactured to class P4 or a similar manufacturer class.
7. Why are spindle bearings mounted in pairs?
Paired or set mounting with preload eliminates internal clearance and increases spindle rigidity. The O, X or tandem arrangement is selected according to the loads and the spindle design.
8. Can I replace only one bearing from a pair?
As a rule, no. Bearings in a pair or set are matched in preload and dimensions, so they are replaced as a complete set with bearings of the same design, precision class and preload.
9. How are spindle bearings lubricated?
Spindle bearings are lubricated with grease or with oil using an oil–air or oil mist system. The correct quantity of lubricant and a run-in procedure after mounting are important.
10. How to choose the right spindle bearing?
Consider the dimensions, dimension series, contact angle, design, precision class, preload and lubrication method. When replacing a bearing, the complete designation of the existing bearing is the most reliable information. At M&M Intercom, we help you choose the right design based on the designation or spindle data.