The Spindle Bearing
Angular contact spindle bearings are bearings of remarkable speed, accuracy and precision. Though seemingly simple, they are in fact complex components that enable and support a remarkable range of sophisticated manufacturing processes.
The great value of angular contact spindle bearings is their ability to handle loads in both the radial and axial directions (see Fig 1). The ability to handle these “combined” loads enables the precision, load-carrying capacity, high-speeds and other running characteristics that make these ultra-precision bearings unique.
The shoulders of angular contact spindle bearings are relieved (see Fig 2) which forces the balls to contact the races at an angle (the feature that enables these bearing to support loads radially and axially). Angular contact spindle bearings can be designed with the inner shoulder (ring) relieved; the outer shoulder (ring) relieved; or both the inner and outer shoulders (rings) relieved (referred to as asymmetrical).
High precision, angular contact bearings are designed to enable a broad range of applications, from dental hand-pieces to car transmissions to precision, high-speed machine tools.
Reliable bearing performance starts with the use of high-quality bearing materials, specifically the rolling elements: the inner and outer races and the balls. Non-contact or low drag seals are also important.
High-quality bearings, such as those furnished by COREDEMAR, feature rolling elements and outer rings made of refined bearing steel, which undergoes a thorough hardening process, resulting in high dimensional stability and resistance to wear and fatigue.
To begin transitioning from austenite (“gamma-phase” iron, containing trace elements of carbon) to martensite (hard steel containing nil carbon traces), the bearings are “low temperature cooled” (quenched) using liquid N2. This is a tightly controlled process that produces high levels of purity with minimal residual austenite.
Next, to reduce the natural brittleness of martensite, the steel goes through a tempering process that strengthens the steel and makes it more malleable (able to be shaped). These two characteristics make the resulting product – chrome steel – ideal for bearing and rolling element design.
To further enhance bearing performance (rotational speed, thermal stability, quiet running, precision, etc.) hybrid bearings (consisting of steel rings with ceramic balls) can be used.
Temperature Operation and Limits
Maximum operating temperature is based upon a (limiting) value below which geometric and/or structural changes to the bearing may occur. The value accounts for factors such as bearing materials, heat treatment applied to the bearing and the type of lubrication used. Generally:
- Rings & balls (hard chrome steel) ~ 150°C
- Cages (polyamide or stratified textile resin) ~ 120°C
As a basic rule, temperature limits will be much higher with oil lubrication than with grease.
Temperature readings at the rings will be much lower than they are inside the bearing. This is most concerning under rigid preloading, where high speeds can cause localized overheating, which – sooner rather than later – will lead to bearing seizure. To prevent this from happening, probe readings should be well below indicated limiting values. For instance, temperature readings at the rings should not be higher than 55oC.
Angular contact spindle bearings, like all precision bearings, are manufactured to specific tolerances set by various worldwide organizations. In the U.S., bearing tolerances are determined by the ABMA and commonly referred to as ABEC standards
Tolerance grades cover a broad range of bearing specifications and characteristics. The higher the tolerance number, the greater the bearing’s precision and running accuracy. The tightest tolerances are used for applications that require (at minimum) high speeds and true accuracy. These applications include electro-spindles, high-speed turbines and turbo-pumps, dental hand pieces, medical devices, robotics and many precision machine tools.
The following tables are based on ABMA standards, which conform to those used by ISO, DIN, JIS and most other standards organizations.
Note that COREDEMAR’s tolerance grades are based on ISO standards. Accordingly: P4 is ABEC7; P4A* is ABEC7/9; and P2 is ABEC9. *P4A represents dimensionally P4 (ABEC7) with functional P2 (ABEC9).
For a complete description of COREDEMAR’s part number nomenclature system, please visit our page: Building the Part Number
The Contact Angle
As discussed, angular contact ball bearings use axially asymmetric races to support “combined loads,” loads in both the radial and axial directions. To achieve this, angular contact bearings have races and balls installed at matching angles, typically in the range of 15°, 25°, 30° or even 40° or higher.
Generally, as the contact angle increases, the axial stiffness of the bearing also increases. However, increased axial stiffness comes with a loss of speed. Understanding the give and take between contact angles and bearing performance, bearings can be designed and arranged to emphasize some characteristics and mitigate the detrimental characteristics of others.
For example, operating and temperature conditions can influence the contact angle, causing deteriorating conditions that can lead to premature failure. Thus, some extremely high-speed applications use ceramic balls to reduce the impact of centrifugal forces on the contact angle; some bearing arrangements use mixed contact angles (different contact angles within the same arrangement) to optimize performance. Indeed, the choice of contact angle is directly related to the choice of arrangement and fit.
COREDEMAR offers a range of contact angles to meet the needs of various customer applications. The most common contact angles are “C” for 15° and “AC” for 25°. Other contact angles available include: 12°, 18°, 21°, 30° & 40° (high speed thrust bearings) and 60° (ball screw support bearings). Contact angle tolerances are +2°/-3°.
Please refer to Building the Part Number for a complete description of COREDEMAR’s naming system.
Due to low friction contact points, high-precision ball bearings can reach extremely high rotational speeds. This is important when considering heat production, working level temperatures and power consumption since bearing limiting speeds are in fact thermal based.
The maximum values included in our product tables assume a constant temperature system, that is, a system in which heat production and heat dissipation are the same. The listed RPM data in the COREDEMAR catalogue is for ABEC7 (P4), single bearing, lightly (spring) loaded.
At working temperature levels, however, several other factors must be considered, including: the number of bearings in the set, mounting directions, preloads, tolerance levels, contact angles and lubrication type and system.
Adjusted speeds are the actual speeds that bearings can achieve during an application, which is provided on the individual product pages in the COREDEMAR Product Catalog. You will note the effects that can influence the working speeds vs. the limiting (catalog) speeds based on the listed tables.
Rated Load and Rated Life
As mentioned, angular contact bearings are designed to support both radial loads (perpendicular to the shaft) and axial loads parallel to the shaft). The maximum load is the load that can be supported before the bearing fails.
Rated Static Loads
The maximum load is further refined when the concept of static and dynamic loads are introduced. The static load is the radial load that a non-rotating (or very-slowly-rotating) bearing can support without fatigue failure (external distortion).
The static load rating must also account for additional external forces exerted during assembly and testing, which can be significant (self-aligning bearings: 4,600MPa; all other ball bearings: 4,200MPa; all roller bearings: 4,000MPa).
Equivalent Static Load/Static Equivalent Radial Load
The radial equivalent static load of angular contact (spindle) bearings is the larger one of the following:Static Load Safety Factor
The permitted static equivalent loads between frictional requirements and actual contact surfaces may be smaller than, larger than or equal to the rated static loads. However, for high-speed precisionbearings, the recommended rated static load divided by the staticequivalent load should not be smaller than three (3). The formula:
Rated Dynamic Loads
The dynamic load rating is the constant radialload at which a group of apparently identical bearings can (theoretically) sustain a rating life of one million revolutions. The “one million revolutions” milestone is also important to determine a bearing’s working life.
Theoretical Life Formulas
Bearing life can be expressed via two formulas: one based upon life in hours and the other based upon life expressed as millions of revolutions. The following are non-adjusted and based upon ISO281 and/or ABMA sections 9 &11:
This is only a partial method of determining “life”. Additional factors that can influence bearing life include lubricant, cleanliness and viscosity, steel purity, overall bearing design and operating conditions.
Both calculations assume 90% reliability. That is, 90% of identical bearings operating under identical conditions can be expected to achieve their full life rating; 10% will fail.
Dynamic Equivalent Radial Load
Using the formula and table below, the axial and the radial loads are replaced by the Dynamic Equivalent Radial Load Factor (Pr):
Preload, Axial Rigidity, Radial Rigidity, Axial Deflection
The high running accuracies and low vibrations achieved with angular contact bearings are only realized through the selection of the right bearing arrangements, mountings and preloading methods. Preloading is achieved by applying an external load to enhance stiffness and eliminate bearing clearances, resulting in:
- Increased rigidity of the bearing/shaft system
- Fewer contact angle changes between inner and outer rings at high speeds
- Prevention of ball skid during high accelerations
- Increased load carrying capacities of the bearing arrangements
Preloading is accomplished via two methods: constant pressure or rigid preload (also known as a position preload). Constant pressure – achieved with springs – is generally the method used for high-speed spindles using ultra-high speed bearings. Even if the bearing position changes during operation, the degree of preload remains relatively unchanged.
Rigid or position preload is achieved with or without spacers and results in system stiffness that is much higher than constant-pressure methods. Rigid preload requires a gap between opposing bearing faces; once the bearings are axially locked, the gap is eliminated.
Note that unlike constant pressure preloading (where preload changes are absorbed thru the springs), rigid preloads may change during operation. In these cases – and in order to prevent a premature failure from occurring – a lower than initial preload should be considered so it will achieve proper value at full operation.
Though not totally recommended, it is possible to change preloads within a given set of bearings, providing that spacers were used to create the preload in the first place and that both spacers have equal widths and parallel flat faces. For instance:
- Reducing the width of the outer ring spacer reduces the preload in DB arrangements and increases the preload in DF arrangements
- Reducing the width of inner ring spacer increases the preload in DB arrangements and decrease the preload in DF arrangements
Deflection or bearing yield is the elastic deformation that occurs between balls and raceways due to external loads; as the external load increases, a non-linear increase in deflection occurs as well.
Adjustments can be made to lower deflection rates (and increase bearing stiffness), including increasing the contact angle, decreasing race curvatures, increasing ball size or increasing ball complement. Contact angle control has the most influence on yield rate and is also the easiest to adjust.
The following formula can be used to calculate spacer thickness and determine axial deflections and rigidities of the preload gap:
One of the most useful characteristics of angular-contact bearings is the ability to arrange or assemble individual bearings into multiple bearing pairs and sets. Angular-contact bearings can be stacked in several ways to greatly increase load carrying capacities, enhance stiffness and alter speeds:
- Preloading bearings in back-to-back (DB) – excellent for radial and axial loads; capable of withstanding tilting moments
- Preloading bearings in face-to-face (DF) – similar to “DB”, but less able to accept tilting moments; provides lower rigidity to the system thereby allowing for greater errors of alignment of the bearing seats.
- Preloading bearings in tandem (DT) allows for heavy axial loads in one direction only. Contact angles of this mounting must be the same, while “DB” & “DF” can accept mixed contact angles.
The mounting process begins with installation – the moment a bearing is removed from its package, actually – and encompasses the obvious (dry and clean assembly room, housing and shaft) as well as the less obvious. For example, as a standard practice, bearing groups sold by COREDEMAR are marked with high point face markings as well as bore and OD deviations (μm) to offer flexible, selective fitting. This is standard (with no additional cost).
With bearings using the same contact angle, virtually any arrangement can specifically fit any application (see “Fits” below). Other than the “DB”, “DF”, “DT” pairing, multiplex arrangements of three to six bearings per set can be provided.
To support heavily loaded applications and greater rigidity, triplex arrangements of TBT, TFT, or TT or quad arrangements of QBC, QFC, QBT, QFC or QT can be considered. For compromising loads to higher speeds, small ball complements can be provided.
Using dissimilar contact angles allows, for example, fitting those applications where space limitations need to be overcome while still meeting stiffness and speed requirements. Using lower preloads within the set itself will result in cooler operation. These types of arrangements require that within the set the tandem portion remains identical. For example:
Bearings can withstand their maximum load only if the mating parts are properly sized. Bearing manufacturers supply tolerances for the fit of the shaft and the housing so that this can be readily achieved. The material and hardness may also be specified.
Types of fit include tight, transition and loose fitting, which may be selected based on whether or not there is interference. The tight fit is generally considered the most effective because it provides uniform load support around the entire circumference of the bearing ring. But tight fits are not easy to install or disassemble and so are not uniformly recommended.
Fittings that are not allowed to slip are manufactured to diameters that prevent slipping and require force fits. For small bearings, this is best accomplished with a press since tapping with a hammer can damage both bearing and shaft. For large bearings, the necessary forces are so great that there is no alternative to heating one part before fitting, so that thermal expansion allows a temporary sliding fit.
If the fit is less than optimal, the bearing may shift or move under load. This shift can occur radially, axially or in the direction of rotation. In high-speed applications, shifting can also occur due to centrifugal dynamic expansion on the inner rings, which can loosen the fit and cause sliding and vibration. In some cases, improper fit may lead to damage and shorten bearing life.
As a rule, spindle bearings are mounted with a slight interference fit. Loose fits are provided for floating bearing locations only.
The tolerances listed in the following tables have been suitable in a wide range of applications, however special operating conditions may require other tolerances than the indicative values listed below:
Machining Tolerances of Spindle Bearing Seats
In angular contact spindle bearings, the bearing seats must be accurately machined for the bearing to reach the high speeds and operating standards required. The machining quality of the bearing seats should therefore match the tolerance class of the bearings.
The roughness of bearing seat surfaces does not have the same influence on bearing performance as the internal tolerances and architecture of the bearing. However, smooth bearing seat surfaces contribute to bearing operation by allowing a smoother fit.
Lubrication plays several vital roles in optimum bearing function: reducing the likelihood of corrosion or contamination from outside the bearing and limiting the effects of friction (and friction-generated heat) from inside the bearing. There are essentially two options: grease and oil.
Grease costs less, is generally simpler to use and is the most common form of bearing lubrication. Additionally, bearings lubricated with grease require no special maintenance or equipment. And with the advent of non-contact (low-drag) seals, speeds are not appreciably affected.
However, at excessive speeds or if over-filled, grease begins to break down and can create the very heat it’s intended to reduce. A good rule of thumb regarding proper bearing fill (%) is that bearings should be 15% of bearing inside void volume. The calculation to assist in determining grease fill:
Oil lubrication is required when grease can no longer meet the speed requirements of an application. There are several different methods of dispensing oil; the three most common are:
- Oil Mist: perfect for many high speed applications due to efficiency in complex bearing arrangements (maintains low temperature, which reduces power consumption). Relatively a low cost, simple construction and assembly; offers good protection against outside contaminants.
- Oil Injection: for applications with very high speeds and very high loads that require constant cooling that can’t be satisfied by oil mist. Nozzles inject the lubricant to the ball to race contact locations with a minimum of oil churning (drainage channels must be provided in order to prevent oil stagnation and heat generation).
- Air-Oil: best with high viscosity oil, which provides a resistant film between the rolling parts and bearing races, even in small amounts, even under high stress. Air-oil is known to have some mild polluting activity due to the low amount of oil consumption and reduced misting effect.
Although oil and grease cost about the same, an oil lube system is much more expensive. The expense is in the delivery systems such as air oil or oil mist. The resultant rotational speeds can reach ndm 3.0 X 106.
Bearings sold by COREDEMAR are generally shipped with an anti-corrosive oil coating. If requested, we can also provide grease specifically for spindle applications. Our suppliers are well versed in providing a wide range of commonly used lubricants including Lubcon L-252, various Klubers, NBU15, LDS SpecA, FB77-22 and so on.
Noise, Vibration & Related Bearing Analyzing Instruments
Noise and vibration are natural consequences of bearing operation and may be the most important dynamic activity of a rolling bearing.
As balls move around races and cages, they generate noise. Specifically, noise and vibration are traceable to ball spin frequencies, inner and outer ball-pass frequencies and cage frequencies (in this case, “frequencies” refers to repeating events that generate noise, such as a ball
spinning around the races).
Although these noises are not particularly loud, they have a cumulative effect that can be significant. Because vibration values are the major “stepping stone” to a comprehensive assessment of bearing quality, increased vibration levels are generally considered the first sign of impending failure and reduced life.
COREDEMAR offers two units specifically aimed at vibration evaluations:
To prepare a spindle for actual operation, the importance of “running-in” the bearings cannot be overstated. Generally, the bearings should be first run-in on the spindles at about 10% of the rated maximum speed of the spindle with the temperature and vibration closely monitored. Temperatures will rise and fall as grease in the ball path decreases due to centrifugal forces and natural displacement.
Once the temperature stabilizes, increase speed by another 10% and continue monitoring. Again, once the temperature stabilizes, increase speed by another 10% and so on. Continue until maximum speeds have been achieved and maintained without incident.
As mentioned earlier, angular-contact spindle bearings are COREDEMAR’s primary business. Have a question? Need more information? Contact COREDEMAR; we welcome the opportunity to speak with you at any time.
The following symbols and abbreviations are used on this page and throughout this Web site. The first few symbols and abbreviations refer to basic dimensional and performance data. They get more complex as you scroll down.
d Inside diameter
D Outside diameter
i Number of bearings
Z Ball quantity
Dw Ball diameter (mm)
n Rotational speed (rpm)
α Contact angle
Fr Bearing radial load (N)
Pr Dynamic equivalent radial load (N)
X A factor of dynamic equivalent radial load
Fa Bearing axial load (N)
Y A factor of dynamic equivalent axial load
C Basic dynamic load rating(s) (KN)
Cr Basic static load rating(s) (KN)
Por Radial equivalent static load
Xo A factor of radial static load
Yo A factor of axial static load
P Calculated equivalent load(s) (refer to chart)
L10 Life (millions of revolutions)
Lh10 Life (in hours)
Q Grease quantity (cm3)
Qb Lubrication factor depending upon bore size
0-40 mm: Qb = 1.5
40-100 mm: Qb = 1.0
So Safety factor
0.6: No shock
1.0: Normal, expected shock
2.0: Excessive or continual shock
δσ Axial deflection (mm)
Ka Axial rigidity (N/mm)
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