Specific bearing features

Areas of application

AXRY-NGS (NGS-SBI) bearings are suitable for applications requiring high load-capacity, ultra-precise and play-free high-speed bearings. Typical applications are machining centres with rotary tables for milling/turning as well as vertical lathes or gear cutting machines.

In order to be able to fully take advantage of the NGS series, the design of the surrounding area is also important. The overall system of the axis, such as lubrication, cooling, heat flow and the components themselves, must be considered in order to achieve the best possible result.

Accuracy requirements

For higher accuracy requirements, the NGS and NGS-SBI series can be supplied with more restricted axial and radial running accuracy.

The inner ring and the axial washer have the same axial runout properties.

Measuring systems

AXRY-NGS bearings can be equipped with inductive angle measurement systems. These are available in incremental or absolute versions, as single or multi-head systems in various accuracies.

myonic only supplies the “mechanical part”, i.e. the bearing including the mounted measuring ring and thread in the outer ring for axial or radial scanning head mounting. For high speed applications only the absolute measuring systems are suitable. Incremental measuring systems are not suitable for high speeds.

By mounting the measuring ring directly on the bearing ring, concentricity errors with respect to the shaft (table) are minimised and thus highest accuracies of a few angular seconds are realised.
More on this in the chapter – Axial/radial bearings with angle measurement system

Lubrication variations

Grease lubrication, relubrication

For high-speed bearings, relubrication should be scheduled at appropriate intervals.

A very good option can be found in controlled relubrication systems that relubricate with small quantities at defined intervals during operation.

For calculation of relubrication quantities and intervals, please contact us, specifying the load spectrum (speed, duty-cycle, load) and the ambient conditions.

Circulating oil lubrication

Mainly used for larger bearings. Due to the larger, cooled oil quantities, cooling and lubrication take place simultaneously.

Due to the large available lubricant quantity these systems also function with lower oil viscosities.

Oil/air lubrication

In a similar way as for spindle bearings, an oil/air mixture is injected directly into or next to the raceways; lubrication takes place with minimal oil quantities. The lubrication takes place axially via 6 holes on the outer ring.

myonic bearings can be supplied with all necessary holes, connecting threads and seals for the use of oil/air lubrication.

The customer must define the parameters such as lubrication cycle, lubricant quantity and air pressure according to the application. The myonic application engineering team can support you with this.

Overlubrication

Overlubrication, whether with grease or oil, leads directly to an increase of the friction in the bearings and to strong increases in temperature. This may lead to premature bearing failure.

If the bearing is overlubricated, repeat the run-in cycle to restore the original friction torque.

Lubrication bores / lubrication grooves

Inner ring rotation

NGS bearings for inner ring rotation can be lubricated on the outer ring via a radial circumferential groove or axially. For error-free positioning of the lubrication hole of the bearing to the lubrication hole in the machine housing, the bearings have a locating pin hole. (See chapter Positioning hole).

For relubrication through the lubrication groove of the outer / inner ring, we recommend to fill the lubrication groove completely with grease before assembly of the bearing. This way grease will enter the bearing faster in case of a relubrication process. The lubrication channel of the housing additionally should be close to the radial lubrication hole of the bearing.

Please ensure that the axial lubrication hole is closed with a set screw when the bearings are delivered.
For axial lubrication, remove the set screw axially and close it radially.

Outer ring rotation

NGS-SBI bearings for outer ring rotation can be lubricated on the inner ring via a radial circumferential groove.

When relubricating via the lubrication groove, we recommend filling the lubrication groove completely with grease. The lubrication hole in the bearing should be located near the lubrication channel of the housing.

Inner ring rotation (oil/air lubrication)

NGS bearings for inner ring rotation are available as a special design for oil/air lubrication. This series has 6 evenly arranged lubrication bores on the outer ring.

The lubricant feed can take place axially on both sides. The lubricant feeds into the bearing 3 times in direction of the inner ring and 3 times in direction of the axial washer.

The exit side in the bearing is marked by the arrow marking on the outer ring.

All lubrication holes are closed with set screws when delivered.
When lubricating, remove the relevant grub screw.

For all bearing sizes, the lubrication holes are uniformly designed as M4 threads.

Seals

When using oil/air lubrication, we recommend fitting the bearing with a contact-free gap seal. Oil/air lubrication thus remains in the working space of the bearing and ensures optimum lubrication. Due to the minimal gap between the seal and the outer ring, the oil/air lubrication additionally functions as sealing air. This effectively protects the bearing from contamination.

Sensor bore / bearing monitoring

AXRY-NGS bearings have a sensor bore in the outer ring as standard, NGS-SBI bearings have a sensor bore additionally in the inner ring. These lead to just under the raceways.

Equipped with a temperature sensor, the current temperature in the bearing system can be used to continuously monitor and control for example cooling or to detect system overheating.

Height tolerances H1 and H2

Both height dimensions, H1 and H2, can be substantially restricted.

The height dimension H1 is restricted as a standard up to and including size 460. Sizes 580 and 650 can be optionally restricted.

H1 refers to the position of the table. Restricted height variation offers the following advantages:

■ Labyrinth seal gap can be optimally adjusted against the penetration of coolant from the machining area.

■ Clamping gap can be optimally adjusted.

The height dimension H2 is not restricted as standard, but can be supplied restricted for all sizes.

H2 refers to the adjacent construction under the bearing, for example for the adjustment of the clearance of a worm gear.

The exact tolerances are located in the product tables.

Customer-specific bearing Adjustment AC

AXRY-NGS (NGS-SBI) bearings can be installed as exposed bearings or with whole-surface support. If the L-section ring is supported across its whole surface by a support ring, the tilting rigidity of the bearing increases by 15 to 20 %.

To prevent an increase in the bearing friction torque, the bearing alignment can be adjusted (suffix AC). If normally-aligned bearings with supported L-section rings are used, the bearing friction torque increases considerably.

The support ring should be at least twice as high as the axial washer.

Customer-specific design Jxxxx

myonic offers customer-specific designs which are designated with J and a four-digit number.

Bearings with J-numbers can, for example, contain the following additional features:

■ Specific application-related preload values
■ Special directives for marking or packaging
■ Customer-specific designs

Limiting speed n_G

The limiting speeds given in the product table are guide values determined on our test benches under the following conditions:

■ Grease distribution run according to set specifications (see run-in cycle)
■ Maximum heating of the bearing by 40 K in the area of the raceway (sensor bore)
■ Active bearing cooling
■ Duty-cycle 2h with limiting speed n_G
■ Bearings fully bolted, without external load, only preload and weight of the mounts.

In order to achieve these limiting speeds, the guidelines for the adjacent construction must be strictly observed. Please also note the chapter – Friction/temperature development.

Friction/temperature development

Axial-radial bearings of the AXRY-NGS (NGS-SBI) series are equipped with cages both in the radial and in the axial part. This ensures the bearings turn under full preload with very little friction. At higher speeds, the friction torque increases only slightly, therefore bearings of the AXRY-NGS (NGS-SBI) series can be operated at high speeds with a long duty-cycle.

At high speeds over a long period of time, the influencing variables that lead to an increase in friction and temperature in the bearing must be avoided or compensated for. For this purpose, it is essential to consider the entire axis, including all drives.

The friction torque of an axis is essentially influenced by the following variables:

■ The bearing friction torque: The bearings are radially and axially play-free and preloaded after assembly and complete bolting. The preload is one factor in achieving the specified rigidity, but at the same time it causes a friction torque.

■ The lubricant used: In high-speed applications, the lubricant of the bearing must be chosen carefully. Few greases with the relevant viscosity are suitable for higher speeds.
The viscosity is dependent on the selected lubricant and the operating temperature. Low-viscosity lubricants may lead to mixed friction, in particular during slow or intermittent operation under high loads.
Excessively high viscosity, on the other hand, leads to high levels of friction and is hardly suitable for fast-rotating applications.
In case of strongly varying loads (high speeds / intermittent operation), please contact myonic application engineering to determine the lubrication.

The following additional points must be observed by the user when designing an axis and assembling it in order to keep an increase in friction torque and thus also an increase in temperature to a minimum.

• Geometrical errors in the adjacent construction lead to distortion of the bearings and thus to higher friction torques. Please note our recommendations in the chapter: Design of the adjacent construction.
• Asymmetrical housings can deform when heated and thus increase the bearing preload.
• Assembly errors can lead to increased friction torques. We recommend rotating the bearing as the assembly progresses and measuring the friction torque. In this way, serious errors can be discovered regarding the geometry of the adjacent construction, the screw connection or the additional parts.
• Touching seals increase the friction torque and transport additional heat into the system. For high-speed axes, touching seals should be avoided as far as possible.
• High accelerations and strong braking processes can introduce additional friction into the system via moments of inertia.
• Machining forces, eccentric clamping and high loads increase the friction torque.
• The heat input from drives should be reduced to a minimum. The following measures are helpful for this:Make the contact surface between the stator of the torque motor and the rotary table housing as small as possible in order to minimise the heat flow between the stator and the rotary table housing. If possible, do not connect the stator cooling jacket to the rotary table housing.Preferably flange the rotor of the torque motor to the rotary table plate instead of the bearing in order to keep the heat flow through the bearing as low as possible.Make the distance between the motor and the bearing as large as possible. A large distance reduces heat transfer from the rotor to the bearing. The stresses between the components due to different thermal expansion are reduced by the higher flexibility of the system.Make the bearing centring of the rotary table plate sufficiently rigid to achieve a high system rigidity. In addition, the risk of the bearing seat being deformed by the heating of the rotor is reduced.

Only by considering the entire system a sufficient level of knowledge can be obtained for the design of suitable cooling or heating/cooling systems.

Our test bench results show the basic performance capacities of the bearing and the lubricant, but only permit limited conclusions to be drawn on the actual operating temperature of a machine tool axis.