Specific bearing features

Areas of application

AXRY-NGX (NGX-SBI) bearings are suitable for applications requiring highload-capacity, ultra-precise and play-free high-rigidity bearings. Typical applications are machining centres with rotary tables for milling as well as milling heads or swivel axes.

In order to be able to fully take advantage of the NGX series, the design of the surrounding area is also important. The overall system of the axis, such as fits or accuracies of the connecting parts, must be considered to achieve the best possible results.

Accuracy requirements

For higher accuracy requirements, the NGX and NGX-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-NGX 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.

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

Run-in cycle – lubrication / L120 / Gxxx

In case of grease lubrication a run-in cycle is to be run during commissioning, in order to distribute the grease in the bearing. Only after complete distribution the bearing achieves full functionality.

In case of incorrect execution of the run-in cycle, high friction torques may occur which overheat the bearing.

In case of slow-running swivel axes, the run-in cycle does not have to be conducted.

Lubrication variations

Grease lubrication

As standard grease, a special grease according to DIN 51825-KPH-C1N-30 is used.

AXRY-NGX-bearings are relubricated via the outer ring, NGX-SBI via the inner ring. We generally recommend annual relubrication. After relubrication, we recommend a run-in cycle.

Further information on grease lubrication can be found in our “Technical basics”.

Please contact the myonic application engineering team regarding relubrication quantity.

Oil lubrication

Of course AXRY-NGX (NGX-SBI) bearings can also be lubricated with oil. Often these bearings are connected to a central oil lubrication system. With the suffix -L120 you get an ungreased bearing (preserved only).

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

NGX 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

NGX-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.

Height tolerances H1 and H2

The height dimensions H1 and H2 are 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.

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-NGX (NGX-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 approx. 15 to 20 % (when using a standard bearing without suffix -AC).

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.

Support ring geometry recommendations for maximum rigidity

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 can be achieved for the selected axial/radial bearing in swivel operation or in short-term continuous operation. In case of prolonged operation in the area of the limiting speed, the bearing increasingly heats up. For high-speed applications such as milling/rotary tables, we recommend bearings in the AXRY-NGS design. The limiting speeds 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
■ for NGX without bearing cooling
■ 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

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 screw connection. The preload is one factor in achieving the specified rigidity, but at the same time it causes a friction torque.

■ The used lubricant. 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 due to the thermal decoupling.

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 lubrication, but only permit limited conclusions to be drawn on the actual operating temperature of a machine tool axis.