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Monorail guidance systems 4.0: intelligent components for feed spindles

13 February 2016

Schaeffler has introduced its fifth generation of linear recirculating roller bearing and guideway assemblies. The RUE-E series now includes sensor units that provide both automatic re-lubrication and bearing condition monitoring.

Where necessary, the sensors monitoring the condition of the lubricant send a message to the machine control system, initiating automatic re-lubrication via, for example, Schaeffler’s innovative FAG 'CONCEPT8' multi-point lubrication system. In order to monitor the condition of the bearing, a piezoelectric accelerometer is mounted to the steel saddle plate on the carriage. This provides the vibration velocity, vibration acceleration and rolling bearing parameters to the attached signal converter, as well as monitoring the pre-set limit values.

With this new concept for the RUE-E, Schaeffler has developed a system that provides users with direct feedback and communication with the unit’s control system, automated lubrication where necessary and automated maintenance. Manual lubrication is no longer necessary and lubricant usage can be reduced by up to 30 percent. Automated condition monitoring provides predictive maintenance which, in turn, increases availability and quality, as well as reducing total cost of ownership.

In its as-delivered condition or when a linear roller bearing and guideway assembly is used in a feed spindle, the guideways are generally in optimum condition with regards to their running characteristics and lubricant condition. This can be seen through the vibration behaviour of the saddle plate when the system is in motion. In order to establish variations and deviations from this ideal nominal value, piezoelectric accelerometers are mounted to the steel saddle plate on the guideway carriage.

The accelerometers are suitable for use with coolants and in harsh, contaminated environments. The permanently connected, two-wire, separately shielded cable is completely encased in polyurethane. The sensor has a sensitivity of 100mV/g, a resolution of 0.001g and a frequency range of 5,000 to 50,000Hz for measuring energy spikes.

The accelerometer measures vibration velocity, vibration acceleration and provides rolling bearing parameters to the attached signal converter via the cable, as well as monitoring the pre-set limit values. The signals are then separated into different frequency ranges or an energy evaluation of the impulse peaks is carried out using spike energy (gSE). 4-20mA and 0-10V signals are passed to the PLC, the Schaeffler Box (controller) and the lubrication unit via analogue outputs. This closes the loop between the monitoring and automated lubrication.

The FAG CONCEPT8 multi-point lubricator is an integral part of the lubrication control loop. This lubrication system for rolling bearings ensures that optimum lubricant is fed to the bearings without the need for any manual intervention. If the actual evaluated signal values sent by the signal converter deviate from the nominal (pre-set) values, FAG CONCEPT8 automatically ensures appropriate maintenance measures or re-lubrication are initiated.

In order to optimise the performance of screw drives in feed spindles, it is vital they are supported by bearings with the appropriate performance, load carrying capacity and dynamics. This is where the benefits of the ZKLN and ZKLF series of double row, preloaded angular contact ball bearings from Schaeffler come into their own. These bearings, which have been used successfully over many years, are now available in premium X-life quality.

As part of the modified X-life manufacturing process, it was possible to increase the basic dynamic load rating by a further 10 percent, which corresponds to a 30 percent increase in the nominal operating life. Tests have also shown that limiting speeds can also be increased by up to 60 percent. The user benefits from reduced friction and the subsequent reduction in heat generated in the bearing. This leads to improved accuracy, increased dynamics and improved machining results.

At the same time, higher speeds can be achieved during rapid traverse, which leads to increased machine efficiencies. Behaviour during changes in the direction of rotation, which often occurs in machine tools during precise positioning, also improves. Uniform frictional torque during the whole positioning process is critical here. Benchmark tests have shown that the new ZKLN and ZKLF bearings have a constant frictional torque during slow changes in the direction of rotation, resulting in extremely accurate positioning.

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