Sensors built into spindle bearings will reduce failures
Schaeffler has developed a range of sensors for monitoring machine-tool spindles that are built into the spindle bearings. It says that the first-of-its-kind system will reduce spindle failures while extending their lives.
Most machine-tool downtime can be traced back to defective spindles – in particular, to spindle bearings being damaged by crashes (collisions) between tools and workpieces, and by continuous overloading that goes undetected.
In milling operations, for example, the combination of high radial loads, long tool protrusions and high speeds, can lead to particularly high loads and unfavourable kinematic conditions on the spindle bearing near the tool. In extreme cases, the bearings can fail suddenly.
Schaeffler’s new system aims to reduce spindle failures by deactivating the spindles rapidly in the event of a collision. It will also allow machine operators to detect unfavourable operating conditions and to make targeted adjustments to the machining process.
The ring-shaped sensor system, developed specifically for this application, measures the displacement of the spindle shaft under load in five spatial directions – three translational and two rotaries. This allows the kinematic conditions in the bearing to be calculated – and thus parameters such as pressure, spin/roll ratio and cage pocket clearance.
If the deflections measured on the rolling elements exceed a threshold – set individually for each spindle and type of machine – the sensor transmits a warning signal to the machine’s control system. Threshold limits can also be defined for other drive components that have lower load limits than the spindle and whose loads correlate with those of the spindle.
All of the necessary software and algorithms are built into the sensor, and no further components are needed. The system’s ability to transmit a warning signal to the machine’s control system can be used to:
• Detect collisions The sensor technology can signal an overload via a digital output within 2ms, thus minimising, or preventing, the risk of serious damage by deactivating the drive rapidly.
• Provide long-term protection for machine-tool spindles Continuous spindle bearing overloads can occur when roughing with a worn tool, for example. If the system triggers a warning signal when this happens, the operator can adjust the machining program and reduce the spindle load by modifying the cutting values, or by using a new or more suitable tool. This result is smaller and less frequent peak loads, extending spindle operating lives and reducing machine-tool downtime, which leads ultimately to increased uptime and reduced repair costs.
The sensor ring with its integrated load monitoring functions is now in pilot production and the first size is available for customers to test. Schaeffler’s engineers have developed an analysis tool for optimising the degree of spindle utilisation. This visualises the deflection measured by the sensor ring during the machining process, as well as helping to define threshold values.
Schaeffler says that, for the first time, the bearing sensor will allow machine operators to know the degree to which the spindle capacity in each machining process is being utilised with a high level of accuracy. They can, therefore, adjust the machining process precisely in terms of capacity utilisation and operating lives. Harmful overloads will be prevented, despite the high spindle loads. Safe operation in the limit range will also allow operators to boost productivity and to achieve longer spindle operating lives with fewer machine shutdowns.