Schaeffler integrates sensor technology into its spindle bearings

At EMO 2017, Schaeffler presented a revolutionary system for maximising the OEE (overall equipment efficiency) of machine tools – a newly developed range of sensors for main spindle monitoring that are integrated into the spindle bearings themselves. For the first time ever, machine manufacturers can now offer their customers a highly effective and easy-to-use device for increasing machine availability, productivity and quality. Using this system will reduce the frequency of spindle failures while simultaneously extending the operating life of the spindles.

The majority of machine tool downtime can be traced back to defective spindles, particularly spindle bearing damage caused by crashes (collisions) and by continuous, undetected overloading. For example, a collision between the tool and the workpiece can cause damage to both the spindle bearings and to other spindle components. In milling operations, the combination of high radial loads, long tool protrusions and high speeds, leads to particularly high loads and unfavourable kinematic conditions on the spindle bearing in the vicinity of the tool. In extreme cases, sudden bearing failures are possible.

From this initial situation, Schaeffler’s engineers have developed a system with the objective of reducing spindle failures by enabling very fast spindle deactivation in the event of a collision. Furthermore, the system allows the machine operator to detect these unfavourable operating conditions and to make targeted adjustments to the machining process.

Spindle displacement as a measure of rolling contact load

The completely new sensor system, which has been developed specifically for this application, measures the displacement of the spindle shaft under load in high resolution and in five spatial directions – three translational and two rotary. This, combined with the matching expertise in rolling bearings, makes the kinematic conditions in the bearing and thus the operation-related parameters such as pressure, spin/roll ratio and cage pocket clearance, to be clearly calculated. The sensor ring transmits an electrical warning signal to the machine’s control system if the deflections measured on the rolling elements exceed a specific threshold, which is set individually for every spindle and machine type. The threshold limit can also be individually defined for other drive components that have a lower load limit than the spindle and whose loads correlate with those of the spindle.

Another special feature in the age of Cloud computing is that all of the software and the required algorithms are integrated into the sensor technology. This means that the system requires no further components. The system is locally functional and transmits an individual warning signal to the machine’s control system, which makes the following applications possible:

- Detecting a crash (collision): the sensor technology is capable of signalling an overload at a digital output within 2 milliseconds. This allows serious subsequent damage to be minimised or even prevented through fast deactivation of the drive.

- Long-term protection for machine tool spindles: in practice, continuous mechanical spindle bearing overloads, e.g. when roughing with a worn tool, are not immediately identified. If the system triggers a warning signal in this or a similar scenario, the operator can adjust the machining program right after the first manufactured part and reduce the spindle load by using a new tool or modified cutting values, or by employing a more suitable tool type. The operator therefore achieves lower and less numerous peak loads, and so benefits from a longer spindle operating life with less machine tool downtime, which ultimately means increased production time and reduced repair costs.

The displacement measurement using the sensor ring and the integrated load monitoring has reached the pilot production stage and the first size is now available for customers to test in practical applications. What is more, Schaeffler’s engineers have developed a new analysis tool for optimising the degree of spindle utilisation. With this system, the deflection collective that is measured by the sensor ring during the machining process is visualised over the time axis in addition to a threshold value being defined. For the first time ever, the machine operator will know the degree to which the spindle capacity in each machining process is being utilised as a percentage with a high level of accuracy and therefore will be able to adjust the machining process even more precisely in terms of capacity utilisation and operating life for each machine. This means that harmful overloads are prevented despite high spindle loads. Safe operation in the limit range allows the operator to increase productivity and to also benefit from longer spindle operating life and less frequent machine shutdowns.