The main spindle is the heart of the machine tool and largely determines the achievable cutting performance, surface quality and precision. The spindle bearing, in turn, is one of the most highly stressed components, as it has to transmit the machining forces precisely at very high speeds over a long period of time and collisions still cannot be ruled out.

It is therefore not surprising that the majority of machine tool failures are due to defective spindles, particularly as a result of collisions and undetected overloading. In milling operations, for example, the combination of high radial loads, long overhanging tools and high speeds creates high loads and unfavorable kinematic conditions, especially on the spindle bearing close to the tool. These sometimes impermissible loads occur because the operator previously lacked a suitable tool with which to monitor borderline spindle bearing loads. Schaeffler solves this problem with the "SpindleSense" monitoring system.

Precisely measure displacements

The first production-ready SRS sensor ring units including radial and axial measuring ring with an inner diameter of 70 mm will be available at EMO 2019. Other sizes with an inner diameter of 80 and 100 mm will be available by the end of the year. The standard width is 16 mm. The scope of delivery also includes the SST setup service tool, which can be used to parameterize and commission SpindleSense.

The sensor system integrated into the spindle bearing measures the displacement of the spindle shaft under load in five spatial directions - three translational and two rotational - with a high resolution. If the deflections detected on the rolling elements exceed a specific threshold, the sensor ring sends an electrical warning signal to the machine control system. The threshold is set individually for each spindle and machine type. It is based on an evaluation of the bearing characteristics relevant to operation.

Software with rolling bearing expertise in the sensor ring

The entire software and all necessary algorithms are integrated into the sensor ring. No additional components are required for the system. The system functions locally and issues an individualized warning signal to the machine control system.

To detect a crash (collision), the sensor system is able to indicate an overload at a digital output within 2 ms. In order to detect permanent mechanical overloads of the spindle bearings, SpindleSense immediately triggers a warning signal if the overload is parameterized accordingly. The operator can modify his machining program after the first part and reduce the spindle load by using a new tool, changing the cutting values or selecting a more suitable tool type.

Two options are available for the output of the measured values. Variant C-A0 outputs alarm messages as soon as the individually defined limit values for the bearing load and kinematics are reached. Variant C-A1 outputs the measured radial and axial displacements, including tilting, via the CAN bus. Machine tool and spindle manufacturers can use these displacement values to develop analysis tools for optimizing spindle utilization, for example by visualizing the deflection collective measured by the sensor ring as a load collective.

Another application of the measured displacements is to use them for zero-point compensation. This makes it possible to produce the same quality with larger cutting values or a higher quality with unchanged cutting values. The monitoring of a maximum displacement can also be used for quality assurance.