Sensing wear on the spindle

PES hears how the system can lead to longer spindle life, reduced downtime, better productivity and a reduction in maintenance costs.

The main spindle on a machine tool is crucial to its performance capability. It is at the heart of the machine and largely defines the achievable cutting capacity, surface quality and precision.

In turn, the spindle bearing support is one of the most heavily loaded components because it must transmit machining forces precisely at very high speeds for long periods of time and the risk of collisions can never be ruled out. It therefore comes as no surprise that the majority of machine tool downtime can be traced back to defective spindles, particularly as a result of collisions and continuous, undetected overloads.

In milling operations for example, 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. These unfavourable and in some cases impermissible loads occur because operators have to date not had a suitable tool with which they could monitor borderline loads on spindle bearings. Schaeffler has now solved this problem with its new SpindleSense monitoring system.

To coincide with the recent EMO exhibition, the first production-ready SRS sensor ring units including radial and axial measuring ring with an inside diameter of 70mm, were available for customers to test in practical applications. Additional sizes with 80mm and 100mm inside diameters will be available at the end of the year. All units have a standard width of 16mm. The scope of delivery includes an SST set-up service tool, with which SpindleSense can be calibrated and put into operation.

The sensor system integrated into the spindle bearing measures the displacement of the spindle shaft under load in a very high resolution and in five spatial directions – three translational and two rotary.

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 is based on an assessment of operation-related bearing parameters such as pressure, spin/roll ratio, and cage pocket clearance, into which Schaeffler has integrated its many years of expertise in rolling bearings.

Software integrated solution

All the software and the required algorithms are integrated into the 16mm wide sensor ring. No further components are required for the system which is locally functional and transmits an individual warning signal to the machine’s control system, which makes the following applications possible.

Firstly, detecting a crash (collision): the sensor technology is capable of signalling an overload at a digital output within two milliseconds which allows serious subsequent damage to be minimised or even prevented through fast deactivation of the drive.

Secondly, long-term protection for machine tool spindles: in practice, continuous mechanical spindle bearing overloads are not immediately identified. In contrast, SpindleSense immediately triggers a warning signal when the overload is parameterised accordingly.

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 thus achieves lower and less numerous peak loads and so benefits from a longer spindle operating life with fewer machine tool downtimes, which ultimately means more production time and reduced repair costs.

Two options are available for outputting the measured values. Variant C-A0 transmits alarm signals 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 CAN bus.

Machine tool and spindle manufacturers can use these displacement values to develop analysis tools for optimising the utilisation of spindle capacity, e.g. by visualising the deflection collective measured by the sensor ring as a load collective.

This means 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 maximum spindle loads. Safe operation in the limit range allows the operator to increase his or her productivity and also to benefit from longer spindle operating life and less frequent machine downtimes.

The measured displacements that are outputted via CAN bus can also be used for zero-point compensation which makes it possible to produce the same quality with larger cutting values or higher quality with unmodified cutting values. The monitoring of the maximum displacement can also be used for quality assurance.

Electronic versus mechanical

Compared to mechanical, i.e. passive overload systems, Schaeffler SpindleSense does not just protect the spindle in the event of a collision; it’s also a revolutionary system for safely maximising the utilisation of the spindle capacity. This means machining manufacturers can now offer their customers a highly effective tool for increasing productivity, machine availability, and quality.

The Schaeffler Group is a global automotive and industrial supplier. By delivering high-precision components and systems in engine, transmission and chassis applications, as well as rolling and plain bearing solutions for a large number of industrial applications, the Group says it is already shaping future mobility.

Schaeffler generated sales of approximately €14.2 billion in 2018. With around 92,500 employees, it is one of the world’s largest family owned companies and, with approximately 170 locations in over 50 countries, has a worldwide network of manufacturing locations, research and development facilities and sales companies. With more than 2,400 patent registrations in 2018, Schaeffler is Germany’s second most innovative company according to the German patent office.

Schaeffler Group www.schaeffler.co.uk