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Measured lubrication for machine tool spindles

2015 February 01, 09:00 GMT

Getting the lubrication level right for high speed machine tool spindles has, for the most part, been an inexact science. A surprising statement perhaps, considering how crucial the accuracy of the finished product is. And given that the lifespan and operational level of bearings depends on adding precise doses of lubricant, there should ideally be no margin for error.
There are a number of accepted methods for lubricating high speed machine tool spindles. These include greased for life bearings, interval lubrication and the use of centralised lubrication systems. The latter generally offers the best option, but traditionally uses a comparative method of monitoring the volumetric flow of lubrication. This means the lubrication system does not respond to the actual operating conditions of the spindle, creating a disparity between how much lubricant is required and the amount dispensed. Additionally, as these systems can also suffer from slow response times, limited dosage accuracy and high compressed air and energy consumption, it has become apparent that a better, more accurate method is required. 
There is now an alternative that is set to make a real difference in the machine tool sector. A computer-controlled microdosage system launched by SKF has the potential to eliminate all of the short falls caused by the other lubrication processes, while increasing efficiency and service life, and reducing energy consumption. So how does this new technology actually function, why is it so vital and how does it directly compare to the tried and tested processes?
How it works
The computer-controlled microdosage system has the capacity to deliver highly precise, nearly continuous volumetric flows with constant quality monitoring in the range of 0.5 to 5.0 mm³/min. Translated into actual oil flow measurements, that is as little as 1/100th of an oil droplet per minute. At the heart of the system are precision microvalves that work in tandem with a flow sensor and a set of complex controls to successfully deliver lubricant quantities down to just a few nanolitres.
Opening for just a few microseconds, the microvalves create a metering pulse that ejects a defined amount of lubricant directly into the lubricant capillary line. This process is repeated in rapid succession, thus creating an almost seamless volumetric flow.
The power that provides the microvalves with sufficient pressure to operate is generated by compressed air that pushes upwards from a designated reservoir, which together with the metering unit is closely controlled and monitored by the machine control unit. This correlation of processes and connection of parts allows the quantity of lubricant to be more accurately delivered, and gives a much better real time performance than previous techniques. In addition, the integration of the microdosage unit components close to that of the actual machine hardware allows the lag between input signals and dosage output to be minimised still further. 
It is also important to note that the use of compressed air for this particular process is far more limited and controlled than in the traditional oil+air method, which relies much more heavily on compressed air to deliver the lubricant.
It is not just the regulated opening times of the microvalves that make a difference. The oil viscosity and ambient temperature also contribute to a more accurate lubrication level. Whereas oil+air systems have suffered from priming pressure fluctuations, the computer-controlled system is in a constant state of monitoring and self-calibration, ensuring this does not occur.
To ensure even greater accuracy, the integrated volumetric flow sensor can record the individual oil quantities from each of the valves in turn. This creates a compare and contrast set of statistics that can be measured against previously stored data and targets, using pre-defined tolerances to automatically determine the open time of each valve. It also offers the advantage of creating a unique operational parameter set for the individual valve, including opening times and how output quantities match up with the target values.
Although the self-calibration process normally takes place every four hours during operation, it can be further activated by specific events, such as when the system is switched back on after a period of inactivity or if the temperature sensor detects a change of five degrees or more.
The new micrometering system has been tested on machine tool spindle bearings. Trials were conducted on a belt-driven spindle bearing, minus temperature control. As it was easy to measure and gauge lubrication levels, the outer ring was chosen as the comparison value. At a speed range of 0 to 2.4 million n×dm the lubricant was passed through to the outer ring in increments of 3,000 r/min. Each speed level was maintained for 30 minutes to make sure that the system reached a steady state. The resulting speed of the bearing with a mean diameter of 54 mm was between 0 and 45,000 r/min.
The results were impressive. Set at a low speed, between 0.5 and 1.0 million n×dm (with a minimum lubricant quantity of 30 mm3/h), the recorded bearing temperature was approximately 2 to 3 degrees lower on the Kelvin scale. At higher speed factors, between 1.8 and 2.4 ‐million n×dm (with metered quantities adjusted accordingly), the bearings remained up to 8 Kelvin cooler thanks to the microdosage system.
The microdosage system offers much more than enhanced accuracy and constant monitoring; it also provides a more intuitive and economical answer to greased for life or intermittent lubrication methods.
The continuous lubrication process not only cuts down on the amount of lubricant used, it vastly reduces the use of compressed air, as it is only needed for the reservoir stage that is so intrinsic to current lubrication methods. This oil+air combination consumes relatively high levels of energy, which can be a considerable addition to both utility costs and environmental impact.
It is estimated that implementation of the microdosage system can create savings of up to £100 per annum for each lubrication point. This could add up to a saving of £400 when considering a spindle with four bearings.
The trials also revealed a lower level of noise emissions. When running at a speed of up to two million nxdm, there was a reduction of up to 10 decibels when compared with results recorded during the oil+air process.
In the industry-wide pursuit of offering a minimal quantity lubrication solution to the engineering sector, the new microdosage solution meets all the requirements and creates a benchmark for future technologies in this field. By introducing components that fulfil their role in measuring and monitoring oil dosing down to the very smallest drop, a longer, more energy efficient life for both high precision bearings and the high speed spindles, is ensured.
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Article was originally published in the February 2015 edition of Plant and Works Engineering.

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