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Accurate ‘micro-dosing’ improves lubrication of high speed spindles

07 June 2015

Phil Burge describes an innovative ‘micro-dosing’ system that can help to deliver accurate quantities of lubricant – in tiny amounts – to high-precision bearings for applications such as high-speed spindles.

High-speed, precision spindles need highly accurate lubrication. Systems operating at speed factors above 2 million n×dm – that’s more than 30,000rpm on conventional tool spindles – require precisely matched quantities of lubricant in order to maintain optimal bearing operation. The frictional moment in the bearing needs to be as low as possible, while mixed friction needs to be avoided to prevent undue wear.

There are several recognised ways of lubricating high speed machine tool spindles, including greased for life bearings and interval lubrication. The favoured method is a centralised lubrication system – though this is far from perfect because it often uses a comparative method of monitoring volumetric lubricant flow.

This means that the system does not respond to the spindle’s actual operating conditions, so there is a difference between the amount of lubricant required, and how much is actually dispensed. On top of this, the systems can suffer from slow response times, limited dosage accuracy and high compressed air and energy consumption. For all these reasons, it has become apparent that a better, more accurate method is required.

One recent development, which could make a big difference to the machine tool sector, is a computer-controlled system from SKF that dispenses ‘micro-doses’ of lubricant. It could bypass the shortfalls of other lubrication processes, while at the same time increasing efficiency and service life and reducing energy consumption.

The system in action
The computer-controlled micro-dosage system delivers precise, nearly continuous volumetric flow with constant quality monitoring in the range of 0.5 to 5.0mm³/min. In terms of actual oil flow measurements, this is as little as 1/100th of an oil droplet per minute.

The critical components within the system are precision micro-valves, which – working in concert with a flow sensor and a set of complex controls – deliver nanolitre quantities of lubricant. The micro-valves open for a few microseconds to create a metering pulse that dispenses a defined, precise amount of lubricant directly into the lubricant capillary line. More pulses are generated in quick succession, creating near-continuous volumetric flow to each lubrication point.

The micro-valves are powered by compressed air, pushing upwards from a special reservoir. The reservoir and metering unit are closely controlled and monitored by the machine control unit. The correlation between processes and connection of parts boosts the accuracy of lubricant dispensing and offers improved real time performance compared with existing techniques. Integrating the micro-dosage unit components close to the machine hardware further minimises any lag between input signals and dosage output.

It should also be noted that the traditional ‘oil+air’ method relies much more heavily on compressed air to deliver the lubricant. The new method uses compressed air in a more limited and controlled way.

It is not just the regulated opening times of the micro-valves that make a difference. Oil viscosity and ambient temperature also contribute to a more accurate lubrication level. While oil+air systems can suffer from fluctuations in priming pressure, the computer-controlled system uses constant monitoring and self-calibration to ensure this does not occur.

The integrated volumetric flow sensor records individual oil quantities from each valve in turn, generating information 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 creates a unique operational parameter set for the individual valve – including opening times, and how output quantities correspond with target values.

Although self-calibration 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. 

Spindle testing
To prove the beneficial effects of micro-metering, the new system was tested on machine tool spindle bearings across a wide speed range. Trials were carried out on a belt-driven spindle bearing, which had no temperature control. The bearing temperature on the outer ring was chosen as the comparison value, because it was easy to measure and gauge lubrication levels there.

Lubricant was passed through to the outer ring in increments of 3,000rpm, in the speed range of 0 to 2.4 million n×dm. Each speed level was maintained for 30 minutes to ensure a steady state. The resulting speed of the bearing with a mean diameter of 54mm was between 0 and 45,000rpm.

At lower speeds (0.5 to 1.0 million n×dm, with a minimum lubricant quantity of 30 mm3/h), bearing temperature was two to three degrees lower on the Kelvin scale. At speeds of 1.8 to 2.4 million n×dm (with metered quantities adjusted accordingly), the bearings remained up to 8K cooler thanks to the micro-dosage system.

This shows that – in addition to enhanced accuracy and constant monitoring – the micro-dosage system provides a more intuitive, economical answer to either greased for life or intermittent lubrication methods. The continuous lubrication cuts the amount of lubricant used, and 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 is also quite energy intensive, which can add considerably to both utility costs and environmental impact. 

Introducing the micro-dosage system can lead to estimated annual savings of £100 per lubrication point – or £400 for a spindle with four bearings. The micro-dosing method also runs more quietly, the trials revealed: at speeds up to two million n×dm, there was a reduction of up to 10dB(A) compared with the oil+air process.

The micro-dosage system meets the engineering sector’s need for a minimal quantity lubrication solution – creating a benchmark for future technologies. Using components that measure and monitor oil dosing down to the smallest drop ensures a longer, more energy efficient life for both high precision bearings and high speed spindles.

Phil Burge is with SKF in the UK

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