Perfect sliding with additional benefits
The lubrication of slideways at machine tools discussed with a practitioner
Ever increasing rates, precision and performances – industry places enormous requirements on modern machine tools. However, even the best machine tool can deliver the performance expected only if the lubricants applied fulfil their tasks efficiently and without any problems.
Thomas Könnecke, Doctor of Chemistry and general manager of the K & W Mineralölgesellschaft Erfurt mbH, knows about the complex characteristics of these lubricants and possesses comprehensive practical experience regarding their application in tool machines. He also recommends high-performance lubricants suited for the single applications to his customers from industry.
Thomas Könnecke and his team do not only advise on the optimum metalworking fluid but also suggest further lubricants fit for the respective case. Often, the lubrication of slideways means a particular challenge. Usually, Thomas Könnecke recommends ADDINOL Guideway oils XG, which he tested in numerous practical applications. To him, the decisive criteria are the oils’ demulsibility, its zinc-free additivation, the protection against corrosion and its anti-stick-slip-effect.
During the operation of the tool machine the usually water miscible coolant absorbs some slideway oil inevitably. To prevent negative effects on the emulsions‘ quality the slideway oil must separate quickly and quantitatively so that it can be skimmed from the surface of the coolant.
If the slideway oil does not possess good demulsifying characteristics, the growth of bacteria in the polluted coolant is encouraged. Normally, the flushing effect of the coolant decreases as well. Furthermore, lifetime of tools and quality of surfaces manufactured is impaired. The coolant needs to be replaced earlier.
In contrast to many other products, the named oils achieve a quantitative demulsification already after five minutes according to the manufacturer ADDINOL. Therefore, demulsibility contributes considerably to the long lifetime of the coolant and the reliable stability of the lubricating film on the slideway.
Guideway oils of the XG range are based on high-quality mineral oil raffinates and a special, zinc-free additivation according to the manufacturer. The additives guarantee excellent load-carrying capacity and are effective against wear. For the disposal of water miscible coolants it is important that they do not contain zinc. Mostly, these emulsions undergo a special conditioning where water phase and oil phase are separated. If the emulsion is loaded with fractions of zinc-containing slideway oil, the separated water phase is also polluted with zinc. This share of zinc must be removed in an elaborate procedure before the water can be reused.
Slide- and guideways of machine tools require lubricants which have long operating lives, form a stable lubricating film also under the impact of water and prevent unwanted reactions on metal surfaces. The use of zinc-free additives achieves a high chemical stability, therefore the respective slideway oils are effective over long periods.
Oils of the XG range meet the corrosion test with distilled water under DIN ISO 7120 and achieve excellent results also with aggressive sea water according to ADDINOL. Their additives against corrosion and oxidation prevent the formation of sticky residues as well as stains and rust on the slideways.
The lubrication of slideways means a challenge to every lubricant manufacturer. The components which need to be lubricated do not move in a simple rotating motion, as for instance with sliding bearings, but linearly in both directions at a defined work cycle and a certain speed. After each stop of the slide immediate movement is called for again. However, this movement is not necessarily as smooth as required as the sledge needs to overcome a resistance which is relatively high. At this point sticking friction prevents its movement initially.
Only if the driving power of the slide exceeds sticking friction it accelerates. Sliding friction takes the place of sticking friction which in turn decreases at the end of the slideway. The slide stops and the cycle starts anew, only in the other direction. This means, sticking and sliding friction take turns constantly. If the effect of sticking friction exceeds sliding friction, the slide moves jerkily and the so-called slip-stick-effect occurs which has negative effects especially for small, precise movements. If the sledge with the work piece or tool cannot move in a smooth and controlled manner the processing quality is impaired considerably.
The lubricants of the XG range are also designed for these problems and promise remedy and a reduction of the unwanted slip-stick-effect to a minimum. The slideway oils contain special additives to improve friction characteristics, to master sliding friction and to ensure starts without jerks, a harmonious transition into full motion and smooth sliding even for heavy work pieces.
Tips from practice
From practice the lubricant expert Thomas Könnecke knows about problems of machine tools in general and respective slide- and guideways in particular and also how to solve these. He noted a relatively strong mixing of guideway oils and coolants with older guideways in particular. To stop this, a counselling with the service of the machine manufacturer might be worthwhile. Often the problem is solved by adjusting lubricating cycles or resetting the impulse for the lubricator.
The change to a higher viscosity of the guideway oil might also be helpful. Guideway oils of the XG series are available in various viscosities. To keep the effort for disposal at a minimum, one should use not only zinc-free guideway oils but also zinc-free hydraulic oils, as for instance ADDINOL hydraulic oil HLP AF or HLPD.
Furthermore, the chemist recommends to make use of the excellent demulsibility of the XG guideway oils and to remove the separated oil from the coolant regularly. Here, there is a range of various skimming devices (belt, disc, coalescence separator, etc.) also offered by K & W. www.addinol.de, www.mineraloelgesellschaft.de
German article published in Betriebstechnik & Instandhaltung February 2012 (view German article here)