molecular size, reactivity and polarity they can be versatilely used to reduce friction and to prevent adhesive and abrasive wear in boundary lubrication.


With the polar centres in their molecular structure, sulphur containing EP additives already adsorb on metal surfaces, forming a friction reducing layer. During metalworking processes, mechanical and/or thermal stress causes chemical reactions between the sulphur atoms and iron or other metal atoms. The result is a very thin metal sulfide layer on the metal surface that is less stable to mechanical shearing than the metal structure itself and thus prevents the metal surfaces from galling and cold welding under a wide range of conditions.


Sulphur carriers are available with various sulphur activity, total sulphur content, polarity, molecular weight and other characteristics to provide performance properties such as lubricity, solubility, viscosity, biodegradability, and so forth. Thus they can be adapted to the requirements of specific metalworking processes.


Compared to other EP additives like chlorinated paraffins, the frictional forces in certain metalworking processes are significantly lower by using suitable sulphur carriers, which also leads to a reduced energy consumption of the manufacturing equipment. In addition to their superior protection against adhesive wear under boundary and mixed lubrication conditions, sulphur carriers show many benefits in terms of toxicity, environmental impact and energy efficiency when compared to other classes of EP additives. Many sulphur carriers are, for example, manufactured by sulphurisation of natural, renewable raw materials like vegetable oils or their ester derivatives. They can be up to 100% biodegradable and non-toxic to aquatic life. Several sulphur carriers are even suitable for the formulation of lubricants that meet the requirements of the European Ecolabel or the United States Vessel Incidental Discharge Act and can be certified as having low or no impact on the environment and showing no health hazards.


Tribological Tests Tribological tests are used to simulate tribological conditions like friction or mechanical shearing under laboratory scale conditions. They help for example to understand the influence of metalworking fluid additives on wear formation or energy consumption in metalworking processes.


Tapping Torque Tests


Some laboratory tests come close to replicating how machines operate in real-life metalworking. The tapping torque test machine, which originated from small series production, measures torque during thread cutting and thread forming processes. The recorded torque is used for the evaluation of the metalworking fluid formulation.


Figure 2 shows the average and maximum torques of different EP additives when thread cutting in high strength steel S960QL. This special steel grade is similar in yield strength, tensile strength and microstructure to high strength steels that are used in vehicle manufacturing to reduce weight and improve energy efficiency.


Figure 2: Thread cutting in high strength steel, average and maximum torques of different lubricant formulations


The additives were dissolved in Group I, ISO VG 46 base oil. The tool diameter was 4 mm, the cutting depth 8 mm and the cutting speed 800rpm. Compared to 10% medium chain chlorinated paraffin, it was possible to reduce the average torque in the test by up to 13.9% by using a combination of 5% sulphur linked polymer and 5% active sulphurised olefin. The reduced torque does not only indicate a lower energy consumption in the tapping process but also a lower mechanical and thermal stress for the cutting tool which results in an extended tool life.


Figure 3: Thread forming in high strength steel, average and maximum torques of different lubricant formulations


Continued on page 48 LUBE MAGAZINE NO.176 AUGUST 2023 47


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