Press releases
Dry yet well lubricated
Published in www.k-magazin.de, 02/2004
Lubricants – Bearings under high loads, at high or very low speeds and with frequent stop-and-go operation, in ultra-high vacuum or outer space. What about the lubrication?
Dry lubricants can be used for extreme operating conditions. They eliminate the risk of oil or grease leakage involved in wet lubrication and make relubrication lines unnecessary. Lifetime lubrication can be attained and running-in behavior improved. Because of today's technical progress and legal requirements the trend towards innovative high-performance lubrication looks set to continue. Higher performance of components normally entails higher operating temperatures causing premature aging of the lubricant. Conventional total loss lubrication is no longer acceptable. Depending on the operating conditions, bearing loads and speeds, there are different types of dry lubrication that may be used: bearing compounds which ensure proper lubrication by "chalking off" during operation, bonded coatings or galvanic coatings which incorporate solid lubricants forming a layer between the surfaces in contact.
High requirements have to be met as dry-lubricated bearings too should offer a long service life. In addition, corrosion protection, resistance to water and chemicals, high speed factors and load bearing capacity are important parameters of dry lubricants.
Dry lubricants are high-performance products suitable for temperatures as high as 280°C and as low as –50°C. Also, they show very little gasification under vacuum.
Klüber has developed a special bearing compound for radial bearings which is very load-resistant and allows lifetime lubrication even with low speeds of up to approx. N x dm = 50,000 mm/min and very high temperatures of more than 250°C. This product contains a friction-reducing solid lubricant combination and an inorganic binder. Eco-friendly application is possible because the product is mixed with water to be filled into the bearings in paste form. Once the compound has hardened and the bearing rotates freely, small quantities of the compound "chalk off" ensuring continuous dry lubrication. The rolling movement provides a consistent transfer of dry lubricant to the friction surfaces of the bearing.
The properties of the compound ensure that the chalking quantity is constant within a certain margin. Excessive chalking would entail risks like bearing jamming, while insufficient chalking would mean increased wear from lubricant starvation. Dry lubrication offers the following advantages for rolling bearings: "clean" lubrication, no dripping of lubricant, thermal stability up to 550°C, no change in consistency or viscosity due to temperature, lifetime lubrication, good corrosion protection, resistance to water and oil as well as easy processing.
Upon completion of its development, this self-lubricating compound was filled into deep-groove ball bearings as well as spherical roller bearings and subjected to ROF and FE8 rolling bearing tests. For steel mill applications, e.g. kilns, the compound was filled into the bearings and tested. All results are positive and give evidence of reliable lubrication.
Double-row, full complement ball bearings running in kilns were also tested at kiln temperatures of 350 – 400°C for 24 hours per day, 5 days per week. After an operating period of 1,350 hours both the bearings and the lubricant were still fully fit for use. The rolling bearing elements and functional surfaces showed minimal wear.
Bearing compounds used as dry lubricant contain a friction-reducing lubricant combination mixed with a binder and are filled into the bearing in paste form
Long service life, corrosion protection, resistance to water and chemicals are vital parameters of dry-lubricated rolling bearings
In bronze sliding bushes and rails the compound acts as lubricant reservoir in lubrication recesses
Photograph: Enzesfeld CARO Metallwerke AG
The functional surfaces were covered with solid lubricant originating from the compound used. Bearing rings and rolling elements were free of damage despite slight grooves on the outer ring. The bearing showed a loss of compound amounting to approx. 7 grams which corresponds to approx. 8 % of the original fill. A special compound is available for the dry lubrication of bronze bearings which acts as reservoir in lubricating grooves, pockets or holes of sliding bushes and rails. Due to the good adhesion of this compound on metal surfaces it can also be used for coating large sliding surfaces. The performance spectrum of this sliding bearing compound is similar to the compound lubrication of rolling bearings, however, in a service temperature range from –40 to +200°C. The hardened sliding bearing compound consists of a two-component, duroplastic binder system. The incorporated solid lubricants and the oil decisively contribute to uniform friction and low wear.
A small quantity of oil release during operation already ensures perfect lubrication condition. Additional lubrication with oil or grease, or relubrication are not necessary. The sliding bearing compound has proven particularly effective at low speeds and in rotating or oscillating operation. Under these operating conditions a bronze bushing achieves a sliding distance several times longer than that of a grease-lubricated bushing. Loads of approx. 50 N/mm² are possible depending on the metallic carrier material. Best running times were achieved on the plain bearing test rig at 10 N/mm² and 0.1 m/s. The poorer performance at 0.5 m/s is most probably due to the higher frictional heat of the bearing which lacks heat dissipation by means of a cooling system or liquid. Prior to application the resin and the hardener have to be mixed to form a paste which is be filled into the lubrication reservoirs or onto the sliding surfaces. The components should be clean and free of grease. This high-performance compound has been especially designed for low-speed plain bearings subject to high loads and covers a wide temperature range.
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