ENGINEERING NEEDLE ROLLER BEARINGS A-19 A (1) Base oil Mineral oil is usually used as the base oil for grease. When low temperature fl uidity, high temperature stability, or other special performance is required, diester oil, silicon oil, polyglycolic oil, fl uorinated oil, or other synthetic oil is often used. Generally, grease with a low viscosity base oil is suitable for applications at low temperature or high rotation speed; grease with high viscosity base oils are suitable for applications at high temperature or under heavy load. (2) Thickener Most greases use a metallic soap base such as lithium, sodium, or calcium as thickeners. For some applications, however, non-soap base thickeners (inorganic substances such as bentone, silica gel, and organic substances such as urea compounds, fl uorine compounds) are also used. In general, the mechanical stability, bearing operating temperature range, water resistance, and other characteristics of grease are determined by the thickener. (Lithium soap base grease) Superior in heat resistance, water resistance and mechanical stability. (Calcium soap base grease) Superior in water resistance; inferior in heat resistance. (Sodium soap base grease) Superior in heat resistance; inferior in water resistance. (Non-soap base grease) Superior in heat resistance. (3) Additives Various additives are selectively used to serve the respective purposes of grease applications. • Extreme pressure agents When bearings must tolerate heavy or impact loads. • Oxidation inhibitors When grease is not refi lled for a long period. Structure stabilizers, rust preventives, and corrosion inhibitors are also used. (4) Consistency Consistency, which indicates grease hardness, is expressed as a fi gure obtained, in accordance with ASTM (JIS), by multiplication by 10 the depth (in mm) to which the cone-shaped metallic plunger penetrates into the grease at 25 °C by deadweight in 5 seconds. The softer the grease, the higher the fi gure. Table A-18 shows the relationships between the NLGI scales and ASTM (JIS) penetration indexes, service conditions of grease. (NLGI : National Lubricating Grease Institute) It is imperative that the bearing operating temperature is always within the temperature range specifi ed for the grease used. Although softer greases provide better lubrication, they are more likely to be churned. Since grease churning tends to cause temperature rise and leakage, this characteristic should be taken into account when selecting grease consistency. For ordinary operating conditions, greases of NLGI No. 0 to 3 are commonly used. When the bearing operating speed is higher, a somewhat harder grease with high mechanical stability should be selected. Table A-18. Grease consistency and service conditions ASTM (JIS) penetration index (25 °C, 60 mixing operations) NLGI scale Service conditions/applications 355 - 385 0 For centralized lubricating 310 - 340 1 For centralized lubricating, at low temperature 265 - 295 2 For general use 220 - 250 3 For general use, at high temperature 175 - 205 4 For special applications [Note] The larger the penetration index, the softer is the grease. (5) Mixing of different greases Since mixing of different greases changes their properties, greases of different brands should not be mixed. If mixing cannot be avoided, greases containing the same thickener should be used. Even if the mixed greases contain the same thickener, however, mixing may still produce adverse effects, due to difference in additives or other factors. Thus it is necessary to check the effects of a mixture in advance, through testing or other methods.