Machinery's Handbook, 31st Edition
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Selecting Lubricants Table 15. Priority Guideline Ratings of Lubricant Functional Attributes for Different Lubricated Components Lubricant Attribute
Sliding Bearing
Rolling Bearing
Wire Rope, Chain, Open Gears
Closed Gears
Friction Reduction Boundary Lubrication Cooling Ability Temperature Range Corrosion Protection Seal Out Contaminants
1 1 2 1 1 0
2 2 2 2 2 2
1 2 0 1 2 1
2 3 3 2 1 0
0 = Low Priority, 3 = High Priority Oil Application.— Oil lubrication can be broken down into two major categories: termi nating (total loss), and recirculating oil systems. Terminating Oil Systems: Terminating oil systems are semi-automated and automated systems that dispense oil at a known rate to the bearing(s) and do not recover the oil. This system can be generally observed in use for lubricating plain bearings, gibs, and slide ways found in small to medium-sized machine tools. Reservoir oil is replenished with new oil on an “as used” basis. Recirculating Oil Systems: Recirculating oil systems pump oil through the bearing(s) on a semi-continuous or continuous cycle, using the oil to cool the bearing as it lubricates. Depending on the system design the oil can be filtered prior to the pump suction inlet, on the pump discharge, and again on the gravity return to the oil storage reservoir. Recirculating systems are used on every kind of small to very large equipment as long as the oil can be contained; reservoirs retain their original charge of oil, which is changed on a condition or time basis. A simple method used by lubrication delivery system manufacturers for determining the bearing oil requirements under normal load and speed conditions uses a volumetric requirement over a specified time period (See Table 16), designated by: (4) where V = oil volume in cubic centimeters, (cc) A = bearing contact surface area, (cm 2 ) R = film thickness replenishment, (mm) Table 16. Lubrication Film Replenishment Rate Guidelines for Oil and Grease Lubricant Delivery Method R - Film Thickness Time Automatic Terminating Oil 0.025 mm (0.001 inch) 1 hour Automatic Recirculating Oil 0.025 mm (0.001 inch) 1 minute Other closed system oiling methods exist: Gearbox splash systems employ a simple recirculative pickup/transfer of oil by a submerged gear tooth from an oil reservoir bath. Constant level oilers maintain a constant level of oil in a specially designed oil bath bear- ing housing. Using air over oil technology, oil can be misted and “rifled” into the bearing allowing very high speeds of over 20,000 rpm at light loads. When replenishing oil reservoirs always use new clean oil of the exact same specifica tion, from the same manufacturer. Mixing different oils of similar specification can cause additive packages to react with one another causing detriment to the bearings. If changing to a new oil specification or manufacturer consult with the new manufacturer for the cor rect change-out procedure. Grease Application.— Because grease is easy to retain in a bearing housing over a long period of time and because it acts as a seal against contaminants, most rolling element bearings are grease lubricated. For most applications a NLGI 1 or 2 rating grease is used. V A R # =
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