Roller bearings in twin screw compressors
Roller bearings, primarily NU type cylindrical roller bearings and tapered roller bearings, are used in twin screw compressors as radial bearings Cylindrical roller bearings can operate at high speed, have high load capacity and are axially compliant, facilitating easy separation of axial and radial load when used in combination with angular contact ball bearings The inner and outer rings can be mounted separately in the compressor Tapered roller bearings are also used, taking combined radial and axial loads .
Cylindrical roller bearings
Bearing design and features
Cylindrical roller bearings of EC design are used in twin screw compressors for their high speed and high radial load capability.
They have a large number and size of rollers, logarithmic roller proile, and optimized lange geometry The bearings are available with ranges of internal radial clearance for optimization of rotor position accuracy The
EC cylindrical roller bearing is produced standard withradial runout .All cylindrical roller bearings are made to
Explorer speciications The NU type cylindrical roller bearing is commonly used in twin screw compressors since it allows separate assembly of the inner ring and outer ring/ roller assemblies onto the shaft and into the housing respectively (ig. 1) The NU type bearing accommodates axial displacement due to thermal expansion of the shaft NU type bearings allow both the inner and outer rings to be mounted with interference its for more precise positioning of the rotors.
The NU cylindrical roller bearings have two integral langes on the outer ring to guide the rollers The bearings have “open” langes, i e the inward face of the lange is inclined by a deined angle (ig. 2) The lange design, together with the roller end design and surface inish, promote the formation of a lubricant ilm to reduce friction and frictional heat The roller logarithmic proile optimizes the stress distribution in the roller/raceway The logarithmic proile also reduces sensitivity to misalignment and shaft delection.
The optimized inish of the contact surfaces of the rollers and raceways maximizes the formation of a hydrodynamic lubricant and minimizes friction .
Needle roller bearings
Needle roller bearings are used in oil-injected air and refrigerant compressors because of their high radial load capability and compactsize This can be an advantage in small compressors, where space is limited Needle
roller bearings are more sensitive to misalignment then cylindrical roller bearings .Compared to cylindrical roller bearings therollers (needles) are guided by the cage instead of being guided by the langes This limits the speed and can lead to higher noise level Needle roller bearings have separable rings like the cylindrical roller bearings, but can also be used without inner rings if the raceways are made as per the speciications below.
Needle roller bearings with special clearances, cages or with higher precision have limited availability To avoid signiicantly reduced bearing life, the angular misalignment should be less than 1,0 minute .
Tapered roller bearings
Ta pered roller bearings are used in oil looded air screw compressors at the male and female discharge positions, and accommodate both axial and the radial load.
They can also be used to support the input gear shaft in both oil looded compressors and high speed dry air compressors High axial and radial load capabilities are the main features of tapered roller bearings .
Tapered roller bearings have tapered inner and outer ring raceways and tapered rollers.They are designed to accommodate combined loads, i e simultaneously acting radial and axial loads The projection lines of the
raceways meet at a common point on the bearing axis to provide true rolling and low friction between the raceways and the rollers. The axial load carrying capacity of tapered roller bearings increases with increasing outer ring contact angle.