TYPICAL MOUNTINGS OF DUPLEX BEARINGS
Back-to-Back Mounting, DB or (“O”) (Contact angles diverging toward shaft centerline)
Before mounting, there is clearance between the two adjacent inner ring faces the bearings. After mounting, these faces are clamped together to provide an internal preload on each bear- ing. This arrangement is well suited for pulleys, sheaves, and in other applications where there are overturning loads, and also in all floating positions where thermal expansion of shaft occurs. It also provides axial and radial rigidity and equal thrust capacity in either direction when used in a fixed location. Back-to-back is the most commonly used of all duplex arrangements. Specify bearing number followed by suffix DU. Examples: 7207W-DU, 2MM207WI-DU. Also available as two single flush-ground bearings, i.e., 7207W SU (2 bearings).
Face-to-Face Mounting, DF or (“X”) (Contact angles converging toward shaft centerline)
Before mounting, there is clearance between the two adjacent outer ring faces. After mounting, these faces are clamped together between the housing shoulder and cover plate shoul- der, providing an internal preload on each bearing. This arrangement provides equal thrust capacity in either direction as well as radial and axial rigidity. Since the face-to-face mount- ing has inherent disadvantages of low resistance to moment loading and thermal instability, it should not be considered unless a significantly more convenient method of assembly or disassembly occurs from its use. Fafnir pairs for face-to-face mounting should be ordered as DU. Examples: 7212W-DU, 2M212WI-DU. Also available as two single flush-ground bear- ings, i.e., 7212W SU (2 bearings).
Tandem Mounting, DT Before mounting, the inner ring faces of each bearing are offset from the outer ring faces. After mounting, when a thrust load is applied equal to that of twice the normal preload, the inner and outer ring faces are brought into alignment on both sides. This arrangement provides double thrust capacity in one direction only. More than two bearings can be used in tandem if additional thrust capacity is required. Fafnir pairs for tandem mounting should be specified as DU. Examples: 7205W-DU, 2M205WI-DU. Also available as two single flush-ground bear- ings with suffix SU, i.e., 7210W SU (2 bearings).
Other Mountings Flush ground (DU) pairs may be mounted in combination with a single flush-ground bearing as a “triplex” (TU) set shown in Figure A. Figure B illustrates a “quadruplex” (QU) set where three bearings in tandem are mounted back-to-back with a single bearing. These arrangements provide high capacity in one direction and also a positively rigid mounting capable of carrying a moderate amount of reverse thrust.
DB Stamped faces of Outer rings together
Clearance between Inner ring faces
Inner ring faces clamped together
These Inner and Outer ring faces are flush
DF Unstamped faces of Outer rings together
Clearance between Outer ring faces
Inner and Outer ring faces clamped together
These Inner and Outer ring faces not flush
Faces flush on both sides
DT One stamped and one unstamped Outer ring faces together
Inner and outer ring faces not flush on either side
Under thrust load equivalent to twice the normal preload. Inner and Outer ring faces are flush on both sides