X and O bearing pairs

Two single angular contact ball or tapered roller bearings that locate a shaft between them work as a pair: under radial load each one's inclined contacts push its rings apart axially, and the other bearing has to hold that induced force as well as the shaft's own axial load. Tell the app which way each bearing takes axial load with Takes axial load towards under Arrangement: "+x (facing positive)" or "−x (facing negative)". Left empty, a bearing takes load towards +x as before, and nothing changes in projects saved before.

The pair and its loads

When both locating supports of a shaft carry such bearings facing opposite ways, the shaft's results get an X pair (the left bearing facing −x, face-to-face) or an O pair (the left bearing facing +x, back-to-back). The pair's axial loads then come from the bearings' own load–displacement behaviour (their internal geometry), not from linear axial springs: each radial load induces at least $F_{r} \cdot \tan α$ , about $1.25 \cdot F_{r} \cdot \tan α$ with half the rollers carrying. The bearing's pair card ("X pair (face-to-face)" or "O pair (back-to-back)") lists each bearing's facing, radial and axial load, the induced force of the catalogue rule ( $0.5 \cdot F_{r} / Y$ with ISO 281's $Y = 0.4 / \tan α$ for tapered rollers, $e \cdot F_{r}$ for angular contact ball bearings), the axial load by that rule for comparison, and how many rollers carry. With the induced forces $F_{A}$ (the bearing the external force $K_{a}$ pushes against) and $F_{B}$ , the rule gives $F_{a, A} = F_{B} + K_{a}$ and $F_{a, B} = F_{B}$ when $F_{B} + K_{a} \geq F_{A}$ , else $F_{a, A} = F_{A}$ and $F_{a, B} = F_{A} - K_{a}$ . At zero clearance the full solution gives the lighter loaded bearing somewhat less than its induced force, as its rings are pushed apart a little.

Preload or clearance

The pair's adjustment is the shaft's Pair preload (N) or Pair axial clearance (mm, end play; negative is a preload displacement), under the shaft's Material and limits, below the thermal growth; with neither the pair is at zero clearance. In operation it changes with the fits and ring temperatures of a bearing that has a mounting, with the shaft's thermal growth against the housing between the bearings (an X pair closes, an O pair opens) and with the shaft's stretch under the axial loads; the card shows the preload or clearance in operation and each of these. A bearing without radial load whose clearance is not taken up lifts off, one at its least axial load runs on the rollers next to the load line, and one with fewer than a quarter of its rollers carrying runs on a small load zone: each is a warning at its support, in every load case too. With a preload, the axial preload check fails when none is left in operation or a bearing lifts off.

Study the preload

Study the preload solves the project at preloads from 0 to 5 % of the bearing's dynamic rating and draws each bearing's modified life against the preload, marking the preloads where a bearing lifts off. The suggested range (shaded) is the preloads within 10 % of the longest life among those where no bearing lifts off or runs at its least load, as entered and in every load case. Apply sets the suggested preload in one step that undoes. For a matched pair (an Arrangement of O or X), the card shows what each of its two bearings carries, and the study varies its Preload.

Speed against load

Speed against load (angular contact ball and tapered roller bearings) draws the thermally permissible speed nθ of DIN 732 as the bearing's loads grow, for grease, oil bath and oil jet, with the thermal reference speed, the limiting speed when one is given, and the operating points of the loads as entered and of each load case. Each point is the bearing's own thermal speed calculation at those loads, so the operating point's nθ is the one under Speeds.

The pair and its loads

Preload or clearance

Study the preload

Speed against load

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