What is misalignment, and how is it counted?
It is the angle the ball can tilt inside the outer ring.
The misalignment angle is measured between the ball centreline and the outer member centreline when the ball is tilted to the extreme position (NHBB defines it this way). The same angle applies on both sides of the centreline, so the total misalignment is double the catalogue angle. Always check whether a figure means one side or both.
What happens if you go past the limit?
The inner ring's edge runs into the outer ring's face, or the seal and washers are crushed or cut. Past that point the load no longer goes through the spherical surface but through a corner, which is why the maximum tilt angle in the maker's tables must not be exceeded, either while mounting or in operation.
Is the catalogue angle the angle you can actually use?
Often it is not. The catalogue angle is the geometric maximum of the bearing alone, measured with nothing around it. It is a design reference for installation, not a working misalignment under load.
In a bearing fitted on a shaft or in a housing, the tilt is usually stopped by something around the bearing before the ball reaches its own limit. Check these:
- Shaft shoulders and spacers. The edge of a shoulder or spacer next to the inner ring can touch the outer ring or housing face while the shaft tilts.
- The housing bore edge. A housing chamfer that is too small, or retaining rings and covers, can block the outer ring's face.
- Seals and washers. A sealed bearing needs room for the seal, especially with 2RS types, so the seal does not touch the surrounding parts (Q1-11).
- Deflection under load. Shaft bending and housing movement reduce the clearance further.
The geometry is simple enough to estimate. The inner ring is wider than the outer ring, so its end face sits a little proud of the outer ring's face. Call the free axial gap between the spacer (or washer) face and the outer ring face g, and the radius at which the tilted spacer edge meets that face r. A tilt of α moves the spacer edge by r × tan α toward the outer ring, so the usable angle is about
α ≈ arctan(g ÷ r)
With g = 2.5 mm and r = 22 mm (illustrative, not a catalogue part) that is about 6.5°. Swap the spacer for a larger-diameter washer that meets the outer ring face at r = 30 mm, and the usable angle drops to about 4.8°. The lesson is that every larger-diameter washer, spacer or seal lip next to the inner ring takes angle away, and the only reliable check is the whole arrangement moved through its full travel, by hand or on a mock-up. For rod ends in a clevis, the usable angle depends on the clevis and the spacers, and is covered in K01-Q1-06 and K03-Q1-09.
How much axial load can it take?
The maker's table says what thrust is allowed.
Radial spherical plain bearings carry high load primarily in the radial direction, and the ratings are written for radial load. When some thrust load is present, it is folded into one equivalent radial load with a factor that grows as the thrust share rises. The formula and a worked example are in Q1-03. A larger thrust share than the maker's table covers is outside the method, and that is the point at which to change bearing type. The axial clearance of the bearing can also be a multiple of its radial clearance, so an axially loaded joint can move more than you expect.
When do you need an angular contact bearing?
When the main load is thrust (along the shaft), and it acts in one direction.
Angular contact types are made for high load in mainly the thrust direction, because their sliding surface is angled so that the contact force acts at an angle to the bore axis. Their best use is with a single-direction thrust load. When the thrust reverses, an angular contact bearing will separate, so for reversing thrust you need a pair. Mount a pair face to face ("DF") if you need moment flexibility, and back to back ("DB") if you need high moment rigidity. The reason is the spread of the two load lines: in a back-to-back pair the lines diverge, so the effective support base is wide and the pair resists tilting moments; face to face they converge, the base is short and the pair tolerates some misalignment.
| Situation | What to do |
|---|---|
| Mostly radial, small thrust share | Standard radial bearing, thrust folded into the equivalent load (Q1-03) |
| Thrust is the main load, one direction | Angular contact bearing |
| Thrust reverses | A pair; DB for rigidity, DF for flexibility |
| Tilt needed is more than the bearing's angle | A series with a larger angle, or change the geometry; do not run past the limit |
How much angle and thrust does a suspension joint really see?
More than most machines, and at the extremes of travel rather than at ride height. A control arm or link swings through the whole suspension travel. With long travel, or arms of unequal length, the bearing axis can end up several degrees out of line with the link at full bump, full droop or axle twist. Check the angle with the real assembly moved through its full travel (bump stop, droop limit, articulation), and include the spacers, bracket faces and tube ends next to the bearing, as above. SYZ's rod end misalignment calculator includes a suspension-geometry example, and the same geometry applies to a bearing in a bracket. For rod end joints, K11-Q1-01 covers spacers that give extra tilt.
Thrust arrives when a link also takes side load, for example a trailing arm that also locates the axle sideways, or a bracket pushed sideways in hard cornering. Illustrative: Fr = 20 kN with Fa = 4 kN gives Fa/Fr = 0.2, Y = 1 in the table quoted in Q1-03, and P = 24 kN. With Fa = 6 kN, Fa/Fr = 0.3, Y = 1.5 and P = 29 kN. A 50% rise in thrust lifts the equivalent load by about 21%. The table assumes a constant load, so on a suspension this is a comparison, not a rating. If the layout puts large, reversing thrust on a radial bearing, change the layout (a separate lateral locator) rather than ask one bearing to do both jobs. The spherical plain bearing buyer's guide lists the bearing types.