What do the static and dynamic ratings mean?
Two different numbers answer two different questions.
The static load rating C₀ is the radial static load that does not cause permanent deformation of the parts while the bearing is stationary, with no pivoting, swivelling or tilting. It assumes normal room temperature and surrounding parts that are strong enough.
The dynamic load rating C is a calculation value for estimating life under dynamic loads. It does not itself give the load the bearing can carry while moving. For that you must also consider the type of load, the swivel or tilt angle, the speed, the allowed clearance or friction, lubrication and temperature. In practice C is used to calculate service life when the bearing oscillates under load, and it is based on the contact pressure of the spherical sliding surface.
Why can't you compare ratings from two makers?
Because there is no common definition. Load ratings of different makers are not easily comparable, since there is no uniform standardised definition for spherical plain bearings. ISO 12240-1 fixes dimension series, dimensions, tolerances and radial clearance. Designations, load ratings and life calculations are left to each maker (Q1-01).
How do you combine radial and axial load?
A spherical plain bearing carries radial and thrust load together. The usual way is to fold the thrust into one equivalent radial load, in the conventional form
P = Fr + Y · Fa
where Fr is the radial load, Fa the axial load and Y a thrust load factor that rises as the thrust share grows. Take Y from the maker's table for your series; the exact equation is each maker's to state. One maker's table (THK), valid when the size and direction of the load are constant, gives Y = 0.8 when Fa/Fr is up to 0.1, 1 up to 0.2, 1.5 up to 0.3, 2.5 up to 0.4 and 3 up to 0.5. In that form the equivalent load works out at 1.08, 1.20, 1.45, 2.0 and 2.5 times Fr at the top of each band. Y is not a multiplier on the whole load: reading 0.8 as "P = 0.8 × Fr" would under-estimate the load.
Example (illustrative): Fr = 40 kN and Fa = 8 kN. Fa/Fr = 0.2, so Y = 1 and
P = 40 + 1 × 8 = 48 kN.
How do you check the static rating?
Require C₀ ≥ S₀ × P, where S₀ is the static safety factor. A common catalogue guide is 3 or more, taking into account the stiffness of the shaft and housing. With P = 48 kN and S₀ = 3 you need C₀ of at least 144 kN. A bearing listed at C₀ = 160 kN (illustrative) would give 160 ÷ 48 = 3.3.
This is the same definition of safety factor as for rod ends: static capacity divided by the load. The factors differ because the products differ. For rod ends, the usual limits start lower and rise with the way the load acts, from 2 to 3 for a constant load in one direction to 5 to 8 for a load in varying directions (K01-Q1-02). For spherical plain bearings the general figure is 3 or more. If your load reverses, do not go below 3, ask the maker for its factor, and note that using the rod-end figures of 5 to 8 would raise the same 48 kN to a required C₀ of 240 to 384 kN.
A bearing that carries a stationary load, or rocks only slightly, can be selected on C₀. Continuous swinging under load is a dynamic case even when slow, and needs the checks below.
How do you check pressure and speed when it swings?
Estimate the contact pressure as the equivalent load divided by the projected area, roughly sphere diameter dk times the outer ring width C, and the sliding speed from the swing angle and rate (Q1-04). Take a bearing with dk = 50 mm and C = 28 mm (illustrative, not a catalogue part), swinging 20° in total, 10 times a minute:
- p = 48,000 N ÷ (50 × 28 mm²) = 34.3 MPa
- V = π × 50 × (20 ÷ 180) × (10 ÷ 60) = 2.9 mm/s
- pV = 34.3 × 2.9 ≈ 100 MPa·mm/s
Compare that pV with the maker's allowed value for the sliding pair and the load direction. The allowed value is the maker's to give: it differs between a greased steel pair and a PTFE liner, and between a one-direction and an alternating load.
What else goes into the life estimate?
The life calculation uses C, the equivalent load, the swing angle, the sphere diameter and factors for load direction, lubrication, temperature and material. Those factors, and how a load that changes direction raises the calculated life, are covered in Q1-04. Gather the inputs now: the load history, the swing angle and rate, the temperature, the lubrication and the clearance you will accept at end of life.
How do you allow for shock and reversing loads?
Put them in the load, not just in the factor. A reversing load and a shock load put more on the bearing than a steady one at the same average. Load factors for shock, vibration and varying speed adjust a basic calculation, and temperature, lubrication and contamination also change the effective capacity. There is no general shock factor. Ask the maker for its factor for your series, and estimate or measure the peak load rather than guessing. Size for the highest load that can really happen, and say which one you used.
What should you give the maker?
The peak and normal radial and axial loads, the direction and any reversals, the swing angle and how often it swings, the speed, the temperature, how the bearing is lubricated, and what happens if it fails. A rating on its own cannot select the bearing.
How do you find the load on an off-road or race suspension link?
Size the bearing for the peak the wheel can push through the link, not for the weight on the wheel. Work back from the wheel force through the geometry. A link pinned at both ends carries only axial force along itself, so for a link and a lever in the same plane (a pushrod or shock link, for example):
link force = wheel force × (wheel-force arm ÷ link arm) ÷ cos φ
where the arms are measured from the lever's pivot and φ is the angle between the link and the perpendicular to the lever.
Example (illustrative). A static corner load of 4 kN, and a landing peak taken as three times that, 12 kN. The wheel force acts 300 mm from the arm pivot, the link attaches 150 mm from it, and the link is 20° off the perpendicular. Link force = 12 × (300 ÷ 150) ÷ cos 20° ≈ 25.5 kN. With S₀ = 3 the static check asks for C₀ of about 77 kN. For a bearing with dk = 50 mm and C = 28 mm (the size used above), the peak pressure is 25,500 ÷ (50 × 28) = 18.2 MPa. The 3× peak and S₀ = 3 are placeholders, not recommended values.
The peak decides the static check; the normal running load decides pressure and pV (Q1-04). Get both from a measurement or from the vehicle designer, and state which. SYZ's motion ratio and link load calculator does the geometry step, and the spherical bearing PV calculator the pressure and sliding-speed check.