What does "life" mean for a bearing that swings?
It is the number of swings until the bearing no longer works properly.
The end point is wear: the radial clearance has grown, or the temperature has risen, until the joint can no longer do its job. Because wear depends on the material pairing, the size and direction of the load, the lubrication and the sliding speed, a calculated life is an empirical estimate for planning, not a promise. The dynamic rating C is one input to the calculation (Q1-03).
How do angle, frequency and speed fit together?
The swing angle and the swing rate give the sliding speed.
Each swing carries the ball surface a certain distance along the ring. For a swing of total angle θ, repeated f times a minute, on a bearing with ball diameter dk (taken from the maker's table, not the bore):
sliding speed V = π × dk × (θ ÷ 180) × (f ÷ 60), in the same length unit per second
For example, a bearing with a 30 mm ball diameter swinging 30° back and forth 20 times a minute slides at about 5.2 mm/s. The ball is larger than the bore, so using the bore in place of dk understates the speed. Makers define angle and frequency in different ways: some use the half angle, some count rocking motions rather than cycles. Always check whether a figure means one side or the total, and a cycle or a half cycle.
What is the pV value?
It is the contact pressure multiplied by the sliding speed, and it indicates how hard the sliding surfaces are working and how much heat they make. Pressure sets how hard the asperities are pushed together; speed sets how often that contact is renewed and how fast the friction work turns into heat.
The contact pressure is the load divided by the area carrying it, roughly the sphere diameter dk times the outer ring width C: p = P ÷ (dk × C). Continue the example above with an equivalent load P of 6 kN and an outer ring width C of 22 mm (illustrative numbers). Then p = 6,000 ÷ (30 × 22) = 9.1 MPa, and pV = 9.1 × 5.2 ≈ 48 MPa·mm/s. That figure is then compared with the maker's allowed pV for that sliding pair.
The allowed pV depends on the load, the lubrication and the cooling. For continuous motion under a load in a constant direction, the heat has nowhere to go and a limited pV applies. Where the load direction changes, the loaded zone moves around the sphere, each spot gets time to cool, and a higher pV can be used.
One bearing maker (THK) gives a speed limit for its own bearings: up to 100 mm/s in oscillating motion, or up to 300 mm/s in rotary motion, in favourable lubrication. It is not a general limit.
There is no single universal formula for oscillating velocity and manufacturers define it differently, so check your supplier's method before final selection.
Why do small angles and very slow or very fast speeds make life worse?
Because grease may not reach the surface. A lubricant film needs sliding to draw fresh grease into the contact. At a very small angle the same strip of surface carries the load swing after swing and nothing is drawn back in. At a very low speed the film that forms is too thin to separate the steel, and at a very high speed heat thins and oxidises the grease. A load acting in one direction only is worse than an alternating one for the same reason: the unloaded side gets fresh grease, the loaded side does not. Advice for lubricating bearings of this type is in Q1-08.
What else changes the life?
A life estimate also carries factors for the direction of the load, lubrication, temperature, the size of the sphere and the material. In one maker's table the load-direction factor is 1 for a fixed load and 5 for an alternating one, and the lubrication factor is 1 with regular lubrication against 0.08 without. A higher factor means a longer calculated life: the alternating load calculates to a longer life than the fixed one, and running without regular lubrication cuts the calculated life to a fraction. These factors are one maker's figures for its own bearings; other makers use other ones.
What do you need to give the maker to get a life estimate?
The load and its direction, the swing angle (one side or total), the number of swings per minute, the speed, the temperature, how the bearing is lubricated, and the clearance or friction you will accept at the end of its life. Without these, a life figure is a guess.
What does the duty look like on a suspension pivot?
Mostly small, irregular movement with the occasional large one, so an average sliding speed hides what matters. On a rough surface a control-arm bearing may move a degree or two around ride height several times a second, and travels its full angle only on a big bump or in axle articulation.
Compare two cases for the 30 mm ball used above (illustrative). 20° in total at 60 cycles a minute gives V ≈ 10.5 mm/s, and each cycle slides about 10 mm there and back over a band about 5 mm wide. 2° in total at 300 cycles a minute gives V ≈ 5.2 mm/s, but only about 1 mm per cycle over a band about 0.5 mm wide. The speeds are the same order of magnitude; the second case keeps working the same narrow strip, under a load that mostly acts one way (the vehicle's weight), which is exactly the case where grease is hard to draw back in. A pV figure can look comfortable while that strip wears.
So describe the duty as a spread: how much time is spent at small angles, the common swing, and the rare full swing, and note that the biggest angles often come with the highest loads. Give the maker that picture rather than one average. SYZ's spherical bearing PV calculator will run pV for each condition.