Below a threshold e (roughly 0.22–0.44 of the radial load) thrust costs a deep-groove bearing nothing. Above it, the equivalent load jumps to 0.56Fr + YFa and life falls with the cube.
The contact angle already carries a share of axial load, so small thrusts do not change the equivalent load at all — people over-design for them. The cliff is real though: e depends on Fa/C₀, and once you cross it the Y factor can be as high as 2, so a thrust equal to the radial load can more than double P and cut life to under a fifth. Find out which side of e you are on before sizing anything.
Not on an angular-contact or taper-roller bearing, which is built for thrust and needs a minimum axial preload to run properly at all. A deep-groove bearing carrying pure thrust with no radial load is also outside the X/Y table the rule comes from — the ratio it depends on is undefined.
At 0.5× the rule says 1,000 N and the exact answer is 1,360 N — 26% low. At the point most people use it the shortcut is already outside its own 5% tolerance. It only holds between 0× and 0.31×, which is not where it gets used.
Under the threshold e the rule is not approximately right, it is EXACTLY right — thrust costs the bearing nothing at all. Cross it and the equivalent load does not ease upward, it steps: the radial term is discounted to 0.56 and the thrust arrives multiplied by about 1.6. At an equal axial and radial load the bearing is carrying more than double what the rule says.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Bearing Calculator →Below a threshold e (roughly 0.22–0.44 of the radial load) thrust costs a deep-groove bearing nothing. Above it, the equivalent load jumps to 0.56Fr + YFa and life falls with the cube. The contact angle already carries a share of axial load, so small thrusts do not change the equivalent load at all — people over-design for them.