Inputs
Radial load Fr, axial load Fa, the threshold e, and the two factor pairs (X, Y) for Fa/Fr≤e and for Fa/Fr>e -- all from the bearing catalogue or from ISO 281.
Rolling bearings · ISO 281
Dynamically equivalent bearing load
Calculate the dynamically equivalent radial load P=X*Fr+Y*Fa to ISO 281 from radial and axial force. The calculator automatically selects the applicable load branch (Fa/Fr≤e or Fa/Fr>e) from the catalogue factors you enter.
Only the entered X, Y and e values are evaluated; no manufacturer or ISO 281 table is embedded in this calculator or invented.
PISO 281
02
Dynamically equivalent load
Set the inputs and calculate.
Inputs and method
Dynamically equivalent bearing load
Calculate the ISO 281 dynamically equivalent radial load P from radial and axial force and your own catalogue factors X, Y and e.
Calculation
The ratio Fa/Fr determines the applicable load branch: if Fa/Fr ≤ e, X1/Y1 apply, otherwise X2/Y2. The equivalent load is P=X·Fr+Y·Fa. This calculator does not set any X/Y/e values itself; it only evaluates the formula with the entered catalogue values.
Example
Fr=1,000 N, Fa=500 N and the catalogue row e=0.44, X2=0.56, Y2=1.00 give Fa/Fr=0.5>e, so P=0.56*1,000+1.00*500=1,060 N. The factors come from the bearing catalogue and are shown here for illustration only.
Sources and limits: ISO 281:2007, formula structure P=X·Fr+Y·Fa; the specific X/Y/e values are manufacturer-specific catalogue data or ISO 281 table values and are deliberately not embedded here.
Dynamically equivalent bearing load in detail
Calculate the ISO 281 dynamically equivalent radial load P from radial and axial force and your own catalogue factors X, Y and e.
What is the dynamically equivalent load P?
A rolling bearing loaded simultaneously in the radial and axial directions does not behave, in terms of rating life, as if simply under the sum of both forces. ISO 281 therefore combines both contributions into a single 'dynamically equivalent' purely radial substitute load P that gives the same rating life as the actual combined load.
Formula and variables
P = X·Fr + Y·Fa
Fa/Fr ≤ e ⇒ X=X1, Y=Y1Fa/Fr > e ⇒ X=X2, Y=Y2
| Symbol / input | Meaning |
|---|---|
| Fr, Fa | Radial and axial load on the bearing at the operating point considered. |
| e | Catalogue threshold of the load ratio Fa/Fr that switches between the two factor pairs. |
| X1, Y1 / X2, Y2 | Catalogue factors for the Fa/Fr≤e and Fa/Fr>e cases respectively. |
| P | Dynamically equivalent radial load -- the direct input for the ISO 281 rating-life calculator. |
Choose the inputs correctly
Radial load Fr, axial load Fa, the threshold e, and the two factor pairs (X, Y) for Fa/Fr≤e and for Fa/Fr>e -- all from the bearing catalogue or from ISO 281.
How to use the calculator
Enter the radial load Fr and axial load Fa at the operating point considered. Then take the threshold e and the two X/Y factor pairs for the Fa/Fr≤e and Fa/Fr>e cases from your bearing's datasheet. The calculator automatically selects the matching pair and outputs P directly.
Worked example
Fr=1,000 N, Fa=500 N and the catalogue row e=0.44, X2=0.56, Y2=1.00 give Fa/Fr=0.5>e, so P=0.56*1,000+1.00*500=1,060 N. The factors come from the bearing catalogue and are shown here for illustration only.
How should P be interpreted?
P is not a physically real force but a calculation quantity: it answers the question 'what purely radial load would cause the same life consumption as the actual combined radial/axial load'. That exact value is what gets substituted as P into the ISO 281 rating-life formula.
Forces in N or kN; X, Y and e are dimensionless catalogue factors.
Typical applications
Determining the P input for the ISO 281 rating-life calculator when a bearing carries a combined radial and axial load and the manufacturer's catalogue X/Y/e values are already known.
Assumptions, limits and common mistakes
ISO 281:2007, formula structure P=X·Fr+Y·Fa; the specific X/Y/e values are manufacturer-specific catalogue data or ISO 281 table values and are deliberately not embedded here.
Common mistake: Don't enter only one X/Y pair and fill the other with zeros when the catalogue gives both cases -- otherwise a wrong load ratio produces a wrong result. e is not a fixed value for all bearings; it depends on the specific bearing type (and, for some designs, on Fa/C0) and must come from the applicable catalogue.
Frequently asked questions
Where do I get the values for X, Y and e?
From the bearing manufacturer's catalogue or from the tables in ISO 281 Annex A, matched to the specific bearing design (e.g. deep groove ball, angular contact ball, cylindrical roller).
How do I check that my entered X/Y/e values belong together?
Catalogue rows are tabulated so that both load branches give the same value at the transition Fa/Fr = e: X1 + Y1*e = X2 + Y2*e. For deep-groove ball bearings with X1=1, Y1=0 and X2=0.56 this reduces to the familiar relation Y2*e ~= 0.44. If your entry differs there by more than 5%, the calculator flags it -- the four factors then most likely do not come from the same catalogue row.
What does the threshold e mean?
e separates two load regions: as long as the ratio Fa/Fr is small enough (≤ e), the axial load is approximated as negligible or only lightly weighted; once Fa/Fr exceeds e, the axial load enters P with a substantially larger Y factor.
Why doesn't this calculator supply its own X/Y/e values?
These values are design-specific catalogue knowledge from bearing manufacturers, or table values from ISO 281, which are not reproduced here without a verified source. The calculator only evaluates the formula with your own, traceable inputs.
What do I do with the result P?
P is directly the 'equivalent dynamic load' input for the existing ISO 281 rating-life calculator, which uses it to determine the basic or modified rating life.