Choose the reference
Use the same reference load for all bands and for any comparisons between cases.
ELECTRICAL & DUTY / TOOL 28
Cubic Load Spectrum Indicator
Compare the severity of four load bands using a clearly defined cycle-weighted cubic indicator.
Before you start: required inputs
- Choose a reference load consistent with the load definition being compared.
- Group the observed or planned cycle history into four representative loads and cycle counts.
Calculation diagram
A PRACTICAL WALKTHROUGH
How to use this calculator
Describe the load history
Assign a representative load and count to each band. Use zero cycles for unused bands.
Read the cubic indicator
Review k, equivalent cubic load and full-load-equivalent cycles together. Keep the cycle weighting explicit.
Four-band 10 t reference spectrum
THE INPUTS
10,000 kg reference; 10 cycles at 10,000 kg, 20 at 7,500 kg, 30 at 5,000 kg and 40 at 2,500 kg.
THE RESULT
For 100 total cycles, k is 0.228125, equivalent cubic load is about 6,110.23 kg and cubic full-load-equivalent count is 22.8125 cycles.
What does the result mean?
01
The cubic weighting gives a heavily loaded cycle more influence than a lightly loaded cycle.
02
Equivalent cubic load is a mathematical constant-load representation of this entered cycle history.
03
Mechanism classification may use time weighting, different load definitions and standard-specific rules; this indicator does not assign an ISO/FEM group.
Watch for this
Copying this cycle-weighted k into a time-weighted duty assessment when heavy and light cycles have different running times.
Formula & model conditions +
k_cycle = Σ[(n_i / Σn_i) × (Q_i / Q_ref)³]
Equivalent cubic load = Q_ref × k_cycle^(1/3)
- Cycle counts provide the weighting. Actual mechanism stress spectra may require duration weighting, dead load and the applicable standard definition.
- This cubic indicator does not assign an ISO/FEM mechanism class and is not a fatigue-life calculation.
- Every entered band must be no greater than the stated reference load.
Common questions
Why is a full-load cycle more influential?
The load ratio is raised to the third power, so the indicator emphasizes higher loads.
Does this calculate fatigue damage?
No. A fatigue assessment needs the relevant stresses, detail category and fatigue resistance model. Load ratios alone are insufficient.