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Load spectra

Using load spectra in the system calculation

To consider load spectra in the system calculation, "consider load spectrum" must be activated in the calculations tab under the gear unit.


(1) Activates the load spectrum calculation in the system calculation.

(2) Number of points within the specified load spectrum for which the system will be solved.

(3) Calculations in which the load spectrum is considered have "load spectrum" in their name.

(4) The selection of the load spectra per gear and the distribution of the total lifetime is made in the "Power Flow" tab.

Number of points calculated in the load spectrum

The system calculation is performed for each load and speed stage. Thus, the influences from the load (such as the face load distribution) and the speed (such as the lubricant film thickness) valid for these stages are taken into account in all stages of the load spectrum. In order to reduce the calculation time for large spectra, the number of operating points for which the system calculation is solved can be specified. These operating points are distributed evenly in the spectrum. For all other load spectrum levels, the loads and speeds are interpolated and the corresponding damage fractions and lifetimes are calculated.  If the number of load points is greater than or equal to the number of stages selected in the load spectrum, all stages are calculated in the system calculation.


When specifying only a speed spectrum, two different calculation options are available. If the torque is specified in the power flow definition of the gearbox, this is kept constant and only the speed is varied. If the gearbox power is specified, the torques in the load stages are adjusted so that the power remains constant.

Specifying a load spectrum and time shares

In the load spectrum calculation, an individual load spectrum can be specified for every gear in the gear unit. The required settings can be made in the Power Flow tab of the gear unit component.


(1) Specification of the total gear unit service life.

(2) A load spectrum can be selected from the Global Database for each gear.

(3) Distribution of the total service life to the individual gears.

Time shares are used to distribute the specified total service life of the gear to the individual gears. Weighting factors are specified, and a time share is calculated from the weighting factor divided by the sum of all weighting factors multiplied by the total service life. Thus, the time shares can be entered as percentages, factors, or hours. The following entries lead to the same result:

Total number of hours: 30,000






Gear 1





Gear 2





Gear 3





If existing percentage or hourly weighting factors are used, the sum does not need to correspond to 100 % or the total number of hours. If the specified total number of hours in the table above was not 30,000 h but rather 60000 h, the individual time shares would be internally converted to 6000 h (Gear 1), 12000 h (Gear 2) and 42,000 h (Gear 3).

Calculation options

Load spectra can be considered in the following calculations in the FVA-Workbench:

Cylindrical gears

Tooth flank & tooth root load carrying capacity acc. to ISO 6336 Part 6:

  • Safety factor (spectrum) acc. to Miner

  • Static safety

  • Continuous safety

  • Various damage accumulation hypotheses can be selected

  • Stress collectives and Woehler S-N curves

  • Maximum permissible scaled total number of load cycles

Gear excitation

  • Excitation level over the load

  • Tooth force level over the load

  • Force amplitudes over the load

  • Excitation amplitude level over the load

  • Transmission deviation amplitudes over the load

  • Maximum transmission deviation over the load

  • Mean transmission deviation over the load

Bevel gears

Standard bevel gear calculation (standard calculation acc. to ISO 10300 (2014))

Comparison of variants for local calculations

  • Mean specific gear tooth stiffness

  • Velocity level at 1000 U/min

  • Efficiency

  • Double amplitude of the total working variation

  • Minimum safety against scuffing

  • Maximum Hertzian contact stress

  • Maximum flash temperature

  • Maximum tooth root stress

  • Minimum safety against pitting of the pinion

  • Minimum safety against pitting of the gear

Local damage accumulation (tooth root/tooth flank damage sum)

Rolling bearings

Rolling bearing lifetime (accumulated lifetimes)


Strength calculation acc. to FKM (2012)

Creating load spectra

Load spectra are saved as data records in the Global Database. Load spectra saved in the database can be then be added to any gearbox model as a database component.


(1) Name of the new load spectrum

(2) Creates a new data record of the type load spectrum.

Importing load spectra as .csv

Load spectra can be imported as .csv files (comma-separated values). These files can be created in Excel or another spreadsheet program. The step frequency must be specified in the first column. Load amplitudes for torque and speed can be specified in the second and third columns. The load spectra can be single or double column. Optionally, the title of the data row can be specified in the first row.


Commas must be used as a decimal separator in the data series. The default decimal separator can be configured in Excel under file -> options -> advanced.

Download sample .csv file


Load spectrum with 2 data rows for torque (factor) and speed (%). The file is shown in Excel on the left and as a .csv file in the text editor on the right.

  1. Figure 7. Import the example file
    Import the example file

    (1) Starts the .csv import

    (2) Select the file

    (3) Selection of which row includes the title and where the data series starts.

  2. Figure 8. Define the type and units of the data series
    Define the type and units of the data series

    (1,2) After successful import, the type must be specified for each data series (torque/speed). It must also be selected whether the values are percentages (%) or factors (--).

    (3) Once all information is complete, the data record can be saved in the Global Database.

Creating synthetic load spectra

Synthetic spectra can be useful in general investigations and preliminary design. They can be described mathematically using the three parameters spectrum scope H0, highest value Ŝa, and shape parameter ν. The larger the shape parameter ν, the greater the damage content of the synthetic load spectra with the same scope H0 and highest value Ŝa.

Figure 9. Synthetic spectra for different shape parameters
Synthetic spectra for different shape parameters

X axis: cumulative frequency H (log)

Y axis: amplitude, relative to the highest value of the spectrum

  1. Figure 10. Create a load spectrum
    Create a load spectrum

    (1) Creates a new load spectrum

  2. Figure 11. Start the load spectrum calculation
    Start the load spectrum calculation

    (1) Starts the load spectrum calculation

    (2) Creates a synthetic load spectrum

  3. Figure 12. Specify the load spectrum parameters
    Specify the load spectrum parameters

    (1) Unit of the load amplitudes ("--" for factor)

    (2) Number of load cycles

    (3) Highest value of the spectrum (here, 1.8 x nominal torque)

    (4) Shape parameter ν of the load spectrum

    (5) Number of steps in the load spectrum