KnowledgeBase

With scaled load spectra, load spectra can be specified as a percentage change to the load and speed as part of the system calculation. Load-time measurement series can be classified using the Global Database and then specified as a load spectrum. Alternatively, synthetic load spectra can also be created.

Number of points calculated in the load spectrum

The influences from the load (e.g., load distribution over the facewidth) and the speed (e.g., the lubricant film thickness) that apply to the load stages are considered as part of the load spectrum calculation.

In order to reduce the calculation time with large load spectra, the number of operating points for which the system is solved can be specified. These operating points are evenly distributed within the spectrum. The loads and the speeds are interpolated, and the corresponding damage percentages and lifetimes are calculated for all other steps of the load spectrum. If the number of load points is greater than or equal to the number of steps in the load spectrum, all steps are included in the system calculation.

Specifying load spectra and time shares

In the load spectrum calculation, an individual load spectrum can be specified for each gear in the gearbox. This can be configured in the Power Flow tab of the Gear Unit component.

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

(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 gearbox to the individual gears by specifying weighting factors. A time share is calculated from the weighting factor divided by the sum of all weighting factors multiplied by the total service life. The time shares can be entered as percentages, factors, or hours. The following entries lead to the same result:

If existing percentage or hourly weighting factors are used, the sum does not need to correspond to 100 % or the total number of hours. For example, if the specified total number of hours in the table above was 60,000 h instead of 30,000 h, the individual time shares would be internally converted to 6000 h (Gear 1), 12,000 h (Gear 2) and 42,000 h (Gear 3).

Creating load spectra

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

(1) Name of the new load spectrum

(2) Creates a new load spectrum data record.

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 can be specified for the torque and speed in the second and third columns. The load spectra can have one or two columns. An optional title for the data series can be specified in the first row.

Notice

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 example .csv file

Load spectrum with 2 data series 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 9. Import the example file

   

   (1) Starts the .csv import

   (2) Selects the file

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

   
   
   
2. Figure 10. Specify the type and units of the data series

   

   (1,2) After a successful import, the type must be specified for each data series (torque/speed), as well as 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 three parameters: spectrum scope H₀, highest value Ŝ_a, 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 Ŝ_a.

Figure 11. 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 12. Create a load spectrum

   

   (1) Creates a new load spectrum

   
   
   
2. Figure 13. Start the load spectrum calculation

   

   (1) Starts the load spectrum calculation

   (2) Creates a synthetic load spectrum

   
   
   
3. Figure 14. 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

   
   
   

With flexible load spectra, any combination of loads and selected gears can be calculated. The time share, operating temperature, gear selection, power flow data, and external loads can be specified for each load case. This makes it possible to consider auxiliary units that are not active in all operating states. Gearbox components with a variable transmission ratio and a specific efficiency curve can be integrated in the power flow using the generic "transmission ratio element" component.

A separate system calculation is performed for each load case, followed by an accumulation of the load cases with their respective time shares. This allows users to subsequently track the effects of the various individual operating states as well as the overall results of the calculation in the FVA-Workbench reporting,

Figure 15. Adding values by columns

Values can be added by columns from Excel by right-clicking on a cell in the Editor.

Note

Cells may not be "selected" in the Editor before right-clicking.

Transmission ratio element components

Any two shafts can be coupled via the generic "transmission ratio element" component, and a transmission efficiency curve can be specified. A transmission ratio can be specified for each load case in the flexible load spectrum.

Transmission ratio elements can be added by right-clicking on a shaft in the Model Tree.

"Measurement data" records can be added via the Global Database. A transmission efficiency curve can be defined in this dataset. The dataset can then be assigned to a transmission ratio element.

The transmission ratio of the "transmission ratio element" can be defined for each load case in the "Power Flow" tab. The associated efficiency is then determined from the characteristic curve.

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:

Gear excitation

Bevel gears

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

Comparison of variants for local calculations

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

Rolling bearings

Rolling bearing lifetime (accumulated lifetimes)

Shafts

Strength calculation acc. to FKM (2012)