Stiffness element file
| Symbol | - |
|---|---|
| Unit | - |
| Attribute-ID | external_stiffness_element |
| Numeric ID | 4934 |
| Value Type | PMFile |
The stiffness element file for specifying an external stiffness matrix should be constructed as follows:
Example file: stiffness_element.txt
NUM_COUPLING_POINTS = 4
PATH_TO_STIFFNESS_MATRIX = C:\yourpath\reduced_matrix_in_harwell_boeing_format.txt
SHAFT_ID = 5 5 7 7
U_POSITION_ON_SHAFT = 440.0 637.0 53.0 446.0
COORD_DEF = 1 2 3
Explanation of attributes:
- NUM_COUPLING_POINTS: The number of coupling points (bearings and/or couplings)
- PATH_TO_STIFFNESS_MATRIX: Path to the stiffness matrix file. The file must be in Harwell-Boeing format. It is assumed that the matrix is complete; therefore, it must be a square matrix with n² entries, where n=6*NUM_COUPLING_POINTS. The reduction should be performed using the unit system [mm] for the geometry.
- SHAFT_ID: IDs of the shafts in the Workbench model at which the coupling takes place. The order of the entries in the stiffness matrix must correspond to the sequence in this line.
- U_POSITION_ON_SHAFT: U coordinate at which the coupling to the shaft takes place (the order of the entries must correspond to the SHAFT_ID line)
- COORD_DEF: The coordinate system of the stiffness matrix is determined using the position of three coupling points. The three numbers in the COORD_DEF line refer to entries in the SHAFT_ID/U_POSITION_ON_SHAFT line.
- In this example, coupling points 1, 2, and 3 in the SHAFT_ID line are used.
The first coupling point is the origin of the coordinate system for the reduced stiffness matrix.
- In this example, the location of the origin is the u coordinate at 440.0 mm on Shaft 5.
The first two coupling points referenced determine the direction of the first coordinate (u direction) of the coordinate system for the stiffness matrix.
- In this example, the u direction points along the u coordinates on Shaft 5 from 440.0 mm toward 637.0 mm.
The third coupling point indicates the third coordinate direction (w direction) of the coordinate system for the stiffness matrix.
- In this example, the w direction points from the u-coordinate at 440.0 mm on Shaft 5 to the u-coordinate at 53.0 mm on Shaft 7.
The default assignment for the selection of the coupling locations is 1 2 3.
If the coordinate system is defined based on the u and v directions, append the keyword USE_COORD_DEF_BASED_ON_UV in a new line at the end of the file.
In the event that all coupling locations are co-linear, the coordinates are defined as follows:
COORD_DEF = 1 2 0
ANGLE = 75.0
An angle (in degrees) by which the third coordinate axis of the stiffness matrix is rotated with respect to the third coordinate axis in the global coordinate system of the gearbox must be specified in the ANGLE line. A positive angle is defined as rotation about the negative first coordinate direction. A zero degree angle indicates that the third coordinate direction of the stiffness matrix points in the direction of the third coordinate direction in the gearbox coordinate system.
For an external stiffness element for a planet carrier, an additional specification must be made for the shafts integrated in the FEM model (carrier shafts and/or planet pins). The SHAFT_REPLACED_BY_FEM line, containing the IDs of all shafts integrated in the FEM planet carrier, must be inserted before the coordinate definition:
SHAFT_REPLACED_BY_FEM = 4 6
In this example, the shafts with IDs 4 and 6 are integrated into the FEM model. For example, these could be the two carrier shafts of a dual-plate carrier assembly that are integrated in the reduced stiffness matrix. The same applies for planet pins integrated into the FEM model.
The challenge for planetary stages is to obtain the IDs of the individual planetary shafts/pins (depending on the coupling). In both the 3D model and the calculation, the number of individual planet shafts and pins corresponds to the number of planets. The IDs of these components, not the ID's of the planetary shafts/pins in the Model Tree, are relevant for the stiffness element. The following procedure is recommended:
The mounting positions of the planetary shafts can be inferred from the following diagram in the planetary stage report.
The mounting angles in this diagram (@0°, @120°, @240°) can be found in the attribute overview of the individual pin or planet shaft components (depending on the coupling) -> geometry -> mounting angle of the individual planet in the model:
The component name contains the ID of the corresponding shaft in brackets (in this figure: 121, 122, 123). When assigning multiple planetary stages, it is helpful to know that the IDs in newly added planetary stages are assigned in ascending order; i.e., the ID of pin 120 in the Model Tree is followed by the IDs of the individual pins 121, 122, and 123. The number of individual pins corresponds to the number of planets. If the individual pins are part of the FEM model, their IDs must be included in the SHAFT_REPLACED_BY_FEM line. The IDs of the individual pins should also be used in the SHAFT_ID line when connecting the planetary bearings.
If there are no shafts integrated into the FEM model of the planet carrier, no entries should be included to the right of the equal sign in the SHAFT_REPLACED_BY_FEM line.
If there are loads on the shafts that are integrated into the FEM model of the planet carrier, such as driving torque on the carrier shaft, a corresponding coupling point must be provided in the reduced stiffness matrix. This must be considered in the lines NUM_COUPLING_POINTS, SHAFT_ID, and U_POSITION_ON_SHAFT.
Refer to the FEM planet carrier tutorial for clarification.
For more information on specifying external stiffness matrices, see:
THEMIS - Abschlussbericht AK Wb 711 I AB 1250 (in German)
The identifiers from the final report can also be used for casings (ANZ_KOPPELPUNKTE, DATEIPFAD_STEIFIGKEIT, IW, UK, KOORD_DEF and WINKEL); the ID's of the shafts in the Workbench model are entered under IW.