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KISSsys 03/2016 – Instruction 020

Inclusion of casing stiffness in KISSsys calculations

09/04/2016

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Contents

1. Introduction .............................................................................................................................................. 3 2. Step by step instructions .......................................................................................................................... 3

2.1 Data preparation and input. ............................................................................................................ 3 2.1 Bearings offsets. ........................................................................................................................... 10 2.2 Solution process. .......................................................................................................................... 15 2.3 Results. ......................................................................................................................................... 17

3. General Comments ................................................................................................................................ 18

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1. Introduction

The scope of this document is to present the steps that must be followed inside KISSsys in order to take

into account the deformations of the housing of a gearbox in its subsequent analysis. The user of these

instructions must be able to interact with the model as an administrator, since the calculation results in

alterations in the model.

The applied methodology in the calculation is based on a reduced stiffness matrix of the housing, as

derived by a Finite Element (FE) package. The stiffness matrix is reduced in the sense that the FE nodes

included in it correspond to the positions of shafts’ supports on the housing. The deformation of the housing

is then included through modifying the offsets of the supports.

2. Step by step instructions

2.1 Data preparation and input.

The first step is to include a housing (kSysCasing) element in the KISSsys model. Right clicking on

this element, we can define all the input necessary for the analysis. As a first step, we select the

ImportStiffnessMatrix function, which opens a file open window, where we can select the file that contains

the FEM nodes coordinates and the stiffness matrix data. The file must also describe the system of units

used in the FEM calculation.

Figure 2.1 Selection to import the FEM stiffness matrix data.

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The stiffness matrix is a text file with a specific format. More precisely, the first lines of this text file must

include information on the system of units used in the stiffness matrix derivation and master node positions.

The following format must be followed:

Figure 2.2 Stiffness matrix file format.

When using ABAQUS, the exported stiffness matrix plus the above definitions will result in a text file as

shown below:

Figure 2.3 Stiffness matrix file including header in required format based on ABAQUS

When using ANSYS, the exported stiffness matrix plus the above definitions will result in a text file as

shown below:

UNIT SYSTEM (1 = SI, 2 = CGS, 3 = BFT, 4 = BIN, 5 = MKS, 6 = MPA, 7 = uMKS)

Active Unit System = 1

---------------------------------------------------------------------------------------

MASTER NODE POSITION

Number x-coord y-coord z-coord

1 ******* ******* *******

2 ******* ******* *******

STIFFNESS MATRIX

******************************

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Figure 2.4 Stiffness matrix file including header in required format based on ANSYS

When using NASTRAN, the exported stiffness matrix plus the above definitions will result in a text file as

shown below:

Figure 2.5 Stiffness matrix file including header in required format based on NASTRAN

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It can be seen that in case of NASTRAN only the active units have to be additionally given, since the nodal

coordinates are already inside the pch file.

After the STIFFNESS MATRIX header, the stiffness matrix data must be present in the format given by the

FE program (i.e. tab separated data for ANSYS, “mtx” data for ABAQUS, or “pch” data for NASTRAN). After

reading the stiffness matrix file, KISSsys opens two new tables with the read in data for review.

It has to be noted here that the procedure to derive the stiffness matrix is different between FEM packages.

In general though, we can say, that the user has to create a substructure (super-element) in the FEM

program, having as external nodes the defined bearings nodes. Then the user can ask the program to

derive the stiffness matrix of this substructure. More information can be found in the documentation of the

FEM package used.

In the next step, we have to position the housing correctly with respect to the KISSsys coordinate system

(CS). This is achieved by selecting the ResetPosition function:

Figure 2.6 ResetPosition function.

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In the window that opens, we can either input directly the origin and the axes orientation of the housing with

respect to the coordinate system of KISSsys, or use the ThreePointsPositioning function, that opens the

following window:

Figure 2.7 Three points positioning window.

There we have to select three KISSsys elements and their respective nodes in the FEM model (to this end

a mapping between KISSsys bearing names and FEM node IDs must be known to the user). The three

points selected must not be collinear. The coordinates of the selected FEM nodes are shown in the window

for validation. Note that the coordinates of the FEM nodes are shown in this window exactly as imported

with the ImportStiffnessMatrix function and we assume that their units are the same as the ones of the

stiffness matrix. The function returns the correct positioning values for the housing in the normal

ResetPosition window, which will result in the exact matching of the origins of the two coordinate systems

and their correct orientation. The function returns an error in case the distance between the given points is

different (within a tolerance) when measured in the two different coordinate systems (meaning that either

the points are wrong, or there is a scaling between the two models).

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This positioning can be visually validated if a step file of the housing geometry has been inserted. This can

be done using the Dialog option under right clicking on the housing:

and select “Read file” in the following window:

Figure 2.8 Selection to import step file of housing geometry.

It is advised to use a simplified version of the housing geometry (e.g. only wireframe), especially for big

housing files, so as not to overload the KISSsys model.

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If it is not possible or not desired to load a step file in the KISSsys model, another way to check the correct

positioning of the housing is using the ShowNodes function when right mouse clicking on the housing

element:

Figure 2.9 ShowNodes function.

This function shows all the imported FEM nodes (and their IDs) as red dots in the 3D view inside KISSsys

as shown below.

Figure 2.10 Nodes of FEM model / stiffness matrix shown in KISSsys 3D viewer

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Two other useful function available when right mouse clicking on the housing element are the

ShowMasterNodesPositionTable and ShowStiffnessMatrix. They can be used to show the imported FEM

node coordinates and stiffness matrix data respectively:

Figure 2.11 ShowMasterNodesPositionTable and ShowStiffnessMatrix functions.

2.1 Bearings offsets.

In case the housing has some initial deformations (e.g. due to external loads applied on it, temperature

gradients, or misalignments), these deformations must be taken into account as initial offsets of the

bearings. These initial offsets can for example be calculated using FEM.

There are several ways to import the initial offsets in the calculation. The straightforward, and most

cumbersome one, is to open all shaft calculation files and enter the offsets directly in the respective

bearings. Another way is to import the default “BearingCalculations” table in the model and type in (or

copy/paste) the initial offsets there. Remember to refresh the model after the initial offsets are set. Finally, a

third way is to read-in the initial offsets through the FEM stiffness matrix file. To achieve this, the following

lines must be added to the stiffness matrix file:

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Figure 2.12 Initial offsets’ lines in the Stiffness matrix file

The coordinate system and the units of these initial offsets refer then to the ones of the stiffness matrix file

and the order is the same of the one of the master nodes. If you want to use these FEM offsets, you must

click “Yes” in the first window that appears after selecting to calculate, and asks if you want to take into

account the offsets defined inside the FEM stiffness matrix file. In this case, these offsets are transferred

automatically to the FEM coordinate system and units and saved as initial offsets.

Since the calculation changes the bearing offsets, the initial offsets are saved in the KISSsys model before

the first calculation. If for any reason you want to get back these initial values in the model, use the

RestoreBearingsOffsets function:

Figure 2.13 RestoreBearingsOffsets function.

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If you want to change the initial offset values that are saved (and keep the new ones as initial for all

subsequent calculations), use the SaveBearingsOffsets function:

Figure 2.14 SaveBearingsOffsets function.

This function uses the currently defined bearing offsets values as initial ones, overwriting any previously

defined initial offsets (it takes a snapshot of the current offsets of the model).

In a similar way, you can also revert back to the latest calculated offset results, by selecting

“RestoreOffsetResults”:

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Figure 2.15 RestoreOffsetResults function.

Finally, you can also reset all bearing offsets to zero, by selecting the ResetBearingOffsets option:

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Figure 2.16 ResetBearingOffsets function.

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2.2 Solution process.

After the above steps are finished, the user can initiate the calculation using the button . The first

window that opens then asks if we would like to take into account in our calculations any already defined

bearing offset values. This can be useful for example if there are any preloads already defined in the

bearings in the KISSsys model. On the other hand, selecting “No”, all initial offsets in the bearings are

ignored and set to zero internally for the solution.

Then we get a message with the nodes that could not be mapped to KISSsys bearings and the respective

distances to the closest bearing for each of them (if all nodes are mapped to bearings, this message will not

be shown):

Figure 2.17 Message with nodes that could not be mapped.

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Decide if the resulting mapping is accepted and continue the calculation, or we prefer to cancel the

calculation and check the model again. If we see that a node is very close to a bearing that should be

mapped on it, then we can change the tolerance used in the mapping from the properties window of the

housing element:

Figure 2.18 Tolerance value in millimetres used in nodes mapping.

In case we want to check the difference between different housing designs, then we can import all of them

in the KISSsys model as separate housing elements and then, before starting the calculation, select the one

to use (this window shows up at the beginning of a calculation, in case more than one housing elements is

present):

Figure 2.19 Housing selection window.

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2.3 Results.

After the calculation is finished, a message appears notifying if the calculation was completed successfully,

or any error was encountered. If the calculation was successful, a new table is generated containing the

resulting offsets and tilting of mapped bearings, due to the housing deformation. The units of the results are

the default ones used in KISSsys, i.e. mm for offsets and rad for tilting:

Figure 2.20 Example output table.

The results are given in both the KISSsys and the FEM coordinate system. In this table, the mapping

between FEM nodes and KISSsys bearings is shown for validation.

At the end of a successful calculation, all the shaft calculations of the KISSsys model include the resulting

offset and tilting values due to the housing deformation and hence they can be used directly to more

accurately analyse the gearbox.

Regarding the accuracy of the final result, the percentage error is shown in the messages window of the

KISSsys screen. Since the offset of the bearings affects in general the applied forces, the whole solution

runs in an iterative way as described in the following diagram:

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Figure 2.21 Iteration diagram of the calculation.

The percentage error shown refers to the difference in the reactions and offsets between the two last

iterations. In case no convergence is achieved in the maximum number of iterations set by the user, then

the program returns a message and keeps the results of the last iteration. The maximum number of

iterations can be set in the properties window of the housing element (it is proposed to use at least 3):

Figure 2.22 Maximum number of iterations setting.

3. General Comments

As it was mentioned in the beginning of this instructions document, the user must have

administrator privileges, in order to follow these steps. After the calculation is finished successfully

by an administrator, then any user can run the calculation again, though without being able to load

another stiffness matrix or change the KISSsys model in any other way.

In case of “soft” housings with large deformations, the number of iterations can be exceeded.

Nevertheless, the solution of the last iteration can still be used, after it is checked by the user

(taking also into account the percentage error).