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vk.com/club152685050Rigid-Body Dynamics and the| vkANSYS.com/id446425943-ADAMS Interface

To supply the above information, ANSYS does a sequence of analyses through a macro file called ADAMS.MAC (see the ADAMS command) in order to generate the required interface constraint modes and interface normal modes.

12.7.2. Adding Weak Springs

During the import of loads from ADAMS to Mechanical APDL, you can instruct Mechanical APDL to add weak springs to the model via the WSPRINGS command. The weak springs are added to the corners

of the bounding box of the component. The stiffnesses of the springs are many orders of magnitude less than the stiffness of the structure, and hence prevent rigid-body motion without influencing the stress results. The program takes the following steps when adding weak springs:

• To define the bounding box, the algorithm finds the nodes with the maximum and minimum coordinates. Six nodes are created by this approach. These nodes define the bounding box of the component. Because a three dimensional model is required for this approach, simple beam models that only have an extension in one dimension cannot be handled by the weak springs options.

COMBIN14 elements are used to link the six nodes of the bounding box to the ground in all three translational directions. The stiffness of the spring element is computed as k = (Emean)(10–6), where Emean is the mean value of all moduli of elasticity defined. This is a very rough approach, but one which has proven to be effective in practical applications. If the stress results are influenced by the springs, you can change the stiffness by changing the corresponding COMBIN14 real constant.

12.8. Example Rigid-Body Dynamic Analysis

This example analysis demonstrates how to model a flexible component in Mechanical APDL and then export the flexible body information to a file for use in ADAMS. The example also provides brief instructions on how to perform the rigid-body dynamic analysis in ADAMS, and details on how to transfer the loads from ADAMS to Mechanical APDL in order to perform a stress analysis.

12.8.1. Problem Description

In the linkage assembly shown below, Link3 is a flexible component. Link3 is modeled as a rectangular rod in Mechanical APDL using SOLID185 elements. The joints in ADAMS will be attached to interface points (nodes) at the middle of the holes at either end of Link3. These middle points are connected to the cylindrical joint surfaces by a spider web of BEAM188 elements.

 

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Example Rigid-Body Dynamic Analysis

Figure 12.5: Linkage Assembly

 

Link 3

U1

 

Crank

U4

Input motion

 

Link 2

U3

Link 1: Output

 

motion

 

U2

12.8.2. Problem Specifications

The figure below shows the Link3 component as it is modeled in Mechanical APDL.

Figure 12.6: Link3 Component

The following are dimensions and properties for the Link3 component.

Radius of holes (radh) = 6mm

Width of rectangular rod (width) = 25mm Thickness of rectangular rod (thick) = 10mm

Length of rectangular rod (length) = 300mm + 4*Radius of holes = 324mm

Young's modulus for rod = 7.22 x 104 MPa Poisson's ratio for rod = 0.34

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Density of rod = 2.4 x 10-9 tons/mm3

Young's modulus for beams = 2.1 x 105 MPa

Poisson's ratio for beams = 0.3

Density of beams = 0.1 x 10-9 tons/mm3

12.8.3. Command Input

/BATCH,list

 

/FILNAME,adamsout

! Define jobname

/TITLE,Export flexible component to ADAMS

!

 

/PREP7

! Enter preprocessor

!

 

! Define Parameters of rectangular rod

radh=6

! Radius of the holes in the rod

thick=10

! Rod thickness

width=25

! Rod width

length=300+4*radh

! Rod length

! Build geometry RECTNG,0,length,0,width CYL4,2*radh,width/2,radh CYL4,length-2*radh,width/2,radh ASBA,1,2

ASBA,4,3

VEXT,1, , ,0,0,thick

!

 

ET,1,SOLID185,,3

! Define SOLID185 as element type 1

ET,2,BEAM188,,,3

! Define BEAM188 as element type 2

!

 

MP,EX,1,7.22e4

! Material of the rectangular rod

MP,PRXY,1,0.34

 

MP,DENS,1,2.4e-9

 

!

 

MP,EX,2,2.1e5

! Material of the beams used for the spider web

MP,PRXY,2,0.3

 

MP,DENS,2,0.1e-9

 

!

 

SECTYPE,1,BEAM,ASEC

 

SECDATA,78.528,490.67,,490.67,,10,,,0.85716,0.85716

!

 

TYPE,1

! Set element type attribute pointer to 1

MAT,1

! Set material attribute pointer to 1

ESIZE,thick/3,0,

! Define global element size

VSWEEP,1

! Mesh rod

!

 

!Define interface points: numbers must be higher than highest

!node number already defined

N,100000,2*radh,width/2,thick/2

! Define interface point 1

N,100001,length-2*radh,width/2,thick/2

! Define interface point 2

!

 

 

NWPAVE,100000

! Set working plane to interface point 1

WPSTYL,,,,,,1

! Set working plane type to cylindrical

CSYS,4

! Activate working plane

NSEL,S,LOC,X,radh

! Select nodes on cylindrical hole

NSEL,A,,,100000

! Also select interface node

!

 

 

! Generate spider web of beams *GET,nmin,node,,num,min *GET,nnum,node,,count *SET,jj,0

TYPE,2

MAT,2

REAL,1 *DO,jj,1,nnum-2 E,100000,nmin NSEL,u,,,nmin

*GET,nmin,node,,num,min

*ENDDO

!

 

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Example Rigid-Body Dynamic Analysis

ALLS

 

 

!

 

 

NWPAVE,100001

! Set working plane to interface point 2

WPSTYL,,,,,,1

! Set working plane type to cylindrical

CSYS,4

! Activate working plane

NSEL,S,LOC,X,radh

! Select nodes on cylindrical hole

NSEL,A,,,100001

! Also select interface node

!

 

 

! Generate spider web of beams

 

*GET,nmin,node,,num,min

 

*GET,nnum,node,,count

 

*SET,jj,0

 

 

TYPE,2

 

 

MAT,2

 

 

REAL,1

 

 

*DO,jj,1,nnum-2

 

 

E,100001,nmin

 

 

NSEL,u,,,nmin

 

 

*GET,nmin,node,,num,min

 

*ENDDO

 

 

!

 

 

ALLS

 

 

!

 

 

/UNITS,MPA

 

! Define units used: millimeter

 

 

! megagram, second, newton

SAVE

 

! Save database

NSEL,s,,,100000,100001

! Select interface points

ADAMS,20,1

 

! Start ADAMS macro,

! adamsout.mnf is written

 

FINISH /EXIT,nosave

At this point you may import the adamsout.mnf file into your ADAMS model and perform a rigidbody dynamics simulation. The ADAMS model should consist of the components shown in Fig-

ure 12.5: Linkage Assembly (p. 327). After the simulation is done, export the loads acting on the Link3 component at five arbitrary “—`time steps. Name the load file loads.lod.

Once you have exported the load file, you can perform a stress analysis for Link3 in Mechanical APDL using the command input shown below.

RESUME,adamsout,db

! Resume model

/FILNAM,adamsin

! Change jobname

/TITLE,Import loads from ADAMS

! Change title

!

 

 

WSPRINGS

! Create weak springs

!

! Enter Solution and solve all load steps

/SOLU

 

/INPUT,loads,lod

! Read in 5 load steps written by ADAMS

*DO,i,1,5

! Use a do loop to solve each load step

LSREAD,i

! Read in load step

IRLF,1

! Activate inertia relief

SOLVE

! Solve current load step

*ENDDO

 

!

 

/POST1

! Enter the general postprocesser

! Write deformation and equivalent stress to graphics file /VIEW,1,1,1,1

/AUTO,1 EPLOT /TYPE,1,4 /SHOW, EPLOT *DO,i,1,5

SET,i

PLNSOL,u,sum

PLNSOL,s,eqv

*ENDDO

/SHOW,term

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FINISH /EXIT,nosave

 

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