Defining an expression for the stiffness of a bushing

Hi everyone, I would like to enter a 3rd-order polynomial, -4e6*x^3-2e-8*x^2-148372x for the stiffness of a bushing. I am using the following expression:

'Poly(DX,0,0,-148372,-2e-8,-4e6)'

or in python API:

bush1.kx.type = 'EXPR'

bush1.kx.expr = "`Poly(DX,0,0,-148372,-2e-8,-4e6)`"

It gives me an error.

Thanks

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Orestis_YB

Hello @AliH,

what error are you getting? You can check the errors also in the log file located in run directory? Is DX() function properly set up ? Which marker's IDs are you using? If you have an entity already defined there (i.e. joint, bushing etc) you can use the DX() of the internal I and J markers (i.e.` {joint_1.DX}`).

Hope this helps!

Best regards,
Orestes

AliH

Hi @Orestis_YB ,

Thanks for the reply.

The DX function was properly set up and tested on a spline. The bushing ID is : 301001

I also tried the following one:

Poly({bush0.DX},0,0,-148372,-2e-8,-4e6)

It didn't work. For the 'Poly(DX,0,0,-148372,-2e-8,-4e6)', the log file shows the following text in the log file:

Reading 608 markers...
Reading 121 rigid bodies...
Reading 15 variables...
Reading 1 xy-curves (splines)...
Reading 1 implicit diffs...
Reading 1 explicit diffs...
Reading 3 revolute joints...
Reading 1 translational joints...
Reading gravity vector...
Reading 117 action forces...
Reading 2 event sensor evaluates...
Reading 3 event sensors...
Reading 238 marker displacement requests...
Reading 238 marker velocity requests...
Reading 238 marker acceleration requests...
Reading 118 marker force/torque requests...
Reading 2182 user-expressions requests...
Reading 117 graphics...
Reading 342 contact force elements...
ERROR: Failure in parsing "Poly(DX,0,-148372,-2e-8,-4e6) - (0.1)*VX(Gen5_LG_L1_Altair_thick_elements_2026.sys_Gen5_LG_L1_Altair_thick_elements_2026.bush0.i, Gen5_LG_L1_Altair_thick_elements_2026.sys_Gen5_LG_L1_Altair_thick_elements_2026.bush0.j, Gen5_LG_
L1_Altair_thick_elements_2026.sys_Gen5_LG_L1_Altair_thick_elements_2026.bush0.j)"!
License OK.

ERROR: Simulation failed due to error encountered in analysis!

Forced termination from API_SendOffCommandSimulate!

run.log

Orestis_YB

Hi @AliH,

if you want to monitor the X displacement of the bodies attached to the bushing then you can use `{bush0.DX} ` as independent variable. Be careful the symbols in the expression are not quotes are backquotes. It's the key under the ESC key. So the polyline expression should look like below

poly.txt

Hope this helps!

Best reagrds,
Orestes

AliH

Hi @Orestis_YB ,

twoseg - busing test.stmod

I tried the expression you sent me, it didn't work.

Please, have a look at the attached file.

Thanks,

Ali

Orestis_YB

Hello @AliH,

I was able to complete the simulation without changing anything. I am using Inspire 2026.

Best regards,
Orestes

AliH

Hi @Orestis_YB ,

You're right. The v2025.1 was the source of issue. I tried writing another expression in form of '-148372*{bush1.DX}-2e8*{bush1.DX}*{bush1.DX}-4e6*{bush1.DX}*{bush1.DX}*{bush1.DX}'. Again, v2026 works, but v2025 does not.

Thanks a lot,

Ali