Select a Units System

You work with two types of units systems in your model:

The Model Units is a dimensionally consistent units system. Simulation Mechanical stores all input and results in the Model Units system. Thus, there is only one Model Units for the model.
Although Autodesk Simulation stores the input and results in the Model Units, you can define any number of Display Units. Any input or result is shown in the Display Units you choose. You can change the units throughout the session to make it easier to enter the numerical values. For example, you know the model pressures in units of psi but the material properties are in metric SI. Activate the English (in) units to enter the known pressures in psi, then switch to Metric SI to enter the material properties.

Click the Unit Systems branch in the tree view to manage the unit systems.

Define display units

Create as many display units as appropriate. Right-click the Unit Systems heading of the tree view and select New. The Unit System dialog displays. For this unit system to appear in each new model, select the Add to tree for new models option. Do not select the option if you only want the unit system to apply to the current model.

You can also change display units for any entry. Right-click the entry in the tree view and select Edit.

Assign model units

Autodesk Simulation assigns the model units as follows:

Model Type	Model Units
CAD Solid Model	Length units transfer when you create the Autodesk Simulation file. The software chooses a units system that most closely matches the length units. As a result, the units for the other dimensions, such as force, time, and more, might not match the CAD model units.
Other Models Includes models you create manually, CAD models that do not have a unit system (such as DXF and wireframe formats), and third-party FEA models.	Click Options to set the default model units before you start a hand-built model. Choose the Analysis tab, then click Default Modeling Units. You can also right-click Model Units in the tree view and select Edit: Default Model Units. Different model units can be set for a new model. Click Override Default Units in the New dialog box. The units set only applies to the current model; the next new model uses the default modeling units. When you open a file without units, Autodesk Simulation prompts you to set the model units.

Model Type

Model Units

CAD Solid Model

Length units transfer when you create the Autodesk Simulation file. The software chooses a units system that most closely matches the length units. As a result, the units for the other dimensions, such as force, time, and more, might not match the CAD model units.

Other Models

Includes models you create manually, CAD models that do not have a unit system (such as DXF and wireframe formats), and third-party FEA models.

Click Options to set the default model units before you start a hand-built model. Choose the Analysis tab, then click Default Modeling Units. You can also right-click Model Units in the tree view and select Edit: Default Model Units.

Different model units can be set for a new model. Click Override Default Units in the New dialog box. The units set only applies to the current model; the next new model uses the default modeling units.

When you open a file without units, Autodesk Simulation prompts you to set the model units.

Note:

You can change the model units after you start the model, but none of the existing numerical values convert to the new unit system. The current values are used with a new unit label. Therefore, only change the model units when necessary and verify all input values afterwards. Consider changing the display units as an alternative to changing the model units.

For example, a 1 inch by 1 inch part created in English (in) units becomes 1 foot by 1 foot if you change the unit system to English (ft). The mesh does not automatically scale to .0833 feet by .0833 feet.

To change the model units, right-click the Model Units branch in the tree view and select Edit: Current Modeling Units. Choose Yes to continue and change the units as appropriate.

Use unit systems to enter values

To change to a different unit system, select the appropriate entry in the tree view, right-click, and select Activate. Set the unit system before you access the dialog box; you can change the units for results at any time.

Unless noted in this documentation, the results transferred from one analysis to another are converted based on the model units of both models. If Autodesk Simulation cannot determine the results file model units, then no conversion applies. Conversion applies to the following types of results:

Temperatures read from a heat transfer result for
- Linear stress analysis
- Mechanical Event Simulation (MES)
- Transient heat transfer analysis
- Electrostatic analysis
Voltages read from an electrostatic result for
- Linear stress analysis
- MES
Results read with Loads from file (temperatures and electrostatic reaction forces) are converted, as are the node coordinates.

Results and new analyses use the same unit system for versions of the software without model and display units. The software does not convert the results.

The following results, when used by another analysis, are not converted. These models must use the same Model Units:

Linear dynamic analyses that read the modal results
MES restarts that read the results from another MES analysis

Unit System dialog box

When you define model or display units, the Unit System dialog box functions as follows:

Use the drop-down for the Unit System to choose one of the predefined units system, or choose Custom to set the units for each variable. The options available are shown in the following table. Other predefined Display Units may be included in the software, but those use a Custom unit system and are not listed in the table.
For Model Units, the base units are force, length, and time. Mass is derived from these and cannot be changed. See Converting Mass Units for more details.
For Display Units, the units of mass can be independent of the force, length, and time units.
For Display Units, optionally choose Add to tree for new models. If activated, the Display Units will be added to the tree view for all new models. If not activated, the Display Units created will not be added as an entry in the tree view to new models.
The Description field is the text shown in the tree view. (This text can also be changed from the tree view. Either right-click and choose Rename, or press the F2 key to edit.)
Press the OK button.

Variable	Options for Custom	Default Variable for Predefined Unit System
		English (in)	English (in Btu)	English (ft)	English (ft Btu)	Metric mks (SI)	Metric cgs
Force	pound force (lbf)	X	X	X	X
	Newton (N)					X
	dyne (dyn)						X
	deciNetown (dN)
	dekaNewton (daN)
	kiloNewton (kN)
	1000 pounds (kip)
Length	inch (in)	X	X
	foot (ft)			X	X
	meter (m)					X
	centimeter (cm)						X
	millimeter (mm)
	micrometer (m)
Time	second s	X	X	X	X	X	X
	minute (min)
	hour (hr)
Temperature (Absolute)	°F (°R)	X	X	X	X
(see Note 1)	°C (°K)					X	X
	°K (°K)
	°R (°R)
Thermal Energy	inchpound force (inlbf)	X
	footpound force (ftlbf)			X
	Joule (J)					X
	erg						X
	milliJoule (mJ)
	British Thermal Unit (Btu)		X
	calorie (cal)
Voltage	Volt (V)	X	X	X	X	X	X
(see Note 2)	milliVolt (mV)
Current	Amp (A)	X	X	X	X	X	X
	milliAmp (mA)
Electrical Resistance	ohm	X	X	X	X	X	X
	milliohm
	megaohm
Mass	lbf*s^2/in	X	X
	slug			X	X
	kilogram (kg)					X
	gram (g)						X
	pound mass (lbm)

Note:

Temperature and Absolute Temperature cannot be chosen separately. The Temperature unit is shown first, followed by the Absolute Temperature in parentheses.
Technically, a Volt is derived from Newton, meter, second, and amp. The only effect in the software is the display of the electrostatic force result.