COMBIN14

Spring-Damper

COMBIN14 Element Description

COMBIN14 has longitudinal or torsional capability in 1D, 2D, or 3D applications. The longitudinal spring-damper option is a uniaxial tension-compression element with up to three degrees of freedom at each node: translations in the nodal x, y, and z directions. No bending or torsion is considered. The torsional spring-damper option is a purely rotational element with three degrees of freedom at each node: rotations about the nodal x, y, and z axes. No bending or axial loads are considered.

The spring-damper element has no mass. Masses can be added by using the appropriate mass element (see MASS21). The spring or the damping capability may be removed from the element. See COMBIN14 in the Mechanical APDL Theory Reference for more details about this element. A general spring or damper is also available in the stiffness matrix element (MATRIX27). Another spring-damper element (having its direction of action determined by the nodal coordinate directions) is COMBIN40.

Figure 14.1: COMBIN14 Geometry

2D elements must lie in a z = constant plane

COMBIN14 Input Data

The geometry, node locations, and the coordinate system for this element are shown in Figure 14.1: COMBIN14 Geometry. The element is defined by two nodes, a spring constant (k) and damping coefficients and . The damping capability is not used for static or undamped modal analyses. The longitudinal spring constant should have units of Force / Length, the damping coefficient units are Force * Time / Length. The torsional spring constant and damping coefficient have units of Force * Length / Radian and Force * Length * Time / Radian, respectively. For a 2D axisymmetric analysis, these values should be on a full 360° basis.

The damping portion of the element contributes only damping coefficients to the structural damping matrix. The damping force (F) or torque (T) is computed as:

where is the damping coefficient given by:

and is the velocity calculated in the previous substep.

The second damping coefficient is available to produce a nonlinear damping effect characteristic of some fluid environments. If real constant CV2 is input, KEYOPT(1) must be nonzero.

The imaginary part of the stiffness constant (k_imag) contributes to the structural damping matrix. The imaginary force (F*) or torque (T*) is computed as:

KEYOPT(2) = 1 through 6 is used for defining the element as a one-dimensional element. With these options, the element operates in the nodal coordinate system (see Elements That Operate in the Nodal Coordinate System). The KEYOPT(2) = 7 and 8 options allow the element to be used in a thermal or pressure analysis.

A preload in the spring may be specified in one of two ways, either through an initial (force-free) length (ILENGTH) or an initial force (IFORCE) input. Only one of the input options may be used to define the preload. If the initial length is different than the input length defined by the nodal coordinates, a preload is presumed to exist. If an initial force is given, a negative value indicates the spring is initially in compression and a positive value indicates tension. For the 3D torsional spring option (KEYOPT(3) = 1), ILENGTH is interpreted as the initial number of turns (rotations) in the spring (the spring is pre-wound) and IFORCE is the torque preload in the spring. The right-hand rule from node I to node J is used to define positive and negative turns as well as positive and negative torque. In a nonlinear analysis, the preload is ramped in the first load step if KBC,0 is set.

A summary of the element input is given in "COMBIN14 Input Summary". A general description of element input is given in Element Input.

COMBIN14 Input Summary

Nodes

I, J

Degrees of Freedom

UX, UY, UZ if KEYOPT (3) = 0

ROTX, ROTY, ROTZ if KEYOPT (3) = 1

UX, UY if KEYOPT (3) = 2

see list below if KEYOPT(2) > 0

Real Constants

K,CV1,CV2, (Blank), (Blank), ILENGTH, IFORCE, KIMAG

See Table 14.1: COMBIN14 Real Constants for a description of the real constants.

In a full harmonic analysis, real constants K,CV1, and KIMAG can be defined as table parameters using the frequency as primary variable (Var1 = FREQ on the *DIM command).

In a full transient analysis, real constants K and CV1 can be defined as table parameters using time as the primary variable (Var1 = TIME on the *DIM command).

In a static analysis, real constant K can be defined as table parameters using time as the primary variable (Var1 = TIME on the *DIM command).

Material Properties

MP command: BETD, DMPR, DMPS

Surface Loads

None

Body Loads

None

Special Features

Birth and death

Large deflection

Linear perturbation

Nonlinearity (if CV2 is not zero)

Stress stiffening

KEYOPT(1)

Solution type:

0 --: Linear damping (default)
1 --: Nonlinear damping (nonzero CV2). The damping coefficient is based on the absolute value of the velocity to avoid negative damping.

KEYOPT(2)

Degree-of-freedom selection for 1D behavior:

0 --: Use KEYOPT(3) options
1 --: 1D longitudinal spring-damper (UX degree of freedom)
2 --: 1D longitudinal spring-damper (UY degree of freedom)
3 --: 1D longitudinal spring-damper (UZ degree of freedom)
4 --: 1D Torsional spring-damper (ROTX degree of freedom)
5 --: 1D Torsional spring-damper (ROTY degree of freedom)
6 --: 1D Torsional spring-damper (ROTZ degree of freedom)
7 --: Pressure degree of freedom element
8 --: Temperature degree of freedom element

Note: KEYOPT(2) overrides KEYOPT(3)

KEYOPT(3)

Degree-of-freedom selection for 2D and 3D behavior:

0 --: 3D longitudinal spring-damper
1 --: 3D torsional spring-damper
2 --: 2D longitudinal spring-damper (2D elements must lie in an X-Y plane)

Table 14.1: COMBIN14 Real Constants

No.	Name	Description
1	K	Spring constant
2	CV1	Damping coefficient
3	CV2	Damping coefficient (KEYOPT(1) must be nonzero)
4, 5	(Blank)	- -
6	ILENGTH	Initial force-free length (Initial number of turns if torsional spring (KEYOPT(3) = 1))
7	IFORCE	Initial force (or torque if torsional spring (KEYOPT(3) = 1))
8	KIMAG	Imaginary part of the spring constant

COMBIN14 Output Data

The solution output associated with the element is in two forms:

Nodal displacements included in the overall nodal solution
Additional element output as shown in Table 14.2: COMBIN14 Element Output Definitions.

Several items are illustrated in Figure 14.2: COMBIN14 Stress Output. A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

Figure 14.2: COMBIN14 Stress Output

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file jobname.out. The R column indicates the availability of the items in the results file.

In either the O or R columns, “Y” indicates that the item is always available, a letter or number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

Table 14.2: COMBIN14 Element Output Definitions

Name	Definition	O	R
EL	Element Number	Y	Y
NODES	Nodes - I, J	Y	Y
XC, YC, ZC	Location where results are reported	Y	1
FORC or TORQ	Spring force or moment (for imaginary result set, this is the force contribution from and KIMAG)	Y	Y
STRETCH or TWIST	Stretch of spring or twist of spring (radians)	Y	Y
RATE	Spring constant	Y	Y
VELOCITY	Velocity (available for ANTYPE,STATIC or ANTYPE,TRANS)	-	Y
DAMPING FORCE or TORQUE	Damping force or moment (zero unless ANTYPE,TRANS and damping present)	Y	Y

Available only at centroid as a *GET item.

Table 14.3: COMBIN14 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) of the Basic Analysis Guide and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 14.3: COMBIN14 Item and Sequence Numbers:

Name: output quantity as defined in the Table 14.2: COMBIN14 Element Output Definitions
Item: predetermined Item label for ETABLE command
E: sequence number for single-valued or constant element data

Table 14.3: COMBIN14 Item and Sequence Numbers

Output Quantity Name	ETABLE and ESOL Command Input
Output Quantity Name	Item	E
FORC	SMISC	1
STRETCH	NMISC	1
VELOCITY	NMISC	2
DAMPING FORCE	NMISC	3

COMBIN14 Assumptions and Restrictions

If KEYOPT(2) is zero, the length of the spring-damper element must not be zero (that is, nodes I and J should not be coincident, since the node locations determine the spring orientation).
The longitudinal spring element stiffness acts only along its length. The torsion spring element stiffness acts only about its length, as in a torsion bar.
The element allows only a uniform stress in the spring.
In a thermal analysis, the temperature or pressure degree of freedom acts in a manner analogous to the displacement.
Only the KEYOPT(2) = 0 option supports stress stiffening or large deflection. Also, if KEYOPT(3) = 1 (torsion) is used with large deflection, the coordinates will not be updated.
The spring or the damping capability may be deleted from the element by setting the spring constant (K) or damping coefficients (CV1 and CV2) equal to zero, respectively.
If CV2 is not zero, the element is nonlinear and requires an iterative solution (KEYOPT(1) = 1).
The preload may not change after the first load step. Any changes are ignored.
The preload is ignored in modal and harmonic analyses.
KIMAG is only supported in modal and full harmonic analyses. In a linear perturbation modal analysis, KIMAG must be defined using RMODIF in the first phase of the analysis (before the SOLVE,ELFORM command). For more information, see General Procedure for Linear Perturbation Analysis in the Structural Analysis Guide.
Tabular input of real constants (K, CV1, and KIMAG) is not supported in modal analysis.
The real constants (K, CV1, and KIMAG) cannot be defined as tabular parameters with respect to frequency in linear perturbation analyses.
Tabular input of real constants (K, CV1, and KIMAG) with respect to time is not supported in the second phase of linear perturbation analyses. Their values are frozen at the end of the base analysis.

The restrictions described below only apply if KEYOPT(2) is greater than zero.

If KEYOPT(2) is greater than zero, the element has only one degree of freedom. This degree of freedom is specified in the nodal coordinate system and is the same for both nodes (see Elements That Operate in the Nodal Coordinate System). If the nodal coordinate systems are rotated relative to each other, the same degree of freedom may be in different directions (thereby giving possibly unexpected results). The element, however, assumes only a 1D action. Nodes I and J, then, may be anywhere in space (preferably coincident).
For noncoincident nodes and KEYOPT(2) = 1, 2, or 3, no moment effects are included. That is, if the nodes are offset from the line of action, moment equilibrium may not be satisfied.
The element is defined such that a positive displacement of node J relative to node I tends to stretch the spring. If, for a given set of conditions, nodes I and J are interchanged, a positive displacement of node J relative to node I tends to compress the spring.

COMBIN14 Product Restrictions

When used in the product(s) listed below, the stated product-specific restrictions apply to this element in addition to the general assumptions and restrictions given in the previous section.

Ansys Mechanical Pro —

Birth and death is not available.
No damping capability (CV1, CV2, and KIMAG are not allowed).
The PRES degree of freedom (KEYOPT(2) = 7) is not allowed.

Ansys Mechanical Premium —

The PRES degree of freedom (KEYOPT(2) = 7) is not allowed.