4.14 COMBIN14 Spring-Damper

4.14 COMBIN14 Spring-Damper (UP19980821 ) COMBIN14 has longitudinal or torsional capability in one, two, or three dimensional 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 Section 14.14 in the ANSYS Theory Reference for more details about this element. A general spring or damper is also available in the stiffness matrix element (MATRIX27) and is described in Section 4.27. Another spring-damper element (having its direction of action determined by the nodal coordinate directions) is described in Section 4.40.

Figure 4.14-1 COMBIN14 Spring-Damper



4.14.1 Input Data

The geometry, node locations, and the coordinate system for this element are shown in Figure 4.14-1. The element is defined by two nodes, a spring constant (k) and damping coefficients (cv)1 and (cv)2. 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 two-dimensional 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:

Fx= -cv dux/dt or T = -cv d/dt

where cv is the damping coefficient given by cv=(cv)1 + (cv)2v.

v is the velocity calculated in the previous substep. The second damping coefficient (cv)2 is available to produce a nonlinear damping effect characteristic of some fluid environments. If (cv)2 is input (as real constant CV2), KEYOPT(1) must be set to 1.

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 Section 2.3.2). The KEYOPT(2)=7 and 8 options allow the element to be used in a thermal or pressure analysis.

A summary of the element input is given in Table 4.14-1. A general description of element input is given in Section 2.1.

Table 4.14-1 COMBIN14 Input Summary

Element Name

COMBIN14

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

Material Properties

None

Surface Loads

None

Body Loads

None

Special Features

Nonlinear (if CV2 is not zero), Stress stiffening, Large deflection, Birth and death

KEYOPT(1)

0 - Linear Solution (default)
1 - Nonlinear solution (required if CV2 is non-zero)

KEYOPT(2)

0 - Use KEYOPT(3) options
1 - 1-D longitudinal spring-damper (UX degree of freedom)
2 - 1-D longitudinal spring-damper (UY degree of freedom)
3 - 1-D longitudinal spring-damper (UZ degree of freedom)
4 - 1-D Torsional spring-damper (ROTX degree of freedom)
5 - 1-D Torsional spring-damper (ROTY degree of freedom)
6 - 1-D 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)

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

4.14.2 Output Data

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

Several items are illustrated in Figure 4.14-2. A general description of solution output is given in Section 2.2. See the ANSYS Basic Analysis Procedures Guide for ways to view results.

Figure 4.14-2 COMBIN14 Stress Output



The following notation is used in Table 4.14-2:

A colon (:) in the Name column indicates the item can be accessed by the Component Name method [ETABLE, ESOL] (see Section 2.2.2). The O and R columns indicate the availability of the items in the file Jobname.OUT (O) or in the results file (R), a Y indicates that the item is always available, a number refers to a table footnote which describes when the item is conditionally available, and a - indicates that the item is not available.

Table 4.14-2 COMBIN14 Element Output Definitions

Name

Definition

O

R

EL

Element number

Y Y
NODES

Nodes - I, J

Y Y
CENT: X, Y, Z

Center location XC, YC, ZC

- Y
FORC or TORQ

Spring force or moment

Y Y
STRETCH or TWIST

Stretch of spring or twist of spring (radians)

Y Y
RATE

Spring constant

Y Y
VELOCITY

Velocity

- Y
DAMPING FORCE or TORQUE

Damping force or moment
(zero unless ANTYPE=TRANS and damping present)

Y Y
Table 4.14-3 lists output available through the ETABLE command using the Sequence Number method. See Chapter 5 of the ANSYS Basic Analysis Procedures Guide and Section 2.2.2.2 of this manual for more information. The following notation is used in Table 4.14-3:

Table 4.14-3 COMBIN14 Item and Sequence Numbers for the ETABLE and ESOL Commands

Name

Item

E

FORC

SMISC

1
STRETCH

NMISC

1
VELOCITY

NMISC

2
DAMPING FORCE

NMISC

3

4.14.3 Assumptions and Restrictions

If KEYOPT(2) is zero, the length of the spring-damper element must not be zero, i.e., 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. KEYOPT(2) must be zero if the element is used with 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 k or cv equal to zero, respectively. If (cv)2 is not zero, the element is nonlinear and requires an iterative solution (KEYOPT(1)=1).

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. 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 one-dimensional 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. If KEYOPT(2) is zero, the restrictions described in this paragraph do not apply.

4.14.4 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/LinearPlus

ANSYS/Thermal