COMBIN37 Element Description

COMBIN37 is a unidirectional element with the capability of turning on and off during an analysis. The element has one degree of freedom at each node, either a translation in a nodal coordinate direction, rotation about a nodal coordinate axis, pressure, or temperature. Similar unidirectional elements (without remote control capability) are COMBIN14, COMBIN39, and COMBIN40. The element has many applications, such as controlling heat flow as a function of temperature (thermostat), controlling damping as a function of velocity (mechanical snubber), controlling flow resistance as a function of pressure (relief valve), controlling friction as a function of displacement (friction clutch), etc. See COMBIN37 in the Mechanical APDL Theory Reference for more details about this element.

Figure 37.1: COMBIN37 Geometry

COMBIN37 Input Data

The functioning of this element is shown in Figure 37.1: COMBIN37 Geometry. The element is defined by two pairs of nodes, these being active nodes (I, J) and optional control nodes (K, L). Generally in the cases using UX, UY, or UZ as the active degrees of freedom, the active nodes should be coincident as this eliminates the possibility of moment disequilibrium. However, for visualization purposes, it may be useful to give node J a slightly greater coordinate value than node I. The element is defined such that a positive displacement of node J relative to node I will stretch the spring. Thus, if nodes I and J are interchanged, the same nodal motions will compress the spring.

Certain parameters associated with the control nodes are used to determine whether the control element is part of the structure (on) or not (off) and, therefore, can be used to disconnect regions of the model during time dependent or iterative analyses. Other input values are stiffness (STIF), damping coefficient (DAMP), concentrated nodal masses (MASI, MASJ), on/off control values (ONVAL, OFFVAL), element load (AFORCE: positive pulls node I in the positive nodal coordinate direction, and pulls node J in the negative nodal coordinate direction), initial on/off element status (START: -1 if explicitly off, 0 if determined from starting value of control parameter, 1 if explicitly on), several nonlinear constants (C1, C2, C3, C4), and a limiting sliding force (FSLIDE).

The FSLIDE value represents the absolute value of the spring force that must be exceeded before sliding occurs. If FSLIDE is 0.0, the sliding capability of the element is removed, that is, a rigid connection is assumed. For structural analyses, units are force/length or moment/rotation for stiffness, force*time/length or moment*time/rotation for damping, force*time²/length or moment*time²/rotation for mass, and force or moment for element load. For thermal analyses with temperature degrees of freedom, stiffness represents conductance and has units of heat/time*degrees, mass represents thermal capacitance with units of heat/degrees, and element load represents heat flow with units of heat/time. Also, in analyses with pressure degrees of freedom, stiffness represents flow conductance with units of length²/time. Stiffness, damping, mass, and element load should be defined on a full 360° basis for axisymmetric analyses.

The active nodes (I, J) have only one degree of freedom each, selected with the KEYOPT(3) option. The control nodes (K, L) can have the same, or a different, degree of freedom as specified with KEYOPT(2). The KEYOPT(1) option assigns to the parameters of the control nodes either the value of the degree of freedom, the first or second derivative of the value, the integral of the value, or time, for example:

Control nodes need not be connected to any other element. If node L is not defined, the control parameter is based only upon node K. If time is the control parameter (KEYOPT(1)), control nodes K and L need not be defined.

When the element is active and used in structural analyses, the element acts like any other spring/damper/mass element (such as COMBIN14, MASS21, and COMBIN40). In addition, the element can exhibit nonlinear behavior according to the function: RVMOD = RVAL + C1|CPAR|^C2 + C3|CPAR|^C4, where RVMOD is the modified value of an input real constant value RVAL (identified by KEYOPT(6)), C1 through C4 are other real constants, and CPAR is the control parameter (see KEYOPT(1)). RVMOD may also be defined by user subroutine USERRC and is accessed by KEYOPT(9) = 1. Note, FSLIDE modified to a negative value is set to zero. In a field analysis, the temperature or pressure degree of freedom acts in a manner analogous to the displacement.

As illustrated in Figure 37.2: COMBIN37 Behavior as a Function of Control Parameter, the KEYOPT(4) and KEYOPT(5) options, when used in combination with ONVAL and OFFVAL, set the control behavior of the element. The element is either on or off depending on the position of the control parameter with respect to the values of ONVAL and OFFVAL. Also, note that when KEYOPT(4) = 0 and the control parameter (CPAR) is within the ONVAL/OFFVAL interval, the element's status depends on the direction of the CPAR (that is, on going from on to off, and vice-versa). If ONVAL = OFFVAL = 0.0 (or blank), the on/off capability is ignored and the element is always active.

A summary of the element input is given in "COMBIN37 Input Summary". A general description of element input is given in Element Input.

COMBIN37 Output Data

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

The active nodal displacements and forces correspond to the degree of freedom selected with the KEYOPT(3) option. For axisymmetric analysis, the element forces are expressed on a full 360° basis.

The element value STRETCH is the relative deflection at the end of the substep less the amount of sliding (for example, UJ-UI-SLIDE). STATUS and OLDST indicate if the element is on or off at the end of the current and previous substeps, respectively. A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

Figure 37.2: COMBIN37 Behavior as a Function of Control Parameter

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 37.3: COMBIN37 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 37.3: COMBIN37 Item and Sequence Numbers:

Analogous thermal and fluid option output items use the same item and sequence numbers. See footnote 2 of Table 37.2: COMBIN37 Element Output Definitions.

COMBIN37 Assumptions and Restrictions

COMBIN37 Product Restrictions

There are no product-specific restrictions for this element.