4.7 COMBIN7 Revolute Joint

Figure 4.7-1 COMBIN7 Revolute Joint Element

4.7.1 Input Data

The active nodes (I, J) are defined to have six degrees of freedom; however, five of these (UX, UY, UZ, ROTX, ROTY) in the local joint system are intended to be constrained with a certain level of flexibility. This level of flexibility is defined by three input stiffnesses: K1 for translational stiffness in the x-y plane, K2 for stiffness in the z direction, and K3 for rotational stiffness about the x and y axes. Joint mass (MASS) and mass moment of inertia (IMASS) input values are evenly distributed between nodes I and J.

The dynamics of the revolute rotation or primary degree of freedom (Figure 4.7-2) include friction torque (TF), rotational viscous friction (CT), torsional stiffness (K4), preload torque (TLOAD), interference rotation (ROT), and two differential rotation limits (STOPL and STOPU). A null value for TF corresponds to zero friction (or free rotation), while a negative value will remove friction capability from the element. Once removed (TF<0), the joint is locked with stiffness K4. The joint will also become locked with stiffness K4 when a stop is engaged. The upper stop (STOPU) represents the allowed amount of forward rotation (node J rotates away from node I), and the lower stop (STOPL) represents the allowed amount of reverse rotation (node J rotates towards node I). Null values for both stops will remove locking action from the element; i.e., rotation damped only by viscous (CT) and friction (TF) damping torques.

Figure 4.7-2 COMBIN7 Real Constants and Dynamic Behavior of Rotation About the Z (Revolute) Axis

The rotational interference (ROT) is intended to correspond to a locally imposed joint rotation if the revolute axis is locked (TF<0) and stiffness is specified (K4>0). A starting status real constant (START) will set the initial behavior of the revolute rotation: START = 0 implies no rotation (locked), START = 1 or -1 implies forward or reverse rotation, respectively. Initial rotation status (START=1,-1) will be overruled if either START=1, STOPU=0, and STOPL#0, or START=-1, STOPL=0, and STOPU#0.

Consistent units should be used. Units are force/length for K1 and K2 and length*force/radian for K3 and K4. CT uses length*force*time/radian, while TF and TLOAD uses length*force. Force*time²/length is used with MASS and length*force*time²/radian is for IMASS. ROT, STOPL, and STOPU use radians.

Feedback control behavior is associated with the control nodes (L,M). The KEYOPT values are used to define the control value (CVAL). KEYOPT(3) selects the degree of freedom for the control nodes, KEYOPT(4) assigns the coordinate system for the selected degree of freedom, and KEYOPT(7) specifies which real constant is to be modified for the subsequent nonlinear analysis. The KEYOPT(1) option assigns to the control value either the value of the degree of freedom, the first or second derivative of the value, the integral of the value, or time.

KEYOPT(2) defines the behavior of the revolute degree of freedom after a stop has been engaged. If KEYOPT(2)=0, the the pin may disengage (or bounce off) the stop. If KEYOPT(2)=1, the pin axis is locked.

The element can exhibit nonlinear behavior according to the function

RVMOD = RVAL + C1|CVAL|^C2 + C3|CVAL|^C4

where RVMOD is the modified value of the input real constant value RVAL (identified by KEYOPT(7)), C1 through C4 are other real constants and give the form of the real constant modification, and CVAL is the control value (see KEYOPT(1)). RVMOD may also be defined by user subroutine USERRC and is accessed by KEYOPT(9)=1. Examples of CVAL are:

CVAL = UX_L -UX_M
CVAL = d(UZ_L -UZ_M )/dt
CVAL = d²(ROTZ_L -ROTZ_M )/dt²
CVAL =
CVAL = t

Control values calculated in the current substep are not used until the next substep. Control nodes need not be connected to any other element. If node M is not defined, the control value is based only upon node L.

A summary of the element input is given in Table 4.7-1. A general description of element input is given in Section 2.1.

Table 4.7-1 COMBIN7 Input Summary

Element Name	COMBIN7
Nodes	I, J, K, L, M (K, L, M are optional)
Degrees of Freedom	UX, UY, UZ, ROTX, ROTY, ROTZ
Real Constants	K1, K2, K3, K4, CT, TF, MASS, IMASS, TLOAD, START, STOPL, STOPU, ROT, C1, C2, C3, C4
Material Properties	None
Surface Loads	None
Body Loads	None
Special Features	Large deflection, Nonlinear (if either stops or friction are specified), Adaptive descent
KEYOPT(1)	Control Value 0, 1 - Control on value (UL-UM) (or UL if M not defined) 2 - Control on first derivative of value with respect to time 3 - Control on second derivative of value with respect to time 4 - Control on integral of value with respect to time 5 - Control on time value (KEYOPT(3) ignored)
KEYOPT(2)	Fixed-stop control behavior 0 - Reverse pin-axis rotation is not prevented when a rotational stop is engaged. 1 - Pin-axis is locked when a rotational stop is engaged (only after the first substep)
KEYOPT(3)	Degree of freedom for control nodes (L and M) 0, 1 - UX (Displacement along X axes) 2 - UY (Displacement along Y axes) 3 - UZ (Displacement along Z axes) 4 - ROTX (rotation about X axes) 5 - ROTY (rotation about Y axes) 6 - ROTZ (rotation about Z axes
KEYOPT(4)	Control node coordinates 0 - Control node degree of freedom is in nodal coordinates 1 - Control node degree of freedom is in element (moving) coordinates
KEYOPT(7)	Used if C1 or C3 is not equal to zero (see Section 4.7.1) 0, 1 - Use K1 real constant for nonlinear function, 2 - Use K2, 3 - use K3, ....., 13 - use ROT
KEYOPT(9)	0 - Use RVMOD expression for real constant modifications 1 - Real constants modified by user subroutine USERRC (see the Guide to ANSYS User Programmable Features for information about user written subroutines)

4.7.2 Output Data

• nodal displacements included in the overall nodal solution
• additional element output as shown in Table 4.7-2.

The following notation is used in Table 4.7-2:

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 4.7-2 COMBIN7 Element Output Definitions

Name	Definition	O	R
EL	Element number	Y	Y
NODES	Active nodes - I, J	Y	Y
CENT: X, Y, Z	Center location XC, YC, ZC	-	Y
ROTATE	Amount of pin rotational sliding	Y	Y
CVAL	Value (see KEYOPT(1)) of the control nodes	Y	Y
STAT	Element status	1	1
OLDST	Stat values of the previous time step	1	1
DUX, DUY, DUZ, DRX, DRY, DRZ	Differential pin displacements and rotations in element coordinates. For example, DUX = UXJ-UXI.	Y	Y
RVMOD	Modified real constant (see Section 4.7.1)	Y	Y
FORCE(X,Y,Z)	Spring forces (in element coordinates)	Y	Y
MOMENT(X,Y,Z)	Spring moments (in element coordinates)	Y	Y
RVOLD	Modified real constant of previous time step	Y	Y

1. Element status values:
0 - No rotation (but no stop engaged)
1 - Forward rotation
- 1 - Reverse rotation
2 - Forward stop engaged
-2 - Reverse stop engaged

Table 4.7-3 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 4.7-3:

Name - output quantity as defined in the Table 4.7-2

Item - predetermined Item label for ETABLE command

E - sequence number for single-valued or constant element data

Name	Item	E
FORCEX	SMISC	1
FORCEY	SMISC	2
FORCEZ	SMISC	3
MOMENTX	SMISC	4
MOMENTY	SMISC	5
MOMENTZ	SMISC	6
STAT	NMISC	1
OLDST	NMISC	2
DUX	NMISC	3
DUY	NMISC	4
DUZ	NMISC	5
DRX	NMISC	6
DRY	NMISC	7
DRZ	NMISC	8
ROTATE	NMISC	9
RVMOD	NMISC	10
CVAL	NMISC	11

4.7.3 Assumptions and Restrictions

The nonlinear capabilities of the element operate only in the static and nonlinear transient dynamic analyses. If used in other analysis types, the element maintains its initial status throughout the analysis. An iterative solution is required when using the nonlinear option.

The precise nature of the element behavior, whether nonlinearities are present or not, depends on several input items. These items include the presence of stops or friction; the selection of a large deflection analysis; and the use of joint control features. Stop input values (STOPL, STOPU) must be greater than or equal to zero. For stops to be engaged, a positive torsional stiffness (K4) should be specified. Negative values are ignored. Stop values represent forward and reverse clearances about the revolute axis.

Revolute friction (TF), when specified, must be positive. A negative friction value removes friction from the element and locks the revolute axis with torsional stiffness K4. A null friction value implies frictionless rotation (unless CT is specified or a stop is engaged).

The element may not be deactivated with the EKILL command. Also, the real constants for this element are not allowed to be changed from their initial values. Only the lumped mass matrix is available.

4.7.4 Product Restrictions