Joint Stops and Locks

Stops and Locks are optional constraints that may be applied to restrict the motion of the free relative degree(s) of freedom (DOF) of most types of joints. Any analysis that includes a valid joint type can involve Stops and/or Locks. For the applicable joint types, you can define a minimum and maximum (min, max) range inside of which the degrees of freedom must remain.

A Stop is a computationally efficient abstraction of a real contact, which simplifies geometry calculations. For Stops, a shock occurs when a joint reaches the limit of the relative motion. A Lock is the same as a Stop except that when the Lock reaches the specified limit for a degree of freedom the Lock becomes fixed in place.

Warning:  Use Joint Stops sparingly. The application treats the stop constraint internally as a "must be imposed" or "hard" constraint and no contact logic is used. As a result, during the given iteration of a substep, the stop constraints activate immediately if the application detects a violation of a stop limit. Depending upon the nature of the problem, the stop constraint implementation may cause the solution to trend towards an equilibriated state that may not be readily apparent to you. In addition, do not use stops to simulate zero-displacement boundary conditions. You should also avoid specifying stops on multiple joints. Finally, do not use joint stops as a substitute for contact modeling. Whenever possible, you need to use node-to-node or node-to-surface contact modeling to simulate limit conditions.

For joints with free relative DOFs, the Details view displays a group of options labeled Stops. This grouping displays the applicable free DOFs (UX, UY, UZ, ROTX. etc.) for the joint type from which you specify the constraint as a Stop or a Lock (as shown below). By default, no Stop or Lock is specified, as indicated by the default option, None. You can select any combination of options. For stops and locks, the minimum and maximum values you enter are relative to the joint’s coordinate system.

Radial Gap Stop

To model clearance in revolute or cylindrical joints, a special type of stop called a radial gap stop can be used. A radial gap stop limits the relative rotation of either the X or Y rotation, limiting the Z axis tilt of the joint's mobile coordinate system with respect to the Z axis of the reference coordinate system. This stop idealizes a revolute joint with a gap between the inner and the outer cylinder that allows the shaft to translate and tilt in the outer cylinder, as shown on the following figure:

Where:

d is the inner diameter.
D is the outer diameter.
H is the height of the joint.

Important Notes:

Stops and Locks are applied to the following Joint Types.

Joint Type Stop/Lock ANSYS Rigid Dynamics Stop/Lock ANSYS Mechanical
Revolute Yes Yes
Cylindrical Yes Yes
Translational Yes Yes
Slot Translational Translational
Universal Yes Yes
Spherical No (Radial Gap is now available through Spherical Gap joint)No
Planar Yes Yes
General Translational, Radial Gap Translational
BushingTranslational, Radial GapTranslational
Spherical GapRadial Gap stop is always definedNot supported
In-Plane Radial GapRadial Gap stop is always definedNot supported
Radial GapRadial Gap stop is always defined, translation along Z if freeNot supported

Note:
  • When using the ANSYS Mechanical solver, Stops and Locks are active only when Large Deflection is set to On (under Analysis Settings). This is because Stops and Locks make sense only in the context of finite deformation/rotation. If Large Deflection is Off, all calculations are carried out in the original configuration and the configuration is never updated, preventing the activation of the Stops and Locks.

  • It is important to apply sensible Stop and Lock values to ensure that the initial geometry configuration does not violate the applied stop/lock limits. Also, applying conflicting boundary conditions (for example, applying Acceleration on a joint that has a Stop, or applying Velocity on a joint that has a Stop) on the same DOF leads to non-physical results and therefore is not supported.


Solver Implications

Stops and Locks are available for both the ANSYS Rigid Dynamics and ANSYS Mechanical solvers, but are handled differently in certain circumstances by the two independent solvers.

Coefficient of Restitution

For the ANSYS Rigid Dynamics solver, Stops require you to set a coefficient of restitution value. This value represents the energy lost during the shock and is defined as the ratio between the joint’s relative velocity prior to the shock and the velocity following the shock. This value can be between 0 and 1. For a restitution value of zero, a Stop is released when the force in the joint is a traction force, while a Lock does not release. A restitution factor equal to 1 indicates that no energy is lost during the shock, that is, the rebounding velocity equals the impact velocity (a perfectly elastic collision).

The coefficient of restitution is not applicable to the stops on the joints when using the ANSYS Mechanical solver.


Release 16.2 - © SAS IP, Inc. All rights reserved.