When linked hierarchies are included in your simulation, you must set locks for the children in the simulation to confine the dynamics results to specific axes. This is done in the Locks rollout of the Hierarchy/Link Info command panel.
The Locks rollout contains three rows of checkboxes affecting the XYZ axes of the three possible transforms: Move, Rotate, and Scale. For the purposes of a dynamics simulation, the Scale transforms are ignored, and only the Move and Rotate locks are used. Generally, when a checkbox is checked, that axis of the specific transform is locked.
Ignoring the dynamics system for a moment, when you manipulate a forward-kinetix hierarchy directly, using the Move or Rotate tools, you might not bother with the Link Info locks, because to can specify axis constraints using the X/Y/Z/XY buttons in the toolbar. However, when that same hierarchy is in a dynamics simulation, where there are various forces at work (gravity, wind, collisions), the only thing that maintains the linkage between the objects is the locks you set in the Link Info/Locks rollout. As a result, no matter what combination you use of Move and Rotate locks, you’ll always want at least one Move lock in place, or your object will not really be linked.
The following lists all of the combinations of Move and Rotate locks that make sense within a dynamics simulation, and the effect on the link of such combinations. An asterisk (*) indicates those combinations that are more typically useful.
1 Move Lock: Any single Move is checked. The physical interpretation of this is of a long pin sliding in a loose, long slot. In this case, the joint can transmit force in one direction only. The objects can slide with respect to each other in two directions and rotate freely.
2 Move Locks: Any two Moves are checked. This is a sliding ball joint; a freely rotating joint at the end of a sliding shaft, which can slide and rotate in a hole.
* 3 Move Locks: All three Moves are checked.This is a ball joint, or the theoretical “pin joint” of the statics and dynamics texts, in that it transmits any force but never transmits any torque.
1 Move + 1 Rotate (unique): Any one Move and any one Rotate are checked, but not in the same column. The physical interpretation of this is two long pins, parallel, sliding in a single long slot. The joint can transmit force in one direction only and restrain rotation about the axis of the “pins.” This combination is of limited application.
2 Moves + 1 Rotate (matching): Two Moves checked, plus one Rotate checked that’s in the same column as one of the checked Moves. The physical interpretation is the same as 1 Move + 1 Rotate, above, except that the pins can no longer slide vertically in their slot. If the assembly rotates so that one pin travels further into the slot, the other must ride higher in the slot. This is of limited application. The possible checkbox combinations are shown below:
XXO XXO XOX or XOX OXX OXX
OXO XOO XOO OOX OOX OXO
* 2 Moves + 1 Rotate (complementary): Two Moves checked, plus one Rotate that’s not in the same column as any of the checked Moves. This is a sliding universal jointlike the splined output shaft between the automatic transmission of a rear-drive car and the drive shaft. It can transmit torque and constrain translation in two directions, both orthogonal to the axis of rotation. The possible checkbox combinations are shown below:
XOX XXO OXX
OXO OOX XOO
3 Moves + 1 Rotate: Three Moves checked plus one Rotate checked. This is a universal joint without the sliding. It’s typical of automative applications where the rear axle is located with the trailing driveshaft. This is an uncommon application.
* 1 Move (complementary) + 2 Rotates: One Move checked that’s complementary to the two checked Rotates. This is like a hockey puck on ice. That is, the joint can slide anywhere on a plane, but cannot fall or tip, and cannot leave the surface of the plane. The possible checkbox combinations are shown below:
XOO OXO OOX
OXX XOX XXO
2 Moves (one complementary) + 2 Rotates: Two Moves checked, one of which is complementary to one of the two Rotates that are checked. This is like a hockey puck with a nail through it, and the nail is sliding along a groove in the ice. It’s free to travel in one direction, and to rotate about an orthogonal axis. One example of the possible checkbox combinations would be as follows:
XXO
XOX
* 2 Moves + 2 Rotates (matching): Two Moves checked and two matching Rotates. This results in a sliding axle. Imagine a shaft that can both slide in and out of a hole, and rotate with the hole. The unchecked Move and Rotate axis specifies the axis along which the joint can slide and rotate. The possible checkbox combinations are shown below:
XXO XOX OXX
XXO XOX OXX
* 2 Moves + 3 Rotates: Two Moves checked and all three Rotates.This is a prismatic or sliding joint. The joint transmits no torque, and force in only one direction. You can use this in conjuction with the Push effect to make a hydraulic cylinder. The one unchecked Move specifies the axis of movement.
* 3 Moves + 2 Rotates: All three Moves checked, and any two Rotates. This is an axle. Probably the most common type of joint. The one unchecked Rotate specifies the axis of rotation.
All Locked: All six checkboxes checked. This is a completely rigid joint.