Most joints are limited in their range of motion along an active axis. For example: a hinge might open only to 120 degrees; a piston slides only within the length of its cylinder. To limit the range of motion allowed on an active axis, you turn on Limited and set values for the From and To fields on the joint rollouts.
You can also view joint limits in the viewport by pressing and holding the mouse button on the From or To label of a limit field. The object will move or rotate to the limit value until you release the mouse button.
Limits for a rotational joint define how far the object can rotate about its parent’s axes. The values in the From and To fields represent the rotation angle about the active axis measured from 0 degrees on the parent object.
For example, an elbow joint rotates the forearm with respect to the upper arm. In the figure the limits on X axis rotation are from 0 to 135 degrees. The Y, Z axes are inactive because an elbow joint rotates about a single axis.
Limits for a sliding joint define how far a joint can move along its parent’s axes. The values in the From and To fields represent a distance for movement along the active axis measured from the pivot point of the parent to the pivot point of the selected object.
For example, a sliding joint on a piston moves the piston in and out of the cylinder. In the figure, the limits on Z axis movement are from 10 to 90. This prevents the piston from hitting the bottom or moving past the end of the cylinder.
Limits for path and surface joints define how far along the path or surface an object can move. The values in the From and To fields represent a percentage of the total distance measured along the path or surface.
For example, a path joint for a house key moves the key along a key ring. Setting the limits on the path joint at 5% to 95% prevents the key from traveling along the ring where the fob is attached.