• HingeJoint does not constrain the absolute rotation or localRotation of the Transform that it is attached to. It constrains the relative rotation between its own RigidBody and the Rigidbody in the ConnectedBody field.

• HingeJoint.angle and HingJoint.axis do not represent the rotation of the Transform that it is attached to. Ex: Quaternion.AngleAxis(hinge.angle, hinge.axis) is not equivalent tohinge.transform.rotation or hinge.transform.localRotation.

These two misconceptions, taken together, can make a HingeJoint behave quite differently from what you might expect.

To quote the official docs:

The rest angle between the bodies is always zero at the beginning of the simulation.

The consequence of this is that HingeJoint.angle does not depend on the starting orientation of either the joint's RigidBody or the connected RigidBody. And because HingeJoint.limits represents limits on HingeJoint.angle, the limits are also independent of any starting orientation of the bodies.

An example: Consider a GameObject with rotation (0, 0, 0) and a HingeJoint on the x-axis with limits of [-90, 90]. When played as-is, this GameObject will be free to rotate between (-90, 0, 0) and (90, 0, 0). However, if you rotate this GameObject in the editor such that it starts with rotation (90, 0, 0) and play it, the GameObject will then be free to rotate between (0, 0, 0) and (180, 0, 0) - even though the angle limits have not changed.

In the vast majority of use cases, your HingeJoint should have a ConnectedBody assigned. This field is optional, but if left empty it causes your HingeJoint to be anchored to the global physics coordinate system. If your HingeJoint happens to be a child of some other object that moves, the joint will continue applying its limits within the global physics coordinate system - not in the joint's local coordinate system. This is usually not what you want.

A good way to ensure that your HingeJoint behaves nicely is to create a RigidBody on the parent object (if one doesn't already exist) and set that as the ConnectedBody. The parent body can have IsKinematic enabled if you don't want it to act like a physics object. In this configuration, the axis and angle of the HingeJoint will actually correspond to the localRotation of the Transform (though it will still be relative to any initial localRotation). This makes the joint behavior much easier to reason about!