Part of the Viking lander hardware is assembled with Tridair LiveLock type fasteners, modeled here:
These are intended for applications where frequent and quick assembly or disassembly may be required. On Viking these were used to attach the top cover of a large housing for each of the lander’s two Radioisotope Thermoelectric Generators (RTGs). Rapidity was not a critical requirement, but the covers were installed and removed many times during overall lander assembly and test.
The LiveLock family of fasteners consist of two principal components: a receptacle and a stud nut. A third component is a retaining ring which enables the stud nut to be captive to the removable panel or object being assembled. Modeled here and used on the lander were the smallest size part number CA2010, where the threading is of type #4-40 (with a two-lead thread pattern, to facilitate faster screwing and unscrewing).
The receptacle contains a very small ratchet mechanism designed to maintain a load on the stud nut, even when loosely screwed into the receptacle. This is intended to minimize progressive loosening due to vibration, and permits use of a modest level of torque for normal installation. A tapered coil spring within the receptacle bears against the bottom of the lower ratchet. (In the 3D model, the spring is shorter than the unit seen in the photograph because the spring has been modeled in its pre-loaded form as it exists within the receptacle housing.) The receptacle’s lower ratchet has tiny tangs or prongs on opposite sides which slide up and down within grooves cut into the inner wall of the receptacle housing. This prevents the lower ratchet from rotating. The central fixed-position threaded core within the housing also has prongs that engage the housing slots to prevent rotation in the event that the core becomes loose from how it is clamped within a groove around the bottom of the housing.
The upper ratchet has a ridge across its top surface, which engages a slot cut into the end of the stud nut. When the stud nut is inserted and screwed into the receptacle, the slot and ridge operate the ratchet mechanism. The upper ratchet rotates with the stud nut, clicking over every 15 degrees as successive ratchet teeth engage. The stud nut has a hex recess in its pan head to enable installation.
The ratchet is a remarkably small mechanism. As seen in the upper right photograph, the complete receptacle is about 10mm (less than 1/2 inch) high. The two ratchet rings are about 4mm (less than 1/4 inch) in diameter.