What mechanical feature do slip type joints primarily require for effective operation?

Slip type joints are designed to allow for movement or expansion within piping systems, accommodating changes in temperature and pressure. For slip type joints to operate effectively, they require appropriate packing and lubrication.

Packing serves to ensure a tight seal around the joint, preventing leaks and maintaining the integrity of the piping system. Without effective packing, the joint might allow for unwanted leakage of fluids, which could compromise the system's functionality.

Lubrication is also crucial as it reduces friction between the moving parts of the joint. This is important because in a slip type joint, there is relative motion that occurs as the pipe expands or contracts. If there is insufficient lubrication, wear and tear can occur, potentially leading to joint failure over time.

Thus, the mechanical feature of packing and lubrication is essential for maintaining both the operational effectiveness and longevity of slip type joints in plumbing designs.