What device is used alongside the unloading valve for automatic air control?

The servo valve is a critical component in automatic air control systems, particularly when used alongside an unloading valve. Its primary function is to control the flow and pressure of air within a pneumatic system. The servo valve does this by receiving input signals (such as pressure changes) and adjusting the position of a valve mechanism to either increase or decrease airflow, thus providing precise control over the operation of pneumatic actuators.

In scenarios where an unloading valve is present, the servo valve ensures that the air pressure levels are maintained within desired limits, thereby allowing for efficient operation without unnecessary energy loss. This is especially effective in systems that require rapid adjustments to air pressure for various operational needs.

Other devices, while having their own specific functions, do not perform the same role as the servo valve in this context. For instance, a pressure relief valve is primarily used for safety to prevent overpressure in a system, while an oil return valve manages the flow of lubricant back to a reservoir. A sump valve is associated with drainage and fluid management rather than direct control of air pressure. Thus, the servo valve's unique capability to modulate and control airflow makes it the correct choice in conjunction with an unloading valve for maintaining automatic air control.