MICO (North Mankato, MN) offers hydraulic-over-air relay valves designed to convert hydraulic modulated input pressure to a proportional output air pressure. The relay valves are suited for various on- and off-highway applications, such as controlling an air-braked trailer with a hydraulically braked towing vehicle that has an air power source. The system originates with a hydraulic vehicle component, such as a master cylinder or hydraulic control valve, which delivers a given hydraulic input pressure. This input pressure sends a pilot signal to the relay valve in order to modulate air brake pressure on the other end.
Three different designs are available: single, dual, and tandem. The single-input design provides one hydraulic pilot port to control modulated air braking pressure. The dual-input features two independent ports, either of which can accept pilot pressure. When pressure is applied to both ports simultaneously, the controlled air braking pressure ratio increases proportionally. The dual-input ports can be set up for different pressure ratios, making the dual design a good fit for certain steering assist applications. Functionally similar to the single design, the tandem input provides redundancy by utilizing two independent pilot pressure ports. This enables hydraulic pilot pressure to be applied to either port while allowing the pressure ratio to remain the same. The valves can accommodate a maximum air pressure of 150 PSI and maximum hydraulic pressure of 2,000 PSI. The valves can be adjusted to operate at anywhere from a 3:1 to 21:1 hydraulic-to-air-pressure ratio.
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