One-way restrictors provide reduced hydraulic flow in one direction only, to limit actuating speed of hydraulic cylinders for the purpose of proper timing or sequence of operation. Also, they provide free flow of fluid in the opposite direction to permit the actuating cylinder to actuate at a faster rate of speed during the reverse action of the cylinder.

One-way restrictors are used in some landing gear systems to regulate the speed and sequence of landing gear retraction or extension. If sequenced action (that is, one cylinder to be actuated before other cylinders on the same line) is desired, one-way restrictors are placed in the line upstream of all cylinders except one. Figure 8-16 shows both the one-way and two-way restrictors. The main parts of a one- way restrictor are the cylindrical body and cap, which contain a spring-loaded poppet, a cage, and a stainless steel filter element.

The one-way restrictor allows free flow in one direction and restricted flow in the opposite direction. Both directions of flow are indicated by arrows found on the body of the valve.

In a restricted direction, pressurized fluid entering port R (fig. 8-16) flows through the filter assembly and enters the cage through drilled passages. Fluid from the interior of the cage is forced through the poppets orifice, thus causing the required metering action. In the free flow direction, pressurized fluid entering port F overcomes poppet spring tension and allows fluid to flow past the poppets seat, through drilled passages within the larger flange of the cage, and out through port R.

Two-way restrictors are used to limit the flow of hydraulic fluid where it is desirable to retard the action of a hydraulic cylinder in both directions. Figure 8-16 shows two types of two-way restrictors, one of which has a machined orifice with two integral stainless steel filters. The other type shown contains an orifice plate between two stainless steel filters. The filters contained within the restrictors are identical in construction and provide protection in both directions of flow. The filter size specification for the two-way restrictor is identical to those found within one-way restrictors.

Two-way restrictors, regardless of whether they are of the machined orifice type or of the plate orifice type, operate identically. Fluid entering either port is filtered prior to flowing through the orifice, thus protecting the orifice from possible stoppage. As the fluid is metered through the orifice, the prescribed rate flow is directed out the opposite port of the restrictor and to the actuating unit.

Maintenance of restrictors is usually limited to checking for external leakage and the required fluid flow. The specific MIM lists the required fluid flow in gallons per minute (gpm) for each size of orifice being checked. It also specifies the correct pressures to use as well as the required procedures during each check.