Tham khảo tài liệu 'mechanism of steam engines part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | HAND-OPERATED REVERSING AND CONTROLLING GEARS 119 reverse the block is moved up or down in the slot. It is possible to design this link to give constant lead by making the radius of the center of the slot equal to the length of the radius rod. 89. Allan Link. In the link mechanism known as the Allan Link the link itself is straight. There is a radius rod as in the Gooch Link and both the link and the radius rod are suspended from arms on the reverse shaft in such a way that when the reverse shaft is turned part of the adjustment is given to the link and part to the link block. The link being straight instead of curved is cheaper to make and for this reason is still used to some extent on small engines. 90. Radial Valve Gears. Another type of mechanism for reversing and hand control of steam distribution includes such gears as the Walschaert Hackworth Marshall and Joy. These are sometimes called radial valve gears. While these gears rarely have more than one actual eccentric and often none the valve motion is approximately that which would be given by the combined action of two eccentrics one with 90 angular advance that is directly opposite the crank or coinciding with the crank according to the type of valve and connection and giving to the valve a movement either side of mid-position equal to the lap plus the lead. This motion is combined with that from another eccentric or its equivalent at right angles to the crank which imparts enough additional motion to the valve to give proper port opening. The connection between this second eccentric and the valve is through some system of adjustable levers and links by means of which the action of the valve may be changed to give different grades of cut-off or to reverse the direction of rotation of the engine. These gears as ordinarily designed give constant lead for all grades. 91. Walschaert Valve Gear. The most important of the so-called radial gears is that invented by Egide Walschaerts in Brussels and patented in .