It will be seen that the instrument shows not only what the valve-gear should, but what it actually does do, and delineates all its imperfections. ======+======+===============+===============+===============+===== | | Width of | | | | | opening | Point of | Point of | | | of steam-port.| cut-off. | release. | No. |Travel+-------+-------+-------+-------+-------+-------+ of | of | Backwd|Forw’rd| Backwd|Forw’rd|Backwd |Forw’rd| curve.|valve.|stroke.|stroke.|stroke.|stroke.|stroke.|stroke.|Lead. ------+------+-------+-------+-------+-------+-------+-------+----- | in. | in. | in. | in. | in. | in. | in. | in. 1 | 2¹⁄₂ | ¹¹⁄₃₂| ⁵⁄₁₆ | 8 | 6³⁄₄ |17 |16⁵⁄₈ |⁹⁄₃₂ 2 | 2⁵⁄₈ | ⁷⁄₁₆ | ¹³⁄₃₂ | 9¹⁄₂ | 9¹⁄₂ |18⁵⁄₈ |18⁵⁄₁₆ |¹⁄₄ 3 | 2⁷⁄₈ | ⁹⁄₁₆ | ¹⁄₂ | 12 | 11³⁄₄ |19³⁄₄ |19⁹⁄₁₆ |⁷⁄₃₂ 4 | 3¹⁄₈ | ¹¹⁄₁₆| ⁴¹⁄₆₄ | 14 | 14 |20¹¹⁄₁₆|20⁹⁄₁₆ |³⁄₁₆ 5 | 3¹⁄₂ | ⁷⁄₈ | ²⁷⁄₃₂ | 16¹⁄₂ | 16¹⁄₂ |21¹¹⁄₁₆|21¹⁄₂ |⁵⁄₃₂ 6 | 4 | 1¹⁄₈ |1³⁄₃₂ | 18¹⁄₄ | 18¹⁄₈ |22³⁄₈ |22¹⁄₄ |¹⁄₈ 7 | 4¹⁄₂ | 1¹⁄₄ |1¹⁄₄ | 19⁵⁄₈ | 19¹⁄₂ |22¹³⁄₁₆|22³⁄₄ |³⁄₃₂ 8 | 5 | 1¹⁄₄ |1¹⁄₄ | 20⁵⁄₈ | 20¹⁄₂ |23¹⁄₈ |23¹⁄₈ |¹⁄₁₆ -=====+======+=======+=======+=======+=======+=======+=======+===== QUESTION 195. _What are the chief dimensions of the valve-gear represented in fig. 128?_ _Answer._ The throw of eccentrics was 5 in., the steam-ports were 1¹⁄₄ in., and the exhaust-port 2³⁄₄ in. wide, the valve had ⁷⁄₈ in. outside and ¹⁄₁₆ inside lap and ¹⁄₁₆ in. lead at full stroke. QUESTION 196. _What relation is there between the distance which the ports are opened by the valve, and its travel when worked by a link?_ _Answer._ As explained in the answer to Question 52, the width which the steam-ports are opened by the valve for the admission of steam diminishes with the travel of the valve. This is shown very clearly by the motion-curves, and also in the above table, from both of which it will be seen that when the valve travels only 2¹⁄₂ in. the steam-ports are opened only ¹¹⁄₃₂ in. for the back stroke and ⁵⁄₁₆ for the front. With 2⁵⁄₈ travel the opening is ⁷⁄₁₆ and ¹³⁄₃₂ in. With 4 in. travel the port is opened 1¹⁄₈ and 1³⁄₃₂ in. and with 4¹⁄₄ in. travel they would be opened wide. With 4¹⁄₂ and 5 in. travel, as will be seen from the motion diagram, the ports are not only opened wide, but the valve throws “over” them, or travels beyond their inner edges. QUESTION 197. _How is the point of cut-off affected by the link?_