Tham khảo tài liệu 'mechanism of steam engines part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SINGLE-VALVE ENGINES 31 1. If the leads are equal the steam laps must be equal and the cut-offs will be unequal. 2. If the cut-offs are equal the steam laps will be unequal and therefore the leads will be unequal. 3. Equal releases or equal compressions require unequal exhaust laps. 4. If the valve has a positive exhaust lap release will occur after the valve has passed mid-position and compression will occur before the valve again reaches mid-position. 5. If the valve has an exhaust clearance release will occur before the valve reaches mid-position and compression will not occur until after the valve again passes mid-position in the reverse direction. 29. Modification of the Slide Valve. In many cases valves are used which while they are slide valves and governed by the same principles are modified in various ways. We will now consider a few of these modifications. The examples mentioned are chosen chiefly because they will illustrate the types which they represent no attempt being made to cover the entire field. 30. Piston Valve. Fig. 33 shows a simple form of piston valve. This is essentially a plain slide valve except that it is cylindrical fitting nicely into a cylindrical chest. The ports spread out around the chest so that steam is admitted or exhausted around practically the entire circumference. A piston valve of a little more complicated construction is shown in Fig. 34. This is a large valve for use on a locomotive and is provided with packing rings to prevent leakage. Here the valve chest is fitted with liners a drawing of which is given in Fig. 35. 31. Balanced Valves. A plain D valve has the full steam-chest pressure on its entire outer surface while its inner surface is either in contact with the seat or subjected only to exhaust pressure except possibly the small area which may be over the ports and subject to whatever pressure is in the cylinder. The result is a heavy unbalanced pressure forcing the valve against its seat. This means a heavy .