27:
166:
138:
219:
Lead is the amount by which a port is open when the piston is at front or back "dead centre". In the case of the steam port this allows admission of steam to fill the clearance space between the cylinder and piston and ensures maximum cylinder pressure before the commencement of the stroke. This both
210:
Negative exhaust lap, also commonly known as exhaust clearance, is the amount the port is open to exhaust when the valve is in mid-position, and is used on many fast-running locomotives to give a free exhaust. The cylinder on both sides of the piston is open to exhaust at the same time when the valve
220:
cushions and assists the mass of the piston to slow down and change direction. At slow speeds no lead is ideal. For engines with pistons of 24 inches plus and masses of over 5 kilos and pressures under 500 psi then cushioning is beneficial. Lead is necessary on locomotives designed for high speeds.
124:
Most piston valves are of the "inside admission" type, where fresh steam is introduced from the boiler via the space between the two piston heads of the valve, and exhaust steam leaves via the space between a piston head and the end of the steam chest. The advantage of this arrangement is that
53:
into the cylinders and its subsequent exhausting, enabling a locomotive to move under its own power. The valve consists of two piston heads on a common spindle moving inside a steam chest, which is essentially a mini-cylinder located either above or below the main cylinders of the locomotive.
202:
side of the piston or slide valve. Exhaust lap is generally given to slow-running locomotives. This is because it allows the steam to remain in the cylinder for the longest possible amount of time before being expended as exhaust, therefore increasing efficiency.
125:
leakage, via the gland which seals the steam chest from the operating rod of the valve gear, is much less of a problem when the gland is subjected to low exhaust pressure rather than full boiler pressure. However, some locomotives, like
Bulleid's
239:'s diagrams) is to pretend that both the valve and the piston have a sine-wave motion (as they would, if the main rod were infinitely long). Then, for instance, to calculate the percent of the piston's stroke at which steam admission is cut off:
153:, manufactured by The Horsely Coal & Iron Company in 1833, shows a very early use of the piston valve. Piston valves had been used a year or two previously in the horizontal engines manufactured by
261:
2-10-0 had lap 2 inches, lead 1/4 inch and valve travel 6 inches in full gear. In full gear the two angles are 48.19 deg and 41.41 deg and the maximum cutoff comes out 49.65% of the piston stroke.
30:
Diagram of cylinder and piston valve. The valve is next opened by moving it to the right, allowing the clear space in the middle of the valve to align with the channel in the cylinder above it.
177:
at precise times relative to the piston's position. This entails controlling the admission and exhaust of steam to and from the cylinders with a valve linked to the motion of the piston.
180:
For timing and dimensioning of slide or piston valves where the valve opens and closes the steam and exhaust ports, a consideration of the "lap" and "lead" is required.
389:
303:
Bristol: Avon Anglia
Publications & Services. Reprinted from the Transactions of the Leicestershire Archaeological Society Volume XXX, 1954.
1104:
228:
Given the valve's lap, lead, and travel, at what point in the piston's stroke does the valve open and close, to steam and to exhaust?
382:
188:
Lap is the amount by which the valve overlaps each port at the middle position of each valve. There are two different types of lap.
129:, used "outside admission" where the reverse was true, in Bulleid's case because of the unusual chain-driven valve gear arrangement.
1264:
1344:
1137:
1274:
532:
1170:
943:
375:
839:
26:
834:
359:
150:
142:
982:
1298:
1210:
195:
side of the piston or slide valve (i.e. the distance the valve needs to move to just begin to uncover the port).
258:
250:
Add the two angles and take the cosine of their sum; subtract 1 from that cosine and multiply the result by -50.
231:
Calculating an exact answer to that question before computers was too much work. The easy approximation (used in
1164:
1339:
1269:
913:
157:
of London, but did not become general for stationary or locomotive engines until the end of the 19th century.
74:. In the 20th century, slide valves were gradually superseded by piston valves, particularly in engines using
1158:
557:
1145:
933:
1283:
1001:
973:
857:
412:
275:
71:
872:
751:
191:
The first kind is the steam lap, which is the amount by which the valve overlaps the steam port on the
1303:
1292:
1150:
849:
462:
93:
With piston valves, the steam passages can be made shorter. This, particularly following the work of
1077:
1009:
198:
Secondly, the exhaust lap, which is the amount by which the valve overlaps the exhaust port on the
1313:
1308:
1229:
1098:
953:
898:
620:
576:
313:
121:
may be used, though standard gears as mentioned above were used as well, by
Chapelon and others.
1116:
877:
1349:
1318:
1216:
1090:
1057:
938:
640:
589:
527:
126:
106:
1183:
908:
829:
802:
599:
517:
427:
254:
154:
102:
1243:
928:
867:
728:
594:
542:
479:
432:
246:
Calculate the angle whose cosine is twice the (lap plus lead), divided by the valve travel
8:
1237:
771:
645:
562:
552:
118:
733:
1034:
882:
821:
776:
756:
743:
723:
705:
650:
604:
474:
417:
355:
174:
165:
94:
1110:
1071:
797:
715:
685:
537:
110:
63:
46:
1016:
958:
766:
761:
675:
630:
1202:
1051:
1023:
700:
522:
512:
422:
236:
199:
1333:
990:
963:
862:
781:
617:
243:
Calculate the angle whose cosine is twice the lap divided by the valve travel
232:
1191:
948:
695:
680:
635:
489:
484:
398:
211:
is passing through the mid-position, which is only momentary when running.
114:
42:
20:
1043:
614:
584:
457:
270:
87:
83:
75:
67:
1122:
1065:
690:
625:
609:
547:
504:
494:
280:
192:
137:
918:
903:
807:
204:
173:
When on the move, a steam locomotive requires steam to enter the
97:, reduces resistance to the flow of steam and improves efficiency
367:
923:
660:
169:
Diagram showing lap and lead and their relation to valve travel
113:, can be used with either slide valves or piston valves. Where
442:
50:
38:
317:
207:
locomotives tended to be equipped with this addition.
145:winding engine of 1833 incorporated a piston valve
70:to control the flow of steam into and out of the
16:Form of valve within a steam engine or locomotive
1331:
354:(London: Hermes Publishing Company, Ltd., 1998)
312:Information plaque on the Swannington engine,
149:The Swannington incline winding engine on the
383:
352:The Ultimate Encyclopedia of Steam & Rail
19:For other applications of piston valves, see
346:
344:
342:
340:
338:
336:
334:
332:
330:
328:
326:
223:
86:slide valves adequately in the presence of
41:used to control the flow of steam within a
390:
376:
101:The usual locomotive valve gears such as
323:
164:
136:
78:steam. There were two reasons for this:
25:
1265:Glossary of steam locomotive components
301:The Leicester & Swannington Railway
1332:
371:
350:Garratt, C. & Wade-Matthews, M.:
160:
117:are used, a different gear, such as
13:
1117:National Museum of Scotland engine
14:
1361:
397:
151:Leicester and Swannington Railway
49:. They control the admission of
1299:List of steam technology patents
1284:Murdoch's model steam carriage
1270:History of steam road vehicles
306:
293:
1:
1345:Steam locomotive technologies
1211:Murray's Hypocycloidal Engine
286:
934:Return connecting rod engine
7:
858:Condensing steam locomotive
276:Steam locomotive components
264:
132:
57:
10:
1366:
1165:"Coalbrookdale Locomotive"
18:
1257:
1228:
1201:
1182:
1171:"Pen-y-Darren" locomotive
1136:
1089:
1042:
1033:
1000:
981:
972:
891:
848:
840:Single- and double-acting
820:
790:
742:
714:
668:
659:
575:
503:
450:
441:
405:
1010:Newcomen Memorial Engine
224:Calculating valve events
1314:Timeline of steam power
1309:Stationary steam engine
1192:Woolf's compound engine
1099:Soho Manufactory engine
954:Steeple compound engine
621:straight line mechanism
314:National Railway Museum
214:
1319:Water-returning engine
1293:Lean's Engine Reporter
1066:Chacewater Mine engine
939:Six-column beam engine
183:
170:
155:Taylor & Martineau
146:
127:SR Merchant Navy class
31:
1340:Locomotive valve gear
1159:London Steam Carriage
299:Clinker, C.R. (1977)
168:
140:
62:In the 19th century,
29:
1105:Bradley Works engine
929:Reciprocating engine
752:Babcock & Wilcox
595:Centrifugal governor
646:Sun and planet gear
143:Swannington incline
119:Caprotti valve gear
82:It is difficult to
1146:Richard Trevithick
744:Water-tube boilers
558:Gresley conjugated
171:
147:
32:
1327:
1326:
1253:
1252:
1132:
1131:
816:
815:
716:Fire-tube boilers
571:
570:
161:Design principles
64:steam locomotives
1357:
1277:fardier Ă vapeur
1111:Whitbread Engine
1072:Smethwick Engine
1040:
1039:
979:
978:
798:Feedwater heater
666:
665:
448:
447:
392:
385:
378:
369:
368:
362:
348:
321:
310:
304:
297:
111:Baker valve gear
37:are one form of
1365:
1364:
1360:
1359:
1358:
1356:
1355:
1354:
1330:
1329:
1328:
1323:
1249:
1224:
1197:
1178:
1128:
1085:
1029:
1017:Fairbottom Bobs
1002:Newcomen engine
996:
968:
914:Expansion valve
887:
873:Watt's separate
844:
812:
786:
738:
710:
655:
631:Parallel motion
567:
518:Stephenson link
499:
437:
406:Operating cycle
401:
396:
366:
365:
349:
324:
311:
307:
298:
294:
289:
267:
226:
217:
186:
163:
135:
60:
24:
17:
12:
11:
5:
1363:
1353:
1352:
1347:
1342:
1325:
1324:
1322:
1321:
1316:
1311:
1306:
1301:
1296:
1289:
1288:
1287:
1281:
1267:
1261:
1259:
1255:
1254:
1251:
1250:
1248:
1247:
1241:
1234:
1232:
1226:
1225:
1223:
1222:
1214:
1207:
1205:
1199:
1198:
1196:
1195:
1188:
1186:
1180:
1179:
1177:
1176:
1175:
1174:
1168:
1162:
1156:
1142:
1140:
1134:
1133:
1130:
1129:
1127:
1126:
1120:
1114:
1108:
1102:
1095:
1093:
1087:
1086:
1084:
1083:
1075:
1069:
1063:
1055:
1052:Kinneil Engine
1048:
1046:
1037:
1031:
1030:
1028:
1027:
1024:Elsecar Engine
1021:
1013:
1006:
1004:
998:
997:
995:
994:
987:
985:
976:
970:
969:
967:
966:
961:
956:
951:
946:
944:Steeple engine
941:
936:
931:
926:
921:
916:
911:
906:
901:
895:
893:
889:
888:
886:
885:
880:
875:
870:
865:
860:
854:
852:
846:
845:
843:
842:
837:
832:
826:
824:
818:
817:
814:
813:
811:
810:
805:
803:Feedwater pump
800:
794:
792:
788:
787:
785:
784:
779:
774:
769:
764:
759:
754:
748:
746:
740:
739:
737:
736:
731:
726:
720:
718:
712:
711:
709:
708:
703:
698:
693:
688:
683:
678:
672:
670:
669:Simple boilers
663:
657:
656:
654:
653:
651:Watt's linkage
648:
643:
638:
633:
628:
623:
612:
607:
602:
600:Connecting rod
597:
592:
587:
581:
579:
573:
572:
569:
568:
566:
565:
560:
555:
550:
545:
540:
535:
530:
525:
520:
515:
509:
507:
501:
500:
498:
497:
492:
487:
482:
477:
472:
467:
466:
465:
454:
452:
445:
439:
438:
436:
435:
430:
425:
420:
415:
409:
407:
403:
402:
395:
394:
387:
380:
372:
364:
363:
322:
305:
291:
290:
288:
285:
284:
283:
278:
273:
266:
263:
248:
247:
244:
225:
222:
216:
213:
185:
182:
162:
159:
134:
131:
99:
98:
95:André Chapelon
91:
59:
56:
15:
9:
6:
4:
3:
2:
1362:
1351:
1350:Engine valves
1348:
1346:
1343:
1341:
1338:
1337:
1335:
1320:
1317:
1315:
1312:
1310:
1307:
1305:
1302:
1300:
1297:
1295:
1294:
1290:
1285:
1282:
1279:
1278:
1273:
1272:
1271:
1268:
1266:
1263:
1262:
1260:
1256:
1245:
1242:
1239:
1236:
1235:
1233:
1231:
1227:
1220:
1219:
1215:
1212:
1209:
1208:
1206:
1204:
1200:
1193:
1190:
1189:
1187:
1185:
1181:
1172:
1169:
1166:
1163:
1160:
1157:
1154:
1153:
1152:Puffing Devil
1149:
1148:
1147:
1144:
1143:
1141:
1139:
1138:High-pressure
1135:
1124:
1121:
1118:
1115:
1112:
1109:
1106:
1103:
1100:
1097:
1096:
1094:
1092:
1091:Rotative beam
1088:
1081:
1080:
1076:
1073:
1070:
1067:
1064:
1061:
1060:
1056:
1053:
1050:
1049:
1047:
1045:
1041:
1038:
1036:
1032:
1025:
1022:
1019:
1018:
1014:
1011:
1008:
1007:
1005:
1003:
999:
992:
991:Savery Engine
989:
988:
986:
984:
980:
977:
975:
971:
965:
964:Working fluid
962:
960:
957:
955:
952:
950:
947:
945:
942:
940:
937:
935:
932:
930:
927:
925:
922:
920:
917:
915:
912:
910:
907:
905:
902:
900:
897:
896:
894:
890:
884:
881:
879:
876:
874:
871:
869:
866:
864:
861:
859:
856:
855:
853:
851:
847:
841:
838:
836:
833:
831:
828:
827:
825:
823:
819:
809:
806:
804:
801:
799:
796:
795:
793:
789:
783:
780:
778:
775:
773:
770:
768:
765:
763:
760:
758:
755:
753:
750:
749:
747:
745:
741:
735:
732:
730:
727:
725:
722:
721:
719:
717:
713:
707:
704:
702:
699:
697:
694:
692:
689:
687:
684:
682:
679:
677:
674:
673:
671:
667:
664:
662:
658:
652:
649:
647:
644:
642:
641:Rotative beam
639:
637:
634:
632:
629:
627:
624:
622:
619:
618:hypocycloidal
616:
613:
611:
608:
606:
603:
601:
598:
596:
593:
591:
588:
586:
583:
582:
580:
578:
574:
564:
561:
559:
556:
554:
551:
549:
546:
544:
541:
539:
536:
534:
531:
529:
526:
524:
521:
519:
516:
514:
511:
510:
508:
506:
502:
496:
493:
491:
488:
486:
483:
481:
478:
476:
473:
471:
468:
464:
461:
460:
459:
456:
455:
453:
449:
446:
444:
440:
434:
431:
429:
426:
424:
421:
419:
416:
414:
411:
410:
408:
404:
400:
399:Steam engines
393:
388:
386:
381:
379:
374:
373:
370:
361:
360:1-84038-088-8
357:
353:
347:
345:
343:
341:
339:
337:
335:
333:
331:
329:
327:
319:
315:
309:
302:
296:
292:
282:
279:
277:
274:
272:
269:
268:
262:
260:
256:
253:As built the
251:
245:
242:
241:
240:
238:
234:
229:
221:
212:
208:
206:
201:
196:
194:
189:
181:
178:
176:
167:
158:
156:
152:
144:
139:
130:
128:
122:
120:
116:
115:poppet valves
112:
108:
104:
96:
92:
89:
85:
81:
80:
79:
77:
73:
69:
65:
55:
52:
48:
44:
40:
36:
35:Piston valves
28:
22:
1304:Modern steam
1291:
1276:
1238:Porter-Allen
1217:
1151:
1078:
1058:
1015:
949:Safety valve
878:"Pickle-pot"
772:Thimble tube
469:
351:
308:
300:
295:
255:Pennsylvania
252:
249:
230:
227:
218:
209:
197:
190:
187:
179:
172:
148:
123:
100:
68:slide valves
61:
43:steam engine
34:
33:
21:Piston valve
1035:Watt engine
835:Oscillating
791:Boiler feed
636:Plate chain
615:Tusi couple
528:Walschaerts
413:Atmospheric
271:Slide valve
107:Walschaerts
88:superheated
76:superheated
1334:Categories
1244:Ljungström
1230:High-speed
1123:Lap Engine
1079:Resolution
983:Precursors
868:Kirchweger
830:Locomotive
777:Three-drum
757:Field-tube
724:Locomotive
706:Lancashire
626:Link chain
610:Crankshaft
577:Mechanisms
505:Valve gear
287:References
281:Valve gear
193:live steam
103:Stephenson
47:locomotive
1275:Cugnot's
1218:Salamanca
919:Hydrolock
904:Crosshead
850:Condenser
686:Egg-ended
84:lubricate
72:cylinders
1258:See also
1184:Compound
1059:Old Bess
899:Blowback
822:Cylinder
808:Injector
767:Stirling
762:Sentinel
676:Haystack
590:Cataract
563:Southern
553:Caprotti
428:Compound
265:See also
237:Realeaux
175:cylinder
133:Examples
58:Overview
974:History
883:Surface
701:Cornish
661:Boilers
543:Corliss
480:Corliss
463:D slide
433:Uniflow
423:Cornish
235:'s and
205:shunter
200:exhaust
1286:(1784)
1280:(1769)
1246:(1908)
1240:(1862)
1221:(1812)
1213:(1805)
1203:Murray
1194:(1803)
1173:(1804)
1167:(1803)
1161:(1803)
1155:(1801)
1125:(1788)
1119:(1786)
1113:(1785)
1107:(1783)
1101:(1782)
1082:(1781)
1074:(1779)
1068:(1778)
1062:(1777)
1054:(1768)
1026:(1795)
1020:(1760)
1012:(1725)
993:(1698)
959:Stroke
924:Piston
909:Cutoff
782:Yarrow
734:Launch
729:Scotch
490:Sleeve
485:Poppet
470:Piston
451:Valves
443:Valves
358:
233:Zeuner
109:, and
892:Other
696:Flued
681:Wagon
605:Crank
548:Lentz
538:Baker
533:Allan
458:Slide
90:steam
66:used
51:steam
39:valve
1044:Beam
585:Beam
495:Bash
475:Drop
418:Watt
356:ISBN
318:York
215:Lead
141:The
863:Jet
691:Box
523:Joy
513:Gab
259:I1s
257:'s
184:Lap
45:or
1336::
325:^
316:,
105:,
391:e
384:t
377:v
320:.
23:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
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