For a while, the PRR favored the “Tailrod” where the piston rod and valve guide extended through the front cylinder heads. I believe the thinking was to reduce wear on the crosshead and piston rings?
It was also a design that was used to a greater extend in Europe.
PRR_K4-3683 by Edmund, on Flickr
PRR K4 3683
Cheers, Ed
i can see how the forward rod extension provides structural support to keep the piston rod in the rear supported. It looks like the crosshead is much smaller than on more conventional locomotives i’m familiar seeing.
the forward piston rod would also make the areas the same so that the force is the same.
i can also see how after some trials, some RRs found the extra effort and expense not worth the trouble. They could live with the asymetric force when piston moves forward and rearward.
since the Walschaerts valve gear was developed in Belgium, i guess it’s fair to say it was first tried on a European locomotive of this type and with slide valves.
I don’t think I fully understand Don Ashton’s page on Walshaerts gear explanation of why outside admission requires a different combination attachment point due to geometry, but it sounds like there is an inherent advantage to inside admission. I’ll guess that it’s better to have a larger/longer opening to allow steam in that to allow it out.
thanks for all the corrections … a learning experience.
Greg,
If you want to play with valve gear, go to this web site and download them:
AH, I did not know that was still around. I downlaoded it years ago, though tit was long gone by now. Great for seeing the differences between valve gear types
–Randy
What was sometimes done was to tinker with the ports in the liner to adjust the steam flow slightly to account for the differential area.
The principal reason for tailrods was to support the piston centered in the bore, for less wear and better ring sealing and longer gland life. You will find different arrangements of ‘slippers’ and other methods of supporting the piston in the bore with additional area ‘down’, and better crosshead precision (particularly in the later multiple-bearing crosshead designs) to prevent flexing of the piston rod in the gland and developing blow there.
Walschaerts didn’t really ‘catch on’ in North America until engine size increase made a fully external valve gear desirable. There are interesting discussions of this in the Angus Sinclair publications, a contemporary view of the issues.