Poppet Valves

[CopCarSS]

Tried this in the main forum without much response. Thought I'd try it here:

I seem to recall that towards the end of steam production there was a small movement towards poppet valves. I think the Pennsy used them on some of their stuff (the S1, T1 and Q2 duplexes maybe?)

I'm curious about their operation, and their advantages/disadvantages. Anybody know much about these creatures?

[J. Edgar]

 

 im not sure of the enitire operation but i know it invovles a prop shaft geared to the main crank ...at the other end ( the valve chest) im under the assumtion there are adjustable rotary valves.    Pennsy used the device on atleast 1 K4s and a couple of T1's..and maybe a S1...the Q's had walcherts gear..what ive read about the Pennsy's use was the device's increased savings in fuel....more presice adjustment and better passage flow....was reneged by the increased cost of a "one off" device not using "off the shelf" parts al la walcherts/baker gears......as i write this i did a  search and low and behold i was wrong.......its a spring loaded piston whose shaft is attached a disk that opens a port..heres a modern valve but the design is pretty much the same on a locomotive except the pilot port and piston "u" cup would have a second set of ports oppisite the ones shown here

 

 

 

 

...the prop shaft on a steam locomotive times the mechanism with the main piston and allows a more precice cut-off.......it seems they were used alot in England and Europe late steam.....

[CopCarSS]

Sweet, thanks for the info and the diagram. After reading the Poppet article that Jay posted in the main forum, I'm thinking that the Q duplexes didn't use poppets because they never saw the speeds where the poppet valving really became advantageous over piston valves. I'm guessing that a locomotive like a Q might be better served by the heavier duty nature of piston valving anyways?

[J. Edgar]

 

 in theory i would agree....the K4s's didnt come close to the Q's almost 6000 HP the T's did maybe but were still a "highspeed" locomotive.....their use in England seems to be on express engines built for high speeds ...here in the States the just plain larger size and hence volume of steam in a large locomotive like the Q's would seem to mean a piston valve....if steam would have had another decade to advance whos to say if a poppet valved Big Boy wouldnt have appeared

[timz]

The PRR S-1 didn't use poppet valves.

Two problems with conventional valves and gears come to mind:

The engineer shortens the cutoff by shortening the valve travel. At less than full cutoff, the valve won't open the full width of the steam port; at 25-30% cutoff it might only give a half-inch opening for the steam to get into the cylinder, which likely isn't enough. Dunno whether the exhaust opening is also constricting.

With conventional gears, shorter cutoff means earlier compression. How much of a problem is that? Dunno, but in the early 1950s the UP Oregon Div Special Rules said their 4-8-4s should not use less than 33% cutoff, in order to avoid hot main pins-- and likely that's why. (So why didn't other RRs say the same? No idea.)

Around 1939-40 the PRR tried poppets on a K-4, along with some other improvements, and got a dramatic increase in high-speed power. But NY Central's poppet 4-8-4 didn't outpull its piston counterparts by much at 80 mph, and not at all at 60.

[CopCarSS]

So it's sounding like the PRR did a lot with poppet valves (which makes sense as they kind of went their own way with steam anyways). I've also seen references to B&O, C&O and now NYC. Did many railroads experiment with poppet valving? It's also seeming like an eastern thing based on the references I've noted so far. Were the eastern roads more "pioneering" when it came to steam development, or were there other reasons that most of the poppet references have been eastern roads?

Thanks as always for any info!

[nanaimo73]

I believe Poppets came too late to see widespread use. ATSF had 4-8-4 3764 in 1929, but it was a failure with the Caprotti system. 4 4-8-4 locomotives came out later with Franklin equipment, CB&Q 5625 (1942), NYC 5500 (1946), ATSF 3752 (1948) and Canadian National 6184 (1951). MP had 4-6-2 6001 equiped with poppets in 1942. The PRR had the T1 fleet built with poppets, and 1 was rebuilt later with Walschaerts.  

[J. Edgar]

 MP had atleast one ....4-6-2 6001 had poppet valves ....dont know much about it just have the picture in a Don Ball Jr. book

[KCSfan]

 

Pennsy used the device on atleast 1 K4s and a couple of T1's..and maybe a S1...

Unless I'm mistaken, all of the T1's had rotary cam poppet valves. I recall hearing they required far more repair that did more conventional valve gear but I feel any of their drawbacks would have been worked out had steam not been replaced relatively shortly after their introduction. Of course by that time Pennsy maintenance was getting pretty slipshod. I have heard the T1's were often sent on the road with one set of sanders inoperational and when starting a heavy train they required both sand and a light touch on the throttle to prevent them from slipping.

Mark

[feltonhill]

Initially, none of the T1's had rotary cam poppet valve gear.  They had oscillating cam gear, better known as Type A.  No. 5547 was rebuilt with Walschaert gear and was reclassified T1a.  No. 5500 was rebuilt with rotary cam gear, sometimes referred to as Type B.

[KCSfan]

Initially, none of the T1's had rotary cam poppet valve gear.  They had oscillating cam gear, better known as Type A.  No. 5547 was rebuilt with Walschaert gear and was reclassified T1a.  No. 5500 was rebuilt with rotary cam gear, sometimes referred to as Type B.

Thanks for the prompt correction. I am not familiar with "oscillating cam" valve gear and apparently had it confused with roatary can driven valving. Did the oscillating cams actuate poppet or conventional steam valves? It's also interesting to learn about the T1a. Any idea why and when in the T1 life cycle (early or late will suffice) it was fitted with Walschaert gear? My guess would be that it was an attempt to overcome the maintenance problems that I have heard plagued the cam actuated valving. Another question that has always puzzled me is how was the cutoff varied with the cam drives, both oscillating and rotary?

Your reply demonstrates one of the great values of this Forum - specifically to correct long held beliefs that we sometimes have come to accept as valid truths.

Mark

[feltonhill]

On the T1, both the oscillating cam and rotary cam drives were mechanical.  Oscillating cams took their motion from a gearbox containing what amounted to a pair of miniature Walschaerts valve gears.  The valve drive cam moved through a small arc but did not fully rotate 360 degrees. Cutoff was  controlled within the gearbox  mechanism.

As the name suggests, on rotary cam drive, the cam operated more like those in IC engines, i.e., it rotated 360 degrees and the valve timing was controlled by the contours machined into the cam.  Others will have to fill in how the cam position was varied to effect cutoff positions.  I know it was done, but don't know exactly how.  I'll keep looking for a good explanation.

Some early poppet valve gear were steam actuated, but I never heard that they were successful, at least in the US environment. PRR had an L1 so equipped IIRC.

No. 5500 was converted to rotary cam valves 7/3/48.  Some of the enginemen considered it the best of the lot.  No. 5547 followed about a year later  entering service 7/30/49 as a T1a.  This was actualy very close to the original duplex concept as designed by Baldwin in the late 1930's before modifications for PRR in 1940-41.  The crews have said they couldn't tell any difference in performance between 5547 and the rest of the T1s, at least at normal operating speeds.

The oscillating cam drive, as it was installed on the T1 was a mechanic's nightmare.  Although the drive boxes themselves were considered fairly reliable, their location was about as inaccessible as one can imagine and routine mainenance suffered accordingly.  The front box was under the "hood" of shrouding on the pilot deck, behind the aftercooler and between the air pumps.  On a conventional locomotive with Type A gear, the drive box usually sat exposed on the pilot beam and was much easier to get to.  For the rear engine set, the box was located behind the cylinder saddle and positioned vertically between the frames.  Nice place to work. Oh, and just for further complication, it was a mirror image of the front drive box.  Both the Walchaerts equipped 5547 and the outside drive rotary cam 5500 were vast improvements regarding accessibility and probably maintenance cost.

[Lost World]

So it's sounding like the PRR did a lot with poppet valves (which makes sense as they kind of went their own way with steam anyways). I've also seen references to B&O, C&O and now NYC. Did many railroads experiment with poppet valving? It's also seeming like an eastern thing based on the references I've noted so far. Were the eastern roads more "pioneering" when it came to steam development, or were there other reasons that most of the poppet references have been eastern roads?

Thanks as always for any info!

C&O's 5 L-1 class Hudsons (rebuilt from older f-19 pacifics) utilized type A poppet valves.  L-2A class Hudsons 310-314 used a type b poppet valve and were ordered with poppet valves due to the success of the valves used on the L-1 class.  From all I've read the poppet valves on either class were pretty reliable, but then one must consider that by this time--1946 and 1948 respectively--poppett valves had advanced quite a ways.