Steam & Excursion > ATSF 3752 w/ Capretti Valve Gear

Pacific BR71000 Duke of Gloucester, is tearing up the record book in preservation on many of the banks over in the UK these days, largely due to her British modified Italian Capretti Valve Gear. I hear ATSF 3752 also had this motion. Does anyone have a link to a clip of her with this motion? Was she an experiment? How was her preformance? This locomotive should have been saved.



Edited 1 time(s). Last edit at 07/11/11 15:35 by OKTrainboys.

The Modified Caprotti valve gear was marketed and licensed by Franklin Ry. Supply in the 1940s.
(Caprotti was Italian Inventor who patented the scheme in the 1930s).

The largest numbers of installation of Caprotti-type gear was on the PRR's T1 Class 4-4-4-4 (Duplex drive) locomotives.
There's a C&O, loco #490, at the B&O Museum in Baltimore, MD that still has its Caprotti-type valve gear.

Its main drawback was that for heavy-duty, low maintenance, service it had a short useful life. The double-beat poppett valves were seated by
large coil springs. At high RPMs, and high valve event frequencies, the springs tended to 'float' --failure to close completely.
The remedy to the valve float was stiffer return springs! Ergo---- the valves and their corresponding valve seat (inserts) tended to crumble and woooshed up the stack ---- leaving the locomotive stranded!

The NYC had a new Niagara 4-8-4 #5500 built & equipped with these type poppett valves and it failed repeatedly because of the problems with the broken return spring mechanisms.

For light trains, like they build in England, they work well --not so much over here with our monster size locos.

Wes Camp1472

Akin to Formula 1 motors that run air springs and why we used pilot operated devices in hydraulics
and pneumatics. You reach a point where the force required and the mechanicals of the springs no
longer co-operate.

Wes is pretty close, but without trying to delve into the really old magazines I'll add that I belive Caprotti was active earlier than the 1930's. Union Pacific fitted Caprotti's gear to 4-8-2 #7006 in 1929 when the engine was seven years old.

Along the lines stated, the gear was found to be too "light" for US service, but I believe it's probably more closely related to Wes' statement that the problem was aligned more with valve spring capabilities than anything else.

By the time my friend Bill Kratville was writing about this Union Pacific experiment the documents related to it were gone and the men associated with it were near retirement. I'm not sure more data related to the Union Pacific experiment exists.

John Bush
Omaha

OKTrainboys Wrote:

> Pacific BR71000 Duke of Gloucester, is tearing up
> the record book in preservation on many of the
> banks over in the UK these days, largely due to
> her British modified Italian Capretti Valve Gear.
> I hear ATSF 3752 also had this motion.

Santa Fe 4-8-4 3764 was built new with Caprotti valve gear in 1929. The conclusions of the test report summarized in Santa Fe Locomotive Development were that the locomotive didn't develop the same horsepower as her Walschaerts-equipped sisters, and that the gear was too lightly constructed to stand up to the service demanded of it. By the early 1930's, she had been re-fitted with Walschaerts gear.

Otto Perry photographed ATSF 3764 with Caprotti gear in 1930:

http://digital.denverlibrary.org/u?/p15330coll22,58331

The engine had Walschaerts gear by the time he shot it again in 1934:

http://digital.denverlibrary.org/u?/p15330coll22,58343

Santa Fe 4-8-4 3752 was rebuilt with Franklin rotary cam poppet valve gear in 1948. This was reportedly a successful installation, and the engine performed well, with improved horsepower and fuel economy compared to her sisters. It evidently wasn't enough to warrant further applications or stem the diesel tide, however. I recall reading somewhere that a problem in the valve gear was what took her out of service for the last time in mid-August 1953, but darned if I can find the reference now.

Otto Perry also photographed 3752 with poppet valves:

1948: http://digital.denverlibrary.org/u?/p15330coll22,58306

Was "modified Caprotti" gear what Franklin marketed as their Type A "oscillating cam" gear, or their Type B "rotary cam" gear?

From Wes Camp1472..

The Franklin licensed Caprotti design used the Oscillating Cam version.
The Oscillating Cam Caprotti type gear used linkage from the crosshead: deriving 'oscillating' motion to the cam shaft(s). The cams were elongated shafts with the cam lobes tapering in width, and also slightly skewed along its length. The skewing provided valve event 'advancing' for higher speed operation. The valve gear 'box' contained the mechanism that allowed the cam shafts to be slid by the roller tappets for advancing speed (the old 'hooking-up' equivalent action) together with allowing for 180 degree phase shifting to allow starting in reverse.

The OC cam version used an unusual cross-drive concept: the right hand crosshead provided the drive for the left side piston valve events & the left side cross head drives the events for the right side piston. The motion derived produces a sine-curve-like variable velocity to the cam shafts. The velocity varies because the piston and crosshead stop at the end of each stroke, suddenly reverse direction and build in velocity as the crank pins pass through
their mid-points (when the crank pins are near the 6 o'clock or 12 o'clock positions) midway through the power stroke. The piston slows in speed as it approaches the closest cylinder head, stops and reverses for the next rotation.

This sine curve-like variable velocity characteristic must be compensated in the design of the Caprotti cam shaft's lobe shapes.

The Rotary Cam Franklin product is easily identified by a large, rigid, elongated triangular support frame between the main driver and the valve housing over the piston cylinders. This frame carried the rotating drive shaft to spin the elongated, continuously variable cam lobes that provides the similar valve event-advancing characteristics. The shaft is driven by the 'return crank' on the main driver's crank pin -- similar to the 'eccentric crank of the common radial valve gears --- Walschaerts and Baker as well as other variants. The cam driven system, however, uses the 'return crank' motion by having the small end of the crank centered over the main diver axle's center line -- the motion thus produced, when converted by bevel gearing (or similar), to a smoothly
rotating shaft drive mechanism.

The rotary motion produced to drive the rotary drive cam for the poppetts is a smooth, constant direction, motion that only varies with the speed of the main axle. Its more like the smooth cam rotation produced in reciprocating (common) automotive engines. The Rotary Cam Poppet valves were driven by the camshaft on the respective side of the locomotive, not a cross-drive like the Caprotti gear.

The Rotary Cam version is simpler to comprehend and understand. I think that an ex-Army 2-8-0 (no. 611?) exists at the TVRM. I believe that its a rotary cam Franklin valve gear. The C&O # 490, at the B&O Museum is a Caprotti-type valve gear.

The main advantage of the poppet valves over classic spool valves is that the size of the port opening of the spool valve systems varies as the valve gear travel is shortened to allow faster running. The valve gear can be 'put in the corner' to get live steam admission, at near full boiler pressure, for almost 80% of the piston stroke. OK at slow speeds, but this drinks horrendous amounts of wet-steam and water when starting. High power valve setting, 50% or more, uses huge quantities of water.

As the locomotive speeds up, two things happen: the steam begins to absorb heat and become superheated and the engine driver shortens the travel of the valve gear to conserve steam and water. When the superheater is working at maximum heat, the heat-saturated steam entering the dry pipe is 'heat-amplified' in the superheater units. More steam comes out of the units than goes into the units! More steam comes out because the gaseous water molecules (steam) are vastly excited and separated from each other at greater spacing--- but the pressure remians the same!. The actual flow of steam through the superheater is slowed down when the locomotive is hooked-up! The really hot steam explodes into the cylinders, the admission valve closes and the steam continues to provide expansion power to the pistons all the way down the stroke. It is this expansive power of the steam that allows the locomotive to merely sip water when cruising at high speed.

Sometimes there is so much power in the cylinders that the engineer squeezes down on the throttle, and lowers the steam chest pressure on the pistons. The engineer will compensate for the lower pressure by dropping the reverse gear control lever for a longer admission period of each stroke. The superheated steam now has a chance to more evenly expand against the pistons, the reduced amount of steam in the cylinders means less crowding at the exhaust events, and a softer bark up the stack.

However, all is not so sweet. The shortening of the valve travel at higher speeds also reduces the port opening less wide and chokes-off both the admission opening as well as reducing the size for the exhaust ports. The reduced intake port-width results in 'wire-drawing' of the steam through the ports. It also reduces the exhaust port opening and thus reduces the draft available to feed the fire! The choked-off exhaust ports increases the back-pressure on the pistons! The increased back pressure REDUCES the effective pressure acting on the piston. Example: If the residual pressure in the cylinder, near the end of the stroke is, say, 80 PSIG and the back pressure is 50 PSIG -- the useable pressure differential across the piston is 30 PSIG!
Good for stack-talk, but beats the running gear mercilessly! Hook her UP, Dummy!

A compromise was struck by squeezing down the opening of the blast nozzle, below the petticoat pipe, of the smoke stack. At starting, this squeezed-down
exhaust jet produces a violent draft through the grates and can pull unburned fuel into the flues and tubes. It also makes great photo run-bys and dynamic audio tracks for the fans!

(Note: The necessary design changes to the exhaust nozzles of late design Poppet Valve locomotives seems to be an area for greater research --- a multiple-port exhaust scheme, like the kinds Dante Porta advocated should yield real draft benefits, and a less fuel-hungry loco!)

Doyle tells the story of a NKP crew storming down the main in a wide-open Berk: The engineer wanted more boiler pressure and chided the combustioneer
(fireman) by saying that there was a buzzard riding on the pop valves (safeties)! The fireman looked out and exclaimed that "the buzzard's not there anymore,
he went to the stack for a drink of water!"

Now, why all the verbiage? The poppett valves always open the to the max lift of the cam lobes! Now we can get full power admission regardless of track speed AND separation of the exhaust events from affecting the admission events. Exhaust only occurs at, or near the end, of the piston stroke! A wide open poppet exhaust valve allows the used steam to escape freely! A free-er breathing engine is one result of the use of poppets.

Another result is a very 'slippery' Locomotive! When locomotive drivers slip on dry rail, the friction between the drivers and the rail approaches ZERO -- like auto tires on icy roads. The free breathing cylinders (poppet valve-fed) continue to get pressurized steam, even with the throttle closed --residual pressure in the steam pipes and valve chests --- the wheels, seeing no drag from the rails are induced to remain spinning on very little steam pressure!

Engineer John Roller, of the PRR, (who drove the NKP 759 during its adventure to Horseshoe Curve in 1971) told me that he could control the PRR T1's slipperiness by letting the T1 boiler pressure come way down during station stops, helped by the injector. Upon departure time, the T1 could be eased out of the station, sticking to the rails, and as speed came up, so did boiler pressure! He had no trouble with the T1s!

That's enough to ponder, for now!

Wes Camp1472

PS...
When I win the BIG lottery, we'll restore and fire up the poppet valve specimen locos that remain!

Tid bit found here: http://www.steamindex.com/jile/jile34.htm


Journal of the Institution Locomotive Engineers

Volume 34 (1944)

Journal No. 177

Alcock, G.W. (Paper No. 444)
Development of the locomotive poppet valve gear in America. 5-25. Disc.: 25-61. 19 figures (illus. and diagrs.)
Third Ordinary General Meeting of the Session 1942-43 held at the Institution of Mechanical Engineers, London, on Thursday, 10 June 1943, at 5.30 p.m., O.V.S. Bulleid, President, occupying the chair.
The Franklin System of Steam Distribution marks a step forward in the scientific development of the steam locomotive. This system stems from two important developments: the use of horizontal poppet valves of a design very similar to that applied to a great many British built locomotives ; and an entirely new design of valve gear developed by the late William E. Wood[w]ard, for many years Vice-president in Charge of Design of the Lima Locomotive Works.
The Franklin Railway Supply Company undertook a major research programme to develop .an entirely new system of steam distribution, which would bring to the American locomotive the full potential advantages possible with poppet valves. The objectives of this research programme were 'as follows :-
Increase mean effcctive pressure to be obtained by a separation of the valve events so that the admission and cut-off, release and compression would be controlled independently, at the same time providing large passage areas for both inlet and exhaust and improved steam flow.
Increase cylinder economy to be obtained by reduced clearance volume, the independent control of the valve events, and by a substantial decrease in back pressure.
Increased mechanical efficiency to be obtained by reduced weight of moving masses, reduced friction and elimination of carbonization.
Reduced maintenance to be obtained by the use of light-weight parts, the entire mechanism to be fitted with anti-friction bearings, running in an oil bath. Absolutely fixed valve events at all speeds and all cut-offs.