Steam limits

quote:

what limits the top speed of a steam engine.

Only the size of the pair on the man in the right hand seat!

Other than the condition of the track (plus curves, grades and location of signals), limiting factors are boiler capacity, driver diameter, counterbalancing, main rod/piston thrust, and side rods. Maybe the valve stem and its valve in the steam chest if they move back and forth too fast. Exceed maximum safe speed and something will give way.

Generally, steam locomotives designed for high speeds have tall (big) drivers and 4-wheel pilot trucks to guide them through curves. Their frames and equalizing systems [allow drivers and trucks to move up and down and side to side] must be up to the task, too.

Nickel Plate Berkshires excelled in fast freight service with 2-wheel pilot trucks.

A rough estimate is 10 mph for every inch of driving wheel diameter.

Shays and other geared locomotives are different because their cylinders drive a telescoping shaft that turns the wheels via gears, not rods. Shays sound as though they are going 100 mph as soon as they exceed walking speed. The best place to hear that is Cass Scenic Railroad. Past the enginehouse, when a Shay begins to push a train upgrade, the engineer widens the throttle and the exhaust from 3 cylinders erupts into a roar.

Exhausts on Heislers and Climaxes are a bit slower because each has only two cylinders turning a driveshaft centered below the boiler.

I suppose a geared locomotive, pushed beyond its limits (15 mph - 20 mph), would suffer a failure in the drive train.

Anecdotal stories say that the PRR T-1's went in excess of their Barco speedometers limit of 120 MPH frequently.
Out the west of Crestline OH.

quote:

Originally posted by ChooChooDennis:
Anecdotal stories say that the PRR T-1's went in excess of their Barco speedometers limit of 120 MPH frequently.
Out the west of Crestline OH.

Actually documented by the late Vernon L. Smith, when he was working for Franklyn Rwy Supply and had to ride the T-1s between Ohio and Chicago. I have seen his notebooks, which his son, Charles, still has.

What limits the maximum RPM of the engine in your car? How fast can you spin it before it throws a rod?

Same thing with a steam locomotive. How fast can the machinery rotate and still stay together.

....or the engineer loses his/her nerve!

Also documented are instances of Milwaukee F7 Hudsons straight-lining the graphs on their 120 MPH speed recorders for several miles on end. C. H. Bilty, the Milwaukee chief mechanical honcho at the time, estimated the actual top speed at 125 MPH.

N&W's Class J 4-8-4 610, with its 70" drivers, was documented as attaining a speed of 111 MPH by PRR supervisors who rode and ran it during its tests between Fort Wayne and Chicago. Although it is certain that other 4-8-4s ran faster than that, there is no other known DOCUMENTATION of a 4-8-4 running that fast. In other words, if anybody ever did, they never bragged about it . . .

EdKing

Yes, I had heard about those notebooks. I am glad to hear they still exist. I hope they have been published or will be!

Top speed of J. Neils #5 at IRM was about 15mph. At that speed, it feels like you've reached escape velocity!

Once had a ground man riding the rear footboards dump the air on me during a move. When I asked him why, he said "You were going too fast!"

I replied, "This is a Shay! It doesn't have a too fast!"

Rusty

quote:

A rough estimate is 10 mph for every inch of driving wheel diameter.

The rule of thumb is 1 MPH per inch of driving wheel diameter.

That being said, I'm pretty sure Rich has mentioned having his favorite 69"-drivered Berkshire near the 79 MPH passenger limit a few times ('93 Akron to Pittsburgh if memory serves), so it really is a rule of thumb and not an absolute maximum.

quote:

Originally posted by ReadingFan:

A rough estimate is 10 mph for every inch of driving wheel diameter.

Ummm . . .

so a locomotive with 80-inch drivers could go 800mph?

Call me skeptical


Just to add on to what others have said, RPM is more of an indicator of top speed than MPH. Check the RPM on all the rod engine examples given, and the top speed will correlate to somewhere around 525-550 RPM. Above that, the forces associated with all those flailing rods tend to result in airborne pieces of steel.

844 has 80 inch and goes faster than 80 more like 100 mph.

Also with the shays in the race you saw the engineer raise the throttle to the roof so was there a governer on it or did the boiler run out of steam for any more speed. If you have the pentrex video on the steam celebration at Sacramento Ca it is on that video. 0 to 18 mph must have been about 1 second.

I have heard that a good rule of thumb was driver diameter +10. But that theory obviously is not a good measure as in the case of the J's and the FEF's definetly bested 90mph many times.

quote:

the engineer raise the throttle to the roof so was there a governer on it or did the boiler run out of steam

1) No, there was/is no governor, the throttle was wide open at that point.

2) Why would you think it would "run out of steam"? There was also a Fireman on duty to maintain working steam pressure during those "drag races".

L&N's "Big Emma" 2-8-4's, with 69" drivers, were once clocked by a trailing dynamometer car at 92 mph on an empty hopper train. This was shortly after the Lima Emmas arrived, in 1949. A test crew was evaluating the Lima, which was added to the 2 Emmas on a s.b. mt hopper move, for return to whatever terminal, maybe Corbin. The fellow in the dyno car decided to cut in the recorder, and got " 90-92 mph ". I would suspect that the 4 passenger Emmas, all Baldwins, prob got the "Flamingo" goin that fast, too, during WWII, when they ran Cincy - Corbin. A retired NKP hogger once told me the fastest he had seen aboard a 700 in regular freight service was 76 mph, tho I would bet some of them got going even faster occasionally. I believe 3-figure speeds for the N&W J's in Eastern VA are pretty well documented. TRAINS' DPM wrote late in the steam era that a ride to Norfolk would reveal that "you will be propelled at speeds such as you have never before moved on rails."

quote:

The faster the pistons go back and forth the more steam you need and the faster you have to get the steam in and out of the cylinders.

To be a little more clear on this subject, "more steam" is NOT necessarily the answer. Of course "the faster you have to get the steam in and out og the cylinders" is definately the key in maintaining/increasing horsepower for acceleration.

Thus enters the Engineer's PROPER use of the power reverse! At full throttle, the key is to maintain the absolute HIGHEST STEAM TEMPERATURE at the valves, as produced by the superheaters. Shortening the valve cut off, reduces steam flow/volume through the superheaters, so that the steam gains high heat instead of simply "rushing through the superheater units".

The "volume of steam", or "steam pressure" is actually NOT what propels a steam engine. The energy is from the HEAT of the steam, which expands in the cylinders. The higher the boiler pressure, the higher the heat of the steam. When the steam passes through the superheater units, the steam is heated even more, i.e. SUPER HEATED, to temps exceeding 700 degrees F.

If the Engineer does NOT have the valve cutoff set correctly, the steam temp will actually DROP, reducing HP and increase fuel and water consumption.

Jack is right on the money with his comments about the setting of the cutoff. That is absolutely critical to the proper handling of a steam locomotive at high speed.

When entering these discussions of speed, you must remember that when you get to REALLY high speeds (over 80 mph) the cutoff is set VERY short. The steam admission port is only open for a very small percentage of the piston stroke, on the order of 20% or even slightly less. However, the throttle is likely wide open in this scenario, providing a wonderful draft for the fire and very high superheat temperatures. A good fireman can easily supply enough steam to take the locomotive to "rod-throwing" speeds...IF the engineer has the cutoff set right.

A steam locomotive is much more likely to be run out of steam on a long, hard pull on a grade at 30-40 mph than it is at 80+ mph. If the engineer sets the cutoff too long in this scenario, the engine is actually using more steam at 40 mph than it is at 80 mph. It's not using it efficiently, but it is using it nonetheless. I could run the 765 out of steam at 30 mph if I wanted to, by leaving the reverse down in the corner at full cutoff and running the throttle wide open. Of course, if I did that, I would likely be banned from the cab forever and hung in effigy by the entire cadre of firemen!

I once fired the 765 in this exact scenario. We had about 30 freight cars and the engineer was running the engine hard. (It was not one of our regular guys at the throttle.) At 30 mph I had the water pump almost wide open, crowding the fire as much as I could and was slowly losing ground on both the water and the steam. When pressure slowly dropped to 210, I knew something was wrong. I finally realized that the engineer had the throttle wide open and the power reverse still full down in the corner. The railfans would have loved it because the 765 was making a deafening racket. It was without a doubt the worst I have ever seen the 765 run. When I realized what was going on, I went over to the right side of the cab, reached in front of the engineer and placed the reverse lever where it belonged. The engine immediately quieted down a little and began to accelerate. I had a brief discussion with the engineer about what the definition of a "hard pull" was and then went back to my seat on the left side. I had already gained back 15 pounds of steam and all was right with the world again.

Just to take this discussion into the asterisk category on things that affect steam loco speed and efficiency, check out the work of Andre Chapelon and Kyost Kylala, which resulted in the Kylchap locomotive exhaust system. This system made the air draw through the firebed more even and reduced back pressure.

ChipR

The 1" of diameter = 1 MPH rule of thumb does give you a good idea of the connection between the size of the driver and the speed an engine can go - but like all "rules of thumb" it's not meant to be precise, just a general estimate.

One big limiting factor on speed is the track and ballast. A train going 100 MPH can pick up ballast stones and fling them around in it's wake, potentially not just endangering people and objects near the track, but could even hit the engine and train itself.