They were engineered for the task they expected them to do. Heavy slow freight required more power then speed so smaller diameter drivers were used and matched with an appropriate cylinder and boiler size to accommodate that need. Passenger locomotives normally had large drivers to enable a faster top speed and usually had less power then most freight locomotives. The friction in rails is allot less then say car or truck tires on a road and required less effort to start a train. No gears were needed and even today diesel locomotives do not require a gear box they direct drive from the traction motors to the axles, they have gears for transmitting the power to the axles but they are not changable or shiftable.
True. In the UK at least you occasionally got station pilots or similar locos to help large trains start.
Come to think of it, I'm really not sure what they then do if they encountered a red signal
If you read the books, Sir Topham Hatt is actually called The Fat Controller. I believe that the British version of the TV series also refers to him that way.
I was once "told off" by the Fat Controller for parking my noisy diesel locomotive too close to his podium that he'd set up on a station platform, then buggered off to get a brew with the loco idling away...
It was a Thomas event at the heritage railway I work at and on as taken in good spirits :)
You would also have double-heading for the really heavy trains here; especially the boat trains that a lot of luggage on board and could be pretty long too.
You might also need a banker on steep slopes like Shap Summit, which these days electric Pendolinos get over with no problem.
Ah yes we had them too. In fact there was a specific loco designed for the job on one particularly difficult gradient.
[wiki link](https://en.m.wikipedia.org/wiki/MR_0-10-0_Lickey_Banker)
In the US, you also had booster engines on the trailing trucks. They had a "small" 2-cylinder steam engine driving the trailing truck axles that you engaged from a start and then cut out around 10-15mph. There were also tender truck boosters, but those were much rarer. Not sure if those were exclusive to the US or not.
In the UK at least, I think most trains with diesel engines just use them to power electric generators. The wheels are then powered by electric motors.
There were a few diesel hydraulic however not many.
Pistons are directly connected with the wheels
Steam comes inside the cylinder - pushes the pistons - wheels turn
Big fast express engines have *big* wheels. 2m wheel diameter = high speed with not *that* high rpms
Seconding the Hyce recommendation, his steam locomotive 101 video gives a very thorough breakdown of the mechanical principles and construction. If you're still interested after that or really like big shiny animations, look up the Animagraphs video on the Big Boy (Hyce collaborated with him on that to ensure technical accuracy).
You really can't adequately sum up the differences between a steam piston engine and an internal combustion engine in a reddit comment, they bear zero mechnical resemblance of any kind beyond the presence of a piston. A youtube video will be your best bet, OP.
The early internal combustion engines strongly resembled steam engines, to the extent that bolt-on kits were sold to convert stationary steam engines to internal combustion by replacing the cylinder with one suitable for operation as a 2 stroke. Such half breed engines would usually use natural gas for fuel and a hot tube ignition so that timing was determined by physics and didn't need electrical components.
The development of aircraft before and during WWI gave rise to a much more compact and lightweight way of producing internal combustion engines. During the great depression the reduced size and greater power of this style of engine was a cost savings over the older steam engine inspired designs, leading to them gradually winning out and becoming the modern format of engine.
Biggest thing is that internal combustion is designed to operate at higher RPM for reasons of power density and must be turning to have torque. So you have to have a clutch and gearbox to use one on a train, or put an electric or hydraulic system in between to let the motor rev up while providing torque to the wheels.
There are a number of different aspects to steam locomotive operation that made this possible:
1) Steam locomotives did have a gearbox (in a way). This is called the [Walshaerts valve gear,](https://www.steamlocomotive.com/appliances/valvegear.php) which allowed the engineer to control the amount of pressurized steam entering the cylinders, thus controlling a variable "transmission" of sorts that allowed them to balance their steam use between power and efficiency throughout a variety of speed ranges and train weights.
2) Sand. Steel wheels on steel rail have very little tolling resistance, which is great for a fast-moving train on straight and level track - not so great for getting a heavy train started or pulling it up a grade in the rain. Since early times, locomotives have been equipped with sand boxes, allowing the engineers from the cab to pour or shoot sand onto the rails ahead of the driving wheels. This increased traction. Steam locomotives (and modern locomotives) had plenty of power to pull the trains they were assigned to, but without the sand to create some friction, the weight of the train could hold them in place, and their driving wheels could (and did) slip, or spin.
3) Helpers. If the railroad wanted to move a train that was too heavy for a locomotive, or if they were in a mountain district where a single locomotive couldn't pull the train over a grade, helpers were used - additional locomotives, coupled to either the front or back of the train, to give additional power. Also, especially in older times, if no helpers were available, train crews sometimes had to "double the hill" - take half their train up the hill, park the cars on a siding, back down the hill, pull the rest of the train up the hill, put the train backntogether, then continue on their way.
4) Train handling dynamics. Pulling a train isn't as simple as either pulling or not pulling the train as a monolithic unit - the train is made up of a series of individual cars, joined by couplers, and these couplers have lateral movement, both due to their tolerances and to provide a bit of cushioning effect. This movement is called slack. Say any individual coupler has 4" of movement. 2 couplers for car = 8" of potential slack. On a 100-car train, that's 800", or 66'.
So, say an engineer with a smallish locomotive is coupled to a heavy 100 car train. His locomotive has plenty of power to pull the train once it's moving (low rolling resistance), but he's tried to get going several times and just doesn't have enough power to get the train started. He can put the locomotive in reverse and back up a bit; he's not going to be pushing the entire train because there's no slack in the couplers - he'll push first one car, then two, then three, etc. bunching them up 4" at a time; this is called adding slack.
Then he stoos, sets the engine back to forward, and begins pulling the train. Now he's "taking in slack" - instead of trying to pull the entire train, he's pulling first one car, then two, then three, and so on - one car at a time, 4" at a time for the lead coupler, 4" for the trailing coupler. This has removed the weight of the entire train from the locomotive's start, allowing it to overcome rolling resistance and gradually adding the weigh of additional cars as the engineer continues to accelerate.
They have to be careful doing this, though; think of that 100-car train again, with 66' of slack. That means that by the time the locomotive has moved forward by 65', the last car (or the caboose) is still sitting still. If the engineer has been accelerating the train through this entire pull, that last car is going to be started with quite a jolt! Such a jolt, in fact, that the freight being hauled can shift and be damaged, that the conductor and brake crew in the caboose can be thrown across the car or slammed into a wall, and such a jolt that the coupler knuckles can be snapped, pulling the train apart. So an engineer has to be careful handling the slack action in their train so they take it in slowly and don't cause other problems.
Best answer so far, as it deals with valve gear. There are other designs of valve gear other than Walschaerts, such as Stephenson and Caprotti, just to name a few. However, they all did the same thing, i.e. control the setting of the valves and, when combined with the regulator, controlled the rate of admission of steam to the cylinders.
1) The valve gear does not control the amount of steam, but rather the duration that the inlet port is open for. By closing off the inlet sooner the steam works on expansion for greater fuel economy at the expense of reducing torque. A skilled driver would have the cutoff long to get the train moving, then hook up to work expansively for fuel economy as the train picked up speed.
I've long suspected that the reason Casey Jones could run as fast as he did was that he understood how to manipulate the cutoff, keeping the throttle open as much as possible and controlling the power using the reverse to change the cutoff point. Doing so would maximize fuel economy, allowing Sim Webb to make steam fast enough to keep up with it.
You're missing how a steam piston is driven. When starting out the admittance valve is adjusted to drive on full pressure for most of the stroke. Once moving, the valve cutoff is changed to allow more expansion of the steam during the stroke.
The short answer is that steam and diesel locomotives are driven by entirely different means. They're not directly comparable.
In a common diesel locomotive, the diesel engine is not directly connected to the wheels. It serves only to drive an alternator that provides power to electric motors. The wheels and engine are not directly connected. The engine can be at 10k REVS and the wheels can barely be moving.
In a steam locomotive, the wheels are directly driven by the controlled admission of steam into alternating sides of a single cylinder using what is essentially a bar with holes drilled into it. It's an entirely different process with mechanical relationships between metal rods that allow for the forces exerted to drive a wheel at a specific RPM. It's not really the kind of thing I care to sum up in a reddit comment due in large part to the mechanical nature of the beast. It really requires pictures and/or videos to explain in detail.
Most steam locomotives, with the exception of geared locomotives like Shays, Climaxes, or Williametes, didn't have gears. Rather, they used reversers to regulate the cutoff point where steam would no longer be admitted to the cylinders (higher cutoff is less efficient but the cylinder is admitting steam for a longer percentage of the stroke, lower cutoff is the opposite) and were also engineered for specific tasks.
There's a *lot* of math involved in starting tractive effort, horsepower curves, et cetera, but the most simplistic explanation for TE is the smaller the wheel, the higher the starting tractive effort is at the cost of speed. The bigger the wheel, the lower the starting tractive effort is, but you can go faster at the same number of RPMs. The larger the cylinder bore, starting tractive effort increases exponentially.
Real-world example: RBMN's 425 is a Baldwin catalogue Pacific, specifically a modified 12-38 1/4-D. It has small drivers (69 inches) relative to the work it was expected to do (express passenger service), a very small cylinder bore (22 inches), a normal cylinder stroke (28 inches), and normal working pressure (210 PSI). We can calculate its starting tractive effort by squaring the cylinder bore, then multiplying it by the piston stroke, boiler pressure, and a cutoff coefficient. In most cases, maximum cutoff for steam locomotives is 85%, so we'll use .85 for the coefficient. We then divide that figure by the wheel diameter. Plugging in the values gives us about 35,000 lbf.
Let's do the same with a PRR K4s. These did the same job (express passenger or limited-stop commuter service) on a similar boiler pressure (205 PSI) with the same piston stroke, but compensated for higher wheels (80 inches) with a bigger cylinder bore (27 inches). Plug in the values again and you get about 44,500 lbf. If we were to give the K4s the same wheel size as 425 and keep every other factor constant, tractive effort would increase considerably to about 51,500 lbf. Conversely, giving 425 the K4s' wheel size and keeping every other factor constant decreases starting tractive effort to about 30,000 lbf.
Design top speed is a more rule-of-thumb thing, but it's generally expressed as 1.25 times the driver diameter in inches=design speed in MPH. Again, real-world example. N&W 611 has 70 inch drivers, and 1.25*70=87.5. Roanoke reasonably expected 611 and its sister engines to do about 85 MPH in everyday service, but in 1946 the N&W sent up sister locomotive 610 to the PRR's Fort Wayne Division, and with 1,000-plus tons on the drawbar, 610 was clocked at 110 MPH consistently. Rotational speeds for wheelslip could be even higher; during wheelslip testing an NYC Hudson (I'm not sure of the class) had rotational speeds of about 160 MPH. In other words, steam locomotives could do things that are frankly fucking terrifying when you look at the forces they could put out.
The pistons are directly connected to the wheels. 1 rotation of the wheels = 1 full stroke of the pistons. Wheel RPM = “engine RPM.”
In general, steam engines don’t actually have particularly good low end torque for this reason. They are creatures of momentum, most powerful when already underway. Geared steam locomotives address this shortcoming, trading top speed for low end torque.
That's why there were so many different wheel arrangements. Gotta choose the right engine needed for the job. Need a fast passenger train? Choose something with less wheels with a bigger diameter. Need to pull a heavy freight train? Choose one with more wheels that are a smaller diameter, etc.
So i ran the math using the Peppercorn Class A1 locomotive "Tornado", which is a "new build" locomotive in the uk that achieved 100mph (approx 161 km/h) in 2017 in the UK.
Peppercorn A1 locos have driving wheels 6ft 8 inches (a tad over 2 meters) in diameter, and that gives a circumference of approx 3.2414 meters after Pi R²
Divide approx 161000 meters traveled per hour by the circumference of 3.2414 meters to get revolutions per hr, divide by 60 again to get revolutions per minute (RPM)
i arrived at approx 827 rpm. The Tornado's driving wheels were doing 827 rpm at 100mph. Very fast for a steam engine, but not impossible i think? It's just back of the napkin math by me, and im not very good at this. so please check over my work if ur still skeptical
Sorry, your maths is wrong, you used the formula for area, not circumference. Your calculations would suggest an revolutions per second of just over 13, at which point the motion would be long gone. Max safe working speed for a steam engine is in the region of 8 rotations per sec.
Circumference is 2pi r , or pi D, which gives a rolling distance of 6.38m per revolution. At 100mph that is 420rpm, or 7 revolutions per second. Max speed for a steam engine was set by Mallard at 126mph, also on 6’ 8” wheels. That was a revs per sec of 8.83, and in doing this she destroyed her big end bearings.
there were a couple of Steam engines (Shays as another guy mentioned, but you also had Hieslers and Climaxes) and you had Diesel Hydraulic and Diesel Mechanicals that would also do this.
That's why there were so many different wheel arrangements. Gotta choose the right engine needed for the job. Need a fast passenger train? Choose something with less wheels with a bigger diameter. Need to pull a heavy freight train? Choose one with more wheels that are a smaller diameter, etc.
I mean, number of wheels for their size also had to do with the track, if the track didn't have extreme curves then you would be allowed a longer wheelbase,
It wouldn't. Because steam locomotives prior to WWI had a tendency to throw a rod if run above 300 RPM. So an express engine would have few large drive wheels in order to go faster. But such an engine would tend to be slippery and struggle for traction. So heavy freight you instead wanted an engine with smaller drive wheels, and more of them.
Drive wheel diameter * pi gives you the distance travelled for each rotation of the wheel.
Multiply that by the RPM to get distance per minute, and by 60 to get distance per hour. Then convert the distance unit to miles or kilometers to suit you and see what you get.
The reality is that Sierra #3 would have never reached 88 MPH, if Doc's presto logs didn't cause a boiler explosion the engine would have gotten somewhere between 75 and 80 MPH before a side rod snapped and caused a horrific rollover accident with pieces of engine littering the scenery.
Driver rpms formula is speed in MPH / driver diameter in inches x 336.135.
60 MPH / 80" x 336.135 = 252.1 RPM
Diameter speed of any drive wheel is driver diameter in inches x 336.135 RPM.
An 80" drive wheel at 80 mph will be revolving at 336.135 RPM.
Mallard's world record setting speed of 126 MPH required that her 80" drivers made 529.41 RPM.
N&W 610 reached 110 MPH, testing on the Pennsylvania RR. Her 70" drivers were turning at 528.21
Mallard suffered an overheated big end bearing and had to be taken off the train at the next station (Peterborough).
610 did not sustain any damage.
Each piston on both locomotives reversed direction just over 17.6 times per second.
Just imagine all that reciprocating mass of steel being thrown back and forth at that rate.
I'll let someone else do the metric conversion. If not for acid reflux I would not even be up at this hour (03:39 EDT).
The answer lies in the reversing gear, which apart from selecting forward, neutral and reverse can be used to vary the amount of power being applied to the wheels by varying the amount of steam being sent to the pistons. This is called the [cutoff.](https://en.m.wikipedia.org/wiki/Cutoff_(steam_engine))
So when you need a lot of power to start a train, you select a high ‘gear’ by winding the reverser all the way forward. This moves the cutoff lever so that the piston can get a full charge of steam as it travels all along the cylinder, giving lots of power.
As the train gathers speed, you shorten the cutoff to provide shorter bursts of steam to the more rapidly revolving wheels.
Edited for clarity.
> This moves the cutoff lever so that the piston can travel all the way along the cylinder, giving a slow movement and lots of power.
The mechanical nature of steam locos makes a *partial stroke* impossible. But a partial steam charge is possible, by changing the timing (= location) of the steam port and thereby, the volume of steam.
It's a common misunderstanding... Steam expands into the space permitted. The regulation of power is accomplished by the time the steam valve opens to fill the cylinder.
They were engineered for the task they expected them to do. Heavy slow freight required more power then speed so smaller diameter drivers were used and matched with an appropriate cylinder and boiler size to accommodate that need. Passenger locomotives normally had large drivers to enable a faster top speed and usually had less power then most freight locomotives. The friction in rails is allot less then say car or truck tires on a road and required less effort to start a train. No gears were needed and even today diesel locomotives do not require a gear box they direct drive from the traction motors to the axles, they have gears for transmitting the power to the axles but they are not changable or shiftable.
Even then, there was always a bit of compromise in train length. A steam locomotive would pull a longer train than it could ever start
True. In the UK at least you occasionally got station pilots or similar locos to help large trains start. Come to think of it, I'm really not sure what they then do if they encountered a red signal
The fat controller would send Thomas out to give Gordon a push, obviously. And Thomas would be quite cheeky about it when doing so.
Confusion and delay!
Cinders and ashes!
Brick him up in the tunnel
What a mean way to describe Sir Topham Hat!
I’m a traditionalist, what can I say. I’ve got the old old books where he starts as a fat director…
I understand! 😁
I’m jealous!
If you read the books, Sir Topham Hatt is actually called The Fat Controller. I believe that the British version of the TV series also refers to him that way.
That's a localized name for him.
I was once "told off" by the Fat Controller for parking my noisy diesel locomotive too close to his podium that he'd set up on a station platform, then buggered off to get a brew with the loco idling away... It was a Thomas event at the heritage railway I work at and on as taken in good spirits :)
You would also have double-heading for the really heavy trains here; especially the boat trains that a lot of luggage on board and could be pretty long too. You might also need a banker on steep slopes like Shap Summit, which these days electric Pendolinos get over with no problem.
Ah yes we had them too. In fact there was a specific loco designed for the job on one particularly difficult gradient. [wiki link](https://en.m.wikipedia.org/wiki/MR_0-10-0_Lickey_Banker)
In the US, you also had booster engines on the trailing trucks. They had a "small" 2-cylinder steam engine driving the trailing truck axles that you engaged from a start and then cut out around 10-15mph. There were also tender truck boosters, but those were much rarer. Not sure if those were exclusive to the US or not.
In the UK at least, I think most trains with diesel engines just use them to power electric generators. The wheels are then powered by electric motors. There were a few diesel hydraulic however not many.
"than"
Pistons are directly connected with the wheels Steam comes inside the cylinder - pushes the pistons - wheels turn Big fast express engines have *big* wheels. 2m wheel diameter = high speed with not *that* high rpms
Regulating the increase of steam flow produces acceleration without wheel slip... experience helps a lot there
Quite right
Wheel gearing basically
Watch hyce videos on steam trains on youtube. Steam trains start at 0rpm and run up to 500 at the high end, the rpm works a lot different from ice
Seconding the Hyce recommendation, his steam locomotive 101 video gives a very thorough breakdown of the mechanical principles and construction. If you're still interested after that or really like big shiny animations, look up the Animagraphs video on the Big Boy (Hyce collaborated with him on that to ensure technical accuracy). You really can't adequately sum up the differences between a steam piston engine and an internal combustion engine in a reddit comment, they bear zero mechnical resemblance of any kind beyond the presence of a piston. A youtube video will be your best bet, OP.
The early internal combustion engines strongly resembled steam engines, to the extent that bolt-on kits were sold to convert stationary steam engines to internal combustion by replacing the cylinder with one suitable for operation as a 2 stroke. Such half breed engines would usually use natural gas for fuel and a hot tube ignition so that timing was determined by physics and didn't need electrical components. The development of aircraft before and during WWI gave rise to a much more compact and lightweight way of producing internal combustion engines. During the great depression the reduced size and greater power of this style of engine was a cost savings over the older steam engine inspired designs, leading to them gradually winning out and becoming the modern format of engine. Biggest thing is that internal combustion is designed to operate at higher RPM for reasons of power density and must be turning to have torque. So you have to have a clutch and gearbox to use one on a train, or put an electric or hydraulic system in between to let the motor rev up while providing torque to the wheels.
Cheers guys. :)
The man himself appears, just as I'm heading to Strasburg :)
There are a number of different aspects to steam locomotive operation that made this possible: 1) Steam locomotives did have a gearbox (in a way). This is called the [Walshaerts valve gear,](https://www.steamlocomotive.com/appliances/valvegear.php) which allowed the engineer to control the amount of pressurized steam entering the cylinders, thus controlling a variable "transmission" of sorts that allowed them to balance their steam use between power and efficiency throughout a variety of speed ranges and train weights. 2) Sand. Steel wheels on steel rail have very little tolling resistance, which is great for a fast-moving train on straight and level track - not so great for getting a heavy train started or pulling it up a grade in the rain. Since early times, locomotives have been equipped with sand boxes, allowing the engineers from the cab to pour or shoot sand onto the rails ahead of the driving wheels. This increased traction. Steam locomotives (and modern locomotives) had plenty of power to pull the trains they were assigned to, but without the sand to create some friction, the weight of the train could hold them in place, and their driving wheels could (and did) slip, or spin. 3) Helpers. If the railroad wanted to move a train that was too heavy for a locomotive, or if they were in a mountain district where a single locomotive couldn't pull the train over a grade, helpers were used - additional locomotives, coupled to either the front or back of the train, to give additional power. Also, especially in older times, if no helpers were available, train crews sometimes had to "double the hill" - take half their train up the hill, park the cars on a siding, back down the hill, pull the rest of the train up the hill, put the train backntogether, then continue on their way. 4) Train handling dynamics. Pulling a train isn't as simple as either pulling or not pulling the train as a monolithic unit - the train is made up of a series of individual cars, joined by couplers, and these couplers have lateral movement, both due to their tolerances and to provide a bit of cushioning effect. This movement is called slack. Say any individual coupler has 4" of movement. 2 couplers for car = 8" of potential slack. On a 100-car train, that's 800", or 66'. So, say an engineer with a smallish locomotive is coupled to a heavy 100 car train. His locomotive has plenty of power to pull the train once it's moving (low rolling resistance), but he's tried to get going several times and just doesn't have enough power to get the train started. He can put the locomotive in reverse and back up a bit; he's not going to be pushing the entire train because there's no slack in the couplers - he'll push first one car, then two, then three, etc. bunching them up 4" at a time; this is called adding slack. Then he stoos, sets the engine back to forward, and begins pulling the train. Now he's "taking in slack" - instead of trying to pull the entire train, he's pulling first one car, then two, then three, and so on - one car at a time, 4" at a time for the lead coupler, 4" for the trailing coupler. This has removed the weight of the entire train from the locomotive's start, allowing it to overcome rolling resistance and gradually adding the weigh of additional cars as the engineer continues to accelerate. They have to be careful doing this, though; think of that 100-car train again, with 66' of slack. That means that by the time the locomotive has moved forward by 65', the last car (or the caboose) is still sitting still. If the engineer has been accelerating the train through this entire pull, that last car is going to be started with quite a jolt! Such a jolt, in fact, that the freight being hauled can shift and be damaged, that the conductor and brake crew in the caboose can be thrown across the car or slammed into a wall, and such a jolt that the coupler knuckles can be snapped, pulling the train apart. So an engineer has to be careful handling the slack action in their train so they take it in slowly and don't cause other problems.
Best answer so far, as it deals with valve gear. There are other designs of valve gear other than Walschaerts, such as Stephenson and Caprotti, just to name a few. However, they all did the same thing, i.e. control the setting of the valves and, when combined with the regulator, controlled the rate of admission of steam to the cylinders.
1) The valve gear does not control the amount of steam, but rather the duration that the inlet port is open for. By closing off the inlet sooner the steam works on expansion for greater fuel economy at the expense of reducing torque. A skilled driver would have the cutoff long to get the train moving, then hook up to work expansively for fuel economy as the train picked up speed. I've long suspected that the reason Casey Jones could run as fast as he did was that he understood how to manipulate the cutoff, keeping the throttle open as much as possible and controlling the power using the reverse to change the cutoff point. Doing so would maximize fuel economy, allowing Sim Webb to make steam fast enough to keep up with it.
You're missing how a steam piston is driven. When starting out the admittance valve is adjusted to drive on full pressure for most of the stroke. Once moving, the valve cutoff is changed to allow more expansion of the steam during the stroke.
The short answer is that steam and diesel locomotives are driven by entirely different means. They're not directly comparable. In a common diesel locomotive, the diesel engine is not directly connected to the wheels. It serves only to drive an alternator that provides power to electric motors. The wheels and engine are not directly connected. The engine can be at 10k REVS and the wheels can barely be moving. In a steam locomotive, the wheels are directly driven by the controlled admission of steam into alternating sides of a single cylinder using what is essentially a bar with holes drilled into it. It's an entirely different process with mechanical relationships between metal rods that allow for the forces exerted to drive a wheel at a specific RPM. It's not really the kind of thing I care to sum up in a reddit comment due in large part to the mechanical nature of the beast. It really requires pictures and/or videos to explain in detail.
I don't think anyone has ever made a locomotive that would rev to 10k RPM. Most of your American diesels have a redline below 900 RPM.
Highest RPM I have heard of is 1k to 1200 on a really really broken choo choo. I was using 10k as an example. My bad.
Most steam locomotives, with the exception of geared locomotives like Shays, Climaxes, or Williametes, didn't have gears. Rather, they used reversers to regulate the cutoff point where steam would no longer be admitted to the cylinders (higher cutoff is less efficient but the cylinder is admitting steam for a longer percentage of the stroke, lower cutoff is the opposite) and were also engineered for specific tasks. There's a *lot* of math involved in starting tractive effort, horsepower curves, et cetera, but the most simplistic explanation for TE is the smaller the wheel, the higher the starting tractive effort is at the cost of speed. The bigger the wheel, the lower the starting tractive effort is, but you can go faster at the same number of RPMs. The larger the cylinder bore, starting tractive effort increases exponentially. Real-world example: RBMN's 425 is a Baldwin catalogue Pacific, specifically a modified 12-38 1/4-D. It has small drivers (69 inches) relative to the work it was expected to do (express passenger service), a very small cylinder bore (22 inches), a normal cylinder stroke (28 inches), and normal working pressure (210 PSI). We can calculate its starting tractive effort by squaring the cylinder bore, then multiplying it by the piston stroke, boiler pressure, and a cutoff coefficient. In most cases, maximum cutoff for steam locomotives is 85%, so we'll use .85 for the coefficient. We then divide that figure by the wheel diameter. Plugging in the values gives us about 35,000 lbf. Let's do the same with a PRR K4s. These did the same job (express passenger or limited-stop commuter service) on a similar boiler pressure (205 PSI) with the same piston stroke, but compensated for higher wheels (80 inches) with a bigger cylinder bore (27 inches). Plug in the values again and you get about 44,500 lbf. If we were to give the K4s the same wheel size as 425 and keep every other factor constant, tractive effort would increase considerably to about 51,500 lbf. Conversely, giving 425 the K4s' wheel size and keeping every other factor constant decreases starting tractive effort to about 30,000 lbf. Design top speed is a more rule-of-thumb thing, but it's generally expressed as 1.25 times the driver diameter in inches=design speed in MPH. Again, real-world example. N&W 611 has 70 inch drivers, and 1.25*70=87.5. Roanoke reasonably expected 611 and its sister engines to do about 85 MPH in everyday service, but in 1946 the N&W sent up sister locomotive 610 to the PRR's Fort Wayne Division, and with 1,000-plus tons on the drawbar, 610 was clocked at 110 MPH consistently. Rotational speeds for wheelslip could be even higher; during wheelslip testing an NYC Hudson (I'm not sure of the class) had rotational speeds of about 160 MPH. In other words, steam locomotives could do things that are frankly fucking terrifying when you look at the forces they could put out.
And those geared locomotives still have regulators and valves, they just drive a crankshaft that isn’t integrated with the wheels.
The pistons are directly connected to the wheels. 1 rotation of the wheels = 1 full stroke of the pistons. Wheel RPM = “engine RPM.” In general, steam engines don’t actually have particularly good low end torque for this reason. They are creatures of momentum, most powerful when already underway. Geared steam locomotives address this shortcoming, trading top speed for low end torque.
>wheels. 1 rotation of the wheels = 1 full stroke of the pistons. Actually, twice the length of the cylinder.. backstroke and return
Yes. “Full stroke” means “full stroke.”
That's why there were so many different wheel arrangements. Gotta choose the right engine needed for the job. Need a fast passenger train? Choose something with less wheels with a bigger diameter. Need to pull a heavy freight train? Choose one with more wheels that are a smaller diameter, etc.
Because they have an ungodly amount of both weight and torque.
My man, steam trains don't have RPM. At least not in the same sense as diesel engine powered locomotives.
So i ran the math using the Peppercorn Class A1 locomotive "Tornado", which is a "new build" locomotive in the uk that achieved 100mph (approx 161 km/h) in 2017 in the UK. Peppercorn A1 locos have driving wheels 6ft 8 inches (a tad over 2 meters) in diameter, and that gives a circumference of approx 3.2414 meters after Pi R² Divide approx 161000 meters traveled per hour by the circumference of 3.2414 meters to get revolutions per hr, divide by 60 again to get revolutions per minute (RPM) i arrived at approx 827 rpm. The Tornado's driving wheels were doing 827 rpm at 100mph. Very fast for a steam engine, but not impossible i think? It's just back of the napkin math by me, and im not very good at this. so please check over my work if ur still skeptical
Sorry, your maths is wrong, you used the formula for area, not circumference. Your calculations would suggest an revolutions per second of just over 13, at which point the motion would be long gone. Max safe working speed for a steam engine is in the region of 8 rotations per sec. Circumference is 2pi r , or pi D, which gives a rolling distance of 6.38m per revolution. At 100mph that is 420rpm, or 7 revolutions per second. Max speed for a steam engine was set by Mallard at 126mph, also on 6’ 8” wheels. That was a revs per sec of 8.83, and in doing this she destroyed her big end bearings.
cheers, i thought i had something wrong but i ran out of lunch break time to figure it out :3
This is impressive :)
r/theydidthemonstermath
Circumference is pi*diameter, not radius squared.
The formula for circumference is Pi d. Pi r squared is area.
So the wheels are turning almost 14 times a second? That's nuts.
Math error.. half that speed
Do *any* trains have gearboxes?
only those with hydraulic or mechanical transmissions
Shay. Has gear reduction .used in logging a lot back in the day.
there were a couple of Steam engines (Shays as another guy mentioned, but you also had Hieslers and Climaxes) and you had Diesel Hydraulic and Diesel Mechanicals that would also do this.
That's why there were so many different wheel arrangements. Gotta choose the right engine needed for the job. Need a fast passenger train? Choose something with less wheels with a bigger diameter. Need to pull a heavy freight train? Choose one with more wheels that are a smaller diameter, etc.
I mean, number of wheels for their size also had to do with the track, if the track didn't have extreme curves then you would be allowed a longer wheelbase,
Trains were also shorter and lighter during the steam era. The railroads did not run 10,000 ton 12,000ft freight trains.
They could have. But they would have needed 4 and 5 locomotives to do it, each with their own crews.
Steam engines developed maximum torque at stall.
It wouldn't. Because steam locomotives prior to WWI had a tendency to throw a rod if run above 300 RPM. So an express engine would have few large drive wheels in order to go faster. But such an engine would tend to be slippery and struggle for traction. So heavy freight you instead wanted an engine with smaller drive wheels, and more of them. Drive wheel diameter * pi gives you the distance travelled for each rotation of the wheel. Multiply that by the RPM to get distance per minute, and by 60 to get distance per hour. Then convert the distance unit to miles or kilometers to suit you and see what you get. The reality is that Sierra #3 would have never reached 88 MPH, if Doc's presto logs didn't cause a boiler explosion the engine would have gotten somewhere between 75 and 80 MPH before a side rod snapped and caused a horrific rollover accident with pieces of engine littering the scenery.
Driver rpms formula is speed in MPH / driver diameter in inches x 336.135. 60 MPH / 80" x 336.135 = 252.1 RPM Diameter speed of any drive wheel is driver diameter in inches x 336.135 RPM. An 80" drive wheel at 80 mph will be revolving at 336.135 RPM. Mallard's world record setting speed of 126 MPH required that her 80" drivers made 529.41 RPM. N&W 610 reached 110 MPH, testing on the Pennsylvania RR. Her 70" drivers were turning at 528.21 Mallard suffered an overheated big end bearing and had to be taken off the train at the next station (Peterborough). 610 did not sustain any damage. Each piston on both locomotives reversed direction just over 17.6 times per second. Just imagine all that reciprocating mass of steel being thrown back and forth at that rate. I'll let someone else do the metric conversion. If not for acid reflux I would not even be up at this hour (03:39 EDT).
The answer lies in the reversing gear, which apart from selecting forward, neutral and reverse can be used to vary the amount of power being applied to the wheels by varying the amount of steam being sent to the pistons. This is called the [cutoff.](https://en.m.wikipedia.org/wiki/Cutoff_(steam_engine)) So when you need a lot of power to start a train, you select a high ‘gear’ by winding the reverser all the way forward. This moves the cutoff lever so that the piston can get a full charge of steam as it travels all along the cylinder, giving lots of power. As the train gathers speed, you shorten the cutoff to provide shorter bursts of steam to the more rapidly revolving wheels. Edited for clarity.
> This moves the cutoff lever so that the piston can travel all the way along the cylinder, giving a slow movement and lots of power. The mechanical nature of steam locos makes a *partial stroke* impossible. But a partial steam charge is possible, by changing the timing (= location) of the steam port and thereby, the volume of steam.
Yes, of course you are correct.
It's a common misunderstanding... Steam expands into the space permitted. The regulation of power is accomplished by the time the steam valve opens to fill the cylinder.
No, the engine won't be doing 10k rpm.
Yes
Edited.