Murphy SidingSo the general consensus is that there is no gearing in a standard steam engine, the relationship between rod and wheel size effectively sets the gear ratio?
In my opinion, the answer is yes in the sense you're asking. The driver tires, as they wear, decrease the diameter appreciably, which increases the effective torque per stroke all else kept equal (schlimm, I pointedly avoided ceteris paribus here) while having a slight effect on diameter-speed-related concerns.
I would add a bit to what tree68 has said: notching up the gear is a bit like varying the spark advance AND fuel-injection modulation on an IC engine. Some of the more advanced poppet-valve gears also vary the timing in the ways systems like VANOS do, and it is possible (although usually done only with dumb relief valves) to alter the analogue of effective compression ratio somewhat. But the mechanical advantage is basically the same for a reciprocating locomotive as it is from piston to crankshaft in a motor, and it seldom if ever pays to play with that aspect of the design.
The history of early steam is replete with approaches that combined gearing with reciprocating engines; even the Fontaine locomotive approximates gears with friction wheels. The second Harrison locomotive used gearing to approximate the effect of large drivers without invoking problems of effective short stroke and very high wheel diameter, and had the state of materials and machine tooling been better at that time, some sort of change-speed gearbox might easily have been provided to allow good starting torque and good high-speed efficiency (of just the kind provided an IC engine by clutch/TC and multispeed transmission). Note the plethora of gear and chain 'assistance' in the locomotives in the original Seraing testing, and how (interestingly) some of the answers to near-zero rigid wheelbase in Europe involved not gears but fancy lateral and swivel arrangements with what was essentially conventional rod direct drive.
As so often expressed with steam automobiles -- the direct-drive steam engine is assumed to be flexible enough to work robustly without the additional cost and maintenance of a geared transmission or connection. Gears of any greater sophistication than those with casual or no lubrication on a Shay are greatly limited in the speed they can reach, even if the gear ratio itself relative to the maximum shaft rpm of the engine driving them does not dictate a slow maximum speed, and that is not helped (much) by even good rotational balancing as with a steeple compound with 120-degree cranks.
petitnj Cylinder, steam pressure, speed and such are all a careful balance. What limits the speed of a locomotive is the movement of steam in and out of the cylinder. You cannot keep letting steam in until the piston reaches its travel end or there would still be steam flowing in when the piston starts to come back to exhause. Cutoff (reducing the time steam enters the cylinder) stops steam flowing well before the end of the stroke so the steam is no longer flowing when the piston reaches the end. Steam is mechanical and moves about the speed of sound. It cannot be started and stopped instantaneously. Comments about larger drivers means the cylinder on a large drivered locomotive still goes back and forth the same as other engines but exerts much less force on the wheel surface. Torque is fixed as radius times force because that's what the driving rod exerts. The large wheel radius means the force at the wheel radius is less. Steam engines were designed to go fast (passenger -- with large fast driving wheels) or pull hard (freight -- with smaller wheels and more force on the wheel radius). This all is a balance of:how much steam pressure pushes steam into the expanding cyliderhow big the pipe is from the throttle to the cylinder valves to allow more steam flowhow fast and where in the cycle you can close the steam and open exhaust valveshow fast you can move the cylinder without excess forces on the rodsand so on Locomotives were a progression of designs that emphasized different characteristics. In the 150 years of steam design many parameters could be designed in and tested. Some worked well, others not so well.
Cylinder, steam pressure, speed and such are all a careful balance. What limits the speed of a locomotive is the movement of steam in and out of the cylinder. You cannot keep letting steam in until the piston reaches its travel end or there would still be steam flowing in when the piston starts to come back to exhause. Cutoff (reducing the time steam enters the cylinder) stops steam flowing well before the end of the stroke so the steam is no longer flowing when the piston reaches the end. Steam is mechanical and moves about the speed of sound. It cannot be started and stopped instantaneously.
Comments about larger drivers means the cylinder on a large drivered locomotive still goes back and forth the same as other engines but exerts much less force on the wheel surface. Torque is fixed as radius times force because that's what the driving rod exerts. The large wheel radius means the force at the wheel radius is less. Steam engines were designed to go fast (passenger -- with large fast driving wheels) or pull hard (freight -- with smaller wheels and more force on the wheel radius).
This all is a balance of:how much steam pressure pushes steam into the expanding cyliderhow big the pipe is from the throttle to the cylinder valves to allow more steam flowhow fast and where in the cycle you can close the steam and open exhaust valveshow fast you can move the cylinder without excess forces on the rodsand so on
Locomotives were a progression of designs that emphasized different characteristics. In the 150 years of steam design many parameters could be designed in and tested. Some worked well, others not so well.
I knew a man who had worked as a fireman (armstrong stoker), who did not like to work with a particular engineer because that engineer liked to work the engine "full stroke," using too much steam, and making fireman work harder to keep the pressure up.
Johnny
Murphy Siding So the general concensus is that there is no gearing in a standard steam engine, the relationship between rod and wheel size effectively sets the gear ratio?
So the general concensus is that there is no gearing in a standard steam engine, the relationship between rod and wheel size effectively sets the gear ratio?
After a fashion, yes. It's rod travel - ie, piston travel, and wheel radius/diameter.
As just noted in a previous post, two locomotives could have exactly the same piston/rod travel, with the crank pins at the same radius, on a yard goat and a top-end mainline speedster. The difference was the wheel radius/diameter.
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blue streak 1 Does higher boiler pressure allow for smaller crank pin radius and a shorter stroke distance of the piston travel ? Would that also allow for higher speeds of a locomotive ?
Does higher boiler pressure allow for smaller crank pin radius and a shorter stroke distance of the piston travel ? Would that also allow for higher speeds of a locomotive ?
In a word, yes, but....Some locomotives' boilers were built in latter years to sustain/provide high pressures over 300 psi. Or, even if their pressure were in the 250 psi range, their cylinder rod loads were high because of the bore in the cylinders (meaning the piston's surface area exposed to the 'work' of the steam) was also high. They actually bent cranks and bent rods. An example was the NYC Hudson, and I believe the ATSF's 5010 series of high-drivered Texas types, their 2-10-4, also had to have their boilers derated for the same reason.
Industrial and freight steamers had smaller drivers, but could have the same size of main crank, than their passenter counterparts so that their cylinders were interchangeable. Same stroke, same bore, same boiler output. However, with the crank's higher ratio of distance to the outer perimeter of the drivers, those being the working tire surfaces, the freights could generate more 'torque' about their axles, and thus place more tractive effort onto the rails for their weight/adhesion. Their levers were longer, if you will. Their greyhound passenger trains had the same cranks in most instances, but they had a much further distance to the rim of the tire, so less traction could be applied. The gearing was higher, as an analogy. Passenger locomotives sacrificed tractive effort, so important when lifting a passenger train, but they made up for it in their ability later to run faster at the same cyclic speed in the cylinders due to those larger perimeters on the larger drivers.
Thanks to Chris / CopCarSS for my avatar.
Euclid They do have variable cutoff that performs a function similar to a transmission, but I would say that it is not a true transmission.
Actually, I believe cutoff is better compared to spark advance on a gas engine than to any sort of transmission. Recall that in the early days, the driver controlled the spark advance.
It is true that there is no gearing on a steam rod locomotive. I assume that the suggestion of gearing in the OP was just an analogy to fixed speed multiplication as is the case with the actual fixed gearing of diesels. With steam rod engines, there is a fixed speed multiplication between the driver diameter and the crank pin circle. The crank pin circle is a result of piston stroke length. If drivers are made larger in relation to the crank pin circle, there is a greater speed multiplication and the effect is like shifting to a higher gear, although there is no actual ability to shift or change this while operating. It is a fixed ratio like the gearing in diesels.
Beyond this comparison to diesels, however, is the fact that diesels do have a transmission in addition to their fixed gearing between the traction motors and the axles. Their transmission is in the control of electrical current from the alternator or generator to the traction motors.
Steam rod locomotives have no transmission of any kind. In effect, they are always in “high gear”, as a fixed speed multiplication between the crank pin circle and the driver diameter. They do have variable cutoff that performs a function similar to a transmission, but I would say that it is not a true transmission.
challenger3980Northwest, By gearing, I was refering more in the context, of fixed gearing such as an automobile having say a 3.05 rear end, a pick up truck typically having a 3.73-4.10 ratio rear end and a rock crawling Jeep having a 4.56 or greater ratio/lower gear rear end. The fixed gear ratio being selected to suit the intended use, not a multiple gear transmission. Diesel locomotives have various ring and pinion combinations for similar reasons, a Passenger locomotive, would have a different ring and pinion combination than would a freight locomotive. In Steam that same effect would achieved by matching driver diameter to intended use, as there is no gearing in a typical side rod locomotive. Doug
By gearing, I was refering more in the context, of fixed gearing such as an automobile having say a 3.05 rear end, a pick up truck typically having a 3.73-4.10 ratio rear end and a rock crawling Jeep having a 4.56 or greater ratio/lower gear rear end. The fixed gear ratio being selected to suit the intended use, not a multiple gear transmission.
Diesel locomotives have various ring and pinion combinations for similar reasons, a Passenger locomotive, would have a different ring and pinion combination than would a freight locomotive.
In Steam that same effect would achieved by matching driver diameter to intended use, as there is no gearing in a typical side rod locomotive.
Doug
Got it. I know some that discuss transmissions by calling them gears, and wanted to make sure that my point was clear. Interestingly, Shays at least came in a wide variety of driver diameters, and some were switched at various times. The big four-truck models had some as large as 40" IIRC.
BaltACDNote wheel slip at the 3 minute mark
Someplace in my collection I have video of NKP765 breaking loose at the rail event in Owosso in 2009. Slight upgrade from a dead stop.
Historically, large drivered locomotives were generally considered 'slippery' by their operators. There is a finite amount 'grip' in the wheel/rail interface and the power required to operate larged drivered locomotives at their top speed could easily overwhem the grip of that wheel/rail interface at slow speeds. Engineers had to be judicious with their throttle use at slow speeds to be able to attain high speeds.
Note wheel slip at the 3 minute mark
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Northwest,
May your flanges always stay BETWEEN the rails
challenger3980 There was NO gearing on a typical rod locomotive(Shays, Heislers and Climaxes, Yes) a conventional side rod locomotive, was/is DIRECT drive, there is no gearing between the piston rod and the wheels. The wheel diameter was in essence the "Gearing" Larger diameter was for higher speed/lower tractive effort/torque, and smaller drivers were used for lower speed Higher tractive effort/torque. Doug
There was NO gearing on a typical rod locomotive(Shays, Heislers and Climaxes, Yes) a conventional side rod locomotive, was/is DIRECT drive, there is no gearing between the piston rod and the wheels. The wheel diameter was in essence the "Gearing" Larger diameter was for higher speed/lower tractive effort/torque, and smaller drivers were used for lower speed Higher tractive effort/torque.
The vagueries of language... Geared steam locomotives such as Heislers did have gearing, but not transmissions. That isn't to say that it would have been impossible or impractical if it were desired. Hopefully RME will chime in on this.
tree68I recall reading somewhere that the diameter of the drivers on a steam locomotives was at least roughly indicative of it's maximum speed. Ie, 60" drivers more or less yield a 60 MPH locomotive.
The rule I've always seen is driver diameter+10, but that rule gradually declined in importance as counterbalancing and frame design improved. Everyone's favorite example seems to be the J 610 hitting 110 MPH with 70" drivers.
Part one is a yes. Higher force on the piston and a smaller crankpin to diameter ratio yields the same force at the railheads.
As for higher speed, I'd say likely. The shorter stroke keeps piston and rod speeds lower. Not sure they higher pressure on the pistons can overcome the leverage for a given speed.
Modeling the Cleveland and Pittsburgh during the PennCentral era starting on the Cleveland lakefront and ending in Mingo junction
Don't forget another key factor in the 'gearing' is the distance between the axle and the crankpin as compared to the radius of the wheel. So that's the amount of leverage the piston thrust has against the resistance to motion down at the railhead. And why slow-speed freight engines had their rods near the outside of the wheel and almost down at the level of the rails.
Since the radius to the crankpin is always less than the radius of the wheel, the force of the piston thrust is always reduced/ 'diluted' by the ratio of wheel radius / crankpin radius.
- Paul North.
I recall reading somewhere that the diameter of the drivers on a steam locomotives was at least roughly indicative of it's maximum speed. Ie, 60" drivers more or less yield a 60 MPH locomotive.
Inasmuch as I don't recall ever hearing of a locomotive with 100+ inch drivers, that's obviously not hard and fast. But all of the other factors already discussed are, indeed, factors.
Speaking of dynamic augment, I also recall reading that the drivers would be balanced for the predicted service speed of the locomotive, meaning a locomotive running well over it's normal speed would provide a rough ride for the crew.
A friend with steam experience once told me that they held a high-drivered loco in place with dimes as chocks. While said dimes would not provide an impediment at speed, from a standing start, they provided a virtually unsurmountable barrier.
Professional engineers will have much better answers than this, but wheel size and the up and down motion of the rods all have an impact.
As an example, 84 inch (7 foot) drivers on the Milwaukee Road F-7 4-6-4 Hudsons used on the Hiawathas traveled 22 feet for every revolution of the wheel.
A typical freight engine such as a Nickel Plate Road S-3 2-8-4 Berkshire with 69 inch (5 foot 9 inch) drivers would travel about 18 feet for every revolution of the wheel.
That is 22.2% farther for one revolution.
In addition, the weight of the rods oscillating round and round, but also up and down, would put pressure on the rails.
So in a very simplistic way, the less "pounding" on the rails should allow for higher speeds, preferable for passenger service.
That is my contribution to the discussion - now I will leave the rest to the professionals!
The main reason is piston stroke. There is a limit to how quickly the pistons can travel in the cylinders without damaging them. Higher wheel diameters allow for lower piston cycle counts at a given speed, which allows faster speeds without damage. Larger wheels also allow for better counterbalancing in order to lessen damage to the track at speed.
Freight locomotives often had smaller diameter drivers as they were better for more tractive effort, which (as pulling power) was the dominant design characteristic for freight locomotives until the superpower era, where speeds and driver diameters grew.
A geared solution would have been possible, such as a large scale high speed Heisler design, but convention won out.
Basically, on steam locomotives, making passenger drivers larger compared to freight locomotive drivers is the "different gearing" equivalent to that of diesels with their fixed gear ratio between the traction motor pinions and the bull gears on the axles. That gearing on diesels is fixed, but it is offered in different fixed gear ratio options for a range of speeds for freight and passenger trains.
I hope this is an easy question. Keep in mind that I haven't had a physics class in 35 years. Why were the driver wheels bigger on steam passenger locomotives than on freight locomotives? I know that the passenger locomotives were set up for speed and the freight locomotives set up for pulling. Couldn't that have been accomplished the same with different gearing? In essensce, isn't that what designers did on diesel locomotives?
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