Why did steam locomotives not transfer power by cogwheels?



Modern cars use cogwheel to transfer power from the engine to the wheels. Steam locomotives used some kind of bars (sorry, I'm not a native speaker) to transfer the power to the wheels.

Why did the engineers not use cogwheels? Would steam locomotives had been faster if they had used cogwheels?

J. Doe

Posted 2018-03-01T18:00:08.887

Reputation: 93


Unless I'm mistaken, a Shay locomotive uses cogs to transfer power from the crankshaft to the wheels. The only "bars" are basically ordinary piston rods.

Hot Licks 2018-03-02T01:55:28.787

This is true for diesel locomotives as well. Only very few (and very small) diesel locomotives use cogwheels.

vsz 2018-03-02T14:44:33.887


Steam locomotives with geared drives were used in specialized situations, mostly involving hauling heavy loads at slow speed on steep grades. Several types were developed, including Shay (named after the original patent holder, Ephraim Shay), Climax (patented by Russ Battles), and Heisler (Charles Heisler patents). See https://en.wikipedia.org/wiki/Geared_steam_locomotive

Bob Jarvis 2018-03-02T16:03:58.740

Comment-answer of @HughMurphy: "If you ever go to Colorado Springs, you might check out the Pikes Peak Cog Railway. https://www.cograilway.com/"

peterh 2018-03-03T08:35:11.883



I would like to point out that modern cars do not use cogwheels for transmission, they use shafts. Cogwheels are used for the gearing and differential.

But bar mechanisms were used mostly because they didnt have the kind of manufacturing facilities as we do today. Bar mechanisms are easy to make, they are flexible and field maintainable. In any case in this particular design also because the entire mechanism would have had to turn the power transmission direction 2 times. See the piston is directly connected to the front wheel and transfering that to the subsequent wheel is pretty simple with a bar while a shaft coupling would have needed more parts, which again were hard to manufacture.


Posted 2018-03-01T18:00:08.887

Reputation: 1 852

3Right- unless there was a need to drive wheels at an rpm different from the steam piston cycle rate, gearing would be pointless.Carl Witthoft 2018-03-01T18:48:30.793

13This is incorrect in most of its points. First, gear cutting long precedes steam engines. And then there's the fact that geared railway steam engines were something that existed. Gears weren't needed in ordinary mainline locomotives, but they were possible and were used where needed.Chris Stratton 2018-03-01T23:20:18.627

1@ChrisStratton Just because a concept is invented before another, doesn't mean the first is perfected to the point that it can be feasibly manufactured to the necessary precision and strength. Furthermore, obviously it was possible to build - that's not the point of the question.pipe 2018-03-02T01:49:37.013


There is a scenic railroad in West Virginia (http://mountainrailwv.com/tour/the-durbin-rocket/) that operates a heisler locomotive. It has a 90 degree V-twin engine turning a drive shaft that runs the full length of the locomotive and the tender. All of the wheels of the locomotive and the tender are powered. It doesn't go very fast, but it can climb steep grades.

james large 2018-03-02T03:17:03.897

@Chris i didnt say thehey did not have gears, i just pointed out that the manufacturing level wasnt really the same as today so they tended to favour the simple and straightforward. A modern design would do different things for architectural reasons as its easy. Anyway i wanted to be succit as i was writing this on a phone.joojaa 2018-03-02T05:02:16.617

6@joojaa - all you're proving is that, even after presumably reading the accurate answers posted by others, you still do not understand the actual design issues involved here. It's not an issue of modern manufacturing (again, precise gears predated steam engines) but of matching the best regime of operation engine to the load. Then and now, gears were used when needed, and not when notChris Stratton 2018-03-02T06:35:05.763

1New steam locomotives were designed and built in the 1950's, and except for a few unsuccessful experimental prototypes, they were all rod-driven. This disproves the claim that they didn't have the technology to manufacture cogwheels.vsz 2018-03-02T14:42:36.950

As a modern example, most electric cars do not have a gearbox, perhaps one gear from the motor to the drive shafts / differential. This is despite there being 8 and 9 gear transmissions readily available.RemarkLima 2018-03-02T22:34:53.957

Minor question - is fexible supposed to be fixable or flexible?Criggie 2018-03-03T12:35:44.580

1@Criggie oops bothjoojaa 2018-03-03T13:13:53.447

@RemarkLima - OTOH, most gasoline cars have gears. Not only a gearbox, but a bevel gear in the differential.Hot Licks 2018-03-03T22:46:05.903

@HotLicks that's my point, gearboxes are readily available, and quite amazing, dual clutch, seamless shifts, etc... But not required for an EV, so gears not being available is incorrect to the question asked.RemarkLima 2018-03-03T22:52:25.097

The main problem with this answer is that it assumes that physical transmission of power from Point A to Point B is the difficulty being addressed in most modern power transmission schemes (and even schemes going back to the dawn off steam locomotives). In fact, the modern front-engine, rear-drive automobile is a major exception. The major issue, as pointed out elsewhere, is torque matching. This answer is just plain wrong, and should never have been accepted.Hot Licks 2018-03-03T23:06:40.043


Steam piston engines can generate a lot of torque from stationary and the pistons can be physically remote from the boiler, so in most cases it is most convenient to have the pistons directly drive the wheels via a crank. Equally as trains don't have a steering mechanism as such and have conical section wheels, you don't need a differential either.

In contrast, internal combustion engines need to be turning at a fairly moderate RPM to generate useful torque and produce most of their torque and power in a fairly narrow rev range so they need both a means of disengaging drive (clutch or viscous torque converter) and a selectable ratio gearbox in order to provide useful torque at a wide range of road speeds.

Also, IC engines tend to work better with multiple cylinders as this smooths out the power delivery over the various different stages of the working cycle and so need a crankshaft with a common output shaft. Steam engines are essentially pneumatic actuators so you can make the working stroke as long as is convenient and get a reasonably consistent linear force.

The external connecting rods on a steam locomotive are a direct analogue of the connecting rods which link the piston of an IC engine to the crankshaft.

The short answer is that the torque characteristic of a steam engine simply means that a gearbox is unnecessary, as torque is more or less independent of RPM for its normal working speed range.

Chris Johns

Posted 2018-03-01T18:00:08.887

Reputation: 12 837

I don't see how turning because of steering versus turning because of curved rails makes a difference as far as whether a differential is needed.Acccumulation 2018-03-02T16:33:22.563

1Because the difference in speed between inner and outer wheels is handled by the wheels themselves being conical ie they can effectively change diameter so the wheel speed can vary despite a constant axle speed.Chris Johns 2018-03-02T16:41:07.247

Yes, it seems to me that the "conical" part is the only relevant part. I don't see how mentioning the lack of steering is relevant. (Although to be nitpicky I believe they're frustrums rather than cones.)Acccumulation 2018-03-02T17:25:11.657

@Acccumulation Rail wheels need a solid connecting axle (no differential) in order to properly maintain centering on the track. There's an explanation here: https://en.wikipedia.org/wiki/Adhesion_railway#Directional_stability_and_hunting_instability

Anthony X 2018-03-03T19:33:30.827

@AnthonyX your comment starts with my username, but I don't see how it's a response to mine.Acccumulation 2018-03-04T00:08:24.483


Here is a picture of the crankshafts inside a modern internal combustion engine:

enter image description here

The purpose of these is to convert the back-and forth motion of the piston into rotational motion. It's very much the same mechanism as used on old steam engines:

enter image description here

The difference is that in the internal combustion engine the power is not transferred directly to the wheels but to a shaft. The reasons for this difference are discussed in other answers --- I just wanted to point out that the underlying mechanism is the same.


Posted 2018-03-01T18:00:08.887

Reputation: 231


for a while, steam locomotives actually used gears and cylinder/piston sets which drove crankshafts. These were called geared locomotives and they were used to haul heavy loads up particularly steep inclines at low speeds. this made them popular for timber logging operations in the western US during the days of steam power.

for higher speed use on more gradual slopes, the direct-drive method (in which the connecting rod engages the driven wheel(s)) is simpler and furnishes an adequate match between the impedance of the load and that of the engine.

niels nielsen

Posted 2018-03-01T18:00:08.887

Reputation: 1 114

These were based on the Italian funicula so far as I know. Are you sure the engine used gears anywhere other than to drive the toothed wheel along the pinion rack?Carl Witthoft 2018-03-01T18:49:36.623

2yes, the common ones had three cylinders in line and drove the big wheels through gears. I think Baldwin was one brand that made these. I'll see if I can get a reference for you- NNniels nielsen 2018-03-01T20:33:44.807

1good example was the 3-cylinder "Shay", lots of info on these on the web.niels nielsen 2018-03-01T20:35:44.773

2This doesn't answer the question, actually the opposite. By stating a narrow example of a use, you've illustration an exception that proves the rule. The question is why cogwheels were not the most common mode of operation.pipe 2018-03-02T01:53:24.570


Steam generates full torque at zero speed as has been mentioned elsewhere, so like in an electric car (which has very much the same sort of characteristic), there is little to be gained from a gearbox, may as well drive the wheels directly.

This is also why the vast majority of diesel locomotives above the very small, are really diesel-electric, it makes the near zero speed part of the performance less annoying, and removes the need to try to cool a very high power clutch.

Incidentally a steam loco does have sort of "gearing", in that the driver can control the valve timing to vary the volume of steam admitted per stroke and thus the torque available, this is subtly separate (but interacts with) from varying the steam pressure.... You see this when a steam train pulls away as initially there will be powerful blasts of steam from the stack because the driver has the valve gear set such that there is still significant pressure in the cylinder as the exhaust valve opens (To maximise torque), as the speed comes up the fraction of a cycle the intake valve is open is reduced to improve efficiency and the exhaust note evens out as the exhaust becomes closer to atmospheric pressure. These variable valve linkages were one of the more patent infested waters back then with fights between all the big players.

Dan Mills

Posted 2018-03-01T18:00:08.887

Reputation: 231

Well yes a steam engine is not really at all like a IC engine, its more like a pneumatic system so the piston in a steam engine is more like the piston on say this excavator, although thats hydraulic.

joojaa 2018-03-03T14:58:21.000

Variable valve timing provides not just torque control (and much improved efficiency at "early cutoff") but also reversing.Brian Drummond 2018-03-03T16:30:02.997

@BrianDrummond Indeed, at least in some of the valve gear variants, a trick that some large diesel engines are also capable of. I would note that care should be taken comparing a steam and a hydraulic power system as only one of them has a significant expansion of the working fluid (with corresponding temperature change) that makes the cycle thermodynamically interesting (And is where the efficiency in early cutoff comes from (Also why compound plants with vacuum in the condensers are such a win in marine applications)).Dan Mills 2018-03-03T17:40:17.990


Steam locomotives use steam pistons, not steam turbines.

Gears/cogs would be pointless since there is no rotary source of power on steam locomotives. They use steam pistons, which go back and forth.

As the physics worked out, direct-drive worked out really well with achievable values of piston diameter, stroke/eccentric, and wheel size. Until it didn't. And what got them was the curves.

Mainline pullers stuck with rods: way too big for gears

As fully superheated boilers became very powerful, fast passenger locomotives used this power at higher speeds. For them, the side-rod design was perfect. But slow lugging freight locomotives needed more weight on the rail to transfer the power at low speeds. This required more driving axles to spread the weight. That made a single rigid group of driving axles too long for curves. So they split into two (rarely, three) groups of driving axles. Power transfer was done with an engine on each group, usually simple, sometimes compound. Union Pacific's Big Boy had 8 drive axles in two groups (each with a simple engine, still avoiding gears), handling curves like a 4-drive-axle locomotive.

enter image description here src

Taken to absurdity. The Virginian Railway finally gave up and electrified.

At these power levels, 4000-6000 horsepower, gear drive was out of the question: it was an order of magnitude too much power for gears. Even the electric GG1 of the era used twelve massive pinions to transfer a similar amount of power to six axles.

Much smaller engines could be geared

Mountain railroads used low power, lightweight locomotives which had to slither up fairly tight curves. Even a very modest side-rod steam engine was too stiff for the curves. They also wasted a lot of precious weight on non-drive wheels, e.g. the pilot truck and the tender. Ephraim Shay solved this problem with, indeed, geared locomotives. Keep in mind these are small locomotives: the largest, Western Maryland #6, has boiler pressure of 200 psi and a top speed of 23 mph.

Ephraim Shay put a drive shaft along one side of the locomotive, gearing to each wheel. The pistons directly cranked the drive shaft. Note the elaborate telescoping drive shafts, most especially important due to its off-center location.

enter image description here

enter image description here Note the gears. sources

Charles Heisler put the drive shaft down the locomotive centerline, and used a "vee-twin" piston arrangement. Note the side rods: that means only one of the two axles is geared to the drive shaft, the side rods transfer power to the other axle. Side rods like that imply perhaps 100 horsepower per axle.

enter image description here

The Climax Manufacturing Co. took Heisler's centerline-shaft arrangement and added a cross-shaft and more gearing to put the steam pistons in an almost conventional location.

enter image description here

Having seen these geared-locomotive arrangements, you can see where they would not "scale" up to multithousand horsepower outputs.


Posted 2018-03-01T18:00:08.887

Reputation: 315