All else being equal.....Is it the height of the drive wheels that determine the "final drive" of a steam locomotive.?
That is to say...will an engine with shorter drive wheels have "lower gearing" than an engine with taller drive wheels.?
Thank You
If the road speed was equal the engine with the smaller wheels would develope more power due to the wheels turning faster, revs are part of the power equation, so the engine with the smaller wheels would be lower geared. Freight engines generally had smaller driving wheels than passenger engines. Just in passing a "rough rule of thumb" is that one inch of driving wheel diameter equals one mile an hour, but this is a very rough rule of thumb. Plenty of engines with 84 inch drivers have got along quite happily at over 100 mph.
Regards, Malcolm
kenny dorham All else being equal.....Is it the height of the drive wheels that determine the "final drive" of a steam locomotive.? That is to say...will an engine with shorter drive wheels have "lower gearing" than an engine with taller drive wheels.? Thank You
All else being equal - yes, driver size equals "gearing". But as steam design evolved, it was found that no steam loco really needed drivers bigger than the 67-80 inch range, and that the smaller side of that range was generally better.
A study of the "Super Power" concept of steam loco design shows that proper balance of power and speed, combined with modern construction methods and modern features made the last of great steam locos truely dual service - powerful and fast.
Examples:
SP GS4 4-8-4
LIMA Berkshire 2-8-4 (NKP, C&O, PM, RF&P, etc)
LIMA 2-6-6-6
N&W Class A 2-6-6-4
N&W Class J 4-8-4
All these locos were both fast and powerful, most had drivers at the lower end of the size range noted above, 67" to 70".
Sheldon
10-4
Thanks guys.....
One problem is that the smaller the drivers, the harder it is to add weight to counterbalance the side rods. An engine with small drivers may in theory be able to spin the wheels fast enough to achieve a pretty good speed, but the up-and-down thrashing of the rods could cause a rough ride, and can even damage the rails as the rods pound down on them. That's why a USRA 2-6-6-2 (like the Bachmann Spectrum engine) with it's small drivers was usually limited to 20 MPH or less, while a UP 4-6-6-4 with larger drivers could be used on passenger trains.
wjstix One problem is that the smaller the drivers, the harder it is to add weight to counterbalance the side rods. An engine with small drivers may in theory be able to spin the wheels fast enough to achieve a pretty good speed, but the up-and-down thrashing of the rods could cause a rough ride, and can even damage the rails as the rods pound down on them. That's why a USRA 2-6-6-2 (like the Bachmann Spectrum engine) with it's small drivers was usually limited to 20 MPH or less, while a UP 4-6-6-4 with larger drivers could be used on passenger trains.
True - but only to a point.
Actually, the USRA 2-6-6-2, and all the similar 2-6-6-2's pioneered and used by the C&O easily ran at speeds more like 35-40 mph with no problem.
The secret is this - because there were only three sets of drivers on each engine, the main rods are short, making them lighter, and the mass of all the rods was small, requiring less weight to offset it.
An example of a loco that did suffer this problem was the USRA light 2-10-2. Because of the five coupled axles, the main rods are longer and heavier and the connecting rods needed to be heavier and stronger and had more total mass. But the small 56" drivers limited the ability to balance them well. They were generally limited to 25-30 mph as built.
UNTIL - more modern hollow cast drivers (boxpok) were invented that allowed heavy metals like lead to be precisely added to the driver to balance even small drivers for such heavy rods.
Several roads invested in rebuilding their USRA 2-10-2 light locos in the late thirties, adding modern boxpok drivers in the main driver position and cast engine bed frames, some with roller bearings, giving these locos better balance and stability and allowing them to run a speeds similar to many Mikados - 35-45 mph.
And, as I listed above, there are many locos with 64" to 70" drivers that easily ran in the 70-100 mph range with no problem, C&O 2-6-6-6, N&W J 4-8-4, B&O EM-1 2-8-8-4, N&W A 2-6-6-4, just to mention a few. But again you will note, two of the four locos listed have only three coupled axles per engine - making them easy to balance and more nimble on curves.
And even the B&O S1a 2-10-2 could easily hit the 60-70 mph mark with its 64" drivers and five coupled axles - it was without question one of the best balanced 2-10-2 types built.
Sheldon, I appreciate your description of the boxpok drivers; I knew of them but had never read any description of them. If I had really been curious I could have delved into the matter and learned their advantage. Thanks again.
Johnny
A critical component of the 'gearing' of the steamer's power train comes from the length of the main crank, or rather its distance from the central axis of the drivers' rotation. All things being equal, two engines with 65" drivers will have the same piston thrust, say, but if one has the longer reach for the main crank, that engine will have more leverage relative to the working circumference of its host driver and should therefore be able to start a heavier tonnage.
Not to be forgotten is the all-important cut-off setting for the reverser because that determines how much steam is admitted to the cylinders during each power stroke. When cut-off is set at 85% during the initial lifting of the consist, the valve will dwell long enough with the inlet port open such that the piston moves about 85% of its mechanical distance down the length of its possible stroke impelled by full pressure steam, thus affording maximum pressure on the working face of the piston.
The J Class 4-8-4 outdid even the marvellously powerful New York Central Niagara in terms of tractive effort, but that was with 70" drivers compared to 79" on the Niagara. Both power plants were capable of approximately the same horsepower at track speeds, although there is insistance that the S1b was the more powerful of the two. Yet, their starting tractive efforts tell the tale about which engine would have done the better job on a typical consist of heavyweight passenger cars on Christianburg Hill. The J reports out at 78K lbs while the vaunted Niagara managed a comparatively anemic 61.5K lbs.
Crandell
Thanks for the Engine/Physics lesson.
Very interesting indeed.....
Driver size and piston stroke are not the only variables to consider when computing the power output of the "drive train" of a steamer. Other factors include designed operating steam pressure, size of the firebox grate area, cylinder dimensions, solid or roller bearings on all axles and weight on drivers to name the most important. All these factors combine to establish the locomotives "power curve". This is a graph showing that horsepower increases as speed develops up to a peak output after which power output drops off even as the engine goes faster and faster. Modern freight locomotives were designed to hit their peak power output at about 35 MPH. Passenger engines peaked out around 50 MPH. Of course, both types could and did go faster but some loss of efficiency resulted. Engines running faster than their peak power output were said to be operating "ahead of the power curve".
Hi Crandell ( howdy ?)
>> The J Class 4-8-4 outdid even the marvellously powerful New York Central Niagara in terms of tractive effort, but that was with 70" drivers compared to 79" on the Niagara. <<
That's by far not the end of that story . Take a look at 1:1 piston thrust at b.p. , take a look at adhesion mass , take a look at valve gear characteristics , take a look at factor of adhesion in each the engines .
The Niagara was 'dual purpose' only in so far as being more powerful in starting t.e. than a Hudson . Outside the N&W , the J class 4-8-4 would have been considered truly dual purpose , yet in relation to an A class 2-6-6-4 it was an express runner .
Definitely the Niagara was superior performing in the higher to highest speed range - although far from marvelling she did quite well , I should say .
Regards
Juniatha