Size of Drivers on a steam locomotive.

[Murphy Siding]

"Steam Dummy"here, with another question: Everything I read about steam locomotives mentions the size of the driving wheels. What influence does that have on the performance of the engine? How did the designers determine what size drivers a certain engine should have?
Thanks

[Modelcar]

...larger dia. drivers: more speed.....Small drivers: Power. How that was calculated specifically for the job will have to be answered by engineering and or design....

[CShaveRR]

Think about it: if everything else (boiler pressure, cylinder bore and stroke) stayed the same, a locomotive with 72-inch drivers would go half again as far in one revolution as one with 48-inch drivers. So engines built for speed would have the larger drivers, and engines built for power would have smaller ones.

Of course, those other factors come into play, too, and somebody far more expert than I will have to explain that stuff.

[beaulieu]

Things like leverage, large enough for proper balancing, etc. all factor in. The was a "Rule of Thumb" that maximum speed would be Driver Diameter plus 10. This doesn't work for very large or very small, but is approximate for medium sizes. Until the Super Power era, freight locomotives had drivers between 56" and 64", except for special circumstances. Passenger locomotives except for mountainous territory were above 69".

[Isambard]

After all these years as a steam enthusiast I should know the answer to this question, but my memory fails me: Is the specified wheel diameter as measured across the flanges or across the tires?

[beaulieu]

Across the tires with half wear.

[Murphy Siding]

So, the size of the drivers is really nothing more than a way to pre-set the gear ratio?

[tree68]

QUOTE: Originally posted by Murphy Siding

So, the size of the drivers is really nothing more than a way to pre-set the gear ratio?

That's one way to look at it.

I'd heard that the top speed was the driver size, but given the other differences between locomotives, either one serves as a handy rule of thumb.

[Tulyar15]

Back in the 1980's the National Railway Museum, York restored a couple of old single driving wheel locos to work ording, both 4-2-2's. I saw the Midland "Spinner" in steam at the Rainhill cavelcade in 1980 (held to mark the 150th anniversary of the Liverpool and Manchester Railway) but I dont think I ever saw the Great Northern Stirling single in steam. It was loaned to the preserved Great Central Railway for a couple of years. With its 8' driving wheels it went like a rocket - they had a job keeping its speed down to 25mph! (the maximum speed most preserved lines are allowed to operate at). They quickly made a rule it had to be pulling at least 4 bogie passenger cars to provide enough brake force!

[Overmod]

Wide variety of issues involved with driver size. Some more complexity is introduced because the mechanical stroke (of the piston) determines the crank circle on a given locomotive.

Remember that (in addition to the valid points made by beaulieu et al.) you have balance issues with rodwork, wear issues with driver tires, suspension, equalization, and balance considerations (including rigid wheelbase), brake-rigging issues, effective frame height and boiler packaging within a given loading gauge ... just to name a few off the top of my head. In a two-cylinder double-acting locomotive, there are relatively 'hard' restrictions on the power available at or close to starting that are related to driver diameter, and the smoothness with which that power can be applied at the railhead and to the trailing load.

It's easy for 'armchair' locomotive designers to start talking about running small-drivered locomotives at higher rps because modern materials and balancing technique (and advanced valve gear) would permit it, but ignore the relative effect of, say, induced pin failure at a given track speed, or take the trouble to calculate whether rotational inertia (and various augment forces) increases or decreases vs. a larger-diameter alternative for a given range of speed. On the other hand, it's easy to misunderstand whether 'high speed' is even an effectively important design criterion for a particular application...

Of course, what you see 'historically' is very often justified for reasons different from straight theoretically-based engineering. Sometimes this is good, sometimes bad. One point made last year in the Trains article is that very often steam locomotives spend much of their working life running at much slower speeds than 'design' would otherwise warrant -- it may therefore be valuable to design them so that higher speed is possible (but not optimal) but the overall 'cost of ownership' is minimized in reality -- which imho is a good and reasonable criterion for locomotive design. If you intend the thing to run at high speed for much of its life -- cf. the Milwaukee class A's, which weren't really suited for much other than that, or the ATSF 4-8-4s -- then you are justified in using larger drivers and tinkering with stuff like multispeed boosters to optimize the main locomotive to run most efficiently. But beware the wrong assumptions -- the Alleghenies and the PRR divided-drive freight power being two important miscalls (the former because the only service that could cost-effectively use their single-unit power ran at speeds well below their best developed power, and the latter because PRR freight speed limits forced the same condition!)

As an interesting exercise, you might consider whether an express engine on most American passenger service (in the days of steam) *required* drivers higher than the 70" non-disc variety found on the N&W J class. My own opinion (not worth much, necessarily) is that the 'sweet spot' for normal fast-service power is somewhere between 72 and 77 inches (this won't change if you use precise valve gear like Franklin type B, but more because of suspension and peripheral considerations than rotational speed or steam consumption) and there's little point in going smaller than the high 50s for anything intended to operate on a normal-speed mainline.

[caseykkkk]

I'm 6ft 2 inches and I think I am a good driver...a bit of a squeeze in the cab sometimes though[:D]

[Anonymous]

QUOTE: Originally posted by Tulyar15

Back in the 1980's the National Railway Museum, York restored a couple of old single driving wheel locos to work ording, both 4-2-2's. I saw the Midland "Spinner" in steam at the Rainhill cavelcade in 1980 (held to mark the 150th anniversary of the Liverpool and Manchester Railway) but I dont think I ever saw the Great Northern Stirling single in steam. It was loaned to the preserved Great Central Railway for a couple of years. With its 8' driving wheels it went like a rocket - they had a job keeping its speed down to 25mph! (the maximum speed most preserved lines are allowed to operate at). They quickly made a rule it had to be pulling at least 4 bogie passenger cars to provide enough brake force!

Yes but what was the starting tractive effort like? It couldn't have been very high.

[Hugh Jampton]

QUOTE: Originally posted by trainjunky29

QUOTE: Originally posted by Tulyar15

Back in the 1980's the National Railway Museum, York restored a couple of old single driving wheel locos to work ording, both 4-2-2's. I saw the Midland "Spinner" in steam at the Rainhill cavelcade in 1980 (held to mark the 150th anniversary of the Liverpool and Manchester Railway) but I dont think I ever saw the Great Northern Stirling single in steam. It was loaned to the preserved Great Central Railway for a couple of years. With its 8' driving wheels it went like a rocket - they had a job keeping its speed down to 25mph! (the maximum speed most preserved lines are allowed to operate at). They quickly made a rule it had to be pulling at least 4 bogie passenger cars to provide enough brake force!

Yes but what was the starting tractive effort like? It couldn't have been very high.

Nope,,, couldn't pull the skin off a rice pudding...

[dldance]

For steam locomotives with out trailing trucks, such as the 4-4-0 American type, the size of the drivers had a significant impact on the size and shape of the firebox and grate area. Grate area then impacts steaming ability - and thus speed and power.

dd

[Anonymous]

Contrary to popular belief, size does matter [:D]

[BR60103]

I heard a story that in the 50s, someone at British Railways saw one of their final steam freight engines (the 9F 2-10-0) bombing along at 90 on a passenger train. Head office was about to issue an edict that maximum speed for a steamer was its driver diameter when someone noticed that this would limit Gresley's A3 and A4 pacifics to 80 mph, including speed record holder Mallard (126 mph). Both of these classes were known to do 100 when given an open track.

[Tulyar15]

It's true about the 9F 2-10-0's reaching 90mph when used on passenger trains - there were several instances when this happened.. I've never heard of any official edict like the one BR60103 describes. In practice at that time each of the BR Regions was a law unto itself anyway.

I think the most common instances of 9F reaching 90mph were on the Great Central line from London Marylebone to Sheffield Victoria. This line had been part of the LNER before nationalisation so at first under BR was part of the Eastern Region. When it was transferred to the London Midland Region in 1960, the LNER A3 4-6-2's (including #60103 "Flying Scotsman" which was allocated to the GC shed at Leicester at that time!) and V2 2-6-2's were transferred away, to be replaced by a smaller number of ex-LMS Black 5 and Jubilee 4-6-0s (later joined by some Royal Scot 4-6-0's too). So when the GC line management found themselves short of express locos, as they inevitably did, they tried using 9F's which they found could keep time. After hearing about this, the LM Region manager sent an inspector to the GC line to question loco crews involved. When he asked one driver how heavy a train he would take with particular classes of locos, the driver just replied: "We dont worry about details like that, we just do the best we can with what we've got!". Needless to say, that went down like a lead balloon!

As for GNR No 1, the Single I talked about earlier, I cant remember what its tractive effort. But as an old Wel***rain driver I know always explains to younger railfans, it's not how much you can pull that matters, it's how quicly you can accelerate. A GW 0-6-0T Pannier tank, or a 350hp Class 08 Diesel switcher can pull a 18 passenger cars, but not very quickly!

During WW2 when freight locos were needed everywhere and express passenger locos surplus, the GWR started using its Saint Class 4-6-0's with 6' 8" drivers on freight trains. At first the crews were worried that they would not cope, as their tractive effort was considerably lower than tha of the GWR 2800 2-8-0's which employed the same boiler and cylinder but had 4' 7" drivers. But they soon found the Saints could pull as much as the 2800's, the main problem was stopping as they'd go so fast. They would try and ensure there were enough vacuum braked wagons on the front to provide extra brake force. (At this time, and ineed up to the 1960's most freight trains in Britain did not have continous brakes!).