Bluebell Motive Power

Not necessarily, it may be subject to the same type of copper weld repair that was carried out on 1638. But contractors who can carry out this work are few and far between, so it may be a little while yet before this happens.

 

Like many "ex-pats", I am unfamiliar with the EG extension's gradients. But I have in the past visited the SVR to be amazed that they expected an Ivatt class 2 mogul to handle NINE, with their gradients - and then they wondered why they couldn't keep time, much less pick up any time lost ... The SVR loading policy is (or possibly was - I don't know if the class 2 is still allowed/expected to handle their largest trains) obviously way behind that of the Bluebell in terms of realism.

 

The Ivat mogul 46443, curently in the engine house, woulb be limited to 6 if running today. Trains of 9 coaches are fairly rare but would need class 4 or above

 

link to gradient profile scroll down the page See http://www.bluebell-railway.co.uk/bluebell/map_grad.html

 

In much the same way that 1638 recently had to be completely dismantled in order to carry out a similar repair …

Tom

 

Soz


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As I recall, the cracks are on the firebox crown, so can be repaired with the boiler in situ. I suspect it will be done when a suitable contractor can be lined up, and when there is space in the workshop - 592 spent a lot of the winter in one of the two roads (and the Q class is in the other, immovable); currently the Schools has replaced the C class preparing for a boiler lift, and Camelot will presumably go in once the Schools comes out for its final assembly. Whether there is a short window for B473 between the Schools coming out and Camelot going in remains to be seen.

Tom

 

Thanks Paul for the link; I see the Bluebell's line is distinctly trickier than the SVR, even excluding Imberhorne.

 

We've discussed load limits quite a lot; if you know the steepest gradient, you can work out at least the gravitational resistance quite easily (weight of train, including loco and passengers / gradient is a very close approximation). That doesn't factor in rolling or wind resistance, but on steep gradients, they tend to be small in comparison to the gravitational pull. It's the 1:55 on the Bluebell that is a killer; in comparison, the published profile on the SVR doesn't get steeper than 1:100 (though I believe there was a suggestion that 1:88 was nearer the mark at one point). In any case, that's a huge difference to the sort of load that engines of equivalent size could pull with equal facility. (https://www.national-preservation.com/threads/gradient-profiles-of-heritage-railways.95457/)

As an example: an Ivatt 2MT tank is 63.5 tons. Taking a Mark 1 as 35 tons on average and allowing an average passenger load of 3 tons per coach (equal to about 40 - 50 people if some of them are children), you get:

(*) Ivatt 2MT + 5 coaches + 15 tons of passengers on a 1:55 gradient = 10324 lbs gravitational force pulling it back down the slope, = 60% of the nominal TE just to overcome gravity
(*) Ivatt 2MT + 9 coaches + 27 tons passengers on a 1:88 gradient = 10321 lbs gravitational force

In other words, almost bang on the same gravitational resistance. Obviously the rolling resistance will be a bit higher with nine coaches, but it shows just what an effect the gradient is. Our hypothetical Ivatt 2MT with nine coaches on the SVR is working little harder than the same loco with five coaches on the Bluebell. In practice, I doubt we'd load a class 2 to that any more: we did when we went just to Kingscote, but the difference between 1:75 and 1:55 is considerable.

I suspect we'd reckon on a four coach load with that sort of engine. That gives 8,780lbs of force, or 50% of the nominal TE. The equivalent loading on the SVR, if the 1:88 figure is correct, would be equivalent to a bit over 7 coaches.

Tom

 

These little calculations are fundamental and little appreciated by most enthusiasts. I had a little 'incident' slipping to a stand with 71000 at Darnholme (NYMR) a couple of years ago. The sands were empty despite assurance from the Loco rep that they had been checked - my fault for taking his word - and he said that they never bothered with them on heritage railways. When I asked him if he would attempt an ascent of Shap with 13 coaches (which was the equivalent of 7 coaches on the NYMR) in damp conditions without sands, he said no, of course not.

Steam locos, and for that matter diesel and electric locos, have a finite tractive effort and haulage capacity which, allied to boiler capacity, lays down what they can and can't do. The only variables are : (1) human and (2) variation of the loco from design specification, especially in cylinder and wheel diameter.

 

Tractive effort is usually quoted at 85 % b.p.; does this actually mean that a loco with 100 % b.p. has a nominal t.e. 17.6 % higher? This is something which has puzzled me on and off since I was a kid, and when I was a fireman, it wasn't one of those things which bothered me unduly - I just got on with the job. But I do remember before joining the footplate travelling behind 46251 when she was stopped at Tebay for water one September morning with the night Perth weighing 620 tons tare. As no bankers were available, the driver opted to head off into the night unassisted, and I have sometimes wondered how he managed to pass Shap summit with that load on a damp rail without slipping in less than 23 minutes. What sort of power must Nottingham have been putting down? More than her nominal t.e., I'd bet...

 

In fact nowhere near the nominal tractive effort (temporarily continuing the thread drift).
Rough calculation along the lines of Jamessquared's above.

Train tare 620 tons
Load say 30 tons (very rough guess)
Loco 105 tons (ignoring fractions)
Tender 56 tons (ditto)
Total 811 tons
811 tons on a 1 in 75 gradient = 24221 lbs gravitational force.

The nominal tractive effort is getting on for twice that, so really not relevant. What is relevant is the adhesion limit.

I believe that the 85% figure is a rough and ready allowance for various factors, including the energy used up in operating the valves and the fact that admission of live steam stops before the end of the piston stroke, even at the latest available cut-off.

 

Remember power and TE aren't the same thing: a BR class 08 shunter has a continuous TE of 35,000lbs, which is about the same as a Gresley A4, but only 350hp, which is maybe only about 15 - 20% of the power an A4 could sustainably produce.

Tractive Effort is a measure of force: it will govern what is the greatest load that can be moved, (because eventually, the sum of the rolling resistance + engine internal resistance + air resistance + gravitational pull will exceed the force applied by the cylinders at whatever speed you are doing; at that point the train will start to decelerate, or, if it is already stationary, it won't be able to start).

Power is a measure of how quickly energy can be expended. Energy is absorbed in internal friction, pushing the air out of the way, gaining height against gravity etc. The quicker you move, the quicker you expend energy, so the more power is needed. Eventually the rate you expend energy just matches the power output of the locomotive, at which point you stop accelerating and move at a constant speed. Uphill, that speed will be lower, because some of the energy expenditure is used in working against gravity (and thereby gaining potential energy). On the flat, there is no work against gravity, so more is available to work against wind and rolling resistance, so the maximum speed will be higher.

Very crudely, you can see those relationships in the load limits we set for different locos. For example, an 08 shunter is allowed a greater load than the Maunsell U class between Kingscote and East Grinstead, because it has a greater TE. But because it probably has only somewhere between a third and a half the power, it isn't able to work that load at anywhere near line speed, so can't keep time with the timetable. (Which is one reason why, with long sections, and section times that require working at close to line speed for most of the way, we don't allow the 08 out during normal timetabled days - it couldn't meet the section times). There was an occasion in the summer when the 08 had to rescue the service and take what I believe was a four coach load; it had no problem moving that load, but I believe was down to about 10mph between Horsted and the Tunnel, and a bit less up to Imberhorne.

Tom

 

In an idle moment, I plotted some resistance graphs to show the impact gradient has on load.

In each case, the y axis shows the overall train resistance, that has to be overcome by the force available from the pistons. The x axis shows the number of carriages.

I've made the following assumptions:
- Carriage weight: 38tons each (includes passengers)
- Carriage resistance: 4lbf per ton (so, for example, 152lbf per carriage)
- Loco resistance: I've guesstimated at 1000lbf for a Terrier / P tank (with slide valves), 1500lbf for an Ivatt 2MT tank / tender engine, and 2000lbf for a Black 5.
- Curve resistance: I've ignored, without good figures for curve radii. That probably does a disservice to the NYMR in my graphs!
- Wind resistance: It's small at heritage line speeds, so I have ignored (probably less than 1% of the total resistance, so within the error).

I've plotted four gradients:

1:49 (NYMR)
1:55 (Bluebell)
1:88 (SVR)
1:264 (A generic figure for a line that is flat enough that the gradient is no longer become the dominant feature. I happened to use 1:264 simply because, the start from Victoria aside, it is the ruling gradient on the London - Brighton mainline).

At the point where the line exceeds the tractive effort of the loco, the loco simply could not pull the train up the gradient. In actual practical terms, the feasible load at, say, 15-25mph would actually be much lower, since TE drops with speed. As an example, an Ivatt 2MT has a nominal TE of 17,400lbf on starting, but doesn't achieve 17,400lbf at 25mph.

There are four graphs:

1) Ivatt 2MT tank, 63.5tons
Nominal TE is 17,400lbf. Note that on the 1:49 gradient, the nominal TE is exceeded at about 7 coaches.




2) Ivatt 2MT tender, 86tons
Nominal TE is 17,400lbf. Note that on the 1:49 gradient, the nominal TE is exceeded at about 6 coaches; the difference is in the tender!



3) Stanier Black 5, 125tons
Nominal TE is 25,500lbf. Note that even without curve resistance, a Black 5 would stall at somewhere round about 10-11 coaches on the Bluebell, and could only manage several fewer if it was expected to run at 25mph on the way up to Imberhorne summit. On our hypothetical Brighton Mainline, no realistic load is going to get close to the limit of TE, and instead there will be other factors (some loco-dependent i.e. power output, some infrastructure-dependent, i.e. platform length) that set an upper limit on the load.



4) Stroudley Terrier / Wainwright P, 28tons
Nominal TE depends significantly on cylinder size (Fenchurch is better than Stepney!) but somewhere in the range 7,000lbf to 8,000lbf.

Note that on our gradients, the physical limit is reached at about 3 coaches, which is why we have a practical limit of two. On the SVR the same locos might reasonably be expected to take 4 (and would be on their physical limit at 5-6 coaches), but they might struggle for water and coal capacity!



Tom

 

Would you care to tell us how two identical tank loco's have a weight difference of 22.5 tons?............................................... is one with coal and water and the other without?..............................................

 

Edit … cut and paste error! The first is the tank engine version, the second the tender version. TE etc is identical, but the tender version is effectively carrying round half a carriage before it even gets around to an actual revenue-paying load.

Tom

 

D'oh!!

I kinda guessed the rest!.................................................. the word tender gave it away.

 

Does anybody know where 80064 has been moved to? Couldn't see it in the shed or anywhere at Sheffield Park earlier today-hasn't moved to pastures new has it?


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I haven't checked, but I think a number of locos have been sheltering over in the Woodpax shed for the winter.

Tom

 

Aha! Didn't know there was space in there, but if there is then it makes lots of sense to use it... Found it very strange pottering around earlier today with the old shed having been flattened...