Another Gresley Thread, middle big end

Tractive effort (or force) at any point in a wheel revolution is a variable. The quoted tractive effort is an average over one wheel revolution and is derived by taking the mean effective pressure (MEP) and the area of the piston (πd²/4) For a loco having a cut off at 75% the MEP is approximately 85% of the maximum steam pressure as the steam expands before being exhausted. For one wheel revolution, the piston moves back and forth in the cylinder so has effectively travelled twice the stroke. However, the locomotive has moved one wheel revolution, which is π x wheel diameter (D) so there is a mechanical disadvantage between piston movement and locomotive movement. There is also more than one cylinder so we have to multiply by the number of cylinders (N). We thus get the following equation for tractive force at the wheel:

T.E. = 0.85 x BP x (π x d² /4) x (2 x L x N/(π x D)) which simplifies to (0.85 x BP x d² /D) x N/2

If the cut off is reduced to 65% then the MEP is reduced accordingly, which is the point I was originally trying to make. I think people don't realise where the 0.85 comes from and assume that it is an allowance for the steam pressure not being at the maximum boiler pressure.

 

That's an interesting statement that I've never heard before but I've not read the green books. If that was the case, how come the cut off was altered to 75% in fore gear in later days? Whislt I don't rate the conjugated gear, it seems to be blamed for a more than its fair share of design problems.

 

As indicated upthread, the middle big end and the alignment of components was improved post second world war, improving matters and allowing for that greater cut off to be used.

 

As you probably know, the locos were given a longer lap c. 1927-31, which required the valves to be given a longer travel. This led to further clearance problems (p20-21) and also re-design of the pivots, plus instructions not to put the locos into full gear with steam shut off at high speed. With the various changes, it was decided to increase the max cut-off to 75% but that was mainly implemented after the War.

 

I have no argument with that except that it applies to a moving engine, and I repeat that Nominal Tractive Effort applies - if it applies at all - to a stationary one. Once moving, all sorts of dynamic forces come into play, forces which are not easy to calculate, such as steam flow through ports, cut-off position, regulator opening and others. It's only when the engine is standing still that all the criteria - all the constants - are known and can be put into a formula. Dynamic T.E. cannot be calculated but must be measured, either at the wheel on a test plant or at the drawbar of a dynamometer car. The m.e.p. doesn't really come into it; at zero mph the Effective Pressure is boiler pressure at all points of the stroke up to the point of release. If the Nominal T.E. was to be applicable at speed, the speed would need to be specified in the formula. It isn't, and the only speed at which it applies is zero mph (or zero degrees per second).

 

Whilst largely agreeing with the individual comments, I don't agree that the mean effective pressure is boiler pressure up to the point of release; it is up to the point of cut-off after which expansion takes place until the point of release. I've got to disagree with this idea that the nominal tractive effort that is traditionally quoted applies only at the moment of starting. For a start, as has been said many times on here, the tractive effort at the point of starting is a variable depending on crank position. The prime purpose of the tractive effort figure is for comparison purposes and design calculations. What else would you use it for? In essence, a steam locomotive is a constant pull machine, unlike a diesel, which is a constant horsepower machine. The (say) 1000hp diesel electric has that horsepower available throughout its range and it is only the practical limitations of adhesion, current carrying capacity and heat that prevent the tractive effort from heading towards infinite as the speed drops to zero. Diesel mechanical drives do this through gearboxes and hydraulics through torque converters. A steam loco, though, has a tractive force defined by the size of the cylinders, boiler pressure and wheel diameter, all of which are fixed parameters independent of speed and it is only the practical limitations of boiler size, passageways, port openings and leakage that prevent that tractive effort being applied continuously as speed increases. At low speeds these practical limitations do not really come into play, other than leakage past piston & valve rings but that is a maintenance function.

 

I agree that Nominal Tractive Effort is of use for comparison only, but suggest it gives an idea of the force delivered at starting so is an indication, nothing more, than the load the engine can get on the move. It's practical value is all but nothing, not least because it makes no account of adhesion.

 

Many thanks to all who have posted replies, even if a couple have drifted a bit. The basic reason for My morbid curiosity was being on the periphery of a conversation between the late Steve Neville and the late Jim Sarney in which Steve stated that Mallard still had the Gresley type middle big end, which to Me suggested that there was a different type of big end used, probably at a later date, but I was at the time unable to learn any more, Now I must stress that My questions regarding Thompson, Peppercorn and Bulleid were regarding the design of the middle big end, rather than fundamental differences between the design and engineering of the locomotive 'Chassis' itself, So may I again ask whether these designers uased a Gresley type big end? and did, in any case their big ends give similar problems?

 

Yes, the basic design remained much the same. Bill Harvey refers to a modified strap with the addition of a web to stiffen the assembly. A bit later, in the early 1950s, K. J. Cook modified the bronze/whitemetal bearing itself and the lubrication which solved the problem. Peter Townsend records that this assembly was dismantled for inspection at 12,000 mile intervals, but they usually ran for 24,000 miles before they needed remetalling. Curiously, the Deutsche Bundesbahn for its 1957 built Class 10 Pacifics inspected these items at 20,000 km which amounts to much the same mileage.