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Garrett sound


thesteambuff

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Pedant mode - neither if it's a Garrett, as they are traction engines. /pedant mode.

 

  A Garratt however, should sound like two engines out of step with each other, although you would tend to find they become synchronised at speed - the WHR ones do anyway, and a friend that has ridden behind big Garratts in South Africa said the same.

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As far as I am aware, Garratt locomotives will sound like two separate engines.

 

Obviously, there is no mechanical linkage between the two engine units, so even if both units had identical driving wheel diameters, they would sound permanently out of sync unless by some fluke they happened to end up at the same crank position.  Even then, it would only take the slightest bit of wheel slip in one and they would be out of sync again, to a greater or lesser extent.

 

In reality, the driving wheel diameters of each pair of Garratt engine units on a locomotive are not usually (or necessarily) the same, both due to wear and as a result of changes/replacement during major services.  Although I cannot lay my hands on it at the moment, somewhere I have an explanation by the doyen of African railways A. E. Durrant, who states (if I remember correctly) that on S.A.R., they permitted up to 1.0 inch difference in diameter, between the driving wheel diameters of the leading and trailing engine sets.  You can do the maths, but inevitably such pairings will exhibit the classic Garratt sound of engine units on the same locomotive going in and out of sync.

 

'New Haven Neil's' observation that they become synchronised at speed is not one I've heard before and I'd be interested to know how that can be possible, even if both wheel sets are the same diameter. Again, I think Durrant does mention somewhere that Garratts had some sort of 'balancing' device in the regulator set-up, to try and obviate wheel-slip on starting (due to the difference in distance from the regulator, of the two engine units) - maybe that has something to do with it?

 

 

HTH

Steve N

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I have traveled behind Union Pacific 3859 which is similar in having two separate engines supplied by one boiler. I also have model SP AC4 which has the same configuration. There is also a short movie on YouTube of the SP AC12 before it was plinthed.

 

In each case the two previous comments are spot on.

 

In theory you could fit two simple two cylinder decodes, one in each end of the Garrett and you would achieve the correct result but having one system do both, as in my AC4, there is a considerable cost saving.

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I have ridden behind and footplated a Zim railways Garratt. Don't know about the 'plumbing arrangements' of the steam circuit but starting was definitely out of synch. and you could feel the usual two cylinder surging from each pair of engines when on the footplate. The driver 'pumped' the regulator on starting from first valve to near closed to limit the potential for a slip. As speed increased the pair of engines did appear to synch up, you hear the difference if there is a slip. (Mud on the rails due to elephants etc walking away from a nearby waterhole, much laughter from the driver.)

 

Going up the Vic falls escarpment (1 in 39) you couldn't hear the beats, it was just a continuous roar; if the reverser scale was to be trusted we were in about 40% cut off and full regulator. What I remember most is the small coal being drawn off the shovels of the two firemen by the draught through the firehole. That and hanging outside the cab as much as possible, to escape the furnace heat within...

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Have now found one of the articles I referenced (the one regarding Garratt synchronisation) -  turned out (perhaps not unsurprisingly) to be in A.E. Durrant's 'Garratt Locomotives of the World' (David & Charles; 1981; ISBN 0-7153-7641-1).

 

On page 28, Durrant launches into a lengthy piece, intended to '...explode a myth about Garratts ....that the two sets of motion on a Garratt allegedly get 'in step' once the locomotive is under-way.'  Those interested in the detail can seek out the book, as I have no intention of reproducing the entire piece here, but the key points confirm my recollections posted earlier and add another I had forgotten.
 

The bit I had forgotten, is that Durrant postulates that the 'myth' (of exhaust synchronisation) probably harks back to the early Garratts, which had '... Z-shaped steam ports and inadequate, short lap, valves'.  This resulted in the muffled exhaust from the rear unit becoming lost in the very long exhaust pipe (on its way to the blastpipe/chimney), with only the exhaust from the front unit being heard distinctly.  He rightly points out in conclusion, that there are recordings available '... clearly demonstrating the fact that Garratts do not, in fact, tend to synchronise.'

 

Just as a matter of interest; that allowable difference between the average wheel diameter of one engine unit and another (on a Garratt), apparrently comes from South African Railways code of practice clause 6.3.1.3 (i.e. 1.0 inches or 25.4 mm).  By way of contrast, Clause 6.2.1 of the same code, states that the difference permitted in diameter between any two wheels, in a set of coupled wheels, is not to exceed 0.031 inches (0.79 mm or 1/32 inch).
 

 

HTH

Steve N

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