Cylinder Positioning

On a 'Conventional' ;locomotive it clearly makes sense to locate the cylinders at the front under the smokebox to minimize the distance to the blast pipe.

On 'Fireless' loco's cylinders are often under the cab as there is no blast pipe & this makes for a more direct steam circuit.

So, why om most Garretts are the cylinders at the outer not inner ends, some German Mallett Tank Loco's have cylinders in the middle. In the same way while it isnt without problems why not place the cylinders on a SAR Class 25 Condensing loco at the rear not the front of the loco. All these would result in shorter steam/exhaust circuits.

 

As the superheater header is at the front the shortest route to the cylinders is to place them at the front also. The distance to the condensing tender on the 25C would actually help in cooling the exhaust steam which has done its work anyway. Or am I misunderstanding the question?
Ray.

 

I've wondered about this as well, and given that K1 (on the WHR) has the cylinders inboard it is not unknown. This was an early example of the type - is there something about the arrangment that causes problems? Articulation of the steam pipes for example? Beyond that I don't know enough to even hazard a guess.

Steve B

 

Not at all, you are spot on with the question & made a helpful point

 

K1 has the cylinders inboard because it is a compound. This reduces the length of the receiver between the high and low pressure bogies. If you look the boiler cradle you'll notice this causes all sorts of design problems. To accommodate the cylinders swinging under the cradle there can't be any metal work under footplate in this location. So it is all above the running plate. This restricts the size of the boiler and also makes the cab quite awkward.

If you compare it with the Darjeeling Garratt they've moved the cylinders outboard which has allowed a much simpler cradle frame with a big fat boiler on top. The ability to fit this huge boiler in is what made Garratt locos so successful.

Tim

 

Thank you, that's helpful.

Steve B

 

If Garratt cylinders had been on the inner end of engine units it would have been nessecary to put the two pivots further apart to get usable weigth distibution.

 

The German articulated locos with cylinders “in the middle” are Saxon Meyer type locos. Mallet locos have cylinders at the front end of each power bogie.

 

I think most of these queries have been covered, but it's also worth noting that on a 25 condenser, the first thing the exhaust does out of the cylinders is drive the draught turbine, which inherently has to be in the vicinity of the smokebox. They had enough trouble with these without going to long articulated driveshafts for a high rpm turbine. In addition, they were built in parallel with the class 25NC, with identical cast steel beds. This fixes the cylinder position to be the same as the conventional version. I believe the original intent was that if the condensers were very successful, the (initially less numerous) non-condensing engines might be converted to create more. In the end the reverse conversion happened, but each original 25NC also had the required exhaust passages for the condensing draught system cast into its frames.

 

I understood it was for weight distribution. Having the cylinders under the smokebox & cab meant that the weight of the loco was mainly in the middle, increasing the axle weight on the nearest wheels, whereas having the cylinders at each end spread the weight over the whole length of the loco so giving a more even axle weight. Also, cabs get hot when the cylinders are underneath.

 

BP tried that layout on the very first Garratt (K1, which survives on the WHR). Can't recall if there was a specific reason why this wasn't repeated.

Image credited to mrweb.co.uk

 

As someone has said before, it's because K1 is a compound, which later Garratts weren't.