Installment No. 2 for the betterment and sharing and making common knowledge good common-sense engineering practices.
Today's subject will address single-crank compound engines and then materials for live steam boilers of large scales.
There are several possible kinds of single-crank (shared crank) compound steam engine. These are most notably the Tandem compound (referred to as a steeple compound if the engine is of the vertical layout), the Vauclain compound, and the Annular compound.
The Tandem compound employs two or more cylinders of compound ratio in line with eachother's centerlines that share either a common pistonrod which passes through them with the appropriate packing glands, or one or more assemblies of split pistonrods that leave the cylinders at opposing ends which hook up in a mechanically force-balanced fashion to the crosshead(s). Sometimes this is accomplished via a tail-rod possessing its own crosshead at the far end of the engine from the crankshaft. This illustrates the most straightforward example of a double expansion tandem compound engine. modernsteampower.files.wordpre…
One notable example was the Willians, here we have a triple expansion tandem (steeple) compound featuring centralised valves, but still clearly illustrating the Tandem principle. upload.wikimedia.org/wikipedia…
The Annular compound employs, as its name suggests, a cylinder located inside a cylinder. The High pressure is always the inner one, with the outer piston being a ring that bears pressure sealing piston rings on both the outer wall of the high pressure cylinder as well as the inner wall of its own bore. The outer ring piston's area is made intentionally larger and requires two or more piston rods, the piston rods here are hooked to the crosshead along with the centerline pistonrod of the HP cylinder in a balanced way. One of the very notable engines in the world possessing this arrangement is the great spider at Cruquius. kalden.home.xs4all.nl/verm/ima…
Several early compound locomotives of the Central Pacific Railroad also employed this arrangement, as did a few early marine engines such as machines designed by Allaire. It should be noted that the length of the ports required from the valves to the high pressure cylinders of these engines is prohibitively long and therefore these engines only excel in performance at low crankshaft speeds.
The Vauclain compound is a side-by-side compound engine in which the high and low pressure cylinders (or more if there are more than two expansions) are placed parallel to eachother and both hook to either side of a shared crosshead via their piston rods. Many examples of this have been built most notably in early locomotives and in traction engines and steam rollers all through the eras, such as this machine that Bartek happened upon.
The first half of this engineering bulletin is to address the alarming shortcomings and design problems happened upon by vauclain type single-crank compound engines, or multiple-crank engines that use a Vauclain setup to drive one or more of its cranks. Barring the usually shared valve, or separate valves that are conjoined to one valve gear which is either way automatically bad practice, The main issue here is that no matter what sort of engine you are operating or designing, you always want the loading on the crosshead to be exactly centered. The crosshead of any steam engine as well as its wrist pin is one of the most high-stress items on the machine and as such, no matter its type or slide bar arrangement, should always be designed with an effort to have all forces centered on it barring of course the inevitable side-loads imparted to it by the transmission of force to the crank off the force centerline, which is of course dealt with by liberally sized slippers that bear upon the slides.
The issue exhibited by the Vauclain type compound is that of course the high pressure cylinder and low pressure cylinder will inevitably during some working characteristic of the engine, impart different amounts of force to the crosshead. Provisions can be made with careful setting of the valve events or indeed having separate sets of valve gear, but in any conditions especially the changing conditions of an engine that is not stationary, the cylinders will most often not be doing exactly the same amount of work as eachother. Usually the case will be that the high pressure cylinder is imparting more force. In the case of a simpleing valve being opened, the case will shift to the low pressure cylinder imparting drastically more force. Either way, the wrist pin of the crosshead will act like a fulcrum and the crosshead a lever, with a greater force on one side of its fulcrum than the other; this results in a sometimes very extreme transverse-torsional load on the crosshead that will skew its alignment from its slippers and pistonrods and can overtime lead to abnormal wear on the slidebars, slippers, wrist pin bearings, and lead to fatigue at where the pistonrods are inserted into the crosshead which can lead to catastrophic failure.
When designing single-crank compound engines, please be mindful and always keep the imparted forces on centerlines especially in terms of crossheads. Vauclain compound engines are a good mental design exercise but in any size above that of model engineering, can be very troublesome machines.
The second section of this engineering bulletin revolves around boiler materials for model engineering and live steam scale applications.
Obviously and for a myriad of reasons, these being but not limited to; heat transfer, longevity, ductility, lack of structural corrosion and lack of disintegration, Copper is the best material to build any conventional boiler out of and is the proper material for the internal fireboxes or tubes of any full sized fire-tube or water-tube boiler regardless of type or use. Copper is only inappropriate for use on super-high-pressure boilers that run in excess of 400 or 500 PSI that require a material with a tensile strength which copper does not have. That being said, some 1750 PSI monotube steamcar boilers were of copper tubing.
The rule of the day that is widely accepted by all is that brass and bronze is never to be used on boilers or fired pressure vessels of any kind due to firstly a dangerous lack of tensile strength and secondly a tendency to leech out zinc and structurally degrade. There is even animosity in the model engineering community regarding the use of brass valves, plumbing and boiler fittings.
In a case of theoretical and mismanaged construction resulting in so called "proof of concept failures" is all well and good, but real results carrying more weight and not being able to be argued with, I will point out a direct contradiction to the above assumptions and an argument that they are in fact completely untrue. Besides bronze castings being used as the downcomers of several successful model engineering boilers in the past as well as staybolts being made out of it, I happened upon quite a discovery when I observed mister Conrad Milster running one of his model boilers.
When I inquired as to how this lovely little gas fired vertical boiler (which is of quite a large size and volume) was constructed, he greatly surprised me by informing me it was entirely of brass construction including even the fire tubes. He surprised me even further still by informing me that the brass sheet thickness was a scant 1/16th of an inch, because this boiler has an operating pressure of the usual 100 PSI. He also informed me that his horizontal boiler, which is several orders of magnitude larger still, is also entirely constructed of brass excluding only its flanged front and backheads, which are of copper. The thicknesses here are also 1/16th of an inch, and the pressure is even higher.
Needless to say I, who come from a background of being told that any model boiler that runs at 60 PSI and over needs to be built of copper and of thicknesses 1/8" or in the case of pressures exceeding 100 PSI, 3/16", was quite surprised and apprehensive at first. I asked him how old the boilers were and how much operation they had seen; to which he replied the little vertical he had made over 50 years prior and it had seen many hundreds of heat cycles. I was astounded, and I can say that above all theoretical hypothesis and retarded forum animosity, actual results do not lie. 1/16" thick brass sheets in use on a 100 PSI boiler of sizable dimensions for half a century with no material failures cannot be argued with in any way, shape or form. This may in fact be due to a certain quality of the brass, but Conrad insisted he did not use any sort of stringent quality check on the brass when it was new nor did he specially treat it in any way. Therefore I must concede that not only is high quality brass an acceptable boiler building material, but also that sheets of copper or brass far thinner than 3/16" can be used for pressures up to and in excess of 100 PSI if properly stayed.
I hope this benefits the engineering community in clearing up some myths.
That will be all-