THE AUTOMATIC CUT-OFF
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The Indispensability of this Simple and Efficient Device to Gas Users.
nURTNG THE PAST few weeks,' especially upon those occasions
". when 'high gusty winds have 'pre: vaned, several mishaps • with gas' bags have been oecorded.„ In one or two instances, the inflated container has been torn eeinpietery adrift front its lashings to bowl down the strcet, the sport of
Dumas, while in others all but one or two of the lashings have given way, placing the car effectively hors de combrit until the wayward container has been retrieved and re-anchored, Such incidents may be regarded as unavoidable, or be,attributed to krises carelessness or -neglect of the holding-down ropes or lugs whereby the hag isasecured to its tray, but as a matter of fact, Such eventualities are really 'due to lack of knowledge s Scarcely eVer` a thought is given to•the strains brought to bear upon the Isolding-dosvn'coscis, especially against the resistance impesed by a severe head Wind. In many instances 'efforts to nibdue_ this tendency' of the. bag to break away are made by increasing the number of :lashings and lugs,. but -such a practice is merely trifling with the
lade: • .
-When Axing a container of this type to the roof of a vehicle, one or other of the following methods are invariably, indeed, must be., -employed. (1) Either the bag is charged to the full capacity of the mains, so that at last the jsaes refuses, to pass from the latter into the former; owing to the establishment of equilibrium' in pressure between the two, in which eVent, the aopekiare left _sleek on the lop of the vehicle j or; (2). the ropes are adjusted so .thats they tighten before any dismalie, arising from excesspressure of the gas within the bag, can be inflicted upon the container.. _Whichever .practice be' adopted the eventual result is the same—damage to or dostrection,of the bag; Thesiresult of. the 'first procedure reveals itself le the formation. of veins' inthe fabric at. the ends of the . bag., this criss-crossing extending fur -clasping distances ,back along the cylindrical section of the .container. In certain'instances this 'Veining is accompanied by puckeringat the junction between each end and, central sectiOns, as if the fabric were shrinking unevenly. It may he mentioned that where -rubber is submitted to tension and to contact with gas, The -life of the rubber becomes set.i6uhly :iiiipetined,' owing to the dee leterioes action exercised by the eonsti
taenta of the•gase . • :•
If the'second practice be followed, and extreme:care be .eet observed; the lasieri, of the fabric 'will laminate, that is, become disrupted. This action also tends to • criss-caossing, but in this instatice it will become -mostsmanifeat -.around the
lugs. Moreover, as time passes, this veining will Spread to the ends as in the previous instance.
Now, no matter' how careful a driver may be, he faintest possibly keep his eye the whole time upon all the ropes hcilding his bag 'in position. When a containeis irrespective of its cubic capaeit,y, is . charged in six or eight-minutes, as in most cases it must he, in order to render :the use of the 'system a coruntereial success, the time elapsing between a rope beginning to tighten and straining with a pull of several hundred pounds is a mere matter of seconds. At night time the. situation is far worse, inasmuch as in the darkness the 'driver cannot possibly observe the condition of his ropes.
The very heavy' stresses which are imposed upon the bags and the anchoring ropes may be gathered from an examination of the accompanying diagrams, which possibly may Surprise treaders whodhitherto have not giveuthe Matter h moment's thought. We will take the case of a bag charged to a pressure ccinal to.,3 ins; of water, which. is Coinretailer experienced, and tO one of 5 ins., which is the .pressure ruling in the mains of many towns . during certain hours of the evening. For the purposes of our illustration we Will take" the length of the' bag to be 25 ft., • with . ii diameter of 6 ft. 6 ins., and assume the condition, to be one with ropes stack and bag fully charged. Under -these conditions' the circumferential pull on the fa-blie (AB, Fig. 1) will be as follows :—At 3-in, water e-pressiiee 516lb., and 850 lb. at 5-in. water. premise.
In the case of the bag we have selected for our purpose, having a diameter of 6 ft. 6 ins., this will be equivalent to 2,5 lb. per ft. of length throughout the 25 ft. at 3-in, water pressure, and of 44 lb. per ft. at 5-in, water pressure. The pressure exerted per square inch by a column of water 3 ins, high is .108 lb., and .180 lb, by a water column 5 ins. high.
The foregoing result is obtained as follows:— Total pressure on material of hag = area of hag X pressure of gas; which equals — X .108 lb. per eq. in. 4 • .
=5151b. or 25 lb. per it. run. In the same' way 5 ins, pressure of water equals 860 lb. or 44 lb, per ft. run.
When the fabric of 'the bag is subjected to pronounced stresses in both directions, the attendant pulling at the ends, together with the pressure on the material due to radial pressure, precipitates ' eriss-evessing: :Moreover, should. there he atia defects in the seam which connects the end Pieces to the pylindricar section of the hag, the matei•jal is likely to: be stressed heavily at this point. Dealing -with the aerond. condition, namely, the pull Or the ropes, it will be gathered from. the following., calculation that the totalstrees on each Side of the bag at, 3 ins: Of' wateris 1264 lb. The pressure which can be imposed upon the Rigs when the bag is inflated-is the equal pressure of gas upon areas ABCD (Fig. 2).
Taking the bag 25 ft. long by 6 ft. diameter, the area—AISCD—is
ler4 sq. ft., or 23,400 sq. ins. The pressure of gas at 3 ins, water being .108 lb. per -eq: in., the ccenbilfecl pressure on both sides, viz., AB nod CD, is 2527 lb, (23,400 x .108) or 1264 lb. per side 23,400 x .108.
if we assume six lags 2 on either side; the pull on each -lug at the above water pressure is therefore 219.5 lb. Taking 5 ins, of water pressure, we get 23,400 x .180 me 4200 lb., the combined pres,ure. or, when divided by two s preseure. of 2106 lb. on either side. Dividing 'this, latter figure by the number'of logs on theleide, say, six, we get a pull of 350 .-lb. on each lug. It will he found in practice that al though the ropes may be set correctly, yet, ;owing to fluctuations in weather conditions, the tension of the ropes is varying; when dry they stretch, while
in wet weather they. contract. This movement is unequal. ,When, very heavy" stress comes on the ropes and legs, as for instance when k wet, either' the eyelets through .Which the holding-down ropes pees:arc turnout or the bag is ripped. Farthermore it is admitted by rubber experts to be detrimental to'the life' of e gas bag for the rubber to be subjected to any gas :pressure, any latent weakness which may exist being readily affected by that pressure Again, it is clear that accentuated dragging and wear must take Place in-theematerial of .a bag which has been highly charged 'and . which is consequently being subjected to more violent rolling' and bumping on the roof between the, fourwooden or canvas sides constituting the walls of the containing cradle. With the connections generally found in practice to be fitted to gas bags, any undue pressure falls; with the result that although the bag may have to be strained to secure a high pressure, no appreciable working advantage is thereby to be gained. From the foregoing,it will be readily seen that it is obviously essential that the gas flowieg, into a coetitincr of the flexible type: should, be, cut off automatically, When • (a) th bag is just .full and before the fabric tightens as a result of pieseitre, and (b) when the lugs Ur ropes at the moment of the cut-off Valve corning into play should be merely slack, and consequently have no appreciable puflon them. •