1846.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOUllNAL. 



Z67 



curate solulion of this -.Toiiderful problem was at last obtained, and was 

 immpdljitely comnuiuicated lo Professor Cliallis, and iu the following 

 month to ihe Aslrononier-Rojal. Both Professors were therefore iu pos- 

 session of the perfectly complete solution upwards of a jear ago. 



MOVEABLE JIB CRANES. 



Remarks or the ■utility and defects of the Moveable Jib Crane, according 

 to the construction now generally used in Glasgow, with proposed If/prove- 

 ments to obviate its defects. By William Gale, Glasgow. Head at the 

 Institute of Civil Engineers. (With Engravings, Plate XIX.J 



The author's attention having been recently drawn to an examination of 

 the causes of numerous accidents (many of them attended with fatal conse- 

 quences) during the erection of some of the public buildings in Glasgow 

 and the neighbourhood, he found that one of the most fruitful sources of 

 these accidents was the defective construction and injudicious use of the 

 movea'de jib crane. This crane, it may be remarked, has nearly superseded 

 all others used by builders in Glasgow, and is at present employed at most 

 of the pu''ic buildings in course of erection. It has, however, undergone 

 material alterations since its introduction by Francis Watt upon Mr. Ste- 

 venson's works, during the erection of the Bell Rock Ligbth) use ; but while 

 undergoing these modifications and changes to suit convenience, the princi- 

 ple of construction has undergone a change, which has increased the strains 

 to a very considerable extent. 



As originally constructed, the post or upright a was from 20 feet to 30 feet 

 long, the jib b being of about the same length ; the upright was supported 

 by gye-ropes or chains, similar to the mode usually adopted in quarries ; but 

 at present the past is reduced to 15 feet in height, and the jib is extended 

 to 50 feet in length, whilst the inconvenience or rather impracticability of 

 getting the gyes fastened in many cases, such as in erecting street buihlings 

 or quay walls, where there is a great trafHc, led to the substitution of the 

 two arms c, d, and the framing (Plate XIX. fig. 1) ; and, in order to prevent 

 it upsetting, the framing is loaded with stones, or other heavy materials, or 

 when placed on the upper itories or roof of a building, which is frequently 

 done, the framing is lashed down with chains to some fixed points beneath. 

 It will at once appear evident that this alteration in the construction, by 

 shortening the post or upright a, and lengthening the jib or derrick b, must 

 liave increased the strain on the jib chain to an enormous extent, and in 

 many instances the accidents occurred from the snapping of the chain. 



No one who has seen this crane in operation can call in question its great 

 utility to the builder, on account of the expedition and ease with which 

 heavy blocks can he bedded over a considerable extent of front, without 

 moving the position of the crane after it has been once fixed down ; but the 

 point to be objected to, is the great amount of strain thrown on the jib 

 chain, even with moderate weights attached, when the jib is worked at a 

 great inclination from the perpendicular, and when it is considered that a 

 weight of 4 or a tons is frequently suspended from it, it is certain that if 

 builders were only made sensible of the risk, they would be more scrupulous 

 in hazarding the lives of those under their charge, and fewer accidents 

 would he heard of. 



From what has been stated it will be obvious, that the total strain thrown 

 on the jib chain depends upon various causes. 1st, The length of the jib. 

 2nd. The height of the post. 3rd, The inclination of the jib. 4th, The 

 weight attached. 5th, The proportion of weight due for the jib itself, with 

 its mountings and chains ; and Cth, The friction. Awa'e of the danger of 

 trusting to theory alone, in making an accurate investigation into these 

 sources r.f strain, the author had recourse to a variety of experiments, by 

 model, being at the same time sensible of the fallacious nature of the results 

 deduced therefrom, unless increased size and weight, and consequently 

 increased leverage and friction, were all accurately calculated and allow- 

 ed for. After numerous experiments, however, it was found, that a near 

 approximate agreement took place betwixt the results brought out by 

 tlie model and those deduced from theoretical investigation for the full-sized 

 machine. 



In the theoretical investigation of the question, the weight to be raised 

 being known (which must include the proportion of weight due for the jib, 

 &c., along with that due for friction), it is only necessary to apply the paral- 



lelogram of forces in the usual way, in order to obtain data wher^jy to 

 ascertain the strains; thus if A B represent the total weight, B D = 

 AC gives the strain on the jib chain, while AD represents the strain on the 

 jib. Or if C D' represent the total weight, A C and A D' respectively re- 

 present the strain on the jib chain, and the jib or derrick. 



Keeping in view, that the great utility of this crane, for street erections, 

 consists in its having a long jib and short post, it became an object to im- 

 prove the acknowledged defective part of the machine, the jib chain, not by 

 adding strength to the chain itself, which had already been done by Luild- 

 ers, until it was rendered quite inapplicable for winding round a barrel of 8 

 or 9 inches in diameter, but simply by introducing a pulley between two 

 rods of iron, bolted to the point of the jib *, as shown in Plate XIX,, figs. 1 

 and 2, and having the end of the chain attached to the top of the post or 

 upright a, instead of attaching it to the point of the jib. A mechanical ad- 

 vantage was thus gained, and a much lighter chain than had hitherto been 

 used, could with safety be adopted. The loss of speed was more than com- 

 pensated by the increased ease with which the jib could be worked ; but; 

 speed in this part of the crane was of little importance, as the jib was 

 generally placed at the required angle, or nearly so, before commencing to 

 raise the block. This is the chief improvement, which it is intended to sug- 

 gest wuc/e this description of crane may be found suitable. It is preferable 

 also to increase the diameter of the pulleys from 10 or U inches to 18 or 

 20 inches. The importance of using large pulleys does not, however, seem 

 to be sufficiently appreciated by the builders, otherwise they would not allow 

 their machines to be fitted up v\'ith smaller ones. Lastly, tne friction caused 

 by the angle of the jib chain, after passing the pulley in the post to either 

 side of the barrel of the wheel and axle, may be obviated to a considerable 

 extent, by confining the chain to a barrel of from 20 to 24 inches in length, 

 (fig. 3.) 



The strength of chain necessary for working the jib of this crane will 

 depend on the nature of the work, but for general purposes a chain made 

 of the best iron, |ths inch or -i^ths inch diameter, will be found amply suf- 

 ficient. 



As some builders might prefer using a rope instead of a chain for working 

 the jib, the back \iew of a crane (fig. 3), and the jib head (rig. 4), are 

 shown. Two pulleys are introduced at the end of the iron rods, the other 

 end being bolted to the end of the jib, as in fig. I, and a third pulley is fixed 

 to the top of the post. The rope is fastened to the barrel of the wheel and 

 axle, thence it passes over a pulley fixed on one side of the post, and then 

 over one of the pulleys at the end of the iron rods; it then returns to the 

 pulley at the top of the post, and passing over the other pulley at the rods, 

 returns to that fixed on the other side of the post, and is fastened to the 

 barrel of the wheel and axle. Thus there is one continuous rope, equally 

 strained by means of the pulley at the top of the post. By having also the 

 barrel of the wheel and axle divided into two compartments, as shown in 

 fig. 3 (back view), the one compartment being about two inches larger in 

 diameter than the other, the whole of the pulleys will be set in motion 

 when the jib is working. The rope necessary to work this crane may vary 

 from 1^ to la inch in diameter, according to the weight of the materials 

 used, and it would be preferable to a chain, where this construction is 

 adopted. Fig. 5 is another view of a jib head, showing II e atlachnient of 

 the rope and pulley to the jib head. The scale of Fi;,'. 1 is drawn to a 

 scale of 10 ft. to | of an inch, and the remainder of the figures double the 



