THE MECHANICAL HANDLING OF MATERIALS 309
measured from centre to centre of the terminal sprockets. It is the common practice to construct chains for these purposes from malleable-iron or steel links; and, as af ter a certain period of working the metal takes up a permanent stretch, it is usually necessary to provide some form of automatic tightening gear. With the simple elevator it will generally be found sufficient if the bottom sprocket is fitted with adjust-able bearings, so that any extension may be periodically taken up. For elevators of larger capacity a single chain is insufficient, and multiple strands have to be resorted to.
An objection to these is that the different strands (usually two) will stretch unevenly, thus throwing the buckets out of alignment with one anotlier. This particularly applies to the malleable-iron type ; and in
long lengths of conveying machinery it is usual to provide some form of adjustment for correcting the défect, distinct from th.at employed for automatically taking up any slack. The device for tightening each. chain which is employed in many con-veyors is shown in Fig. 225, whilst Fig. 226 illustrates the means introduced for taking up the total increase of length of the chain. In modern elevators for heavy
i'lG. 226. - COMPENSATING DEVICE FOR
taking up Slack in Draw-bab Con-VEYORS AND ÜRTVING CHAINS OR ROPES.
purposes wrought-iron links connected togetlier by turned bolts passing through machine-bored holes are almost exclusively employed.
On gasworks, the principal materials requiring transport from one point to an-other are coal, coke (eitlier hot or cold), clinkers, and in some cases oxide of iron, lime, and such-like materials. It is cliiefly, h.ow-ever, materials of moderately high density which have to be considered. This faet is of
importance in that it has an important effeet upon the angle of slope of the elevators. Machinery dealing with vegetable matters of low specific gravity can be driven at very much higher speeds than that made use of for such substances as coal. Elevators for coal, moreover, sliould be inclined so that part of the load is borne by the rubbing strips attached to the guide supports. The whole weight is not then hanging from the driving sprocket, as is the case with. the vertical elevator. For heavy materials the angle of slope (with the horizontal) should lie between 45 and 60 degrees, alth.ou.gh many gasworks