ELEVATING MECHANISMS.
There are two general classifications of gun carriages according to the manner in which the laying for elevation is effected. The 3” gun is an example of one type in which the total quadrant angle of departure is laid off as one angle necessitating at each resetting of a range a new resetting of the range bubble. It is evident that this militates against both speed and accuracy in laying for elevation. The other type is illustrated by the American 75 and by the French and British 75-mm gun carriages which have what is called the “independent line of site.” It differs from the other type in that there is placed between the top carriage and the cradle an intermediate carriage or rocker arm pivoting on the gun trunnions at one end, and carrying at the other the support for the elevating device which gives to the cradle and the gun the proper elevation for the range. This intermediate carriage or rocker arm has a toothed edge geared with a pinion fastened to the trail flask, by means of which, gun cradle and elevating device may be moved in elevation without disturbing the relation between itself and the gun, this being done by gears independent of the elevating mechanism. An angle of site may be set off for the intermediate carriage and after the bubble is centered this actuating pinion may be blocked and no further concern be paid to the angle of site. A cannoneer therefore has only to set his range accurately on the index to give the changes in range ordered.
Elevation is accomplished by telescoping screws, by circular racks and pinions, and by worms and arcs. Telescoping screws are good for from 15 to 20 degrees in elevation only, as for greater angles they become rather too large and cumbersome. The elevating arc is attached on the top, on the bottom or on the side of the cradle and, if possible, at its center. When the latter is not possible, two arcs are generally used to prevent torsional strains developing. Top racks are exposed to fire, side circular racks or arcs interfere with the traverse, with the placing of the sights, and with the general handling of the carriage. The bottom of the cradle is perhaps the best location for the rack. As in the traversing mechanism, a train of bevelled gears transmits the power from the hand wheel to the elevating mechanism.
In both the traversing and elevating mechanism, strength, simplicity, power, accessibility, non-interference and absence of lost motion are the features sought. To secure these is one of the most difficult things in gun-carriage design; for, since the traversing and elevating are the last two elements considered, they must, therefore, be the ground for modification and the means of adapting the great main elements—namely, wheels, axle, trail, recoil mechanism and gun—into a unit.