950

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Fig. 1306

Fig. 1307.

Fig. 1308.

the cross, which is of iron, and contained within theboxes, and cause them to revolve; at the same time theother two screws d,d press against the pieces b, b andput the box F and shaft B into motion.

'Where there is only one bearing for every length of ashaft this may be used. 'Die two shafts have at each enda pivot fitted into a coupling piece, bored to receive theends of them, with a neck turned on the outside, whichworks in a bearing. 'Die two shafts are connectedwith the coupling piece by means of keys passed througheach shaft, and there are notches within the coupling-box which receive the ends of the shafts. The shaftsdo not fit tight any where but at the pivots, so thatthere is some freedom of motion allowed, and a slightdeviation from a right line does not produce muchstrain.

Hooke's universal Joint is a beautiful means of uniting shafts which in-cline towards each other. By this motion the rotary may be conveyed,when there is considerable inclination from one shaft to another: whenthis angle is more than 15 degrees of inclination, then bevelled wheelsare used to prevent the friction and great irregularity of motion whichare the consequences of too great an inclination.

This ingenious contrivance consists of two shafts or axes, terminatingin a semicircle, and connected by means of a cross: as the brancheshave motion round their pivots, it is obvious that when one shaft isturned round, a similar motion will be communicated to the other ; suchjoints are used when the inclination of the shaft does not exceed 40 orrather 140°, its complement. When the inclination is between 50° and 90°, the doubleuniversal joint is employed, which has two crosses, with their extremities moving on theirpivots in the semicircles at the ends of the rafts. These joints may be constructed withfour pins, fastened at right angles upon the circumference of ahoop or a solid ball.

Friction Couplings are employed when the connection is merelythe contact of the surface, and is not applicable to heavy ma-chinery ; it is employed much in boring instruments. The shaftA has at the end a circular plate, which fits into a loose box atthe end of the shaft B. A collar of leather bb within the end ofthe hollow part of the shaft B, screwed up by the nuts a, a, causesthe box D to press against this collar, and the friction producedputs the shaft B in motion.

Engagement and Disengagement of Machinery is usually performedby either bands, belts, or chains, the particular object being to com-municate motion as steadily as possible, for if given suddenly, frac-ture of the machinery is too often the consequence. The fast andloose pulley is one of the most simple used, and consists of twopulleys, one fixed on the axle, the other loose, so that the beltwinch conveys the motion ma) be placed upon either of them, thusputting the pulley either in or out of motion. The belt inclining tothat part of the pulley which has the greatest diameter, it is requisiteto have the rim rounded towards the middle, in order to prevent itsslipping off.

The common Clutch or Gland has a pulley revolving upona shaft, to which it can be connected when it is required to givemotion to the machinery. The axle has a gland or cross-pieceattached firmly to it, and upon the wheel, and is so formed as tohe able to slide along the shaft when motion is required; bymoving the handle the wheel is pressed towards the gland, andthe cogs or teeth at the side engage the gland, turning itround and thus put the axle it is connected with into motion.

The pulley P, made to revolve upon the shaft A B, is oftenused for disengaging or re-engaging a machine, moved by abelt or band. DE is a cross-piece or gland attached to theaxle, for the purpose of receiving the teeth of the pulley, P,when it is slided along the shaft for the purpose of engaging it,

Fig. 1310.

Fig.1311.

and which is easily performed by the handle EG. It is necessary that the teeth placed at theside of the pulley should be made of sufficient strength, and that they nicely fit the gland theyare intended to engage, otherwise the motion in some machines would be rendered irregular.