Straps are used for connecting the pivot-ends of crossheads with their respective side-rods; also for connecting crank-pins with connecting-rods belonging to pumps, steam-engines, and several other classes of machines.

The simplest class of straps, and the easiest to make, are those having arms of equal thickness. A pair of straps of this shape are shown in Fig. 972. Straps are of all sizes, being used for machines of all sizes; and the metal of which they are made is forged iron or steel; the preferable mode of forging consisting in bending a straight bar, as stated on page 33.

Straps of ordinary shapes are denoted by Figs. 973 and 974. These have arms which are thicker at the ends than at the bent portions, to provide strength for the key-way portions without causing the other parts to be too heavy. The strap shown by Fig. 973 is one with comparative thin arms, being suitable for a pin or pivot whose bearing is of great length, the strap-brasses being also of great length, thus causing the strap to be of considerable thickness.

To ensure a proper fitting for a couple of strap-brasses, it is necessary to first accurately shape the strap. The gap or opening of any strap may be either parallel or slightly taper. A taper form is preferable ; and the smallest part of the gap should be the bottom. But when it happens that the bottom is the largest, the brasses will require to be forcibly driven along the length of the tight or small part, every time they are put into or removed from the strap, unless they are so much reduced as to fit the strap very loosely when in their places. The small amount of taper is also suitable for an easy fitting of the strap to the square end or boss of the connecting-rod, or other rod, to which the strap belongs. The strap should also be rather taper in its thickness; but in this respect it should be smallest or thinnest at the entrance of the gap or extremities of the arms.

Large straps are sometimes made of blocks of steel or iron in which gaps are formed by drilling and slotting. This mode avoids a tedious bending of thick heavy pieces of metal; and the forging consists in well closing the particles of the lumps and trimming them with chiselling, to produce the curved form for the ends which will be the outer portions of the straps when made. A strap-lump of this class is shown by Fig. 979.

The first treatment of straps after forging consists in shaping their gaps; the outer surfaces not being shaped until the gaps are finished, or at least finished excepting filing. The gaps are formed by drilling or lathe-boring, and by planing or slotting. Those straps that possess solid lumps which are to be cut out, require a sort of preliminary planing of their two broad sides to make them parallel with each other and fit for lining. Two flat surfaces are thus formed on which the intended shape and dimensions for the strap can be scribed. The lumps are next drilled or bored, to accurately form the desired half-round bottoms of the gaps. This process makes a round hole at the bottom of the intended gap, and causes the strap lump to resemble Fig. 980. A row of holes are next drilled along each arm of the strap, as denoted in the Figure, extending to the already finished circular hole. The piece is now ready to have the superfluous middle removed by a grooving-tool of a planing-machine or a slotting-machine, according to circumstances. When the strap happens to be small enough, chiselling is adopted for separating the middle piece instead of employing a machine.

The gap of a strap can be almost finished with planing, if proper care is exercised at the fixings, and the measurements are properly conducted. The strap should have both sides of its gap finished while it remains in one position on the table ; this will cause the gap to be parallel, and the cutting tools can be easily adjusted to the already finished half-round surface at the bottom of the gap. After a gap has been planed parallel, the small amount of enlargement at the mouth to taper it as directed, can be easily performed with filing.

After the gap is completed the entire outer surface should be smoothly reduced to the desired form, and made parallel with the finished gap-surface. Each strap is to be next fitted to its rod or bar by reducing the rod's end to the exact width required. For an easy fitting of a large rod it is convenient to support it on vee-blocks on a planing-machine; and after scribing the width of the strap's gap upon the rod's end, the metal is planed off with regard to the lines, and by using inside callipers and outside ones near the conclusion of the planing. The strap is next tried upon the square end while the rod yet remains fastened as when being planed; consequently, the strap can be placed on and taken off several times without removing the rod from the machine. By this mode, a large rod's end can be accurately reduced until the strap will slide a short distance upon the end, and a very little subsequent filing will suffice to complete the fitting.

As soon as the strap is fitted to its rod, the keyway can be made, and the keys partly fitted thereto. During this operation the strap is kept in its proper place on the rod by means of a packing piece which is fixed between the surface at the bottom of the gap and the flat extremity of the square boss. The length of this piece is about the same as that of the brasses; consequently, while in position it is fixed by the act of tightening the key. A strap keyed to its rod by means of a packing-piece is shown by Fig. 974 ; in which condition it is held while the edges are planed.

The next step is to plane both the broad sides of the rod's end and the narrow sides or edges of the strap's arms; this being done while the strap is keyed to its rod and the rod supported on vee-blocks, in a situation similar to that occupied while it was being planed to fit the strap. This planing makes the rectangular boss of the rod parallel, and, consequently, the narrow sides of the strap's arms are also made parallel with each other. It is therefore needful to afterwards taper the strap slightly with filing, when it is apart from its rod or bar. This tapering is that before referred to for making the strap thinner at its extremity, which will allow an easy fitting of the flanges belonging to the brasses.

It is next needful to ascertain the exact intended length of the connecting-rod or bar in progress ; the particular distance here referred to being the length between the centre of the hole in the brasses at one end and the centre of the hole in the brasses at the other end, supposing that the rod is being fitted with a strap at each end. If a fork-end connecting-rod is in hand, the distance referred to is the length between the centre of the brasses at the strap-end and the centre of gudgeon-hole at the fork-end. For a rod having a strap at each end, the length is indicated as shown in Fig. 975. A radius-gauge is adjusted to the length, and its points can be used for reference, and for scribing the length at two opposite sides, by means of an ender or packing-piece of some kind which is fixed for the purpose. It is also needful to show the length on the narrow sides of each arm. Therefore, after the centres have been scribed, an el-square is used, and its blade is put to the centre while the pedestal is put to the planed outer surface of the strap. When the square is placed, a line is scribed along the blade s edge upon the arms of the strap. This line is seen on the narrow surfaces of the straps denoted by Figs. 976 and 977 ; both sides of the strap are thus marked, and the lines become gauge-lines to be afterwards used when fitting the brasses. These lines may be termed length-marks.