Fig. 2836.
“When the plates are thin, small side pieces are riveted on the sides of the ends to be clamped, as shown in [Fig. 2836]. These stiffen those ends and afford a larger bearing for the securing pins. The connection with the shackles is made by means of steel pins passing through the end holes, and when specimens like [2835] are properly prepared, the direction of the stress on them must be in a line with their axis. [Fig. 2837] shows another form of plate specimen in which the holes are dispensed with, the ends being held in the machine by friction clips, as shown. These specimens are more easily prepared, and from the absence of holes may be made of a very narrow strip of plate.
Fig. 2837.
“In [Fig. 2837] the jaws or forked arms of the shackle are closed to form a rectangular ring, as shown in section in the figure. Two of the interior faces are tapered inwards to the same angle as the back of the wedges or clips a a′, which are perfectly smooth and free to slide upon the inclined or tapered surfaces of the shackles. The faces of the wedges, however, which come in contact with and grip the specimen to be tested, as b, are fluted or grooved, so that the friction of the edges against the specimen is much greater than against the inside surfaces of the shackles. The result of the arrangement is, that when the shackles are pulled, the wedges a a′ are tightened against the specimen with a degree of force proportionate to the load on the specimen, which is prevented from slipping through the clips by the ‘bite’ of their fluted faces. The grooves on the faces of the clips need not be deep—a depth of a little more than 1⁄16, with about the same distance apart, answering well for ordinary loads. With deep grooves and a wider pitch apart, the danger of the specimen breaking in the clips is increased. The inclination of the backs of the wedges a a′ to the faces may be at an angle of 5 or 6 degrees. When the taper is too small, the removal of the halves of the specimen after breaking is sometimes difficult, while on the other hand, when too great, the specimen is apt to slip between the wedges while being tested. The wedges exert a very considerable outward pressure, and the jaws of the shackles must be made strong enough to resist any strain likely, under extreme conditions, to fall on them, otherwise they will speedily become unfit for use. In securing a specimen care must be taken that its axis is in the direct line of strain, and the opposite clips should be driven in equally so that the stress may act fairly upon it. Parallel planed strips of metal, without any enlargement at the ends, may be tested in these friction clips, though, of course, there is a chance of the specimen breaking within them. Turned specimens may also be held by such clips; as also may rough, unturned round and square bars, an advantage when it is desired to immediately ascertain approximately the strength of metal samples.”
Open fires for hand forging purposes are mainly of two classes, those having a side and those with a bottom or vertical blast.
