JOINTS, in engineering, may be classed either (a) according to their material, as in stone or brick, wood or metal; or (b) according to their object, to prevent leakage of air, steam or water, or to transmit force, which may be thrust, pull or shear ; or (c) according as they are stationary or moving ("working" in technical language). Many joints, like those of ship-plates and boiler-plates, have simultaneously to fulfil both objects mentioned under (b).
All stone joints of any consequence are stationary. It being uneconomical to dress the surface of the stones so as to be flat accurately, a layer of mortar or other cementing material is laid between them. If the ingredients of the mortar are chosen so that when hard it has about the same coefficient of compressibility as the stone or brick, the pressure will be nearly uniformly dis tributed. The cement also adheres to the stone or brick, and allows a certain amount of tension to be borne by the joint. It likewise acts to prevent the stones from slipping one on the other. The joints are made partially impervious to air or water by "pointing" their outer edges with a superior quality of cement.
Wood joints are also nearly all stationary. They are made par tially fluid-tight by "grooving and tenoning," and by "caulking" with oakum or similar material. The wood swells when it is saturated with water, the edges of the joints press closer together, and the joints become tighter. Relatively to its strength, wood is a better material than iron for transmitting a thrust past a joint, for when a heavy pressure comes on the joint all the small irregularities of the surfaces in contact are crushed out and there results an approximately uniform distribution of the pressure over the whole area (i.e., if there be no bending forces). To attain this result the abutting surfaces should be well fitted together, and the bolts binding the pieces together should be arranged so that they will not interfere with the timber surfaces coming into this close contact. Owing to its weak shearing strength parallel to the fibre, timber is peculiarly unfitted for tension joints. If the pieces exerting the pull are simply bolted together with wooden or iron bolts, the stresses become intensely localized in the im mediate neighbourhood of the bolts. A tolerably strong timber tension joint can, however, be made by means of iron plates covering the joint and bolted together through the wood. These plates should have their surfaces which lie against the wood ribbed in a direction transverse to the pull. The bolts should fit their holes slackly, and should be well tightened up to make the ribs sink into the surface of the timber. There will then be very little
localized shearing stress brought upon the interior portions of the wood.
Iron and the other commonly used metals possess in variously high degrees the qualities desirable in substances out of which joints are to be made. The joint ends of metal pieces can be fashioned to any desired form and size. Metallic surfaces can be cut smoothly and evenly to ensure the closest contact over their whole areas of contact. This is of the highest importance, espe cially in joints designed to transmit a force or an electric current. Wrought-iron and mild steel are especially suitable for joints where there is not continuous rapid motion. Where such motion occurs, a "bush" or layer of brass is inserted. The joint then possesses the good frictional qualities of brass. Leakage past moving metal joints can be prevented by cutting the surfaces very accurately to fit each other. Steam-engine slide valves and their seats, and piston "packing-rings" and the cylinders they work to and fro in, may be cited as examples. A subsidiary com pressible "packing" is sometimes employed in the "stuffing boxes" which prevent the escape of steam from cylinders through the piston-rod hole of steam engines. Fixed metal joints are made fluid tight—(a) by caulking a riveted joint, i.e., by hammering in the edge of the metal with a square-edged chisel; (b) by the in sertion between the surfaces of a layer of various cements; (c) by the insertion of a layer of soft solid substance called "packing" or "insertion." Apart from cemented and glued joints, most joints are formed by cutting one or more holes in the ends of the pieces to be joined, and inserting pins in these holes. The word "pin" is technically restricted to mean a cylindrical pin in a movable joint. The word "bolt" is used when the cylindrical pin is screwed up tight with a nut so as to be immovable. When the pin is not screwed, but is fastened by being beaten down on either end, it is called a "rivet." The pin is sometimes rectangular in section, and tapered or parallel lengthwise. "Gibs" and "cottars" are examples of the latter. Fixed joints are seldom subject to simple compression in the direction of their length, though they are frequently subject to simple tension in that direction. A good example is the joint between a steam cylinder and its cover, where the bolts have to resist the whole thrust of the steam and at the same time to keep the joint steam-tight.