_Answer._ They are made of flat bars of wrought iron. The rods which connect the cross-heads with the driving-wheels are called _main connecting-rods_, and the rods which connect or couple the driving-wheels together are called _coupling-rods_.[47] Fig. 98 represents a side view and a plan of a main connecting-rod. In the side view the end _B_, and in the plan both ends of the rod are shown in section. It is attached to the wrist-pin at _A_ and to the crank-pin at _B_. Fig. 99 represents similar views of a coupling-rod. To save room in the engraving each of these rods is represented with a part of the middle broken away. The main rods are usually made wider at _G_, next the crank-pin, than at the other end, as it has been found that they are most liable to break at that end. The coupling-rods are now made either straight or somewhat wider in the centre. [47] They are also often called _side_ or _parallel-rods_, but the term _coupling-rods_ is considered the best. [Illustration: _Fig. 98._] [Illustration: _Fig. 99._ Scale ³⁄₄ in. = 1 foot.] QUESTION 178. _How are these rods prevented from getting loose on the pins from the wear of the latter in the inside of the holes of the rods?_ _Answer._ The ends of the rods are provided with what are called _brass-bearings_,[48] or “_brasses_,” _c_, _d_, and _e_, _f_. These brasses are made in pairs, so as to embrace the pins, from each side. They are held by ~⊃~-shaped clamps, _s s s_, called _straps_, which are bolted to the rods. When the brass-bearings become worn, they are taken out of the straps, and a portion of their surfaces of contact with each other is filed away, thus allowing them to come nearer together, and thereby reducing the size of the hole which receives the pin or journal. In order to prevent their being loose in the straps, tapered or wedge-shaped _keys_, _k_, _k_, which bear against the brasses, are fitted in the straps and rods. By driving down these keys the brass bearings are forced together, thus reducing the size of the hole for the journal, and making the rods fit tightly on the pins. A hard steel plate, shown by dark shading in the engraving, is sometimes interposed between the keys and the brasses to prevent the key from indenting the surface of the soft brass. As the keys are very liable to get loose and fall out, they are held either by screws and nuts, _x_, _x_, as shown in the engraving, or by _set-screws_ on the side of the rods. The whole arrangement of straps, keys and brasses is called a _stub-end_. [48] The portion of a shaft pin or spindle subjected to friction is called a _journal_, and the surface which presses or “bears” against it is called a _bearing_. QUESTION 179. _How are the journals of the crank-pins oiled?_