Sliding bearings in steam engines

Construction, 1812-1813

An excellent contemporary description of early 19th c. bearing design is given in "The Cyclopædia; or universal Dictionary of Arts, Sciences and Literature"; Rees, A.; Longman, Hurst, Rees, Orme & Brown, London; 1802-1820. It is not all that easy to find it, however. The most obvious lemma - "Steam engine" - offers no information at all on the subject .... In fact, you have to turn to the following two lemmata: "Machine" and "Mill". These appear in, respectively, Volume 21, Section II (1812); and Volume 23, Section II (1813).

Rees starts with the following observation: The construction of bearings, pivots, gudgeons, or centres, or spindles, as they are indifferently termed, is a most important point; these parts being the principal seats of that friction which is the destruction of all machinery.

Material

Pivots are always made of iron or steel. Both because these substances are better adapted for rubbing surfaces, and that their strength admits the pivot being as small as possible. The bearing, or bed, to receive the gudgeons or pivots, should be of a softer material, as brass or tin. The bearing should be kept well supplied with oil when at work.

The term "iron" is to be interpreted as a generic term, comprising both cast and wrought iron, as well as soft steel (less than 0.4 carbon, non-hardenable). While "steel" is to be interpreted as hardenable steel, with more than 0.4 carbon. The term "brass", finally, is used to denote the copper-tin alloy we nowadays generally call "bronze".

Hardened steel is a most admirable substance for pivots, which have a great strain to bear, and a rapid motion. The bearing or bed may then also be made of the same material, and this is the only instance where two bodies, having friction against each other, can with propriety be made of the same substance. Soft steel or iron pivots can not be worked against bearings of the same substance, because it is found that the friction and abrasion are far greater than when a softer material as brass, tin, hard wood, ivory, horn &tc. is used.

The great difficulty of making hardened steel pivots is the only reason they are not generally used; but there are some cases, in which nothing else can be employed: where steadiness and accuracy of motion are required, and great velocity at the same time. It is necessary that a hardened steel pivot should be fitted accurately with the interior surface of the hardened steel pivot-hole so as to ensure a sufficient access of oil, to prevent the spindle from burning or heating by the friction, when in rapid motion. Such accurately fitted spindles, if getting heated, tend to become fixed in the spindle-hole, due to thermal expansion. The spindle will then rather twist itself off than turn round in the hole. A failure in the supply of oil, even for a moment, can thus be fatal.

Shape

The best form of a gudgeon or pivot, is that of a cylinder, with a flat shoulder, to prevent it from shifting its position endways. This form will bear most fairly and steadily. But it is necessary that the socket, or "brass" which contains the pivot, should be made in two halves, and put together with screws. These so that the halves may be screwed closer as the socket enlarges by wearing. This naturally is only an imperfect method, because the pivot can never fit accurately after wear. When the pivot is given a slightly conical shape, it can always be made to fit a worn hole accurately by pressing it sideways into the pocket. In many cases such sideways drift is not allowable, however. Heavy and slow-moving works, such as the gudgeons of waterwheels, seldom have a top brass screwed down over them, their own weight being sufficient to keep them down. These bearings are always true as they wear away.

Construction

The old kind of bearing called "brasses" is shewn in Fig. 12. A lump of brass a, with a semicircular notch in it, was let into the piece of timber A, which was to support it; and two screw-bolts b b were fixed through the timber, being half received in notches formed in the sides or ends of the lower brass a. The upper brass b, was exactly similar to the lower, and over it a plate of iron d was placed, with two holes through it to receive the two bolts b b and keep them together. The nuts c c upon the tops of the bolts confined the upper brass down, and made all fast and tight.

This kind of brass is not sufficiently strong or steady for all purposes, and, therefore, the bearing shewn in Fig. 13 has taken its place.

In this, a a is a cast iron plate, which is held by two or more bolts r r down upon the timber or framing of the mill. This piece of cast iron has two pieces b b rising up from it, between which a piece of brass l is bedded, and has a semicircular notch in it. Another similar piece of brass is fixed into the cast-iron cap-piece B which is fitted into the space between the two pieces b b and is drawn down by nuts upon the two bolts C C. The brasses are prevented from getting out sideways by small fillets projecting from the middle of them, which are received into proper notches in the cast-iron work. In the same manner, the cap B is fitted between the pieces b b with a tongue or fillet, and groove, so that it cannot deviate sideways, and then the bolts have only to draw the brasses down together.

Sometimes a bearing of this kind is fitted up, so that it is adjustable in its position a little: to adjust two toothed wheels to work accurately with each other; or for other purposes where nicety is required. In this case, an iron plate D is bolted down to the framing and the bearing a a lays upon it, the same bolts r r going through both, and also through the framing beneath. But the holes through which they pass in the piece a a are oblong, to admit the whole bearing being adjusted sideways. This is done by two wedges o o inserted at the end of the piece a a, between the two ends of D which rise up for the purpose, as at n n. The bearing rests upon two wedges at g g and is drawn upon them by the bolts r r. By these two wedges it can be raised up at pleasure, and by the other two, at the ends, it can be adjusted endways; and the bolts r r, when screwed fast, hold all tight.

The best way to make the interior surface of the brasses for a bearing exactly true, is to have them cast solid, that is, the two halves of the brass in one, with a notch which very nearly, but not quite, separates them. In this state, it can be bored or turned out true in a lathe. Then it may be sawn in two halves, and put into its place. To prevent the upper brass from clamping down on the journal when the cap-bolts are tightened, filler sheets to the thickness of the saw cut have to be placed between the two brasses; these can be partially removed to allow for wear.

For larger brasses, the halves are cast separately and fitted together and screwed down and then bored in situ to the exact dimension required. A borer is used, the same as is employed to bore pump barrels.

Light spindles

Very light spindles can be supported on centres. The axis has a small conical hole made in each end of it; and the supports are formed by sharp conical points, received into the holes. At least one of them must be adjustable by a screw, to make it always fit the length of the spindle. It is however usual to make the conical points on the ends of two scews, either of which may then be adjusted. The same thing may be accomplished by making conical points at the ends of the spindle, and forming the holes for its reception in the ends of the two fixed screws. This is the most perfect of all methods, but it is not adapted to bear any great strain, because the screws will get loose. Also, these constructions do show sideways drift with wear.

Thrust bearings

The pivot at the lower end of a vertical shaft usually has to sustain a great weight, as in a horsewheel or gin; or in a windmill. Most properly it is made of a hemispherical figure, and received into a properly shaped cavity. These pivots are also made with a cylindrical bearing with a flat end supported on a flat plate. But it is difficult to keep oil supplied to them, as the great weight of the upright axle presses the oil out from between the acting surfaces, and the flat end burns. To avoid this, some mechanics make a spiral cleft across the flat face of the gudgeon. This getting full of oil, is constantly supplied to the acting surfaces.

Lubrication

Ample lubrication of all these bearings is of vital imoortance. Go visit this page for more information on this topic.