Replacement OR Repair of Worn Bearings

Replacement OR Repair of Worn Bearings

Replacement OR Repair of Worn Bearings - Machine tool spindles are supported in bearings which wear with the use of the machine. The problem of making good a worn bearing is a common maintenance job. According to the type of bearing the job may be one of extracting the old bearing and fitting a replacement (as with ball and roller bearings), of relining a shell with white metal and boring out to size before refitting, of driving out the worn bush and fitting a replacement, or if the shaft is carried in the machine frame, of enlarging this hole to accommodate a bush with a suitable bore.

Wheel Extractor

At the same time the portion of the shaft which runs in the bearing may require cleaning up on a cylindrical grinder to remove grooves, unevenness and inaccuracy.  Often the major part of a job of this nature is that of stripping down to remove the shaft and bearing.

There may be an outside wheel to remove, and if this fits well to the shaft an extracting tool will be required to draw it off; if it fits well on a taper the extractor plus hammer blows on the end of its screw and some heat on the wheel may be necessary.

When the shaft has been taken out and the bearing bush cleared of surrounding impediments it may be driven out or pulled out. Driving out involves a heavy hammer with a short shaft of suitable diameter to hold against the bush and communicate the hammer blows to it.

A better method which does not involve the risk of bending or fracturing anything with the hammer blows is to draw the bush with a bolt and distance piece as shown at (b). If the screw used has a fine thread, there should be no difficulty in moving the bush and the same method, without the spacing bush, may be used to fit the replacement.

Before the replacement bush is finished in the bore, attention should be given to any renovation necessary to the shaft. The best treatment is a light skim between the centres of a cylindrical grinder. Failing that it may be smoothed up between the centres of a lathe, and, as a last resource, held in the vice and polished with fine emery cloth, measuring and correcting for any errors in roundness or parallelism.

If the bush is plain bronze or cast iron this may now be turned, the outside being made similar to the one extracted and the bore to suit the shaft for a running fit, remembering that when the bush is driven in the casting its bore will close in by an amount about equal to that allowed on the outside diameter for the driving fit.

Removing a Bush

The bush may then be driven or drawn into its hole, and if the shaft is at, all tight it is an easier matter to ease the shaft than the bush since if the former has been reduced slightly in the renovation process a standard reamer will render the bush too large. Any oil feed holes and grooves should, of course, be put in the replacement bush.

If the bearing consists of a bronze shell with a white metal lining the shell may be used again, but it must be relined and rebored to suit the shaft. Before relining, the old metal must be melted out and the inner surface of the bush thoroughly cleaned. The bush must now be supported on a plate with a mandril inside it, the diameter of the mandril being such as to allow metal for boring out (Fig).

After fluxing the bore of the bush and warming it up to a reasonable temperature, the white metal may be poured in from a small ladle. The lining may then be bored out concentric with the outside diameter and the finished bush replaced.

Babbiting by sand moulding 

The sand mold (4) is prepared with the internal sand core (1). Babbit (melting point ≈450ºC) is melted in a coal fired furnace and poured at place (2) between the sand core (1) and the rough (bored) prepared body of the bush (3).

Bimetalling a bush with stellon compound 

The bore of the old bush is roughly threaded (2). The bush body (3) is held in lathe chuck (1) and rotated. Stellon compound (4) is poured through a paper cone (5) from the open end, while the bush is rotating.

The bush is kept rotating (for about an hour) and due to centrifugal force, the stellon compound adheres evenly to the bush walls. When 2-3 mm thick compound wall forms, the bush is removed from the lathe chuck and allowed to set for 12 hours.

When bearings are in two halves with a cap, a certain amount of wear may be taken up by removing metal from the joint face, assembling the shaft, bearings and cup (after smearing the shaft with prussian blue) and scraping the bearing surfaces at the points where the blue is deposited until the shaft is bedding reasonably at all points without play.

The limit to this method of taking up wear is when the joint faces of the cap and main casting come together since metal must not be taken from these surfaces. This necessitates replacement bearings, and since they form a circle with their joint faces in contact they cannot be made as one bush and sawn through, but must be processed as two separate parts held together during the turning and boring operations.

Although it is rare for a bearing of any importance to be formed in a casting without a bush, cases do arise when the fit of a shaft in an unbushed hole necessitates being taken up. This involves boring out the hole to a large size and bushing back to suit the shaft.

The boring is often rather particular since it may be necessary to preserve a centre location; in addition, a good, parallel hole is necessary to ensure a reliable fit for the bush. Depending on the size and shape of the component, the hole might be bored on a horizontal borer, milling machine, lathe, or by some form of boring head attached to the casting.

Arrangement For pouring White Metal to re-line a Bearing Shell

Babbitting By Sand Molding

Bimetalling a Bush with Stellon Compound