The bearings in which moving parts of machinery are supported have to withstand many different kinds of stress. The utmost care, therefore, has to be taken in the manufacture of these vital components
BATTERY OF PRESSES in which are formed the cages that house the balls or rollers in the rings of a bearing. The cages are pressed from strips of steel plate with dies of a complicated pattern. Cages sometimes require four or five pressing operations before they are made.
NOTHING has contributed so much to the reduction of work, wear and tear in all classes of machinery, as well as in almost every vehicle used on the roads, as the use of ball bearings and roller bearings. Their application is almost universal in these fields.
The types of bearings and the particular uses for which they are designed are numerous. Their uses are more or less confined to withstanding either radial loads, such as in a plain line shaft or bearing not subject to any end pressure, or axial loads — those in the direction of the axis or shaft. In many instances the bearings have to take a load which is a combination of these two conditions.
The principal classes of bearings include single-row bearings with flat tracks for light loads; single-row bearings with deep-grooved tracks, which will take heavier loads and also carry a certain amount of end thrust; doublerow self-alining bearings which automatically adjust themselves to any mis-alinement of the shaft; cylindrical roller bearings for heavy radial loads; taper roller bearings to carry heavy radial and thrust loads ; and thrust bearings for axial loads alone.
There are three distinct stages in the manufacture of ball and roller bearings. The stages correspond to the manufacture of the three main component parts forming a complete bearing, apart from the housing in which it is eventually to be fitted. These components are the rolling elements, the balls and rollers; the rings, outer and inner, which carry the races on which the balls or rollers run ; and the cages, which serve only to keep the rolling elements in place and to prevent them from crowding against one another.
The raw material from which balls and rollers of the most common smaller sizes are made is a special quality of chrome steel which is “through” hardened. This comes into the factory in large coils of wire of various diameters. One of these coils is placed on a rotary stand from which one end of the wire is fed into a machine. Short pieces are cut off to the exact length required for the size of ball or type of roller which is to be made.
After the cutting-off operation, the machine performs another function by which two hard-steel hemispherical dies squeeze the ball to a rough spherical shape, leaving a thin “beard” or fin on it, which has been squeezed out into the small space left between the dies when they are pressed hard up on the ball. The next operation is to remove the beard and any slight roughnesses. The machine for this purpose is loaded with numbers of these spheres in the making and gives them a severe treatment. As they are yet relatively soft, this is fairly easy. Now comes the important function of hardening the balls, for when they have to run in a bearing the pressure is distributed, not over a large surface as in a long cylindrical bearing, but on a number of what are virtually small points. The gas-fired hardening furnace consists of an inner and an outer casing, between which the exact temperature required is maintained and automatically controlled by a thermostat. The balls are placed in the inner casing. The furnace is made to revolve in a horizontal position. The balls work their way slowly along spiral grooves inside until the correct temperature is attained.
The balls are then discharged automatically into an underground quenching tank of salt water, which at once hardens them. To leave them in this state of hardness would make them too brittle; so they are next taken from the cold bath to be warmed up again, at a certain temperature, for tempering.
The grinding machine, for which the balls are now ready, operates on some 10,000 balls at a time and treats them for several hours They are ground round and round between two flat disks. One of the disks has a surface of, or is made of, “Carborundum”, which is a powerful abrasive substance similar to emery powder but harder. The other disk is made of cast iron. The continual grinding eventually wears perfectly concentric grooves in the “Carborundum” wheel, made to a true radial form in section, thereby contributing to still further accuracy of shape in the product.
The grinding is now all but complete, leaving only two or three stages of polishing. Each successive stage gives a finish finer than the previous one. The first of these polishing methods uses a sort of iron pot, into which a large number of balls is placed, with a fine abrasive fluid. The pot is slowly rotated on its axis at an angle of about 45 degrees, and as it turns the contents adhere to the sides of the pot enough to be carried part of the way round it. Gravity causes the mass continually to slide down the rising side of the pot and the balls get turned over and over in this manner. The balls are then turned out and pass on to another rotary drum, containing sawdust, which has a cleaning effect, removing all the abrasive material and leaving the balls with a fair polish on them.
From here they are put into small oaken barrels which are rotated at fairly high speed. With the balls are mixed some cuttings of chamois or soft leather which wipe the balls as they are spun round in the barrel. When the balls have been long enough in these barrels they are turned out with the pieces of leather on to a sieve. The leather is removed and used over and over again. The balls are now in their finished state, burnished and sparkling brightly.
THE INNER RACE of a huge roller bearing set up on the workhead of an enormous grinding machine used for finishing the components of large bearings. The workhead is mounted on a swivelling base so that it can be rotated in a complete circle. The faceplate, on which the race is mounted, revolves, as does the grinding wheel which is brought against the race.
Although at every stage of the various processes through which the material has passed a careful inspection is made by the regular inspectors and by the operators themselves, for which purpose hundreds of delicate measuring instruments are provided, the balls now undergo a strict ocular examination and grading process. The room in which this is carried out has a subdued fight throughout, but each examiner has a slightly shaded electric light over her work. In front of her are placed about a gross of balls lying in rows on a flat board. With one hand she slips a piece of white cardboard under the balls, giving them a slight rotary movement so that every spot of the surface is exposed in turn.
The balls in this light appear to be of pure white enamel, resembling pearls with no lustre. This makes it easier to detect the tiniest flaws, especially those known as “pinholes”. Only a highly trained eye can see such minute defects. With her other hand the examiner holds a magnetic needle with which she can pick up any defective ball and put it aside.
A more recent apparatus carries a large magnifying lens so that the chances of missing the minutest flaw are still further reduced. The balls are fed into this appliance in an ingenious manner. They pass over a plate with 144 holes in it, each hole being just large enough to hold the ball in position. The plate is made to slide through a slot. As no more balls can go through the aperture with the plate than those in the pockets, the plate takes this exact number and delivers them on to a rotating board in groups of a gross each. This board passes at a regular speed in front of the examiner, the passed balls being delivered as they are dealt with and the rejects removed. Another ingenious and interesting machine is that for grading the balls, and another on similar lines for grading rollers, either parallel or tapered. Such balls are guaranteed to an accuracy of one ten-thousandth of an inch and the grading has therefore to be extremely accurate. The grading machine will automatically detect a difference of one twenty-five-thousandth of an inch. The hopper of this machine is provided with an automatic feeding mechanism. This consists of a circular disk round whose outer edge is a single row of holes large enough to hold the size of ball dealt with.
Grading Machine
The disk is revolved slowly and in doing so carries the circular row of balls round to the top of the feeder, where they are discharged one by one on to the gauge. This gauge consists of two steel “knife-edge” plates which are fixed slightly wider apart at the bottom end of the slope than at the top. This space is divided along its length into five or six sections by narrow dividing plates across the slot.
As each ball is fed in at the top of the slope it rolls down slowly and drops into the first section of the slot which is just wide enough to pass it through. The larger balls get farther down as the slots increase in width. Each section is connected by a tube to a drawer in the table cabinet below the machine, so that each selected grade of ball travels into its own drawer. Oversize balls pass right down the slope, as they will not get through the slot, and are delivered into a special pipe at the end which diverts them to the reject section.
The roller grading machine is similar in principle, but instead of passing downwards through a slot, the rollers are fed on to a pusher plate. This plate presents the rollers, now standing on end, to corresponding gates or spaces formed by steel pegs placed at the required distances apart. The pusher plate, which holds several rollers, is arranged to move a step farther down each time it presents the batch to the gates. If a roller will not pass through at the first attempt it may do so the next, or next again, and so on, until it finds its proper home.
ONE OF THE LARGEST ROLLER BEARINGS IN THE WORLD has a bore of 2 ft. 6 in. and an outside diameter of 4 ft. 3 in. Its weight is 3¼ tons, and it has a capacity at normal speed of 6,070,030 lb. A pair of smaller roller bearings is also shown. Note the tiny bearing which has been placed on the upper edge of the huge bearing in the centre.
With tapered rollers it would not do for them to be standing sometimes with the smaller end lip and at other times with the bigger end on top. An ingenious device in the feeding arrangement tips the rollers right way up as they pass along. They are then always
presented to the gates in the one way and are gauged at a constant distance from the larger end. No chances are taken in the degree of taper. Every tapered roller passes a test for this particular measurement.
The examiner has a small plate of hardened steel, with a tapered slot cut out of it. The taper roller is tried into this slot in front of an electric light. If it fits the taper gauge correctly the light will not show through, but if it is not ground correctly the light will show through along just so much of the tapered edge as is out of truth.
Alongside this examiner is another who inspects the ends of the rollers, whether of the parallel or of the tapered type. A crack of microscopic dimensions, caused, say, by a flaw in the core of the steel rod from which the roller is made, would be more likely to show itself at the clean ground end than on the surface of the cylindrical portion. A tool, similar in shape to an oversize hand mirror, has a gross of holes or pockets, evenly distributed over its surface, and a cover plate is provided for either side of it. With the bottom cover plate in position, and the top one off, the examiner inspects the ends of the rollers for defects.
Having completed the inspection of that batch of ends and removed any defective ones, she slides the top cover plate on, turns the implement upside down, removes the other cover plate and repeats her search at the opposite end of the rollers.
This completes the rolling element of the ball or roller bearing. The rollers differ from the balls in their manufacture, apart from the inspection and grading, only in the methods of grinding.
Machine grinding of cylindrical objects of various lengths may be done by either of two common methods. The piece to be ground may be fixed between two “centres”, as in a lathe, and rotated on those centres while being ground, or it may be ground in what is called a “centreless” grinder. In this event the work to be operated on is not held by centres but is placed in a V groove on the grinding machine, and is free to rotate in that groove. The grinding wheel, which is made of “Carborundum” or other suitable abrasive material, revolves at a high speed and is fed by automatically controlled mechanism. This brings it down closer and closer on the surface of the object until the required diameter is reached. Exactly at that moment the feeding mechanism automatically stops the feed. The grinding is done under a constant stream of water to keep the surfaces cool. The rollers, after the grinding processes have been completed in centreless grinding machines, can be mixed with the balls when passing through the rotating drums and polishing barrels. They are separated from the balls by suitable sieves at the various stages.
Electric Tempering Furnace
In every radial bearing is a pair of rings, one forming the inner and the other the outer race. The raw material of the rings may be in the form of a bar or of a tube, or it may be a ring already forged approximately to the finished size. Large rings are made as forgings because it would be expensive to bore out thick disks cut from a bar of great size. If the ring is made from a bar, the bar has to be bored through the centre, turned on the outside and then cut off to the length required for the ring. This work is generally done in a four-spindles automatic lathe, a machine which can operate on four bars at the same time. These operations are, roughly speaking, turning to required diameter, boring the hole and cutting the ring off to the required length, besides one or two minor trimming operations, such as bevelling the edges.
THE RACE of a large bearing being prepared on the grinding machine. The outer race of a bearing is fixed to the stationary housing when assembled.
This part of the process may also be done on a chucking lathe, or else on a capstan lathe, but these machines deal with only one ring at a time. The capstan lathe is so called because the tool post, which carries a number of cutting tools of various shapes, is turned round in the way a ship’s capstan is worked as each tool is presented to the job. The rings, having been machined by one of these methods, are now put into a large gas-fired heating furnace. They are placed in a wide single layer on to a travelling conveyer which automatically takes them slowly through the red-hot chamber. By lifting the end door it is possible to see the regular gradation of colour of the rings from a dull steel colour to a glowing red at the far end where they are about to emerge. The temperature is controlled by thermostats and continuously recorded on automatic instruments. The gas pressure is also automatically regulated.
As the conveyer, with its load, approaches the exit the rate of traverse is hastened at the point of discharge, and the red-hot rings are dropped through a chute into an adjacent quenching bath. This fluid would soon get hot but for an ample provision of cooling coils through which a constant stream of cold water passes. Another conveyer, one end of which is in the quenching tank, removes the rings and carries them up an incline to be tipped over at the end into iron boxes.
The rings are by now coated with a skin of more or less burnt oil, and could not in that condition be put into the tempering furnace, which is electrically heated ; they are therefore first passed through a cleaning machine. The electrical tempering furnace is used also for balls, rollers and rings. The temperature here is again controlled automatically. Thermometers placed alongside give a continuous record on charts of what is happening in the furnace at every minute of the day.
Cages of Extruded Metal
Having left the tempering furnace, the rings are put into, a large casing, where they are subjected to sandblasting, to remove the thin hard scale which has formed during the heating processes.
The first grinding operation is to do the flat surfaces. This is carried out on a vertical spindle grinding machine. The job is placed flat on the machine table and held firmly by a magnetic chuck. The usual form of chuck grips the work by two or more “jaws” which are screwed up tightly against the part to be machined, but for certain jobs a magnetic chuck is better, because it does not tend to mark the piece held and it is more quickly applied and released. All that is necessary is to switch the current on and off as required for gripping and releasing the work. After the rings have been ground on this machine they are demagnetized and, as they have been flooded with water during grinding, they are further cleaned and dried.
The rings then go to a large centreless grinding machine, which is similar in principle to those used for the rollers. A number of rings may be placed in a row at one time. These machines grind the outside diameter only, the bore being done on a different form of grinder. For certain types of bearings the bore is parallel, but in those of the deep-groove type there is a hemispherical groove running round the centre of the bore, in which the balls run. In the parallel, or flat-track type, the track is the central portion. It is on these parts of the surface that the balls run.
THE GRINDING OF RACES for large bearings. The exterior surfaces of the rings are ground by this machine. The rings are fitted on to the faceplate of the grinding machine. The grinding wheel, of “Carborundum ” or some other abrasive material, revolves at high speed. A constant stream of water is fed on the work to keep it cool.
Some bearings have a single row of balls, others have two rows. For taper roller bearings the track has to be ground on outer and inner rings to the correct angle. The rings are in pairs, and the ball track is the inside of the outer ring but the outside of the inner one. Where a circular-shaped groove track is used the grinding is not done by a grinding wheel made just to the shape of the groove, because if it were done that way an accurate form would not always be ensured. The work is better done by a “generating” machine.
Although the rings, in the same way as the balls and rollers, have been inspected and gauged many times during their several operations, they now undergo an examination in an accurate set of gauges. These show not only whether the rings are correct to size but also if they have their outer and inner diameters truly concentric. The points of the gauges used in this tool are set with small diamonds so as to exclude all possibility of wear.
The rings are now ready for assembling. A pair of inner and outer rings are laid on the bench, being taken from a number of finely graded sizes on pegs alongside. A few balls are placed in the annular space between the rings and spun round the circle by hand to test the degree of freedom of running. Much greater selectivity is thus obtained, because of the extremely delicate sense of “feel” that a trained hand can develop.
The third element of the bearing is the cage. Its function is to retain the balls or the rollers in their correct position in relation to one another when placed in the rings and running on their daily work. For the smaller bearings the cage may be made of a light steel or brass stamping. For some types it may be made from metal which has been “extruded” to the shape it finally takes. Extrusion is a forming operation somewhat similar to what happens when tooth paste is squeezed out of its container tube. It comes out in the shape of the hole through which it has been squeezed.
The Final Scrutiny
When cages are made in two pieces they have to be riveted or otherwise secured together. An ingenious machine is used for inserting the rivets in the cage before the operation of closing the rivets, which may be done in another machine. The two halves of the cage are placed together, one on top of the other, in a holder. A tray holds a large number of rivets and these are fed out of the tray on to a tiny carrier bar, along which they are passed and dropped into the holes in the cage. Then a little plunger comes down and pushes them home ready to be closed in the appropriate press. The closing of the rivet may be effected by different methods according to its size. If the rivet is small the head is formed by a fast rotating tool pressing in the end of the rivet. In larger examples the end of the rivet is heated to a red heat by an electric current and then closed by a die.
In some of the smaller types of cages there is enough spring in the cage to push the balls into their pockets. This feature also serves the purpose of preventing them from dropping out when being handled, and yet leaves the balls quite free when in place. In others the balls or rollers are placed in the pockets, and the cover ring is riveted up afterwards.
The insertion of the completed cage, with its rolling elements, into the pair of rings completes the assembly. The complete bearing unit is finally given the strictest scrutiny by trained inspectors. It is then passed through a bath of heated mineral grease which covers every part of the bearing, inside and out, with a protective coating. From there it is lifted out by hooks and placed on the packing table, carefully packed in a box with the appropriate label, and placed on to a travelling conveyer to the stores ready for dispatch to its destination in any part of the world.
GRADING MACHINES automatically sort the finished balls of ball bearings into sizes distinguished from one another by only one twenty-five-thousandth of an inch. Balls roll down from the hoppers until they reach a slot which is just wide enough for them to fall through into the appropriate drawer.
