PREFACE
"Where do we go from here?" is an expression characteristic of what we of the navy subconsciously are asking ourselves as life unrolls its phase of duties ashore and afloat. Very often we must unceremoniously change from an operating to a designing engineer. Naturally, in such changes we find "nuts to crack." If they are covered by good technical writings we are fortunate; if not, it's a case of dig. That's when we wish some one had blazed the trail with some notes, no matter how meager.
Duty, not long ago, brought me face-to-face with the necessity of determining upon the type of ball bearing to choose for certain high-speed instruments. This was the "nut to crack." From notes made in the investigation of the subject, certain general principles, upon which the theory of ball bearings depends, became apparent, and these are presented in the following paragraphs.
It is understood in a general way that the purpose of the ball bearing is to reduce friction. What else we may expect of it is rather hazy. If the question is asked, "Why are there different types of bearings?", can we answer intelligently? What would be the answer to these questions with reference to ball bearings for a certain machine?
1. Size of bearing required.
2. Type of bearing.
3. Accuracy of bearing.
4. Kind of lubricant.
5. Methods of lubrication.
A little investigation shows that the subject is one that is a profession in itself; consequently, this subject is taken up, not with the idea of writing a treatise on ball bearings, but with the idea of noting certain general principles upon which the theory and practice of ball bearings depend.
DEFINITION-PURPOSE
A ball bearing is a bearing device consisting of hardened balls and ball paths. (See Sketch 1 for names of parts.) This design reduces friction of motion as compared with ordinary surface bearings, because:
1. Reduction of surface of contact.
2. Bearings consist of accurate spheres rolling in accurate paths.
Surface contact is reduced, as the ball has either two or three points of contact with the races instead of large surfaces in contact.
The reduction of friction, as claimed for accurate spheres rolling in accurate paths, may be illustrated by comparing the friction of running a motor-car on a smooth asphalt pavement (smooth path) with same on mud road (no defined path).
The friction coefficient of a well-made annular ball bearing is from 0.001 to 0.0002 of the load referred to the shaft diameter and is independent of speed and load.
The ball bearing, therefore, has the following advantages over plain bearings, viz.:
1. Reduction in friction.
2. More adaptable to high-speed work because of 1.
3. Less vibration because of accuracy.
4. Occupies less space along the shaft.
HISTORY
Ball bearings in modern use date from introduction of the bicycle. This brought in the adjustable cup and cone (three-point contact) type. Under the demands of greater load resistance and reliability the two-point contact type, without adjustability, was evolved for use, carrying loads of from a few pounds to many tons.
SIZE OF BEARINGS
The ball diameter determines the size of the bearing. The load a bearing can carry depends upon the size and number of balls and the speed of rotation. Races must be heavy enough to carry the balls at their designed load without deformation.
As the load is taken up between the races by the balls it can be readily seen from Sketches 1 and 2 that the carrying capacity of the bearing must depend upon the strength of the balls and the number of the balls. This is expressed by formula:
L=Knd2 in which
=Load capacity in pounds.
n= No balls.
d= Ball diameter in ? an inch.
K=Varies with the type of bearing; also with material and speed. The radial load up to 2000 r. p. m. is unaffected by speed. Above that speed the radial load sharply increases.
The strength of a ball depends upon its size (diameter) and is expressed by formula:
The strength of a bearing can be increased by increasing the number of rows of balls.
TYPES OF BEARINGS
Ball bearings are of many types, each of which has its champions to sing its praises. There are certain sound reasons
why there are different design, or types of ball bearings. These may be enumerated as follows:
1. Duties.
2. Method of assembly.
3. Method of ball spacing.
4. Method of alignment.
5. Number of rows of balls.
DUTIES
The following is the division made according to duties, viz.:
(a) Radial bearings. (See Sketch 2.)
(b) Thrust bearings. (See Sketch 3.)
(c) Combined radial and thrust bearings. (See Sketch 4.)
In radial bearings there can be a slight variance in size of balls, depending on the accuracy required.
In thrust bearings, variance 0.001" or above in ball diameter seriously affects the bearing, because the whole load is taken up by the large ball, tending to fracture the races at point of contact. Careful selection of balls is consequently essential in this type of bearing.
METHOD OF ASSEMBLY
The following divisions are made according to method of assembly, viz.:
(a) The open type.
(b) The closed type.
The open type bearings are of several kinds, with the principal differences in refinements of retainers, races, etc. The general divisions of this type are:
(a) The box or square bearings, as shown in Sketch 7.
(b) The cone bearing, as shown in Sketch 5.
(c) The retainer bearings, as typified by the Norma bearing. (See Sketch 6.)
The closed type are of several kinds, the difference being in method of filling balls in races. The general divisions under this type are:
(a) Slotted (interrupted race), and
(b) Non-slotted type (uninterrupted race).
The slotted types (interrupted race) are of two kinds, viz.:
(a) One race slotted. (Sketch 9.)
(b) Both races slotted. (Sketch 8.)
These types may be either full ball or retainer types. This type of bearing cannot take end thrust, as balls tend to ride over slot edges; and this type cannot be disassembled without tending to wear the slots into the ball paths.
The constant for the carrying capacity of bearing with one race slotted is K=5, 2000 r. p. m. in formula L=Knd2.
The constant for the carrying capacity of bearing with both races slotted is K=2.5, 2000 r. p. m. in formula L=Knd2.
The non-slotted (uninterrupted race) types differ in design in the method of filling, and are divided into three principal types:
(a) SKF self-aligning bearing filled by means of retainer.
(b) Fafnir type in which races are tapered so that the balls may be entered by springing the races apart. (Sketch 11.)
(c) Hess-Bright type in which the race diameters are such that six balls can be entered by springing the races, as shown in Sketch 10.
METHODS OF BALL SPACING
There are two methods of ball spacing, the non-retainer type and the retainer type.
The non-retainer type or full ball type is a bearing in which centrifugal force is depended upon for spacing the balls. In this type the race is filled with balls, so that there is a space of about one-fourth (¼) the ball diameter between any two balls when they are forced apart. Centrifugal force tends to keep the ball spaced when the bearing is in operation at high speed; but, due to there being no retainer to cushion balls and reduced vibration, this type is noisier than the retainer type. It is generally considered that this type of bearing is undesirable for high-speed work where great accuracy is necessary, but in actual practice this type is efficient; examples, torpedo gyro, Sperry gyro-compass, Fafnir bearing. It makes a better bearing for slow speeds with great weight, for according to the formula for carrying capacity of a bearing, it will be noted that the greater the number, of balls the greater the carrying capacity of the bearing.
The retainer types differ in the design and construction of the ball retainers.
Construction.—The retainers may be either a built-up retainer or a solid retainer. There is not much choice between the two when comparing well-made samples of each.
The material used in manufacture of retainers does make a great difference in the life of the bearing. Retainers are made of the following materials: Steel, grey iron and bronze. Steel is good for low speeds and low heat. At high speeds, it loses its temper and breaks up, thus ruining the bearings. Not considered good for high-speed work. Grey iron is better than steel for high-speed work, but is not in any way as efficient as bronze for high-speed work. Bronze is considered the best material for retainers. It is less noisy than retainers of other material, has longer life and makes a successful retainer for high-speed work.
Design.—The design differs in the method of spacing the balls. To space the balls, the retainers must touch the balls at some point or points. All points on the surface of the balls traveling about the race have either motion of translation or motion of rotation, or both. The axis of the ball at right angles to the plane of the race (plane of translation) has but one motion, i. e., translation. Therefore a retainer touching only at the axis of rotation of the ball will cause less friction from contact with the ball than a retainer touching at any other point.
The type that spaces balls by touching at points A and B, i. e., points other than axis of rotation, is objectionable, because it causes friction (heat) and tends to shorten life of bearing by wearing retainer, the particles of which, tracked into races by the balls, destroys the concentricity of the bearing.
The type that spaces balls by holding them at axis of rotation, i. e., at points C and D, is, therefore, the most approved type, because there is a minimum of wear and friction.
In general, it is better, if design allows, to use the spaced ball type instead of full ball type; if necessary, increase size of balls and decrease number to within practical limits to perform duty.
The disadvantages of the full ball type are:
1. Increase of friction.
2. Increase of cost (more balls).
3. Increase of noise of operation.
4. Increase of lubrication needed, because there is nothing to retain the oil in contact with balls, as in case of retainer bearing.
METHOD OF ALIGNMENT
There are, in reference to alignment, two types of bearings, viz., the non-aligning and the self-aligning bearings.
The non-aligning bearings are the ordinary types of bearing whose races must be in the same plane to operate without binding.
The self-aligning bearings are the types of bearings which allow certain freedom of movement of the races independent of each other without binding.
There are two kinds of self-aligning bearings: (a) Cone and cup type; and (b) the SKF type. (See Sketch 5 for cone and cup type.)
It will be noted that outer race of SKF bearing is ground on a radius from the center of the bearing and the balls ride on the inner race on this radius so that the planes of the two races do not have to coincide for proper operation. Of these two types the SKF is the better. It does not have to be adjusted for ball clearance when installed and is a successful high-speed bearing. Generally speaking, the cone and cup type of bearing is not considered a good bearing, because of frictional resistance of three-point contact and difficulty in making accurate adjustment of bearing on the shaft.
NUMBER OF ROWS OF BALLS
In order to increase the carrying capacity of bearings without increasing the size of the bearing radially, the multiple race type is adopted. For practical purposes the races are seldom more
than doubled, because of increase of frictional factor and the practical difficulty of manufacturing double races, exactly similar; for over-sized, balls, or races out of parallel, throw whole weight on one side.
Accuracy.—The accuracy of the bearing depends on the accuracy of the balls and races.
Balls.—Balls for ordinary commercial work are accurate to
Grade "A" 0.0025” in diameter.
Grade "B" 0.001” to 0.002" in diameter.
High duty or special 0.0001" to 0.00005" in diameter.
i. e., the difference between the greatest diameter and the least diameter.
Races.—The accuracy of races or completed ball bearings for ordinary commercial work are from 00.0015" to 0.00015" eccentricity.
ACCURACY OF BEARINGS
By tolerance in bearings we mean the measured variations allowed in bearings. The dimensions are as shown in Sketch 15.
The tolerances in these measurements would be expressed as plus or minus so many thousandths or hundredths of an inch or millimeter, as the case maybe.
These measurements can be made accurately by plug gages and snap gages.
End Play.—By rigidly securing the outer race the movement of the inner race in line with the bore can be measured. Thetolerance for end play must depend upon duty to be performed. For instance, the bearings of a gyroscope should not have any end play, as end play would change the horizontal balance, while end play of a few thousandths of an inch would not make any difference in a generator.
Eccentricity.—By placing a bearing in a measuring machine, such as is shown in Sketch 16, the total movement of the shaft caused by the inequality of the races and balls of the bearing being tested will be indicated by the movement of the needle of the measuring instrument. The total movement of the needle during a complete revolution of the shaft is the eccentricity measured in thousandths or ten thousandths, as the case may be.
There are bearings of all degrees of tolerances on the market from poorest grade bearing, such as is used in baby carriage wheels, to the highly specialized bearing, such as is used in gyroscopic wheels. The degree of perfection is also an indication of the price, so that it behoves the designer to get the proper bearing for the work; i. e., be satisfied when he gets a bearing that runs smoothly and does not wear out quickly.
In determining the tolerances of ball bearings for a certain machine, bear in mind the following:
1. Avoid vibration, as it shortens life of bearing.
2. Vibration is caused by inaccuracies of races or balls.
3. Vibration is caused at high speed by dynamic unbalance of the load.
4. That perfect bearings will vibrate with load dynamically unbalanced.
5. With inaccurate bearings, the higher the speed the greater the vibration.
6. That the proof of the selection is a smooth running bearing.
7. The life of a bearing depends on the quality of the races, balls and retainer—a breakdown in any one of which will destroy the bearing.
KIND OF LUBRICANT
Oil is the greatest friend, and dirt the greatest enemy of the ball bearing.
The lubrication of a ball bearing is absolutely necessary. A thin film of oil has by practice given the best results. Too much oil causes friction, but the heat is carried off by the oil and immediate bad results do not arise from this source. The main objection to too much oil is that the oil becomes broken down rapidly, thus losing its lubricating qualities, which entails loss of money and labor. Too little oil results in friction and damage to bearing by over heating.
Two general kinds of lubricant are used: Grease and oil. It is necessary that either kind be chemically neutral, to prevent rusting of bearing.
Grease is used where it is necessary to close in bearing with no chance of oiling. This method is used in high-speed grinding machines of 30,000 r. p. m.
Oil is probably the best lubricant. A light grade oil, such as Arctic engine oil, has proven successful in medium and high-speed machines. It has good chemical qualities and can stand temperatures to 400° F. One thing that must always be kept in mind is that the lubricant must be kept clean, as dirt reduces life of bearing.
METHODS OF LUBRICATION
There are many methods of lubrication. With grease in use the bearing is packed in grease. With oil lubrication the lubricant is transferred to the bearing by surface attraction (wick method), by splash or by oil rings.
The wick method is the most efficient, as the oil flow can be accurately determined by the number of strands in the wick and all oil reaching the bearings is filtered. The wick should not touch the moving parts of the bearing, otherwise the filtering feature is destroyed, as lint would be carried into the bearing.
Oiling rings and splash system have two disadvantages: Firstly, they break up the oil, thus shortening its life; and secondly, they do not filter the oil and tend to flood bearings with oil sediment and chips from oil rings.
SUMMARY
In summation, the points to be borne in mind in reference to choice and use of bearings are:
1. A ball bearing has less friction than ordinary bearing.
2. Two-point contact bearing best.
3. For high speed use retainer type bearing.
4. For tow speed, heavy load, use full ball type bearing.
5, Slotted type bearing objectionable where there is end thrust.
6. Double or multiple row bearings not so satisfactory as single row bearing.
7. Use bearings with 300 or 400 per cent safety factor.
8. A well-made bearing of good material will give a long life.
9. The tolerance of bearing must be determined by the amount of vibration allowable.
10. Long life for high-speed bearings depends on load and accuracy of bearing.
11. Radial bearings should not be subjected to more end thrust than 10 per cent of designed load capacity.
12. Use self-aligning bearings where perfect alignment is not essential.
13. Lubrication of ball bearings is necessary.
14. Too much oil is better than too little oil.
15. Wick oiling best.