Ball (bearing)
Encyclopedia
Bearing balls are special highly spherical and smooth ball
s, most commonly used in ball bearing
s. The balls come in many different grades, as defined by the American Bearing Manufacturers Association (ABMA), which defines the precision of the balls. They are manufactured in specially designed machines for the job.
In 2008, the United States produced 5,778 million balls.
. The grades range from 2000 to 3, where the smaller the number the higher the precision. Grades are written "GXXXX", i.e. grade 100 would be "G100". The grades are divided into two categories: semi-precision and precision. Grades 100 and greater are semi-precision balls and lower than that are precision balls.
The specification defines three parameters: surface integrity
, size, and sphericity. The surface integrity refers to surface smoothness, hardness, and lack of defects, such as flats, pits, soft spots, and cuts. The surface smoothness is measured in two ways. surface roughness and waviness
.
Size refers to how tight the tolerances are on the size, as measured by two parallel plates in contact with the ball surface. The starting size is the nominal ball diameter, which is the nominal, or theoretical, ball diameter. The ball size is then determined by measuring the ball diameter variation, which is the difference between the largest and smallest diameter measurement. For a given lot there is a lot diameter variation, which is the difference between the mean
diameter of the largest ball and the smallest ball of the lot.
Sphericity, or deviation from spherical form, refers to how much the ball deviates from a true spherical form (out of roundness). This is measured by rotating a ball against a linear transducer with a gauge force of less than 4 gram (0.141095848420448 oz). The resulting polar graph is then circumscribed with the smallest circle possible and the difference between this circumscribed circle and the nominal ball diameter is the variation.
balls start as a wire
. The wire is sheared
to give a pellet with a length approximately the size of the desired ball outer diameter (OD). This pellet is then headed
into a rough spherical shape. Next, the balls are then fed into a machine that de-flashes them. The machine does this by feeding the balls between two heavy cast iron
or hardened steel
plates, called rill plates. One of the plates is held stationary while the other rotates. The top plate has a section an opening to allow balls to enter and exit the rill plates. These plates have fine circumferential grooves that the balls track in. The balls are run through the machine long enough so that each ball passes through multiple of these grooves, which ensures each ball is the same size, even if a particular groove is out of specification. The controllable machine variables are the amount of pressure applied, the speed of the plates, and how long the balls are left in the machine.
During the operation coolant
is pumped between the rill plates because the high pressure between the plates and friction
creates considerable heat. The high pressure applied to the balls also induces cold working, which strengthens the balls.
Sometimes the balls are then run through a soft grinding
process afterward to improve precision. This is done in the same type of machine, but the rill plates are replaced with grinding stone
s.
If the balls are steel they are then heat treated. After heat treatment they are descaled to remove any residue or by-products.
The balls are then hard ground. They are ground in the same type of machine as used before, but either an abrasive is introduced into coolant or the rotating plate is replaced with a very hard fine-grain grinding wheel
. This step can get the balls within ±0.0001 in (0.00254 mm). If the balls need more precision then they are lapped
, again in the same type of machine. However, this time the rill plates are made of a softer material, usually cast iron, less pressure is applied, the plate is rotated slowly. This step is what gives bearing balls their shiny appearance and can bring the balls between grades 10 and 48.
If even more precision is needed then proprietary chemical and mechanical processes are usually used.
The inspection
of bearing balls was one of the case studies in Frederick Winslow Taylor
's classic Principles of Scientific Management.
and zirconium oxide.
, chrome steel
, brass
, aluminium
, tungsten carbide
, platinum
, gold
, titanium
, plastic
. Other less common materials include copper
, monel
, k-monel, lead
, silver
, glass
, and niobium
.
. The building is supported by 267 columns, each of which rests on a steel ball with a diameter of 5 feet (1.5 m). The ball sits in a concave foundation. If an earthquake
occurs, the ground can move up to 20 inch (0.508 m) in any direction, as the columns roll on their bases. This is an effective way to separate the building from the movement of the ground. After the earthquake has ended, the columns are re-centered on their bases by the force of gravity.
Ball
A ball is a round, usually spherical but sometimes ovoid, object with various uses. It is used in ball games, where the play of the game follows the state of the ball as it is hit, kicked or thrown by players. Balls can also be used for simpler activities, such as catch, marbles and juggling...
s, most commonly used in ball bearing
Ball bearing
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit...
s. The balls come in many different grades, as defined by the American Bearing Manufacturers Association (ABMA), which defines the precision of the balls. They are manufactured in specially designed machines for the job.
In 2008, the United States produced 5,778 million balls.
Grade
Bearing balls are manufactured to a specific grade, which defines its geometric tolerancesTolerance (engineering)
Engineering tolerance is the permissible limit or limits of variation in# a physical dimension,# a measured value or physical property of a material, manufactured object, system, or service,# other measured values ....
. The grades range from 2000 to 3, where the smaller the number the higher the precision. Grades are written "GXXXX", i.e. grade 100 would be "G100". The grades are divided into two categories: semi-precision and precision. Grades 100 and greater are semi-precision balls and lower than that are precision balls.
The specification defines three parameters: surface integrity
Surface integrity
Surface integrity is the surface condition of a workpiece after being modified by a manufacturing process. The term was coined by Michael Field and John F. Kahles in 1964.The surface integrity of a workpiece or item changes the material's properties...
, size, and sphericity. The surface integrity refers to surface smoothness, hardness, and lack of defects, such as flats, pits, soft spots, and cuts. The surface smoothness is measured in two ways. surface roughness and waviness
Waviness
Waviness is the measure of the more widely spaced component of surface texture. It is a broader view of roughness because it is more strictly defined as "the irregularities whose spacing is greater than the roughness sampling length"...
.
Size refers to how tight the tolerances are on the size, as measured by two parallel plates in contact with the ball surface. The starting size is the nominal ball diameter, which is the nominal, or theoretical, ball diameter. The ball size is then determined by measuring the ball diameter variation, which is the difference between the largest and smallest diameter measurement. For a given lot there is a lot diameter variation, which is the difference between the mean
Mean
In statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....
diameter of the largest ball and the smallest ball of the lot.
Sphericity, or deviation from spherical form, refers to how much the ball deviates from a true spherical form (out of roundness). This is measured by rotating a ball against a linear transducer with a gauge force of less than 4 gram (0.141095848420448 oz). The resulting polar graph is then circumscribed with the smallest circle possible and the difference between this circumscribed circle and the nominal ball diameter is the variation.
| Grade | Size range [in] | Sphericity [in] | Lot diameter variation [in] | Nominal ball diameter tolerance [in] | Maximum surface roughness (Ra) [μin] |
|---|---|---|---|---|---|
| 3 | 0.006–2 | 0.000003 | 0.000003 | ±0.00003 | 0.5 |
| 5 | 0.006–6 | 0.000005 | 0.000005 | ±0.00005 | 0.8 |
| 10 | 0.006–10 | 0.00001 | 0.00001 | ±0.00005 | 1.0 |
| 25 | 0.006–10 | 0.000025 | 0.000025 | ±0.0001 | 2.0 |
| 50 | 0.006–10 | 0.00005 | 0.00005 | ±0.0002 | 3.0 |
| 100 | 0.006–10 | 0.0001 | 0.0001 | ±0.0005 | 5.0 |
| 200 | 0.006–10 | 0.0002 | 0.0002 | ±0.001 | 8.0 |
| 1000 | 0.006–10 | 0.001 | 0.001 | ±0.005 |
| Grade | Sphericity [mm] | Lot diameter variation [mm] | Nominal ball diameter tolerance [mm] | Maximum surface roughness (Ra) [µm] |
|---|---|---|---|---|
| 3 | 0.00008 | 0.00008 | ±0.0008 | 0.012 |
| 5 | 0.00013 | 0.00013 | ±0.0013 | 0.02 |
| 10 | 0.00025 | 0.00025 | ±0.0013 | 0.025 |
| 25 | 0.0006 | 0.0006 | ±0.0025 | 0.051 |
| 50 | 0.0012 | 0.0012 | ±0.0051 | 0.076 |
| 100 | 0.0025 | 0.0025 | ±0.0381 | 0.127 |
| 200 | 0.005 | 0.005 | ±0.025 | 0.203 |
| 1000 | 0.025 | 0.025 | ±0.127 |
Manufacture
The manufacture of bearing balls depends on the type of material the balls are being made from.Metal
MetalMetal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
balls start as a wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
. The wire is sheared
Shearing (metalworking)
Shearing, also known as die cutting, is a process which cuts stock without the formation of chips or the use of burning or melting. Strictly speaking, if the cutting blades are straight the process is called shearing; if the cutting blades are curved then they are shearing-type operations...
to give a pellet with a length approximately the size of the desired ball outer diameter (OD). This pellet is then headed
Heading (metalworking)
Heading is a metalworking process which incorporates the forging, extruding and upsetting process. It is often performed in the cold state, resulting in cold working...
into a rough spherical shape. Next, the balls are then fed into a machine that de-flashes them. The machine does this by feeding the balls between two heavy cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
or hardened steel
Hardened steel
The term hardened steel is often used for a medium or high carbon steel that has been given the heat treatments of quenching followed by tempering. The quenching results in the formation of metastable martensite, the fraction of which is reduced to the desired amount during tempering. This is the...
plates, called rill plates. One of the plates is held stationary while the other rotates. The top plate has a section an opening to allow balls to enter and exit the rill plates. These plates have fine circumferential grooves that the balls track in. The balls are run through the machine long enough so that each ball passes through multiple of these grooves, which ensures each ball is the same size, even if a particular groove is out of specification. The controllable machine variables are the amount of pressure applied, the speed of the plates, and how long the balls are left in the machine.
During the operation coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
is pumped between the rill plates because the high pressure between the plates and friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
creates considerable heat. The high pressure applied to the balls also induces cold working, which strengthens the balls.
Sometimes the balls are then run through a soft grinding
Grinding (abrasive cutting)
Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool.A wide variety of machines are used for grinding:* Hand-cranked knife-sharpening stones * Handheld power tools such as angle grinders and die grinders...
process afterward to improve precision. This is done in the same type of machine, but the rill plates are replaced with grinding stone
Grinding Stone
Grinding Stone is a 1973 debut album by Gary Moore, released under the "The Gary Moore Band" moniker.-Track listing:All songs by Gary Moore.# "Grinding Stone" – 9:38# "Time to Heal" - 6:19# "Sail Across the Mountain" - 6:58...
s.
If the balls are steel they are then heat treated. After heat treatment they are descaled to remove any residue or by-products.
The balls are then hard ground. They are ground in the same type of machine as used before, but either an abrasive is introduced into coolant or the rotating plate is replaced with a very hard fine-grain grinding wheel
Grinding wheel
A grinding wheel is an expendable wheel that is composed of an abrasive compound used for various grinding and abrasive machining operations...
. This step can get the balls within ±0.0001 in (0.00254 mm). If the balls need more precision then they are lapped
Lapping
Lapping is a machining operation, in which two surfaces are rubbed together with an abrasive between them, by hand movement or by way of a machine.This can take two forms...
, again in the same type of machine. However, this time the rill plates are made of a softer material, usually cast iron, less pressure is applied, the plate is rotated slowly. This step is what gives bearing balls their shiny appearance and can bring the balls between grades 10 and 48.
If even more precision is needed then proprietary chemical and mechanical processes are usually used.
The inspection
Inspection
An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity...
of bearing balls was one of the case studies in Frederick Winslow Taylor
Frederick Winslow Taylor
Frederick Winslow Taylor was an American mechanical engineer who sought to improve industrial efficiency. He is regarded as the father of scientific management and was one of the first management consultants...
's classic Principles of Scientific Management.
Ceramic
Ceramic bearing balls are made of sintered materials that are then ground to size and shape as above. Common materials include: silicon nitrideSilicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...
and zirconium oxide.
Materials
Common materials include carbon steel, stainless steelStainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
, chrome steel
Chrome steel
Chrome steel is one of a class of non stainless steels such as AISI 52100, En31, SUJ2, 100Cr6, 100C6, DIN 5401 which are used for applications such bearings, tools and drills.-Popular culture:...
, brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
, aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, tungsten carbide
Tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
, platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
, gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
, titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
. Other less common materials include copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, monel
Monel
Monel is a trademark of Special Metals Corporation for a series of nickel alloys, primarily composed of nickel and copper, with some iron and other trace elements. Monel was created by David H. Browne, chief metallurgist for International Nickel Co...
, k-monel, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, and niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
.
| Material | UNS 52100 | Stainless steel 440C | M50 | BG-42 | REX-20 | 440NDUR | Haynes 25 | Si3N4 | BECU | 455 | C276 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Hardness [HRC] | 60 | 58 | 62 | 66 | 60 | 50 | 70 | 40 | 50 | 40 | |
| Temperature limit [°F] | 300 | 300 | 400 | 600 | 300 | 1200 | 1500 | 400 | 500 | 1000 | |
| Corrosion resistance | 1 | 3 | 1 | 2 | 1 | 4 | 5 | 5 | 1 | 4 | 5 |
| Cost | 1 | 1 | 1 | 2 | 3 | 1 | 5 | 5 | 3 | 2 | 4 |
| Availability | 1 | 1 | 2 | 2 | 2 | 4 | 5 | 3 | 3 | 2 | 4 |
| Magnetic | Magnetic | Magnetic | Magnetic | Magnetic | Magnetic | Magnetic | Non-magnetic | Non-magnetic | Non-magnetic | Magnetic | Magnetic |
| Size limit | None | None | None | None | None | None | 1.5 in (38.1 mm) | None | None | 5 in (127 mm) | |
| Relative load capacity | 3 | 2 | 4 | 4 | 5 | 3 | 1 | 5 | 1 | 1 | 1 |
| Relative fatigue life | 3 | 2 | 4 | 4 | 5 | 3 | 1 | 5 | 1 | 1 | 1 |
Atypical uses
One interesting atypical use for bearing balls is at San Francisco International AirportSan Francisco International Airport
San Francisco International Airport is a major international airport located south of downtown San Francisco, California, United States, near the cities of Millbrae and San Bruno in unincorporated San Mateo County. It is often referred to as SFO...
. The building is supported by 267 columns, each of which rests on a steel ball with a diameter of 5 feet (1.5 m). The ball sits in a concave foundation. If an earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
occurs, the ground can move up to 20 inch (0.508 m) in any direction, as the columns roll on their bases. This is an effective way to separate the building from the movement of the ground. After the earthquake has ended, the columns are re-centered on their bases by the force of gravity.