Project commonly used. SPECIFICATIONS: The most common materials

Project 2- Material Selection

Date:  December 8, 2017

To:  Professor Michael Jenkins

From:  Gregory Angel

Re:  Project 2 – Material Selection


Attached please find my Project 2 report on
materials selection entitled “Ball Bearing Material Selection” which you
requested on November 2, 2017.  This
report satisfies all the requirements of the as detailed in the project


Please contact me if you have any questions or
comments ([email protected], or mobile 559-871-2509)






Bearing Material Selection



A ball bearing is a component, typically made of 52100
chrome steel or used to reduce rotational friction.  This is accomplished by two concentric races with
a set of balls between them held at equal increments along the races by a cage
as seen in Figure 1.  For radial
bearings, which this report will cover, there are grooves in the races that
keep the balls rotating in the same plane. 
The inner race and outer race rotate more freely than a ring sliding
around an axle because the coefficient of rolling friction is lower than the coefficient
of sliding friction.  Typically, the
inner race is fixed to an axle and the outer race rotates about it.  Due to the reduction of rotational friction,
ball bearings of various materials are present in most appliances that have
rotating components.  This paper will
discuss conventional radial steel ball bearings, why ceramic and plastic ball
bearings are substitutes and their applications, and why steels are most
commonly used. 



The most common materials used in ball bearings are stainless
steels and chrome steels, due to their corrosion resistance properties, however
this report will focus on 440C stainless steel and 52100 chrome steel pdf.  These materials are both appealing due to their
relatively low cost, capability of withstanding high loads, and capability of being
precisely machined.  They are also fairly
corrosion resistant with 440C stainless steel having better corrosion
resistance due to its 18% chromium content compared to 52100 steel’s 1.5% AST.  Passivation occurs in oxidizing atmospheres
by creating a thin film of oxide on the surface to prevent deeper corrosion or
mechanical wear TEXT.  The ability of a
steel to passivate increases with its percent chromium content text.  52100 chrome steel has a hardness ranging
from 59 to 61 HRC and a modulus of elasticity of 210 GPa, while 440C
Martensitic stainless steel has a hardness ranging from 60.5-63 HRC and a
modulus of elasticity ranging from 204 to 215 GPa at standard temperatures
MAT.  The high hardness and the high
modulus of elasticity help prevent the balls from deforming while loaded, allowing
the bearing to maintain high speeds and reducing the wear rate, even while
loaded as well as resisting abrasion from particulates that enter the races text.  While they can function in many environments,
steel ball bearings require lubrication to prevent them from seizing, which
occurs when aspects of the bearing’s geometry change sufficiently to prevent
the balls from rolling. To prevent bearing seizure and keep lubricant on the
balls, it is common for bearings to have shields that shield the bearing from
debris.  Debris entering the races can cause
abrasion, which shifts surface material causing stress concentrations, which
will lead to an increased wear rate.  Another
problem that occurs for stainless steel bearings is magnetization.  While the steel bearings alone may not be magnetic,
when introduced to a magnetic field, they will be experience a drag force, due
to Lenz’s Law PHYS.  This can cause
bearings to slow down or stop if a sufficiently strong magnetic field is



Chrome and stainless steels are most commonly used, because
of their lower price and properties that function in standard conditions.  Because so many manufacturers make them,
there is sufficient supply to meet the ever-present demand.  They also perform well in a wide variety of
applications, so they are a “one size fits most” solution for rotational
friction reduction.  Their relatively
high modulus of elasticity and hardness grant them lifetimes sufficiently long
for most applications, because they only experience small stresses relative to
their yield strengths.  Everyday use does
not occur for extreme temperatures or loads, so steel bearings suffice
especially in blenders and pull start lawn mowers app.  Their low price is desirable for luxury items
such as toys like fidget spinners and yoyos, which apply loads on the order of 10
grams, making load bearing capabilities irrelevant.  Even in skateboards, loading is relatively
low, allowing for the use of steel bearings. 
Additionally, most common applications do not exist in sufficiently
strong magnetic fields. 



The second most common material used in ball bearings is ceramic.  Ceramic bearings tend to be made of silicon
nitride, are stronger than steel, can spin faster, nonconductive, and are more
corrosion resistant, due to ceramics being inherently more corrosion resistant
than metals.  Silicon nitride’s elastic
modulus of 320 GPa text, is higher than those of the steels, which subjects
it to less deformation when loaded and thus creates less unwanted vibration and
noise than steel bearings text.  The
reduction in unwanted vibration increases the lifetime of the ball
bearing.  Silicon nitride also has a
higher hardness than both steels at 75 to 80 HRC, which decreases the wear rate
and increases lifetime TEXT. 

There are both “hybrid” bearings consisting of steel races
and ceramic balls, and full ceramic bearings. 
Both types are lighter than steel, due to the density of silicon nitride
being 3.20g/cm3 which is less than half of both 51200 steel’s density of
7.81g/cm3 and 440C steel’s of 7.81g/cm3 mat, so they can spin faster while experiencing
less centrifugal force and having less angular momentum.  This is crucial for medical applications,
such as dental drills and surgical saws, where the surgeons must be precise.  The lower the angular momentum, the easier it
is for a surgeon to rotate a drill or saw and thus give them more control, preventing
errors in the surgery leading to injury or death.  This allows ceramic bearings to operate in
more extreme temperatures without significant changes to geometry.  The lower coefficient of friction for silicon
nitride reduces wear and operation temperature elevation caused by friction,
especially when operating at high RPMs TEXT 511.  Its coefficient of thermal expansion at 2.80
?m/m-°C is
lower than those of both 52100 chrome and 440C stainless steels, which have
coefficients of thermal expansion of 10.2 ?m/m-°C and 11.9 ?m/m-°C
respectively.  This along with its high
corrosion resistance allows silicon nitride bearings to operate in extreme
temperatures and corrosive environments.  Ceramic bearings cannot be magnetized, and
thus do not experience any drag when in the presence of a magnetic field,
allowing them to operate in such conditions. 
The low coefficient of thermal expansion prevents the components from
drastically changing their geometry, insuring proper function of the bearings
and reducing wear.  Ceramic bearings are
far more expensive, which greatly limit their use and making plastic bearings seem
more attractive if a lighter bearing is required. 


Common plastics to use for ball bearings are glass
reinforced nylon, PTFE, and POM, buy this report will focus primarily on PTFE
plast.  Plastic ball bearings are self-lubricating,
corrosion resistant, and operate quietly text.  Self-lubrication and corrosion resistance
greatly reduces maintenance costs and reduces the chances of corrosion caused
by unlubricated balls to occur.  As with
ceramics, plastics are inherently more corrosion resistant than metals. PTFE
particularly is resistant to acids, bases, solvents, and water, which allow it
to operate in extremely corrosive environments.  The only substances that corrode PTFE are, “molten
alkali metals, turbulent liquid… gaseous fluorine… and fluorochemicals” fric.  PTFE has an extremely low coefficient of kinetic
friction of 0.1, which is much lower than the coefficient of kinetic friction
for steel on steel, which is 0.57 fricword. 
This allows them to operate more quietly and wear at a slower rate than
steel bearings fric.  Glass reinforced
PTFE has a density of 2.22 g/cm3, which is less than a third of that of steels
and just over two thirds that of silicon nitride, which decreases centrifugal
force and angular momentum. 

A major drawback to plastic ball bearings is that they cannot
withstand high loads or extended use in elevated temperatures.  Glass reinforced PTFE has a much lower modulus
of elasticity than both silicon nitride and steels at 1.36 GPa, which is over
150 times lower than steels and over 230 times lower than silicon nitride.  This causes the geometry of PTFE balls to
significantly change when loaded, which can cause sudden catastrophic failure.  Polymers tend to experience scission at
elevated temperatures, which destroys the molecular chains of which they are
composed, progressively weakening all properties of the material until catastrophic
failure occurs TEXT.  They also cannot
be magnetized, which is important in the medical field.  Magnetic resonance imaging (MRI) machines use
magnetic waves to analyze tissues in the body to identify damaged or cancerous
tissues mri.  Steel bearings cannot be
used in these machines, because they can become magnetized by the magnetic
field and thus disrupt the imaging process med.  They are also used in fluid flow machines and
pumps for medical applications due to their low friction coefficients,
self-lubrication, and corrosion resistance such as in fluid pumps, which rely
on bearings to run smoothly to deliver the correct amount of fluid to the
patient med. 



Steel bearings are most common due to their low cost and
ability to perform in most applications. 
Because it is a relatively old technology, fabrication techniques have
been perfected over the years, driving down the cost.  There is also an overwhelming supply of steel
bearings, due to their widespread use.  Stainless
steel bearings work for most application, with only extreme and unique
conditions requiring materials with different properties.  The lifetime of stainless steel bearings is
sufficient for most applications as well, especially if well maintained.  For most applications, a ceramic ball bearing
would function, however their price hinders their use in many applications.  Plastic bearings offer very little in terms of
strength, especially in extreme temperatures which is crucial in aerospace
applications, although have low coefficients of friction and high corrosion resistance.  The low price of steel bearings and the
relatively mild conditions of standard operation do not justify the use of ceramic
or plastic bearings. 


A Guide to Ball Bearing Materials










NASA pdf:

Bearing corrosion:

– Physics for Scientists and Engineers with Modern


Plastic bearings:

Plastic bearings in medicine:


Ptfe friction:

Fric word:

Figure 1: The anatomy of a ball bearing with all
components labeled and a cutaway to reveal the geometry of the races, balls,
and cage.  The cage is referred to as a
“Separator” in the diagram.


Figure XXX: A shielded stainless steel ball bearing
in a yoyo. 

Yoyo bearing pic: