It is estimated that there are more than
500,000 medical products on the market today. Many of these devices rely
on rolling element bearings to achieve required performance levels.
Engineers and designers are faced with unique challenges developing
these products, and bearings play a key role in both simple and complex
devices.
Bearing materials
All bearings should be manufactured using rings produced from
high-purity material. For most medical applications, martensitic
stainless steel, similar to AISI 440C, is recommended and often required
due to regulatory requirements. This material, which can be specified
differently depending on its manufacturer, provides for good corrosion
resistance and has fine, evenly dispersed carbides, which result in
lower noise and vibration levels than 440C. This type of stainless steel
is very desirable, particularly in high-speed devices.
Nitrogen-enhanced martensitic stainless steel is also available, and
while it is more expensive, it offers up to five times the corrosion
resistance when compared to traditional 440C type materials. Other
benefits of this material are very low noise levels, extended fatigue
life due to its fine structure that contains smaller chromium nitrides
(as opposed to chromium carbides), and a high resistance to corrosion
resulting from exposure to blood.
In some cases, balls produced from ceramic materials, such as silicon
nitride, can prove beneficial. Ceramic balls exhibit high hardness and
are lightweight, highly polished, nonmagnetic, and resistant to attack
from most liquids and chemicals. Ceramic balls greatly improve the
high-speed capability of the bearing. Bearings made of steel rings and
ceramic balls are commonly called hybrid bearings. While ceramic balls
have an impressive list of beneficial characteristics for bearing
applications, they are not a cure-all medicine. Due to the high hardness
of the ball, contact stress is increased, so fatigue life is
compromised. When the typical failure mode is characterized by fatigue,
it's usually best to stick with steel balls.
Retainers, or ball separators, are typically produced from a 300
series stainless steel. In high-speed applications, it is often
necessary to use a plastic or phenolic resin snap-in or crown style
retainer. For very high speeds, an angular contact bearing with a
full-machined retainer is recommended. These types of retainers provide
increased stability at higher speeds.
Phenolic resin cages have a porous structure and can be impregnated
with oil for additional lubricity. Some of the plastic materials, such
as polyamide-imides, contain additives such as graphite and Teflon for
additional lubricity in emergency running conditions. A wide array of
lightweight plastic materials are available that can handle temperatures
up to 500F and are autoclavable.
An autoclave is a device used to sterilize surgical tools, dental
drills, or other devices by subjecting them to high-pressure saturated
steam for around 20 to 30 minutes depending on the size of the load.
This is a common practice that can hurt the bearing materials and
lubrication.
Lubrication
Lubricant selection may be the specification most overlooked by
designers and engineers. Bearing life depends on proper lubrication in
terms of both type and amount. In many cases, miniature and smaller
instrument bearings are lubricated once for the lifetime of the device.
Thousands of greases and oils are available that are designed to
function in a variety of conditions and environments.
Operating temperature is the primary consideration when selecting a
lubricant. Temperature directly impacts the base oil's viscosity, which
in turn impacts the ability to support loads. In the world of medical
devices, bearing lubricants are subjected to sterilization, temperature
extremes, high-speed rotation, saline washdown or irrigation, chemicals
and reagents, blood, and radiation
Lubricant
selection not only depends on the operating conditions the bearing will
face, but may also be subject to regulatory requirements. Manufacturers
of medical devices are often required to use what are known as
food-grade lubricants, which are broken into categories based on the
likelihood they will contact food.
H1 lubricants are food-grade lubricants used in food processing
environments where there is some possibility of incidental food contact.
H2 lubricants are used on equipment and machine parts in locations
where there is no possibility that the lubricant or lubricated surface
will contact food. Finally, H3 lubricants, also known as soluble or
edible oil, are used to clean and prevent rust on hooks, trolleys, and
similar equipment.
Due to the wide array of products, prices, and availability, both a
lubrication specialist and the bearing manufacturer should be consulted
before making a final lubricant selection.
Surgical and dental tools
Some of the more demanding medical applications include surgical and
dental tools (drills and saws), laboratory and diagnostic equipment, and
imaging equipment. Bearings of special design, or catalogue bearings
with modifications or enhancements, are typically required. These
handheld tools, particularly dental drills, generally operate at very
high speeds -- rotational speeds of 400,000RPM or higher are common. Low
speed is 125,000RPM, which is quite fast. High-precision (ABEC 5 and 7)
miniature and instrument series ball bearings are used.
However, for ultra-high speeds, these bearings are modified further
and have improved raceway surface finishes. In addition, the raceways
and surfaces that guide the retainer have tighter dimensional and
geometrical tolerances, in some cases ABEC 9. More expensive angular
contact designs are often recommended for their high-speed stability,
and they also allow for the use of full machined type retainers. This,
again, enhances speed capability.
Running at high speeds also presents challenges with instrument noise
levels and heat generation. High audible noise during a dental
procedure is a problem for both the dentist and the patient. When
bearings are assembled, it is necessary to have a certain amount of
internal clearance, or radial play, built in. This allows for one
bearing race to move both radially and axially relative to the other.
Application of a preload across a pair of bearings is recommended.
Preload can be defined as the application of an axial load across a pair
of bearings to force the rolling elements to assume a contact angle for
the purpose of removing the internal clearance. The result is constant
ball-to-race contact. This reduces ball skidding, vibration, and noise.
However, disadvantages of preload include torque, heat, and reduction in
fatigue life. Preload determination is a balancing act where the goal
is to apply the least amount of axial preload force possible while
meeting the performance requirements of the instrument.
During surgery, the bearings in tools are regularly exposed to harsh
conditions and liquids, including blood and saline, as well as
particulate debris. When space permits, shielded bearings should always
be used. The speed of these tools is generally too high for seals, but
when conditions permit, seals should be used. Seals are the best option
for keeping foreign debris out of the interior of the bearing and
keeping lubricant in.
Laboratory and diagnostic equipment
The laboratory work that goes into the testing of blood, urine, tissue,
and other specimens is critical for the timely diagnosis and treatment
of millions of patients every day. Thousands of tests are prescribed in
hematology, immunochemistry, and histology every hour in hospitals and
laboratories around the world.
In the area of hematology, samples are
typically subjected to a variety of conditions during testing and
analysis. This includes light scatter analysis techniques for counting
cells, mechanical motion and agitation, controlled temperature and
humidity cycles, and the addition of reagents. Medical technologists use
advanced laboratory and diagnostic equipment to conduct and catalogue
these tests and results.
Due to the high volume of tests and the requirements for reliability,
these test systems are often highly automated and programmable and have
full data management and storage capabilities. In addition, they can
handle hundreds of samples (oftentimes open vials) and conduct multiple
tests during an automated cycle. These systems have demanding
positioning requirements and utilize a variety of different types of
bearings, including linear, angular contact, thin section, and miniature
and instrument ball bearings.
Bearings for these applications should be manufactured from the type
of martensitic stainless steels described previously. The bearings are
often exposed to high humidity or moisture resulting from condensation.
In addition, they are exposed to fluids during testing that include
blood and the reagents used for the test, so good fatigue life is
critical.
Sealed bearings should be considered whenever the potential for
contamination exists. The most common bearing seal material is a nitrile
rubber. However, this may not be well suited or permitted, due to
regulatory requirements. Teflon seals are often used in medical devices.
They have outstanding chemical resistance and high- and low-temperature
capability, and they exhibit less torque than nitrile rubber seals.
Viton is also available when a more robust seal is required. The seals
found on most types of bearings are not designed for immersion, and
fluid penetration will eventually take place. They offer excellent
protection from particulate contaminants or a fluid splash and wipedown
situation.
These systems move test samples (most often vials) to various
locations within the machine for scanning, testing, or the addition of a
reagent prior to analysis. In addition, samples may be spun, shaken, or
otherwise agitated for various reasons. These movements and motions are
then repeated over thousands of cycles. To achieve the precise
positioning and repeatability requirements, in most cases, bearings
should be ABEC 3 or better. Housing and shaft design should allow for
very precise fitting to minimize any eccentricities or the chance for
slippage or fretting. When fitting bearings that have thin cross
sections, such as miniature bearings, line-to-line fits are commonly
specified. Interference fits can reduce the internal clearance in the
bearings. If this reduction is excessive, bearing life will be
compromised.
In these applications, where positional accuracy must be controlled
to precise levels, radial (and axial) play in the bearing is usually
unacceptable. Application of a preload, described previously, is
recommended. The principal benefits are precise shaft positioning (no
free motion), control of axial and radial compliance, and shared loading
between bearings. In addition, shaft rotational accuracy is greatly
improved, minimizing runout characteristics.
Pharmaceutical, dental, and medical device applications present many
challenges for bearings. These include high speeds, low noise, long
service life, and resistance to harsh environments or aggressive
chemicals or fluids. Bearing manufacturers conduct continuous research
into materials for components such as retainers, special lubricants, and
optimization of the bearing geometry to in order to meet the
ever-increasing demands of the industry. In addition to superb product
quality, bearing suppliers must offer a flexible approach to problem
solving.
