Lubrication, Bearing Design Considerations in Medical Technology

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 life time 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 wash down 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 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, price and availability, both a lubrication specialist and the bearing manufacturer should be consulted before making a final lubricant selection.

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Bearing Design Considerations in Medical Technology as 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 5 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 are light weight, highly polished, non-magnetic, exhibit high hardness, 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. So, 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 material are available, which, 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 20-30 minutes depending on the size of the load. This is a common practice that can have a negative impact on the bearing materials and lubrication.

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Corrosion-Resistant Precision Ball Bearings Eastmachinery

EASTMACHINERY Bearings LLC now offers a range of solutions allowing precision ball bearings to withstand harsh environments, including exposure to moisture, chemicals, high temperatures and vacuum conditions.

Among the options available are stainless steel (AISI, 440C, AISI 304, SV30) rings and balls; balls made of silicon nitride or titanium-carbide-coated steel; rings specially coated with thin dense chromium (TDC); and special seals, retainer materials and lubricants.

Applications for these enhanced bearings include thermal processes, chemical processing, hydraulic pumps, vacuum environments, fuel pumps and pharmaceutical and food processing.

A standard chrome bearing (left) versus an extreme environment, corrosion-resistant bearing (right) following nitric acid testing. The corrosion-resistant bearing showed no signs of corrosion, which meets the design requirement of 0% corrosion in a nitric acid submersion. Alternatively, the chrome steel bearing was severely corroded, and the ball retainer was deteriorated to the point of non-functionality. If the chrome steel bearing was put in service in such an atmosphere, catastrophic failure would occur in <1min.

You can rely on AST for our applications expertise and our commitment to provide exceptional service and value.  Call us with your next design… we can help with the solution.

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Bearing Grease Consistency White Paper in Eastmachinery

Bearing grease consistency can greatly affect the performance of the bearing and consequently the performance of the mechanical device or product.

Most people are familiar with the term "viscosity" as the measurable consistency of oil lubricants.  However, the average person probably does not know what the equivalent measurement for the consistency of grease (hint: it's not viscosity). Bearing grease consistency is measured in "penetration levels".

Eastmachinery Bearings has created a mini white paper which covers the topic of "Bearing Grease Consistency".  In it the following topics are covered:

Grease Penetration Definition
How is Unworked Penetration Measured?
How is Worked Penetration Measured?
Comparing Worked and Unworked Penetration
Channeling Grease Characteristics
Grease Consistency Plays Role in Preventing Fretting Corrosion


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New Seals Protect Bearings from Harsh Environments - Eastmachinery

Eastmachinery Bearings LLC now offers specially formulated rubber seals with unique geometries to protect 6000, 6200 and 6300 series ball bearings from moisture, wash-down chemicals, mud and dirt. Applications include outdoor environments, lawn and garden equipment, snow blowers, pumps and compressors, pulleys and idlers and rollers and conveyors.

Bearings are filled with a special grease lubricant, featuring synthetic base oil with a lithium complex thickener. The grease contains additives to protect against wear and corrosion, and can withstand temperatures from -40°C to +150°C (-40°F to +302°F). The sealed bearings meet ABEC 1 tolerances per A.B.M.A. standard 20, and have modified internal clearances that allow for easy interference fitting of both rings if desired.

A 6203 bearing, for example, could be treated with this sealing and lubricant technology and the nomenclature would be referred to as 6203-2RES. Other bearing specifications for a sealed 6203-2RES remain the same: bore diameter (17mm), outer diameter (40mm), width (12mm), static load capacity (1,070 lbs.), dynamic load capacity (2,150 lbs.), and maximum speed (9,000 rpm).

You can rely on Eastmachinery for our applications expertise and our commitment to provide exceptional service and value.  Call us with your next design… we can help with the solution.

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Spherical Bearings in Eastmachinery about Accommodating Misalignment

Spherical bearings come in many different types to accommodate a wide variety of applications. What all of them have in common is the ability to accommodate misalignment from mounting or shaft deflection.
Spherical Ball Bearings
Spherical Roller Bearings
Spherical Plain Bearings
Spherical Rod End Bearings
Spherical Ball Bearings
More commonly known as Self-Aligning Double Row Ball Bearings, spherical ball bearings are designed to work in applications where severe misalignment exists whether from mounting or shaft deflection. The concave raceway of the outer ring in combination with two rows of balls operates over a wide range of contact angles. Furthermore it aligns itself correctly with the shaft and assembly without causing undue stress on the rest of the bearing assembly. Noteworthy is that the self-aligning ball bearing has the lowest friction of all rolling bearings, which enables it to run cooler even at high speeds. Furthermore, the design allows for easy mounting and dismounting.
Spherical Roller Bearings
Spherical Roller Bearings are designed to work in applications where severe misalignment exists whether from mounting or shaft deflection and with relatively heavy radial loads and some axial loads in either direction. They are also extremely resistant to shock loads and their self-aligning feature allows full capacity loading despite shaft deflection. They are very similar in purpose to Self-Aligning Double Row Ball Bearings, except the Spherical Roller Bearings are designed to be more robust by carry heavier loads but at lower maximum speeds.
Spherical Plain Bearings
Spherical Plain Bearings (also known as “Spherical Plane Bearings” “Spherical Ball Bushing Bearings” and “Ball Bushings”) are spherical bearings that have an inner ring with a sphere convex outside surface and an outer ring with a correspondingly sphere, but concave inside surface. Their design makes them particularly suitable for bearing arrangements where alignment movements between shaft and housing have to accommodate, or where oscillating or recurrent tilting movements must be permitted at relatively slow sliding speeds.
Spherical Rod End Bearings
A Rod End is spherical bearing that consists of an eye-shaped head with integral shank forming a housing and a standard spherical plain bearing, or a spherical plain bearing inner ring, or a spherical plain bearing inner ring and a sliding layer between the bore of the head and the inner ring. As the name implies, they are used at the end of rods and have either a male or female thread for mounting.
Which Is Best for Shaft Misalignment?
If you need help in selecting how to best accommodate for shaft misalignment and which products may be best for your particular application, please remember that design engineering and application assistance are available from Eastmachinery as part of our value-added services designed to make your job easier.  Contact us for more information.
Category: Ball Bearings, Bearings - General, Roller Bearings, Spherical Plain Bearings
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Eastmachinery Bearing Design Considerations for the Food Processing Industry - Seals

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In the fifth (and final) portion of our examination of bearing considerations in the food processing industry, we will be taking a closer look at seals. In food processing plants, bearings are continuously exposed to liquids and various types of particulate debris. Sealed or shielded bearings are the best option for keeping foreign debris out of the interior of the bearing and keeping lubricant in.
The most common bearing seal material is a nitrile rubber. This type of seal is typically Buna-N bonded to a steel insert. The seal is fixed into a groove in the outer ring. The maximum operating temperature is 240° F. This type of seal makes contact with the inner ring providing better protection in contaminated environments than a metal shield. However, this results in an increase in torque and reduces the maximum speed capability of the bearing, but in almost all cases is a design tradeoff that improves the product life.
Certain lubricants and chemicals react with rubber. Aside from the Buna-n and nitrile rubber material, other materials are available for higher temperatures and chemical resistance. These include FDA approved silicone rubber seals that can handle a maximum temperature of 400° F and Viton that has good chemical resistance and a maximum operating temperature of 400° F.
The seal construction can greatly improve its ability to function in extreme environments. Seals can be shrouded in metal, such as stainless steel or galvanized steel. Often called slingers, this type of seal is often found on mounted bearings and the metal shroud helps protect the rubber from abrasion.
The portion of the seal that contacts the inner ring of the bearing is known as the seal lip. Seals can be designed as single, double or triple lips. The seal lips flare out where the contact the ring provides effective protection against lubricant loss and wet or particulate contaminants. Triple lip seals provide the most protection, but also increase rotational torque substantially compared to single lip designs.
When lower torque is a design requirement, a non-contact rubber seal based on the labyrinth effect can be utilized. These seals function by creating a path the fluid or debris must navigate in order to penetrate the seal. In bearings, this is typically just a groove in the inner ring. A contact lip seal can be used very effectively in combination with a labyrinth design to provide excellent protection and low torque.
Teflon and glass reinforced PTFE seals are an excellent material choice in food processing applications. They have outstanding chemical resistance, high and low-temperature capability and exhibit less torque than rubber seals. However, depending on the type of bearing and their construction, these seals are not as robust and can be dislodged if hit directly with a high-velocity stream of fluid.
Keep in mind, 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 wipe down situation.
In the food processing industry, bearing life is a major factor contributing to uptime and reliability. The environments bearings encounter in these applications are extremely harsh. Bearings are selected based in operational loads and desired life, but designers must also give careful consideration to protecting bearings from corrosion and contamination and maximizing lubricant life.
As evidenced, there are a wide range of bearing options to take into account for optimal application performance. Food processing engineers and designers are advised to consult a bearing applications specialist for assistance with calculating environmental factors, load capacity, bearing life and attributes.
Due to the wide array of products along with price and availability, both a lubrication specialist and the bearing manufacturer should be consulted before making a final lubricant selection.
There are a wide range of lubrication options to take into account for optimal application performance. Food processing engineers and designers are advised to consult a bearing applications specialist for assistance to best navigate these regulations.
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