Why Medical Device Designers Should Care about Fasteners

Fasteners come in a variety of shapes, sizes, and styles, but for medtech applications, they must all offer corrosion resistance, strength, and holding power.

March 19, 2015

6 Min Read
Why Medical Device Designers Should Care about Fasteners

Bob Michaels

Granted, fasteners aren’t the sexiest components in the world. But try building a single piece of medical equipment without them, or try doing it if the fasteners aren’t just right.

Clinching It

What does it take to design and manufacture fasteners for medical device applications that are just right? “The first issue is corrosion resistance,” remarks Jon Brunk, applications engineer at Danboro, PA–based PennEngineering. “In medical device applications, you never want to run into any kind of rusting, staining, or other issues stemming from the material from which the fastener is made.” Thus, the company typically makes its fasteners from Type 304 or Type 316 stainless steel.

However, when self-clinching fasteners are to be installed into stainless-steel sheet metal, they must be harder than the panel material itself so that they can clinch adequately. In such cases, the fasteners must be made from heat-treated stainless. Because Type 304 and Type 316 stainless steels can’t be heat-treated, fasteners for such applications are made from precipitation-hardened stainless steel. Precipitation-hardened stainless steel, according to Brunk, provides sufficient strength while maintaining high corrosion resistance.

“In addition to the choice of materials, we are usually tasked with maintaining tight tolerancing,” Brunk comments. The question of tolerance is particularly critical for standoffs. Self-clinching, internally threaded fasteners of a defined length, standoffs are used to raise and attach one assembly above another. These defined lengths must meet exacting tolerances because the panels must rest at certain distances from each other and must be perfectly aligned. “For example, if two panels are stacked on top of each other using four standoffs, the standoffs must be almost exactly the same length,” Brunk adds. “Thus, the length tolerance must be tight.”

Besides meeting tight tolerance specifications, medical device fasteners must also exhibit a range of physical properties, including load-bearing ability, torqueability, strength, and vibration resistance. Crucial among these properties is proofload. To meet the durability and load-bearing requirements of such electronic packaging applications as front panels, the company’s nuts typically meet a property Class 10 rating, Brunk says.

“The basic premise for our fasteners is that they are self-clinching, meaning that they become a part of the panel itself,” Brunk says. “This allows manufacturers to eliminate loose hardware and thus meet design for manufacturability needs.” Moreover, if a piece of equipment has to undergo secondary repairs, the repair personnel don’t have to worry about lost or missing hardware because loose hardware has been replaced with captivated hardware, such as nuts or studs. This capability is especially advantageous in tight equipment spaces in which there is little room to maneuver wrenches or other tools.

Collar ’Em

Besides nuts, bolts, and screws, many other types of fasteners are used in the medical device industry. One such type, the shaft collar, is commonly used for guiding, spacing, stopping, and component alignment. Because it offers a precise face-to-bore relationship, it is well suited for such medical device applications as blood and fluid analyzers, as well as MRI/CT machines and their positioning systems.

Choosing the right shaft collar for the application is critical to overall system performance, remarks Bill Hewitson, vice president of operations at Marlborough, MA–based Ruland Manufacturing Company, Inc. For example, while setscrew shaft collars are economical, they have limited holding power. In contrast, clamp-type shaft collars provide good holding power, do not mar the shaft, and allow for simple positioning adjustments. Clamp-style shaft collars distribute compressive forces evenly around the shaft, ensuring a tight fit and better holding power than setscrew-style shaft collars.

Shaft collar strength is largely a function of material, construction, and design, Hewitson says. Well-sourced raw materials ensure consistency and reduce the likelihood of flaws during the manufacturing process. Low-grade materials, in contrast, can crack or deform under torque, resulting in failures. These failures, in turn, can cause burrs to build up and transfer to other areas of the medical device, resulting in contamination. “Shaft collars made from traceable raw materials will have a longer useful life and perform consistently over time,” Hewitson adds.

Like the other types of fasteners used in medical device applications, shaft collars must be corrosion resistant. While steel is the most common shaft collar material, it may not be suitable for medical applications that are exposed to the environment or that undergo frequent corrosive washdowns. “Type 303 and Type 316 stainless steels are better options than steel, Hewitson notes. “And for consistent corrosion resistance, shaft collars must be supplied with hardware of like material. For example, if a Type 316 stainless collar is supplied with Type 303 hardware, the screw will prematurely fail, necessitating the replacement of the entire shaft collar.”

The screw is the primary source of shaft-collar holding power, Hewitson explains. Low-quality hardware or a poor pocket design significantly reduces the collar’s clamping ability and performance. “To ensure holding power, we apply a proprietary black oxide finish on steel shaft collars to prevent stick-slip, which provides a false impression that the screw has been tightened to its appropriate stress level. This finish ensures smooth screw operation during torqueing.”

One of the most important features of shaft collars is their face-to-bore perpendicularity. “Shaft collars,” according to Hewitson, “are often used to align or locate other shaft components or accommodate axial thrust loads. Face-to-bore perpendicularity ensures the squareness of the component to the shaft with no shifting or tilting relative to the shaft axis.” To achieve this precise relationship, the shaft collar face and bore are machined in a single operation.

To ensure that load forces are applied evenly to the face of the collar, designers must select shaft collars with high face-to-bore perpendicularity, the measure of which is a low total indicated run-out. By providing an even distribution of clamping forces over the entire face of the collar, high face-to-bore perpendicularity allows for higher axial loads. It also ensures that such mounted components as gears, sprockets, and bearings will align properly and run smoothly.

“A common misconception is that increasing the shaft collar outer diameter will increase its holding power,” Hewitson notes. “But this actually has the opposite effect because more screw torque is required to bend the extra material around the shaft before the remaining forces are applied to the shaft, requiring a change of screw location in relation to the change in OD to achieve the desired effect. However, because such OD modifications are common in medical device applications in which larger screws or hidden hardware are required, designers should be aware of this misconception in order better select the proper shaft collar.”

Bob Michaels is senior technical editor at UBM Canon. Reach him at [email protected].

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