New Ways to Mate Parts

In any assembly, you must connect pieces together–somehow. Exactly how you accomplish this goal opens the door to innovative thinking. Here’s a look at several recent developments.

Joint makeover
When a major medical device manufacturer received discouraging feedback from surgeons about their surgical scissors, they went into action to redesign them. The surgeons reported that the scissors were difficult to control. It seems the screw and small nut used in the hinge mechanism, although secure and reliable, was either too loose or too tight to allow pinpoint operation accuracy.

Endoscopy surgical scissors need a consistent “feel” so that opening and closing the blades is sensed the same way every time. It was evident to the surgeons during delicate procedures that, although they visually positioned the scissors and cut by sight, of equal and critical importance was the minimal friction experienced when adjusting the blades.

With specially designed spindle, slide, and servo motor arrangement,
this orbital forming machine from the Orbitform Group controlled
pivot joint diameters to 0.058 in.

In an effort to make medical surgical operations go smoother, the scissor manufacturing engineers sought out a new method to fasten and assemble the blades together. They turned to Orbitform Group’s Application Lab engineers to analyze their options.

Surgeon feedback on the lack of tension control
when adjusting these surgical scissors prompted
the manufacturers to seek out a new method to
fasten and assemble the blades together. 
They turned to the Orbitform Group for their answer.

With screws, even with the most precise process control the screw and nuts weren’t holding with a consistent tension because of small thickness variations within the blade halves.

Orbitally forming the scissor parts to a clamp load
eliminated part variations and gave the surgeons better control.

Rivets did not solve the problem either. Since the blades needed to be “firm yet flexible,” simply riveting them together to an exact stop point would not allow for needed flexibility.

It turned out that orbitally forming the scissor parts to a clamp load eliminated part variations.

Through multiple load cells on an Orbitform Model 125 bench-top orbital forming machine, a form-to-force process was created that allowed the torque on the scissor joint to correspond to a pre-determined load, thus eliminating part size as a quality factor.

The orbital forming machine used a specially designed spindle, slide, and servo motor arrangement. This arrangement provided accurate control of the 0.058 in. diameter pivot joints and permitted the forming machine to dwell at a position instead of a positive stop. Most importantly, extensive testing by the scissor manufacturing engineers verified that each pair of medical scissors was able to repeat the opening and closing well within the desired force range.

The orbital work station featured Watchdawg process control digital readouts for orbital and anvil positioning and exact force monitoring. Also, a custom fixture for positioning the assembly helped increase the throughput and decrease the fastening time.

The resulting application specific machine is now running these surgical scissors 24/7. And thankfully for surgeons, and their patients, the cuts have never been better.

Supplier partnerships accelerate innovation
Driven by the need to produce ultra-reliable products – and the desire to capture a bigger share of their respective markets – more of today’s medical manufacturers are forging “innovation partnerships” with trusted component suppliers. What makes these alliances different is that they are not just focusing on specific design challenges. They are targeting the overall advancement of the technology and improvement of the product as a whole.

Bal Conn electrical connectors, from Bal Seal Engineering, can be custom made to accommodate specific requirements for contact resistance and insertion/removal forces, and can also be designed with radial or axial compression properties. Diameters are available to fit any lead configuration, from IS1 and IS4 down to 0.040 in.  The compact design allows for greater connector density in applications where space is limited.

OEM/supplier collaboration is especially prevalent among manufacturers of the electronic devices used in cardiac rhythm management, neurostimulation, and sensing therapies. Bal Seal Engineering, Inc., Foothill Ranch, CA, is involved in such a collaboration. The company, a provider of custom-engineered connecting, conducting, shielding and sealing systems, used the model to work with a major manufacturer of implantable neurostimulation devices. The two companies took Bal Seal’s electrical contact product, the Bal Conn™, and developed a product that dramatically simplified the process surgeons use to connect lead wires to implantable electronic units across the therapy spectrum.

Here the Bal Conn connector is shown in use in a deep brain sensing application.

The Bal Conn™ is an electrical connector consisting of a precision-engineered, canted-coil™ spring retained in a metal housing. It is installed in the implantable device header, and its spring coils adjust independently to maintain maximum contact with lead electrodes that are inserted there. The canted-coil spring exerts a near-constant force over the working deflection, so it compensates for alignment issues, surface irregularities — even temperature changes without significant deviation from its original force. The spring element provides multi-point conductivity and exhibits excellent compression set resistance.

“Early-stage collaborative partnerships,” said Jim Sittler, Vice President of Global Sales and Marketing, Bal Seal Engineering, “enable you to intensely focus the resources of two or more organizations on a function or design. In many cases, what you end up with is a game-changing product.”

Such partnerships also offer a less obvious benefit: component suppliers, drawing from their knowledge and experience in a variety of industries, can lend a big-picture perspective to the early product development process. They can often see opportunities for the adaptation and application of proven technologies that exist outside the manufacturer’s worldview.

This ability to bring the “un-obvious” to the table is exactly what NuVasive Inc. was seeking when it began collaborating with Bal Seal on the development of the Helix™ anterior cervical plating system in 2007. NuVasive, a San Diego, California-based manufacturer of products for the treatment of spine disorders, sought to enhance its cervical plate by integrating a locking system to prevent screw fasteners from shifting and “backing-out” over time.

NuVasive, a developer of products for the treatment of spine disorders, used the Bal Seal canted-coil™ spring in a locking application that helps prevent screw fasteners from “backing out.”

NuVasive engineers proposed using the Bal Seal canted-coil™ spring system combined with a unique screw design as the integrated locking mechanism in the new Helix cervical plate. This sparked an innovation partnership between the two that resulted, after significant prototyping and testing, in the successful release of a breakthrough locking system. This locking system makes fasteners more resistant to axial and torsional loads and thus less likely to loosen and back out.

Collaborating with suppliers at a higher development level, say industry experts, is one of the few ways medical OEMs of all types and sizes will be able to successfully battle competitive pressures and remain technologically relevant in the future. 

At a summit hosted by the Cleveland Clinic in mid-November, a panel of medical and financial professionals advocated a “more holistic” approach to technology development. In support of increased collaboration, the panelists cited that manufacturers who cling to a “not invented here mindset” will be more likely to overlook important solutions that lead to innovation.

Meanwhile, the Bal Conn is at work in many implantable electronic devices, including those used in pain management, cardiac rhythm management and sensing therapies. It can also be used in cochlear stimulation, gastric stimulation and epilepsy treatment.

Improving joint consistency
The AKH FAS-NER is a simple wasp-waisted cylinder of steel or aluminum, where the FAS-NER punches its own hole through two or more sheets of metal and then uses a simple coining process to flow metal into the concave “waist” locking them together.

Machining grooves to accept retaining rings can reduce waste and cutting fluid, as is show here in this Rotor Clip example.

The fastener can join metals of different thicknesses, dissimilar metals, and multiple layers of metal. The resulting mechanical joint is strong, tight, corrosion resistant, and flush with both surfaces. The fastener also comes in a headed version that provides the same benefits except that it will not be flush with connecting surfaces.

You can often replace cover plates and bolts through the use of a Rotor Clip internal retaining ring.

Unlike spot welding, the fastening process does not produce heat or plating bleed out. It does not require pre-cleaning, and has virtually no adverse environmental effects. Only basic press tools are required and the process suites high-speed, automated assembly operations.

FAS-NERs are manufactured by AKH FAS-NER Systems, Indianapolis. After heat-treating they are sorted by height and diameter and cleaned prior to packaging. Lot certifications are offered for critical, high-precision applications.

For one Louisiana-based manufacturer of air conditioning systems, the FAS-NERs replaced spot welding in the assembly of pre-painted commercial air handlers. Pre-painted FAS-NERs match the housing color and produce a finished surface in one operation. The previous process involved spot welding the air handling housings then painting them after cleaning up weld spatter and other surface imperfections.

Set screws, like this one from Rotor Clip, can replace shaft collars in many applications.

David Caulk of AKH says the customer “adopted the FAS-NER process after testing showed the joints it produced were more consistent and stronger than the spot welds. An additional benefit was the relative simplicity of the process, which eliminates years of study and apprenticeship as is often needed with spot welding.”

The FAS-NER system accommodates process with material variability. But there is always room for improvement.

While the FAS-NERs themselves deliver quality and consistency, the company engineers’ worked on improving the insertion system.  AKH originally offered two different press systems for the fasteners, an air/oil series and a pure hydraulic series, both with capacities ranging from 5 to 30 tons. While the pure hydraulic units were equipped with a self-diagnostic control module, both were essentially fixed stroke/fixed force machines. They simply repeated the same operation regardless of process variations.

What was needed was an intelligent press system that could monitor the insertion and coining processes in real-time and make force and position adjustments “on the fly” to compensate for variations. The first requirement is a press that can be controlled precisely and the second is for it to be fully instrumented to provide feedback on the operation it is performing, thus eliminating the need for hydraulic or pneumatic presses.

Simple fastening is easy with the AKH FAS-NER. The wasp-waisted cylinder of steel or aluminum punches its own hole through two or more sheets of metal and then uses a simple coining process to flow metal into the concave “waist” locking them together.

Promess Inc., was able to meet both requirements with an electro-mechanical assembly press (EMAP) that uses a servomotor to drive a ball-screw ram. The servomotor connects to the ram with gears or a timing belt so there is little or no backlash in the system. The servomotor’s encoder provides a precise measurement of the ram’s position.

EMAPs are available in 16 different sizes with outputs ranging from 225 lb to 75,000 lb and strokes to suit the full range of fastener assembly operations. When equipped with force sensors and process monitoring and control software, an EMAP-based system provides a fully integrated quality assurance and process monitoring and control system.

The insertion systems also use “signature analysis” to “clone” good parts and processes. The system records the force/distance signature of a known good operation, and then compares the values generated by subsequent operations and makes real-time adjustments as necessary. You establish upper and lower process tolerance limits, and the system accepts all operations that fall between them.

But, the exact shape of the signature also provides information about the individual parts being assembled. These data also can be used as a process control input. For example, the signature of the piercing phase of the FAS-NER insertion provides information about both the hardness and thickness of the joined materials.

By feeding that data into the control software, the coining phase of the process can be matched exactly to the actual characteristics of the joint being formed. The result is a perfect “clone” of the desired joint every time, regardless of any variations in the materials being joined, or the individual FAS-NER itself.

All of the necessary calculations are performed by software running on a PC-based electro-mechanical multi-axis controller (EMAC), an easily programmed, fully integrated, multi-axis motion controller, data acquisition and analysis system. The EMAP and EMAC work together to monitor and control the dynamics of each FAS-NER insertion and graphically display the results to the operator.

Promess and AKH also are developing jointly a series of retrofit monitoring kits for existing hydraulic and air/hydraulic presses. While these will not deliver the same level of real-time process compensation provided by the EMAP/EMAC systems, they will still significantly improve the consistency of the joining operations performed on these presses.

Simple retaining
And of course, don’t forget to consider retaining rings in your fastening applications.  They can reduce costs, including material costs. Here’s a look at a few examples.

Machined Shoulder:  Machining a shoulder and screw threads onto a ½-in. cold-rolled steel shaft to accept a washer and nut retainer can generate 0.021 lb of waste. Machining two grooves to accept SH-50 (1/2 in.) retaining rings, on the other hand, will produce just 0.003 lb of waste and use correspondingly less cutting fluid.

Cover plate and screws: Often, threads are machined into a housing to accept a cover plate and six bolts to retain a bearing. The machining as well as the bolts can be replaced by a single groove and one Rotor Clip internal retaining ring.

Cotter pin and washer:  You can hold a shaft to a brace by drilling a hole through it, installing a washer and fastening the system with a cotter pin. Or, you can machine a groove onto the shaft and install a retaining ring. It creates a “shoulder” that retains the component or assembly.

Set Screw Collars: Shaft collars are used to position and retain parts on a shaft. However, they are bulky and expensive. The set screw can also dig into the shaft when tightened causing damage. These collars can be replaced by machining a groove and using a retaining ring.

Lock Nut vs. Retaining Ring: Rotary unions perform the critical sealing function between fixed plumbing and machinery that is constantly rotating. Integral to this function is the inner cartridge, containing the seal and bearing, which must be periodically replaced. A threaded lock nut previously held the assembly in place. Removal required a special wrench. The customer replaced the threads with a simple machined groove. A Rotor Clip internal retaining ring replaced the lock nut. With this improved design, a much less expensive fastener replaced a costly, machined part. It also saved time in assembly for the manufacturer and eliminated environmentally damaging waste from the machining process.

AKH FAS-NER Systems
www.akhfas-ner.com

Bal Seal Engineering Inc.
www.balseal.com

The Orbitform Group
www.orbitform.com

Promess Inc.
www.promess.com