O-rings are ubiquitous, uncomplicated sealing solutions for millions of products. They seal moisture and environmental contaminants out, separate surfaces to mitigate friction, and keep in essential fluids. However, despite their seeming simplicity, O-rings used in aerospace take on an unexpected level of material and performance complexity when engineering and design standards and industry regulations are factored into purchasing decisions.

The number of O-rings found on an average commercial jetliner reaches into the thousands, and each must compensate for different environmental conditions – high and low pressure, aggressive lubricants and fluids, extreme temperature changes, industrial performance requirements, and sometimes challenging installation procedures.

With no one-size-fits-all O-ring solution, manufacturers must rely on their suppliers’ material, testing, and industry expertise to identify the best seals for every use.

Dr. Robert W. Keller, with 38 years of experience in the specialty elastomer and sealing products industry, joined Freudenberg-NOK Sealing Technologies in 2005. Following, he answers five frequently heard customer questions.

1) Can a supplier quote a single part or package of parts?

In some cases, this is straightforward, such as with the AS3209-214, which is an industry-standard size O-ring with an industry-standard materials specification and Qualified Products list requirements. There’s also a large variety of customer-part number standards and drawings with customer-approved sources of supply. This requires researching customer drawings, dimensional tolerances, and materials specifications to assure that we have the correct dimensions and are authorized to supply the part or material. The vast number of industry standards and customer part series with hundreds of specific sizes in multiple materials makes this one of the most complex challenges with aerospace O-rings.

2) Can you help diagnose O-ring failure via a picture?

We also want details on the mating parts’ dimensions and tolerances. We also need as much application detail that can be provided – temperature extremes, pressure extremes, fluids and gases the ring is exposed to, and possibly assembly practices. We typically want to get the failed part(s) back for microscopic examination which usually provides a lot of information. Our 3D microscopes can measure dimensions and depth of any damage or unusual surface conditions of the O-ring. This is a typical question concerning O-rings installed in critical applications.

The most prevalent cause of leakage we see is installation damage. The O-ring is typically installed over metal splines, which can cut or gouge its surface. Likewise, misalignment of mating parts can cause localized pinching or cutting of the O-ring surface. Considering the millions of critical aerospace O-rings we ship per year, the actual incidence of failure is less than 0.04 parts per million O-rings produced and shipped.

3) Can you advise if an O-ring and gland design is appropriate?

Gland and O-ring designs that are very old and have been in place for many years should be reviewed. While there are many O-ring design software packages available, they don’t always take into account many of the variables and peculiarities of actual customer gland designs. Some software can also introduce numerical rounding and truncating, resulting in calculation errors. We can typically do a more thorough analysis with our in-house calculation routines and can factor in temperature extremes and coefficients of thermal expansion of the mating gland and O-ring material to get a more detailed look at the designs. Our in-house calculation routines can perform ‘what-if?’ calculations, and we can look at and recommend changes to gland design, O-ring dimensions, O-ring dimensional tolerance, or material recommendations.

Since O-rings are relatively inexpensive and physically compliant, we’ve seen them used as soft, conical seals or high-force shut-off seals in high-pressure hydraulic systems. This type of non-traditional application is considerably beyond what off-the-shelf software packages can calculate. Often, we need to perform finite element analysis (FEA) to fully understand how the O-ring will perform in these applications. So, we need as much detail about the application as possible – mating part dimensions and dimensional tolerances; materials the mating part is made from; fluid contact; pressure extremes for normal and proof testing; and temperature extremes. We have refined our techniques for elastomer property modeling and have found our FEA to be very accurate in predicting performance.

4) With typical fluids or gases that could interact with the O-ring, what would the best material be?

Parent industry standard materials specification or customer materials specifications will often define the material in sufficient detail. However, there are still questions that the industry standard or customer standard doesn’t address in detail. Our extensive library of the chemical compatibility of our materials with various fluids and gases is our first point of reference. If our library information doesn’t sufficiently answer the question, our laboratories can test to define the best elastomer material for the application.

It’s not uncommon for fluids, temperatures, and pressures to produce situations where no material exists in our current portfolio or our competitors’. We have extensive expertise and laboratories for developing newer, high performance materials. We typically pursue major aerospace materials development projects annually and we are currently finalizing development of an ethylene propylene diene monomer (EPDM) material with improved low-temperature performance for aircraft hydraulic systems and a fluoroelastomer (FKM) material with improved low-temperature flexibility for high-pressure aircraft fuel systems.

Enhanced low-temperature performance is currently a major technology driver in aerospace O-rings for higher altitude flight with prolonged low-temperature soak periods. Since the design is simple, most of the O-ring technology is in the material and material performance.

5) If an O-ring was designed to seal fluid X, but some customers use fluid Y in the field, what is the risk and should we replace the O-rings?

As much detail as possible about the different fluids, pressure extremes, and temperature extremes is typically needed. We may run cyclic accelerated aging studies to look at exposure to fluid X followed by exposure to fluid Y then back to fluid X or a mixture of both. We look at standard properties such as volume change, property degradation, or compression set resistance. Test results will determine if O-rings must be fully replaced or not.

Freudenberg-NOK Sealing Technologies

Adobe Stock; Photos/images
courtesy of Heidenhain

Many 5-axis control users only access a tiny fraction of the power at their fingertips. The demand for quicker, more efficient machining of complex parts has been met with powerful 5-axis CNC control functions, and the full features are astounding. As research and development (R&D) continues in this area, expert tips are emerging – CNC control users now have access to many tool compensation forms unavailable before. These 5-axis machining tools enable CAM programmers to better control their project. One underused control technique to increase flexibility is vector programming.

A vector is a directed variable made up of components in the X, Y, and Z axes – which specify where the beginning and end points of the vector lie – that indicate known direction and length. However, sometimes code will be output in a CNC control without the tool orientation (TXTYTZ) or the surface normal (NXNYNZ). When pieces are missing, flexibility is limited.

There are nine components that make up one line of standard code and the M128, which tells the control that it is in full 5-axis compensation. A normalized vector is a vector with a value of 1 – vector value is calculated from the root sum of the squares of its components.

With vector programming, users don’t need to re-post the program or guess the compensation when a tool wears or a new tool is loaded into a machine. Compensation is made for re-worked tools and, if the correct output is generated, can be used to create uniform oversize or undersize on the workpiece, regardless of the tool’s contact angle. All these benefits result in less machine maintenance and more accurate parts.

The following are 5-axis programming tips for CAM vector output.

1) Compensate a re-worked tool

With peripheral (swarf) milling, the operator shouldn’t have any problem if using cutter compensation in standard code format (G-code/Heidenhain code) or if using vector output. Users can use any form of vector output because compensation takes place as normal/orthogonal to the tool’s contact edge if not specified. In this case, normal to the tool axis (TXTYTZ) or having surface normal (NXNYNZ) will yield the correct result. If only tool axis vectors are available, the user must use radius compensation and specify the direction of the cutter (cutter comp. left or cutter comp. right) because there isn’t a normal vector to specify the proper direction.

Face milling involves using all parts of the ball end mill (L/R). Think of a tool moving back and forth 3x or 5x across a 3D dome. At the top of the dome, the operator is using more of the bottom of the ball. At the sides of the dome or near the bottom, the operator is using more of the radius or edge of the ball. Many mold finish passes use this technique. If they then want to compensate a re-sharpened tool, perhaps running along and using the L and R of the tool to cut, they will need a nine-place output vector code, allowing the user to compensate along the normal vector and the tool axis.

2) Create a constant oversize/undersize

When vector code isn’t used, problems such as gouges on the machined surface can occur because the machine doesn’t have enough data to make the proper moves.

If the operator is running 5-axis code, wants to create a constant oversize or undersize, and is using all of the ball end mill (face milling), the most efficient solution is vector code. The programmer should specify the radius and length oversize using delta length (DL) or delta radius (DR). Most likely, these specifications will be the same if using a face milling technique. This will ensure a constant removal or oversize of the machined surface.

3) Compensate using tool contact angle

Every ball end mill has slight deviations in its radius, meaning the radius on a 6mm ball end mill is never uniformly 3mm around the center point. A Heidenhain control can measure part deviations in the radius using a laser tool measuring device inside the machine and can find as many as 30 points to define the radius. This is a special cycle, normally Blum cycle 588 (L/R/DR/DR2). This information is tabulated in a table designated DR2.3DTC and requires a vector program with surface normal blocks (NX NY NZ). For each tool angle, the control reads the appropriate angle and deviation on the ball to create a more accurate surface.

When an angle lies between two data points, interpolation takes place. This can allow precise mold makers, aerospace manufacturers, and medical manufacturers to get closer to true part surface integrity. Validation can take place with Cycle 444 (probe free form surface) and the results should be closer to nominal part surface.

4) Use IS Contour

With Heidenhain’s IS Contour function, the leading point is where the tool touches the workpiece, enabling the user to see the points with coordinates to describe the surface. Without this, operators or programmers don’t know where they are because a vector program is driving the tool.

Output in the CAM system must be tool-neutral – the surface points must be output with N vector or T vector.

In addition, the user can read contour points describing the surface directly from the program without having to calculate back to the surface coordinates. The user can also use a point directly from the NC program and check it with a measuring cycle to see if it is within a certain tolerance. A good cycle to validate the surface is cycle 444 – probe-free form surface.

In the actual value display of the Heidenhain TNC, the user will always see the tool leading point.

If a 5-axis control operator is programming finish passes in vector format and uses a reworked or re-sharpened tool, there is no need to recalculate or post new code to the machine – proper vector output takes care of this, providing improved accuracy and time savings.

Heidenhain

In milling machines, retention knobs used with tapered toolholders can engage drawbars that secure the toolholder firmly in the spindle. The tapered toolholder has threads designed to accept the retention knob, but improper seating of V-flange toolholders lowers productivity and causes runout and vibration that reduce machining precision, tool life, and spindle life.

Machining exceeding 20,000rpm spindle speed is often used in aerospace, medical, and other industries that process exotic alloys and harder metals such as titanium. At higher speeds, precise and secure seating of tapered toolholders in the spindle (with the help of the retention knob) is even more critical. Failing to pay attention to this detail can be costly.

Avoid failures

Strong, well-manufactured retention knobs increase setup rigidity, increasing tool life and speeding cycle times. Beware of retention knobs made of low-quality steel that may be prone to break during machining. Newer CNC milling machines exert significantly more drawbar pressure on retention knobs than in the past. As drawbar tension increases, so much pressure is put on the retention knob that it may be more likely to break if not manufactured to the highest standard.

Retention knob failures may occur due to other reasons, including improper knob configuration, misaligned machines, metal fatigue, over-torquing, and exceeding machine capability. It’s essential to periodically inspect each retention knob for signs of excessive wear, damage, or corrosion, and if found, replace it.

“We deal with a lot of aerospace companies, some with high-speed CNC applications with harder metals or alloys,” says Bob Selleck, manager at SAG Supply, a Long Island, New York-based provider of high-quality industrial tools, machinery, and accessories. “For that, you need a reliable retention knob that will last without breaking or moving. It needs to hold everything tightly in place with precision.”

Although retention knobs often look very similar, they are not interchangeable. Materials, applied coatings, and manufacturing quality can be important differentiators. Another consideration is where the item is manufactured.

“Some [retention knob] imports do not always fit snugly or precisely in the flange and may wobble or move a little bit under high-speed applications,” Selleck adds.

The most reliable retention knobs are those made with superior steel and coated with black oxide to prevent corrosion. In addition, the retention knobs should be subjected to the most rigorous quality control standards, including frequent inspection by the manufacturer.

Selleck points to Ohio-based high-quality retention knob manufacturer T.J. Davies, a company s that uses certified 86L20 and 9310 steel drawn in the United States. He adds that SAG Supply carries T.J. Davies retention knobs for their reliability at higher spindle speeds during CNC machining.

A low alloy nickel, chromium, molybdenum case-hardening steel, 86L20 has high hardenability without temper brittleness along with good external and internal strength and wear-resistance. If CNC operators need a higher quality steel, retention knobs are available made of 9310, a low alloy steel composed mostly of nickel and chromium. This alloy also has high hardenability, core hardness, and fatigue strength, making it ideal for heavy-duty machinery.

“We have found the domestic T.J. Davies retention knobs to be of higher quality and more reliable than imports,” Selleck says. “They do not break; they last longer even at higher speeds with harder metals. We have carried them for more than 10 years, and I have never heard of a problem or complaint.”

Black oxide benefits

Long-term reliability also depends on corrosion prevention so each retention knob is coated with black oxide. The top layer of the ferrous material is treated with a chemical that converts the surface to prevent rust. Black oxide also increases abrasion resistance while providing a decorative finish.

If a retention knob ever needs to be replaced, it also helps to have quick shipping from a domestic supplier rather than dealing with lengthy overseas shipping and logistics. T.J. Davies can ship items the same day with no minimum order and will drop-ship to end users on behalf of distributors.

“When I need retention knobs, I do not want to wait for overseas shipping,” Selleck adds. “When I order domestically, I usually get what I need within two days.”

Because contract machine shops and original equipment manufacturers (OEMs) require CNC precision and productivity at all times, using reliable retention knobs made with superior steel, coated with black oxide, can serve as a competitive manufacturing and production advantage.

SAG Supply Co.

T.J. Davies Co. Inc.

Complex parts with multiple features to be machined demand high quality computer aided manufacturing (CAM) software to manage processes, but designers still develop simpler parts with manual processes.

Engineers at Umbra Cuscinetti S.p.A., however, decided that using CAM on all parts could optimize operations and ensure the company was using the best possible approaches for even the simplest products.

Since 1972, aerospace supplier Umbra Cuscinetti has designed and manufactured ball screws for aircraft. Umbra also manufactures electric spindles, milling heads, and electromechanical actuators for aerospace and industrial applications. The company’s growth has transformed its parent company in Foligno, Italy, into the multinational UmbraGroup, with four offices in Italy, Germany, and the United States, and a worldwide workforce of more than 1,000 employees. Most of the company’s employees are located at the Perugia factory in Italy and handle the design and manufacture of ball screws from 10mm to 10m long, mainly intended for aviation.

“We export more than 90% of what we produce here in Foligno, Italy to every airline in the world. Every airplane with more than 100 seats has at least one component inside it that has been made in our factory,” says Angelo Radicioni, production manager at Umbra Cuscinetti.

Nowadays, digital drawings are usually provided for production, but many tools and CNC programs for manufacturing simple components are still manually developed, with more complex parts being machined using CAD/CAM software.

“The goal we set for ourselves was to implement a CAM solution for even the easiest parts in order to provide us with the total cycle time to determine the overall efficiency of our processes,” explains Mauro Trasciatti, production supervisor at Umbra Cuscinetti.

Umbra implemented Esprit CAM software by DP Technology for improved CNC programming speed and increased productivity on the shop floor.

High performance CAM

To meet the demands of customers and members of Umbra’s internal design team, the company increasingly pushed for more advanced machine tools to reduce cycle time and improve productivity.

“We needed a machine tool solution that would allow flexibility and speed in standard operations,” says Gianluca Mercuri, turning & milling specialist at Umbra Cuscinetti. “Esprit CAM software provided us with everything we needed to run our advanced machine tools and gave us the ability to save customized milling and turning operations, both for the processes we use most as well as our non-standard processes.”

The field of electromechanical actuators is growing fast for the company so they continue to advance internal and external turning processes and multi-axis milling.

“One of the most complex operations that we accomplished with Esprit CAM was wrap pocketing using a simultaneous 4th axis,” Mercuri adds.

With Esprit CAM software, the company has optimized their CNC machining centers and benefited greatly from certified post-processors. This has allowed Umbra to access full functionality of their CNC machine tools, increase their productivity, and continue to make the high-quality parts their customers demand.

DP Technology Corp.

UmbraGroup