When it comes to designing and developing seals, the aerospace and industrial industries need a basis to allow production anywhere in the world.
One of the first PTFE (Teflon) standards, AMS3678, describes Teflon and the addition of fillers. This was used in conjunction with Mil-R-8791, which is one of the Mil specs describing a backup ring device.
The origin of all these specs dates back to the creation of the O-ring.
In 1939, Niels A. Christensen was granted a U.S. Patent for “new and useful improvements in packings and the like for power cylinders.” These referred to improved packing rings made of “solid rubber or rubber composition very dense and yet possessive of great liveliness and compressibility.” These products were suitable for use as packings for fluid medium pistons (liquid or air). The improved packing ring is the modern O-ring.
There was a progression of standards for the O-rings created by individual countries, such as AS568, BS 1806, DIN 3771, JIS B2401, NF T47-501, and SMS 1586. Eventually, AS568 became more accepted in the industry.
The backup ring was originally created to help improve the O-ring’s ability to resist extrusion. Teflon was widely used as one of the materials for backup ring devices. Standards were created to unify the production of this Teflon device.
The Progression of Mil Specs
The progression of standard changes has led to AMS3678/1 for Virgin PTFE through AMS3678/16. These standards describe a group of Virgin- and filled-PTFE materials accepted by the industry for manufacturing seals and back-up ring devices.
Mil-R-8791 was canceled in February 1982. This spec was superseded with AS8791, which eventually evolved into AMS3678.
AMS3678 is a tool used by customers and Teflon suppliers to create uniformity in the manufacturing and processing of seal and bearing materials. The standard is inclusive of most of the compounds upon which the industry was built.
When customers approach with an old “mil spec”, they are pushed to the new AMS spec which is currently active. Eclipse manufactures to the spec so their customers will have the confidence that they manufacture to a known standard.
When crossing custom materials from well-known sources, customers are driven to an accepted spec that is equivalent to the original source of the material. This helps customers sell their products with internationally-known materials rather than custom, home-grown compounds that are often intended to single source those materials.
There are several qualifications of the spec that suppliers must observe. This includes dimensional stability tests. This test ensures the material has been properly annealed, and that the seal or backup ring will fit and function as it was originally intended.
Eclipse is uniquely qualified to supply parts to the latest AMS3678 specification. They understand the scope of the specification which allows us to ship parts with fully traceable certification.
AMS3678 helps validate a material to a customer to ensure they get the same material processed the same way with each order. Beyond this, there are other ways to determine what makes a part process-capable.
Making a Part Process-Capable
Machining plastics is as much a skill as it is an art form. It takes understanding that whenever you cut a part, it will probably have some motion or energy still within the material.
This is largely because our parts have more rebound than steel in the cutting process. And while plastics are mostly thermally stable, they’re not dimensionally, thermally stable.
Changes in temperature from the time the part is machined, inventoried, and put into service show that parts are constantly changing in size. PTFE or Teflon suffer the greatest change in thermal instability where we machine the part, around 74 degrees Fahrenheit.
What Makes a Machining Operation or a Specific Dimension Process-Capable?
You might think that if you can make a part and verify that it’s intolerance, then the part is process-capable. But the reality is that a large tolerance range doesn’t make a part process-capable.
At Eclipse, a rigorous process is performed to ensure parts remain process-capable after machining is complete.
By measuring the parts after a run and reviewing all the critical dimensions, standard deviation for a dimension is found. Then an estimate can be made with a high degree of certainty if, during the process, there will be parts with dimensions that will fall outside the tolerance range.
Eclipse’s with this process helps ensure accuracy. But without doing an actual analysis based on real parts that would actually run on a job, it can’t be known with complete certainty if a particular dimension will be within tolerance.
One thing is certain: If you dimension a plastic part and apply metal tolerances, it will most certainly fall outside the process-capable range for that part.
Certain materials like PEEK will hold tolerance during a machining run far better that Teflon. But there are some dimensions we hold in Teflon which can be maintained to a high degree.
Usually, if a part is designed with Eclipse’s standard manufacturing tolerances, it will certainly have a bigger tolerance range than if it’s designed with steel. Most often, making the tolerance range smaller has little to no impact on plastic parts. Under pressure, the parts get pushed around, and are constantly changing shape to some degree.
So why bother with tolerancing at all?
Eclipse designs the part to fit the application, so products have to fit into the required hardware. And depending on the type of elastomer, tolerances may need to be held on a particular dimension so the seal will perform in the entire range in which a customer wants to use it.
What Is and Isn’t Critical
When looking at why a seal preforms, the wall dimension (cross-section) generally has the most influence on success or failure. This is due to the seal being energized by either an O-ring or a spring, and the impact of the extrusion gap on the sealing system.
The width is usually the second most important factor, as the seal must be able to move freely in the gland. The least important dimension is usually the diameter. The seal needs to sit in the gland so that it can be assembled; but in general, this is the one dimension in which you’ll see the greatest change due to temperature.
Unless the application is operating in a very cold environment, the diameter must allow the seal to sit in the gland and not protrude too much for installation. Tolerances on large diameter seals (over a couple of feet) often are +/-.060 inch. This may seem like a lot, but throw in a little temperature and you find that fluctuation at this scale isn’t impossible.
How Do We Determine Process Capability?
The use of process capability calculations became common for suppliers to the automotive industry due to the large quantity of parts being made, and the need for upper-tier suppliers to know with certainty that they were getting good product.
A process capability study will tell you how likely the process is to make a good part. It’s enough to know that given dimensional data from a run, equations can be used to find out a number that will indicate how likely it meet print tolerances.
The main index that is looked at is labeled “Cpk”. The Cpk is calculated for critical dimensions using the tolerance limits and the data from a sample run. The higher the Cpk value, the more capable the process and the likelihood of making good parts.
Industry standards aim for a value of 1.33 for industrial parts and 1.67 for aerospace. In order for a process to reach high Cpk levels, the parts must be made using a small portion of the tolerance range and run at the middle of that range.
Generally speaking, a Cpk less than 1.0 indicates that the process isn’t capable, and is likely to produce parts that are out of print tolerance.
Process Capability Put to Use
How does Eclipse make use of Cpk? In practice, the machinery, tools, and processes are all known well before a part is designed. So putting a non-capable tolerance on a print will place undue burden on the inspection required to supply parts to that print.
A much better approach is to consider the capability of the process first, allow for that variation, and then design the part to accommodate the required tolerance. Having run Cpk on all our processes, Eclipse has an excellent understanding of what is process-capable and uses that knowledge to design and manufacture seals accordingly. In the case of customer-designed parts, they can offer process-capable tolerancing in quotes.
Gallagher Fluid Seals is a preferred distributor of Eclipse Engineering. Call us at 1-800-822-4063 for more information on Eclipse seals.
Article written by Cliff at Eclipse Engineering, Inc. For the original article, visit their website.