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Flanges & Fittings
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Q: Why use silver plated bolts?
A: Silver plating is an alternative to using a thread lubricant. The silver plating prevents bolt threads from seizing with its mating threads during temperature excursions. MDC recommends applying an anti-seize lubricant or using silver plated bolts in all metal seal applications.
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Q: Why have a "dished head" on a wire seal flange?
A: A dished head provides a blank flange without the significant weight that a solid blank flange would have. The dished head is usually 3/16-inch thick, whereas a solid blank ranges from 1.125- to 1.750-inch thick. Although the primary savings is in weight, a comparison of prices shows the dished head blank to be less expensive than the solid flat blank. The curved surface of the dished head absorbs the pressure of the atmosphere without deformation. In a similar manner, a cylindrical vacuum chamber can be constructed with walls that are much thinner than a rectangular chamber. Again, the savings are in a significant weight reduction.
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Q: Why are there no leak check grooves on Mini flanges?
A: The main reason is the difficulty of machining a groove into the small flange without damaging the knife-edge. End mill machining could be quoted as a special.
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Q: Why are there no leak check grooves on flanges larger than 10"
O.D.?
A: Industry standards for metal seal flanges include sizes up to and including the 10-inch OD (8-inch OD tube size) flange. Leak check grooves on the larger size flanges are omitted as a cost savings, but may be quoted as a special product.
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Q: Why use a gasket material other than copper?
A: While copper gaskets satisfy the vast majority of vacuum applications, some processes may use a gas that is corrosive to copper. In those cases, a gasket material that resists the effect of a gas should be selected. MDC offers nickel, aluminum and silver plated copper gaskets as alternatives to pure copper.
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Q: Can a copper gasket be used more than once?
A: Some users have reported success in using a copper gasket more than once. However, before attempting to reuse a metal gasket, know that the time required to disassemble a seal once a leak has been detected can be very frustrating, especially if this means interrupting a process or experiment. In most applications, there are more important issues to deal with than a faulty gasket. MDC recommends using a new copper gasket for each seal.
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Q: What is the difference between a matte finish and an electropolished finish on vacuum tubing?
A: A matte finish is a bead blast finish on tubing exterior only. The surface has a uniform dull appearance. Most fittings are constructed with a matte finish. An electropolished finish removes a small portion of the surface of the vacuum tubing, usually both inside and out, which results in a surface that traps less water vapor than an unpolished surface. In most high vacuum and many ultrahigh vacuum applications, a matte finish is acceptable. For ultrahigh vacuum applications requiring a minimum of trapped gases in a chamber's surface, an electropolished finish is desirable. Additional time and expense accompany electropolishing.
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Q: Why would someone choose a metal seal system over an elastomer seal system and vice versa?
A: The main reason for choosing a metal seal system is in the level of vacuum that can be achieved. Elastomer seals allow air and other gases to permeate through them faster than through copper, and therefore are restricted to high vacuum applications. Metal seals have lower rates of permeability and can be baked to higher temperatures than elastomers, and are therefore used for ultrahigh vacuum applications. If achieving ultrahigh vacuum is not critical to a process, an elastomer seal system provides high vacuum capability at the least cost.
Disadvantages of a metal seal system are single use of gaskets, more time required for assembly and disassembly, and a greater expense.
Disadvantages of an elastomer seal system are limitations in vacuum level and restrictions in baking temperatures.
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Q: What is a plate nut and where is it used?
A: A plate nut is a convenient alternative to standard hex head nuts when the space behind a flange is so limited that it is difficult to install a nut and perhaps even impossible to apply a wrench to a nut. Plate nuts eliminate the need for a wrench on the nut side of the flange. They are also made as thin as possible to allow installation in tight spaces.
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Q: Why use a "rotatable" flange if it can't rotate after being installed?
A: The outer portion of a rotatable flange, called the receiver, includes the bolt holes necessary for joining two flanges. The receiver can be rotated prior to insertion of bolts until bolt holes in the two mating flanges line up. The "rotatable" feature of a metal seal flange is for alignment purposes only.
For a component that can be rotated after installation, see the Rotatable Axis stages
section
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Q: What is the difference between a flexible coupling and a flexible hose?
A: The primary difference is overall length. A flexible coupling is used for short misalignment connections and is defined as a fitting less than 12-inch overall length. In general, a flexible hose is constructed of the same material, but starts at a minimum length of 18-inch and varies in wall thickness. Flexible hose is also available with a protective braiding to provide structural support and to minimize abrasion damage to the formed bellows.
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Q: What is the difference between formed bellows and welded bellows?
A: Formed bellows are made from a single continuous piece of very thin wall tubing whereas welded bellows are made of multiple leaves of very thin flat discs. The convolutions on formed bellows are smooth curves versus the sharp, welded edges of the welded bellows. Formed bellows offer some flexibility and are used mainly for static misalignment purposes. Welded bellows offer great flexibility and are used primarily in devices requiring repetitive motion such as valves and motion feedthroughs. Formed bellows can be used for some motion applications, but mainly where repetition and stroke are both minimized.
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