Inexpensive, reliable sealing of large UHV ports utilizing a progressively-deformed wire.

J. Savino, E. Fontes, R. Seeley, S. Smith, E. Kathan, J. KopsaCornell High Energy Synchrotron Source

AbstractOn SR beam lines, pressure levels approaching ring vacuum are necessary in large vessels having large access ports. Metal gaskets and sealing features for such large ports are often complex and expensive, and installation requires special care and skill. A method is herein described where an ordinary aluminum wire and an ordinary flat-surfaced enclosure lid are fixtured with simple jigging. The enclosure lid is then intentionally elastically flexed, capturing the wire in a manner that causes a controlled, progressive deformation. The method and fixturing result in simple, rapid sealing of large enclosures as performed by personnel with ordinary levels of mechanical skill. Fabrication costs are significantly reduced. Design features enhancing the utility of such wire-seal joints are presented.

UHV sealing of large doors and ports has often been a problematic and costly undertaking, with complex schemes still in common use. Inexpensive wire seals on ordinary flat surfaces would seem to be a desirable solution, but numerous problems make wire seals troublesome on the larger scales, especially where the seal is on a vertical surface.As CHESS progressed from elastomer O-ring seals to UHV seals on optical enclosures, a more practical method of sealing was needed. It was also desirable that the new method be retrofittable to existing boxes with the O-rings removed. The advantages of wire sealing were tempting, but certain problems would have to be overcome. These included:

• wire elongation resulting in sagging and uncontrolled extrusion between random “pinch points” (Fig 1) • damage to the wire as the result of unintended motion of a heavy lid, and • general uncontrolled motion of the wire away from the desired sealing surface.

Thus, some means of keeping the wire’s position under control as it elongated and deformed during the clamp-up operation became the central design challenge.

Fig. 1Uncontrolled pinch.

Wire capture is random, causing

sealing wireto sag away from sealing surface

and extrude into a bolt hole.

The inherent flexibility of the heavy stainless-steel lids used on these vessels is not intuitively obvious. However, the larger lids, supported on the four corners, can be measurably deformed in the center by the small force exerted by one’s hand. This characteristic is exploited in the new sealing method. (Fig 2). Experiment revealed that a controlled elastic deformation of the vessel lid, and subsequent relaxation back to the flat condition moving from the center of each edge outward to the corners, could be used to effect a progressive capture of the sealing wire over almost any practical length. It completely controls the wire and obtains a reliable UHV seal.

Fig. 2Elastic deformation of a lid suspended at corners

and bolted at center(highly exaggerated

for clarity; bolts not shown)

The new method can be summarized as follows: Stretch the wires straight, mount the lid slightly off the wire, then use the inherent elasticity of the lid to roll the sealing wire into the corners, keeping tension on the wire as it is captured.

The key steps are:• Soft aluminum seal wires are accurately positioned over flat sealing surfaces by fixtures. Springs stretch the wires initially straight, and maintain the straightness as the lid is clamped up. (Fig 3)• The vessel lid is positioned directly over the wires without clamping or even touching them. Instead, fixing the corners with ordinary set screws gives the lid a very small clearance over the wires and prevents it from touching them. (Fig 4)• The lid is then fastened lightly in the centers of each run. This pinches and captures the wires in the center and only in the center. Since the lid corners are mounted with some clearance, this fastening causes the centers of all edges to bend inward slightly. (Figs 2 and 5). We are not trying to crush the wire at this stage, only to capture it and fix it in the right position. • The lid bolts are then further installed and lightly tightened, working outwards from the center to the corners. This progressively captures and crushes the sealing wire and causes it to elongate. • Spring tension takes up this elongation, keeping the remaining free wire taut, straight and correctly positioned on the sealing surface.• As the wire capture reaches the corners, the corner set screws are removed, allowing the lid to fully return to its original flat state. • The corner bolts are installed and all bolts fully tightened. The seal wire pressure-welds to itself where it crosses in the corners.

Fig. 3Seal wires positionedover flat

sealing surface.Rotatable arms enable exact positioning.

Springs maintain tension

on wire.

Fig. 4Cutaway of hold-off setscrews installed in box (in back)

and lid (in front). Clearance at wire crossing point is exaggerated for illustration.

Fig. 5Top view of initial clamp-up. Box centerline at left, hold-off shown at right.

Bending is exaggerated for clarity.

Special mechanical design considerations will enhance the reliability and usefulness of this method, as follow. Note: Presence or absence of these features will determine whether this method can be reliably retrofitted to existing enclosures. • The sealing surfaces must be flat, contiguous, and made of (or surfaced with) stainless steel or other hard material. This method has been initially tested and shown to work on GlidCop AL-15. Aluminum enclosures have not been tested, but they may still be suitable if a very soft wire such as indium is used (at significantly greater cost), or a hard surface is provided. • The sealing surface should be smooth, but extreme flatness and surface smoothness are not required. A fly-cut finish of 0.8µm (32µin) roughness and total flatness on the order of 0.5mm with not more than 0.1mm occurring in any 200mm of length is sufficient.• Excessive rigidity of the vessel lid and sealing flange is actually detrimental and should be avoided. We have found 19mm (3/4”) flange thickness to be optimal.• The wire is 99.999% pure, soft aluminum, 0.75mm in diameter. Smaller diameter wires have given lower bolt torque requirements or allow the use of fewer bolts, but these have not been extensively tested.• Bolt spacing must be sufficiently close to generate enough force to crush the wire. Experience has shown that 37mm (1.5”) spacing of M10 fine-thread bolts is sufficient, with the corner bolts being M12 or larger. Extra-thick washers are used under all bolts. • Bolts should be closely spaced in the corners to allow for greater clamping force where the wire crossing point requires it. (Figs 4 and 6)• Stainless steel lid bolts must be silver-plated for resistance to galling and seizure. In applications where its use is permissible, DuPont Krytox 1645 oil (Pvap at 20C = 10e-12 Torr) used as a bolt lubricant will improve wire crush for a given bolt torque.• Use of hardened thread inserts in the flange threaded holes greatly reduces rework due to seized bolts. • Clearance holes in the lid should be generous. 0.8mm greater than bolt diameter has given good results.• Outboard of the wire crossing points, the remaining wire does not contribute to the seal. Crushing this excess wire adds significantly to the amount of force the corner bolts have to exert. It may therefore be helpful to “undercut” the box or the lid in this small region, so as not to contribute to excessive bolt forces. (Fig 6).• In new design, it may be useful to retain an O-ring groove to ease leak checking and allow quick access to other box internal components prior to the final wire sealing with O-ring removed. • Any leak checking of large enclosures should be done with the actual lid installed, and not with any temporary fixturing such as flat plates, etc. Inadvertent permanent deformation of large boxes has occurred when boxes were evacuated for leak-checking, but the lid was not present to play its part in resisting pressure loads.

Fig. 6Wire crossing point detail.

Undercuts reduce bolt torque requirement.

Close bolt spacing in corner enables easy pressure-welding of

the wire at the crossing point.

CHESS has not had any wire seals slip, roll, crumple or otherwise lose position and seal integrity on a sealing surface. There is reason to think that the high-pressure embedding of the aluminum into the stainless surface does afford some “grip” and is capable of transmitting some presently-unquantified level of force from box to lid without loss of seal. Nevertheless, additional design precautions may be taken as follow: • Whenever localized heat sources (i.e., scattering) are present, boxes should incorporate cooling features to prevent large temperature differentials from imparting shear loads to wire seals.• Stiffening members, lid-mounted load-transfer features, etc., should be considered whenever atmospheric-pressure-induced box or lid deformation may be sufficient to impart shear loads to wire seals.

Numerous UHV boxes sealed using this method have been in use at CHESS, some for 7+ years, without any leakage or other difficulty. Vessel size does not appear to present an upper limit; initial tests showed that the existing tooling can maintain practical wire straightness to lengths exceeding 4m. Training of personnel in installation of wire seals is straightforward and does not require more than ordinary mechanical skill. This approach significantly reduces vessel costs by allowing much looser tolerances, less material, less rigidity and fewer machined features on box sealing surfaces.

The authors wish to thank the CHESS Operations and Machine Shop Staff, whose many suggestions greatly helped this idea to reach a useful, practical level.

This work is based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR-0225180.

The Problem

The Solution