High vacuum and UHV
A practical guide to the pockets a fastener traps, the hardware that empties them, and the platings and dry films that keep stainless and other alloys from galling in vacuum.
Where the gas hides
Ordinary screws in blind holes are a common limit on pump-down. The fastener does not leak through the chamber wall. It traps a pocket of air, water, and whatever else was in the hole at assembly, and that pocket can reach the pumped volume only through a very small path. The pressure trace looks like a real leak. A helium spray from outside the chamber finds nothing. The gas keeps coming, slowly, and a joint that shifts with bakeout or vibration can release another burst later.
A vent is useful only when it connects the pocket that is actually trapped to a surface the pump can see. One style of vented screw does not cover every pocket.
Published comparisons of the same bolted joint with and without a vent path show shorter pump-down and lower base pressure. Take that as evidence that the pockets matter. It is not a rating for a particular chamber.
Five pockets
- End volume. Past the screw tip, at the bottom of a blind hole.
- Thread void. The flanks never fill the tapped form. A long channel runs the engagement.
- Side volume. Beside the screw: a counterbore, a relief, or the gap along an unthreaded shank. The hole can be open at the far end and this pocket can still be sealed.
- Head volume. Under the head, or under a plain washer, once the head clamps down.
- Face film. Between two broad mating faces. A load plate on a bearing block is the usual case.
Blind hole
Open hole, trapped side
Clamped faces
Purple is the end volume. Teal is gas beside the screw or under the head. Gold is the film between two faces. The thread void is the helix itself, too thin to draw at this scale.
Center-vented screws
A center-vented screw has a small hole on its axis, from the head through the threaded end. This is the usual catalog vacuum screw. It is the right fastener when the trapped gas sits at the bottom of a blind hole that the screw tip opens into.
What the bore reaches
Advantages
- Direct path out of the end volume. Air and water at the bottom of the hole pump out through the head instead of weeping up the thread.
- The outside is a normal thread. Same tools, same feel, usual head styles in inch and metric. Other heads can be made.
- On a simple blind hole, pair it with a vented washer.
Disadvantages
- The bore misses any pocket that does not open onto the screw tip: a side volume, a closed counterbore, and the gap under the head.
- Gas in the thread still travels the helix to reach the bore. A side slot vents the threads more directly.
- The bore reduces the screw's load-bearing section. Keep it small; on small screws this is the limiting factor.
- Burrs, plating, and debris can close it. A closed bore is no vent.
Side-slot screws
A side-slot screw has a narrow groove along the outside of the thread. On a proper vacuum slot the groove continues across the underside of the head. Gas in the threads, in the side volume, and under the head then has one continuous path into the chamber. Leveling feet, motor mounts, and gusset screws often need this, because the trapped gas sits beside the fastener rather than beyond the tip.
Slot carried under the head
Advantages
- Vents the side volume a center hole misses, including a screw whose tip never opens into the pocket.
- Carried under the head, one feature covers the end volume and the head volume. A separate washer is less critical.
- The cut is at the outside, so the loss of section is smaller than a center hole of useful diameter. Prefer this when the screw is small or highly loaded.
- The slot can be inspected before the screw goes in.
Disadvantages
- The slot crosses thread crests and leaves a row of edges. Those edges gall and shed particles, especially stainless on stainless. Coat slot-vented screws.
- Less thread contact where the groove runs, and the screw may start differently. The groove is a designed feature.
- The channel is enough to empty a virtual leak. It is not a roughing port.
- A slot that never reaches under the head still leaves the head volume trapped. Once tight, a side slot is easy to mistake for an ordinary screw.
Vented flat washers
A vented flat washer has radial grooves in one face, sometimes both. Tightening the screw no longer seals the top of the hole. Without that path, the same screw can land sealed or leaking, depending on the head and the surface. The leak rate is then a matter of chance, and it changes if the screw relaxes a few degrees.
Grooves under the head
Advantages
- Opens the head volume and the clearance between parts, which a center vent does not touch.
- Smooth faces. A vented flat washer does not scrape the way a split lock washer or a star washer does.
- Catalog parts, in the same materials and finishes as the screws. The usual simple joint is a center vent plus one of these.
Disadvantages
- Does not vent the bottom of a blind hole. The end volume still needs a center vent, a side slot, or a weep hole.
- Grooves reduce bearing area. Check contact stress on a soft part or a small head.
- Grooves on the wrong face, or grooves packed with plating, seal the head again.
Split lock washers and external star washers sometimes leave a leak path. They are a poor substitute. The split edge and the star teeth generate particles as the screw turns, and the path they leave is accidental. Where a washer is required, use a vented flat washer. A nut traps the same kind of volume. A vented washer, or a slot across the nut face, covers it.
Weep holes
A weep hole is a small drilled passage from a pumped surface into the trapped volume. The fastener can then be an ordinary, full-strength screw. Use one when the pocket is in the part, not in the screw: beside a fastener, in a closed counterbore, or under a plate. It is also the practical fix when the joint has to keep a specific commercial screw.
Advantages
- Reaches a pocket no screw vent can see.
- The screw keeps its full section and its full thread.
- Can be added to a hole that is already machined.
Disadvantages
- The hole has to intersect the real pocket and exit into vacuum. A hole that stops short does nothing.
- Another burr, another surface to clean, and another place for plating to close.
- Hidden after assembly. If the drawing is silent, the next person cannot confirm it and may machine it away.
- On a thin wall it can break into a seal face or a water, gas, or cryogen passage.
Pump-out grooves
Two flat faces clamped together trap a film of gas over the whole contact. No screw vent reaches the middle of that film. A shallow groove, or a deliberate scribe, has to run from the contact out to a pumped edge. Together with a vent on each blind hole, that stops the face from acting as a virtual leak. Bearing blocks, load plates, gussets, and stacked spacers are the usual places.
Advantages
- Treats the contact a fastener cannot vent.
- A machined groove has a known depth and a known exit. It is still there at the next teardown.
- Can be added when the faces are accessible.
Disadvantages
- Cuts bearing area, and can change stiffness, thermal contact, or a ground path. Put it where the load does not need the full face.
- A light scribe has almost no conductance. A deep groove becomes a dirt trap and a stress raiser.
- Invisible once clamped. The drawing is the confirmation.
Through holes
When both ends of the fastener see vacuum, a clearance hole through the part traps no end volume. Use ordinary hardware, still cleaned for vacuum, and still finished against galling if the materials need it. This is the preferred joint when the geometry allows it. It is not available on a blind hole, behind a captured nut, or where the screw bottoms on a shoulder.
Choosing a scheme
Match the vent to the pocket. Use more than one method when the joint has more than one pocket.
| Trapped volume | What reaches it | What misses it |
|---|---|---|
| End of a blind hole | Center vent, side slot, or a weep hole into that end | Washer alone, face groove |
| Thread helix | Side slot along the engagement | A center hole, until the gas has traveled the helix |
| Annulus beside the screw | Side slot, or a weep hole into that annulus | Center vent |
| Under the head | Side slot carried under the head, or a vented washer | Center vent alone |
| Large flat contact | Pump-out groove or scribe to a free edge | Any screw vent |
A center-vented screw and a vented washer cover a simple blind hole. A side-slot screw covers a blind hole plus the side and head volumes. A weep hole or a face groove is added when the pocket is in the part.
Finishes
Venting and finish are specified together. Stainless against stainless in vacuum galls. The oxide that prevents cold-welding in air is not replenished, and a tight thread can seize on the way in or the way out. A side slot makes that worse by adding edges. Oils and hardware-store anti-seize are not a fix inside a baked UHV system.
MTS can have hardware plated or coated for vacuum service with silver, gold, MoS2, WS2, hBN, or DLC. The finish has to leave the vent open, and the added thickness has to stay inside the thread allowance.
Plating
Silver
The general-purpose anti-gall finish for stainless vacuum fasteners. Soft, conductive, and it smears instead of cold-welding. Tolerates bakeout. The film is relatively thick, so thread class and the vent are part of the plating spec. Silver transfers onto the mating thread. Some processes cannot accept it. Tarnish from air is mostly cosmetic.
Plating
Gold
Stays free of oxide. Choose it when the fastener is also an electrical contact, and when a noble surface is worth the cost. A good anti-gall film, typically thinner than silver, and soft enough to transfer if the joint is cycled hard. A poor choice where abrasion dominates and a hard film would last longer.
Dry film
MoS2
Molybdenum disulfide, the usual dry film on vacuum screws. Friction in clean, dry vacuum is very low. Binderless films are the UHV form. A resin-bonded coating can outgas. Humid air raises the friction and ages the film, so this is a weak choice for hardware that lives in air between runs. Dark, not an insulator, and it transfers. Debris matters near optics and wafers.
Dry film
WS2
Tungsten disulfide does the same job and is often the better choice when the hardware also lives in humid air, or runs warm in air. Films are very thin, so thread fit and the vent barely move. Friction in hard vacuum is low. MoS2 has lower friction in the driest service. A transfer film, and not an insulator.
Dry film
hBN
Hexagonal boron nitride is the lamellar lubricant that does not conduct. Use it when the joint must stay electrically isolated, or when a dark sulfide film is unwelcome. It remains a lubricant at high temperature in vacuum and inert gas, past the point where MoS2 oxidizes in air. Friction is higher than MoS2 in vacuum. Specify a thin, adhered film. Loose powder sheds.
Hard coating
DLC
Diamond-like carbon is a hard vacuum-deposited film, not a soft plating. It cuts adhesive wear and particles on threads and fits that come apart more than once. Hydrogenated DLC can be very low friction in dry vacuum and then lose that in air, and the hydrogen belongs in a strict residual-gas budget. Many grades graphitize at a few hundred degrees Celsius, so bakeout picks the grade. Hydrogen-free films are harder, highly stressed, and thin. Stainless needs a bond layer. Thickness must not close the thread or the vent.
No single finish is the vacuum finish. Silver or MoS2 covers most stainless fasteners. Gold is for clean electrical contact. WS2 is for hardware that also lives in air. hBN is for electrical isolation. DLC is for wear and repeated assembly.
How MTS helps
These rules are already part of how MTS designs the in-vacuum end of manipulators, sample holders, heaters, and transfer tools. The same help is available on your parts.
- Review the joint, from a drawing, a model, or the hardware, and name the trapped volumes.
- Choose center vents, side slots, vented washers, weep holes, and pump-out grooves, including combinations.
- Source catalog vented screws, washers, and nuts in inch and metric sizes, in the head style and material the joint needs.
- Manufacture custom vented fasteners when a catalog part does not fit, and machine weep holes and pump-out grooves into your parts.
- Have the hardware plated or coated for vacuum service with silver, gold, MoS2, WS2, hBN, or DLC, with the vent left open and the thread still in class.
- Clean and pack the hardware for vacuum installation.
Send the joint
Include the screw size, the materials, a sketch of the blind features, the bakeout temperature, and any limit on the finish: silver-free, particle-sensitive, electrically live, or electrically isolated. We will recommend the vent and the finish before anything is made.
Contact the factory for sizes, materials, coatings, and custom vent features.