Prdouct Category

Tech Info

Typical dynamic Seals Used in Vacuum

Bellows seal

Principles. The bellows seal uses the principle of elasticity to transfer rotational force from the atmosphere side of the seal to the vacuum side.

Advantages. The atmosphere and vacuum sides are isolated from each other by the welded bellows. Rubber is not used, so baking at high temperatures can be performed. This type of seal is used in ultra-high vacuum equipment.

Disadvantages. The bellows and welded section undergo repeated expansion, contraction and twisting, causing damage from fatigue that shortens working life. The vacuum side must be fitted with a bearing.

Magnetic coupling

Principles. The atmosphere side and vacuum side are separated by a thin barrier wall. The magnetic force of the magnet on the atmosphere side rotates the shaft on the vacuum side.

Advantages.The vacuum and atmosphere sides are shut off from each other by a nonmagnetic barrier wall. Since rubber is not used, this type of seal can be used in high-temperature baking environments. This seal is widely used in ultra-high vacuum equipment.

Disadvantages.Motion is transferred by magnetism, causing idling if the load is great. Moreover, a bearing must be installed on the vacuum side.

Rotary feedthrough

Principles. A magnetic fluid is held in the gap between a shaft and a pole piece along magnetic flux lines generated by a magnet. The magnetic fluid held in the gap by the magnetic force does not flow out even in the presence of a pressure differentials, so it works like a liquid O-ring to form a seal.

Advantages. The seal is formed using a liquid, so contact between solids is avoided.
This structure virtually eliminates friction, yielding long service life, eliminating particle generation and supporting high rotational speeds.
Moreover, the vacuum-side bearing is moved to the atmosphere side, creating a cantilever structure that further ensures a clean ambient environment.

Disadvantages. The seal is for rotational movement only. It cannot be used as a translatory motion seal.

Basic Vacuum Concepts

Our concept of solids and liquids depends largely on our ability to see/touch them. If we have two lumps of solid, roughly the same volume and one lump is light while the other is heavy, we say the heavy lump has a higher density - mass per unit volume (lb/in 2, g/cc, kg/m3, etc.). Gases present a challenge to our ability to see/touch and new terms have been introduced to describe the "gaseous state".

Note: The gas laws used to derive the values quoted below are correct only for ideal gases. However, in room temperature chambers as pressure decreased, all gases approach ideal behavior. For vacuum applications, the appropriately scaled value - to allow ofr pressure change - will be sufficiently accurate for precise calculations.

Number Density

Avogradro determinesd that equal volumes of gas at the same temperature and pressure contained equal numbers of molecules. It does not matter if the gas is pure N2, CO2, Ar, H2, or a mixture of all four. Later, Loschmidt determined that 22.4 liters of gas at 760 Torr and 0 ° contain 6.022 × 10 23 molecules (the present day value, often called Avogadro's number).

Since gas fills any volume that contains it, its "density" (in g/cc units) depends on that volume, the gas composition, and molecular weights of the components. If instead of density (mass per unit volume) we use number density (number of molecules in 1 cc) we can describe a "quantity" of gas without knowing anything about composition or molecular weights. For Avogadro's number (which refers to 22.4 liters) we know the number density (which refers to 1 cc) of any gas at 760 Torr and 0 ° C is 2.69 × 1019 cm-3.

Mean Free Path

The huge number density at atmospheric pressure and the high velocities of the gas molecules mean that in each cc there are many, many gas phase collisions every second. Expressed another way, even though a molecule travels at high speed, on average it travels a very short distance before hitting another gas phase molecule. This average distance is called the mean free path (mfp). For air at 760 Torr the mfp is 6.5 × 10 -6 cm.

Particle Flux

In addition to colliding with each other in the gas phase, gas molecules hit the containing vessel walls and every other surface inside the enclosure.

The rate at which they hit these surfaces, called particle flux, depends on the gas's number density.

The flux of air at 760 Torr and 0°C is 2.9 × 1023 cm-2 s-1.

Flow Regimes

The mean free path (described above) and the chamber/ component dimensions determine the gas's flow conditions. If mfp is:

  • Very short compared with the chamber's 'characteristic dimension's' the gas is in continuum flow
  • Shorter than the chamber's characteristic dimension's but approaches them, the gas is in transitional flow
  • Equal to or longer than the chamber's characteristic dimension's then the gas is molecular flow

The flow regime is used to identify the appropriate equations needed to calculate conductances, pump down times, and other characteristics.

Vacuum Doesn't Suck!

There is a common misunderstanding that vacuum pumps suck. There is no such force as suction. If the gas molecules in one "section" of a vacuum volume could be instantaneously removed, molecules from the remaining section, in their normal high-speed flight, would randomly collide and bounce off walls until they filled the whole volume at a lower pressure.

For vacuum pumping this means, until a gas molecule in its random flight enters the pumping mechanism, that molecule cannot be removed from the volume. If effect the pump acts like a one-way valve: gas molecules may enter but not return. But for that to happen, molecules must first arrive at the cannot reach out and grab them. Understanding that vacuum doesn't suck makes the basic aspects of vacuum technology much easiter to grasp.