Ultrahigh Vacuum Non-Coaxial Linear-Motion Feedthroughs

Abstract

Analysis of linear-motion feedthrough schemes (see Figure 4.2, Chapter 4) shows that sliding speed (V parameter) in kinematic pairs of feedthrough can vary from the maximum value corresponding to the traditional nut-screw couple to zero. The sliding speed variation from maximum value to minimum value leads to a great variation in the design schemes of the feedthroughs. In general, it makes the design of feedthroughs more complex.

One of the ways to decrease sliding speed is the use of bellows sealing elements. These elements directly transmit linear motion fromthe atmosphere into the vacuum chamber. Let us consider the mechanisms of linear motion which correspond to the upper line of Figure 4.2. In the mechanisms of this group, there is no friction in vacuum (Vc = 0). The simplest design of the mechanism consists of one bar sealed with a bellows [1]. The maximum length of the travel depends on allowable strain (stress) in the bellows [1] which can be determined from the following equation:

$$ \Delta L = \frac{{\left[ \sigma \right] \cdot n \cdot (1 - \mu )^2 \cdot R_B^2 }} {{E \cdot h \cdot .k_{2B} }} $$

where [σ] is the allowable strain in the bellows; n is the number of crimps; μ is the Poisson coefficient; RB is the internal radius of the bellows; k2B is the coefficient which depends on the goffer depth.