Laddertron Charging Chain

The charging system of the Stony Brook FN tandem van de Graaff accelerator was converted from the original rubber belt to a modern "Laddertron" system in 1980. The Laddertron was manufactured by High Voltage Engineering Corporation (HVEC) following a design developed at the pioneering large vertical tandem (now closed) at Daresbury Laboratory (UK). Similar Laddertron installations may be found in the horizontal tandems at Orsay (France), Legnaro (Italy), Beijing (China), and Ile-Ife (Nigeria).

The Laddertron chain consists of about 300 modular units, each containing two cylindrical stainless-steel casting (rings), one aluminum crossbar (rung), two polished steel bearing pins and two insulating links ("dogbones") with replaceable bushings. In the original HVEC design the link material is a blue monocast nylon and the bushings are Garlock DU; the lubricant in these bushings is lead-loaded Teflon held in a phosphor-bronze layer backed by steel. Concerns about the cost and durability of the nylon links and Garlock bushings prompted the development of a new dogbone design with an improved link material (Techtron PPS) and a Delrin AF plastic bushing. The new design has been in continuous use throughout the Laddertron since 1993.

This figure shows an illustrative dogbone containing one original Garlock bushing (right) and one Delrin bushing (left). Other than the larger opening for the bushing there is no change to the link dimensions with the new design. The Techtron PPS links are machined from rod stock with an exact 1.000-inch diameter.

Why use Techtron PPS (Polyphenylene sulfide)?

• Techtron PPS is very stable dimensionally under high tension and with changes in temperature.
• The more stable dimensions make for an easier and more precise assembly process. During the assembly lengths of 7-link chain sections are measured under tension and matched in pairs. With the Techtron PPS there is a much narrower distribution of measured lengths than with the nylon.
• The electrical strength of Techtron PPS is at least equivalent to nylon.
• Techtron PPS's tensile strength exceeds that of nylon. This made possible the larger diameter Delrin AF bushing in the same link (see dogbone drawing).
• Machinability is comparable to or better than nylon.
• Techtron PPS is available as rod stock with an exact 1.000 +/- 0.001 inch diameter. This eliminates a turning down step from 1-3/8 inch necessary with nylon rod.

Why use Delrin AF?

• With the new Delrin AF bushing the lubricant is disbursed throughout the material. This means that there is hardly any possibility of wearing entirely through to a non-lubricating surface.
• The old Garlock bushings tended to shed lead-loaded Teflon flakes, especially for the first weeks of operation. The Delrin AF bushings are much cleaner and contain no hazardous materials.
• Delrin AF bushings can easily be inserted by hand, while the steel-backed DU bushings needed to be pressed in and out.
• The Delrin AF bushings are available at much lower cost than the DU bushings, less than $2 a piece in a quantity of 1000.

For more information ...

• More about Techtron PPS and Delrin AF.
• Our report on the improved Laddertron design from the Rochester (1993) SNEAP meeting (197 KB postscript file).

[Stony Brook University home page] [Stony Brook Physics & Astronomy home page] [Stony Brook Nuclear Structure Laboratory home page]