ion at regular intervals of small blades
of hard wood (fig. 1). Modern worm conveyors usually consist of a
spindle formed of a length of wrought iron piping, to which is fitted
either a broken or continuous worm. In the former case (fig. 2) the worm
is composed of a series of blades or paddles arranged like a spiral
round the spindle; each blade is fixed, by means of its shank, in a
transverse hole in the spindle, and the shank is held in position by
being tapped and fitted with a nut. In this way is formed, out of
separate blades, a practically complete screw, technically known as a
"paddle worm." The lengths or sections of the worm run to about 8 ft.,
the various lengths being coupled by turned gudgeons, which also serve
as journals for the bearings. In the so-called continuous worm conveyors
the screw is formed of a continuous sheet-iron spiral (fig. 3).
Sometimes a narrow groove is cut in spiral form on the spindle, and in
this groove the sheet-iron spiral is secured.

[Illustration: FIG. 1.--Early Flour Mill Conveyor.[1]]

[Illustration: FIG. 2.--Paddle Worm Conveyor.]

[Illustration: FIG. 3.--Continuous Worm Conveyor.]

The _spiral_ or _anti-friction_ conveyor (fig. 4) was introduced about
1887. In this case a narrow spiral, which passes concentrically round
the spindle, with a space between both, is fixed to it at set intervals
by small blades, each of which is itself fixed by its shank and a nut to
the spindle. The spiral may be made of almost any section, from a round
bar about 1/2 in. in diameter to L or T section, but is preferably a flat
bar. Worms are fitted into wooden or iron troughs leaving a clearance of
1/8 to 1/4 in. The spindle must be supported at suitable intervals by
bearings, preferably of the bush type. A continuous worm, being more
rigid than a paddle worm, needs fewer supports. The lid of the worm
trough should be loose, not screwed on, because in case of an
accumulation of feed through a choke in a delivery spout the paddles of
a paddle worm would be broken, or a continuous worm stripped, unless the
material could throw off the lid and relieve the worm. The ratios of the
pitch of the worm to the diameter must be regulated by the nature of the
material to be conveyed, and will vary from one-third to a pitch equal
to, or even exceeding, the diameter. The greater the pitch the larger
the capacity, but also the greater the driving power required, at the
same speed. For handling materials of gr