In a hydraulic packing press, the work done by the ram during one
stroke may be roughly divided into two periods, in the first of which
the resistance, although gradually increasing, may be called light,
while in the second the resistance is heavy. The former of these two
periods embraces the greater part of the stroke, and it is only a
small proportion at the end which requires the exercise of the full
power of the press to bring the material to the determined degree of
consolidation. Consequently, if a hydraulic press is to be worked so
as to waste no time, it requires to be provided with means by which
its table may be made to rise rapidly during the portion of the stroke
when the resistance is small, and afterward more slowly when the
entire power of the pumps is being expended upon the final squeeze.
Many methods of obtaining this end have been devised, and are in
common use both here and abroad. It is, however, more particularly in
the packing of raw material that such appliances are useful, since the
goods pressed into bales in this country are not usually of a very
yielding nature, and consequently do not require a long stroke to
bring them to a high state of compression. In India and Egypt, from
whence cotton is sent in bales, presses must have a long stroke; and
unless they can be worked rapidly, a very considerable amount of plant
is required to get through a moderate quantity of work.
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