IMPROVEMENTS IN APPARATUS FOR RAISING AND LOWERING HEAVY BODIES
Year: 1876

page_text: PROVISIONAL SPECIFICATION left by Alfred Todd at the Office of the
Commissioners of Patents on the 17th November 1876.
ALFRED TODD, of Birmingham, in the County of Warwick, Mechanical Engineer.
"IMPROVEMENTS IN APPARATUS FOR RAISING AND LOWERING HEAVY BODIES."
My Invention consists of the construction and arrangement, herein-after described,
of the parts of mechanism for raising and lowering heavy bodies, whereby heavy
bodies may be raised and lowered with great facility, and left suspended at any
required height without risk of running down.
An apparatus for raising and lowering heavy bodies made according to my
Invention is constructed essentially as follows:-Two sheaves or pulleys carried by
axes situated the one over the other are supported in a frame in which the ends of
the axes work. Each axis, besides carrying a sheave or pulley, carries a toothed
wheel. The toothed wheels are in the middles of their respective axes, and gear
with one another, while the sheaves or pulleys on the axes are on opposite sides
of their respective toothed wheels, and are consequently not over one another. The
toothed wheels have not the same number of teeth, the toothed wheel on one of the
axes (say the upper) having one or more teeth in excess of those on the lower
toothed wheel. The diameter of the acting part of the sheave or pulley on the
upper axis is less than that of the sheave or pulley on the lower axis. An endless
chain forming two loops is arranged on the pulley in the following manner:-One
of the loops of the chain carries a snatch block, to which the heavy body to be raised
is attached. The chain from one side of the loop carrying the snatch block passes
over the larger pulley of the lower axis, while that from the other side of the said
loop passes over the smaller pulley on the upper axis. The chain descending from
the said pulleys forms a second loop, by pulling one or other side of which the
raising or lowering of the heavy body can be effected.
motion is given to the pulleys in one direction, and by pulling the other he pulleys
The action of the apparatus is as follows:-By pulling one side of the second loop
side of the
said-loop motion is given to the said pulleys in the other direction. As t
are geared together by the toothed wheels they move simultaneously and in opposite
directions, the result of their motion being that one side of the loop in which the
snatch block is situated is ascending while the other is descending, the rate of ascent
and descent in the two sides of the loop depending on the circumference and rate

page_text: of rotation of the two pulleys over which they are respectively passing; thus, sup-
posing the large pulley to have a circumference of eight and to perform four
rotations in a given time, while the small pulley has a circumference of seven and
performs three rotations in a like time, the former during four rotations will deliver
a length of chain equal to thirty-two, while the latter will draw in a length of
chain equal to twenty-one, the difference, namely, eleven, representing the virtual
lengthening of the chain forming the loop, in which the snatch block is supported.
The heavy body supported by the snatch block is raised or lowered according as the
motion of the pulleys is in one or other direction.
The relative sizes of the pulleys and numbers of the teeth of the toothed wheels
may be varied. When they have the proportions described the load will rest in
any position at which it may be left, but to prevent any risk of the load running
down one of the pulleys may be fixed upon its axis excentrically, in which case, if
the apparatus be stopped when the portion of the pulley of shortest radius is at or
about its highest point, there will be no tendency to run down. Where it is required
to lift a heavier load than can be conveniently raised by the arrangement described
I combine therewith a moveable snatch block having two grooves.
I prefer to combine the said moveable snatch block with my improved pulley by
fixing side by side with the pulley a fixed snatch block. The chain passes over the
said fixed snatch block as well as over the pulley, and in the loops formed between
the pulley and fixed snatch block the moveable snatch block to which the weight is
suspended is placed.