The Mechanical Handling and Storing of Material
Forfatter: A.-M.Inst.C E., George Frederick Zimmer
År: 1916
Forlag: Crosby Lockwood and Son
Sted: London
Sider: 752
UDK: 621.87 Zim, 621.86 Zim
Being a Treatise on the Handling and Storing of Material such as Grain, Coal, Ore, Timber, Etc., by Automatic or Semi-Automatic Machinery, together with the Various Accessories used in the Manipulation of such Plant
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54
THE MECHANICAL HANDLING OF MATERIAL
of this chapter, as the trough though stationary laterally, is revolving, but it is essentially
a worm conveyor. The advantages are the absence of internal bearings, which are the
principal cause of trouble when using ordinary worm conveyors for material of a cutting
nature.
The spiral is best chosen of a width equal to one-third of the diameter of the cylinder,
i.e., for a 12-in. worm the spiral strip would be 4 in. wide, and thus leave a clear space of
4 in. in the centre of the tube. The best pitch for the screw is two-fifths of the diameter.
The supporting rollers need not be so close together as the internal bearings of
ordinary worm conveyors, as the cylindrical body of the tubular conveyor is so much
more rigid. The distance apart of the supporting rollers depends upon the strength of
the construction, but as a general rule they may be about twice as far apart as the bearings
in ordinary worm conveyors.
Intermediate outlets are not impossible, but they are not advisable owing to the
complication and expense.
This form of conveyor is more expensive in initial cost, and the fixing is also more
costly than that of ordinary worms, but its life is longer. The introduction of this
Fig. 58. Tubular Worm Conveyor.
conveyor may be considered to be the first step in the direction of providing a more
economical conveyor for fine material.
One of the essential features is that the speed must be chosen within the correct
limits. If too high the capacity diminishes, as the centrifugal force counteracts the
propulsion ; for it must be remembered that with this appliance the forward movement
of the material is effected by gravity, inasmuch as through the revolution of the tube
the material contained in it is carried up the side in the direction of the revolution until
the angle of repose is „exceeded, when it is compelled to fall, and is then guided in a
slightly forward direction by the spiral attached to the inside of the tube.
The capacity of this tubular conveyor is not very large, as the pitch of the screw
must be smaller than with most ordinary worms, say about 0-33 to 0'4 of the inner
diameter of the tube, and the number of revolutions is also limited, as already mentioned.
It is thus obvious that the greater the pitch of the screw the greater the tendency of the
material to cling to the inside of the tube, which tendency is still further encouraged
when the centrifugal force begins to exert itself, so that a point is reached where the
material is carried too far up the side of the tube, and there is then a tendency for it
not to slide forward in its appointed channel but to fall, more than slide, back from its
higher position, and so some of the mateiial may fall back into the previous thread of
the screw, which reduces the capacity more and more as the speed is increased, and
eventually the centrifugal force being greater than the gravity, the material revolves
round and round on the inner periphery of the tube and conveying ceases altogether.