_MACHINERY FOR MOVING AND HANDLING MATERIAL._
Steam and other machinery applied to the transport of material and
travel, in navigation and by railways, comprises the greater share
of what may be called engineering products; and when we consider
that this vast interest of steam transport is less than a century
old, and estimate its present and possible future influence on
human affairs, we may realise the relation that mechanical science
bears to modern civilisation.
To follow out the application of power to the propulsion of vessels
and trains, with the many abstruse problems that would of necessity
be involved, would be to carry this work far beyond the limits
within which it is most likely to be useful to the apprentice
engineer; besides, it would be going beyond what can properly be
termed manipulation.
Marine and railway engineering have engrossed the best talent in
the world; investigation and research has been expended upon these
subjects in a degree commensurate with their importance, and it
would be hard to suggest a single want in the many able text-books
that have been prepared upon the subjects. Marine and railway
engineering are sciences that may, in a sense, be separated from
the ordinary constructive arts, and studied at the end of a course
in mechanical engineering, but are hardly proper subjects for an
apprentice to take up at the beginning.
In treating of machinery for transport, as a class, the subject,
as far as noticed here, will be confined to moving and handling
material as one of the processes of manufacturing, and especially
in connection with machine construction. If the amount of time,
expense, labour, and machinery devoted to handling material in
machine shops is estimated, it becomes a matter of astonishment to
as many as have not previously investigated the subject; as an item
of expense the handling, often exceeds the fitting on large pieces,
and in the heavier class of work demands the most careful attention
to secure economical manipulation.
It will be well for an apprentice to begin at once, as soon as he
commences a shop course, to note the manner of handling material,
watching the operation of cranes, hoists, trucks, tackle, rollers;
in short, everything that has to do with moving and handling. The
machinery and appliances in ordinary use are simple enough in a
mechanical sense, but the principles of handling material are by
no means as plain or easy to understand. The diversity of practice
seen in various plans of handling and lifting weights fully attests
the last proposition, and it is questionable whether there is any
other branch of mechanical engineering that is treated less in a
scientific way than machinery of this class. I do not allude to
the mechanism of cranes and other devices, which are usually well
proportioned and generally well arranged, but to the adaptation
of such machinery with reference to special or local conditions.
There are certain inherent difficulties that have to be encountered
in the construction and operation of machinery, for lifting
and handling, that are peculiar to it as a class; among these
difficulties is the transmission of power to movable mechanism, the
intermittent and irregular application of power, severe strains,
also the liability to accidents and breakage from such machinery
being controlled by the judgment of attendants.
Ordinary machinery, on the reverse, is stationary, generally
consumes a regular amount of power, is not subjected to such
uncertain strains, and as a rule acts without its operation being
controlled by the will of attendants.
The functions required in machinery for handling material in a
machine shop correspond very nearly to those of the human hands.
Nature in this, as in all other things, where a comparison is
possible, has exceeded man in adaptation; in fact, we cannot
conceive of anything more perfect than the human hands for handling
material--a duty that forms a great share of all that we term
labour.
Considered mechanically as a means of handling material, the human
hands are capable of exerting force in any direction, vertically,
horizontally, or at any angle, moving at various rates of speed,
as the conditions may require, and with varying force within the
limits of human strength. These functions enable us to pick up or
lay down a weight slowly and carefully, to transport it at a rapid
rate to save time, to move it in any direction, and without the
least waste of power, except in the case of carrying small loads,
when the whole body has to be moved, as in ascending or descending
stairs. The power travelling cranes, that are usually employed in
machine-fitting establishments, are perhaps the nearest approach
that has been made to the human frame in the way of handling
mechanism; they, however, lack that very important feature of a
movement, the speed of which is graduated at will. It is evident
that in machinery of any kind for handling and lifting that moves
at a uniform rate of speed, and this rate of speed adapted, as it
must be, to the conditions of starting or depositing a load, much
time must be lost in the transit, especially when the load is moved
for a considerable distance. This uniform speed is perhaps the
greatest defect in the lifting machinery in common use, at least in
such as is driven by power.
In handling a weight with the hands it is carefully raised, and
laid down with care, but moved as rapidly as possible throughout
the intervening distance; this lesson of nature has not been
disregarded. We find that the attention of engineers has been
directed to this principle of variable speed to be controlled at
will. The hydraulic cranes of Sir William Armstrong, for example,
employ this principle in the most effective manner, not only
securing rapid transit of loads when lifted, but depositing or
adjusting them with a care and precision unknown to mechanism
positively geared or even operated by friction brakes.
The principles of all mechanism for handling loads should be such
as to place the power, the rate of movement, and the direction of
the force, within the control of an operator, which, as has been
pointed out, is the same thing in effect as the action of the human
hands.
The safety, simplicity, and reliable action of hydraulic machinery
has already led to its extensive employment for moving and lifting
weights, and it is fair to assume that the importance and success
of this invention fully entitle it to be classed as one of the
most important that has been made in mechanical engineering during
fifty years past. The application of hydraulic force in operating
the machinery used in the processes for steel Bessemer manufacture,
is one of the best examples to illustrate the advantages and
principles of the hydraulic system. Published drawings and
descriptions of Bessemer steel plant explain this hydraulic
machinery.
There is, however, a principle in hydraulic machinery that must
be taken into account, in comparing it with positively geared
mechanism, which often leads to loss of power that in many cases
will overbalance any gain derived from the peculiar action of
hydraulic apparatus. I allude to the loss of power incident to
dealing with an inelastic medium, where the amount of force
expended is constant, regardless of the resistance offered. A
hydraulic crane, for instance, consumes power in proportion to
its movements, and not as the amount of duty performed; it takes
the same quantity of water to fill the cylinders of such cranes,
whether the water exert much or little force in moving the pistons.
The difference between employing elastic mediums like air and
steam, and an inelastic medium like water, for transmitting force
in performing irregular duty, has been already alluded to, and
forms a very interesting study for a student in mechanics, leading,
as it does, to the solution of many problems concerning the use and
effect of power.
The steam cranes of Mr Morrison, which resemble hydraulic cranes,
except that steam instead of water is employed as a medium for
transmitting force, combine all the advantages of hydraulic
apparatus, except positive movement, and evade the loss of power
that occurs in the use of water. The elasticity of the steam is
found in practice to offer no obstacle to steady and accurate
movement of a load, provided the mechanism is well constructed,
while the loss of heat by radiation is but trifling.
To return to shop processes in manufacturing. Material operated
upon has to be often, sometimes continually, moved from one place
to another to receive successive operations, and this movement
may be either vertically or horizontally as determined, first,
by the relative facility with which the material may be raised
vertically, or moved horizontally, and secondly, by the value
of the ground and the amount of room that may be available, and
thirdly by local conditions of arrangement. In large cities, where
a great share of manufacturing is carried on, the value of ground
is so great that its cost becomes a valid reason for constructing
high buildings of several storeys, and moving material vertically
by hoists, thus gaining surface by floors, instead of spreading
the work over the ground; nor is there any disadvantage in high
buildings for most kinds of manufacture, including machine fitting
even, a proposition that will hardly be accepted in Europe, where
fitting operations, except for small pieces, are rarely performed
on upper floors.
Vertical handling, although it consumes more power, as a rule costs
less, is more convenient, and requires less room than horizontal
handling, which is sure to interfere more or less with the
constructive operations of a workshop. In machine fitting there is
generally a wrong estimate placed upon the value of ground floors,
which are no doubt indispensable for the heaviest class of work,
and for the heaviest tools; but with an ordinary class of work,
where the pieces do not exceed two tons in weight, upper floors if
strong are quite as convenient, if there is proper machinery for
handling material; in fact, the records of any establishment, where
cost accounts are carefully made up, will show that the expense
of fitting on upper floors is less than on ground floors. This is
to be accounted for by better light, and a removal of the fitting
from the influences and interference of other operations that must
necessarily be carried on upon ground floors.
For loading and unloading carts and waggons, the convenience of
the old outside sling is well known; it is also a well-attested
fact that accidents rarely happen with sling hoists, although
they appear to be less safe than running platforms or lifts. As
a general rule, the most dangerous machinery for handling or
raising material is that which pretends to dispense with the care
and vigilance of attendants, and the safest machinery that which
enforces such attention. The condition which leads to danger
in hoisting machinery is, that the power employed is opposed
to the force of gravity, and as the force of gravity is acting
continually, it is always ready to take advantage of the least
cessation in the opposing force employed, and thus drag away the
weight for which the two forces are contending; as a weight when
under the influence of gravity is moved at an accelerated velocity,
if gravity becomes the master, the result is generally a serious
accident. Lifting may be considered a case wherein the contrivances
of man are brought to bear in overcoming or opposing a natural
force; the imperfect force of the machinery is liable to accident
or interruption, but gravity never fails to act. Acting on every
piece of matter in proportion to its weight must be some force
opposing and equal to that of gravity; for example, a piece of iron
lying on a bench is opposed by the bench and held in resistance to
gravity, and to move this piece of iron we have to substitute some
opposing force, like that of the hands or lifting mechanism, to
overcome gravity.
As molecular adhesion keeps the particles of matter together so
as to form solids, so the force of gravity keeps objects in their
place; and to attain a proper conception of forces, especially in
handling and moving material, it is necessary to familiarise the
mind with this thought.
The force of gravity acts only in one direction--vertically, so
that the main force of hoisting and handling machinery which
opposes gravity must also act vertically, while the horizontal
movement of objects may be accomplished by simply overcoming the
friction between them and the surfaces on which they move. This is
seen in practice. A force of a hundred pounds may move a loaded
truck, which it would require tons to lift; hence the horizontal
movements of material may be easily accomplished by hand with
the aid of trucks and rollers, so long as it is moved on level
planes; but if a weight has to be raised even a single inch by
reason of irregularity in floors, the difference between overcoming
frictional contact and opposing gravity is at once apparent.
One of the problems connected with the handling of material is to
determine where hand-power should stop and motive-power begin--what
conditions will justify the erection of cranes, hoists, or
tramways, and what conditions will not. Frequent mistakes are made
in the application of power when it is not required, especially
for handling material; the too common tendency of the present day
being to apply power to every purpose where it is possible, without
estimating the actual saving that, may be effected. A common
impression is that motive power, wherever applied to supplant hand
labour in handling material, produces a gain; but in many cases
the fallacy of this will be apparent, when all the conditions are
taken into account.
Considered upon grounds of commercial expediency as a question of
cost alone, it is generally cheaper to move material by hand when
it can be easily lifted or moved by workmen, when the movement
is mainly in a horizontal direction, and when the labour can be
constantly employed; or, to assume a general rule which in practice
amounts to much the same thing, vertical lifting should be done by
motive power, and horizontal movement for short distances performed
by hand. There is nothing more unnatural than for men to carry
loads up stairs or ladders; the effort expended in such cases is
one-half or more devoted to raising the weight of the body, which
is not utilised in the descent, and it is always better to employ
winding or other mechanism for raising weights, even when it is to
be operated by manual labour. Speaking of this matter of carrying
loads upward, I am reminded of the fact that builders in England
and America, especially in the latter country, often have material
carried up ladders, while in some of the older European countries,
where there is but little pretension to scientific manipulation,
bricks are usually tossed from one man to another standing on
ladders at a distance of ten to fifteen feet apart.
To conclude. The reader will understand that the difficulties
and diversity of practice, in any branch of engineering, create
similar or equal difficulties in explaining or reasoning about the
operations; and the most that can be done in the limited space
allotted here to the subject of moving material, is to point out
some of the principles that should govern the construction and
adaptation of handling machinery, from which the reader can take up
the subject upon his own account, and follow it through the various
examples that may come under notice.
To sum up--We have the following propositions in regard to moving
and handling material:
1. The most economical and effectual mechanism for handling is that
which places the amount of force and rate of movement continually
under the control of an operator.
2. The necessity for, and consequent saving effected by,
power-machinery for handling is mainly in vertical lifting,
horizontal movement being easily performed by hand.
3. The vertical movement of material, although it consumes more
power, is more economical than horizontal handling, because less
floor room and ground surface is required.
4. The value of handling machinery, or the saving it effects, is as
the constancy with which it operates; such machinery may shorten
the time of handling without cheapening the expense.
5. Hydraulic machinery comes nearest to filling the required
conditions in handling material, and should be employed in cases
where the work is tolerably uniform, and the amount of handling
will justify the outlay required.
6. Handling material in machine construction is one of the
principal expenses to be dealt with; each time a piece is moved
its cost is enhanced, and usually in a much greater degree than is
supposed.
(1.) Why has the lifting of weights been made a standard for
the measure of power?--(2.) Name some of the difficulties to
contend with in the operation of machinery for lifting or
handling material.--(3.) What analogy exists between manual
handling and the operation of hydraulic cranes?--(4.) Explain
how the employment of overhead cranes saves room in a fitting
shop.--(5.) Under what circumstances is it expedient to move
material vertically?--(6.) To what circumstances is the danger of
handling mainly attributable?