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 [61] 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 ma............