Few engineers who have to do with the steam engine are ignorant of the trouble which is met with in obtaining a really good piston-rod packing. Sound hemp, property "laid up," and copiously lubricated, makes a tight join enough for a time, especially if the rod is in first-rate condition; but the period of tightness is usually short, and the gland requires constant screwing up, and much friction results which is very prejudicial in small engines. If hemp is bad in the case of low-pressure engines, it is infinitely worse when we have to do with high steam, especially if the steam is slightly superheated. A process of slow cabonisation appears to go on, the hemp packing loses its elasticity, and becomes nearly useless for its intended purpose. All manner of schemes have been tried to get over the difficulty, combinations of cotton, india-rubber, and wire gauze, such, for example, as Crickmer's patent packing, have hitherto given on the whole the best results. One inventor, indeed, dispenses altogether with cotton and rubber, and uses copper wire gauze alone. In this case the tightness of the joint is no doubt secured by the presence of water and oil lodged in the meshes of the gauze; and we have received very favourable reports from those who have tried this packing. It is still certain that something better than anything hitherto in use is required, and we have a strong belief that this something is supplied by asbestos.
Asbestos is a mineral fibre consisting of silicate of magnesia, silicate of lime, and protoxide of iron and manganese. In mineralogical parlance, it is a fibrous variety of actinolite or tremolite. It exists in vast quantities in the United States, also in the Tyrol, Hungary, Corsica, Greenland, Wales, Cornwall, Banffshire, and in the north and east of Ireland. It is found under various forms, from that of soft silky fibres to that of a hard block capable of taking a polish. As a rule, the lumps or blocks taken from the vein are easily broken up and separated into fibres extremely flexible, and elastic in the sense that each fibre admits of great extension in the direction of its length without contracting again, greasy to the touch, and very strong. The fibres vary in length from a couple of inches to about two feet. They can easily be spun or woven if proper precautions are used. Furthermore, asbestos is an admirable non-conductor of caloric, and it is practically indestructible by heat. All these conditions are just those which are required in a material for piston-rod packing; and it is therefore somewhat strange that until a very recent period no one thought of utilising asbestos for this purpose. The credit of suggesting it as a piston-rod packing is due, we believe, to Mr. St. John Vincent Day, C. E., who on the 5th instant read a very interesting paper on "Asbestos, with special reference to the use as steam engine packing," before the Institution of Engineers and Shipbuilders in Scotland. The new packing, we learn from this paper, was first used in America with much success, and it has since been tested in this country with results of which we shall speak in a moment. In referring to the value of the new packing, Mr. Day said :-" The packing used for piston and valve rods or spindles has as we all know, three prime elements of destruction to contend with, namely, an elevated temperature, friction, and moisture, and one of them only, namely, frictien, has any appreciable effect on asbestos packing when the mineral is pure and properly prepared. No matter how high the temperature of the steam, how rapid the stroke of the piston, or how great the pressure of the steam, the packing seems to be unaffected by those conditions. In America, where the new packing was first used, some of it was taken from the piston-rod stuffing-box of a locomotive engine, after having been in, and the engines at constant work, for three months, with steam at 130 lb. pressure per square inch, and making an average daily run of 100 miles, including Sundays; and, as you can see by the sample shown, the fibre, with the exception of being discoloured by oil and iron, is just as flexible and tenacious as originally. After having been once disintegrated, it appears impossible to so pack or mat the fibres together that they are not easily separable by the fingers."
Asbestos packing was first used in Great Britain by Mr. Benjamin Conner, locomotive superintendent of the Caledonia Railway, and Mr. Day exhibited to the members of the Institution the packing of a locomotive stuffing-box which had been used on that line from the 27th of July, 1871, to the 18th of Nov. The engine in which it was used has outside cylinders, and single drivers 8ft in diameter. The piston stroke is 2ft. The engine was employed in working the fastest train on the Caledonian line; to wit the 10 a.m. express from Glasgow, reaching Carlisle at 1 p.m., with three stops on the journey. The best ordinary packing lasts, under these conditions, two months at most, rarely so long, and the gland requires constant screwing up. The asbestos packing was apparently as good as when it was put in, and the engine had run a distance of 2,000 miles in three weeks, during which the gland screws had never been touched. The following letter from Mr. Connor to Mr. Day contains valuable testimony to the excellence of the packing:-
The box herewith contains the asbestos packing put into a piston-rod stuffing-box of one of our main line service passenger engines on the 27 July, and taken out on the 18th November; in the time the engine had run 14,070 miles.
As the packing was put in coiled instead of being cut into rings, the gland was nearly home on 12th September, and an additional ring was put in at the date.
In the course of the discussion, Mr. Connor stated that, "The advantages of the asbestos packing over the soapstone packing was, that with the latter, at the high temperatures of steam from 125 lb to 130 lb, the lover portion of the packing got thoroughly charred, and another ring had to be put in after the first week; so that in course of a month the packing had almost entirely changed. The asbestos packing being practically incombustible did not waste; he suggested that the covering of the packing should be made of incombustible material also. At first he had applied it coiled round the piston-rod continuously; but he thought it should be applied in rings. The inside of the packing seemed to him as fresh as when first put in. He believed it took less oil to lubricate the piston-rod, for the oil remained on the rod, not being absorbed by packing. It kept the rod beautifully polished, more so than with any other packing."
We think that with such testimony as this before us, supported further by that of Mr. David Rowan , who spoke to the value of asbestos packing for the belief that this mineral will supply a way out of one of the most troublesome obstacles to the use of very high-pressure steam. There is, further-more, not the slightest chance of the supply being exhausted; on the contrary, it is likely to last as long as our coal-fields. We are unable to say at present what the price of the asbestos packing is, or where it can be obtained. It is, probable, however, that when once the value of the material as a packing is recognised, its regular manufacture in this country will follow.