Image
|
Title: |
1880 Article-Ashcroft Mfg. Co., Tabor Steam Indicator |
|
Source: |
American Machinist, 08 May 1880, pg. 1 |
|
Insert Date: |
8/11/2015 12:55:53 PM |
The New Steam Engine Indicator
Our readers may have noticed, that, at sundry times within the last year, we have alluded to a new engine indicator as being prepared for the market.
These allusions have led to such lively inquiry among such of our readers as are interested in steam work, that it is with peculiar pleasure we are enabled in this issue to lay before them complete illustrations of the new instrument, and also such description of its points of novelty as will enable them to judge of its merits.
The instrument is now in the market, and, we should judge, not prematurely, since our first brief announcement of the existence of this new instrument, the whole interval has been consumed in the construction of special machinery for the systematic manufacture of it, upon plans which are likely to insure a uniformity of excellence in construction, and, at the same time, insure an economical production.
While we are not advised as to the prices of the new instrument, we are led to believe that the purchasers will receive a full share of the benefits arising from the system of manufacture referred to. If the system of manufacture proposed, will give to purchasers an instrument of such high character as those we have seen, at a price less than is usual for indicators, we feel that we may congratulate those among our readers who are interested.
In our issue of December 20th, 1879, our correspondent, "Chordal," in reviewing the growth and modifications of indicators, briefly described, among others, the Tabor indicator, and ventured the prediction that it had a future. The Tabor instrument, so briefly referred to by "Chordal," is the new indicator, and the two accompanying illustrations will give a good idea of its general make up.
The most striking feature, and in fact, the main point of novelty in design, about the instrument, is the parallel motion. In describing this parallel motion, we can hardly say that it consists of few pieces, or many pieces, for in fact no pieces are introduced, especially to give the motion parallelism. If parallelism were not desired the same number of pieces would be required. It will be noticed that the piston rod, which is jointed to the piston and the pencil lever, as usual, is slotted. The slot is curved and works over a guide roller set in the cylinder cap. The rear end of the pencil lever is pivoted to the usual radius link.
The slot curve is that peculiar curve, which would be described by the guide roller asa a scribing point, while the pencil was being moved in a true line. This it will be seen insures an absolutely correct parallel motion to the pencil. No moving parts are added and those essential to the motion are peculiarly light and substantial.
The guide roller is journaled in a free collar, held in the cylinder cap. This allows all the motion work to be freely revolved as the pencil is brought to the paper. By simply unscrewing the cylinder cap, the whole motion work is removed as one piece. No small parts of the motion work need be removed, when springs are changed, or when a general interior inspection is required. This feature seems to be of peculiar value and should be appreciated by those indicator users, who have had the misfortune to lose small parts, which were necessarily removed when the indicator was opened. This feature permits the use of substantial and permanent steel screws for all the joints, as they seldom or never require separation. A section of the pencil lever would be a flat oval; it is very light and solid. The pencil socket is arranged to fit Faber's Siberian leads. The pencil lever, piston rod, and radius link are all of steel, spring tempered. The small number of motion parts, and the lightness of those used, reduce the error due to momentum, to a point beyond which it seems impossible and unnecessary to go. The friction, which might at first be thought to exist at the guide roller, is found not to exist. What little friction there is, is that due to the conversion of an arcal motion into a parallel one, and this much friction is inherent in any possible device for so converting the motion.
The friction of any of these parts may be called immaterial in any of the well-known indicators, as its amount is an inconsiderable trifle, compared with the energy of the steam acting on the piston. The momentum of the parts, however, becomes a matter of grave concern, and every effort should be made to keep it at a minimum. The piston spring (40 lbs. per inch) and parallel motion in the Richards Indicator weigh 2.875 ozs. (Avoirdupois), and in the Thompson 2.125 ozs., while those of the Tabor instrument weigh 1.427 ozs. The piston, piston rod, and pencil lever together weigh only 9.655 drams or ¾ ozs.
The piston is much longer than usual. It is very light, being made of nickel casting, very much lighter in fact than the illustration shows. The piston is brought to the exact area o£ one-half a square inch, and the lever multiplies the extent of motion in the usual proportion. The piston has a single capillary packing groove, and its whole action is remarkably nice. The springs, both as to range and general structure, are similar to those in the Richards and Thompson.
The rotation of the top work is effected by a handle, as usual, but a controllable limit is put upon this motion, by means of a convenient thumb screw in the top band, bearing against a projection on the drum bracket. This screw is so located that it may be adjusted by the hand, while putting the pencil to work.
There is nothing peculiar about the structure of the cylinder, save in its peculiar external plainness. The usual form of cock, not shown in the illustration, is provided, but a change has been made in the plan of attracting the indicator to the cock. The usual compound screw coupling, on account of the difficulty of properly timing the two threads, is omitted, and a simple union nut with finger levers substituted. This nut, never being removed, serves to protect the taper and ground surface of the tail piece, which fits the socket of the cock. Upon loosening this nut it separates the joint without danger of injury to the parts.
The paper drum is made very much lighter than usual, and the paper clips are of tempered steel and quite flexible. The lower guide pulleys are self-adjusting in their line, as the bottom nut is arranged not to pinch on the swiveling piece, which holds them. The paper drum revolves upon a steel spindle, upon which the bottom nut is screwed. The nut inside the drum is simply a milled head, firmly screwed on the upper end of the spindle.
The recoil spring is seated in a cup on the bracket, and the outer end of the spring is fixed to the cup. The inner end of the spring is hooked on by the hub of the drum base. A stop-block on the cup, engaging with a lug in the drum base, forms the stop for the recoil motion. If the spindle be slacked somewhat, the drum base may be revolved over the stop-block, and more or less tension given to the recoil spring.
The whole instrument is very much lighter and more portable than any others we have seen. Its weight complete, with cock attached, is three pounds, six ounces. The Thompson indicator weighs about one pound more than this, and the Richards indicator about a pound and a half more than this. This reduction of weight, of course, becomes a matter of special value only when circumstances require hand carriage of instruments. The workmanship of the instruments is excellent, and their behavior in use is very fine. We might, perhaps, have done well to present diagrams as produced by this instrument, but we shall have occasion to do so in a future issue.
The diagrams produced are admirable. A seventy-five pound card, taken with a thirty spring, at two hundred and sixteen revolutions, and retracing its line continuously for fifty revolutions, shows a clean, sharply cut, unvarying card of the most delicate accuracy.
A seventy pound card, taken with a thirty two spring, at three hundred and eighty-five revolutions, gives results so perfect as to almost lead to the conclusion that high speed need not interfere with accurate indicator work.
The construction of this instrument seems to open up a way for high pressure diagrams made with low tension springs, even at the highest of modern high speeds. This instrument may be used either right or left-handed, the necessary changes requiring only a few moments. The Tabor Indicator is made by the Ashcroft Manufacturing Co., 53 Sudbury St., Boston, Mass., to whom all inquiries should be addressed. |
|
1880 Ashcroft Mfg. Co., Tabor Steam Indicator
|
|
|
|