Modern Machine-Shop Practice Part 176

[Ill.u.s.tration: Fig. 2587.]

First prepare a number of rude wooden frames, such as shown in Fig.

2585. They are called targets, and are pieces of wood nailed together, with the outer edge face A planed true, and having a line marked parallel with the planed edge and about three-quarters of an inch inside of it. Upon this frame we hang a line suspending a weight and forming a plumb-line, and it follows that when the target is so held that the plumb-line falls exactly over and even all the way down with the scribed line, the planed face A, Fig. 2586, will stand vertical. To facilitate this adjustment, we cut a small [V] notch at the top of the scribed line, the bottom of the [V] falling exactly even with the scribed line, so that it will guide the top of the plumb-line even with the scribed line at the top; hence the eye need only be directed to causing the two lines to coincide at the bottom. To insure accuracy, the planed edge A should not be less than a foot in length. Then tightly stretch a strong closely-twisted and fine line of cord beside the line of shafting, as shown in Fig. 2587, placing it say six inches below and four inches on one side of the line of shafting, and equidistant at each end from the axial line of the same, adjusting it at the same time as nearly horizontally level as the eye will direct when standing on the floor at some little distance off and sighting it with the line shaft.

In stretching and adjusting this line, however, we have the following considerations:--It must clear the largest pulley hub on the line of shafting, those pulleys having set-screws being moved to allow it to pa.s.s. If the whole line of shafting is parallel in diameter, we set the line equidistant from the shafting at each end. If one end of the shafting is of larger diameter, we set the line farther from the surface of the shafting, at the small end, to an amount equal to one-half of the difference in the two diameters; and since the line is sufficiently far from the shafting to clear the largest hub thereon, it makes, so far as stretching the line is concerned, no difference of what diameter the middle sections of shafting may be. The line should, however, be set true as indicated by a spirit-level.

We may now proceed to erect the targets as follows: The planed edge A in Fig. 2585 is brought true with the stretched line, and is adjusted so that the plumb-line B in Fig. 2586 will stand true with the line or mark B. When so adjusted, the target is nailed to the post carrying the shafting hanger. In performing this nailing, two nails may be slightly inserted so as to sustain the target, and the adjustment being made by tapping the target with the hammer, the nails may be driven home, the operator taking care that driving the nails does not alter the adjustment.

[Ill.u.s.tration: Fig. 2588.]

In Fig. 2588 A A represents the line of shafting, B, B two of the hanger posts, and C, C two of the adjusted targets.

[Ill.u.s.tration: Fig. 2589.]

We have now in the planed edges A of the targets a rigid subst.i.tute for the stretched line, forming a guide for the horizontal adjustment, and to provide a guide for the vertical adjustment we take a wooden straight-edge long enough to reach from one post to another. Then beginning at one end of the shafting, we place the flat side of the straight-edge against the planed edge of two targets at a distance of about 15 inches below the top of the shafting; and after levelling the straight-edge with a spirit-level, we mark (even with the edge of the straight-edge) a line on the planed edge of each target, and we then move the straight-edge to the next pair of targets, and place the edge even with the mark already made on the second target. We then level the straightedge with a spirit-level, and mark a line on the third target, continuing the process until we have marked a straight and horizontally level line across all the targets, the operation being shown in Fig.

2589, in which A represents the line of shafting, B the hangers, and C the targets. D represents the line on the first target, and E the line on second. F is the straightedge, levelled ready to form a guide whereby the line D may be carried forward, as at E, level and straight, to the third target, and so on across all the targets.

[Ill.u.s.tration: Fig. 2590.]

[Ill.u.s.tration: Fig. 2591.]

The line thus marked is the standard whereby the shafting is to be adjusted vertically; and for the purpose of this adjustment, we must take a piece of wood, or a square, such as is shown in Fig. 2590, the edges A and B being true and at a right angle to each other. The line D, in Fig. 2589, marked across the targets being 15 inches below the centre line of the shaft at the end from which it was started, we mark upon our piece of wood the line C in Fig. 2590, 15 inches from the edge A (as denoted by the dotted line); and it is evident that we have only to adjust our shaft for vertical height so that, the gauge being applied at each target in the manner shown in Fig. 2591, the shaft will be set exactly true, when the mark C on the piece of wood comes exactly fair with the lines D marked on the targets.

[Ill.u.s.tration: Fig. 2592.]

For horizontal adjustment, all we have to do is to place a straight-edge along the planed face of the target, and adjust the shaft equidistant from the straight-edge, as shown in Fig. 2592, in which A is the shaft, B the target, C the straight-edge referred to, and D a gauge or distance piece. If, then, we apply the straight-edge and wood gauge to every target, and to the adjustment, the whole line of shafting will be complete.

There are several points, however, during the latter part of the process at which consideration is required. Thus, after the horizontal line, marked on the targets by the straight-edge and used for the vertical adjustment, has been struck on all the targets, the distance from the centre of the shafting to that line should be measured at each end of the shafting, and if it is found to be equal, we may proceed with the adjustment; but if, on the other hand, it is not found to be equal, we must determine whether it will be well to lift one end of the shaft and lower the other, or make the whole adjustment at one end by lifting or lowering it, as the case may be. In coming to this determination we must bear in mind what effect it will have on the various belts, in making them too long or too short; and when a decision is reached, we must mark the line C, in Fig. 2590, on the gauge accordingly, and not at the distance represented in our example by the 15 inches.

The method of adjustment thus pursued possesses the advantage that it shows how much the whole line of shafting is out of true before any adjustment is made, and that without entailing any great trouble in ascertaining it; so that, in making the adjustment, the operator acts intelligently and does not commence at one end utterly ignorant of where the adjustment is going to lead him to when he arrives at the other.

Then, again, it is a very correct method, nor does it make any difference if the shafting has sections of different diameters or not, for in that case we have but to measure the diameter of the shafting, and mark the adjusting line, represented in our example by C, in Fig.

2590, accordingly, and when the adjustment is complete, the centre line of the whole length of the line of shafting will be true and level. This is not necessarily the case, if the diameter of the shafting varies and a spirit-level is used directly upon the shafting itself.

In further explanation, however, it may be well to ill.u.s.trate the method of applying the gauge shown in Fig. 2590, and the straight-edge C and gauge D shown in Fig. 2592, in cases where there are in the same line sections of shaftings of different diameters. Suppose, then, that the line of shafting in our example has a mid-section of 2-1/4 inches diameter, and is 2 inches at one, and 2-1/2 inches in diameter at the other end: all we have to do is to mark on the gauge, shown in Fig.

2590, two extra lines, denoted in figure by D and F. If the line C was at the proper distance from a for the section of 2-1/4 inches in diameter, then the line D will be at the proper distance for the section of 2 inches, and E at the proper distance for the section of 2-1/2 inches in diameter; the distance between C and D, and also between C and F, being 1/8 inch, in other words, half the amount of the difference in diameters.

In like manner for the horizontal adjustment, the gauge piece shown at D in Fig. 2592 would require when measuring the 2-1/4 inch section to be 1/8 inch shorter than for the 2 inch section, while for the 2-1/2 inch section would require to be 1/8 inch shorter than that used for the 2-1/4 inch section, the difference again being one-half the amount of the variation in the respective diameters. Thus the whole process is simple, easy of accomplishment, and very accurate.

If the line of shafting is suspended from the joists of a ceiling instead of from uprights, the method of procedure is the same, the forms of the targets being varied to suit the conditions. The process only requires that the faced edges of the targets shall all stand plumb and true with the stretched line. It will be noted that it is of no consequence how long the stretched line is, since its sag does not in any manner disturb the correct adjustment, but in cases where it is a very long one it may be necessary to place pins that will prevent it from swaying by reason of air currents or from jarring.

The same system may be employed for setting the shafting hangers, the bores of the boxes being used instead of the shafting itself.

CHAPTER x.x.x.--LINE SHAFTING.

LINE SHAFTING.--A line of shafting is one continuous run or length composed of lengths joined together by couplings. The main line of shafting is that which receives the power from the engine or other motor, and distributes it to other lines of shafting, or to the various machines to be driven. In some practice each line of shafting is driven by a separate engine or motor, so that it may be stopped without stopping the others. This same object may be obtained by providing a clutch for each line. It is obvious that in each line of shafting the length nearest to the driving motor transmits the whole of the power transmitted by the line, and that the diameter of the shafting may, therefore, be reduced as it proceeds from the engine in a proportion depending upon the degree to which the power it is required to transmit is reduced. It is desirable, therefore, so far as the shafting is concerned, to place the machines requiring the most power to drive as near as possible to that end of the shafting that receives power from the motor. Line shafting is supported in bearings provided in what are termed hangers, which are brackets to be bolted to either suitable framing, to walls, posts, or to the ceiling or floor of the building.

The short lengths of shafting that are provided to effect changes of speed, and to enable the machine to be stopped or started at pleasure, are termed countershafts. When there is interposed a countershaft between the motor and the main line of shafting, it is sometimes termed a jack shaft.

Shafting is usually made cylindrically true either by special rolling processes as in what is known as "cold-rolled," or "hot-rolled"

shafting, or else it is turned up in the lathe. In either case it is termed bright shafting. What is known as black shafting is simply bars of iron rolled by the ordinary process and made cylindrically true only where it receives its couplings, and for its journal bearings, &c. The diameter of black shafting varies by a quarter of an inch, and is usually above its designated diameter by about 1/32 inch.

The main body of the shafting not being turned cylindrically true and parallel, the positions of the pulleys cannot be altered upon the shafts, nor can pulleys be added to the shaft as occasion may require without the sections being taken down and seatings turned for the required pulleys to be added. Furthermore black shafting does not run true, and is in this respect also objectionable. Nevertheless, black shafting is used for some special cases where extra pulleys are not likely to be required and the shafting is exposed to the weather, as in the case of yards for the manufacture of building bricks.

The diameters of bright or turned shafting (which is the ordinary form in which shafting is made, unless otherwise specified) vary by 1/4 inch up to about 3-1/2 inches in diameter; but the actual diameter is 1/16 inch less than the denominated commercial diameter, which is designated from the diameter of the round bar iron from which the shafting is turned; thus a length of what is known as 2-inch shafting will have an actual diameter of 1-15/16 inches, being parallel, or as nearly parallel as it is practicable to turn it in the ordinary lathe.

Cold-rolled shafting has its actual diameter agreeing with its designated or commercial diameter, and is parallel throughout its length.

In England the diameters of shafting vary by eighths of inches for diameters of an inch and less, and by quarters of an inch for diameters above an inch, the commercial and the actual diameters being alike.

The strains to which a line of shafting is subject are as follows: The torsional strain due to rotating the line of shafting, independent of the power transmitted; the torsional strain due to the amount of the power transmitted; and the transverse strain due to the unequal belt pressures and distances from the bearings of the driving or transmitting pulleys. The first and the last are, however, so intimately connected in practice that they may be considered as one: hence we have, 1st, the torsional strain due to driving the whole load, and, 2nd, the transverse strain due to the belt pressures being exerted more on one side than on another of the shaft, and to the belt pulleys being at unequal distances from the hanger bearings.

The first may be reduced to a minimum by so proportioning the strength of the line of shafting that it shall be capable of transmitting the required amount of power at the various sections of its length without suffering distortion of straightness beyond certain limits, and shall be at the same time as light as is consistent with this duty and a certain factor of safety.

Referring for a moment to the above limitation, the weight of the shaft itself will cause it to deflect between the hanger bearings, and the amount of this deflection will depend upon the distance apart of the points of support, or, in other words, of the distance apart of the hanger bearings.

The second may be reduced to a minimum by so regulating the distance apart of the hanger bearings that the deflection of the shaft from the belt pressures shall not be sufficient to produce sensible irregularities in the axis of rotation of the shaft; by so connecting the bearings to the hangers that they shall be rigidly held, and yet capable as far as possible of automatically adjusting their bores to be true with the shaft axis, notwithstanding its deflection from any cause; by placing the pulleys transmitting the most power as near to the hanger bearings as practicable; by so disposing the driving belts as to deliver the power as near as possible equally on all sides of the shaft; and by having the shafting and the pulleys balanced so as to run true, so that the strains on the pulleys shall be equal at each point in the shaft rotation. From this it appears that the distance apart of the shafting hangers may vary according to the amount of power transmitted by a shaft of a given diameter. The following table (given by Francis) gives the greatest admissible distances between the bearings of continuous shafts subject to no transverse strain except from their own weight, as would be the case were the power given off from the shaft equally on all sides, and at an equal distance from the hanger bearing.

+----------------+------------------------------------+ | Diameter of | Distance between bearings, in feet.| |shaft in inches.+--------------------+---------------+ | |Wrought-iron shafts.| Steel shafts. | +----------------+--------------------+---------------+ | 2 | 15.46 | 15.89 | | 3 | 17.70 | 18.19 | | 4 | 19.48 | 20.02 | | 5 | 20.99 | 21.57 | | 6 | 22.30 | 22.92 | | 7 | 23.48 | 24.13 | | 8 | 24.55 | 25.23 | | 9 | 25.53 | 26.24 | +----------------+--------------------+---------------+

These conditions, however, do not usually obtain in the transmission of power by belts and pulleys, and the varying circ.u.mstances of each case render it impracticable to give any rule which would be of value for universal application.

For example, the theoretical requirements would demand that the bearings be nearer together on those sections of shafting where most power is delivered from the shaft, while considerations as to the location and desired contiguity of the driven machines may render it impracticable to separate the driving pulleys by the intervention of a hanger at the theoretically required location. The nearer together the bearings the less the deflection either from the shaft's weight or from the belt stress, and since the friction of the shaft in its bearings is theoretically independent of the journal-bearing area, the closer the bearings the more perfect the theoretical conditions; but since it is impracticable to maintain the true alignment of the shaft, and as the friction due to an error in alignment would increase with the nearer proximity of the bearings, they are usually placed from about 7 to 12 feet apart, according to the facilities afforded in the location in which they are to be erected.

It is to be observed, however, that the nearer together the bearings are the less the diameter, and, therefore, the lighter the shafting may be to transmit a given amount of power, and hence the less the amount of power consumed in rotating the shafting in its bearings.

COLD-ROLLED SHAFTING--This is shafting made cylindrically round and parallel by means of cold rolling, which leaves a smooth and bright surface. The effects of cold rolling upon the metal have been determined by Major Wm. Wade, U.S.A., Sir William Fairbairn, C.E., and Professor Thurston, of the Stevens Inst.i.tute, as follows:--

The experiments were made upon samples of cold-rolled shafting submitted by Messrs. Jones and Laughlins, of Pittsburgh, Pennsylvania.

SUMMARY OF THE RESULTS OBTAINED BY MAJOR WADE FROM NUMEROUS EXPERIMENTS WITH ORDINARY HOT-ROLLED BAR IRON, COMPARED WITH THE RESULTS OBTAINED FROM THE SAME KINDS OF IRON ROLLED AND POLISHED WHILE COLD BY LAUTH'S PATENT PROCESS.

------------------------------------------+-------------+---------+--------- | Iron rolled | Ratio of|Average | while | increase|rate per +------+------+ by cold |cent. of | Hot. | Cold.| rolling.|increase.

------------------------------------------+------+------+---------+--------- TRANSVERSE.--Bars supported at both | | | | ends; load applied in the middle; distance| | | | between the supports, 30 inches. Weight | | | | which gives a permanent set of one-tenth | | | | of an inch, viz. | | | | } 1-1/2 inch square bars | 3,100|10,700| 3.451 |} } Round bars, 2 inch diameter| 5,200|11,100| 2.134 |} 162-1/2 } Round bars, 2-1/4 " " | 6,800|15,600| 2.294 |} | | | | TORSION.--Weight which gives a permanent| | | | set of one degree, applied at 25 inches | | | | from centre of bars. Round bars, 1-3/4 | | | | inch diameter, and 9 inches between the | | | | clamps | 750| 1,725| 2.300 | 130 | | | | COMPRESSION.--Weight which gives a | | | | depression, and a permanent set of | | | | one-hundredth of an inch to columns 1-1/2 | | | | inches long and 5/8 inch diameter |13,000|34,000| 2.615 | 161-1/2 Weight which bends and gives a permanent | | | | set to columns 8 inches long and 3/4 inch | | | | diameter, viz. | | | | } Puddled iron |21,000|31,000| 1.476 |} } Charcoal bloom iron |20,500|37,000| 1.804 |} 64 | | | | TENSION.--Weight per square inch, which | | | | caused rods 3/4 inch diameter to stretch | | | | and take a permanent set, viz. | | | | } Puddled iron |37,250|68,427| 1.837 |} } Charcoal bloom iron |42,439|87,396| 2.059 |} 95 Weight per square inch, at which the same | | | | rods broke, viz. | | | | } Puddled iron |55,760|83,156| 1.491 |} } Charcoal bloom iron |50,927|99,293| 1.950 |} 72 | | | | HARDNESS.--Weight required to produce | | | | equal indentations | 5,000| 7,500| 1.500 | 50 ------------------------------------------+------+------+---------+---------

NOTE.--Indentations made by equal weights, in the centre, and near the edges of the fresh cut ends of the bars, were equal; showing that the iron was as hard in the centre of the bars as elsewhere.

GENERAL SUMMARY OF THE RESULTS OBTAINED BY SIR WILLIAM FAIRBAIRN'S EXPERIMENTS.

--+-------------------+-----------+-------------------+------------ | | Breaking | | Strength, | Condition of bar. | weight of |Breaking weight per| the un- | |bar in lbs.| square inch. |touched bar | | | |being unity.

--+-------------------+-----------+-------------------+------------ | | | In lbs. In tons. | 1| Untouched (black) | 50,346 | 58.628 26.173 | 1.000 3| Rolled cold | 69,295 | 88.230 39.388 | 1.505 4| Turned | 47,710 | 60.746 27.119 | 1.036 --+-------------------+-----------+-------------------+------------

NOTE.--In the above summary it will be observed that the effect of consolidation by the process of cold rolling is to increase the tensile powers of resistance from 26.17 tons per square inch, to 39.38 tons, being in the ratio of 1:1.5, one-half increase of strength gained by the new process of cold rolling.

Extract from the general conclusions arrived at by Professor R. Thurston from experiments.