Chapter II: Hand and Power Looms (2)

We see from Fig. 61 that in a loom with a 4-inch crank and 11-inch arm, the velocity of the slay is much greater when beating up than with the 3-inch crank and 12-inch arm.

The effect of the length of the crank-arm on the velocity of the slay can easily be shown by a diagram or by calculation. If the length of the crank-arm be altered without altering the length of the crank, there will be found a somewhat quicker movement of the slay at the beat up in the loom with the shorter arm. The difference is not so great when the crank-arm is a long one in proportion to the crank. The chief cause of the difference in the velocity of C in Fig. 61 is the difference in the length of the _crank_. It is obvious that the longer the crank the greater the angle which it will cause the arm to make, and therefore the greater will be the acceleration of the velocity of C when the angle is closing and the slay moving forward. Likewise, it is obvious that the shorter the arm the larger will be the angle to close, but the _principal_ thing to notice is that an increase in the length of the _crank_ causes an increase in the velocity of the slay _owing to the extra distance which it has to travel in each revolution_; so that even if the crank-arm were lengthened in exact proportion to the increase in the length of the crank, so as to keep the angle to be closed in beating up the same, there would still be a considerable increase in the velocity of the slay, caused by the extra distance it has to travel. This lengthening of the crank has obviously much more to do with the increase in velocity of the slay than the shortening of the arm has.

The longer the crank the further back from the cloth will the slay be taken, and assuming that the shed is open for the shuttle when the crank is at the bottom centre, a long crank is obviously more suitable for a wide loom, as, having to move further back, it will allow a longer time for the shuttle to pass through the shed than a short crank would; therefore the wider the loom, the longer the crank is required to be to allow time for the shuttle to pass.

The time allowed for the passage of the shuttle may also be increased by using a short arm so as to increase the eccentricity of the slay.

The longer the crank, the greater the velocity of the slay, therefore a long crank is suitable for _heavy_ work, as it stores up more force in the slay than a short one. The force may also be increased by shortening the crank-arm, thus increasing the eccentricity of the slay.

The position of the crank-shaft in relation to the connecting pin has some effect upon the _eccentricity_ of the slay’s movement. Fig. 62 shows this, but to see clearly the effect it would be advisable to make an accurate drawing to a large scale. Four positions of the crank-shaft are shown. The one on the line A is just a little below the level of the connecting pin, so that the pin moves as nearly as possible on the line A when making the front quarter of its stroke. The circle on the line B is the position where the pin moves as nearly as possible on line B when at the back quarter of its stroke; D is any higher plane, and C any lower one. Divide the stroke of the connecting pin LR into four equal parts, and from S, with the crank-arm in the compasses, cut the circles with the arc E, and from T cut the circles with the arc F. It will be found that in the circle A, OP is slightly longer than in any of the other circles; therefore this is the position where the beat up is slowest. It will also be found that in the circles B and C there is scarcely any difference in OP, therefore sinking the crank-shaft from within reasonable limits makes very little difference; if anything, there is a slight decrease in the size of OP as the plane is lowered, but it is very slight, and the increase in the velocity of slay would also be very slight. On the other hand, by raising the crank-shaft to D a considerable increase in the velocity of the slay in beating up takes place, as it will be found that in this circle OP is much less than in the others.

At the back of the stroke it will be found that in the plane B the distance XY is least; therefore there is here the least dwell of the slay at the back of its stroke with the shaft in this position. This is because the pin moves as nearly as possible on the line B whilst the crank is at the back part of its stroke. As the crank is raised or lowered the dwell at the back increases slightly.

Reversing the direction of the loom makes a difference in the beat-up.

It will be found that in the circle A, OP and ON are about equal, therefore there will be scarcely any change in the velocity of beat-up by reversing the loom; but as the shaft is lowered ON will be found to become less than OP, and therefore a quicker blow is given by reversing the loom if the shaft is in this position. If the shaft is raised, as in the case of circle D, it will be found that ON becomes _greater_ than OP; therefore with the crank above A, reversing the direction of the loom will cause a slower and weaker beat-up.

In the diagram, Fig. 62, the crank and crank-arm are the same length for each position, the centre of the shaft being indicated by the dotted arc.

_Timing of the Primary Movements._

The primary movements, shedding, picking, and beating up, are timed differently in relation to each other in weaving different classes of fabrics. For plain cloths, or other cloths where a good cover is required--that is, where the warp has to be spread--the crank should be set about the top centre when the healds are crossing each other. At Fig. 38 the loom is timed in this manner. When so timed it is obvious that the shed will be considerably or altogether open when the reed is in contact with the cloth. By sinking the centres of the healds below a line drawn from the temple to the back rest, the upper portion of the shed is always slack, and if the pick is beaten up in a crossed shed, the loose ends of the warp are spread between the taut ones. In Fig. 63 the straight line AB is drawn from the front carrier A to the back carrier B. The centres of the healds when level are on the line ACB, the point C being a little way below the line AB. When one stave is lifted a certain distance and the other goes down the same distance, it is obvious that the upper portion of the warp will be slacker than the lower portion, because the line ADEFB is shorter than ADGFB, and when the reed beats up with the warp in this position the slack ends are spread between the taut ones, thus giving a good cover to the cloth and preventing the reed marks from showing. Each set of ends alternately becomes slack.

Another advantage of beating up when the shed is crossed or partly open for the succeeding pick is that the pick is held more firmly in position than when the shed is not crossed, and therefore the picks can be got in better.

In twilled cloths the boldness of the twill is somewhat affected by the warp being spread, and these cloths are often preferred when made without the healds having been sunk.

If the dwell on the tappet is equal to one-third of a pick, as in Fig. 64, the line D will mark the point of the tappet when the crank is at the top centre. When the crank has made one quarter of a revolution and is at the front centre with the reed in contact with the cloth, the point E will be acting on the treadle bowl. It will be seen that here the shed is almost fully open. When the crank is at the bottom centre the point G will be acting on the bowl, and the shuttle should just be entering the shed. When the point H of the tappet is acting on the bowl the shed will be commencing to close, and the shuttle must be just leaving the shed. When the point I is acting on the bowl the crank will be at the back centre, and when the crank reaches the top centre the healds will be again level.

If the dwell on the tappet is more than one-third pick, and at the commencement the crank is set on the top centre with the healds level, the shed will keep open longer for the shuttle to pass through, and would be more open when the crank reached the front centre. It will be obvious that for a wide loom a longer dwell is required than for a narrow loom.

By having the shed fully open before the shuttle enters the shed, the warp is spread and a good cover put on the cloth, but all this dwell is taken off the time which would otherwise be allowed for opening and closing the shed, and therefore means extra strain on the warp.

If it is not necessary to spread the warp, the shed need not be fully open until the shuttle is entering the shed. In this case the greatest possible amount of time is allowed for opening and closing the shed, thus putting as little strain as possible on the warp.

_Speed of Tappets._

As previously stated, the bottom shaft in the loom, being the one used for picking, revolves at one-half the speed of the crank-shaft, and therefore plain cloth tappets may be fastened on the bottom shaft. Tappets of more than two picks to the round are usually fixed on a counter-shaft, S (Fig. 65), in looms with inside tappets. Sometimes the wheel E is geared directly into the wheel C on the bottom shaft, but usually a carrier-wheel, D, is used to convey the motion from the bottom shaft. The number of teeth in the carrier wheel has no effect on the speed of the tappets, as it is used simply to fill up the space between the bottom and counter-shafts.

If the wheel on the crank-shaft A contains 45 teeth, and the wheel B 90 teeth, C 40 teeth, and E 60 teeth, the tappet-shaft S will be making one revolution for three revolutions of the crank-shaft; therefore these wheels will do for three-end twill tappets. This may be proved by multiplying the drivers together and the drivens together, and dividing one by the other, thus--

(90 × 60)
--------- = 3
(45 × 40)

It is usual to place two or three wheels on the bottom shaft of the loom, so that any one of them may be geared into the carrier wheel D, each giving the required speed for different tappets. If a 40 wheel, a 30 wheel, and a 24 wheel are placed on the bottom shaft in such a manner that they can be moved along the shaft and any one of them be geared into the carrier wheel, any 3, 4, or 5 pick tappets can be driven with these wheels. We have seen that a 40 wheel at C gives three picks to the round.

Suppose the 30 wheel at C is geared into the carrier wheel, we get--

(drivens 90 × 60)
----------------- = 4,
(drivers 45 × 30)

or the relative speed of the tappets and crank-shaft are as 1:4; therefore these wheels may be used for any tappets with four picks to the round.

If the 24 wheel is at C, we get:

drivens 90 × 60
------- = 5,
drivers 45 × 24

and thus we get the proper rate of speed for tappets five pick to the round.

Some loom makers use the wheel E as a change wheel. With a 24 wheel C and a 36 wheel E we get three picks to the round, thus--

drivens 90 × 36
------- = 3
drivers 45 × 24

With a 24 wheel C, a 48 wheel E gives 4 picks,

With a 24 wheel C, a 60 wheel E gives 5 picks,

With a 24 wheel C, a 72 wheel E gives 6 picks.

_Example._--Find the number of teeth for the wheel C on the bottom
shaft to drive tappets seven picks to the round, wheel on tappets
63 teeth.

90 × 63
------- = 18 wheel. _Ans._
45 × 7

Woodcroft’s tappets, as a rule, are driven directly from the crank-shaft. As these tappets are usually of a large circumference, a large wheel on them is of no disadvantage, although sometimes intermediate wheels are used.

If the tappets are twelve to the round, and the wheel on the tappets contains 192 teeth, a driving wheel of 16 teeth will be required on the crank-shaft.

192
--- = 12 picks to the round
16

For driving outside tappets, as in Fig. 39, a driving wheel on the crank-shaft and two intermediate wheels are generally used. The tappets are placed on the bottom shaft outside the loom, but they are loose upon the shaft, and can, of course, be made to revolve at a different speed to the shaft, either in the same or in the opposite direction. This system of driving the tappets is shown at Fig. 66. The wheel A, on the crank-shaft, drives the wheel B, on an intermediate stud; the wheel C, on the same centre, drives the tappet wheel D.

To find the wheel on the crank-shaft, or the first driver, the other wheels being as follows: first driven wheel, B, 36 teeth; second driver, C, 12 teeth; tappet wheel, D, 120 teeth.

Multiply the two driven wheels together, and divide by the given driver multiplied by the picks to the round, thus--

36 × 120
-------- = 40 first driver, A.
12 × 9

To find the second driver for eight picks, the other wheels being: first driver, A, 20; first driven, B, 40; second driven, D, 60.

The given driver multiplied by the picks to the round, 20 × 8 = 160; the drivens multiplied together, 40 × 60 = 2400; 2400 ÷ 160 = 15 wheel required.

To find either of the driven wheels, multiply the two drivers and the picks together, and divide by the driven given wheel, thus--

_Example._--Find the wheel for the tappets, D, for 10 picks to
the round, the other wheels being: first driver, 16 teeth; first
driven, 32 teeth; second driver, 20 teeth.

16 × 20 × 10
------------ = 100 wheel required
32

To find both intermediate wheels, multiply the given driver by the picks to the round, and as the product is to the teeth in the tappet wheel, so is the required driven to the required driver.

_Example._--Find the two intermediates for 10-pick tappets, if the
wheel on the crank-shaft has 18 teeth, and the wheel on the tappets
120 teeth. The 18 × 10 = 180, and therefore the two required
wheels must be in the proportion of 180 to 120, the former being
the driven wheel. Thus a 36 driven and a 24 driver will give the
required speed to the tappets. That this is correct may be seen
from the following:--

18 × 24
-------- = 10 picks
36 × 120

That the required wheels must be in this proportion will be apparent from the fact that if the wheel B has ten times the number of teeth in A, then B is revolving at the speed at which the tappets are to move; therefore if the wheel C has the same number of teeth that D has, the speed of the tappets will remain the same.

FAST-AND LOOSE-REED LOOMS.

One of the most important motions in the power loom is that by which the loom is stopped automatically when the shuttle is caught in the shed or for some reason does not enter the shuttle-box. A motion of this kind has always been considered necessary since the introduction of the power loom. If the shuttle be caught in the shed as the reed is beating up, it is obvious that great damage to the warp must result unless the loom is brought to a sudden stop or the reed thrown out. The oldest form of protector is the “stop rod.” In this the reed is fast, and if the shuttle is caught in the shed or flies out, the loom is brought to a sudden stop before beating up. Fig. 67 will illustrate the principle of this motion. If the shuttle enters the box safely it presses back the swell S, which projects inside the box and is held there by a spring. As the swell is pressed back it raises the lever B above the frog F as the slay beats up. If the shuttle for any reason does not enter the box, the swell is not pressed back, and as the slay moves forward in beating up, the lever B catches the frog F, which is moved a little and applies the brake G, and also knocks off the loom handle H, which removes the belt on to the loose pulley. Before the application of the brake to this motion the frog was fixed to the framework of the loom, and it will easily be understood that the concussion caused many breakages. A stop rod protector was patented in 1791, but the brake was not applied until 1840 or thereabouts.

The loose reed is a better way of preventing damage to the warp by the shuttle being caught. If the shuttle is caught in the shed it throws out the reed and stops the loom. Its action will be understood from Fig. 68. A rod, C, runs underneath the shuttle-race at the back of the slay, and the finger B is fastened to it. The reed is held in position by a board, A, which is also connected to the rod C, as shown in the diagram. If the shuttle is caught in the shed, it presses back the reed and the board A, and lifts the finger B to the upper side of the frog F, and as the slay moves forward it throws the board A further back and the reed out at the bottom, and the lever H is brought into contact with the loom handle, and the loom is stopped. If the shuttle passes safely through the shed, the reed is not pressed back, and the finger F comes under the frog as the slay gets to the front of its stroke, and holds the reed comparatively fast. The disadvantage of the loose reed is that the reed is not sufficiently firm to put a large amount of weft into the cloth, but improvement is being made in this respect, and loose-reed looms are to-day made for weaving fabrics for which it was formerly necessary to have fast reeds.

The invention of the loose reed is generally attributed to Mr. James Bullough. It was invented about 1842.

THE WEFT FORK STOP MOTION.

One of the most useful adjuncts to the power-loom is the motion for stopping the loom when the weft breaks or runs out. Fig. 69 will explain the principle of this useful contrivance. The grid A is placed at the side of the reed between the reed and the shuttle-box, and the fork is so placed that as the grid moves forward the prongs of the fork pass through it. When the weft comes between the fork and the grid it raises the end of the fork E, out of the way of the hammer H, which is moved forward every two picks by a cam or lifter, D, on the bottom shaft of the loom. If the weft breaks or runs out the fork will of course pass through the grid, and it is so balanced that the hook E will be caught by the hammer and the loom handle knocked off. The invention of the Weft Fork Stopping Motion is claimed by several persons, but it was perfected about the year 1842, when the brake was applied to it. The action of this brake is illustrated at Fig. 70. When the handle is pushed sideways in starting the loom it lifts the rod R and the lever L, and thus takes the brake off. When the handle is knocked off by the weft fork being caught, the lever L drops and the brake is applied. The brake power can be regulated by altering the position of the weight on the levers.

In looms with change boxes at both sides the weft fork is often placed in the middle of the loom. It is obvious that when several shuttles are used there will always be some weft threads opposite the grid in the ordinary weft fork motion, and this renders it inoperative in this class of looms. It is therefore necessary to have a fork to feel for each pick separately.

TAKING-UP OR COILING MOTIONS.

There are two distinct classes of taking-up motions--the positive, and the negative or drag motion. In the former the cloth is taken up a small but regular distance each pick, and the number of picks per inch can be regulated to a fraction. Fig. 71 is the common form of positive take-up motion. A ratchet wheel or “rack wheel,” A, is moved forward one tooth every pick by a click or catch, M, operated by a projection, G, on the slay sword. As the slay moves forward the rack wheel is moved one tooth, and the holding catch or detent N prevents it from going back. There are five wheels in the train, and the names usually given to them are as follows: A, rack wheel; B, change wheel; C, stud wheel; D, stud pinion; E, beam wheel. The emery taking-up roller is marked F. The cloth, as it is woven, is drawn forward by the emery roller and is wound upon the cloth roller, which is pressed against the emery roller by weighted levers, and is turned by friction.

The speed at which the emery beam roller is turned regulates the number of picks per inch, and as changes are constantly required in most weaving mills, the wheel B is usually taken as a change wheel. As this wheel is a driver, a smaller wheel will make the emery roller move slower, and therefore more picks will be put in the cloth, and a larger wheel will drive the emery roller quicker, and as a consequence a smaller number of picks will be put in. If the rack wheel has 50 teeth, the stud wheel 120 teeth, the stud pinion 15 teeth, and the beam-roller wheel 75 teeth, the beam roller being 15 inches in circumference, and if the change wheel used has 25 teeth, the number of picks per quarter-inch will be 20.

This may be proved by multiplying the drivers together and by the circumference of the emery beam roller in quarter-inches for a divisor, and multiplying the drivens together for a dividend: the quotient will be the number of picks per quarter-inch.

DRIVERS.
25
15
---
125
25
---
375
60 quarter-inches
----- in beam
22500

DRIVEN.

50
120
----
6000
75
------
22500)450000(20 picks per
45000 quarter-inch
------
0

When the cloth is taken out of the loom, rather more than this number of picks will be counted, as there is not the same tension as when the cloth is being woven. It is usual to allow about 1½ per cent. for this shrinkage.

For the purpose of easy calculation the dividend of the loom is obtained; that is, the change wheel required to give one pick per quarter-inch. By using this as a dividend and dividing by the number of picks required in a quarter-inch, the quotient will be the change wheel required; and, _vice versâ_, by dividing by the change wheel, the number of picks given by that wheel can be obtained.

To find the dividend of a loom--

Multiply the rack, stud, and beam wheel together for a dividend, and the stud pinion and the number of quarter-inches in a circumference of the emery beam for a divisor, and the quotient will be the mathematical dividend. Add 1½ per cent. to this for the practical dividend.

With the wheels given in Fig. 71 the dividend will be as follows:--

15 stud pinion
60 quarter-inches in circumference of beam
---
900

50 rack wheel
120 stud wheel
----
6000
75 beam wheel
----
900)450000(500 mathematical dividend
4500
------
00

500
7 = 1½ per cent. for shrinkage
---
507 =practical dividend=.

Having the dividend, it is only necessary to divide by the picks to obtain the change wheel required, or to divide by the teeth in the change wheel to obtain the picks which it will give, thus--

507 507
picks --- = 39 change wheel change wheel = --- = 25⅓ picks.
13 20

The following are the wheels used by various loom makers:

+------+------+-------+------+-------------+---------+
| | | | |Circumference| |
|Rack |Stud |Stud |Beam | of beam in |Dividend.|
|wheel.|wheel.|pinion.|wheel.| inches. | |
+------+------+-------+------+-------------+---------+
| 50 | 120 | 15 | 75 | 15 | 507 |
| 60 | 120 | 15 | 75 | 15 | 609 |
| 50 | 146 | 14 | 90 | 15 | 794 |
| 50 | 100 | 12 | 75 | 15 | 528 |
| 60 | 100 | 12 | 75 | 15 | 634 |
+------+------+-------+------+-------------+---------+

_Example._--Find the dividend of a loom with a rack wheel 60 teeth,
stud wheel 100 teeth, stud pinion 12 teeth, beam wheel 75 teeth,
beam 15 inches circumference.

rack stud beam wheel
60 × 100 × 75
------------------ = 625 mathematical dividend
12 × 60 9 = 1½ per cent.
stud quarter-inches ---
pinion in beam 634 practical dividend.

It is not possible by changing one wheel only to obtain any number of picks or fraction of a pick, as will be seen from the following examples:--

507
picks --- = 12·67
40

507
picks --- = 12·12
41

507
picks --- = 12·07
42

For the lower number of picks the motion does fairly well, but for the higher numbers of picks the changes cannot be made with sufficient exactitude by changing a single wheel. Even in the lower picks it is now required to make the smallest fractional changes.

An improved arrangement of wheels is now largely adopted. This is Pickles’ motion. Fig. 72 shows the train of wheels. The change wheel B is in this case a driven wheel, and therefore if a larger wheel is used it will give a larger number of picks in the cloth, and if a smaller wheel is used it will give a smaller number of picks; so that if the wheels are so proportioned that the change wheel B has the same number of teeth that there are picks per quarter-inch, it will always remain so, whatever size the wheel is. If a 20 driven wheel gives 20 picks, a 30 will give 30 picks, and so on.

The wheel A is also changed, and this is usually called the “standard” wheel. This is a driver wheel, and therefore a smaller wheel gives more picks, and _vice versâ_. The wheels are so proportioned that if A, the standard wheel, has nine teeth, each tooth in B, the change wheel, represents one pick, and therefore, this wheel being a driven, the number of teeth in it will also represent the number of picks per quarter-inch. If an 18 standard wheel is used, it is obvious that the emery beam will be driven twice as fast, therefore each tooth in the change wheel B will then represent half a pick per quarter-inch. With a 27 standard each tooth in the change wheel B will represent one-third of a pick. With a 36 standard each tooth in B will represent a quarter of a pick per inch.

The wheels mostly used are those in the diagram, and supposing we have a 36 standard and a 45 change wheel, and taking the emery beam as 15·05 inches in circumference, we get--

B
24 × 45 × 89 × 90
-------------------- = 11·088
36 × 24 × 15 × 60·20
A ·166 = 1½ per cent. for shrinkage
------
11·254 picks per quarter-inch.

Thus with a 36 standard a 45 change wheel, B, gives 11¼ picks per quarter-inch, or each tooth in the change wheel gives a quarter of a pick per quarter-inch.

By changing these two wheels any fraction of a pick can be obtained. Thus if 13½ picks per quarter-inch are required, the wheels used would be an 18 standard and a 27 change wheel. For 13⅔ picks a 27 standard and a 41 change wheel would be used, and so on.

The following examples will fully illustrate the principle of this motion:--

+-------------+--------+------+
| Picks per |Standard|Change|
|quarter-inch.| wheel. |wheel.|
+-------------+--------+------+
| 20 | 9 | 20 |
| 15½ | 18 | 31 |
| 14⅓ | 27 | 43 |
| 14⅔ | 27 | 44 |
| 13¼ | 36 | 53 |
| 13¾ | 36 | 55 |
| 12⅕ | 45 | 61 |
+-------------+--------+------+

It is not always customary to change the wheels in the above manner, as a different value is often given to each tooth in the change wheel by altering the standard wheel, otherwise than by multiples of nine.

Any number may be made the basis of a train of wheels of this kind; there is no reason why it should be nine more than any other number, and in adapting looms from the ordinary five-wheel motion to this principle, it is not necessary to get all new wheels, as sow of the old ones may be made to form part of the train.

There are several kinds of negative or drag take-up motions. One of the older forms is that given in Fig. 73. A lever, AB, centred at C is weighted on the arm B. A small cam, D, on the crank-shaft presses down A every pick and lifts the catch E, which operates the ratchet wheel F. As the weights drop they act as a drag upon the ratchet wheel. A small pinion on the same centre as the ratchet wheel drives the wheel G on the cloth beam. The cloth in a negative motion is wound directly on to the cloth beam, and thus there is no risk of damaging the finer fabrics, as is the case when an emery beam is used, as in a positive motion. The number of picks put in the cloth is regulated by the weights on the lever B; the greater the weight the less the number of picks, and _vice versâ_.

The action of a negative motion is as follows:--As the slay beats up, the cloth between the cloth beam and the reed is slackened a little, and the weights on the lever at that moment act as a drag upon the ratchet wheel F. The holding catch is usually a double one, and will hold the ratchet wheel when taken forward the space of half a tooth.

By increasing the drag upon the ratchet wheel, a slighter blow from the slay will enable the weights to act, and thus less weft is put into the cloth. If a loom is regulated so as to put a certain number of picks per inch into the cloth of a given count of weft, and weft of a finer count is then used, it is obvious that the number of picks per inch would be increased. If the weft varies in thickness the negative motion compensates for this somewhat, by putting more picks in where it is thinner, and thus a more even thickness of cloth is produced than where a positive motion is used.

As the cloth is wound on the beam the circumference of the latter gradually increases, and consequently there would be a gradual alteration in the amount of weft put into the cloth, owing to the difference in leverage. It is necessary, therefore, to count the cloth and adjust the weights at intervals in order to keep the number of picks regular.

Another kind of negative take-up motion is shown at Figs. 74 and 75. This is now more generally used than the other kind. The cloth beam A is driven by a screw, S. The ratchet wheel B is fastened to the screw-shaft, and the method of operating the ratchet wheel will be seen from Fig. 75, which is another view of the mechanism. A short lever, E, is attached to the rocking shaft K, and as the slay moves backwards from the cloth the weights W are lifted a little, and when the slay moves forward, the weights, acting through the catch M, will take the ratchet wheel forward a tooth, or half a tooth, as the case may be. There is usually a double-holding catch N, which will hold the wheel if taken forward half a tooth. When the ratchet wheel has made one revolution, the wheel on the cloth beam will only have been moved one tooth by the screw, so that the required slow movement of the cloth beam is obtained by very simple means. There is a hand wheel, P, for unwinding the cloth readily. The negative motion is used principally in weaving the heavier classes of cotton fabrics and those in which there is a large number of picks per inch, such as velvets, and similar fabrics. Its advantages are that the cloth is wound directly on to the cloth beam, and cannot therefore be injured by an emery beam, and that it makes the cloth of a more even thickness, as it compensates for any variation in the thickness of the weft; and its disadvantages as compared with a positive motion are that it requires frequently adjusting (less frequent, of course, when a very large number of picks are put in, as in velvets), and that it does not put a perfectly regular number of picks in the cloth, as a positive motion does. This latter is the chief objection to it, as even in the lighter makes of common velvets a positive motion is preferred on account of its giving a more evenly picked cloth. In silk looms, where it is absolutely necessary to dispense with an emery beam, a very large cloth beam is used, and the cloth is wound directly on to the cloth beam although the take-up is positive. The cloth beam is sometimes over a yard in circumference, so that it will hold a fair length of cloth without making much difference in the number of picks. The cloth is taken off the beam frequently, or the gradual change in the thickness of the cloth beam would cause the piece to get too thin. This would, of course, not do for cotton goods.

Another ready method of obtaining any required pick in a positive motion is used in the East Lancashire districts. Seven wheels are used, as in Pickles’ arrangement, but the ordinary wheels of a 507 dividend (or other dividend) are used, and in addition the two wheels B and C, as in Pickles’ (Fig. 72), are introduced. The wheel B, the driven wheel, is called the standard in this arrangement; and suppose it is required to put 15 picks per quarter-inch in the cloth with the rack wheel 50 teeth, stud wheel 120, stud pinion 15, beam wheel 75, beam 15 inches circumference. The standard used is a 24--this, it must be borne in mind, is in this case a driven wheel. Then by multiplying the dividend of the five-wheel motion, viz. 507·5, by 24, the teeth in the standard, and dividing by the picks per quarter-inch required, we get the product of the two drivers, A and C, thus--

507·5 × 24 standard B
--------------------- = 812
15 picks

This 812, then, is the product of the two drivers, and any two convenient wheels which, multiplied together, give this number can be used--thus 812/28 = 29. Therefore the two drivers may have 28 and 29 teeth respectively. The two wheels are found by experiment. If the dividend of the five wheels is 609 a 20 standard wheel is used, and the same drivers as in the preceding case will do. If it is required to change only one wheel, and to have the arrangement such as to give an exact number of picks, or half-picks, or quarter-picks, in the quarter-inch of cloth, by taking the two drivers A and C of such numbers that their product amounts to 507, the number of teeth in the driven wheel B will always equal the number of picks per quarter-inch exactly. Thus 507/13 = 39. Therefore if the drivers A and C have respectively 13 and 39 teeth, every tooth in the driven wheel B will represent one pick per quarter-inch.

Suppose half-picks are required exactly, the method of obtaining the wheels is as follows:--Multiply the 507·5 by 2, which equals 1015, then find two convenient wheels which, multiplied together, produce this number; 35 × 29 = 1015, and the two drivers A and C may be 35 and 29. This will cause every tooth in the driven wheel B to represent half a pick exactly.

Thus with a 35 wheel A, and a 29 wheel C, a 31 wheel B will give 15½ picks per quarter-inch, the other wheels being the same as in an ordinary 507 dividend motion.

The following examples will prove this:--

50 rack × 31 B × 120 stud × 75 beam wheel
------------------------------------------- = 15·27
35 A × 29 C × 15 pinion × 60 quarter-inches

and 15·27
0·23 = 1½ per cent. shrinkage
-----
15·50 picks.

When quarter-picks are required exactly, by changing one wheel only--multiply 507·5 by 4, and the product of the two drivers A and C must equal this. Then every tooth in the driven wheel B will represent a quarter-pick per quarter-inch.

There are many methods of letting off the warp positively, but none are likely to succeed in displacing the older and quite satisfactory method of levers, ropes, and weights. The very fact of making the let-off positive, causes too great a rigidity in the hold of the warp, which is detrimental to the yarn. The frictional let-off is not likely to be replaced in cotton goods weaving unless it be in some of the heavier kinds of fabrics. Where it is a question of putting in as much weft as possible, the positive let-off has an advantage.