_DOBBIES_
The tappet shedding motion is the simplest and most perfect for a
small number of shafts. They may be made to work an indefinite number
of shafts, but it is seldom that above eight or ten are worked with
ordinary tappets, and about sixteen with Woodcroft’s or other plate
tappets.
With dobbies, a higher number of shafts may conveniently be worked, but
it is not only for this reason that dobbies are so extensively used.
They are extensively used for weaving twills, satins, and other simple
weaves, on four or five shafts. The chief advantage they possess is
that any number of shafts within their capacity may be used without
extra trouble or cost; whereas ordinary tappets have to be made
specially for each pattern; whilst section tappets, and oscillating
tappets, are inconvenient.
Dobbies are made to weave up to 40 or more shafts, but 16 and 20 are
the commonest numbers. Most dobbies now used are on the double-lift
principle; indeed, the single-lift dobby or witch machine is almost
obsolete in cotton weaving. The chief kind of double-lift dobby is the
“Hattersley” or “Keighley” dobby. The principle of this machine was
invented by Messrs. Hattersley & Hill, of Keighley, Yorkshire; hence
its name. Since the original patent rights have expired, almost all
loom makers have their own particular form of this dobby, embodying
many more or less minor improvements on the original. The principle
of this dobby will be understood from the lecture diagram, Fig. 91.
The dobby is placed at one side of the loom, and is therefore in
a convenient position for being attended to. The upright rod R is
connected to a crank on the bottom shaft of the loom, and therefore the
rocking lever AB, centred at C, will make one complete movement to and
fro, every two picks. The knives D and E slide along, always retaining
a horizontal position, one going inward as the other comes outward.
[Illustration: FIG. 91.]
The shaft or stave is connected to the jack lever FGH at F, and the
upright MN is fastened to this lever at H, the fulcrum being at G. The
upright MN has two hooks, P and Q, connected to it at opposite ends,
and suppose that when the knife D is in its innermost position, as in
the diagram, the hook P is dropped on to the knife; when the knife
begins to move it will take the top of the upright MN with it until MN
assumes the position indicated by the dotted line M′N, and the stave
is lifted. If it is required to lift the same stave for the succeeding
pick, the bottom hook is then dropped on to the knife E, which at that
moment will be in its innermost position just commencing its outward
movement, and is taken forward by it until the upright MN assumes the
position indicated by the dotted line MN′; and it will easily be seen
that as the top of the upright is moving back from M′ to M whilst the
bottom of the upright is moving forward from N to N′, the centre of
the upright H remains stationary at H′, with the exception of a slight
movement caused by the knife going further back than the hooks, and
thus the stave remains up all the time. The character of the shed is,
therefore, what is termed “open shed”--that is, if a stave is required
up for several picks in succession, when it is lifted it remains up
until it is required to come down again. This is what is meant by “open
shed” as compared with “centre shed,” the characteristic of which is
that the lifted stave, instead of remaining up, is let down halfway
every pick and taken up again if required.
The method of dropping the hooks is as follows:--Two levers, S, T, of
different shapes are employed for each pair of hooks; these levers
are centred on a rod, X. One of the levers, viz. T, is bent from the
fulcrum to touch the bottom hook, and the lever S projects straight out
from the fulcrum, and an upright needle O rests upon it, the top hook
resting upon the upright needle. When the lever SY is lifted a little
at Y it will drop the top hook, and when TY is lifted at Y it will drop
the bottom hook.
In a 16 shaft dobby the parts shown in the diagram are duplicated
sixteen times--that is, there are sixteen uprights MN, each with two
hooks, sixteen levers SY, sixteen like TY, and sixteen of other parts.
The levers SY and TY are operated by lags pegged so as to lift the
staves to give the required pattern. These lags work round a cylinder
or barrel, which is turned round the space of one lag every two picks
intermittently. Each lag operates the hooks for two picks, one row of
pegs operates the top hooks P, and the other row of pegs the bottom
hooks Q. The method of pegging the lags will be understood from Fig.
92, where two lags are shown with the pegging for a two and two twill.
Of course care must always be taken that the pegs are put opposite the
proper levers, as when only a portion of the jacks are used, say eight,
it is often preferred that the staves be connected to eight jacks in
the middle of the machine.
[Illustration: FIG. 92.]
[Illustration: FIG. 93.]
Dobbies constructed with single jacks, as indicated in Fig. 91, are
only suitable for narrow looms. Those constructed with double jacks
are preferable for wider looms, as they not only keep the healds under
better command, but they also move them in a perfectly vertical plane
without the tendency to a slight side movement such as occurs when the
healds are controlled by single jacks, in consequence of the ends of
these describing an arc of a circle as they rise and fall. One of the
best adaptations of double jacks to the Keighley type of dobby is that
exemplified in the “Climax” dobby made by Lupton and Place, Burnley.
This is represented in Fig. 93, in which A and B are complementary
jack-levers operated from the same baulk-lever, J, controlled by
hooks P and Q, to govern the same heald. The distinctive features
of this dobby are the construction of the outer jack-lever A in one
part, instead of two parts, and its attachment with the inner jack,
B, by means of a link, C. This modification is a great improvement on
double-jack dobbies in which the connections are made with streamer
hooks or rods, or those in which the jacks are geared by means of
toothed segments, as these increase the number of parts that are liable
to wear and to get out of order.
The Keighley dobby is decidedly the most popular one at the present
time, but what is known as the “Blackburn” dobby is preferred by some.
This is a double-lift dobby, which gives a centre shed--that is, the
staves which are required up for a number of picks in succession are
let down halfway every pick and taken up again. The principle of this
dobby is illustrated at Fig. 94. The staves are lifted by the two jacks
A and B; when B is lifted it causes A to lift the same distance. There
are two hooks, D and F, for each double jack, and the lags are divided
into two parts, all the odd numbered picks being fastened together, and
the even picks forming another chain. The pegs in the lags press back
the hooks, the back part of each of which forms a spring, so that when
the hook is pressed back it leaves the stave down.
[Illustration: FIG. 94.]
The knives lift alternately. When one is going up the other is going
down, and when one hook of a pair is lifted, as in the diagram, a lag
operates the other hook, and if the same stave has to be lifted for
the next pick, the hook is left over the knife, and the second hook
will be taken up whilst the stave is being let down, and will catch it
halfway and take it up to the top again. This is the advantage of all
double-lift machines over single-lift. The staves which are required
up for a certain pick are being taken up whilst those which were up
for the previous are coming down. A saving of time is thus effected,
and the looms can be run quicker than with single-lift machines. A
crank L on the end of the bottom, or picking, shaft is connected by
means of a lifting-rod E to the end of a horizontal arm M, mounted
on a shaft G, which constitutes a fulcrum for the arm. On one end of
shaft G, at the rear of the machine, there is fixed a toothed quadrant
H, which, through the medium of a small wheel, transmits motion to a
similar toothed quadrant H′ fixed on the end of a shaft G′; and each
quadrant is connected, by means of rods K and K′ to the rear ends of
the respective griffe-bars D and F. On the opposite ends of the shafts
G and G′, there are fixed, one on each shaft, two short arms which are
respectively connected to the fore ends of the griffe-bars. Therefore,
as the crank L revolves, the griffes are raised and depressed
alternately, and in a contrary manner.
Another thing to be borne in mind is that in a single-lift machine all
the staves come to the bottom every pick, and therefore the character
of the shed is different from that of a double-lift. In double-lift
machines there are the “open-shed” like the Keighley dobby, and the
“centre shed” like the Blackburn dobby. It is important to remember
these points, as the cover and appearance of the cloth is affected by
the beating-up being done in different kinds of sheds.
[Illustration: FIG. 95.]
The loom crank is usually set at the top centre, or thereabouts, when
the rising and falling staves are level, so that the shed will be
partly open for the next pick by the time the loom crank gets to the
front centre. In single-lift dobbies the beat-up is made when the shed
is closed, and so the warp has not the same chance of being spread
as with the timing of double-lift dobbies. This difference in the
character of the shed when the beat-up is made is caused by the fact
that in a double-lift machine the knives, being in the middle of their
stroke, are moving at their quickest speed when the shed is closed,
and in a single-lift the knife is almost stationary when the shed is
closed. The same thing occurs in Jacquards, and the matter may be
better understood by a reference to the chapter on Jacquards.
Dobbies can be made “positive” in various ways. Keighley dobbies are
made with a pin fixed on the upright MN (Fig. 91) at the point H. A
wire is hooked on to this pin and connected to an ~L~ lever at the
side of the loom opposite the dobby; this is connected to a lever at
the bottom of the loom. By connecting the bottom of one stave to this
lever the stave will be pulled down as the upright MN is taken forward,
and so the knife whilst taken one stave up is pulling another down,
rendering the dobby positive. This, of course, will only do for certain
simple patterns, such as twills, satins, and similar weaves, and would
not do for patterns where different numbers of staves are lifted every
pick. Positive dobbies are not much used in the cotton trade.
The ordinary form of dobby is non-positive, the stave being kept
down by springs in some form or other. One reason which may be urged
against ordinary springs, or “jack boxes,” is that the pull on the
heald increases as the stave is lifted and the spring opened. It is
obvious that this is just the reverse of what is required, as the stave
is lifted positively, and the pull on it may therefore conveniently
be decreased as it is lifted, and the healds would last longer. The
use of the spring is to keep the stave down, and therefore it should
exert its greatest force when the stave is at the bottom. A simple
method of accomplishing this has long been in use. Something on the
same principle has been used on hand looms for generations, and very
cheap and convenient undermotions of this kind have long been available
for power looms; but, strange to say, cotton manufacturers have been
very slow at adopting them. An undermotion of this kind is illustrated
at Fig. 96. The spring is fixed at A, and a wire hook connects the
spring with the quadrant at B. It will easily be seen that as the stave
lifts, the direction of the pull of the spring is gradually moved over
the centre of the quadrant at C. If the stave were lifted until the
spring was in a direct line from A to C, the pull on the stave would be
_nil_, as all the force would be exerted on the fulcrum. Each stave is
connected at both sides in the same manner, the springs and other parts
are all arranged in a very compact manner, and the cost is very small.
[Illustration: FIG. 96.]
Another form of undermotion on the same principle is much used in
Yorkshire in the woollen and worsted trades. This is illustrated at
Fig. 97, and is known as Kenyon’s undermotion. In this the springs are
arranged horizontally, and therefore longer springs can be used. The
quadrant is centred at C, and a strap is fast to the quadrant at D.
The spring is connected with the quadrant at F. The strap passes from
the quadrant under the bowl B, and then to the stave. Another quadrant
serves in the same manner for the opposite side. The spring is fastened
to a bar at E, and as the stave is lifted, the pull of the spring is
gradually moved over the centre C, and therefore the pull on the heald
gradually decreases as it is lifted.
[Illustration: FIG. 97.]