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The Start-Stop Mechanism

  • In terminating the spinning operation on a rotor spinning machine , it is desirable to stop machine so that at all the spinning positions, the end yarn is not withdrawn from the doffing tube but terminates at all predetermined position & has the desired tail shape.
  • These features are required so that on restarting the machine , successful piecing-up is achieved at all spinning positions & the yarn piecing have the desired strength. Most of the latest models of rotor spinning machines will be found to have a stop start mechanism.
  • Although there are variations between one manufacturer's  device & another's, their basic stop-&-start procedures are similar.
  • The stop procedure :-.
    •  In the stop procedure, the sliver feed rollers & the yarn delivery rollers , will, immediately a predetermined speed is reached, initiate the stopping of the feed rollers.
    •  After a further few seconds, determined by a time-delay circuit, the decelerating yarn-delivery rollers are braked; The rollers come almost to rest before they are braked.
    • Finally, the drive to all the spinning rotors is switched off,& the drive to the traverse guide is also switched off.
    • The tension on the yarn between the delivery rollers , the traverse guide & the yarn wind-up drum is maintained.
    •  It is important to note that the delivery rollers & wind-up drum are stopped at a time when the yarn end, resulting from the break in the sliver feed, still remains in the doffing tube under the suction effect of the partial vacuum in the spinning rotor.
    •  Because the yarn end will be given a strong twist, this suction must be sufficient to prevent the yarn end from snarling after the rotor stops & thereby shrinking upwards out of the doffing tube.
    • In order to ensure the yarn end remains in the doffing tube, a yarn holding device can be used to keep the yarn end in the rotor until the residual twist causing snarling falls off; The yarn end can then remain in the doffing tube under the suction present in the rotor.
    • On starting, the yarn end is not released by the device until the suction in the rotor reaches substantially the same value as that present during normal spinning operations.
    •  This results in a greatly increased success rate in yarn-piecing on restarting the machine.
  • The start procedure :-.
    •  On starting, all the spinning rotors begin rotating simultaneously; The yarn-take-up rollers are then rotated in the reverse direction to push the yarn ends down the doffing tubes & into the spinning rotors.
    • The yarn end is next attached to the ring of fibres left in the rotor groove during the stop procedure.
    •  As so0n as the ring of fibres begin to be peeled from the rotor groove, the sliver-feed rollers are set in motion, the take-up rollers are reversed to their correct direction of rotation & the spinning process begins.
    •  Because of the high spinning speeds  involved, this sequence of operations has to be performed within fractions of a second.
    • However, because of the considerable mass & moment of inertia of the take up rollers & the bobbin winding-up drum , instantaneous start up at delivery speeds is practically impossible.
    • To prevent the shearing of the drive shaft during start up , the initial speed of the take up system is comparatively slow,& so too is the reversal to correct spinning direction.
    •  Once the yarn is pieced up, the delivery rollers & wind up drum accelerate to their actual pre-selected spinning speeds. In order to ensure a successful piecing within the fraction of second in which the yarn end enters the rotor & is removed again, the speed of the rotor at instant at which piecing takes place must be lower than the actual rotor spinning speed.
    •  In other words, the twist at piecing up must be be comparable to the spinning twist.
    • Thus, because the delivery speed is less at start up, the rotor speed must also proportionally lower than normal spinning speed.
    •  This requirement of a lower rotor speed presents a problem in that fibres in the collecting groove must experience a certain centrifugal force in order to sufficiently compact for twist to propagate into the rotor groove
    • As one solution to the problem, the rotor is initially accelerated to a speed n2, twice the value of rotor speed required for piecing-up, n1{see fig.}.
    •  The rotor speed is then rapidly decreased until it falls within the lower-speed region, n2-n1, in which yarn-piecing can be effected.
    •  Thereafter, the speed of the spinning rotor is  increased to the required spinning speed, n0. During rotation of the rotor at n2, a sufficient vacuum is produced in the rotor to stretch the yarn end & prevent it from being snarled.
    •  If a large package is present on the machine at the time of starting up, then another problem can arise. A large inertia force will be imposed on the yarn package because of the  quantity of yarn on the package & slip can occur between the package & wind up roller at start up.
    •  Owing to the slip , the tension on the yarn is increased between the package & the nip point of the delivery rollers, so that a yarn break can occur in this area.
    • Naturally, such a yarn break renders the yarn piecing up operation difficult at the start of the spinning in cycle.
    • Again, owing to slip, when the direction of winding up drum is reversed to the true spinning direction, reversal of the package direction will occur a second or so later.
    • Accordingly, the timing of drawing out the yarn from spinning rotor is upset, and the number of successful yarn Piecings is remarkably reduced.
    • In order to alleviate this effect & enhance the ratio of success in the yarn piecing operation, the drives to the sliver feed roller, the winding up drum & the delivery rollers are all operated at an optimum timing sequence for the established spinning conditions.
    • The optimum timing sequence is determined from the amount of yarn wound on the package during normal spinning.
    • Thus, the quantity of yarn wound on the yarn package is detected at a time just before the machine is stopped.
    •  The time for complete stoppage of the winding drum is set on the basis of the quantity of yarn on the package.
    • As a result, the delay in stopping winding-up rollers after the delivery rollers is greater the larger the quantity of yarn on the yarn package.
    •  In starting up , this time delay is also used.
    •  Consequently, the possible yarn breakage & mis-piecing owing to a slip are prevented.
    • As a further aid to ensure an adequate twist level for piecing up during the start up of the machine, several devices can be attached to the start up mechanism.
  • The automatic starting & stopping of the rotor machine are effected by controlling different motors, clutches, & brakes according to so called stop start programme, i.e., The delay time sequences.
  • On high speed modern machines, such complicated control functions are performed by electronic control units, which can also perform additional auxiliary functions, such as determining the quantity of yarn produced, monitoring the vacuum in the rotor,& monitoring the electrical voltage.
  •  However, care has to exercised in using such equipment, since vibrations.
  •  Humidity, electrical disturbances, faults in material & inappropriate handling can cause premature failures.

 



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