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Developments In Rotor Drives

  • As increasingly higher rotor spinning speeds become commercially realistic , m/c manufacturers have realized that new bearings have to be designed to meet the required low-noise , low vibration & acceptable working life specifications.
  • Present rotor spinning m/cs have the design concept of central control drive , that is , all the rotor s are driven by a common belt running the length of the m/c , the driving motor & transmission mechanisms being housed at one end of m/c.
  • However , with increasing speeds , individually driven rotors are becoming an attractive alternative.

Individual Drives

  • Advantages :-.
    • Noise reduction .
    • Reduced energy consumption.
    • Simplified construction .
    • Improved economy.
    • Improved regulations during transitions.
    • Higher efficiency of spinning machines.
    • Possibilities for simplified & inexpensive operation & yarn quality control.
  • With an individual drive , most of the energy at the rotor & stator surface is converted into frictional heat.
  •  The spinning conditions are therefore thermally influenced by energy losses, since air- flow through the unit is not sufficient to cool the driving mechanisms .
  • To overcome this problem , some modified designs have an additional cooling system provided.
  • The  additional air-flow {the cooling air } is separated from the fibre transporting air-flow & is conveyed into space between the spinning rotor & the stator of an electrical motor, which thus reduces the temperature of the rotating components.
  • With the additional cooling system , a constant temperature of 20 c can be kept within the rotor system , which is suitable for spinning most fibre types.
  • Above a rotor speed of 30000 rpm , the rotation of the rotor must be free of resonance , & the upper limiting speed is then not dictated by the drive .
  • The maximum spinning speed is instead limited by the yarn breaking tenacity or the maximum allowable rotor temperature or both.
  • To achieve accuracy of rotational speed , the design of the drive system must ensure a stable nominal frequency input & continuous frequency regulation

Central Drives

  • Most central drive systems utilise a common tangential belt-drive arrangement.
  •  This drive is cheaper & more simple than individual-drive system.
  • However in order to operate at high speeds , the problems of vibration, high temperature , noise,& wear have also be resolved. The solution lies primarily in the improvement of the rotor bearings.
  • The ball bearing system :
    • Reported improvements in the ball-bearing system were obtained by using elastically mounted arrangements similar to that shown in fig. 1.
    •  Resiliently yieldable rings are sandwitched by bearing casing & a holder so that any vibrations can be absorbed.
    • The modulus , the number , the size ,& the sucuring positions  can be arbitrarily changed.
    • At least two such rings , spaced apart along the axis of the spindle , should be disposed between the bearing casing {19} & the holder {5} & at least one{8b} or {13}detachably secured to the bearing casing.
    • The elasticity mounted bearings gives high reliability in operation,reduces the noisiness of the machine , & eliminates the negative effect of oscillation on the other parts of soinning unit.
    • The high temperature generated in the rotor area will shorten the life of the anti-friction bearings & also that of the resilient support members of the bearing arrangement.
    • In order to cool a bearing arrangement of the type shown in fig. 1 with maximum efficiency & the lowest possible cost , a heat condusive body of high thermal conductivity , made in the form of a sleeve, was positioned between the bearing casing & the rotor housing [see fig. 2] .
    •  As the fig. Shows [a-e] , a number of thermally efficient profiles can be used to effect good heat transfer from the bearing casing to the rotor housing.
    •  Fig. 3 shows that a finned design to the outer bottom surface of the rotor base [15] can be used to give extra cooling of the bearing casing.
    •  Further cooling is also obtained with the oil-feed device[18].
    • The device includes a reservoir, which is responsive to reduced pressure caused by rotation of the spinning shaft.
    • Thus no oil is supplied when the shaft is stopped.
    •  However , during operation , the bearings are kept reasonably cool while being lubricated.

Air Bearings

  • With current rotor speeds up to 60000 rpm ,rotors fitted with ball bearing drives are operating at the practical limit for central  drive system.
  • Thus , with constant developed aimed at achieving faster & faster rotor spinning speeds , air bearings have become an attractive alternative , particularly because of their low power consumption.
  • At present air-bearings are not in wide usage , although the constraints in using them are not so much economic as technical.
  • This is because of the difficulties in ensuring the high m/c tolerances, the fear of breakdown during long running times, & the high rates of air consumption.
  •  Several reported modifications have been made to reduce the air consumption to acceptable levels.

Indirect Bearings Drive

  • A second alternative to using direct bearing drives is the use of indirect bearing drives & because the technology is simpler than that of air bearings & facilitates certain operational advantages like replacement of spinning components such as rotor, some machinery manufacturers have already opted for this system.
  • The majority of commercial open end spinning machines capable of spinning at a rotor speed of 100000rpm utlize indirect-bearing drives.
  •  In this system the shaft of the rotor is driven by a tangential belt &  rests  in a cradle formed by four supporting discs .
  • The discs acts as the bearings for the rotor shaft &themselves fitted with direct ball bearing drives.
  • The outer circumference of each disc is fitted with a synthetic fibre ring, which acts as a damper to give vibration-free running of the rotor.
  • The complete system is then supported on the elastic base as one block to prevent any problems due to rocking motions at right angles to the rotor shaft.
  • It would appear, from the number of patented design modifications , that the elastic support block en masse had not totally solved the problem.
  • The rotor shaft itself is subjected to a twisting-rocking motion, resulting from the relationship between the tangential belt drive & pivot points on the disc, the result being constant defluxions of the rotor shaft from the true axis of rotation.
  • A completely new approach  to the problems of indirect-bearing drives is illustrated in figure.
  •  Here three tapered rollers , 4 ,5,6 are positioned in a housing {9} with their axes forming a tetrahedron to support a shortened tapered rotor shaft.
  • The shaft is driven by the tapered roller[4],which is that the full length of the rotor shaft is in contact with the rotating discs & this will ensure a constant axial location.

 



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