Aprons in the drafting system are, with the top roller cots and the ring travelers, those components having a strong influence on the quality of the yarn and the efficiency of the ring spinning machines.
Increases in output of the ring spinning machines, updated design of components of top weighting arms and nose bars have increased the demands put on aprons considerably. If one considers that over the last 15years the speeds of spindles have increased by about 40%, yet the lifespan of aprons whose working life is defined by the frequency of flexing, has remained practically the same, the improvements in polymer materials are obvious.
Construction of aprons
Synthetic rubber, mainly NBR based, is the most widely used material for the manufacture of top and bottom aprons. There are also leather, synthetic leather, PU and PU-coated materials. Synthetic rubber has become a leading material for this purpose because of its long life, a wide range of uses and because of its resistance to fibre finishing additives and abrasion.
A typical rubber compound for the outer or inner layer of an apron contains the base polymer as well as between 10 and 15 different additives, which also affect the various physical and mechanical properties of the various aprons.
Basically, aprons made of synthetic rubber are made in endless tubular form whereas leather or synthetic leather aprons are made open in strips and subsequently glued together to form an apron. The advantage of the tubular construction is the lack of a seam. The apron is uniform along its entire circumference.
Aprons for use on ring spinning machines and roving frames are made of three layers; in some special applications used in the long staple spinning they only have two layers.
- Outer layer
- Reinforcement layer
- Inner layer
The outer layer is finish ground to give it a defined roughness for optimum fibre guidance, but also must not be so rough as to prevent the release of the fibres at the turning point. The heaviest load is placed on this layer in the traverse zone. Here abrasion and the pressure from the bundle of fibres lead to smoothing and the formation of grooves.
This effect is especially noticeable in machines, which work with a very narrow traverse zone, such as for compact yarn or core yarn. The length of time for which an apron can create optimum yarn quality therefore depends on performance of the rubber compound. Resistance to abrasion and elasticity determine how long the initial structure of an apron outer layer lasts. (Fig. 2)
The reinforcement gives the apron its dimensional stability. Modern methods of apron manufacturing with defined parameters for the reinforcement layer achieve very tight internal diameter tolerances. These in their turn play an important part in determining the lifespan of a top apron or a short bottom apron. The reinforcement in a bottom apron should have a nominal fracture point so that when the apron is clamped between the bottom steel roller and the nose bar the tension device is not damaged.
Optimum slippage over cradle and nose bars is expected of the inner layer. In the case of the bottom apron the inner layer must also withstand abrasion and notching of the knurled bottom roller. The inner layer should also provide good friction, so as to ensure that both aprons are consistently driven, which contradicts the need for it to be slippery.
Emphases of new developments of aprons
Since the number of spindles on short staple ring spinning machines has continually increased, the energy consumption of the top weighting arms was initially more crucial for the standing times of drives; today more emphasis is put on torque values so as to improve the whole machines energy balance. Low apron torque values help in achieving it.
In recent years extensive testing has been carried out in the ATPG laboratory on test benches developed especially for this purpose. The tests have included both a variety of different compounds and various surface structures. On the test benches torque values can be monitored and compared over an extended period. The findings have also helped in the ongoing development of aprons for the large variety of conditions nowadays for machines by various manufacturers. ATPG is thus following a concept that should continue to provide the spinner with a product that can be used across the entire range of fibres, drafting system settings and machine types. Here the need for low torque values and good slippage go hand in hand. (Fig. 2)
To achieve improved yarn regularity, closer settings on the top arms are used, requiring greater flexibility in the aprons so as to accommodate the narrow radius at the nose bar. Here the task was to reduce the thickness of the apron layers in order to increase the aprons flexibility and reduce the tension at the point of turn, but without losing the elasticity of the outer layer, which is essential for good fibre control.
Results of these developments, which have been tested in the field under different conditions for a long time, are today a constituent part of Accotex aprons produced by ATPG.
About the Author:
The author is associated with ATPG Textile Products Group GmbH.
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