Spinning a good yarn

Hansgeorg Binz, from SKF Textilmaschinen-Komponenten, Research and Development in Stuttgart traces the development of a novel solution to yarn making

Ring spinning is a basic technique used to produce fabrics from wool, cotton, man-made fibres, or blends of fibres. In a ring spinning machine, yarn is produced by a mechanism known as a drafting system that reduces the thickness of the raw material and twists it into a yarn. One key component in the drafting system is the weighting arm. A crucial component in determining yarn quality, the weighting arm is responsible for the drawing of the material, decreasing its width by up to 20 times.

In a conventional system (Figure 1), a weighting arm presses three top rollers onto three bottom cylinders that rotate at different speeds. The first bottom cylinder and top roller rotate very slowly. The middle ones move 10% to 20% faster and the front bottom cylinder and top roller about 12 to 30 times faster, depending on the fibre material. Because of these different speeds and the loads on the top rollers, the raw material fed into the drafting system is drawn, and in the case of wool, becomes about 12 to 30 times thinner.

The top rollers are coated with rubber cots to improve the clamping effect. Between the middle and the front roller, the fibre mass is guided between two small rubber belts, the top and bottom aprons. The top apron is tensioned and guided by a top apron cradle. Smooth and consistent running of the aprons is very important to yarn quality.

While leaving the last top roller and bottom cylinder of the drafting system, the fibre mass is given a twist by a metal ring with a rotating traveller and a rotating spindle. These are located underneath the drafting system. The twist creates friction between the fibres, increasing the strength of the fibre mass. In this state, the fibre mass is called yarn. The yarn is then wound onto a bobbin attached to the spindle.

The arm must allow different loads to be set on the top rollers. Normally, this is achieved by helical springs that can be adjusted mechanically in steps. The distance between the rollers must also be adjustable. This distance must be larger than the length of the longest fibres to avoid tearing single fibres between the clamping points of the top rollers and bottom cylinders.

Recently, design engineers at SKF in Stuttgart found that there was room to improve the design of existing systems. This was especially evident in the way that the weighing arm on the ring spinning machine was mounted and handled.

Because all the spindles of one machine are required to produce yarn of identical quality, constant load on the top rollers and small load deviations are significant characteristics of the weighing arm.

Existing opening and closing forces were considered to be too uncomfortable for operators. During longer periods of machine standstill, such as weekends, the loads of all the weighting arms had to be reduced in a time-consuming process to avoid marks in the soft rubber cots of the top rollers, which would otherwise damage the yarn.

Different raw materials, types of yarn and desired yarn quality mean that a number of options for load setting at the top rollers were required. One existing weighting arm offered three load steps for every top roller. But the setting has to be performed manually on each arm, requiring a special tool.

The underlying idea for a new weighting arm, called the PK 6000, was to use pneumatic pressure instead of metal springs to generate the load on the top rollers, which can then have infinite adjustment. A pneumatic ring tube in the machine supplies each weighting arm with compressed air. When the machine is in operation and the weighting arm is closed, a pneumatic valve is opened so that the compressed air can enter a pneumatic rubber spring. The pneumatic energy is transformed into mechanical forces on the top rollers by weighting elements with pressure plates.

When the weighting arm has to be opened, for example if the yarn is broken, the operator has to push back the handle that is connected to the push rod. The pneumatic valve is pushed down, the air inlet is blocked and the compressed air flows out of the rubber spring. The helical spring moves the weighting arm upwards automatically. This locking device has the advantage that the arm can be opened and closed without load, resulting in a minimal amount of effort by the operator. A new top apron cradle that provides individual tensioning of each apron, completes the new design.

The surface of the new top apron cradle reduces the friction during apron operation and helps to preserve the aprons. Individual apron tensioning makes the system less sensitive to tolerances in the apron length, improving the fibre guidance and, consequently, yarn quality.

The air pressure is set centrally for all arms at just one point. Thus, load setting and the partial load reduction necessary during longer standstills, can be done in a second. It is no longer necessary for the user to operate hundreds of weighting arms. The infinite pressure setting also allows the machine to be adapted precisely to the requirements of the yarn.

Unlike previous designs, no great strength is required to open and close the system. The weighting arm opens automatically after unlocking and the height of the opened arm is reduced. The top rollers can be easily exchanged. The new top apron cradle also saves up to 40% of the time spent on changing aprons. There is no need to remove and dismantle the top apron cradle for service work. Mounting and setting of weighting arms is easier too, thanks to the improved accessibility of the fastening and setting elements.