The drum of the bag turning centrifuge is divided into a fixed part and an axially movable part, as shown in figure 1. Wherein the barrel cover, the barrel cover connecting rod, the filter cloth fixing plate, the inner shaft and the filter bag are axially movable parts. The feed pipe is used to transport the suspension. When feeding or centrifugal filtering, the drum cover closes the drum, as shown in figure1a. The barrel cover is connected with the filter cloth fixing plate through the barrel cover connecting rod, and the filter cloth fixing plate is pulled by the inner shaft. The drum is connected with the outer shaft and driven by the motor to rotate. The inner shaft and the outer shaft can slide relatively. In the solid-phase discharging stage, the inner shaft drives the cylinder cover, the cylinder cover connecting rod, the filter cloth fixing plate and the filter cloth to move out of the cylinder, and the inner surface of the filter cloth becomes the outer surface, and the precipitate is thrown out in a high-speed rotating state (figure 1b).
Comparison of two kinds of bag turning mechanisms
The opening and closing of the drum cover comes from the relative movement of the inner shaft and the outer shaft. Different inner shaft pulling methods will produce different filter bag overturning mechanisms. The turning mechanism of filter bag determines whether the drum can be sealed in dehydration state and whether the filter bag can be turned over and discharged smoothly in slag discharge state, so it is a key component in the development of this machine.
2. 1 cylinder-driven filter bag turnover mechanism
Cylinder-driven filter bag turnover mechanism is the earliest structure of bag turnover centrifuge. The inner shaft driving the filter bag to turn over is integrated with the piston rod of the oil cylinder and connected with the piston. The oil cylinder and the drum rotate at the same speed to control the direction of hydraulic oil in and out, so as to control the filter bag to turn out or return. Disadvantages of this structure: ① The rotating oil cylinder is in dynamic contact with the static oil supply device, which is easy to leak oil and pollute the environment. ② When the piston rod enters and exits the oil cylinder, it is easy to pollute the separated products and the environment. (3) The large moment of inertia of the rotating cylinder will reduce the critical speed of the shaft.
In recent years, the cylinder-driven filter bag turnover mechanism has been eliminated. However, some domestic centrifuge manufacturers are still planning to produce bag-turning centrifuges with cylinder-driven filter bag turnover mechanism, because the relevant patents have just expired, and it is justified to produce centrifuges with this structure. The disadvantage of this approach is that it reduces the competitiveness of products.
2.2 Heinkel mechanical drive filter bag turnover mechanism
Starting from 1994, Heinkel Company of Germany gradually replaced the filter bag turnover mechanism driven by oil cylinder with the filter bag turnover mechanism driven by machinery, as shown in Figure 2. The motor drives the outer shaft through the pulley, and the other motor drives the threaded shaft through the pulley. When the speed of the motor driving the outer shaft is higher than that of the motor driving the threaded shaft, the threaded shaft will rotate relative to the sliding nut. The sliding nut can't rotate due to the restriction of the guide cylinder and the guide key, but only makes the threaded shaft move axially and pulls the inner shaft to move axially. Under the pull of the inner shaft, the drum cover seals the drum, so that the drum is in a closed state. At this time, charging or centrifugal filtration can be carried out.
On the contrary, when the speed of the threaded shaft motor is higher than that of the external shaft motor, the drum cover leaves the drum and the centrifuge is in a discharging state. In Figure 2, all parts on the right side of pulley 1 except the bracket are high-speed rotating parts, which are at the same speed as the drum under the working conditions of feeding, filtering and washing. In order to control the forward and backward movement of the inner shaft, it is necessary to correctly control not only the respective speeds of the threaded shaft motor and the outer shaft motor, but also their relative speeds. The problem of this structure is that there are many rotating parts, which affects the vibration performance of the shaft, increases the manufacturing difficulty and machine cost, and is difficult to control.
2.3 isolated mechanical filter bag turnover mechanism
After in-depth study of the above two mechanisms, we designed an isolated mechanical filter bag turnover mechanism, as shown in Figure 3[5]. The feature of this mechanism is that the number of rotating parts is greatly reduced. In fig. 3, the bearing between the pulley 1 fixed on the outer shaft and the guide cylinder stops the guide cylinder from rotating with the outer shaft. The bearing between the sliding nut and the inner shaft makes the sliding nut and the threaded shaft no longer rotate with the inner shaft. The bearing can transmit axial force, but it isolates the rotation between the front and rear parts.
The pulley drives the threaded shaft to rotate clockwise or counterclockwise, thus driving the inner shaft to slide in the outer shaft. The characteristic of this structure is that the advance and retreat of the inner shaft has nothing to do with the rotation speed of the outer shaft, which makes the mechanism easier to control. The number of rotating parts is reduced, which reduces the manufacturing difficulty and improves the reliability of the machine. Shortening the length of the rotating shaft improves the vibration performance of the shaft.
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