Screw Speed Control

The first method is used most frequently. The next preference is for the last method, mostly used in small compressors due to problems with speed control of electrical motors. Other means of capacity control are very seldom utilized due to thermodynamic inefficiencies and design difficulties. Energy losses in a compressor, when capacity regulation is provided by lifting the suc tion valves, are due to fric tion of gas flowing in and out the unloaded cylinder. This is shown in Fig. 11-84 where the comparison is made for ideal partial load operation, reciprocating, and screw compressors. 

These complex systems demand strict screw speed control for all the individual machines in the setup, to balance the extrusion characteristics of the different rubber compounds forming the multiunit component. 

Figure 12.5 An extrusion process with a downstream gear pump with the screw operating in inlet pressure control and followed by the screw in manual operation (constant screw speed). The large level of variation in the motor current during constant screw speed control suggests that the extruder process is unstable, and the control algorithm is not the root cause for the variation in the motor current...

A variable speed feeding screw must be used to continuously and uniformly discharge material from the bin and feed it to the extruder, as uniform feeding of the raw material is essential for the consistent and uniform operation of an extruder. The extrusion rate of the food extruder is typically controlled by a feeder screw or other metering devices. Therefore, the production of the single screw is dependent on the screw speed. This feeding screw must meet a variable speed to achieve the desired feed rate for extrusion. This feeder screw can feed directly into the extruder inlet throat or into a preconditioning cylinder (31). 

A, had cell B, speed control for the motor drive C, support bar (a hole in this served as the confining chamber) D, support disc used to secure the load cell to the apparatus. (This initial load on the load cell—i.e., before compression—was regulated by adjusting the tension in these screws.) E, displacement transducer F, motor-driven micrometer screw G, power supplies for the load cell and the displacement transducer H, recorder. 

In Enplex ABS/PVC blends, ABS contains some a-methyl styrene and a methylmethacrylate, MMA, copolymer. An in-line screw injection machine for molding general-purpose ABS, PS and similar resins can be used. The machine should have multi-stage injection speed control unit and a cooling blower unit to control shearing... 

A host of electrical, mechanical, and hydraulic variable speed drive devices are available for screw speed control. The ubiquitous inverter offers many convenient features, not least the facility to be mounted in a safe area for feeders in hazardous zones. As with hydraulic drives, the overload capacity is small. Frame sizes are usually de-rated to compensate for lack of fan cooling at low operational speed. Auxiliary fans can be fitted if sustained, low-speed usage is expected. 

Extruder drive motors must turn the screw, minimize the variation in screw speed, permit variable speed control (typically 50 to 150 r/min), and maintain constant torque. In selecting drive motors, the three major factors are (1) base speed variation, (2) the presence or absence of brushes, and (3) cost. The speed variation of a drive motor is based on the maximum speed available for the motor. Since this variation does not change when the speed is reduced, screw speed. 

Initial die gaps are set to about 20 percent greater than the final film thickness, and then adjusted to accommodate changes in polymer flow which are resin and rate sensitive. Higher screw speeds increase extruder output, overall film thickness, the tendency toward melt fracture, and may alter the flow pattern. Thus, extruder speed is not a recommended control. In contrast, increased chill-roll speeds decrease film thickness, reduce film width due to increased neck in, increase uniaxial orientation, and alter the optimum air gap or drawdown distance. The optimum air gap, which produces the best orientation, crystallization, and surface properties, depends on the material and chill-roll speed. At 23 to 30 m/min (75 to 100 ft/min), the air gap for low-density polyethylene is about 100 mm (4 in), but when the line speed increases, the air gap is found by trial and error, Since the chill-roll speed controls film stretching, the take-off speed has little effect on the film dimensions. 

For fast injection and consistency of moulding, the melts are momentarily ( 0.1s) pressurized by the screw rams before injection into the mould - this feature is programmed into the moulding cycle. Fast injection also helps avoid weld lines due the less time available for the melt to cool before welding takes place. Ten-stage programmable injection speed controls for both cylinders is incorporated on the machine together with closed-loop facilities. 

Belt drives use adjustable sheaves. The axial distance between the sheaves can be varied this changes the effective pitch at which the belt contacts the sheave. This, in turn, changes the transmission ratio. The speed is usually varied by a vernier screw mechanism, which is hand cranked or activated electrically. Belt drives are used up to 100 hp. The largest speed ratio is about 10 1, and a maximum speed is typically 4000 rpm. Belt drives have a reasonable efficiency, tolerate shock leads, and provide optimum smoothness in a mechanical drive. Disadvantages are heat generation, possibility of slippage, and relatively poor speed control. In addition, belt drives are subject to wear and, thus, are maintenance-intensive belts generally have to be replaced every 2000 hours. 

Important issues with gear reducers are the reduction ratio, energy efficiency, power transmission capability, cost, and backlash of the gearbox. Backlash is basically the slop in the gears. If the screw speed is maintained at a constant value, backlash is not a big issue. However, if the screw speed is changed quickly, as is done in some advanced control schemes, then the backlash should be minimal to maintain good speed control and to avoid rapid wear of the gears. 

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