Screw torque speed

Figure 10.1. Schematic representation of the processing window for the extrusion process. The four curves identify limits to the process Une 1 indicates the maximum screw rotation speed, curve 2 represents the torque limitation of the drive system (region to the right and below of curve exceeds the allowable torque on the drive system), curve 3 represents the maximum temperature allowable to avoid the thermal degradation, and Une 4 shows the minimum screw rotational speed to provide the minimum throughput, acceptable by the process economics.

High screw torque required therefore medium screw speeds should be applied. 

The load acquired from the pneumatic pressure sensors was negligible. This force and velocity data was sufficient to find a screw lead and motor such that the requirements were within the torque speed curve of the motor. Of the screw leads available, a 10.16mm screw lead was chosen. 

As the screw rotates at fixed speed, the back pressure affects the screw torque and hence the work done and so the melt temperature the melt temperature increases if the back pressure is increased. 

Most extruders have a so-called constant torque characteristic. This means that the maximum torque obtainable from the drive, for all practical purposes, remains constant over the range of screw speed. The torque-speed characteristic can be used to determine the power-speed characteristic by using the well-known relationship between torque and power ... 

Extmsion accounts for about 30% of nylon produced and is used in various processes (24). Nylons can be extmded on conventional equipment having the following characteristics. The extmder drive should be capable of continuous variation over a range of screw speeds. Nylon often requires a high torque at low screw speeds typical power requirements would be a 7.5-kW motor for a 30-mm machine or 25-kW for 60-mm. A nylon screw is necessary and should not be cooled. Recommended compression ratios ate between 3.5 1 and 4 1 for nylon-6,6 and nylon-6 between 3 1 and 3.5 1 for nylon-11 and nylon-12. The length-to-diameter ratio, T/D should be greater than 15 1 at least 20 1 is recommended for nylon-6,6, and 25 1 for nylon-12. 

In another case where the twin-screw extruder was used, the rubber and plastic were melt mixed with all ingredients in a similar manner as described in blend compositions for static vulcanizations. The product was then dumped, cooled, and granulated. The premixed granules were then fed into a twin-screw extruder where a very narrow temperature profile was maintained with a relative high compression (2 1), and the screw speed was adjusted depending on the final torque and the flow behavior of the extruded stock. The stock was cured by shear force and temperature enforced by the twin-screw extruder. The dynamically crosslinked blend was taken out in the form of a strip or solid rod to determine the... 

Screw conveyors are suited to transport of even sticky and abrasive soiids up inciines of 20° or so. They are iimited to distances of 150 ft or so because of shaft torque strength. A 12 in. dia conveyor can handie 1000-3000 cuft/hr, at speeds ranging from 40 to 60 rpm. 

Movement of the Teflon sweeping barrier is effected through a worm screw assembly mounted beneath the aluminum base that supports the trough during operation. The screw is driven by a sealed reversible motor with a precision feedback controller to maintain constant speed and torque. Force-area isotherms are actually gener-... 

Figure 5.18 Torque as a function of discharge pressure for select resins. The temperature of the barrel and screw were maintained at 35 °C. The screw speed was maintained at 50 rpm...

At a screw speed of 56 rpm and a rate of 120 kg/h and with the motor operating at full current (full torque), the extruder motor would be inputting a specific energy of 1440 J/g to the resin via the screw. Since the 80 mm diameter extruder is expected to require only 750 J/g, the motor is expected to operate at about 52% of the maximum current. Thus, the extruder has enough power to operate the proposed screw design at 120 kg/h and a screw speed of 56 rpm. 

The material of construction for the screw is critical in order to provide the proper strength in the event of an improper startup procedure such as a cold start and to minimize cost. The choice of the material of construction depends on the motor size, the maximum screw speed at the base motor speed, and the depth of the feed section. The maximum torque that the motor and gearbox combination can supply to the screw, (Nm) is computed as follows ... 

The availability of power and torque to the shank of the screw is extremely important to the success of an extrusion process, especially if modifications to the line are planned to increase the rate. That is, the proper level of power must be available at the proper screw speed. Thus, the motor size and speed, belt sheaving if used, and gearbox reduction must be specified to provide enough power to the screw to plasticate and pump the resin at a high rate. 

Figure 10.12 Pow/er and torque as a function of motor speed for a 100 kW DC motor w/ith a maximum speed of 1750 rpm w/ith a full voltage on the field and at the maximum motor current. The motor can be operated at screw speeds up to 2100 rpm by weakening the field voltage...

At a maximum rate of 440 kg/h, the extruder was rate limited by motor torque and not by the quality of the extrudate. This extruder was fabricated with a gearbox and sheave ratio such that the maximum screw speed was 105 rpm. Since the process was operating at a screw speed of 45 rpm, the sheave ratio could be changed such that the maximum screw speed would be about 75 rpm. This might allow the process to operate at rates up to 700 kg/h. Changing the sheave ratio will increase the torque to the inlet side of the gearbox. Care must be taken to make sure that the additional torque does not harm the gearbox. 

The power train on the extruder was also limiting the rate of the process. The extruder was driven by a 190 kW motor with a maximum speed of 1750 rpm and a maximum current of 262 A. The motor was directly coupled to a gearbox with a ratio of 13.91, providing a maximum screw speed of 125 rpm. The maximum torque available to the screw from the drive system was 14,200 Nm. As will be shown later, this gearbox-motor combination did not provide a high enough torque to the screw for this process. 

In order to provide a higher torque to the screw, a belt sheave system was installed between the motor and the gearbox. This sheave system had a reduction ratio of 1.95, reducing the maximum screw speed from 125 to 65 rpm. Sheaving... 

Figure 11 Torque of extrusion as a function of moisture content of wet mass of MCC at varying screw speeds a-19 rpm, -28 rpm, c-38 rpm, d-4S rpm. Source From Ref 26.

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