Plastic Gears

Tolinski M. Tougher resins drive gears. Plastics Eng [trade journal—Society of Plastics Engineers] March 2008. 

Gaskets, printed, in hydraulic control equipment, 147 base plates, 151 failure in, 155 load-deflection curves, 149 material selection, 150 polyurethane seal, 149 production, 150 screen printing, 152 specification development, 155 testing and quality control, 154 Gears, plastics, 42-64 bending stresses in, 47, 52 Lewis formula, 52-4... 

The reaction between urea and Aiming sulfuric acid is rapid and exothermic. It may proceed with violent boiling unless the reaction temperature is controlled. The reactants are strongly acidic. Therefore, operators should wear suitable protective gear to guard against chemical hazard. Special stainless steel, mbber lining, fiber-reinforced plastics, and polyvinyl chloride and carbon equipment are used. 

Disk Filters A disk filter is a vacuum filter consisting of a number of vertical disks attached at intervals on a continuously rotating horizontal hollow central shaft (Fig. 18-127). Rotation is by a gear drive. Each disk consists of 10 to 30 sectors of metal, plastic, or wood, ribbed on both sides to support a filter cloth and provide drainage via an outlet nipple into the central shaft. Each sector may be replaced individually. The filter medium is usually a cloth bag slipped over the sectors and sealed to the discharge nipple. For some heavy-duty applications on ores, stainless-steel screens may be used. 

Industrial equipment is a continuing area of development for plastics. Pipes, pumps, valves and sight glasses, made from such materials as PVC, PTFE and poly-4-methylpent-l-ene, have become well established on account of their corrosion resistance. The nylons are used for such diverse applications as mine conveyor belts and main drive gears for knitting machines and paper-making equipment. These and other materials are widely used where such features as toughness, abrasion resistance, corrosion resistance, non-stick properties, electrical insulation capability and transparency are of importance. 

This tower uses fans at the top of the tower to draw air in the base of the tower through the fill and out the fan discharge (Figures 9-103-105). In this type of mechanical draft tower the hot moist air discharges vertically (usually) to the atmosphere with such a velocity as to eliminate the possibility of recirculation of this air in at the base of the tower. This moist air is corrosive to the fan parts and therefore requires protection of coated plastic or special metal blades and sealed motors and reduction gears. 

The blades are usually fixed pitch up to 48-in. diameter with applications for adjustable pitch above this size. Fixed pitch is used up to 60-in. diameter with aluminum fen blades when direct-connected to a motor shaft. Variable pitch is used with belts, gears, etc., between the fen shaft and the driver to allow for the possibilities of slight unbalance between blades due to pitch angle variation. Aluminum blades are used up to 300°F, and plastic is limited to about 160°-180°F air stream temperature. 

The types of wear and failure that occur in metallic gears are distinctive and have been subjected to close analysis over many years. The more common types of failure are abrasion, scuffing, pitting, corrosion, plastic yielding and... 

Most of the engineering plastics reproduce faithfully and easily conform to the mold configuration, and when processing parameters are appropriately controlled, they will repeat with excellent accuracy tolerance wise, etc. As an example for the past many decades, we see plastic gears and other precision products made of acetal, nylon, polycarbonate,... 

Fig. 3-30 Design of the solid steel gear (top) is redesigned using plastic (bottom).

The factors that cause a gear to mesh tightly are (1) tolerance on concentricity of shaft hole with pitch diameter, (2) tolerance on center distance, (3) tolerance on quality, (4) coefficient of thermal expansion, and (5) change in dimensions due to moisture absorption, which is a consideration/detriment in some materials. The first three apply to gears of any material. Item 4, and in some cases Item 5, for plastic gears deserves special consideration. 

The subject of transmitting motion and power by means of gears, their construction, and detail requirements are fully covered in textbooks, technical handbooks, and industrial literature of gear suppliers. The knowledge of gear fundamentals is a prerequisite for the understanding of where and how to insert the appropriate plastic behavioral information into the gear formulas so that the application results in favorable operation. 

To quiet a noise-generating mechanism, the first impulse is often to enclose it. Sometimes an enclosure is in fact the best solution, but not always. If it can be determined what is causing the noise, appropriate action can be taken to be more specific and provide a cost-effective fix. In some cases the problem is caused by a component such as a stepper motor or gear set that does not produce objectionable noise by itself. The trouble typically develops because a small noise is transmitted to a metal frame or cabinet that then serves to amplify the sound using a plastic cabinet can isolate the noise problem. 

Although plastics may not be as hard as metal products, there are those that have excellent resistance to wear and abrasion. Plastic hardware products such as cams, gears, slides, rollers, and pinions frequently provide outstanding wear resistance and quiet operation. Smooth plastic surfaces result in reduced friction, as they do in pipes and valves. 

The wear characteristics of one plastic as opposed to another vary widely, even among materials that have good natural lubricity. When an application calls for plastic-to-plastic bearings, shafts, gears, or other wear members, the combination of materials must be chosen carefully. Because plastics are not rigid, they do not behave according to the classic laws of friction. It is these deviations that cause some of the unexpected results when plastics are run against metals. 

Acetal This crystalline plastic is strong, stiff, and has exceptional resistance to abrasion, heat, chemicals, creep and fatigue. With a low coefficient of surface friction, it is especially useful for mechanical products such as gears, pawls, latches, cams, cranks, plumbing parts, etc. It is chrome platable. 

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