Wear problems

Wear occurs in all machinery with moving parts. Unfortunately, extruders are no exception. Wear problems in extrusion can take many shapes and forms. A good general text on wear in polymer processing is the book by Mennig [80]. 

Laminar wear occurs when the shear strength in the heterogeneous interfacial layer is higher than the shear strength of the homogeneous portion of the interfacial layer. Laminar wear takes place only at the thin outer layers of the interface. Laminar wear is sustained only if the outer layer of the heterogeneous interface continu- 

In corrosive wear, a chemical reaction attacks at least one of the sliding surfaces. Corrosive wear in extrusion occurs usually in combination with one or more of the other wear mechanisms. The combined chemical and mechanical attack of the sliding surface can cause wear rates far in excess of what would be expected based on their individual contribution. In extrusion, the most important wear mechanisms are adhesive, abrasive, and corrosive wear. 

There are basically two ways to test the wear characteristics in the extrusion process. One method is to run the actual machine under normal operating conditions and to measure the progress of wear at regular intervals. This approach is time consuming and expensive, but it does yield accurate and representative results. However, it does not allow a simple analysis of the parameters that influence the wear process. 

Another method involves testing on model systems. In such a test, a test specimen is subjected to certain load conditions to simulate actual service conditions. Such wear 

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Table 19-2 includes impacl velocities for some internal rotating devices in tumblers as well as other mixers. Contamination and wear problems of internal rotating devices are discussed under Performance Characteristics. ... 

Maintenance The only moving parts in spiral concentrators are those in the pumps that supply the feed and recirculate intermediate products. However, there are sometimes minor maintenance problems associated with the spiral trough itself. Some ores contain sharp particles of veiy abrasive minerals. The presence of these minerals in some ore causes rapid formation of deep grooves in the surface of cast-iron spirals. Wear grooves can be patched with a variety of plastic and metallic cements. Most spirals presently in service are made of cast iron with molded and vulcanized liners. These liners have successfully solved most wear problems. 

Other than the wear problems, actual in-plant maintenance usually involves removal of wood, pieces of blasting wire, and other trash from the ports. When a reagentized feed is used, layers of oily reagents can build up on the spiral surface and sometimes require scrubbing for removal. With feeds containing oily reagents that attack rubber, ra-sion-resistant alloy spiral sections are used. 

Some 70-85 per cent of failures and wear problems in lubricated machines are caused by oil contamination. Clean oil extends machine and oil life and gives greater reliability, higher productivity and lower maintenance cost. Hence, some type of filter is an essential part of virtually all lubrication systems. 

It is believed that the air velocities in a large-diameter dilute-phase system can be 50 to 100% higher than an equivalent well-designed dense-phase system. Hence, much greater wear problems are expected in the dilute-phase system, although significant advances have been made in the technology of wear-resistant materials and bends (Wypych and Arnold, 1993). Other features involved with dilute-phase transport systems include ... 

Methanol is not miscible with hydrocarbons and separation ensues readily in the presence of small quantities of water, particularly with reduction in temperature. On the other hand, anhydrous ethanol is completely miscible in all proportions with gasoline, although separation may be effected by water addition or by cooling. If water is already present, the water tolerance is higher for ethanol than for methanol, and can be improved by the addition of higher alcohols, such as butanol. Also benzene or acetone can be used. The wear problem is believed to be caused by formic acid attack, when methanol is used or acetic acid attack when ethanol is used. 

Chemical wear problems infiltration of alkali salts, redox effects, hydration cracks, corrosion of chrome ore. 

The high precision with which structures can be etched using DRIE allows for a precise alignment of the structures with minimal clearance between scrolls. Wear problems can be avoided by deposition of polymer films. First experiments have been done with a pump with 10 mL/min pump capacity. 

Weld Metal Overlay a Cost-effective Solution to High-temperature Corrosion and Wear Problems... 

An in-depth analysis is made of the wear behaviour of Krauss-Maffei s twin-screw extruders for manufacturing PVC pipes. Wear mechanisms and factors for minimising wear and influencing wear behaviour are discussed and the development of a new series of 36D double degassing extruders, which are less prone to wear, is reported. The energy balance, output rates and configuration of these extruders are briefly described and a policy for dealing with wear problems in extruders is outlined. 

L. Halm, Note on refractory wear problems in blast furnaces due to the presence of alkaline compounds. Br. Iron Steel Inst. 8003 C.D.S. Circ. 4, 955-964 (1969). 

Because glass reinforced grades often give wear problems, bimetallic liners QCyloy 800) should be used. Abrasive wear occurs primarily... 

In addition to the above inductive technologies for information storage, devices based on the Faraday or Kerr effects are being developed. The aim is to increase two of the most appreciated characteristics in storage technology devices data density and access time. The storage capacity of a magnetooptical disc is currently 600 MB (files, 1991). An additional characteristic of these methods is that the wear problem is virtually eliminated. 

High-Temperature Corrosion and Wear Problems High-temperature corrosion and wear occurs in (i) waste incineration, (ii) fossil energy, (iii) pulp and paper (iv) petroleum refining (v) the chemical and petrochemical industries (vi) mining and smelting operations. 

Another typical feature associated with ceramics is the cyclical occurrence - often associated with the season of the year - of laminations due to variations in the consistency of the material to be processed. Frequently the origin can also be traced back to wear problems in the preceding grinding and mixing operations. 

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