Intermittent load application

Products that are subjected to a load have to be analyzed carefully with respect to the type and duration of the load, the temperature conditions under which the load will be active, and the stress created by the load. A load can be defined as continuous when it remains constant for a period of 2 to 6 hours, whereas an intermittent load could be considered of up to two hours duration and is followed by an equal time for stress recovery. The temperature factor requires greater attention than would be the case with metals. The useful range of temperatures for plastic applications is relatively low and is of a magnitude that in metals is viewed as negligible. 

Performance. Carbon—zinc cells perform best under intermittent use and many standardized tests have been devised that are appropriate to such applications as light and heavy flashlight usage, radios, cassettes, and motors (toys). The most frequently used tests are American National Standards Institute (ANSI) tests (6). The tests are carried out at constant resistance and the results reported in minutes or hours of service. Figure 3 shows typical results under a light load for different size cells, whereas Figure 4 shows results for different types of R20 "D"-size cells under a heavy intermittent load. 

Most mechanical conveyors are designed for continuous operation and may exhibit problems in intermittent-service applications. The primary problem is the startup torque for a fully loaded conveyor. This is especially true... 

FIGURE 8.15 Zinc-carbon batteries, discharged under simulated cassette application (10 ohm intermittent load) at 20°C. 

Figure 8.15 shows the same four battery sizes discharged through a relatively low resistance of 10 fl (about 150 mA) on a simulated intermittent cassette application. AAA- and AA-size batteries deliver about 30% less service when continuously discharged at this load. 

Load factor is particularly important with air-cooled machines where sustained full-load operation results in an early build-up of deposits on valves and other parts. This build-up increases the frequency of maintenance required to maintain compressor reliability. Intermittent operation is always recommended for these units. The frequency and duration of unloaded operation depends on the type, size, operating pressure of the compressor. Air-cooled compressors for higher than 200-psig-pressure application are usually rated by a rule that states that the compressing time shall not exceed 30 minutes or less than 10 minutes. Shutdown or unloaded time should be at least equal to compression time or 50 per cent. 

There are no electrolyzers developed specifically for operation with wind turbines. However, the rapid response of electrochemical systems to power variations makes them suitable "loads" for wind turbines. Industrial electrolyzers are designed for continuous operation, mainly because their elevated investment cost requires high-capacity factors for reasonable payback times, but they are subject to a considerable number of current interruptions through their lifetime due to occasional power interruptions, accidental trips of safety systems, and planned stops for maintenance. Current interruptions are more frequent in specialty applications, where electrolyzers supply hydrogen "on demand." Therefore, the discontinuous use of the equipment is not new, and most commercial electrolyzers may be used in intermittent operation although a significant performance decrease is expected with time. In fact, it is not power variation, but current interruptions that may cause severe corrosion problems to the electrodes, if the latter are not protected by the application of a polarization current when idle. 

In addition to indentation curves, the residual imprints after the application of a defined load force can be images using intermittent contact mode AFM. This is shown below for indentation tests performed on PE, in which the size of the imprint was shown to be reduced to the level of individual lamellae (Fig. 4.23). 

---