Screw Breaks

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  

In general, F, should be greater than 1.5 to provide an acceptable strength for the screw. If the safety factor is less than 1.5, then the designer should consider a different base metal alloy with a higher shear strength. 

Base Alloy for the Screw Ultimate Tensile Strength, MPa Shear Strength (S ), MPa (assumed to be 60% of the ultimate tensile strength) 

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The manager of a truck manufacturing company was shown a screw that cost five cents to make and sold for 30 cents. He asked What happens if that screw breaks The workers explained it was a vital part of the engine and that without it the truck wouldn t work. The manager then proposed to raise the price of that screw to 100. The employees were appalled, but the manager explained that the manufacturing price should not determine the product price the perceived benefit of the clients should. 

All pilot and igniting piping shall be provided with break flanges located just below the burner tip flange. If the pilot is equipped with an air inspirator, the latter shall be located as low as practicable but not within 500 mm of the burner tip flange. Screwed connections shall not be used for pilot and ignition piping. 

Brass water fittings give no trouble except that dezincification may occur in acid waters or waters of high chloride content, especially when hot. This dezincification has three effects. Firstly, the replacement of brass by porous copper may extend right through the wall of the fitting and permit water to seep through. Secondly, the zinc which is dissolved out of the brass may form very voluminous hard corrosion products and eventually block the waterway —this is often the case in hot soft waters. Thirdly, and often the most important, the mechanical properties of the brass may deteriorate. For instance, a dezincified screwed union will break off when an attempt is made to unscrew it and a dezincified tap or ball-valve seat is readily eroded by the water. 

Centrifuging the screw-cap vial can easily break emulsions, which often form during extraction. The vial will survive up to 6000 g if rubber stoppers are inserted into the centrifuge cup to provide a flat base for the vials. The required phase (usually the top layer) can be easily removed with a pipet or, if it is to be discarded, it can be removed using a disposable pipet connected by tubing to a suction flask and vacuum line. 

I spent the next two weeks while on my breaks, during lunch, and whenever I could sneak away from my work area for a few minutes removing the locking screws from several window frames. Never mind how many. . . gee, who the heck was counting ... 

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