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The Selection Process environments and the graphite makes the plastic self-lubricating. Carbon-graphites were not used because of availability problems. The polyimide molding resin selected cost $9 per pound, but there are about 400 parts per pound. The material cost per roller is only about 2 cents. Material: Injection molding polyimide + 20% graphite; shall be ARUN grade 407. Poly Plastics, Henrietta, NY. 747 lium copper would involve more process steps. It would have to be heat treated. Wrought spring materials such as full-hard cold-rolled wrought steel or copper alloys were considered, but heattreated springs last longer and are more likely to deliver the desired 107 cycles. Material: 1080 steel, hardened and tempered to spring condition (47 HRC). Fabricate complete and black oxide coat all over. Problem Flat springs are needed to hold plastic film sampIes against a platen in a test rig. The springs are 0.5 mm thick, 10 mm wide, and 50 mm long. A material is needed for these springs. Requirements: l. Must properties: Black color, will not rust in 50% RH, 70° F room environment. 2. Want properties: Would like 107 flexes at a stress level of 3000 psi (20.68 MPa). 3. Quantity: Twenty springs. 4. Delivery: Springs must be made within 30 days. 5. SelVice life: The 107 flexes equate to about a year service life. 6. Cost: Should not cost more than $10 each after fabrication. Availability controls this selection to a significant degree. There are not many materials in warehouses in the form of 0.5-mm-thick sheet or strip. Two common flat spring materials that have good availability are 1080 steel and beryllium copper. We opted for 1080 hardened and tempered spring material. It can be easily blackened with a black oxide treatment. This spring stock is widely used for flat springs, steel rule dies, and pallet banding. The springs can be cut from the hardened strip with a water jet laden with particles or by EDM machining. The beryl- Summary Our discussions of various material systems should have supplied enough information so that the designer can quantitatively compare the candid ate materials and know what heat treatments and processing to use when specifying the material on the drawing. Obviously, it is not possible to compile a list of all the components that are found on machines. Similarly, the materials listed are not the only ones that will work, but the examples reflect a wide range of service experience. The material selection process should be started as early as possible in a machine or structure design. The key to proper material selection is to establish a checklist of the properties required for a serviceable design. Does the part have to be hard? Does it require corrosion resistance? Are any special physical properties required? Once the required use properties are established and prioritized, designers can draw on their repertoire of available engineering materials to select one or more candidate materials and treatments. Remember to state the function of the part. What does it do? What are the apparent forces on it? What are the potential unforeseen forces? Calculate stresses based upon the perceived forces. List potential corrodents; determine if tribological properties are to be a consideration. Doing these steps should lead to
