Bevel Gear for a Riding Lawn Mower
Figure CS 29 is a bevel gear for the transaxle of a heavy duty riding lawn mower. The gear has an outer diameter of 3.50 inches and a maximum thickness of 1/2 inch. At the root of the teeth, the thickness is approximately 3/16 inch. The minimum material properties have been estimated to be a yield strength of 125 ksi and a surface hardness of Rockwell C 25. The maximum operating temperature should be less than 250°F. Since the mower has a multi speed transmission, the transaxle gearing may be subject to sudden applications of load due to improper engagement of the clutch. The manufacturer has proposed a nonstandard test in which a 10 pound weight is dropped onto the gear from a height of 15 inches. While this does not translate to a Charpy test value, it is clear that impact resistance will be an important property. The gear will operate in an oil-filled enclosure so corrosion resistance need not be outstanding. A smooth surface finish is desirable (especially on the teeth), and the total production run has been placed between 25,000 and 50,000 units.
1. Based on the size and shape of the product, describe at least three reasonable ways in which the component could be produced. For each method, briefly discuss its relative pros and cons.
2. What type of engineering materials might be able to meet the desired requirements? What would be the pros and cons of each general family?
3. For each of the shape generation methods in part 1, select an appropriate material from the alternatives discussed in part
2. (Note: Casting alloys should be matched for the casting processes, high machinability alloys would be favored for cutting applications, etc.).
4. Which of the above alternatives do you feel would be the "best" solution to the problem? Why? For this system, outline the specific steps that would be necessary to produce the part from reasonable starting material. Include any necessary heat treatments and/or surface treatments.
5. Your supervisor has asked you to evaluate the possibility of producing this part as a near net shape ferrous forging, with forged gear teeth. Investigate this process and determine if it would be a feasible approach. If so, would you recommend that the forging be performed cold, warm, or hot? Would subsequent heat treatment or surface treatment be required? What concerns might be associated with these processes?
