194 AGRICULTURAL ENGINEERING 



4. What are the three essentials of a practical machine? 



5. Why is the selection of a machine important? 



6. What is friction? 



7. Define the coefficient of friction. 



8. What are some of the conditions that modify the coefficient of 

 friction? 



9. What would be the draft on ice of a sled weighing 4000 pounds 

 if the coefficient of friction between the runners and the ice is 0.025? 



10. Mention several instances where friction is especially useful. 



11. Why is friction of rest greater than friction of motion? 



12. What is the cause of rolling friction? 



13. What are the purposes of lubrication? 



14. What kind of lubricant should be used on a machine like a 

 harvester? 



15. Of what value is graphite as a lubricant? 



16. What should be taken into account in the design of a bearing? 



17. Why should the material used for the bearing be different from 

 that of the journal? 



18. When are roller and ball bearings best? 



19. Explain the process of babbitting a box. 



20. How should a bearing be adjusted? 



