Chapter 11

1. Chapter 11 Crankshaft, Bearings, and Engine Balancing

2. Objectives • Analyze wear and damage to crankshaft and bearings • Select most appropriate repairs to perform • Explain theory of engine balancing • Describe related theory of bearings, crankshaft, and related parts

3. Counterweights • Opposite of each rod journal • Precisely balances combined rotating mass of offset rod journals and rod • Compensates for weight of connecting rod, bearings, and piston assembly

4. Counterweights

5. Cast or Forged • Forged cranks are stronger but costly • Cast cranks are high enough quality to do an adequate job • Cast cranks have larger counterweights • Cast metal is not as dense as forged steel so it is lighter

6. Cast or Forged

7. Checking Crank Condition • Check crankshaft for warpage before grinding • A forged, bent crankshaft can be straightened • Peen shaft in fillet radius area with a punch then regrind crankshaft • A bent cast crank is typically replaced

8. Checking for Cracks • “Ring” crankshaft with light tap of a hanger • Dull sound indicates presence of a crack • Use Wet Magnaflux magnetic particle inspection method

9. Checking for Cracks • Magnetic field is induced in crank • Spray crank with a mixture of iron particles suspended in a liquid • Use black light to detect cracks

10. Vibration Damper • Referred to as a harmonic balancer • Heavy outer ring and an inner hub separated by a synthetic rubber strip • Two parts stretching against rubber strip absorb vibrations

11. Checking Damper • Faulty damper could cause crankshaft failure • Outer ring may slip causing: • Out of balance condition • Inaccurate ignition timing marks • Make sure damper is correct for engine

12. Checking Damper • Check for looseness on crankshaft • Loose keyway • Ensure pulleys are straight • Check seal area for any damage

13. Checking Damper

14. Crankshaft Wear • Check crank for: • Out-of-round • Tapered • Journal tolerance • Nicks in journals • Oil groove wear

15. Crankshaft Wear

16. Polishing the Crankshaft • Polish after grinding to remove burs • Done in opposite direction of grinding • Three steps require an increasingly finer grit sandpaper • 280 • 320 • 400

17. Crankshaft Hardness • Some crankshafts are specially hardened • Must be rehardened after regrinding • Crankshafts can be surface hardened • Two types of surface hardening • Gas nitriding • Tuffriding

18. Measuring Crank Hardness • Types of hardness testers: • Brinell • Measures diameter of an impression left by a steel ball • Rockwell • Measures depth of an impression

19. Measuring Crank Hardness • Types of hardness testers: • Vickers • Measures depth of impression left by diamond pyramid • Shore scleroscope • Measures height of rebound of a dropped ball

20. Measuring Crank Hardness • Types of hardness testers: • Electronic hardness • Measures penetration and changes in frequency

21. Checking Bearing Clearance Using Plastigage • Oil bearing surface so Plastigage won’t stick • Apply to bearing journal then torque bearing cap • Loosen cap bolts and remove • The wider the flattened string, the tighter the clearance

22. Checking Bearing Clearance Using Plastigage • Look for variations in thickness that would indicate a tapered journal • Measure widest part of Plastigage to determine minimum clearance • Use oil to remove Plastigage

23. Bearing Properties • Three primary properties that make it suitable for use in an engine: • Ability to embed • Ability to conform • Fatigue strength

24. Bearing Properties

25. Multilayered Bearings • Can carry at least three times as much load as babbitt • Typical multilayer • Consists of a steel back covered with a layer of copper and lead covered by a thin overlay of babbitt

26. Multilayered Bearings • Babbitt protects against dirt • Multilayers are choice for high performance applications

27. Bearing Inspection • To check for metallic particles in bearings: • Scrape bearing surface with sharp tool • Place scrapings on clean paper • Rub a permanent magnet on underside of rag • If particles are iron or steel, they will follow the magnet

28. Types of Bearing Load • Pressure from flame front against piston • Centrifugal force from rotating weight of rod and piston • Inertia from up and down motion of piston and rod assembly

29. Thrust Bearing Wear • Thrust bearings wear • Two flat surfaces contacting flat surfaces • Most have concave reliefs for lubrication • Thrust surface can only handle intermittent loads • Thrust bearing failure can be traced to both engine or transmission problems

30. Thrust Bearing Wear

31. Engine Balancing • Balance all moving parts • Imbalance can cause vibration and worn parts • Two types of vibrations • Primary • Secondary

32. Engine Balancing • Counterweights are added to crankshaft • Counteracts up-or-down vibrational force

33. Balance Shafts • Silent shafts • Secondary imbalance can be corrected • Use two counterweighted balance shafts driven by crankshaft in opposite direction • Many are located in lifter valley • Many are supported by needle bearings

34. Balance Shafts

35. Engine Balancing • Every time engine speed doubles • Imbalance is multiplied by four • Balance rotating parts • Spin on a balance machine to find heavy spots

36. Engine Balancing • Heavy counterweights can be lightened by drilling or machining • Light counterweights can be welded

37. Types of Imbalance • Centrifugal force • Causes an object to travel in the same direction as it rotates • Centripetal force • Perpendicular load to direction of rotation