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Machining 101: Rotating assembly
In a traditional engine the motion that the crankshaft experiences along with the big end of the connecting rod is qualified as rotation while the piston and the small end of the connecting rod reciprocate. But engine parlance lumps these together as the rotating assembly. This, the second installment in Successful Farming’s three part series on hiring an engine machine shop services will explore the rotating assembly.
Inspecting the crankshaft . . .a must do list
When the crankshaft is removed many shops just measure it for size to confirm if it is reusable and if it can be machined. If it passes that cursory examination it is assumed that it is ready to be reinstalled when the rest of the work is done. That is a mistake that can cause a good many headaches.
The following are the steps your machinist should take to inspect a crankshaft:
- Inspect keyway condition and the washer under the damper bolt if so equipped.
- Visually inspect snout threads for damage.
- Visually inspect the flywheel bolt-holes.
- Inspect rear main oil seal surface for wear and if a sleeve will be required.
- Check main bearing and crankpin surface by measuring with a micrometer.
Mount the crankshaft in “vee” blocks and check for bend, snout trueness and flywheel flange run-out. If the crank needs to be straightened confirm the method employed. Some shops straighten a crankshaft by striking the fillet area with a rounded chisel and hammer. This is NOT a method prescribed by any engine manufacturer. The crank needs to be straightened in an application specific press. Most shops send the crankshaft out to a special job operation for this service.
Visually inspect the entire crankshaft for any nicks, dents or pitting in the journal area.
Magnetically inspect the crankshaft for any cracking.
There are two schools of thought when it comes to crankshaft service. If the crankshaft has been in use for some time the shop automatically grinds the journals and fits the proper bearings to compensate for the material removed. The other belief is if the journals check out fine then the crankshaft is just polished before reinserting.
I prescribe to the theory of leaving as much material on the crankshaft as possible without sacrificing quality. I am not against having a crankshaft ground undersize. It is a well -accepted procedure. I just do not automatically do it unless it is required.
Depending on the application the engine may have either a flywheel (manual transmission) or a flexplate (automatic transmission or fluid drive).
The face of the flywheel can be considered the other half of the clutch disk. It needs to be in good shape, true and have the proper surface finish for the clutch to work as intended.
A visual inspection is only the first step in flywheel service. It needs to be measured since it is designed to work as a parallel surface. A dished, crooked or worn flywheel can cause release issues with the clutch and is not easy to access once the engine is reinstalled.
A flywheel needs to be inspected for:
- Face run-out: This is done with a dial indicator while the flywheel is mounted on the crankshaft.
- Dishing: As its name implies, the surface of the flywheel can warp or distort to resemble a dish or bowl.
- Hard spots and discoloration: Excessive clutch slippage or localized overheating will cause nucleation and growth of carbide hard spots on the flywheel surface.
- Groove wear: This condition is caused by the interaction of the clutch friction disc on the flywheel face.
- Ring rear wear (if applicable): Worn teeth on the flywheel ring gear mean that weight has been lost on a large diameter and will impact balance. It is not recommended to reverse the ring gear but replace it instead.
Proper rotating assembly service will include checks of all these areas, corrective machining if necessary along with resurfacing (a requirement) of the friction area.
Connecting rods requirements
The connecting rods are subjected to a number of different forces. The big end (crankshaft side) is a rotating member while the small end (piston side) reciprocates. The rod is subjected to literally tons of force in both compression and stretch as the piston changes direction. The load and stretch along with thermal cycling eventually deform the big end of the connecting rod. Thus, the connecting rod needs to be serviced properly before put back in service.
The connecting rod needs to be checked for twist and misalignment (bending). If necessary IT can be straightened if new parts are not offered. The rod needs to have the big end resized and the small end measured and honed to the proper dimension and surface finish. Connecting rod service is often where many shops fall short in providing the proper procedure for their customers.
A connecting rod that is resized is often identified as reconditioned. To do this the cap is removed and a small amount of material is precisely removed (ground) from both the cap and the rod to eliminate any error in concentricity. Then the rod is assembled and honed back to the proper dimension so that it fits on the crankshaft journal. Any engine being rebuilt should have either the connecting rods reconditioned properly or new ones installed. If the rods are going to be reused new bolts and studs should be installed.
Pistons – when to replace
If the engine block does not have sleeves then the pistons will need to be replaced to correct for the larger cylinder diameter created by the boring procedure. If the engine has a sleeve that is being changed, then in theory the piston can be reused since the bore will be the same size.
If the piston is going to be put back in service it must be thoroughly cleaned and inspected along with being measured. The same holds true for the wrist pins if they are going to be reused.
A general rule though is if the bore is worn, the piston, which is made from a softer material, needs to be replaced. There are rare instances when the piston can be reused or must be placed back in service since a replacement is no longer offered. This would be rare for an active engine and is more of a case when restoring an antique engine.
The damper’s purpose is to quell the frequency of impulses (vibration) that occurs from the firing events of the cylinders. There are many different theories employed to accomplish this. It is important to recognize that the harmonic damper will become less effective at this task from age, use and thermal cycling. A good rule to apply is that if the engine has been used enough to require a rebuild then the harmonic damper should be replaced.
The need for balance on engines is a topic that fuels aggressive debate. By industry definition the engine balancing procedure first has the pistons and rods all weight matched (made the same) by removing material on their respective balance pads. Then the crankshaft is spun much like a tire on a special balance machine and material is added or removed to bring it into equilibrium. If the engine is V-shaped then bob weights that simulate the mass of the reciprocating components are attached. For an in-line engine bob weights are not required.
Many shops only weight match the parts and never balance the crankshaft and call the engine balanced. In this case the only true statement is that the pistons and rods all weigh the same but the engine is not balanced. The shop sites the low rpm nature of farm engines not requiring a real balance as a higher speed application would. The author does not prescribe to that notion since harmonics are present at all engine speeds. The harmonics are transferred into the rotating assembly, through the timing chain or gear drive into the valve train and to the transmission. As an aside, the frequency is also transmitted into any accessories such as a hydraulic pump, alternator, power steering or air conditioner compressor. The cost to perform a proper balance of the rotating assembly in relation to the entire rebuild is so small that the benefit it pays in longevity and smooth operation makes it a real value.