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Machining 101: Rotating assembly

Updated: 09/08/2014 @ 10:27am

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.

Flywheel servicing

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.

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