Whether you think diesel crankshafts are the same as or different from their smaller gasoline counterparts, you’re right. Diesel crankshafts and rotating assemblies like their cousins in the gas engine world turn a linear motion from combustion into a rotating motion that can move a vehicle, supply power through a generator, pump oil from an underground or otherwise give measurable activity.
They require balancing and attention to selection of appropriate components. But doing so requires a lot more of everything. They’re big, they’re heavy and they’re expensive. They may be harder to find in certain applications.
But whether your shop handles heavy-duty diesel engine repair, light or medium duty diesel service or you build engines designed to produce as much performance as possible, understanding the crankshaft’s importance in the equation and your ability to affect that can be a rewarding challenge.
According to the 2011 Machine Shop Market Survey conducted by Engine Builder magazine (reported in these pages in June of this year), diesel crank production decreased in 2011 relative to 2010, falling from 6.4 to 4.8 total units per month. Gasoline crankshaft regrinding fell less, percentage-wise, going from just over 21 total units produced monthly during 2010 to just over 20 total units produced in 2011.
A downturn in the economy (fewer trucks on the road at the time) may be partially responsible for the decline in production, but we expect the number to jump back again and continue to climb as the economy continues to recover.
Despite their similar appearances, diesel crankshafts have some primary differences from their gasoline counterparts. A gasoline engine uses a spark to ignite the fuel. A diesel engine, operating at a much higher compression ratio, uses the heat produced by compression to ignite the fuel. Diesel engines typically have a longer stroke than the bore diameter, so to handle this load diesel crankshafts are much larger physically.
Diesel crankshafts can be found in a variety of materials. From strongest to (relatively speaking) weakest, you’ll find billet steel, steel forgings, cast steel, nodular iron, malleable steel and (in some cases) cast iron. Heavy-duty diesel cranks have hardened bearing surfaces (case hardened) with the most common form of hardening being the induction process, according to industry experts.
Induction surface hardening is a well-known and widely used process in the global diesel engine manufacturing industry. It was originally used primarily to harden bearing journals, improving their wear resistance but it is recognized also as a viable technology for improving the fatigue resistance of highly-stressed diesel crankshafts. Induction hardening and induction tempering are rapidly becoming the processes of choice for manufacturing crankshafts for diesel engines of all sizes.
In addition to the physical requirements of dealing with diesel crankshafts, industry experts caution that availability of replacement bearings will have a direct impact on whether you should or can service these behemoths. Unlike bearings in the automotive aftermarket, diesel bearings may not be available in convenient oversizes so it’s a good idea with out-of-the ordinary shafts not to grind them until you have the bearings in hand.
But whether you repair or replace a crank ultimately comes down to how badly it was damaged. If it was broken, more than likely replacement is your only option (but remember that in most cases, a broken crankshaft is the result of some other issue).
Additional common causes of damage to diesel crankshafts include spun bearings, resulting in a loss of journal hardness; viscous dampeners wearing out, resulting in a broken crank; failure of the balance box assembly, resulting in a broken crank; and cracks in the journals.
In most instances, cracks in the journal of a diesel crankshaft will prevent its repair. While some diesel cranks can be welded or chromed back to standard, it often becomes an issue of economics. The higher the value of a crank the more work you can put into it. What you would do to fix a $30,000 crank may not make much sense on a $1,000 crank.
Balancing the Equation
Top diesel engine builders have told us for years that diesel design improvements that have resulted in smoother running engines mean attention to engine balancing is more important than ever. Far from being just a “comfort” issue, a balanced engine can reduce vibrational fatigue.
Balancing the rotating assembly of a diesel engine has been more important since the update from aluminum pistons to a one-piece steel slug. The piston design went from an articulated piston to a heavy one-piece piston body, variations in the weights of the components were realized.
Many problems engine builders find in other areas may be due to vibrational fatigue as well. Balancing helps improve the performance of the engine when the bottom end and the main running components are balanced together.
From a technical point of view, every engine regardless of the application or its selling price can benefit from balancing. A smoother-running engine is also a more powerful engine. Less energy is wasted by the crank as it thrashes about in its bearings, which translates into a little more usable power at the flywheel.
Reducing engine vibration also reduces stress on motor mounts and external accessories, and in big over-the-road trucks, the noise and vibration the driver has to endure mile after mile.
Though all engines are balanced from the factory (some to a better degree than others), the original balance is lost when the pistons, connecting rods or crankshaft are replaced or interchanged with those from other engines.
The factory balance job is based on the reciprocating weight of the OE pistons and rods. If any replacements or substitutions are made, there’s no guarantee the new or reconditioned parts will match the weights of the original parts closely enough to retain the original balance.
One well-known diesel engine facility tells us that every crankshaft is carefully cleaned and thoroughly visually and magnetically inspected. If they’re doing a crank with counterweight bolts the old bolts are discarded and new counterweight bolts are put in. The counterweight mounting areas are inspected for any fretting as well.
Look very carefully at the radiuses of the crankshaft. Make sure they’re properly sized and there are no stress risers or areas of concern there.
Some shops are straightening the cranks as well, and while this may be out of the skill set of many shops, when they match all these things up the straightness of the crankshaft, the straightness of the main line, the bearing clearances one expert says he is then confident there is good bearing clearance and no fatigue spots on the crank. It’s a real good riding surface for that crank to lie in the block.
It may take more time and cost more money, he says, but customers with these engines rely on him to provide maximum performance for the life of the engine. They trust that he will provide the very best that he can produce. Downtime is lost money, and your careful attention to each step of the reconditioning process is an investment in their business.