Crankshaft Engine Problem
Crankshaft engine problem - CRACKED
Crankshafts are under stress and vibration by their very nature of design. Some crankshafts crack more frequently than others. The most common cracks appear in the crankshafts out of the oil holes and in the radius areas. Some minor cracks can be removed when grinding to an under-size. Shafts that are prone to cracking should be crack tested before and after grinding. Some causes of cracks can be a faulty harmonic balancer, revving the engine above manufacturer specifications or out of balance shaft and associated components. As a common rule all diesel shafts should be crack-tested at time of engine overhaul.
Crankshafts will crack in the radius area as a result of incorrect grinding. If the correct radius is reduced or the grinder produces a step edge this crank will be more likely to crack and become unserviceable than one ground correctly. Great care has to be taken to ensure no scratches or marks are evident in the radius area.
Oil holes have to be re-chamfered after grinding and all burrs removed. Caution has to be taken to retain the correct size and chamfer. This area is very thin due to the angle of the oil drilling and will crack if not reshaped correctly after grinding.
The reduction of vibration by balancing engine components will reduce the likelihood of crankshaft cracks. Shot peening and rolling of the radius area will also strengthen these areas against increased stress produced in high performance diesel's and racing engines.
: Every care has been taken in writing this information and procedures, but no responsibility can be excepted for errors, omissions or misuse of this information and procedures. The information available on this site is for your instruction only and cannot be copied for sale, © copyright 2020 UMR Engines www.engineproblem.com.au
Crankshaft engine problem - PICKED UP / SEIZED / FLOGGED
When crankshaft and bearings seize it is the end result of major damage caused by a breakdown in lubrication or assembly bearing clearances being under minimum specifications.
The clearance is referred to as "vertical oil clearance". As the lubrication breaks down and allows metal to metal contact between the shaft and the soft bearing material, heat created by friction melts and welds bearing material on to the shaft. In a very short period the bearing and shaft can be destroyed. This same scenario takes place during a dry start after re-assembly.
Engines have to be primed to obtain oil pressure before the initial start after a rebuild. This involves cranking the engine until oil pressure is obtained. Removal of spark plugs in a petrol or removal of injectors or glow plugs in a diesel allow oil pressure to be obtained at starter cranking speed.
The results of a dry start may not always destroy the bearings immediately but even minor bearing contact with the shaft will damage the bearing enough to result in premature failure. (Note well, if an engine will not turn freely by hand or under normal starter motor action the engine should be dismantled until the source of incorrect clearances is identified and repaired. Common causes are incorrectly fitted wick rear mains, misaligned con rods, dirty assembly, overlapped oil rings, insufficient ring end or back clearance etc, etc.)
If crankshaft-bearing seizure occurs many thousands of miles after initial engine assembly the failure is usually related to a breakdown in lubrication. (ENGINE DRIVEN WITH LOW OIL LEVEL OR NO OIL PRESSURE)
Pre maturely worn out crankshaft and bearings can be the result of dirty or diluted engine oil. When unsure and a cause must be established take a sample of the used oil from the engine in a clean glass container and have it analysed. The result of the oil analysis will give definite answers to the cause of the failure.
: Every care has been taken in writing this information and procedures, but no responsibility can be excepted for errors, omissions or misuse of this information and procedures. The information available on this site is for your instruction only and cannot be copied for sale, © copyright 2020 UMR Engines www.engineproblem.com.au
Crankshaft engine problem - BENT
Most new crankshafts will attain some bend during normal operation. Up to.004" to.006" may be detectable at time of regrinding. This amount of bend is acceptable and can mostly be removed by regrinding. Crankshafts with more bend should be straightened before regrinding.
Crankshafts with sever bend should be replaced. Crankshafts with bend can be straightened in a special jig. The shaft should always be mildly heated to reduce the chance of developing a crack. All straightened shafts should be crack-tested after straightening.
Crankshafts with badly localised overheating on a flogged journal may have bent at the position of where the web and journal meet. This type of bend can be removed by the use of special shaped punches used against the radius. Again after straightening the radius must be reclaimed and crack-tested. All of these straightening process's can damage the shaft beyond repair and this should be considered with the customer before being attempted.
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: Every care has been taken in writing this information and procedures, but no responsibility can be excepted for errors, omissions or misuse of this information and procedures. The information available on this site is for your instruction only and cannot be copied for sale, © copyright 2020 UMR Engines www.engineproblem.com.au
Crankshaft engine problem - BROKEN
When a crankshaft breaks it is usually a continuation of a crack that has developed due to excessive harmonic vibration. If these cracks go undetected eventually the shaft will suffer a major break. This usually occurs as the normal load is exceeded and the weakened shaft finally lets go. The crack usually starts around the fillet radius and continues until the journal breaks away from the web.
The design of every shaft has a built in critical point of breakage and will usually break in a similar position every time. This is referred to as the nodal point. The design and weight of the flywheel, shaft and balancer determine this point.
For more information see: Cracked Crankshaft
: Every care has been taken in writing this information and procedures, but no responsibility can be excepted for errors, omissions or misuse of this information and procedures. The information available on this site is for your instruction only and cannot be copied for sale, © copyright 2020 UMR Engines www.engineproblem.com.au
Crankshaft engine problem - WORN OUT
Severely worn out crankshaft journals are normally associated with worn bearings and all the reasons and symptoms are explained under big-ends/mains / worn out. However the incorrect selection of bearings can cause excessive wear of the crank while the bearing shells are reasonably normal.
Some cranks must be nitrided during the grinding process and run fairly hard bearings. The introduction of embedded abrasive material can wear the crank pins and journals more than expected leaving the bearings at first glance relatively unscathed. When the lack of nitriding is the suspect cause all journals will generally be similarly affected. Check hardness.
The rear main seal area can be severely worn where the seal lip contacts the shaft. This can be the result of hardening of the seal over time. Most shafts suffer this problem and this area can be reground provided the shaft diameter stays within the seal specified shaft size. If this is not possible by grinding a speedy sleeve can be fitted to reclaim this area.
A loose front pulley or balancer can also damage the front snout of the crankshaft. Welding and re-machining can reclaim the keyway and snout area. This process also has its problems and all repaired shafts should be crack-tested. Again this process should only be carried out after discussion with the customer. Good service procedures can prevent this problem occurring.
: Every care has been taken in writing this information and procedures, but no responsibility can be excepted for errors, omissions or misuse of this information and procedures. The information available on this site is for your instruction only and cannot be copied for sale, © copyright 2020 UMR Engines www.engineproblem.com.au