Welding of assembled crankshafts

Monday, October 07, 2013

Tags :  crankshafts

In 2010 I wrote a RET-Monitor article on assembled or ‘split’ crankshafts. In it I mentioned some of the applications of this type of crankshaft assembly, and discussed some of the advantages and disadvantages. There was a brief mention of the use of welding in the assembly of such crankshafts, but no further explanation. Although somewhat late in doing so, I’d like to explain more about the reasons for welding.

The 2010 article mentioned that welding is sometimes used to prevent the various parts of the crankshaft from losing their angular alignment if the interference between crankpin and crankwebs is insufficient. In addition to angular movement being possible, it is sometimes found that the crankwebs tend to move axially in relation to each other.

There are two main problems with this particular axial movement. The first is that the rod end float can be diminished to zero, with the rod thrust faces in constant contact with the crankwebs. This can lead to rapid thrust washer wear and an increase in friction. If the movement is significant enough, it can cause the piston to be thrust to the side, effectively rotating the area of maximum contact pressure from its usual location. That can mean the piston wears more quickly as the ‘bridge’ between inlet or exhaust ports, against which the piston usually bears, is no longer loaded as intended. The second problem with axial movement of the crankwebs is that the rod thrust clearance can increase; again this can allow unintended off-centre loading of the piston-to-cylinder contact.

It is common to find axial movement of the crankwebs combined with some angular misalignment too, which should not be surprising given that there is insufficient interference to prevent movement.

Welding is a common way to prevent movement, but there are some pitfalls in welding the crankpin to the web(s). If there is too much heat added to the crankshaft during the operation, there is a danger of distorting the crankshaft, thereby making it unfit for use. The crankshaft must be assembled accurately before welding, otherwise we may simply be condemning an engine to run with an inaccurate assembly, even if the welding is sound. If too much heat is introduced then the weld itself might weaken the crankshaft components to the extent that the fatigue life of the crankshaft as a whole is reduced.

The welding is done on the outside face of the crankweb, where the crankpin is pushed through. If proper weld preparation is done on both components, and a skilled welder does the job, it may not be necessary to dress the weld, but it is common to see the weld dressed either for neatness or to prevent contact with adjacent components. Often the weld does not go right around the crankpin, especially where the engine is relatively short stroke. It is common to have just a short run of weld closest to the outside diameter of the crankweb, or to create an intermittent weld around the crankpin.

Even if the process is carried out perfectly, there is the disadvantage that the weld has to be ground away before the crankshaft assembly can be serviced.

Written by Wayne Ward

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Comments

It's an old article (I'm replying in late 2017) but I do have a little experience of built up cranks and crank welding. It isn't an issue on majority of single throw (motorcycle) engines but can help on multi cylinder machines with relatively high outputs (around 110bhp/litre) and above) On some motors, it only affects the time before significant destruction due to flex of flywheels on crank-pin, The crank counterweights cause pin to fret inside it's bore. Although I aven't tested it, I believe a 'pendulum' type balancer would allow much higher rpm with less movement of friction fit components?