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Joint Face Design

con-rodsLast month we looked at some of the aspects concerning the design of the big end and this month we pick up with further aspects of this critical area, namely those concerned with the joint faces. The big end joint is an important area of rod design, as it affects the durability of the rod bolts. If there is a problem with this joint, it will surely lead to a catastrophic engine failure – it would be a real surprise to find a loose rod bolt with no significant damage found elsewhere.

There may be some among you who aren’t familiar with the workings of a cyclically loaded joint and who don’t possess a good textbook which covers the matter, but we can say that the greater the ratio of stiffness of the clamped members to the clamping member (i.e. the bolt), the smaller the proportion of the cyclic load that the bolt will see. This will be covered in greater detail in articles on fasteners, but it is worth knowing this useful morsel when considering the design of the joint. It would be quite easy to design a con rod assembly with the rod and the cap closely sculpted around the bolt head and shank for minimum mass, but this is a sure way to disaster where significant cyclic loads are present. As can be seen from the picture accompanying the previous article on big-end design, the joint is deliberately made quite stout in the area of the joint face.

By keeping a high stiffness ratio as described, we can use a smaller bolt, which in turn has benefits for the rod assembly, the rest of the engine and ultimately the performance of the car. The smaller bolt is, of course, lighter and, being smaller, can be placed closer to the big end axis which has two main benefits. The first benefit, which is obvious to the naked eye, is that the rod becomes narrower. On vee-engines, the width of the rod at this point can become the limiting factor in lowering the crankshaft axis in the engine. A lower crankshaft height allows packaging and performance benefits in the car, not least from having a lower centre of gravity for the engine. Prior to the Formula One engine homologation rules, it was not unknown to have the sump machined locally to provide minimum clearance to the rod locus; the lowest point of which was defined by the big-end bolt. Another recently used solution to this problem was to use a big-end joint which is angled. The cap is still applied axially to the rod, but each joint face is angled, with the outer edge above the nominal centreline, thus angling the fasteners and providing more latitude to lower the crankshaft. Of course, the edge of the joint at the inner edge must still be on the nominal joint centreline.

The second good reason for keeping the bolt axis close to the big end axis is the reduction of bending load in the bolt. As is obvious from examining the con rod, the load is not applied to the joint in a simple axial fashion, but is a combination of axial and bending loads. To mitigate the effects of this bending stress, it is possible to make design changes to joint face, involving judicious removal of material from the mating faces. Another factor which can allow the fasteners to be placed closer to the rod axis is by not using a ring-dowel around the bolt, although this remains a popular solution to the problem of locating the cap accurately with respect to the rod. However, this can be achieved with a pin dowel and good-quality small diameter individual rollers as found in needle roller bearings, often used for this purpose. An alternative is to use a bolt with a tight-tolerance shoulder only slightly larger than the bolt thread.

Written by Wayne Ward.

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