How Does Stress Concentration Affect A Motorcycle Engine?

Описание к видео How Does Stress Concentration Affect A Motorcycle Engine?

Cycle World Technical Editor Kevin Cameron explains how he learned about stress concentration in cylinder studs.

Yamaha manufactured for many years what were called "production" racers. They were a grand thing in the two-stroke era because they made it possible for a much-larger number of people to go racing.

In 1974, Yamaha decided to enter the 750cc class in a big way. It produced a machine with two cylinder blocks, side by side, so that it was an inline-four. There were 16 hold-down studs, eight in each block, plus three little ones.

If these things were steel and there were 16 of them, they would tear your pockets out if you had to carry that stuff. And they would slow down your motorcycle. So Yamaha made them out of aluminum.

This part has a mighty stress concentration right under the nut. If I take a magnifying glass and look at the transition from the straight shank to the head of the bolt, on some of them it’s a smooth radius. On others, there are little lines.

Any place where there is a line or a scratch is a place where a crack can form—and it did. Yamaha immediately made sets of steel sleeve nuts, which were distributed to all the machine owners, and, later on, improved aluminum replacements.

It’s good for this part to be aluminum because not only does it save weight but when the cylinder block expands as the engine warms up the stud expands with the block. Steel, which expands about one-third as fast as aluminum, would overstress the studs.

This is another case of stress concentration leading to failure. There are recognized ways of dealing with stress concentration, which we’ll talk about another time.

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