What does "throw sand into the works" mean here?
In this passage:40 percent : Decline in the price of a barrel of oil from its 2014 peak this summer. Oil prices are now at their lowest levels in five and a half years due to slowing global demand and expanded alternative energy supplies. The Financial Times has called the price decline “by far the biggest shock for the global economy this year,” noting that its effects could “throw sand into the works of the usual economic relationships.”
What does "throw sand into the works" mean in the last sentence?

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The "works" refers to the processes and/or items that makes something run. For example gears in an engine. When you throw sand into the works (gears) it causes things to break down and not run as expected. Another common expression that means the same thing is "throw a wrench in the works"

So that sentence is basically saying that the effects of the oil price decline could cause the usual economic relationships to not work as expected.

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This phrase could mean that there is a big problem with the drop in oil prices. This metaphor comes from the Industrial Revolution, and if someone throws sand into a machine often ruins it.