If the thickness of the wall of a shell is greater than 1/10 of its diameter, it is known as thick shell.

The thick shells are, generally, used to withstand high pressures. Sometimes, even, compound thick shells are used to withstand very high pressures or to contain chemicals under high pressure.
The problem of thick cylinders is, somewhat, complex and is solved by Lame's theory.

Acme thread is trapezoidal thread.