The cow’s teats are attached to the teat cups. Each teat cup contains a rubber or silicone liner inside a plastic or stainless-steel shell.
The liners are the only parts of the machine that touch the cow. They form a seal between the teat and the short milk tube, used to transport the milk. All of the liners are worked by a pulsator valve, which in turn is connected to a vacuum pump. The area between the liner and the shell is the pulsation chamber.
The pulsator pulls a vacuum on the pulsation chamber, causing the liner to open up.
This definition expresses the importance of shell as a chamber for milking action. Stainless steel part has another duty: To make enough heaviness to milk claw for optimum milking.
So the thickness of the shell must be standard and the quality must be high as it in contact with Alkaline and Acid during washing the milking machine. A low quality shell changes in color (black) after some usage. On the other hand, oxidized shell can damage the liner wall easily and tear it up.