Rubber expansion bellows, especially for double sphere expansion bellows have two types of reinforced ring: inside and outside rings, their function is totally different:

Inside ring(Vacuum ring)

Inside ring’s function is improving the vacuum resistance. Our rubber thread type expansion bellow can resist 800mmHg vacuum without steel inside ring. If the working pressure is less than 800mmHg, there will be no inside ring; If working pressure is more than 800mmHg, inside ring(Vacuum ring) is necessary, especially suit for inlet of the pump. Popular speaking, the vacuum ring can prevent the sphere from huddled, like the vacuum food we usually seen.

Outside ring:

Outside ring’s function is not to improve the vacuum, it’s for positive pressure, when the positive pressure exploded suddenly, the outside ring can protect the double spheres from exploded to one bellow, even a straight pipe.