Bellofram Rolling Diaphragms are, in effect, pressure vessels having a variable volume and flexible moving sidewalls. As in any other pressure vessel, their strength should be considered with respect to safety factors. Generally, they are designed with a large safety factor. This means the maximum safe working pressure will be a fraction of the pressure that could cause failure of the sidewall.
The accompanying illustration shows the pressure reaction of the rolling diaphragm. Here it can be seen that almost the entire pressure load is supported by the piston head. Only a small amount of the liquid or gas pressure is supported by the narrow convolution of the rolling diaphragm.
In the second illustration, particularly, to that portion of the convolution in immediate contact with the cylinder and piston walls, it will be noted that the lines of unit pressure (acting in horizontal plane because they must be normal to the surface) force the diaphragm against the piston and cylinder sidewalls. The lines of pressure acting on that portion of the diaphragm not in immediate contact with cylinder and piston walls (semi-circular segment of the convolution) are shown. Each line of unit pressure acts in a direction normal to the semi-circular segment; hence all of the pressure lines can be replaced by their horizontal and vertical components. The sum of the vertical components is the total force acting on the semi-circular segment.