The principles of In-line pressure balanced bellows expansion joints is more or less similar to the Pressure balanced bellows except that the axial pressure thrust is reacted by the pressure acting on a cross sectional area equal to the area of the working of primary bellows.

A constant volume arrangement created by two balancing bellows whose difference in cross sectional area is twice that of the in-line bellows thus eliminating the pressure-thrust without change in direction.

As this expansion joint is entirely axial and there being no directional changes in the pipe, the cross sectional area needed to balance the pressure is placed around the outside of the unit. Moreover as the pressure forces are generated by the pressure acting on the annular surface between the primary and the outer balancing bellows, the arrangement of the tie rods transfers and balances the pressure thrust created in the pipe on each end.