DY47 series of mechanical seals are designed with wave springs which occupy smaller space than multipe springs,but special tooling must be made to reach the best manufacturing results, besides the tempering required on this design limits materials to the high grade stainless steel and Hastelloy groups.Thirdly, a greater change in loading for a given deflection must be tolerated. A great deal of force loss or force gain with comparatively small axial movement must be expected.