Issue 32

N. Golinelli et alii, Frattura ed Integrità Strutturale, 32 (2015) 13-23; DOI: 10.3221/IGF-ESIS.32.02 17         f c un f F F F F D F F F (7) Figure 5 : The total force of MRDs can be obtained by summing: friction (dotted blue line), viscous (dashed blue line), magnetic (dash- dotted blue line) and pressure (solid red line) driven forces. Eq. (4)-(7) were manipulated taking into account the geometrical constraints and the design parameter of the remaining components were determined. In particular, considering that a fluid gap h=1 mm was chosen, the annular area A A is 819.24 mm 2 and the viscous forces can be calculated as follow:                              7 3 3 12 122.26 1 100 12 3 10 81954 90 819.24 1 1 191 2 2 81954 122.26 1 D A whV QLA F N Q wh (8) In which the velocity V D =100 mm/s and the viscosity η=0.3 Pa·s. Assuming that the friction forces F f = 250 N and the total force F tot = 2000 N, the required controllable force F τ is:              2000 2000 191 250 1559  f F F F N (9) and the dynamic range turns out to be:             1559 191 250 4.53 191 250 f f F F F D F F (10) Once the controllable force was found, the yield stress of the fluid τ B = 20 kPa was set, considering the nominal working current value I = 1 A. The total active pole length is obtainable by manipulating Eq.(4):         tot P B A F h 1559 1 L 41.32mm c A 2.30 0.020 819.24 (11) where the coefficient c = 2.30 [12]. The activation areas are four, which implies a single axial length   tot P P L L / 4 10mm. The chosen yield stress implies, by means of the Eq. (1)-(2), a magnetic field density B mrf along the active pole of 0.35 T. After that, the piston head (Fig. 6a) along with the flange (Fig. 6b, c), the rod (Fig. 6d) and the bottom-rod (Fig. 6e) were manufactured. The piston head and the flange were made of AISI 430. Conversely, the rod and the bottom-rod were made of brass because they do not have to influence the magnetic flux during operations. The flange is coupled with the rod by a drilled screw in order to let the coil’s wire passing through it (Fig. 6f).