3.5. Boundary Conditions and Loading

Two load steps are defined (parts (b) and (c) in Figure 3.2: Sealing System and Finite Element Model).

Loading is applied as follows:

Load Step 1

U_x rigid shaft = 0.2794 mm
U_x rigid groove = - 0.05715 mm
The housing compression is analyzed in the first load step. The rigid shaft surface (left side) moves 0.2794 mm in the x direction and the rigid groove surfaces (right side) move 0.05715 in the negative x direction, simulating the housing assembly.

Load Step 2

PRES = 4.1 MPa
In the second load step, a fluid penetration pressure of 4.1 MPa is applied to the contact pairs (via the SFE command with the load key value LKEY set to 1), as shown:
esel,s,real,,6                      ! select rigid-flexible contact pair
esel,r,ename,,172                 ! reselect contact elements only
esel,a,real,,8                    ! select flexible-flexible contact pair
sfe,all,1,pres,,4.1               ! apply fluid pressure 
allsel
The following input prevents fluid penetration loads from being applied multiple times on overlapping contact elements from different pairs:
esel,s,real,,8                      ! select flexible-flexible contact pair
nsle
esln,s,1
esel,r,real,,6                      ! reselect rigid-flexible contact pair
sfedele,all,all,all                 ! remove overlapping fluid pressure loads
allsel

Figure 3.5: Sealing System with Applied Fluid Pressure to Contact Pairs

The fluid-penetration starting points are also specified in order to model the system fluid pressure exposed from the bottom part (the opening between the shaft and the lower part of groove). For the rigid-to-flexible contact pair, no default starting points exist because the contact surfaces are in a closed loop, as shown in (a). To activate the fluid penetration pressure, two contact elements are chosen as starting points initially exposed to the fluid (via the SFE command, with the load key value LKEY set to 2 and the first starting point status value VAL1 set to 1), as follows:

SFE,3121,2,PRES,,1				! starting point for o-ring  
SFE,3308,2,PRES,,1				! starting point for cap

One starting point is for the contact surface of the o-ring, and the other is for the contact surface of the cap, respectively.

For the flexible-to-flexible contact pair, the ending points of the contact and target surfaces are the default starting points. To force the fluid to penetrate from the bottom only, the two default starting points on the top part of the seals are suppressed (via the SFE command, with the load key value LKEY set to 2 and the first starting point status value VAL1 set to -1), as follows:

SFE,2529,2,PRES,,-1
SFE,2625,2,PRES,,-1