6DOF for moving piston
 

hello everybody;
i have a problem with 6DOF.

Imagine a cylinder closed by a piston mass (eg 1 kg) and filled with a gas (eg air) . if I let the piston, it will move down.

and it stops if:

the piston weight (p = m.g ) = pressing force ( pressure force = pressure. area)

here is the UDF that I use :

#include "udf.h"
#include "math.h"

DEFINE_SDOF_PROPERTIES(mass_6dof, prop, dt, time, dtime)
{
prop[SDOF_MASS] = 1;
prop[SDOF_ZERO_TRANS_X] = TRUE;
prop[SDOF_ZERO_TRANS_Y] = FALSE;
prop[SDOF_ZERO_TRANS_Z] = TRUE;
prop[SDOF_ZERO_ROT_X] = TRUE;
prop[SDOF_ZERO_ROT_Y] = TRUE;
prop[SDOF_ZERO_ROT_Z] = TRUE;
prop[SDOF_LOAD_F_Y] = -9.81*1;

}

the problem is that the piston does not stop :confused::confused::confused:

the real case :
http://s15.postimg.org/4a6r3ife3/piston.gif

my case:
http://s13.postimg.org/yaulhc1yv/ppppp.gif


thank you in advance