38
7.2 Design
The proposed design is to employ LN load limiter in the retractor system instead of traditional torsion bar. The LN load limiter
allows for webbing payout without any plastic deformation of the whole device once the retractor is locked during vehicle crashes.
Note this design only replaces the torsion and all other parts in the traditional retractor system remain unaltered. Figure 7.2 shows
the schematic of the LN limiter consisting of an LN-filled chamber and a threaded piston. The LN-filled chamber, with one end of
the wound webbing fixed to it, engages with the retractor housing through a gear mechanism, ensuring the LN-filled chamber to
freely rotate with the fastened webbing. The threaded end of the piston is partially screwed into the LN-filled chamber. The other
end of the piston should be engaged to the inertia locking mechanism in a traditional retractor system. In normal operation, this end
should be completely free to rotate until crashes occur. For the purpose of demonstration, in the current study, this end of the piston
is locked up by the retractor housing to mimic the condition vehicle crashes.
As the retractor system locks up during a vehicle crash, the piston cannot rotate as the webbing is pulling out. Therefore,
the LN-filled chamber rotates towards the piston direction, leading the piston to screw into the LN-filled chamber.
Consequently, the LN inside the chamber is subjected to hydrostatic pressure. When the hydrostatic pressure reaches the
working pressure, i.e., the infiltration pressure, of the LN, the tensile force of the webbing is maintained at a constant value.
Continuing compression of the LN provides additional webbing payout for the moving occupant. As the pores in the LN are
used up, the LN becomes incompressible. The rotation of the chamber stops and no further webbing payout is allowed.
The working load limit of the webbing, F w , is thus determined by the working pressure of the LN based on the following
analysis. According to the force relation in Fig. 7.3, we have
F
D
D
F
F
D
D
F
w
i
o
n
f
i
o
n
=
×
+ ×
(
) =
+ ×
(
)
sin
c os
sin
cos
q
q
q m
q
F
F
F
F
P
D
LN
n
f
n
i n
i
= ×
+ ×
=
+ ×
(
)= ×
cos
sin
sin
cos
q
q
q m
q
p
2
4
Therefore,
F
D
D
F
w
i
o
LN
=
×
+
- ×
×
×
=
×
+
- ×
×
m
q
q
q m
q
m
q
q
q m
q
cos
sin
cos
sin
cos
sin
cos
sin
p p
a
D
D
P
P
i
o
in
in
3
4
× = ×
The webbing payout is determined by the nanopore volume of the LN,
L
D V
D
V
=
×
×
= ×
4
3
o
n
i
n
p
q
b
sin
Fig. 7.1 (a) Seat belt in a
vehicle, (b) cumulative
estimated number of lives
saved by seat belt use [1], (c)
commercial seat belt retractor
and (d) torsion bar load
limiter in commercial seat
belt retractor
M. Li et al.
7.2 Design
The proposed design is to employ LN load limiter in the retractor system instead of traditional torsion bar. The LN load limiter
allows for webbing payout without any plastic deformation of the whole device once the retractor is locked during vehicle crashes.
Note this design only replaces the torsion and all other parts in the traditional retractor system remain unaltered. Figure 7.2 shows
the schematic of the LN limiter consisting of an LN-filled chamber and a threaded piston. The LN-filled chamber, with one end of
the wound webbing fixed to it, engages with the retractor housing through a gear mechanism, ensuring the LN-filled chamber to
freely rotate with the fastened webbing. The threaded end of the piston is partially screwed into the LN-filled chamber. The other
end of the piston should be engaged to the inertia locking mechanism in a traditional retractor system. In normal operation, this end
should be completely free to rotate until crashes occur. For the purpose of demonstration, in the current study, this end of the piston
is locked up by the retractor housing to mimic the condition vehicle crashes.
As the retractor system locks up during a vehicle crash, the piston cannot rotate as the webbing is pulling out. Therefore,
the LN-filled chamber rotates towards the piston direction, leading the piston to screw into the LN-filled chamber.
Consequently, the LN inside the chamber is subjected to hydrostatic pressure. When the hydrostatic pressure reaches the
working pressure, i.e., the infiltration pressure, of the LN, the tensile force of the webbing is maintained at a constant value.
Continuing compression of the LN provides additional webbing payout for the moving occupant. As the pores in the LN are
used up, the LN becomes incompressible. The rotation of the chamber stops and no further webbing payout is allowed.
The working load limit of the webbing, F w , is thus determined by the working pressure of the LN based on the following
analysis. According to the force relation in Fig. 7.3, we have
F
D
D
F
F
D
D
F
w
i
o
n
f
i
o
n
=
×
+ ×
(
) =
+ ×
(
)
sin
c os
sin
cos
q
q
q m
q
F
F
F
F
P
D
LN
n
f
n
i n
i
= ×
+ ×
=
+ ×
(
)= ×
cos
sin
sin
cos
q
q
q m
q
p
2
4
Therefore,
F
D
D
F
w
i
o
LN
=
×
+
- ×
×
×
=
×
+
- ×
×
m
q
q
q m
q
m
q
q
q m
q
cos
sin
cos
sin
cos
sin
cos
sin
p p
a
D
D
P
P
i
o
in
in
3
4
× = ×
The webbing payout is determined by the nanopore volume of the LN,
L
D V
D
V
=
×
×
= ×
4
3
o
n
i
n
p
q
b
sin
Fig. 7.1 (a) Seat belt in a
vehicle, (b) cumulative
estimated number of lives
saved by seat belt use [1], (c)
commercial seat belt retractor
and (d) torsion bar load
limiter in commercial seat
belt retractor
M. Li et al.