Side airbag device

1. A side airbag device adapted to be mounted on a back portion of a seat of a vehicle, the airbag device comprising:

an airbag that is housed in a housing in a folded-up configuration and is configured to be deployed forward when fed with an inflation gas by an inflator, the folded-up airbag having an elongated outer contour extending generally in an up and down direction as mounted on the seat;

a limitation member that is arranged around the folded-up airbag for constraining the airbag from protruding forward at airbag deployment, the limitation member being formed of a flexible sheet member and having a band shape, the limitation member comprising:

a pair of joint regions that are located at opposite ends of the limitation member and joined with the folded-up airbag; and

a loose region that is disposed between the joint regions and arranged around the folded-up airbag in such a manner as to be remote from the folded-up airbag, the loose region being disposed on a side of the folded-up airbag towards which the airbag protrudes, wherein a length of the loose region between the joint regions is longer than a length of an outer circumference of the folded-up airbag and shorter than a film length of the airbag as deployed, and wherein the loose region includes, in a region thereof in a deployment direction of the airbag, a tearable region that is tearable in such a manner as to split up the loose region between the joint regions at the airbag deployment,

wherein:

the tearable region includes a starting point of tearing at one end thereof in the up and down direction;

the inflator is generally cylindrical in outer contour and is housed in a rear end region of the folded-up airbag;

the inflator is provided with gas discharge ports at a lower end thereof;

a lower edge of the loose region is located in a vicinity of a front region of the gas discharge ports;

the loose region includes a cut-out region that tapers towards the tearable region, the cut-out region constitutes the starting point of tearing of the tearable region;

the airbag includes a pelvis protecting portion for protecting a pelvis of an occupant seated in the seat at a lower end region of the airbag at deployment; and

the pelvis protecting portion protrudes out of the limitation member downwardly in the folded-up airbag.

2. The side airbag device of claim 1 , wherein the loose region is wrapped around the folded-up airbag in such a state that is doubled on a mountain-fold crease.

3. The side airbag device of claim 1 , wherein the length of the loose region of the limitation member between the joint regions is configured such that a top of the loose region is disposed at a position corresponding to a vicinity of a front end of the back portion of the seat in a horizontal cross-sectional view depicting a situation in which the loose region as mounted around the folded-up airbag and mounted on the seat is extended forward.

4. The side airbag device of claim 1 , wherein the length of the loose region of the limitation member between the joint regions is configured such that the tearable region as the airbag device is mounted on the seat is located inside the back portion of the seat before being torn in an initial stage of the airbag deployment.

5. The side airbag device of claim 1 , wherein the airbag further includes:

a thorax protecting portion that is located above the pelvis protecting portion at the airbag deployment for protecting a thorax of the occupant; and

a regulating member that is disposed inside the thorax protecting portion and helps reduce a width in a front and rear direction of the thorax protecting portion compared to that of the pelvis protecting portion at the airbag deployment.

6. The side airbag device of claim 1 , wherein the airbag further includes:

a thorax protecting portion that is located above the pelvis protecting portion at the airbag deployment for protecting a thorax of the occupant;

a front-rear partition wall that is located inside the thorax protecting portion and partitions the thorax protecting portion into a front region and a rear region; and

a valve mechanism that is formed on the front-rear partition wall, wherein the valve mechanism opens at the airbag deployment and closes when the rear region cushions a shoulder of the occupant.

7. The side airbag device of claim 1 , wherein:

the airbag further includes a wrapping member that is wrapped around the folded-up airbag;

the limitation member is mounted around the folded-up airbag which is wrapped by the wrapping member; and

the wrapping member constrains the airbag from protruding forward at the airbag deployment.