The present invention relates to a web sensor for a vehicle
safety restraint retractor.
A vehicle safety restraint traditionally comprises a seat
belt in a so-called three point configuration. The seat belt comprises a strong
woven webbing material attached to the vehicle so that it passes horizontally over
the lap of a vehicle occupant, and diagonally across the torso. Usually one end
of the webbing is fixed to a structural part of the floor of the vehicle, typically
between a seat and a side of the vehicle. The belt then passes across the lap of
an occupant and is releasably fastened to a buckle mounted to a structural part
on the other side of the seat. Subsequently the belt passes diagonally across the
torso of the occupant and through the loop of a ring attached to a side pillar at
approximately shoulder height for the occupant. Then the belt passes vertically
downwards and the other end of the belt is attached to a retractor which is bolted
to the floor of the vehicle.
The belt webbing is wound on a cylindrical spool of the
retractor which is spring biased to keep the webbing wound onto the retractor and
thus to keep the belt secure about the occupant's body. Under normal conditions,
the spool is free to rotate to pay out webbing to provide comfort to the vehicle
occupant by allowing movement within certain limits, for example to adjust a car
radio or access a storage compartment.
However, when a crash occurs the spool must be locked against
further payout of webbing so as to prevent the occupant moving forwards and sustaining
injury such as by collision with the dashboard or front windscreen. Traditionally
this is achieved by providing sensors to detect accelerations or decelerations over
predetermined limits. A so-called vehicle sensor detects rapid acceleration or deceleration
of the vehicle and a so-called web sensor detects a sudden pull on the belt webbing.
Both are indicative of crash conditions.
A web sensor comprises an eccentrically pivoted inertia
disc mounted to the spool, to rotate with the spool under normal conditions, but
biased by a calibration spring so that when the spool moves suddenly, indicating
a crash, the inertia disc lags behind the spool and effectively pivots relative
to the spool. The inertia disc incorporates an integral locking pawl which then
engages with teeth on a ratchet wheel (known as a lockcup) attached to the locking
system. This locks the spool against further payout of belt webbing and thus secures
the vehicle occupant.
US 5,682,224 discloses a web sensor according to the preamble
of claim 1 with a locking pawl shaped to correspond to the shape of the ratchet
wheel teeth when in full locking engagement.
However, the integral pawl is at a relatively long distance
from the pivot point of the disc and is thus at a particularly acute angle to the
load vector. It has been found that this arrangement increases the possibility of
the retractor jamming due to the pawl and the lockup teeth hitting each other tip
to tip. Jamming is decreased if a separate locking pawl is used but this increases
the cost of the retractor.
The present invention aims to eliminate the tendency to
jamming in a cost effective manner.
According to the present invention there is provided an
improved arrangement and shape for the locking pawl and of the teeth of the lockcup.
According to the present invention there is provided a
web sensor for a vehicle safety restraint retractor comprising an inertia member,
a locking pawl and a lockcup, the inertia member being eccentrically pivotally mounted
to the lockcup, the locking pawl being integrally formed with the inertia member,
the lockcup comprising a set of radially inwardly pointing locking teeth for engaging
with the locking pawl, characterised in that the shape and profile of the locking
pawl complements the shape of the locking teeth, so that the angular position of
the lockcup when one of the teeth is in a tip-to-tip locking engagement with the
locking pawl is the same as the angular position of the lockcup when said one of
the teeth is in a full locking engagement with the locking pawl.
Preferably the angle subtended at the centre of rotation
of the lockcup by the position of the locking pawl in full locking engagement with
one of the teeth of the lockcup, and the position of the locking pawl in tip-to-tip
engagement with one of the teeth of the lockcup is around 360 degrees divided by
the total number of teeth of the lockcup. Typically for a retractor with 24 teeth
on the lockcup this results in an angle of around 15°, for example between
14° and 16°.
The lockcup teeth each have a leading edge, a trailing
edge, a tip between the leading and the trailing edge and in one embodiment an intermediate
section between the trailing edge and the leading edge of the subsequent tooth.
The locking pawl has a locking tip between an engaging edge and a trailing edge.
According to one embodiment the locus of the trailing edge of the locking pawl matches
(in complement) the locus of the trailing edges of the lockcup teeth.
In another embodiment the lockcup teeth are shaped like
shark's teeth in that they are longer, and the leading edge comprises two sections
of different gradients, a first section adjacent the tip and a second section of
the same (complementary) profile to the leading edge of the locking pawl.
Strength is improved if the profile of the locking pawl
tip in such that the angle between the leading and trailing edges of the locking
pawl is higher than 60 degrees and preferably higher than 70 degrees or even higher.
In effect, in this configuration, the tooth gap is filled in.
For a better understanding of the present invention and
to show how the same may be carried into effect, reference will now be made to the
accompanying drawings, in which:
- Figure 1A is a cross sectional view of a web sensor according to one embodiment
of the prior art: showing the locking pawl in a full locking position;
- Figure 1B is a cross sectional view of the web sensor of figure 1A showing the
locking pawl in a different position;
- Figure 2 is a cross sectional view through a web sensor according to another
embodiment of the prior art;
- Figure 3 is a cross sectional view through a web sensor according to one embodiment
of the present invention;
- Figure 4 is a cross sectional view through a web sensor according to a second
embodiment of the present invention.
- Figure 5 is a cross sectional view through a web sensor according to a third
embodiment of the present invention;
- Figure 6 is a cross sectional view through a web sensor according to a fourth
embodiment of the present invention;
- Figure 7 is a cross sectional view through a web sensor which does not form
part of the invention.
In figures 1A and 1B a traditional design of web sensor
is shown comprising an inertia disc 1 pivotally mounted at pivot point 2 to a retractor
spool (not shown). The pivot point 2 is offset from the axis of the spool which
coincides with the axis 3 of a ratchet wheel 4. The ratchet wheel 4 has teeth 5,
of saw-tooth cross section, having a long side 7 and a short side 8. They are arranged
in a regular repeating pattern around the circumference, pointing inwardly of the
ratchet wheel 4. The ratchet wheel teeth 5 are arranged to engage with a tooth 6
on the inertia disc 1 to lock the spool (not shown) against rotation when a crash
condition is detected. A crash condition is detected when a sudden change in the
speed of rotation of the spool occurs, usually a rapid acceleration, for example
as the vehicle occupant exerts a sudden higher force on the seat belt when the vehicle
brakes. The inertia disc 1 cannot change speed as rapidly as the spool and hence
tends to lag the spool (and the ratchet wheel 4). There is then a phase difference
between the inertia disc 1 and the ratchet wheel 4 and the inertia disc pivots the
disc tooth 6 engages with the teeth 5 on the ratchet wheel 4, thus locking the spool
and securing the vehicle occupant. This is shown in figure 1A where the pawl 6 is
fully engaged between the teeth 5 of the ratchet wheel 4. When the force on the
occupant, and thus on the belt, abates the teeth 5 and pawl 6 disengage and the
inertia disc 1 returns to its normal position relative to the ratchet wheel 4, under
the influence of a biasing or calibration spring (not shown).
A problem occurs in the situation shown in figure 1B when
the disc tooth 6 engages the
A problem occurs in the situation shown in figure 1B when
the disc tooth 6 engages the tip of a tooth 5 on the ratchet wheel 4 because the
angular position of the spool relative to the locking systems is incorrect; the
parts then tend to jam and will not easily release when the additional forces abate.
It can also happen that this condition causes shearing of the tip of the teeth and
thus degrades the performance and safety of the retractor. The angular position
of the spool in figure 1B is incorrect by an angle of 10.71 degrees when tip-to-tip
locking occurs. 10.71 degrees corresponds to two-thirds of a tooth spacing. This
situation occurs frequently in use of a traditional retractor and the pawl does
not easily slide off the tips of the teeth 5 and the retractor jams.
In figure 2 a known attempt to improve the retractor is
shown. In this embodiment the shape of the teeth 5 is changed compared to the shape
in figure 1. The teeth 5 are smaller than the teeth in figure 1 and the long sides
7 have two parts, a first part 7A of a relatively steeper gradient and a second
part 7B of a relatively shallow gradient.
In this case the angular position of the spool is incorrect
by only 6.88 degrees between tip-to-tip engagement of the disc tooth 6 and full
engagement of the disc pawl 6 but jamming still occurs.
Figure 3 illustrates a new design of web sensor in which
the teeth 5 are shaped to follow the locus of the inertia disc tooth 6. In this
embodiment the angular position of the spool is the same no matter where the inertia
disc 6 locks, ie whether it locks fully at 6 or at the tip-to-tip position 6'.
In figure 4 a variant of the invention is shown in which
the disc tooth gaps are "filled in" so that the disc tooth takes the form of a square
hammer head. This does not degrade the performance but does improve the strength.
Again the position of the full engaged pawl is shown at 6 and the tip-to-tip position
Figure 5 shows yet another variant of the invention in
which the tip of the disc tooth 6 is shaped to dig in to the material of the ratchet
wheel 4 to resist skipping. The design of the teeth 5 comprises a corner 9 formed
between an arc 10 centered on the pivot point 2 of the inertia disc 1, and a flatter
portion 11, ie the tip of one tooth 5 and the through 9 of the adjacent one are
equidistant from pivot point 2.
In the embodiment of figure 6 the corner 9 is formed at
a less shallow angle and the face 11 is made the same shape as one face 12 of the
tooth 6 of the disc 1. Thus the teeth 5 of the ratchet wheel 4 take a slightly eccentric
zig-zag cross section and thus have a stronger compressive strength. The embodiments
of figures 3 to 6 demonstrate the principal of maintaining the angular position
of the spool irrespective of the contact position of the inertia disc tooth tip.
In practice this type of tooth shape has a limited torque capacity and can also
result in skipping of the pawl 6 of the inertia disc 1.
The web sensor of figure 7 has longer ratchet teeth 5 in
a shark's tooth shape and the disc tooth 6 is also modified. The angle between the
position of the tip of the disc pawl 6 correctly engaging a ratchet tooth and the
position of the disc pawl 6 engaging the tip of an adjacent ratchet tooth is 15
degrees, which means that there is no phase change. In this way an elimination of
jamming with no reduction in torque capacity, is achieved. The chosen value of 15
degrees may be approximated by 1 to 2 degrees either side depending upon the particular
configuration of the retractor components to allow for some flexure of components.
The arc subtended by a line between the full engagement position 6 of the inertia
disc pawl and the tip-to-tip position 6 is centered on the pivot point 2 of the