Seat belt retractor with curved tooth auxilliary gear

Retractor for front seat belt having inertia responsive pivoted latch bar for engaging main ratchet sprocket gears to arrest belt extraction, and an auxilliary sprocket gear with radially elongated teeth engaged by pivoted lever finger actuated by inertia pendulum for raising lever and latch bar into main gear arresting engagement characterized by auxilliary sprocket gear teeth each having a concave surface for finger point engagement adapted to increase the lever actuating moment arm at the extremity of tooth contact relative to that of prior straight tooth surfaces.

BACKGROUND OF THE INVENTION 
Automotive safety belt retractors having an inertia actuated latch against 
belt extraction conventionally employ a pivoted latch bar for engaging the 
teeth of a pair of one-way ratchet sprocket gears actuated to engaging 
position by an inertia element responsive to vehicle deceleration. The 
latch bar is in a normally disengaged position and must be raised through 
a substantial arc to reach a fully engaged ratchet tooth position. 
In order to minimize the size and stroke of a suspended pendulum or other 
type of inertia element for actuating the latch bar, an auxilliary 
sprocket gear is normally employed in fixed concentric relation to one of 
the dual ratchet gears with an equal number of elongated teeth adapted for 
preliminary engagement by initial pivotal movement of a lever produced by 
the inertia element when initially displaced, such lever in turn raising 
the latch bar to its ratchet gear tooth engaging position. The extended 
tooth form is adapted to produce additional arcuate lever actuation 
sufficient to pivot the latch bar into full latch engagement. 
The relative geometry of the pivoted lever having a narrow finger for 
engaging the elongated auxilliary tooth is such as to produce a radial 
sliding of the lever finger contact point radially inward along the 
elongated tooth which multiplies the angular displacement of the lever, 
and corresponding displacement of the latch bar, relative to the angular 
travel of the elongated tooth as required to produce full engagement of 
the latch bar with minimal lost motion following pendulum displacement. 
The conventional prior art form of the elongated auxilliary gear tooth has 
been a narrow straight tooth form with the engagement face in radial 
alignment with the gear axis. The narrow finger of the lever arm has an 
elongated tapered finger form terminating in a sharp point adapted to 
initially engage the actuating side of a sharp point of the elongated 
tooth. 
In order to maximize the ratio of arcuate travel of the lever arm and latch 
bar relative to the sprocket gears, and thereby minimize lost motion to 
produce belt locking against extraction, it is desirable for the angle 
between the line passing through the lever pivot and its finger point of 
initial contact with the elongated tooth extremity and a line normal to 
the initial engaging contact surface of such tooth be as small as possible 
consistent with free-sliding contact of the lever engagement point with 
the elongated tooth surface following the initial engagement. The 
effective "angle of friction" for the plastic materials involved defines a 
limit for reducing such angle since any possible sticking upon initial 
contact may cause fracturing of the tooth or lever finger, or both. 
Moreover, even if a sliding angle is provided relative to the effective 
"angle of friction" of the plastic parts, an impact stress may be involved 
at the contact points proportional not only to suddenness of belt 
extraction and retraction spool acceleration, but also proportional to the 
impact force vector which is ineffective to produce lever pivoting. 
Repeated impact of the tips at an angle close to the "angle of friction" 
may dull the points and enhance the possibility of tip engagement at a 
jamming angle. In practice a 14.degree. design-angle has been employed on 
the assumption that it provides an adequate margin of safety for assuring 
sliding upon initial contact as well as an acceptable rate of latch bar 
engagement; however, due to impact of contact points, tolerance 
extremities, irregularities including wear at the contact points, and 
sliding tooth surface, as well as other possible unknown factors, jamming 
of the contact points, and fracturing of the tooth or finger element, or 
both have been found to occur occassionally resulting in malfunction and 
service requirement. 
SUMMARY OF THE PRESENT INVENTION 
Applicant has found a way to produce a substantially greater safety angle 
of contact relationship, without any loss of ratio in the angle of lever 
to tooth displacement, by the simple expedient of modifying the auxilliary 
gear tooth form from a straight slide surface to a curved slide surface 
with a center of curvature displaced from the lever pivot to increase the 
lever actuating moment arm of contact force normal to tooth surface at the 
point extremity of the elongated tooth. Such normal line of force is 
displaced almost 20.degree. from a line normal to the straight radial line 
of the prior art tooth form. No change in the position or construction of 
the lever, or any other change in the component parts or their geometry 
has been necessary to achieve this improvement in contact angle which has 
substantially eliminated the problem of occassional misengagement, jamming 
and tooth or finger fracturing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
With reference to FIG. 1, retractor 10 is provided with a pair of 
conventional main ratchet-form sprocket gears 11, latch bar 12 pivoted at 
13 for engaging the sprocket gears 11 shown in disengaged portion, 
actuating lever 14 pivoted at 15 having an upper latch bar engaging 
surface and lower projection 16 actuated by inertia element 19, to raise 
lever 14 and its finger projection 18 into engagement with a curved tooth 
17 of auxilliary sprocket gear 20 of the present invention in reponse to 
predetermined vehicle deceleration sufficient to tip inertia element 19 
from its neutral position shown. 
With reference to FIG. 2, the functional elements of FIG. 1 are shown in 
operating relationship with a substitute prior art auxilliary sprocket 
gear 20a having each tooth 17a formed with straight radial engaging 
surface 21a, and illustrated with lever 14 finger 18 elevated by initially 
tipped condition of inertia element 19, not shown in FIG. 2, to an initial 
auxilliary sprocket tooth engaging position having clearance 22 
illustrated in FIG. 1 taken up, but with finger point 23 in bypassing 
relation to tooth point 24a as would occur when the timing of inertia 
element 19 tipping was such as to just miss lever tip engagement with a 
passing auxilliary sprocket tooth. 
With reference to FIG. 3, the elements of FIG. 2 are illustrated with point 
23 shown in initial engaging relationship with tooth surface 21a. 
With reference to FIG. 4, the elements of FIG. 3 are shown after clockwise 
rotation of auxilliary sprocket gear 20a has produced full actuation of 
finger 18 to the engaging position of latch bar 12 with ratchet teeth 25 
of main sprocket gears 11. 
With reference to FIG. 5, auxilliary sprocket gear 20 of the present 
invention is illustrated in geometric relationship to actuating lever 14 
when the respective tips of curved tooth 17 and lever finger 18 are in 
point-to-point contact. Radial line 26 passing through point extremity of 
curve tooth 17 corresponds to radial line 26a in FIG. 2 which is 
coincident with the contact surface 21a of the prior art elongated gear 
teeth. Accordingly, the beginning and end positions of finger point 23, 
shown respectively in FIGS. 5 and 7 are the same as in the prior art shown 
in FIGS. 2 and 4 and it is only the curved path of such point along the 
curved tooth surface 21 that differs from the prior art resulting in an 
equal average ratio of lever 14 to gear 20/20a angular travel in each 
case. 
The significant difference of the curved tooth form contact surface 21 
shown in FIGS. 5-7 from the straight surface 21a of the prior art shown in 
FIGS. 2-4 lies in the relative relationship of a geometric line normal to 
the curved contact surface 21 at its extemity point as shown at 27 to the 
corresponding normal line 28 to the straight tooth contact surface 21a and 
the respective difference of such normal lines 27 and 28 relative to the 
line 29 passing from the contact tip 23 of finger 18 through pivot 15. 
Thus, the almost 20.degree. increase of arc 30 over the approach angle of 
arc 31 between the line 29 and the line normal to the contact surface 
21/21a at the extremity contact tip assures a sliding rather than possible 
sticking and jamming initial contact of point 23 with the actuating 
contact surface 21 of gear tooth 17. 
Even though the initial ratio of amplified pivotal actuation of lever 14 is 
somewhat less with a curved tooth form of FIG. 5, the average ratio will 
end up the same by the time the contact point 23 reaches the position of 
FIG. 7 in a continuously increasing departure of the line 29 from the 
angle of friction relative to contact surface 21 as the contact point 
progresses from FIG. 5 to FIG. 7 position assuring a non-sticking 
actuation throughout the actuation of lever 14. 
With reference to FIG. 8, details of geometric dimensions expressed in 
millimeters serve as a specific example of a typical auxilliary gear 
constructed with curved teeth in accordance with the present invention 
which has served to effectively solve the problem of occassional tooth 
and/or finger breakage experienced with the prior art straight contact 
surface tooth form.