Stud unit and skid-proof tire having the same

The stud unit of the present invention is used for skid-proof tires of vehicles. On a snow- or ice-covered road, a force acts on a front end of a stud contacting the road when the vehicle is braked or accelerates and the tire treads consequently slip, so that the stud is kept projecting from a cylinder due to mutual engagement of the stud and the cylinder. On a dry road, the treads of the tire hold the road without slipping, so that no force acts on the stud to cause it to incline, and the stud can smoothly retract into the cylinder.

BACKGROUND OF THE INVENTION 
The present invention relates to a stud unit and a skid-proof tire having 
the stud units for preventing vehicles from slipping on snow- or 
ice-covered roads. 
Conventionally, snow tires having multiple studs on the outer 
circumferential face are used as skid-proof tires. 
However, studs of the snow tires grind surfaces of roads when vehicles 
having the snow tires run on a road whose surface is dry and exposed. 
Powdered dust, which is worn from roads by grinding with studs of the snow 
tires, is blown up in the atmosphere and causes air pollution. The 
powdered dust is harmful for our health because it includes such harmful 
heavy metals as cadmium, lead, and the like. Further, traffic signs 
painted on roads are worn off by studs of the snow tires, which will be a 
factor in traffic accidents, and a heavy expenditure for repairing the 
road signs will be required. 
To avoid the above noted disadvantages, recently, studless-tires are being 
used. The studless-tire is a type of snow tire. It is made of specially 
composed rubber which does not become hard even at low temperature and has 
multiple narrow grooves on the outer circumferential face thereof so as to 
increase contact area with the road and to increase friction therebetween. 
Note that the friction of the studless-tire with the road is not less than 
that of conventional snow tires having studs when the temperature is quite 
low; the friction of the studless-tire with the road decreases when the 
temperature comes close to 0.degree. C. The frictional function is 
undermined by ice or snow on the road. Mixing a low-temperature 
plasticizer with tire rubber to soften treads of tires at low temperature 
is know but this property continues for only about one year, after which 
the tread becomes quite hard. 
As another means, melting snow and ice on roads by spraying calcium 
chloride and the like is attempted but roadside trees are blighted and 
secondary pollution, such as water pollution, occurs. 
To solve these disadvantages, a skid-proof tire having stud units was 
disclosed in the Japanese Patent Provisional Publication (Kokai) Gazette 
No. 59-186704. Each stud unit thereof shown in FIG. 23 has a cylinder 2 
and an elastic member 3 of such material as rubber therein. A stud 5 is 
passed through a hole 3a, which is bored in the center of the elastic 
member 3. The rear end of the stud 5 is engaged with an inside face of 
rear wall 2a of the cylinder 2. There is formed a large-neck section 6 at 
the front end of the stud 5, and the large-neck section 6 projects forward 
from the front opening of the cylinder 2. 
In operating a vehicle having tires with many studs 5 on a road whose 
surface is clear, when the brakes are applied each stud 5 is inclined and 
the large-neck section 6 engages the front end wall of the cylinder 2 to 
prevent the stud 5 from being pushed inwards, whereby the front end of 
each stud 5 projects from the tread face of the tires until the vehicle 
stops, so that the surface of the road is substantially ground. 
It is necessary that the diameter of the shaft of the stud 5 be large 
because a great force is applied to the stud 5 in the direction of its 
inclination when the vehicle accelerates or is braked. But, if the shaft 
of the stud 5 is larger, the elastic member 3 and the cylinder 2 must be 
larger, so that each stud unit must be heavy. 
When the stud 5 is inclined, the elastic member 3 is sometimes clipped as 
in a jaw between the underside of the large-neck section 6 and the front 
end wall of the cylinder 2. 
Because the durability and stability of the elastic member are low, the 
elastic member is apt to be permanently deformed by frequent inclination 
of the stud 5, so that the stud 5 sometimes will not return to the center. 
Further, studs made of shape-memory alloy have been proposed so as to 
project and to retract by themselves but they have problems about stable 
function, cost, and durability, so that they have not been realized. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a stud unit which is 
capable of adjusting the function of a stud with respect to condition of 
roads according to the coefficient of the friction between the surface 
thereof and the tire treads, and a skid-proof tire having the stud unit. 
To overcome the above described disadvantages, the present invention has 
following structure. 
A stud unit according to the invention comprises a stud axially 
reciprocally received in a hollow cylinder having a closed end and an open 
end and biased outwardly by an elastic element in the cylinder between the 
closed end of the cylinder and an end of the stud so that when the stud 
unit is installed in a hole bored in a tire tread radially with the inner 
end of the cylinder being the closed end, the stud projecting from the 
tire tread when not in contact with a dry road but being forced into the 
cylinder by the weight of the vehicle to be substantially flush with the 
tread when in contact with the dry road. The stud and cylinder are 
provided with mutually engaging surfaces to prevent the stud from being 
projected entirely out of the cylinder. The stud is received in the 
cylinder with play and the cylinder has an axially extending section in 
which the inner circumference of the cylinder is non-circular and the stud 
has a corresponding section also having a non-circular circumference. When 
the tread slips, for example on ice or snow, a horizontal force is applied 
to a stud in contact with the icy or snowy road surface by that surface, 
and in the direction opposite slipping of the tread, to incline the stud 
so that a portion of the end thereof radially outward relative to the tire 
engages a portion of the non-circular inner circumference of the cylinder 
to provide sufficient friction between the stud and the cylinder to retain 
the stud in a position in which it projects outwardly from the tire tread, 
biting into the icy or snowy surface. 
A skid-proof tire is made by fitting a plurality of the studs having above 
noted structure in holes bored in an outer circumferential face of a tire.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will now be described with 
reference to the accompanying drawings. 
A tire 11 is composed of, as shown in FIGS. 1 and 2, a carcass 11a, 
breakers 11b superimposed as several layers on the outer face of the 
carcass 11a and a rubber layer 11c superimposed on the outer face of the 
breakers 11b. The rubber layer 11c is formed thick so as to support the 
weight of a vehicle and to bear shock and friction. There is formed a 
tread 11d on the outer circumferential face of the rubber layer 11c. 
There are bored multiple holes 15 (see FIG. 1) in all treads 11d, and stud 
units 10 are respectively fixed into the holes 15 (see FIG. 3). 
As seen in FIG. 1, a cylinder 12, of which each stud unit 10 is composed, 
has a flange section 14 at the rear end (the end on the tire-center side). 
A stud 32 is inserted into each cylinder 12 with play. 
The stud unit will be explained with reference to FIGS. 4 and 5. Three 
grooves 32a are formed on the circumference of the stud 32 in the axial 
direction thereof, so that three projections 32b are formed between the 
grooves on the circumference of the stud 32 in the axial direction 
thereof. The rear end of each groove 32a is closed; the front end thereof 
is open. The rear end section of the stud 32 is a stopper section 20. The 
radius of the stopper section 20 is slightly larger than the radius of the 
projections 32b. There is provided a hard tip 24 at the center of the 
front end face of each stud 32. 
From midway to the front end of the cylinder 12 is a small-diameter 
generally cylindrical section 12A the inner circumference 13 of which is 
non-circular (FIG. 5). The circumference 13 corresponds to a non-circular 
circumference 33 of the stud 32. There is a narrow annular gap between the 
stud 32 and the small-diameter section 12A. The rear section of the 
cylinder 12 is a large-diameter cylindrical section 12B. There is formed a 
step 16 at the border between the small- and the large-diameter 
cylindrical sections 12A and 12B. The step 16 (FIG. 6) engages with the 
stopper section 20 so as to retain the stud 32. 
There is provided a spring 30 as an elastic member between the rear end 
face of the stopper section 20 of the stud 32 and the bottom face 12b of 
the cylinder 12 so as to bias the stud 32 outward. 
The stud units 10 are, as described above, fitted in the treads 11d of the 
tire 11 but the tip 24 and the front end section of each stud 32 project 
from the tread 11d. Alternatively, the stud units 10 may be so fitted in 
the treads 11d or so designed that only the tip 24 of the stud 32 projects 
from the tread 11d. 
Successively, the function of the stud units, which are adapted for 
installation in a vehicle's tires, will be explained. 
First, the case of dry road and fixed speed will be explained. 
Rotating the tire 11, the front end of the studs 32 consecutively contact a 
surface of a road together with the tread 11d. The weight of the vehicle 
pushes the stud 32 contacting the road. The stud 32 contacting the road is 
pushed into the cylinder 12 against the elasticity of the spring 30 by the 
weight of the vehicle (see FIG. 6). At that time the stud 32 is biased 
outward by the elasticity of the spring 30 but the elasticity thereof is 
so small that the stud 32 does not damage the road. 
If the stud 32 leaves the road with the rotation of the tire 11, the front 
end of the stud 32 is projected from the tread 11d by the spring 30. 
On the dry road, the friction is large, so the tread 11d holds the surface 
of the road, so that whole of the torque of the tire 11 acts as a thrust. 
The stud 32 contacting the dry road receives the counter force from the 
road toward the center of the tire without receiving horizontal force for 
inclining the stud 32, whereby the stud 32 is smoothly retracted into the 
cylinder 12. 
On the dry road, when the brake is applied, the friction is large, so the 
treads 11d hold the surface of the road without slip. Note that the 
braking shock is almost totally absorbed by the treads 11d, so only a 
quite small horizontal force acts on the front end of the stud 32 but the 
stud 32 is retracted into the cylinder 12 against the elasticity of the 
spring 30 by the weight of the vehicle when the stud contacts the road. 
But on a snow- or ice-covered road, the tire is apt to slip because of low 
friction. 
At fixed speed the tire 11 does not slip even on the snow- or ice-covered 
road. The front end of the stud 32 contacting slightly bites the snow or 
ice surface because of biasing the stud 32 outward by the spring 30 (FIG. 
7). Therefore, the stud 32 can maintain the frictional force even if the 
treads 11d of the tire 11 slip when the brake is applied. 
If the brake is applied on such slippery road, the part of the treads 11d 
which contact the road slips in the S direction. At that time counter 
force f acts on the section of the tire contacting the road. The force f 
acts on the front ends of those studs 32 contacting the road when the 
treads 11d slip thereon. 
The force f acting on the front end of the stud 32 contacting the road 
causes the stud 32 to incline in the direction counter to S and a section 
of the non-circular circumference of the stud 32 engages trailing section 
12a of the non-circular inner circumference 13 of the cylinder 12 at the 
mount of the latter. The spring 30 partially compresses with inclination 
of the stud 32. With this compressing and the aforementioned mutual 
engagement of sections of the non-circular circumferences 13 and 33, the 
stud 32 is kept projecting despite the weight of the vehicle acting to 
push the stud 32 inwardly of the tire. It is apparent that friction 
between the mutually engaging portions of the circumferences 13 and 33 
resists pushing in of the stud 32. Note that, as described above, the 
front end of the stud 32, which is biased by the spring 30, slightly bites 
the surface of the snow- or ice-covered road, so that the studs 32 secure 
the braking action of the tire 11 (see FIG. 7). 
As the speed of the tire 11 is reduced by braking the force f acting on the 
studs 32 contacting the road is also reduced and the treads 11d hold the 
surface of the snow- or ice-covered road at low speed and slipping is 
prevented. As the force f acting on the studs 32 contacting the road 
decreases, the projected length of the studs 32 contacting the road 
increases. 
When the treads 11d of the tire 11 do not slip, the force f acting on the 
stud 32 contacting the road becomes small, so that the stud 32 retracts 
into the cylinder 12. When the vehicle stops, the force f disappears and 
the stud 32 receives the weight of the vehicle only. The front end of the 
stud 32 slightly bites the surface of snow or ice due to the elasticity of 
the spring 30. 
When the inclined stud 32 biting the surface leaves the road, the 
horizontal force f disappears, and the compressed spring 30 springs back 
to the original form, whereupon the spring 30 presses the stopper section 
20 to quickly return the stud 32 to the position in which the axis of the 
stud 32 coincides with the axis of the cylinder 12, so that the stud 32 
projects. 
In case of acceleration or starting on the snow- or ice-covered road, 
especially on a slope, the torque of the tire 11 acts on the road and the 
vehicle moves forward. If the tire 11 slips in the direction of S, the 
force f in the counter direction acts on the stud 32 contacting the road 
so that the stud 32 engages trailing section 12a at the mouth of the 
cylinder 12 as described hereinabove, and the stud 32 is thereby kept 
projecting to provide friction. Therefore, the slip of the tire 11 is 
quickly stopped. 
In the above description, the stud 32 inclines and a portion of the 
non-circular circumference 33 of the stud 32 engages a portion of the 
non-circular inner circumference 13 of the cylinder 12 to keep the stud 32 
projecting when the force f acts on the front end of the stud 32. Further, 
the non-circular circumference 33 of the stud 32 engages the trailing edge 
12a of the non-circular inner circumference 13 of the cylinder 12 at the 
mouth of the latter. 
Even if sand or the like enters the gap between the cylinder 12 and the 
stud 32 of the stud unit 10, the sand is removed because of the 
centrifugal force of the tire 11 rotating at high speed and the 
reciprocative movement of the stud 32. Also, when the brake is quickly 
applied, inertia causes engagement of a portion of the non-circular 
circumference 33 with the non-circular circumference 13 to prevent the 
stud 32 from retracting into the cylinder 12. 
Next, other embodiments of the stud unit will be explained. 
In FIG. 8, the transverse sectional shape of the tip 24 at the front end of 
the stud 32 is substantially triangular. 
In FIG. 9, the cylinder 12 of the stud unit 10 has a mouth piece 12g, which 
is made of material harder than the material of the cylinder 12, at the 
mouth of the cylinder 12. In this embodiment, stopper section 20 is 
annular and projects radially inwardly of the small diameter section 12A 
of the cylinder 12 and the step 16 of the cylinder 12 with which the 
stopper section 20 engages is also annular. 
FIG. 10 is a longitudinal sectional view of the stud 32 of another 
embodiment of the stud unit 10. There is formed an annular engagement step 
33a at the midway of the section of the stud 32 of non-circular 
circumference 33. The engagement step 33a engages with the lip of the 
mouth of cylinder 12 when the stud 32 inclines whereby in this embodiment 
not only friction between mutually engaging portions of the circumferences 
33 and 13 maintains projection of the stud 32. In alternative embodiments, 
step 33a may be formed only on the projections 32b or the grooves 32a. 
In FIG. 11(a) and (b), the cylinder 12 of the stud unit 10 has projections 
12f, which support both sides of the projections 32b of the stud 32, at 
the step of the cylinder 12. The side faces of the projections 32b 
therefore do not contact the non-circular circumference 13 of the cylinder 
12, so that the stud 32 can be moved smoothly. 
As seen in the embodiment of FIG. 12, the number of the projections 32b and 
grooves 32a of the stud are not limited but may be many. 
In the embodiment of FIG. 13, projection 32b on the outer circumferential 
face of the stud 132 does not extend to the lower end. The projection 32b 
of the stud 132 is slidingly received in a corresponding groove in the 
small-diameter section 12A of the cylinder 12. 
In the embodiment of FIG. 14 a hole 232a is bored in the axial direction of 
the stud 232 at the rear end thereof. The front end of the spring 30 is 
inserted into the hole 232a; the rear end of the spring 30 is fitted onto 
a projection on the bottom face 12b of the cylinder 12, so as not to move. 
In the embodiment of FIG. 15, the both edges 12h of each projection 12e of 
the small-diameter section 12A are formed by walls of the projection 
meeting at an acute angle and are, therefore, sharp. When the stud 32 
inclines, the front ends of the sharp edges 12h engage portions of the 
non-circular circumference 33 of the stud 32 to keep the stud projecting 
due to friction between the edge 12h and the portion of the non-circular 
circumference engaged thereby. 
In the embodiment of FIG. 16, four narrow grooves 32c are formed in the 
stud 32 in the axial direction and on the outer circumferential face 
thereof; four projections 12i, whose ridges are formed sharp so as to fit 
in the grooves 32c of the stud 32, are formed on the inner face of the 
small-diameter section 12A of the cylinder 12. The front end of the ridge 
of a projection 12i engages the stud 32 so as to prevent the stud 32 from 
retracting into the cylinder 12 when the stud 32 inclines. 
In the embodiment of FIG. 17, as compared to the embodiment of FIG. 16, 
sharp ridges are also formed on the stud 32 and grooves cooperating 
therewith on the cylinder 12. 
In FIGS. 18-20, the ridges of the projections of the cylinder 12 are formed 
as sharp as in the embodiments of FIGS. 16 and 17. 
FIG. 21 is another embodiment of the projections. There are formed three 
grooves 12j on the inner face of the small-diameter section 12A; there are 
formed three projections 32d corresponding to the grooves 12j and each of 
which is comprised of two ridges. The two ridges of each projection 32d 
are sharp. When the stud 32 inclines, the front end of the ridges of a 
projection 32d engages the stud 32 so as to prevent the stud 32 from 
retracting into the cylinder 12. 
FIG. 22 is another embodiment of the stud unit 10. There are formed 
projections and grooves in zigzag form on the inner face of the 
small-diameter section 12A of the cylinder 12, and the projections and 
grooves on the outer circumferential face of the stud are also formed in 
zigzag form. The stud 32 is inserted into the cylinder 12 with play. The 
largest outer diameter of the stud 32 is smaller than the smallest inner 
diameter of the small-diameter section 12A, so that the stud 32 has much 
play. With this much play, substantial inclination of the stud 32 is 
required before a portion of the front end of the non-circular 
circumference 33 of the stud 13 engages a portion of the non-circular 
inner circumference of the cylinder 12. 
Note that, in all embodiments, a projection may be formed on the front end 
face of the stud instead of providing a separate tip. The cylinder and the 
stud may be made of ceramics or plastics. The elastic member can be not 
only the spring but a rubber member or the like. 
Further, there may be formed slits in the axial direction of the cylinder 
on the outer circumferential face thereof so as to reduce the weight of 
the stud unit.