Laminated metallic belt for torque transmission device and method of manufacture thereof

Disclosed is an improved laminated metallic belt consisting of at least two layers of an endless metallic band for use in a torque transmission device, which can transmit torque between two pulleys having variable effective radii, in combination with a plurality of metallic blocks which have surfaces contacting the innermost layer of the laminated metallic belt and are arranged along the laminated metallic belt. The metallic belt according to the present invention may be made of precipitation hardening martensitic stainless steel or maraging steel which have favorable properties as to toughness, mechanical strength and the adaptability for welding required for making an endless laminated metallic belt, but lack the surface hardness to ensure relatively favorable durability. The durability of the steel is improved according to the present invention by a nitride layer and/or an electroless nickel plating layer formed on the side edges of the endless metallic band. Optionally, the innermost and/or the outermost surface of the laminated metallic belt may be hardened in a similar manner.

TECHNICAL FIELD 
The present invention relates to a metallic belt for a torque transmission 
device such as a continuously variable transmission device and in 
particular to a laminated metallic belt which forms part of such a 
metallic belt in combination with metallic blocks attached thereto and a 
method of manufacturing such a laminated metallic belt. 
BACKGROUND OF THE INVENTION 
According to a known continuously variable transmission device using a pair 
of pulleys and an endless V-belt, each pulley comprises a pair of members 
having coaxial and mutually opposing conical surfaces which are supported 
opposite to each other in a mutally axially slidable manner so as to 
define a V-groove whose width can be varied by relative axial motion of 
the two members. By thus varying the effective radius of the two pulleys 
in a corresponding manner, torque or rotational power can be transmitted 
between the two pulleys by the endless V-belt at a continuously variable 
transmission ratio. 
In such a continuously variable transmission device, the endless V-belt is 
subjected to both repeated flexing and considerable tension as it travels 
between the two pulleys, and, in order to assure power transmission 
efficiency and reliability, the V-belt must be both flexible and durable. 
V-belts using synthetic resin, leather and other flexible materials may be 
sufficiently flexible for efficient power transmission but will wear out 
in a relatively short time when the power to be transmitted is great. 
Metallic links such as chains could be used for such a V-belt, but the 
lack of flexibility and the uneven contact between the belt and the 
pulleys would give rise to the problems of low power transmission 
efficienty and noise. 
Japanese Patent Application No. 58-70920 (Japanese Patent Laying-Open 
Publication No. 59-197641) filed by one of the Assignees of the present 
application proposes a continuously variable transmission device which 
uses an endless V-belt comprising a plurality of of blocks having a 
V-shaped cross-section and a laminated metallic belt which is passed 
through the blocks and formed into a loop. Since the laminated metallic 
belt is sufficiently flexible to assure high power transmission efficiency 
and has a sufficient tensile strength to endure the tension applied 
thereto in transmitting power while the metallic blocks assure uniform 
contact between the V-belt and the pulleys and can transmit compressive 
force by their mutual contact, very favorable overall results can be 
obtained. However, it has been found that when such a continuously 
variable transmission device is subjected to severe load conditions for an 
extended time period, the laminated metallic belt tends to suffer uneven 
wear, particularly along the side edges. 
The metallic tape or band used for forming such a laminated belt is 
required to be extremely thin, on the order of 0.1 mm to 0.2 mm, in order 
to be sufficiently durable against repeated bending stress, to be 
flexible, and contains a sufficient number of layers to endure the tensile 
stress. The material for such a metallic band is selected from quenched 
and annealed steel such as AISI 4340, precipitation hardening 
semi-austenitic stainless steel such as 17-7PH stainless steel, 
precipitation hardening martensitic stainless steel and maraging steel. 
The former two materials are relatively poor in toughness, while the 
latter two materials are relatively great in toughness and mechanical 
strength and are adapted to welding, but relatively poor in wear 
resistance. 
The material for the metallic blocks is required to have high surface 
hardness and to be wear resistant since the blocks transmit torque by 
frictionally engaging with V-groove pulleys under severe load condition. 
When a laminated metallic belt travels around the pulleys, relative 
velocity differences arise between the metallic band layers and between 
the laminated metallic belt and the metallic blocks, and the lateral side 
edges of the metallic band layers and the innermost layer of the metallic 
band are subjected to relative slips while receiving high surface 
pressure. The surface pressure tends to be high only because the metallic 
band is thin but also because the side edges of the metallic band are 
chamfered for the purpose of reducing stress concentrations in the 
corners. As a result, the lateral side edges of the metallic band tend to 
wear out faster than other areas and the durability of the metallic belt 
is impaired accordingly. 
A nitriding process is well known as a means for improving the wear 
resistance and the fatigue resistance of metallic materials, but it is 
also known that if a nitriding process is attempted on an elongated member 
such as a metallic band for an endless laminated belt, problems arise 
because the dispersion of nitrogen in the metallic material causes 
dimensional increases and twisting deformation. Furthermore, a gas 
nitriding process will produce a brittle layer on the surface which must 
be removed by grinding. Tin plating and nickel plating may be used as an 
alternative to nitriding, but plating has the disadvantages of causing 
dimensional changes when performed on elongated thin band material. 
Plating is also costly. 
BRIEF SUMMARY OF THE PRESENT INVENTION 
In view of such problems of the prior art, a primary object of the present 
invention is to provide a laminated metallic belt, consisting of at least 
two layers of endless metallic band for use in a torque transmission 
device in combination with a plurality of metallic blocks which have 
surfaces contacting the innermost layer of the laminated metallic belt and 
are arranged along the laminated metallic belt, which is highly durable 
and adapted to efficient torque transmission. 
Another object of the present invention is to provide a laminated metallic 
belt for a torque trasmission device which is economical. 
Yet another object of the present invention is to provide a laminated 
metallic belt for a torque transmission device which is easy to 
manufacture. 
According to the present invention, such objects are accomplished by 
providing a laminated metallic belt consisting of at least two layers of 
endless metallic band for use in a torque transmission device in 
combination with a plurality of metallic blocks which have surfaces 
contacting the innermost layer of the laminated metallic belt and are 
arranged along the laminated metallic belt, wherein at least the side 
edges of the endless metallic band are hardened. 
According to the broadest concept of the present invention, since the side 
edges of the endless metallic band which have a great tendency to wear are 
made more durable, the overall durability of the laminated metallic belt 
is improved. 
According to one aspect of the present invention, since the hardening 
process comprises a nitriding process or an electroless nickel plating, 
the hardening process will not affect the toughness and mechanical 
strength of the metallic band. 
According to another aspect of the present invention, since the endless 
metallic band consists of a material selected from precipitation hardening 
martensitic stainless steel and maraging steel, the laminated metallic 
belt will have sufficient toughness and mechanical strength and favorable 
workability particularly for welding. 
According to yet another aspect of the present invention, since the 
innermost and/or the outermost layer of the metallic band which is also 
prone to wear is also hardened, the improvement in the durability of the 
laminated metallic belt is further enhanced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
FIGS. 1 to 3 show an endless metallic belt 1 passed around a pair of 
V-groove pulleys 2 and 3 to which this invention may be applied. As shown 
in FIGS. 2 and 3, an endless laminated metallic belt 4 made by laminating 
a plurality of endless metallic band layers 4a to 4n is received in a 
groove 6 defined between a pair of projections 5a integrally formed in the 
top surface of a metallic block 5. A pin 7 press fitted into holes 
provided in the two projections prevents the laminated metallic belt 4 
from falling out of the groove 6. The bottom surface 8 of the groove 6 is 
slightly curved along the lateral direction thereof for centering the 
laminated metallic belt 4 in the groove 6 during the operation of the 
device according to the principles well know in the art of flat belt 
transmission. 
A plurality of such metallic blocks 5 are arranged along the laminated 
metallic belt 4 substantially without any gaps between neighboring 
metallic blocks. The bottom surface 8 of the groove 6 is slightly curved 
along the longitudinal direction thereof to provide even contact between 
the laminated metallic belt and the bottom surface 8 of the groove 6 as 
shown in FIG. 2. The lateral side surfaces of each metallic block 5 are 
inclined downwardly so as to be snugly received by the pulley grooves 2a 
of the V-groove pulleys 2. 
Thus, the metallic blocks 5 are attached to the laminated metallic belt 4 
but are freely moveable along the longitudinal direction of the laminated 
metallic belt 4. Therefore, as a driving force is transmitted from one of 
the pulleys 2 to the laminated metallic belt 4, the driving force is 
transmitted to the other pulley 3 mainly by way of the metallic blocks 5 
as compressive pressure therebetween. 
FIGS. 4 to 6 show another metallic belt 1 to which the present invention 
may be applied. A laminated metallic belt 4 is received in a groove 11 
defined between a pair of projections 10a which are integrally formed on 
the top surface of a metallic block 10 and held therein by a clip 12 which 
is bent in a three-dimensional manner. As shown in FIGS. 4 to 6, the clip 
12 comprises a middle portion 12a which is curved in a plane parallel to 
the major surface of the laminated metallic belt 4 for even contact 
therewith, and end portions 12b which are downwardly bent in the shape of 
a letter L for engagement with the lower surfaces of undercut portions of 
the projections 10a. 
The front and rear surfaces of each metallic block have depressions 13 
which have arcuately curved bottom surfaces as best shown in FIG. 4, and 
roller members 14 are interposed between the neighboring metallic blocks 
10 and received by these depressions 13. The roller members 14 may be 
elastically deformable so as to accommodate the impulsive forces which may 
act between the metallic blocks 10. 
Thus, as the laminated metallic belt 4 undergoes bending deformation so as 
to negotiate the curvature of the pulleys 2 and 3, the curved bottom 
surfaces of the depressions 13 of the metallic blocks 10 roll over the 
outer circumferential surfaces of the roller members 14 whereby the 
metallic blocks 10 move in conformity with the deformation of the 
laminated metallic belt 4 without the neighboring metallic blocks 10 
interfering with each other. 
As best shown in FIG. 6, the metallic blocks 10 also have an inclined 
surface on both side surfaces 15 for even contact between the metallic 
blocks 10 and the internal lateral side surfaces of the pulley groove 2a. 
In the above-described examples, the thickness of the metallic band layers 
4a to 4n of the laminated metallic belt 4 is preferred to be on the order 
of 0.1 mm to 0.2 mm and the lateral side edges of the metallic band layers 
4a to 4n are provided with nitride layers formed by an ion nitriding 
process. The thickness of the nitride layers is preferred to be on the 
order of 0.5 to 30 microns. If the thickness exceeds 30 microns, the 
rigidity of the nitride layer becomes excessive and cracks may develop 
when the belt passes over the pulleys or when the radius of curvature of 
the belt is reduced below a certain level. On the other hand, if the 
thickness of the nitride layer is less than 0.5 microns, the wear 
resistance thereof may not be sufficient for practical purposes. 
The ion nitriding process may consist of accelerating nitrogen ions by glow 
discharge in a vacuum environment and impinging the nitrogen ions on to 
the material on which a nitride is to be deposited. Alternatively, it is 
possible to use salt bath and plasma processes for nitriding. In nitriding 
the lateral side edges of the metallic band layers 4a to 4n which are 
combined into a laminated metallic belt 4, it is possible to conduct the 
nitriding process by placing a pair of soft steel plates against the 
outermost surfaces and the innermost and laminated metallic belt and 
performing the above mentioned nitriding process on the exposed side 
surfaces of the metallic band layers 4a to 4n. 
Optionally, it is possible to conduct a similar additional nitriding 
process to the outermost surface and/or the innermost surface of the 
laminated metallic belt. This may be conducted before hardening the side 
edges of the laminated metallic belt and the depth of the nitride layer 
should be on the order of 0.5 to 5 microns to ensure the toughness 
requirements, while the thickness of the nitride layer along the lateral 
side edges of the laminated metallic belt may be on the order of 0.5 to 50 
microns. In order to compensate for the dimensional changes resulting from 
such a nitriding process, the metallic band of the innermost or outermost 
layer of the laminated metallic band may initially be provided with 
smaller dimensions. The surface hardened layer on the innermost and 
outermost surfaces of the metallic band will prevent wear which may 
otherwise occur due to the relative motion between the innermost or the 
outermost layer of the laminated metallic belt and the corresponding 
surfaces of the metallic blocks. 
A nitride layer thus formed is so strongly attached to the base material 
and durable that the wear resistance of the base material can be much 
improved without compromising the original properties of the base 
material. The base material for the metallic band may be selected from 
precipitation hardening martensitic stainless steel, maraging steel and 
their equivalents which have sufficient mechanical strength and toughness 
and suitability for welding. 
As an alternative process for improving the durability of a laminated 
metallic belt, it is also possible to form surface hardened layers on the 
lateral side edges of the metallic band layers by electroless nickel 
plating. Such an electroless nickel plating layer may extend from the two 
lateral side edges over the whole innermost surface of the laminated 
metallic belt and optionally over the whole outermost surface of the 
laminated metallic belt. This can be accomplished by conducting 
electroless plating to a fully assembled laminated metallic belt since the 
plating liquid can not reach the interfaces between neighboring layers of 
the laminated metallic belt. 
Since an electroless nickel plating layer likewise has high hardness and 
strong attachment to the base material, it can produce a high surface 
hardness and provide the laminated metallic belt with high durability. The 
surface hardened layer on the innermost surface of the metallic band will 
prevent wear which may otherwise occur due to the relative motion between 
the innermost layer of the laminated metallic belt and the corresponding 
surfaces of the metallic blocks. 
It is also effective to combine the iron nitriding process and the 
electroless plating process in improving the durability of the laminated 
metallic belt. This can be accomplished by first conducting a nitriding 
process and then conducting the electroless plating. Since the oxide layer 
which will be formed on the nitride layer is extremely hard, the 
preparation of the surface for the electroless plating must be carefully 
performed. 
According to one experiment conducted by the inventors, a laminated 
metallic belt provided with the nitride layer along the lateral side edges 
thereof according to the present invention was passed around a pair of 
pulleys each having an effective radius of 30 mm, and a tensile force of 
500 to 1,000 kg was applied between the two pulleys. After 400 hours of 
operation, the laminated metallic belt of the present invention suffered 
very little wear while a similar conventional laminated metallic belt 
without such a hardened surface showed 1.0 to 2.0 mm of wear after the 
same time period under the same conditions. 
Although the present invention has been shown and described with reference 
to the preferred embodiment thereof, it should not be considered as 
limited thereby. Various possible modifications and alterations could be 
conceived of by one skilled in the art to any particular embodiment, 
without departing from the scope of the invention.