Variable strength spring

A variable strength spring includes a tubular body, at whose inner surface is coaxially provided a concave contact seat, a ring made of elastomeric material rigidly engaged in the tubular body and a connection tang rigidly engaged in the elastic ring. The elastic ring has a radial groove that defines, on the longitudinal length of the ring, a first portion rigidly engaged inside the tubular body opposite the contact seat, and a second portion slidably engaged in the tubular body. On the outer circumferential end of the second portion is situated a rigid rabbet edge positioned opposite to a stopping seat inside the tubular body.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to a variable strength spring, comprising a 
tubular body in which is situated a contact seat, concave and coaxial in 
respect to the tubular body, a ring made of elastomeric material coaxially 
engaged in the tubular body and having a checking surface turned towards 
said contact seat and a connection tang rigidly engaged in the elastic 
ring opposite the checking surface. 
2. Background of the Related Art: 
It is known that springs of said type are particularly suitable to be used 
in suspensions for transport vehicles, for example railway wagons which 
are required to support considerable variations of static load. Such 
springs must present, as an essential characteristic, a rigidity growing 
with the increase of static load to which they are asked to support. 
Considering the behavior of suspensions in railway wagons, springs must 
present a relatively reduced rigidity when the wagon is unloaded (load 
acting on each spring equal to 800-900 kg., approximately) or when the 
transported load is of relatively low weight. This is a necessary 
condition to avoid, during running, undesirable disengagements between 
wheel and rail which might occur due to unavoidable unevenness, 
particularly in connection with junctions between rails. In conditions of 
a limited static load, a low strength of the spring is essential, also to 
avoid various impacts supported by wheels during running being entirely 
sent to the suspended mass. 
Conversely, when the static load is of a considerable value (4200-4800 
Kg.), springs must have a high strength, so that situation of static load 
variations does not produce excessive variations of spring flexure and, 
hence, excessive vertical movements of the vehicle center of gravity. 
To satisfy these needs, springs which comprise essentially a shaped elastic 
ring are utilized; this shaped elastic ring is realized by elastomeric 
material of suitable rigidity, which is rigidly and coaxially engaged 
inside the tubular body. 
The tubular body is shaped in order to present coaxially, at its interior, 
a concave contact seat connecting with a cylindric wall inside the tubular 
body. The contact seat is turned towards the elastic ring and is suitably 
spaced from the same. 
The elastic ring is also rigidly engaged a connection tang, whose fastening 
portion protrudes from the ring on the side opposite to the contact seat. 
In the assembly of the spring, the tubular body and the connection tang are 
respectively fixed to parts under relative movement, i.e. to the suspended 
mass and to the non-suspended mass of the vehicle, and vice versa. In such 
a situation the loads sent to the spring are opposed by the consequent 
elastic deformations of the ring. 
Choosing suitable constructions for the various spring components, it it 
possible on confer to said spring a rigidity that within certain limits, 
grows with the static load increase. 
When the conventional spring must support relatively reduced loads, the 
entire section of the deformable ring is substantially stressed by 
shearing stress. In such a situation the elastic ring is susceptible to 
deformations which are relatively high under the action of load variations 
and the spring shows a relatively low rigidity whose value remains 
substantially constant within certain limits of load. In these limits the 
elastic ring is deforming by moving in the tubular body without 
interferring with the contact seat in said tubular body. When the load 
increases beyond the above mentioned limits, the elastic ring, while 
deforming itself, goes progressively in contact, by its duly shaped 
checking surface with the above mentioned contact seat. 
In such a situation, the elastic ring is also stressed to compression, with 
stresses increasing little by little while the contact surface inhibits 
further deformations. As a result there is a considerable increase of 
spring rigidity with increased load increase. 
It is noted that known springs, although they allow rigidity variations 
with an increase of static load, have not shown, up to now, a fully 
satisfactory behavior. 
As a matter of fact, as evident from the above, the rigidity of said 
springs is substantially subjected to increase with the increase of load, 
only after that same load has exceeded a predetermined value. 
SUMMARY OF THE INVENTION 
The main purpose of the present invention is of eliminating such drawback, 
realizing a spring also having variable strength when it must support 
relatively reduced loads. 
This aim and others, which will now be better understood from the following 
detailed description, are substantially achieved by a variable strength 
spring, including a tubular body having an inside surface defining a 
concave contact seat coaxial to the tubular body and an elastic ring in 
the tubular body. The elastic ring has a checking surface facing the 
contact seat and a circumferential groove extending from an outer 
periphery of the ring towards the interior thereof, the groove being 
positioned along the length of the ring such that the groove 
longitudinally divides the ring into a first part located on a side of the 
groove opposite the contact seat, and a second part. The first part is 
fixed to the tubular body and the second part includes the checking 
surface and is slidable relative to the tubular body. Means are provided 
for defining a rigid rabbet edge movable with the outer periphery of the 
second portion and a connection tang is rigidly connected to the elastic 
ring. A stopping seat is positioned in the tubular bodies such that it is 
spaced from the rabbet edge when the spring is not subjected to a load, 
and such that movement of the rabbet edge due to flexure of the elastic 
ring in response to a load on the spring is stopped by the stopping seat.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the figure, 1 generally indicates a variable strength 
spring, according to the present invention. 
The spring 1 includes a tubular body 2 of circular shape, provided with a 
connection flange 3 by which the tubular body 2 can be fixed to the 
non-suspended mass of a railway wagon. 
Inside the tubular body is coaxially housed, at one end of the body, 
contact seat 4 and an inner cylindrical surface 5. 
Between seat 4 and the inner cylindrical surface 5 is positioned a stopping 
seat 6, which extends radially inside tubular body 2, whose purpose will 
be described further on. 
Inside tubular body 2 is coaxially engaged an elastic ring 7, made of 
elastomeric material of suitable rigidity, which is provided with a 
circumferential groove 8 radially extended from outside towards inside. 
The groove 8 substantially divides the longitudinal length of elastic ring 
7 in a first portion 7a and in a second portion 7b. 
Considering a preferred embodiment, groove 8 is positioned on the 
longitudinal length of ring 7 so that the area of ring section 7 is 
equally divided between first portion 7a and the second portion 7b. 
Preferably, the ratio value between the groove depth 8 and the elastic ring 
thickness 7, near the groove, is between 0.4 and 0.6. The groove width 8 
is not of considerable importance to the suitable functioning of spring 1. 
It is however foreseen that groove 8 terminates, inside ring 7, at a 
suitable curved end 8a, to avoid regions of major vulnerability in which 
ruptures of elastic ring 7 might occur. 
The first portion 7a is fastened to the tubular body 2 and is disposed on 
the opposite side to the seat 4. More precisely, the first portion 7a is 
fixed by a rubber-metal connection to a flange 9, which is, in turn, 
fastened to the tubular body 2. 
The second portion 7b presents a suitably shaped check surface 10 turned 
towards the seat 4. The check surface 10 is outwardly provided with the 
annular rigid element 11, which is fastened to the contact seat 4 by a 
rubber-metal connection. 
The annular element 11 is slidably engaged along the inner cylindrical 
surface 5 and presents a rigid rabbet edge 12 to the stopping seat 6. 
Rabbet edge 12 is normally spaced from said stopping seat by a 
predetermined distance. 
It is also foreseen that between the annular element 11 and the inner 
cylindrical surface 5 of the tubular body 2 are interposed two or more 
sliding rings 13 made of low friction coefficient material. 
The spring 1 also comprises a connection tang 14 which connects spring 1 to 
the suspended mass of a wagon; this connection tang is fixed inside the 
elastic ring 7. 
The connection tang 14 is preferably constituted by a trunco-conic element 
15 rigidly engaged inside the elastic ring 7 by a rubber-metal connection. 
Furthermore, the connection area between elastic ring 7 and the 
trunco-conic element 15 is substantially identical to the fastening area 
of first portion 7a to the flange 9 fixed to the tubular body 2. 
To the trunco-conic element 15 is removably associated, by a bolt 16, a 
connection portion 17. The latter presents an annular shoulder 17a 
radially extending over elastic ring 7 and a threaded portion 17b disposed 
on the the opposite side with respect to the trunco-conic element 15. 
After having described the above structure, the functioning of the spring 
will now be described. 
When the spring is required to support a load, the connection tang 14 and 
the tubular body 2 approach one another while being opposed by the elastic 
ring 7. 
For relatively light loads, the mutual approach between connection tang 14 
and tubular body 2 is only opposed by the elastic reaction of first 
portion 7a, which is substantially deformed due to the shearing stresses. 
The second portion 7b does not flex at all under the load, as it is freely 
slidable along the inner cylindrical surface 5. 
In such a situation, the static load variations sent to spring 1 will cause 
relative displacements between the connection tang 14 and the tubular body 
2 of relatively high value. In other words, spring 1 will present a 
relatively low rigidity. 
As load grows, a progressive approaching of rabbet edge 12 towards stopping 
seat 6 within tubular body 2 will occur. In this brief phase the spring 
rigidity remains almost constant until, when a predetermined load value is 
achieved, a mutual contact between rabbet edge 12 and stopping seat 6 
occurs. 
When reaching and exceeding said value, the load acting on spring 1 is also 
opposed by the consequent deformation of second portion 7b which is under 
shearing stress. The spring 1 rigidity appears therefore to be increased, 
as the loads that act on the spring are opposed by the elastic reactions 
exercised by the whole section of elastic ring 7. For this purpose, the 
rabbet edge must be sufficiently ridged that it is capable of being 
stopped by the stopping seat 6. 
The strength of spring 1 is therefore subject to be further increased when, 
by a further increase of load acting on the spring, the checking surface 
10 is put in contact with the contact seat 4, as clearly outlined in chain 
lines in the drawing. In such a situation, the mutual approach between 
connection tang 14 and tubular body 2 is also opposed by compression 
stresses induced in elastic ring 7 by the contact between contact seat 4 
and the elastic ring 7. 
The checking surface 10 and the contact seat 4 are shaped in order to enter 
in contact according to an area progressively growing with increased load. 
As a consequence of the above, said compression stresses, and consequently 
the rigidity of spring 1, are subject to grow progressively by the load 
increase acting on spring 1. 
The invention aims are thus achieved. As a matter of fact, whereas known 
springs have a constant rigidity during a phase of shearing stress and a 
rigidity increase during a phase of shearing stress and compression, the 
present spring presents a first shearing stress value of constant 
rigidity, during a starting phase of shearing stress, a second value of 
constant rigidity, greater than the first value, during the second phase 
of shearing stress, and a further rigidity increase during the phase of 
shearing stress and compression. 
It is also to be noted that, advantageously, the rigidity values of the 
spring according to the invention are substantially maintained less than 
the correspondent rigidity values of the known springs during the phases 
of shearing stress, in order to reach the same values of the traditional 
springs during shearing stress and compression working phases. 
It is also understood that the above means a better behavior of the spring, 
particularly in functioning conditions comprising low and medium loads. 
Obviously, the invention is susceptible of possible modifications and 
variations, without going beyond the inventive idea. More specifically, to 
satisfy particular requirements, it is also possible to foresee additional 
movable portions 7b separated by grooves 8 and interfering in succession 
with as many stoppings areas 6.