The present invention relates to a method for checking the correct assembling of a twin- tube hydraulic shock-absorber, particularly for a vehicle suspension.

With reference to Figures 1 and 2 of the attached drawings, a twin-tube hydraulic shock- absorber, for use in particular on a vehicle suspension, is generally indicated 10 and comprises, in per-se-known manner:

- an outer cylindrical tube 12 with a closed bottom end and an open top end, the bottom end being closed by a cover 12a which may be a piece separate from the outer cylindrical tube 12 and attached to this tube (for example, by welding) or, alternatively, may be integrally formed by the outer cylindrical tube 12 itself;

an inner cylindrical tube 14 which is arranged coaxially with the outer cylindrical tube 12 and defines with the latter a reservoir chamber 16;

a piston 18 slidably received in the inner cylindrical tube 14 so as to split the internal volume of this tube into an upper working chamber 20, or rebound chamber, and a lower working chamber 22, or compression chamber;

a rod 24 attached to the piston 18 and projecting from the top end of the outer cy- lindrical tube 12;

a valve assembly 26 arranged at the bottom end of the inner cylindrical tube 14; a guide body 28 interposed between the top end of the inner cylindrical tube 14 and the top end of the outer cylindrical tube 12 to guide the rod 24 in the sliding movement thereof along a longitudinal axis of the cylindrical tubes 12 and 14; and

- a sealing member 30 arranged to sealingly close the outer cylindrical tube 12 at the top end thereof.

A hydraulic shock-absorber of the above-mentioned type is assembled first by inserting the various components of the shock-absorber (valve assembly 26, inner cylindrical tube 14, piston 18 with relating rod 24, guide body 28, sealing member 30 etc.) into the outer cylindrical tube 12 and finally by closing the outer cylindrical tube 12 at the top end thereof, for example by folding radially inwards a top edge 12b (or at least a portion of the top edge 12b) of the side wall of the outer cylindrical tube 12 or, more generally, by deforming at least a portion of the top edge 12b, so as to lock the sealing member 30 in its position and thus ensure that the outer cylindrical tube 12 is sealingly closed at its top end.

Figures 3 and 4 of the attached drawings show the top end of the shock-absorber 10 before and after the aforesaid final closing step, respectively. As can be seen from Figures 3 and 4, as a result of the final closing step (which is carried out, in the embodiment proposed herein, by folding radially inwards of the top edge 12b of the side wall of the outer cylin- drical tube 12) the sealing member 30 is clamped with a given axial load against the guide body 28, which is in abutment against the top end of the inner cylindrical tube 14, the latter being in turn in abutment against the valve assembly 26. In order for the shock-absorber 10 to be sealingly closed, it is necessary to ensure that once the final closing step has been carried out a given axial compression load is applied onto the sealing member 30, for example by the folded top edge 12b of the outer cylindrical tube 12. Such an axial compression load onto the sealing member 30 brings about, by reaction, an axial tension load onto the outer cylindrical tube 12.

It is an object of the present invention to provide a method for easily, quickly and reliably checking the correct assembling of a twin-tube hydraulic shock-absorber, particularly for a vehicle suspension.

This and other objects are fully achieved according to the invention by a method comprising the steps set forth in the enclosed independent claim 1.

Advantageous modes for carrying out a method according to the invention are defined in the dependent claims, the subject-matter of which is to be intended as forming an integral and integrating part of the following description. In short, the invention is based on the idea of measuring, in either static or dynamic mode, the so-called Barkhausen noise on the outer cylindrical tube of the shock-absorber, both before and after the final closing step, and checking, once the final closing step has been carried out, whether the difference between the Barkhausen noise measured before the final closing step and the Barkhausen noise measured after the final closing step is higher than a given minimum level (in which case, the shock-absorber is to be regarded as being correctly assembled, in that the outer cylindrical tube has been properly closed at its top end) or not.

The Applicant has in fact observed that the final closing step leads to a change in the Barkhausen noise that can be measured on the outer cylindrical tube, which change is due to the stress state (namely, the tensile stress state) that is generated in the outer cylindrical tube of the shock-absorber as a result of the reaction force applied by the sealing member on that tube once the sealing member is compressed by the folded top edge of the outer cylindrical tube. Starting from this observation, the Applicant has experimentally checked the existence of a correlation between the amount of change in the Barkhausen noise measured on the outer cylindrical tube of the shock-absorber as a result of the final closing step and the correct or non-correct assembling condition of the shock-absorber, in that a change (namely, an increase) in the Barkhausen noise lower than a given minimum level means that the shock-absorber has not been correctly assembled, and more specifically that the outer tube has not been correctly closed at the top end thereof.

Such a way of checking the correct assembling of a twin-tube hydraulic shock-absorber is very easy and quick to carry out. It is therefore possible for the shock-absorber manufacturer to check all the shock-absorbers that have been produced, instead of only making spot checks based on random sampling. Furthermore, the checking method of the invention is very reliable and allows to avoid on the one hand that correctly assembled shock- absorbers, i.e. shock-absorbers whose outer cylindrical tubes have been correctly closed, are discarded and on the other that non-correctly assembled shock-absorbers, i.e. shock- absorbers whose outer cylindrical tubes have not been correctly closed, are passed. Further characteristics and advantages of the present invention will result more clearly from the following detailed description, given purely by way of non-limiting example with reference to the attached drawings, where:

Figures 1 and 2 are a perspective view and an axial section view, respectively, of a twin-tube hydraulic shock-absorber, in particular for a vehicle suspension, to which the checking method of the present invention is applicable;

Figures 3 and 4 are axial section views showing in detail the top end portion of the shock-absorber of Figures 1 and 2 before and after the final closing step, respectively;

Figure 5 is a graph showing the values of the Barkhausen noise measured along the outer cylindrical tube of a shock-absorber before and after the final closing step; and

Figure 6 indicates, for each shock-absorber of a batch of thirty shock-absorbers on which the Applicant has carried out tests, the value of the difference between the Barkhausen noise measured in a given point of the outer cylindrical tube of the shock-absorber after the final closing step and the Barkhausen noise measured in that point before the final closing step. With reference to Figure 5, the graph shown therein illustrates the change over time of the Barkhausen noise, measured by a suitable detection probe (not shown, but of per-se-known type) moving at a constant speed along the outer cylindrical tube 12 of a shock-absorber 10, such as the one described above with reference to Figures 1 and 2, both before the final closing step (line designated by CI) and after the final closing step (line designated by C2). The Barkhausen noise is indicated in the graph in terms of the physical quantity known as magneto -elastic parameter (mp). The measure path along which the values of the Barkhausen noise as shown in the graph of Figure 5 have been measured is indicated with p in Figure 1. The measure path p is preferably a straight path extending parallel to the longitudinal axis (indicated z in Figures 2 to 4) of the shock-absorber along a non-occupied portion of the outer cylindrical tube 12 of the shock-absorber, between a mounting bracket 32 and a spring plate 34 which are welded to the outer cylindrical tube 12 at the bottom and top ends thereof, respectively. The length of the measure path p may be selected depending for example on the length of the outer cylindrical tube 12. As can be readily noticed from the graph, along substantially the whole measure path p the Barkhausen noise measured after the final closing step (hereinafter simply referred to as "final noise") is higher than the Barkhausen noise measured before the final closing step (hereinafter simply referred to as "initial noise"). More specifically, the final noise is higher than the initial noise both in the starting point and in the end point (indicated Pi and Pf in Figure 1 , respectively) of the measure path p. Furthermore, the maximum value of the final noise along the measure path p is higher than the maximum value of the initial noise along the measure path p.

As already mentioned above, the Applicant has realized that the condition of correct assembling of the shock-absorber, i.e. of correct closure of the outer cylindrical tube, de- pends on the difference between the final noise and the initial noise (which difference is indicated with Amp in the graph of Figure 6), in that a difference lower than a given minimum level means that the outer cylindrical tube of the shock-absorber has not been correctly closed, and that therefore the shock-absorber has not been properly assembled. The checking method of the invention comprises therefore the following steps:

a) a first step wherein the initial noise (i.e. the Barkhausen noise before the final closing step) on the outer cylindrical tube of the shock-absorber is measured;

b) a second step wherein the final noise (i.e. the Barkhausen noise after the final closing step) on the outer cylindrical tube of the shock-absorber is measured;

c) a third step wherein the difference Amp between the final noise and the initial noise measured at steps b) and a), respectively, is calculated; and

d) a fourth step wherein the difference calculated at step c) is compared with the above-mentioned minimum level to determine whether the outer cylindrical tube of the shock-absorber has been correctly closed or not.

The Barkhausen noise on the outer cylindrical tube of the shock-absorber is measured by means of a measurement apparatus of per-se-known type, which will not be described herein. In this connection, it is to be pointed out that the method of the invention does not require a specific measurement apparatus. Any suitable measurement apparatus may there- fore be used to carry out the method of the invention. By carrying out preliminary measurements on a suitable number of samples with a given measurement apparatus, the shock-absorber manufacturer will be able to determine the above-mentioned minimum level of the difference between the Barkhausen noise measured after the final closing step (i.e. the final noise) and the Barkhausen noise measured before the final closing step (i.e. the initial noise) such that a difference lower than this minimum level means that the shock-absorber has not been properly assembled and must therefore be discarded, whereas a difference higher than this minimum level means that the shock- absorber has been properly assembled and can therefore be accepted. The Barkhausen noise may be indifferently measured in static or dynamic mode.

In the first case (static mode) the measurements at steps a) and b) above are carried out only in a given point on the outer surface of the outer cylindrical tube 12, for example in the above-mentioned starting point Pi, and the difference Amp is calculated taking into account the Barkhausen noise measured in that point only. In this connection, Figure 6 shows the values of the difference Amp between the final noise and the initial noise measured in a given point on the outer surface of the outer cylindrical tube 12 during some tests carried out by the Applicant on a batch of thirty shock-absorbers. The shock-absorbers having a difference Amp lower than 5 have turned out not to be correctly assembled, whereas those having a difference Amp higher than 5 have turned out to be correctly assembled.

In the second case (dynamic mode), on the other hand, the Barkhausen noise is measured before and after the final closing step along the whole measure path p and the difference Amp between the final noise and the initial noise is calculated considering for example the maximum value of the final noise and the maximum value of the initial noise. Also in this case, a difference Amp lower than a given minimum level (which minimum level, as already explained, may be determined by carrying out a number of tests on the shock- absorber) means that the shock-absorber has not been correctly assembled and does not therefore comply with the prescribed requirements.

Naturally, the principle of the invention remaining unchanged, the modes of carrying out the invention may be greatly varied with respect to those described and illustrated here purely by way of a non-limiting example, without thereby departing from the scope of the invention as defined in the accompanying claims.