Method for testing the resistance of a tyre to pressure loss

Abstract

A method for testing a punctured tyre's resistance to a loss in pressure includes: creating a plurality of punctures in a wall of the tyre by inserting a plurality of puncturing objects through the wall; running the tyre with the puncturing objects in the wall thereof over a given distance with a regulated inflation pressure; stopping the running of the tyre; and determining, for each puncture, a pressure loss resistance index based on an estimate of a leak rate of the puncture.

Claims

1. A method for testing a resistance of a tyre to a loss in pressure following puncturing of the tyre, the method comprising steps of: creating a plurality of punctures in a wall of the tyre by inserting a plurality of puncturing objects through the wall; running the tyre with the puncturing objects in the wall thereof over a given distance; regulating an inflation pressure of the tyre during the running step; stopping the running of the tyre; and, after completing the running step, determining, for each puncture, a pressure loss resistance index based on an estimate of a leak rate of the puncture, wherein the puncturing objects are ejectable from the wall of the tyre during the running step, and wherein the puncturing objects include a combination of 8 to 30 screws and nails of different diameters.

2. The test method according to claim 1, further comprising a step of, when the tyre does not include any self-sealing product, after the stopping step and prior to the determining step, placing a sealing product in an internal cavity of the tyre.

3. The test method according to claim 1, wherein, when at least one of the puncturing objects has remained in place during the running step, for each puncture corresponding to a puncturing object that has remained in place during the running step, a pressure loss resistance with the puncturing object in place index (I.sub.P) is determined based on an estimate of a leak rate.

4. The test method according to claim 1, wherein, when at least one of the puncturing objects has been ejected from the wall of the tyre during the running step, for each puncture corresponding to a puncturing object that has been ejected during the running step, a pressure loss resistance after ejection of the puncturing object index (I.sub.E) is determined based on an estimate of a leak rate.

5. The test method according to claim 1, further comprising steps of, after the running step: extracting each puncturing object still in place after the running step; and determining, for each puncture corresponding to a puncturing object that has been ejected during the running step or that has been extracted in the extracting step, a pressure loss resistance index (I.sub.E) based on an estimate of a leak rate.

6. The test method according to claim 1, further comprising a step of redetermining, after a given time of 5 to 20 minutes, for each puncture corresponding to a puncturing object that has been ejected during the running step or that has been extracted after the running step, a pressure loss resistance index (I.sub.10) based on an estimate of a leak rate.

7. The test method according to claim 1, further comprising a step of calculating a mean pressure loss resistance index (I.sub.M) for all punctures.

8. The test method according to claim 1, wherein, during the running step, the tyre is run a distance greater than 200 km.

9. The test method according to claim 1, wherein, during the running step, the tyre is run on a rolling road.

10. The test method according to claim 9, wherein a developed length of the rolling road is greater than 16 m.

11. The test method according to claim 1, wherein the tyre is inflated before the puncturing objects are inserted through the wall of the tyre.

12. The test method according to claim 1, wherein the regulated inflation pressure during the running step is a pressure between 1.8 and 3 bar.

13. The test method according to claim 1, wherein, during the running step, the tyre is run at a speed in a range of 90 km/h to 160 km/h.

14. The test method according to claim 13, wherein, during the running step, the speed varies in steps of increasing speed.

15. The test method according to claim 5, further comprising steps of, after the running step, after the extracting step, and after the determining step: running the tyre in an inflated condition for an additional distance without a puncturing object in place; and determining, for each puncture, a pressure loss resistance index based on an estimate of a leak rate.

16. The test method according to claim 15, wherein, during the additional distance run by the tyre is between 100 and 500 km.

17. The test method according to claim 1, wherein a diameter of at least one of the puncturing objects is from 1 to 5 mm.

18. The test method according to claim 1, wherein at least one of the puncturing objects is inserted through a crown portion of the tyre.

19. The test method according to claim 18, wherein the at least one of the puncturing objects is inserted through the crown portion of the tyre from an exterior surface of grooves of a tread pattern of the tyre.

20. The test method according to claim 18, wherein the at least one of the puncturing objects is inserted through the crown portion of the tyre from an exterior surface of a tread block of a tread pattern of the tyre.

21. The test method according to claim 1, further comprising a step of using a surfactant to visualize and qualitatively assess a leak rate of a puncture.

22. The test method according to claim 21, wherein a scoring scheme is used to assess the leak rate of a puncture, in which a score of 100 corresponds to no leak, with no bubble visible; a score of 80 corresponds to a nanoleak, with very small bubbles having a diameter smaller than 0.1 mm and visible only with a magnifying glass; a score of 60 corresponds to a microleak, with small bubbles having diameters between 0.1 and 1 mm and visible by naked eye; and a score of 0 corresponds to a leak, with growing bubbles having a diameter greater than 1 mm, with or no bubble at all due to an excessive air flow rate.

23. The test method according to claim 1, further comprising a step of determining an overall index by combining scores corresponding to the puncturing objects and weighting the scores using a curve that indicates a frequency at which the puncturing objects appear within a customer base.

Description

DESCRIPTION OF THE FIGURES

(1) The attached figures illustrate various aspects of the method for testing the resistance of a tyre to a loss in pressure in the case of a tyre comprising a self-sealing layer on its interior wall:

(2) FIG. 1 shows a number of puncturing objects;

(3) FIG. 2 is a curve, as a cumulative frequency, of the distribution of nail diameters observed in China and in the United States;

(4) FIG. 3 is a partial view from above of the crown of a tyre comprising three punctures;

(5) FIG. 4 illustrates the case of a puncture with a zero leak rate;

(6) FIGS. 5(a) and (b) illustrate the case of a puncture with a very low leak rate;

(7) FIG. 6 illustrates the cases of a puncture with a low leak rate;

(8) FIGS. 7(a) and (b) illustrate the cases of a puncture with a rapid leak; and

(9) FIG. 8 shows the result of a puncture resistance test.

DETAILED DESCRIPTION OF THE INVENTION

(10) A Michelin Energy 3 tyre 1 of size 205/55 R 16 provided with a layer of self-sealing product as set out in the aforementioned patent application WO 2008/080556 A1 is tested.

(11) FIG. 1 shows a few examples of puncturing objects commonly used for the test method. These are nails 21 of diameter 3 mm, nails 22 of diameter 4 mm and nails 23 of diameter 5 mm as well as screws 25 of diameter 3.5 mm.

(12) The diameters of these puncturing objects are entirely realistic with respect to the puncturing objects encountered under actual running conditions. FIG. 2 shows, as a cumulative frequency, the distribution of the nails found on the roads in China and in the United States. It may be observed that nails with diameters less than or equal to 5 mm together account for more than 90% of the objects encountered.

(13) Once the tyre has been mounted on an appropriate wheel and inflated to 2.5 bar, the tyre and the wheel are rigidly attached to a rotary hub, not depicted, and a plurality of puncturing objects is inserted through the crown 3 of the tyre 1.

(14) FIG. 3 shows a partial view from above of the crown 3 of the tyre 1. The tyre tread pattern comprises two longitudinal grooves, inboard 7 and outboard 9, and an outboard shoulder 5 with a set of lateral grooves 11. Inboard or outboard refers to the side of the tyre intended to be mounted towards the inside of the vehicle or towards the outside of the vehicle, the tread pattern of this tyre being asymmetric. FIG. 3 shows three punctures by nails 21 arranged in the inboard longitudinal groove 7, the outboard longitudinal groove 9 and the lateral groove 11 of the outboard shoulder 5.

(15) Three nails 21 of diameter 3 mm and with lengths of between 45 and 60 mm, three nails 22 of diameter 4 mm and of similar length, and three nails 23 of diameter 5 mm and of similar length as well as three screws 25 of diameter 3.5 mm and of lengths between 35 and 50 mm have been inserted across the entire crown. The puncturing objects are evenly distributed about the circumference of the crown. The nail 21 pushed into the groove 11 of the shoulder is positioned a distance of between 20 and 30 mm from the outboard longitudinal groove 9.

(16) It is also possible to puncture the crown of the tyre through the tread blocks of the tread pattern but that requires greater penetration force. It also alters the conditions of ejection of the puncturing objects during running.

(17) The inflated tyre and wheel assembly is then fixed to the hub of a roller with a diameter in excess of 16 m in order to get close to the conditions of running on flat ground.

(18) The running conditions are as follows: the inflation pressure is regulated, for example to 2.5 bar, the applied load is of the order of 90% of the load rating of the tyre, and the temperature in the rolling road chamber is regulated to around 20° C., running being in a straight line without torque and with no applied cornering or camber.

(19) The tyre is run under these conditions at speeds of from 100 to 150 km/h in 10 km/h steps, each speed level lasting for 1 hour. The complete test thus goes on for 6 hours and 750 km.

(20) During running, approximately 70% of the 5 mm diameter nails are expelled as too are around 30% of the 4 mm diameter nails. The 3 mm diameter nails normally remain in the crown of the tyre. The screws are not expelled during running either as the screw thread increases the force necessary for extracting them.

(21) It should be noted that in the case of certain types or sizes of tyre, the 3 mm diameter nails may also be expelled.

(22) After running, a cooling phase lasting a minimum of 4 hours is observed.

(23) The result of the test is a qualitative observation of the leaks of each puncture, prior to extraction (if the puncturing object is still present after the running), after extraction and approximately 10 min after extraction.

(24) The leaks are assessed using a surfactant, for example an aerosol canister of the “1000 bubbles” make. The product is sprayed onto the puncture and the assessor notes the presence, size and number of bubbles using a magnifying glass under bright lighting.

(25) FIGS. 4 to 7 illustrate the various cases observed with the puncturing objects in place (FIGS. 4, 5(a), 6 and 7(a)) and after they have been extracted or ejected (FIGS. 5(b), 7(b)).

(26) FIG. 4 shows a nail 21 passing through a puncture 41 positioned in the longitudinal groove 9 of the tyre. No bubble can be seen; there is no leak; the puncture is scored 10 or 100%.

(27) FIG. 5(a) shows a puncturing object 21 passing through a puncture 51 located in the longitudinal groove 9 of the tyre. The application of the surfactant reveals a great many very very small bubbles 51, visible only under a magnifying glass and of a diameter less than 0.1 mm. This is a very small leak scored 8 or 80%.

(28) FIG. 5(b) shows a puncture 52 made by a puncturing object which has been expelled or extracted. The puncture 52 is likewise situated in the outboard longitudinal groove 9 of the tyre. The application of surfactant also reveals a great many very very small bubbles 51, visible under a magnifying glass and of a diameter smaller than 0.1 mm. This is given the same score 8 or 80%.

(29) FIG. 6 shows a puncturing object 21 passing through a puncture 61 located in the outboard longitudinal groove 9 of the tyre. There, the application of the surfactant reveals a collection of small bubbles 63 of a diameter roughly comprised between 0.1 mm and 1 mm. This is a small leak scored 6 or 60%.

(30) FIG. 7(a) shows a puncturing object 21 passing through a puncture 71 still located in a longitudinal groove of the tyre. The application of the surfactant reveals a single large bubble 73 of diameter greater than 1 mm. This is a leak that scores 0 or 0%.

(31) FIG. 7(b) shows, in the longitudinal groove of the tyre, a puncture 72 the puncturing object for which has been expelled during running or extracted after stopping. Likewise, just one single large bubble 73 of a diameter greater than 1 mm can be seen. This is a leak that scores 0 or 0%.

(32) The following tables indicate the results of the test.

(33) TABLE-US-00001 TABLE 1 Before running Nail ø 5 mm Nail ø 4 mm Nail ø 3 mm Screw ø 3.5 mm Position ILG OLG TG ILG OLG TG ILG OLG TG ILG OLG TG Score 10 10 10 10 10 10 10 10 10 0 0 0 ILG Inboard longitudinal groove OLG Outboard longitudinal groove TG Transverse groove

(34) Table 1 above indicates the results observed when the puncturing objects are pushed into the crown of the tyre. Twelve puncturing objects were inserted, of four different types, each type at three different positions as indicated.

(35) It will be noted that the insertion of the screws 25 in this example causes an immediate leak to occur. However, this leak usually disappears when the tyre is driven on.

(36) TABLE-US-00002 TABLE 2 Table 2 - Index I.sub.P After running, puncturing objects in place in the tyre Nail ø 5 mm Nail ø 4 mm Nail ø 3 mm Screw ø 3.5 mm Position ILG OLG TG ILG OLG TG ILG OLG TG ILG OLG TG Score x x x 10 10 10 10 10 10 10 10 10

(37) TABLE-US-00003 TABLE 3 Reminder of the scoring scheme 10 no leak  8 very small leak  6 small leak  0 leak x object ejected

(38) Table 2 gives the results obtained for the puncturing objects that have remained in place in the crown of the tyre. As indicated previously, the 5 mm diameter nails were ejected, but no puncture with the objects in place reveals a leak. The indices I.sub.P for all of the punctures with the puncturing object in place are all equal to 10.

(39) TABLE-US-00004 TABLE 4 Indices I.sub.E-I.sub.10 After running, puncturing objects ejected or extracted Nail ø 5 mm Nail ø 4 mm Nail ø 3 mm Screw ø 3 m 5 mm Position ILG OLG TG ILG TG OLG ILG OLG TG ILG TG OLG Time t0 t10 t0 t10 t0 t10 t0 t10 t0 t10 t0 t10 t0 t10 t0 t10 t0 t10 t0 t10 t0 t10 t0 t10 Ejected 8 10 8 0 10 10 Extracted 10 10 8 10 10 10 8 10 8 10 10 10 8 10 8 8 8 8

(40) Table 4 gives the results obtained after running and following extraction of all the puncturing objects still in place. Note that there are two scores, the first at the time t0 immediately following extraction and t10, 10 minutes thereafter.

(41) It may be noted that the greatest spread on the results obtained is obtained for larger diameter objects and that the results are better 10 minutes after extraction than immediately after extraction.

(42) FIG. 8 provides a graphical indication I.sub.10 of the results obtained at t10 as a function of the nature of the puncturing objects.

(43) No leak is observed for nails of diameters 3 and 4 mm and screws remaining in the tyre, but there is a degradation to 67% for 5 mm diameter nails and to 87% for screws following extraction thereof.

(44) A level of cover can also be calculated by weighting the indices I.sub.10 using the nail diameter distribution found in the customer base (see FIG. 2). In the case described, that leads to an overall value of 94%, which is an excellent result.

(45) The same tyre then underwent additional running after the indices indicating resistance to loss of pressure following ejection or removal of the puncturing objects had been determined for all the punctures. At the end of this additional running it was found that all the indices were 10 or 100%; there were no longer any leaks.

(46) The combined score as described hereinabove is not the only conceivable score. Other combined scores are possible, for example combining the nails score and the screws score, with a certain weighting. Obviously the scores for the various stages of the test can also be used separately (for example the score for the nails or for the screws before pulling out and after pulling out).

(47) The test described was for a tyre equipped as original equipment with a layer of self-sealing product. As has already been indicated, the test described also allows the other solutions such as tyre inflators and repair kits to be tested.

(48) Tests were conducted with these other solutions. It is found that the sealing performance is practically 100% for all the solutions in the event of a puncture with instant removal of the puncturing object. By contrast, if the tyre is run with the puncturing object in place, after as little as 200 to 300 km of running, the performance of tyre inflators becomes zero, the product escaping through the punctures. As far as repair kits are concerned, these perform better but the performance drops off very greatly also with the length of running performed with the puncturing objects still in place.

(49) The test thus described has the advantage of being highly selective and of being based on an analysis of the leak rates of each puncture rather than on a loss in pressure, and this allows numerous results to be obtained with one single tyre.