Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M13/00—Testing of machine parts
  • G01M13/02—Gearings; Transmission mechanisms
  • G01M13/021—Gearings

Definitions

• the present invention regards a method for testing the life-time of gear drives and the like.
• simulation methods enable to calculate the tensions and deformations of the virtual model at discrete points, known as "nodes", of the aforesaid finite elements. In such a way it is possible to obtain a generally rather precise prediction of the behaviour of the mechanical component to be verified.
• Rapid prototyping methods are also known, which provide for realizing small-scale mod el s of the mechanical components to be verified. Nevertheless such scale prototypes are especially designed to the verification of the manufacturing and geometrical aspects of the design of mechan ical com ponents, more than of the dynam ic and structu ral behaviours. Therefore prototyping is generally not suitable for simulating the behaviour as to the stresses, in particular to the fatigue stresses to which the gear teeth are usually subjected, to predict their working life or life-time.
• EP 1 930 855 discloses a method for estimating the life of a gear box using a simulation model.
• DE 197 13 583 discloses a general method for evaluating the remaining working life of a machine.
• DE 10 2007 017614 discloses a method of estimating the life of a gear device by determining stresses in several sections of the gear device.
• the task of the present invention is that of solving the aforementioned problems, devising a method for testing the life-time of gear drives and the like, the method being able to perform in an efficient and reliable way the testing of the life-time of gears until the arising of a damage or failure.
• Another object of the present invention is that of providing a use of a scale model of a gear drive for actuating the method for testing the life-time of gear drives according to the invention.
• figure 1 shows a front view of a gear drive suitable to be tested by means of the method according to the invention
• figure 2 shows a longitudinal section view of the gear drive in figure 1 ;
• figure 3 shows a diagram of the damage curve defined by the standard ISO 6336.
• the gear drive 1 illustrated in figures 1 and 2 is of the epicyclic type, but different types can be also provided .
• the gear drive 1 illustrated for indicative purpose comprises four planet wheels 2 carried rotating by a planetary member 3.
• the gear drive 1 further comprises a solar pinion 4, mounted centrally and geared with the planet wheels 2.
• the satellites 2, the planetary member 3 and the central pinion 4 are mounted inside an internal-toothing crown wheel or ring 5, which is in its turn geared with the planet wheels 2.
• the central pinion 4 is mounted at the end of a shaft 6, which according to the application can be driving or driven.
• the planetary member 3 is mounted freely rotating and coaxial to the shaft 6, such that the rotational motion can be transferred from the central pinion 4 to the planetary member 3 or to the ring 5 or vice versa as a function of the type of application and of the desired reduction ratio. More precisely, in the case wherein the ring 5 is mobile, driven or driving, the planetary member 3 is fixed, so the functioning does not result epicyclic.
• the testing method according to the invention provides for verifying the lifetime of the gears of the real gear drive 1 through the building of a scale model of the same gear drive 1 or of the pair of wheels which results most stressed from the calculation.
• the scale model is suitable to be subjected to a direct test with operating cond itions equivalent to the conditions provided by the design for the real gear drive 1 , as better explained in the following.
• Such verification suitable to be performed not on the real gear drive specimen, but on the corresponding scale model is therefore an indirect verification aimed at testing a pair of wheels consisting of the central pinion 4 and any of the geared planet wheels 2 for example.
• the scale model is subjected to operating conditions equivalent to the conditions provided by the design, that is suitable to cause, on the basis of the theoretical calculation, the same level of damage.
• the method provides for prearranging at first for the model a first load value and for calculating the life-time thereof until a damage, then for identifying an incremental coefficient to be applied to such first load value to obtain an equivalent load value, any other operating condition remaining the same, for example the angular speed , such as to obtain through calculation a calculated life-time until the damage which is substantially equal to a desired testing time and shorter than the initially calculated lifetime for the real gear drive to be tested.
• the method preferably provides for calcu lating in an intermed iate computational step, by means of an intermediate incremental coefficient, an intermediate load value suitable to determine the same life-time calculated for the gear drive to be tested, until the arising of the damage event.
• the method provides for successively identifying a corresponding incremental coefficient suitable to determine the cited shorter calculated life-time for the scale model, substantially equal to the desired testing time.
• the method also applies in the case wherein the design provides variable loads, in particular different load levels that can be represented according to a determined load histogram.
• the design provides variable loads, in particular different load levels that can be represented according to a determined load histogram.
• Successively, equivalent load levels are calculated, incremented preferably in proportional way, such as to determine a calculated life-time which is reduced to a desired shorter time.
• the model is then subjected to the incremented load levels, for the reduced calculated life-time that represents the testing time for the scale model.
• the cited equivalence is based on a calculation proceeding which takes into account all the usual circumstances which condition the life-time of the gears, on the basis of the determinations for example of the standard ISO 6336/2006, which allows to identify life-time curves for the gears, and correction factors to take the particular surface conditions of the gears to be verified into account.
• the first tract of the curve is defined in an unequivocal manner by the cited standard ISO 6336.
• ISO 6336 defines for such tract the cited surface factors that enable to take into account all the circumstances of the different applications which can affect the arising of the damage.
• the cited equivalence concept of the method according to the invention also refers to the concept of damage D n for a determined load level n, the only one provided in the design or the nth of a series of multiple load levels, which can be expressed such as the ratio between the required life-time N L , for example expressed in number of functioning hours, and the calculated life-time until the arising of the damaging failure N Lf or failure number of cycles expressed in hours as well, according to the formula [27]
• the life-time to the arising of the damaging failure N Lf for the calculation of the damage D n can be calculated, known the unique provided load level or each load level of the multiple provided ones, from the life-time curve of the type illustrated in figure 3, for the WV tract relative to the only finite life-time (see figure 3).
• various load levels it is possible to calculate a cumulative damage, assuming that each load level causes a partial damage which can be calculated according to the previous formula, the cumulative damage being given by the sum of such partial damages, according to
• first operating cond itions for the scale model wh ich is intended to be realized are applied to the same calculation method.
• Such conditions are defined by a single load level or by a series of multiple load levels, according to the case, and determine a corresponding calculated life-time.
• the first determined operating conditions, or the intermediate ones if provided, a re then incremented, through successive iterations of the calculation, until obtaining equivalent operating conditions for a reduced calculated life-time, that is such as to lead to a calculated life-time equal to a desired testing time, for example 500 hours maximum.
• a desired testing time for example 500 hours maximum.
• the entity of the equivalent operating conditions for example in terms of load or of the multiple load levels, to be applied to the scale model so that the predetermined life-time is equivalent, as for the damage effects on the model, to the life-time of the real gear drive.
• incremented operating conditions as an example in terms of incremented loads, or in alternative of incremented rotational speeds or of both conditions, allow to perform on the scale model an indirect but reliable verification of the life-time of the real gear drive 1 in testing times reduced, that is shorter, at will.
• the manufacturing techniques of the gears of the scale model are the same as the corresponding wheels of the real gear drive 1 to be tested .
• the mechanical workings are carried out by different specimens of tool machines, but of the same type, for example by means of gear cutting machines or grinding machines.
• the geometrical features of the gears of the model to be realized for the test must be in a fixed scale ratio with respect to the features of the corresponding gears of the real gear drive 1 . That means, for example, that the number of teeth, the dimension of the centre distance of the pair of gears to be tested, as well as the face width and the corresponding tip diameters must be in the same scale ratio with respect to the design values of the real gear drive 1 .
• the construction material is the same for the scale model and for the real gear drive 1 .
• thermal treatments if provided for one or both wheels of the gear to be tested , they must be of the same type, to obtain the desired equivalence, namely the same probability of failure or damage level in the reduced scale model and in the real gear drive 1 .
• the method can be applied to any type of material and of surface treatment provided for the pair of wheels of the real gear drive 1 to be tested.
• the material of one or both wheels of the pair to be tested can in fact lack of any hardening treatment, as in the case wherein for example the pin ion of the pair results cemented, while the wheel suitable to be coupled thereto is made of simply drained steel.
• an epicyclic gear drive 1 designed with centre distance equal to 203 mm, net face width of 190 mm, modulus equal to 10, tooth number equal to 19, 20 respectively for the central pinion 4 and for a planet wheel 2 coupled thereto.
• the manufacturing technology of the gear and the accuracy grade must be, for the scale model, the same as for the real gear drive 1 .
• a surface hardening treatment is provided for one or both wheels of the real gear drive 1 , as an example a case-hardening treatment, the corresponding effective case-hardened thickness of the scale model, which geometrically depends on the diameter of the wheels and on the face width.
• the effective case- hardened thickness depends also on the value of the load or on the applied load levels, confirming the fact that as a geometrical data to be considered for the scale model , the case-hardened thickness is not proportional, according to the adopted scale, to the case-hardened thickness of the real gear drive.
• the corrective factors are successively calculated as a function of the dimensions of the scale model, preferably according to the standard ISO 6336, in order to characterize the slope of the life-time curve in the WV tract of the finite life resistance for the three first loads, at least one of which must result comprised between the points W and V of the same curve (fig. 3), in order to obtain, as previously indicated, the corresponding partial damage D n .
• the desired testing life-time is fixed, for example equal to 500 hours, and the sa me l ife-time calculation process is applied, with incremental load coefficients greater at each calculation iterated step, until a test incremental coefficient, in the case equal to 1 ,597, which determines a calculated total life-time equal to the testing life-time desired, namely around 500 hours (see table 4 of figure 4).
• the method for testing the life-time of gear drives and the like according to the invention allows to perform in an efficient and reliable way the testing of the life-time of gear drives.
• Such a scope is attained mainly through the use of a scale model of the gear drive to be tested, to which are applied, accord ing to the method, equivalent functioning conditions for a predetermined testing time, preferably reduced at will with respect to the calculated life-time until the damage for the designed gear drive 1 .
• a characteristic of the invention is therefore the fact that it allows to perform an indirect test of the resistance and of the functioning of the real gear drive 1 , on the model of which constant or variable loads according to the case are applied, or rotation speeds, proportional or equivalent to the values of the corresponding operating conditions accord ing to the design.
• Such equivalence refers to the evaluation of the same damage for each load of the real design and for each load to be applied to the model.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Testing Of Devices, Machine Parts, Or Other Structures Thereof (AREA)
• Lubricants (AREA)
• Diaphragms For Electromechanical Transducers (AREA)

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Citations (4)

• 2010
  • 2010-02-17 IT ITBO2010A000084A patent/IT1398482B1/it active
• 2011
  • 2011-02-14 WO PCT/EP2011/052107 patent/WO2011101306A1/en active Application Filing

Patent Citations (4)

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