JP4367666 | Screw tightening device |
WO/2023/117881 | SUPPLY DEVICE, SUPPLY SYSTEM AND SYSTEM |
COLLARES ANDRE RENATO (BR)
ZUFFO CESAR HENRIQUE (BR)
CIOTO RUBENS (BR)
COLLARES ANDRE RENATO (BR)
ZUFFO CESAR HENRIQUE (BR)
DE4024577A1 | 1992-02-06 | |||
US4375121A | 1983-03-01 | |||
US5284217A | 1994-02-08 | |||
US4267629A | 1981-05-19 |
1. | A system for automated execution of bolted joints consisting of two or more joint members and threaded fasteners, said system comprising a power wrench provided with realtime sensing means that continuously measure torque and turn angle values applied to said fasteners, said power wrench being bidirectionally coupled to a computer by communication means which continuously communicate said values from said power wrench to said computer, the latter comprising a first memory means for storing said values, a second memory means for storing a program for the computation of the magnitude of the torque and turn angle parameters as a function of the target clamping force, a third memory means for storing the values of the acceptance limits and numerical constants used in said computation and command means for inputting said parameters to said power wrench. |
2. | A system as claimed in claim 1, wherein said threaded fasteners comprise bolts, nuts, studs and machine screws. |
3. | A method for automated execution of bolted joints using a power wrench provided with realtime sensing means that continuously measure torque and turn angle values, a computer connected by communication means to said power wrench, said computer being provided with memory means wherein said method comprises sequential tightening steps which apply increasing stress forces to said fasteners without exceeding the elastic limits of the fastener material, the data relating to torque and turn angle collected in a given step being used to compute the torque or turn angle applied by said power wrench in the following step. |
4. | A method as claimed in claim 3 wherein the method comprises an algorithm, which comprises of a sequence of computing operations and a sequence of checking operations. |
5. | A method as claimed in claim 4, wherein said algorithm comprises a first preliminary tightening step and a second final step, the torque and turn angle data relating to said first tightening step being used to compute the value of torque applied by the wrench to the fastener in said second final step. |
6. | A method as claimed in claim 5, wherein said first step comprises tightening the fastener to a preload which is a fraction of the estimated final value of preload. |
7. | A method as claimed in claim 6m wherein said fraction is between 60% and 80%. |
8. | A method for automated execution of bolted joints as claimed in claim 4, wherein said algorithm comprises the following steps: initial tightening to a preestablished torque which is a fraction of the final torque value, storing continuously the data collected by sensing means provided in the wrench to generate a curve relating torque and turn angle; determination of the best fitting straight line passing through two predefined points in the straight portion of said curve; determination of the slope of said straight line which relates the values of turning moment ΔMA and the angular displacement θ; computation of the K factor using the formula (1) below: computation of the torque to be applied in the final tightening step using the target value of the clamping force (preload) according to formula (2): MAds = FMdK (2) checking whether the relation MAmin. < MAds < MAM3X holds true, where , MAmin. e MAMax are previously defined, rejecting the junctions that do not fit this criterion; applying torque MAds to the fastener; plotting a new substitutive straight line and corresponding angular coefficients ΔMA' e θ'; using formula (1) recalculation of valor de K' using the new angular coefficients; checking whether the relation Dmin < (XZK') < Dmax holds true rejecting the junctions that do not fit this criterion; recalculation of the clamping force variation according to the expression FM' = MAds / (K' d); checking whether the relation Dmin < (FM / FM') < Dmax holds true, rejecting the junctions that do not fit this criterion; computing the friction coefficient μges by means of the formula K (0,16 p / d) dw3 dh3 μges = where dkM == C4 (0,58d2 + 0,5dkM) / d dw2 dh2 checking whether the relation μgeSMin. ≤ μges ≤ μgesMax. holds true rejecting the junctions that do not fit this criterion; detemiining the angle θt between the horizontal axis and a straight line passing through the points of the curve corresponding to MAds and MA 0. calculating the value θti = ( MAds / ΔMA ) • θ' checking the whether the relation DnUn < (θt / θu ) < Dmax holds true, rejecting the junctions that do not fit this criterion; computing FM" = 360 • (δs + δp) checking whether the relation D1nJn < (FM" / FM) < Dmax holds true, rejecting the junctions that do not fit this criterion. |
9. | A method as claimed in claim 8, wherein the data related to the tightening process of each bolt are stored and a statistical report based on said data is printed at the end of the operation. |
Lastly, the invention has the object of providing a tightening method, which can be advantageously applied to either single- or multi- spindle computer controlled power wrenches allowing real-time process control for each individual fastener.
Brief summary of the invention The preceding aims and objects are accomplished by the invention by providing a system consisting of a power wrench provided with real- time means that measure torque and turn angle values, said power wrench being bi-directionally coupled to a computer which comprises storage means for said values, as well as a resident program for computing the final values which will achieve the desired preload force and command means for the command of the power wrench to apply said torque or turn values to the fastener.
According to another aspect of the invention, the instantaneous values of the turn angle and torque measured in the course of the tightening operation are inputted to the resident program in order to determine the behavior of the elements comprising the individual joint and compute the final values of torque or turn angle that are to be applied by the power wrench in order to achieve the specified preload force.
According to another aspect of the invention, the tightening operation is based on an algorithm that in addition to computing the parameters of torque and turn angle comprises verification steps in which the conformity of the joint to preset standards is checked. According to a further aspect of the invention, said algorithm comprises successive tightening steps, the torque and turn angle parameters collected during each step being inputted to the algorithm to compute the parameters to be used in the next step. According to yet another aspect of the invention, the method comprises a first and a second tightening steps, the parameters collected during said first preliminary step being used as input for computing by means of the resident algorithm the parameters applied during the second final step. According to another aspect of the invention, said algorithm comprises corroborative steps performed after said second final steps, the acceptance or rejection of the joint being contingent on the conformity to pre-established conditions.
Brief description of the drawings For a better understanding of the invention, its operating advantages and the specific results obtained by its use, reference should be made to the following detailed description of a preferred non-limiting embodiment taken in conjunction with the accompanying drawings in which: Fig. 1 shows a bolted joint in a highly schematic and idealized form. Fig. 2 shows an exaggerated view of a bolted joint emphasizing the deformations undergone by the bolt and the joined pieces. Fig. 3 is a histogram showing the preload scatter for a lot of 25 identical bolts when torqued to 530 N.m (400 lb-ft). Fig. 4 depicts the general idealized behavior of a bolt being tightened, the curve comprising at first the running down phase (segment O-A'-B'), snugging (segment B'-C), the linear portion (C-D') in which the bolt behaves elastically and the plastic portion (D'-E'-F').
Fig. 5 shows the bolt behavior by means of the torque x angle curve during the first tightening phase in a real-life situation according to the present invention.
Fig. 6 depicts the bolt behavior in the final tightening phase according to the invention.
Figs. 7-a, 7-b and 7-c detail by means of a flow graph the algorithm used in the present embodiment of the invention.
Detailed description Referring now to the drawings and particularly to Fig. 4, that depicts in a schematic form the curve that relates the torque and the turning angle of the bolt or nut, it can be seen that said torque remains negligible in the first portion A' -B' of the curve, corresponding to the running down of the nut, said portion having no bearing to the present method. From point B' on, the nut starts to pull the joint members together. Joint members may not be completely flat or there may be a bent washer, etc.. As a result, most of the input is absorbed by the joint and the bolt sees only a small increase in preload: this process is called snugging, and the amount of turns required to reach point C varies unpredictably.
After all joint members are in contact, all joint members start to to the present preferred embodiment, said preliminary torque value is a fraction, usually between 60% and 80% of the estimated final torque, said estimate being based in existing knowledge or in previous tests with the fastener being used. The data collected as the torque is increased from zero to said preliminary value yields the curve 21, which relates the turn angle to the inputted torque. As indicated in Fig. 5, this curve has an initial snugging portion, which will be disregarded. According to the proposed method, two points I and II are chosen in the linear portion, and the best theoretical straight line 22 passing through said points is mathematically determined using known methods, such as, for instance, linear regression. Said line 22 makes an angle α with the horizontal axis, the tangent of this angle being equal to the relation between the increments of the turning moment ΔMA and the angular displacement θ. The next step consists of calculating the K factor by means of formula (1), below:
where d is the nominal diameter of the bolt p is the pitch of the thread δs is the bolt's elasticity δp is the joint's a elasticity, and α is the angle between line 22 and the horizontal axis With the K factor, it is possible to calculate the final torque to reach the target clamp load (preload), by means of the expression (2) as follows:
MAdS = FM - K - d (2) where FM is the target preload, and MAds is the final torque The above values are stored in the computer's memory to be used in posterior checking steps. Said torque should fall between previously defined limits, according to the relation (3), below:
MAMn < MAds < MAMaχ (3) The joints that do not meet this criterion are rejected.
In the following final step, the power wrench applies said torque MAds to the fastener, yielding the curve 21' shown in Fig. 6, which is a continuation of the previous curve 21. Point III, at the end of curve 21' will be used to calculate a new straight line 22', now adjusted to points I and HI, which defines the new values of θ' and ΔMA' as well as the new angle α'. These new values, now related to the final tightening of the fastener, will be used to compute K' using formula (1) as well as the new clamping force FM' - MAds / (K' • d), which are compared with the corresponding preliminary values. The allowable scatter, which lies between the limits designated as D1nU1 e Dmax is a function of the confidence level related to the specific application. In the present exemplary embodiment, said limits are equal to 0.97 and 1.03, meaning that the allowable scatter range will be ±3%. The joint will be accepted when the following relations are complied with:
0,97<-~ < l,03 e 0,97<-^r <l,03 Should said relations fall outside the allowable scatter range, the junction will be rejected and should be made over again with another fastener. Once more, it should be stressed that the above-mentioned limits are merely illustrative; different limits can be set in accordance with the intended application. In addition to the above-mentioned checks, the algorithm comprises further tests, in order to eliminate joints with abnormalities such as excess friction, thread defects, etc. For the first test, the following friction value is computed: K - (0,16-p/d) dw3 - dh3 μ = (4) in which dkM = 2/3 • (5) (0,58-d2 + 0,5-dkM) / d dw2 - dh2 where d2 is the diameter at the root of thread (primitive) dw is the diameter of the effective nut's or bolt's head contact area, and dh is the hole diameter. The joints which do not comply with the following relation will be rejected: J-lgesMin — M"ges — MgesMax \P) where μgesMin. and μgeSMaχ are pre-established parameters.
The second additional check refers to the angle θt between a straight line passing through the origin (MA= 0) and the extreme of curve 21' corresponding to the final tightening moment MAds. The joint will pass the test if the relation below is complied with:
0,97 < (θt / θtl ) < 1,03 (7) where θtl = ( MAds / ΔMA ) • θ' (8)
A further additional check uses a new value of the clamping force FM" computed by means of the formula shown below: θf P FM" = - (9) 360 - (δs + δp) The value of this new clamping force is compared with the target value, and the joints which do not comply with the following relation will be rejected:
0,97 < (FM" / FM ) < 1,03 (10) The numerical values of FM, FM', FM", K5 K', μ^ e MAds for each joint are stored in the computer memory, allowing the compilation of a statistical report for quality control purposes.
The method of the invention is appropriable for computer controlled power wrenches having one or more spindles, and is particularly suitable for automated assembly lines due to the precision, reliability and repeatability of the results.
Notwithstanding the fact that the invention was described with reference to a particular embodiment, additional advantages of the present invention will readily occur to those skilled in the art, while keeping within the conceptual bounds of the invention. For instance, in junctions that comprise elastic parts such as gaskets more than two tightening steps may be employed, in which case the data collected in each step being used as an input for the subsequent tightening step. A different algorithm than the one employed in the detailed exemplary description may be used without overstepping the bounds of the inventive concept.
It should be also stressed that actual scatter limits of 0,97 and 1,03 are only exemplary, not being limitative in any way of the invention. In addition, some constant parameters in the formulae may differ from the ones employed in the embodiment shown without departing from the invention.
Furthermore, the final tightening may be performed by control of the final turn angle, as φ^s and MAds are mathematically related by the angle α'. Consequently, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact instruction and operation shown and described. All suitable modifications and equivalents that fall within the scope of the appended claims are deemed within the present inventive concept.