JPH02224936A - Fastening method for plastic zone of screw - Google Patents

Abstract

PURPOSE:To obtain a stabilized high axial force irrespective of the fluctuation in the friction coefficient of a screw by finding a torque rate in the proportional relation to the friction coefficient of the screw and adjusting the fastening amount in the plastic zone of the screw according to this torque rate. CONSTITUTION:A torque rate RT of the rate of the fastening torque increase part to the fastening rotation re-increase part in the plastic zone on the way of fastening a screw is found and a set angle thetaa determined according to this torque rate RT is found as well. A CPU 17 decides the straight lines Ya, Yb having the same slopes as those of the torque rate lines La, Lb in an elastic zone and apart at the set angle thetaa(thetaalphaa in case of La and thetaalphab in case of Lb in the figure) in the horizontal axial direction in the correlation diagram taking a fastening azimuth theta in the horizontal axis and a joint torque in the vertical axis. In case of intersections Pa, Pb intersecting the straight lines Ya, Yb and torque rate lines La, Lb, the operation of a nut runner 1 is stopped. The higher the friction coefficient of a bolt is, the larger the fastening amount in a plastic zone becomes and the fastening axial force is uniformized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for tightening screws in a plastic region when screwing mechanical parts.

(Prior art) Generally, when screwing machine parts, the screws are tightened within the elastic range, and this elastic range tightening method is as follows:
For example, as disclosed in Japanese Patent Publication No. 60-14675, there is a torque method for tightening to a preset setting torque, and as disclosed in Japanese Patent Publication No. 61-5857, a torque method for tightening to a preset set rotation angle is used. The angle method of tightening only by the angle is well known. In addition, in JP-A-63-52976, in order to keep the tightening axial force of the screw constant, a torque rate, which is the ratio of the increase in the tightening torque to the increase in the tightening rotation angle during tightening of the screw, is disclosed. An elastic region tightening method is disclosed in which the tightening angle is decreased as the torque rate increases.

Recently, a plastic region tightening method has been proposed in which the screw is tightened to the initial state of the plastic region, which exceeds the elastic limit of the screw. A method is known in which a screw is tightened using the angle method, and then the screw is tightened by a predetermined angle using the angle method. The plastic region tightening method stops tightening in the plastic region, a region where the tightening axial force increases relatively little as the tightening rotation angle increases. Stable and high tightening axial force can be obtained.

(Problem to be Solved by the Invention) However, even in the conventional plastic region tightening method described above, variations occur in the tightening axial force due to the yield axial force difference based on the friction coefficient of the screw. This is due to the following reasons.

That is, in general, when a screw is tightened, torsional stress acts in addition to tensile stress. In this case, according to the screw failure law shown in equation (1) below, the larger the torsional stress τ, the lower the axial force will yield. The higher the friction coefficient μ, the larger it becomes.

Therefore, the higher the friction coefficient μ is, the lower the axial force is required for the screw to yield.

221/2 σ − (σ 13τ ) ... (1) v
t −8F d 2 (L, 15μ+ianβ)/yrd8・
...(2) However, σ: Simple tensile failure equivalent stress of the screw, σ■ t = Tightening tensile stress of the screw, T: Tightening torque, d: Diameter of effective cross section of the screw, F: Tightening axial force, d2: Effective diameter of the screw, β: Lead angle of the screw.

FIG. 4 is a correlation diagram showing the relationship between the tightening rotation angle θ and the tightening axial force F of two types of bolts that are threaded members having different coefficients of friction μ, that is, the A bolt with a high μ and the B bolt with a low μ. A bolt and B bolt both have F = lc in the elastic region
Although they rise with the same slope due to the relationship of θ, these yield axial forces Fa and Fb differ due to the difference in the friction coefficient μ (Fb
>Fa). The gradient in the plastic region after yielding becomes gentle, and the axial force F gradually increases until it reaches the maximum axial force.

The variation in tightening axial force in the conventional plastic fastening method is
Yield axial force Fa, F based on such difference in friction coefficient μ
There was no consideration given to the difference in b, and the variation reaches Fl in FIG.

Here, in order to eliminate variations in the tightening axial force, as can be seen from Figure 4, for bolts with a high friction coefficient μ (A bolts), the amount of tightening after yielding should be greatly varied. It is difficult to directly detect the friction coefficient μ and yield axial force of the screw. On the other hand, from the relationship between the screw tightening axial force F and the tightening rotation angle θ, and the relationship between the tightening torque T and the tightening axial force F, it is found that The following relational expression holds true between Dorkley 1-RT, which is the rate of increase in applied torque T.

μmm#RT-n (3) where m and n are coefficients determined depending on the shape of the screw and the shape of the object to be tightened.

The present invention has been made in view of these points, and its purpose is to focus in particular on the fact that the larger the torque rate RT is, the larger the friction coefficient μ of the screw is, as seen from equation (3) above. Based on this torque rate, the present invention provides a plastic region tightening method that can obtain a very stable and high axial force regardless of fluctuations in the friction coefficient μ of the screw.

(Means for Solving the Problems) In order to achieve the above object, the solving means of the present invention has the following configuration as a method for tightening a screw in a plastic region.

That is, the torque rate, which is the ratio of the increase in tightening torque to the increase in tightening rotation angle in the elastic range during tightening of the screw, is determined, and the set angle determined according to this l·le rate is determined. Next, in a correlation diagram in which the horizontal axis represents the tightening rotation angle and the vertical axis represents the fastening torque, define a straight line that has the same slope as the torque rate line in the elastic region and is separated by the above set angle in the horizontal axis direction, Tightening of the screw is stopped at the intersection point where this straight line intersects with the torque rate line in the plastic region.

(Function) With the above configuration, in the present invention, the torque rate which is proportional to the friction coefficient of the screw is determined, and the screw is tightened in the plastic region after yielding by an angle corresponding to this torque rate, so the friction coefficient of the screw is (The higher the friction coefficient, the greater the amount of tightening in the plastic region, and the more uniform the tightening axial force.)

(Example) Embodiments of the present invention will be described below based on the drawings.

FIG. 2 shows the overall configuration of a bolt tightening device used in the method for tightening a screw in the plastic region according to the present invention, in which 1 is a nut runner, and the nut runner 1 has a socket 2 that fits into a bolt head. , a motor 3 that rotationally drives the socket 2 via a main shaft (not shown), a torque transducer 4 that detects the torque of the main shaft, and an angle encoder 5 that detects the rotation angle of the main shaft.

In addition, 11 indicates torque T1, and 12 indicates torque T2 (however,
Torque setting devices 13 and 14 are comparators, respectively, for setting T, <T2, where both T1 and T2 are values at which the tightened bolt remains in the elastic range.
The comparators 13 and 14 respectively compare the tightening torque detected by the torque transducer 4 with the set torques TI and T2 set by the torque setters 11 and 12, and when the two match, the gate 15. C through 16
A signal is sent to PU17. 18 is CPU
An angle gate receives a control signal from 17 and sends a rotation angle signal detected by the angle encoder 5 to the CPU 17, and 19 is a servo amplifier that drives and controls the motor 3 of the nut runner 1.

Next, a method for tightening a bolt as a screw member using the bolt tightening device described above will be explained with reference to the flowchart shown in FIG.

In FIG. 1, first, in step S1, in response to an external nut runner 1 signal, the CPU 17 rotates the motor 3 of the nut runner 1 in the forward direction via the servo amplifier 19 to tighten a bolt.

Then, in step S2, the tightening torque T detected by the torque transducer 4 and the set torque T1 set by the torque setting device 11 are compared, and the tightening torque T is determined to be the set torque T1.
If the tightening torque T has not reached the set torque T1, the bolt is further tightened, and if the tightening torque T matches the set torque T1, the angle gate 18 is turned on in step S3.

Next, in step S4, the bolt tightening is continued, while in step S5, the tightening torque T is compared with the set torque T2 set by the torque setting device 12, and it is determined that the tightening torque T has not reached the set torque T2. If so, the bolt is further tightened, and when the tightening torque T matches the set torque T2, the angle gate 18 is turned off in step S6, and then the tightening rotation angle increment Δθ is calculated in step S7. This tightening rotation angle increment Δθ is during the period from when the angle gate is turned on to when it is turned off, that is, until the tightening torque T increases from T to T2.

Then, in step S8, the CPU 17 first
The torque rate RT, which is the ratio of the tightening torque increase to the tightening rotation angle increase, is determined, and the set angle θα determined according to this torque rate TR is determined. here,
Torque rate RT and setting angle θα are determined by the following equations.

RT-(T2-T+)/Δθ θα-RT-K However, K is a coefficient determined by the shape of the bolt, the tensile strength of the bolt, the shape of the object to be tightened, etc.

Next, the CPU 17 calculates the elastic region in a correlation diagram between the tightening rotation angle θ and the tightening torque T, in which the horizontal axis is the tightening rotation angle θ and the vertical axis is the tightening torque T. Straight lines Ya and Yb are defined which have the same slope as the torque rate lines La and Lb and are separated by the set angle θα (in the figure, when La is θαa and when SL b is θαb).

The functional expression T- of this straight line is T'=f (θ) -RT·θ-RT -K (4).

After the calculation in step S8 is completed, the bolts are continued to be tightened in step S9, while step SIO
The current actual bolt tightening torque T is read, and in step Sll, this tightening torque T is compared with T- determined from the above equation (4) for the current tightening rotation angle θ. If T and T' do not match (TNT- in this case), tighten the bolt further, and T and T'
(T=T-), that is, at the intersections Pa and Pb where the straight lines Ya and Yb and the torque rate lines La and Lb intersect, the operation of the nut runner 1 is stopped and the bolt is removed. Tightening finishes the work.

When tightening bolts using the method described above,
As can be clearly seen from Fig. 3, the larger the slope of the torque rate lines La and Lb of the bolt, that is, the torque rate RT, the more the bolt is rotated in the plastic region after yielding, and the tightening rotation angle θ becomes larger. Become. Since the torque rate RT is proportional to the friction coefficient μ of the bolt, the bolt with a high friction coefficient μ can be tightened by a large amount in any plastic region, and the tightening axial force is made uniform. As shown in Fig. 4, in the case of the conventional tightening method in the plastic region, the tightening between the A bolt with a high friction coefficient μ and the B bolt with a low friction coefficient μ (=j-axis force difference Fl is considerable). In contrast, in the case of the plastic region tightening method of the present invention, the tightening axial force difference F2 is extremely small, which clearly shows that.

Furthermore, the plastic region tightening method of the present invention does not directly detect the friction coefficient μ and yield axis 1 of the screw, which are generally difficult to detect, but simply detects the screw tightening torque T and tightening rotation. Since it only detects the angle θ, it can be easily implemented.

(Effects of the Invention) As described above, according to the method for tightening a screw in the plastic region of the present invention, the torque rate that is proportional to the friction coefficient of the screw is determined, and the tightening in the plastic region of the screw is performed according to this torque rate. Since the amount of attachment is adjusted to increase or decrease, the tightening axial force can be reliably made uniform regardless of fluctuations in the coefficient of friction of the screw. Furthermore, the present invention does not directly detect the friction coefficient and yield axial force of the screw, which are difficult to detect, and can be easily implemented.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention, and Fig. 1 is a flowchart of the plastic region tightening method, Fig. 2 is an overall configuration diagram of the tightening device used in the above tightening method, and Fig. 3 is a tightening method. FIG. 4 is a correlation diagram between the rotation angle and tightening torque. FIG. 4 is a correlation diagram between the tightening rotation angle and the tightening axial force. 1...Nut runner, 4...Torque transducer, 5...Angle encoder, 11.12...Torque setter, 13.14...Comparator,

Claims (1)

[Claims] (1) Find the torque rate, which is the ratio of the tightening torque increase to the tightening rotation angle increase in the elastic range during tightening of the screw, and also find the setting angle determined according to this torque rate. In the correlation diagram where the shaft is the tightening rotation angle and the vertical axis is the tightening torque, a straight line is defined that has the same slope as the torque rate line in the elastic range and is separated by the above set angle in the horizontal axis direction, and this straight line and the plasticity A plastic range tightening method for screws that stops tightening the screw at the intersection of the torque rate lines in the range.