DD147642A1 - METHOD AND DEVICE FOR INDIRECTLY MEASURING THE SCREW TENSION - Google Patents

Abstract

The invention relates to the indirect measurement of the screw bias with the ultrasonic pulse-echo method when connecting machine elements. The aim is a fast and precise determination of the preload. For this purpose, a method and an apparatus are to be provided in which the measuring process takes place indirectly and while avoiding mechanical means. The bias voltage is calculated by the relative Laengenaenderung according to the Hooke's law, the change in length is determined from the transit time difference of an ultrasonic pulse with computer and compared with the change in length of a test screw with known bias voltage Assembly of internal combustion engines.

Description

Method and device for the indirect measurement of clean-up bias

The invention relates to the indirect measurement of the screw bias with the ultrasonic pulse-echo method when connecting machine elements! -!

Characteristic of the known technical solutions

The Schraubenvorspannung, z, B, in cylinder head screw, is usually determined directly by measuring the torque it by mechanical means. However, one must accept a range of Hh 25 % , which depends mainly on the dependence of the friction coefficient of external conditions (rust, lubrication, etc.) on the surface of the screw. This must be regarded as unsatisfactory condition for mass production.

.Ziel. "The., ^ ^ F ^ ^u . ⁿ / j.

To address this problem, the invention has the object to develop a method and a device for faster and much more accurate determination of the Scliraubvorspamiung.

Explanation of the essence of the invention

The invention has for its object to provide a Verfahren'und apparatus for indirect measurement of the screw bias, to provide with the ultrasonic pulse-echo method, in which the measuring process is carried out indirectly and avoiding mechanical means.

2 1 760 1 - ² "11.3.1980

WP B 25 B / 217 601 56 680/26

According to the invention, this object is achieved in that the length of a test screw before and after the application of a certain bias is measured by a tractor · The lengths correspond to a particular duration of the ultrasonic pulses «From the difference of the lengths results in the relative change in length of the screw , The preload force of the mounting screw, which is the same type as the test screw, is increased by tightening until the relative change in length, indicated by the duration of ultrasonic pulses, is identical to that of the test screw. To perform the procedure, the ultrasonic device used is calibrated to the speed of sound of the material in the unloaded screw.

By this method, it became possible to narrow the deviations of the actual bias from the desired bias to a value of ± 5% instead of ± 25 % according to the conventional methods. This is possible by releasing the propagation of uncertainty factors such as the friction coefficient, which fluctuates greatly depending on the surface condition of the screw. The dependence of the method according to the invention on the speed of sound of the screw material is determined by calibrating the ultrasonic device to the speed of sound of the screw bypassed unloaded state · Another advantage of the method is the use of normal Vollschaftschrauben instead of expensive expansion screws, as you are on the achieved preload in the picture and you need therefore fear no loosening of the screws by alternating load.

A device for carrying out the Verfalirens is characterized by the fact that in an inserted between a screw head and 'a ratchet nut and this concentrically an ultrasonic probe is installed, that this Ultraschallprüfkopf via a spring with the interposition of a Kopp-

-3- 11.3.1930

VT / P B 25 B / 217 601 56 680/26

Medium is pressed against the screw head, and that this probe is connected via an ultrasound device to a computer.

By such a device can be reliably and without additional time screws tighten so that they have a predetermined bias. This is achieved by measuring the propagation time differences of sound pulses from the test head, which are fed to a computer after prior amplification in an ultrasound machine, the computer resulting in the elongation of the screw resulting in the preload. It takes the operator so only before putting the nut some coupling medium painted on the screw head and the screw to be tightened until the computer indicated a certain elongation of the screw

Particularly advantageous is the design of the device, when in the arranged between the screw head and the ratchet nut screw and concentric to an 'electrodynamic probe is installed, wherein between the probe and the screw head, an air gap is provided. The probe is connected to the computer via an ultrasound device.

This type of device is independent of a coupling medium, but offers the same advantages as the previously described device. However, as an additional advantage, it offers applicability at higher temperatures (over 330 K), since instead of the heat-sensitive coupling medium, only a small air gap is required for coupling.

-4- 11.3.1930

WP B 25 B / 217 601 56 680/26

Execution example

The invention will be explained in more detail below using an exemplary embodiment. In the accompanying drawing show:

1 shows the section through an attached to a screw head nut with test head;

Fig. 2: a calibration diagram for determining the Schraubenvorspannkraft.

The inventive method consists of the following successive process steps to determine the length of a Prüfscheibe by means of ultrasonic pulse, then applying a fixed bias on the test screw by means of a tractor, this results in a relative change in length of the screw _{6 is} followed by measuring the length of the preloaded test screw by means of ultrasonic pulse » The difference between the ultrasonic pulses corresponds to the change in length. During assembly, the preload force of the mounting screw, which is the same type as the test screw, is increased until the duration of the ultrasonic pulses indicates a relative change in length which is identical to the relative change in length of the test screw.

To carry out the process, the ultrasonic device used must be calibrated to the speed of sound of the material in the unloaded screw ©

The method is based on the fact that when a screw is preloaded there is an elongation _% which is proportional to the pretension according to Hook's law. To measure this strain "one uses the ultrasound, which is introduced at the screw head, passes through the screw and is reflected at its end

21760 1

VZP B 25 B / 217 601 56 680/26

Length of the screw are determined, it should be noted that the speed of sound YQ * is a puncture of the voltage. If V is the speed of sound in the unloaded state, then Vg * becomes:

V ₀ * = V ₀ (1 + a ff) (1)

Here a is also a material constant. If now a screw of length 1 · is stretched by the amount ff by an amount, then a transit time difference Δt results:

2 (1 + Al) ^{2 '1} O

At = 2 - - (o).

V ₀ (1 + a · ο) ο

With Hooke's Law, which is transformed:

(2.1),

results with a. ö 4. ^. 1 and Ό ^ ^ E the earning formula:

f & ls AJL ·. (i) a) E) (3) o

The JYj material constants ν, Ε occurring in this formula are known or easily determinable, but not a ₀ The causes for the change in the speed of sound, which depends on the Ε modulus, the density, and the Poisson's transverse contraction coefficient, are difficult to grasp, in order to obtain a to calculate. Therefore, it is better to determine the direct relationship between & t and Δ 1 by experiment. With regard to the usual technique, it is not the t shift that is observed on the screen of the ultrasound machine

1 760 1 " ⁶ - 11.3.1980

WP B 25 B / 217 601 56 680/26

A s of the echo with respect to the pulse that was sent out ο This shift Δ s is proportional to A t.

s ^ l ^ -Ai. b (1 - ae) (4). ο

The factor b is a device constant of the ultrasound device and is the scanning speed of the electron beam on the lines of the screen,

The factor 2 appearing in the formulas 2, 3, 4 is usually omitted, since the devices used for ultrasonic testing are set up so that the factor 2 is already incorporated into the device as a result of the double path which the sound has to cover in reflection β For a better understanding of the actual occurring quantities factor 2 was incorporated into the formula.

In the apparatus for carrying out the. Method, a screw 3 is tightened by a ratchet 1 with the interposition of a nut 2 inside the nut 2 is a test head 4 (in the illustrated case, an ultrasonic probe 5), which by means of a spring 6 to a screw head 7 In the case of the ultrasonic test head, the screw head has been coated with a coupling medium prior to fitting the screw nut. The test head 4 is connected to a computer via an ultrasonic device 8, which takes over the control of the screw nut during series production and prevents further tightening of the screw 3 when the stored value is reached β

For simpler cases, an ultrasound machine will suffice to indicate the elongation of the screw 3, and by comparing it to the leaching of a screw of the same type in a tractor, one knows when the desired preload force has been reached. "

-7- 11.3.1980

WP B 25 B / 217 601 56 680/26

In place of the ultrasonic probe 5, an electrodynamic probe operating according to the magnetostrictive principle may also be used; this is not coupled with coupling medium, but via an air gap; the rest of the system remains unchanged

Fig. 2 shows a calibration diagram in which the change in length ^ 1 of the test screw is plotted against the force P applied by a tractor. The calibration is carried out so that it is a test bolt, which resembles the screws to be installed in series and clamped and loaded with the same clamping length as in the installed state in a tractor. Now the length change Δ 1 of the screw is determined as a puncture of the force P with an ultrasonic test head and an ultrasonic tester. The screw of the same type provided for installation thus only needs to be stretched when tightened by the length .DELTA..sub.1 that can be taken from the calibration diagram and measured with the device according to the invention to get to a certain preload force.

Claims (3)

-8- 11.3.1980 WP B 25 B / 217 601 56 680/26 Invention sanspruc h · Method for the indirect measurement of the screw bias with the ultrasonic pulse-echo method, characterized in that the length of a test screw before and after the application of a certain bias by means of a tractor by the duration of an ultrasonic pulse is determined that the difference in both lengths Length change is determined, and that the biasing force of a certain screw for installation (3) of the same type v / ie the test screw is increased by tightening until the also determined with ultrasonic pulse from the running relative change in length with the determined in the tractor relative change in length Test screw is indentisch, wherein an ultrasonic device used for this purpose (8) is calibrated to the speed of sound of the material in the unloaded screw _c 2 »device for carrying out the method according to item 1, characterized in that in a between a screw head (7) and a ratchet (1) turned screw nut (2) and to this concentric an ultrasonic probe (5) is installed, that this Ultraschallprüfkopf ( 5) via a spring (6) with the interposition of a coupling medium to the screw head (7) can be pressed, and that this probe (4) via an ultrasonic device (8) with a computer (9) is connected « - ⁹ ~ 11.3.19.80 WP B 25 B / 217 601 56 680/26 3. Apparatus for carrying out the method according to item 1, characterized in that in the between the screw head (7) and the ratchet (1) turned screw nut (2) and to this concentric instead of the Ultraschallprüfkopfes (5) an electrodynamic probe is installed , and that between this probe and the screw head (7) an air gap is provided, and that this probe via the ultrasound device (8) to the computer (9) is connected » For this 1 page drawing