GB2090976A

GB2090976A - Apparatus for measuring bolt prestressing

Info

Publication number: GB2090976A
Application number: GB8201013A
Authority: GB
Original assignee: MAN Maschinenfabrik Augsburg Nuernberg AG
Current assignee: MAN AG
Priority date: 1981-01-14
Filing date: 1982-01-14
Publication date: 1982-07-21
Also published as: FR2497950A1; IT8219026A0; JPS57137830A; IT1149417B; DD207039A1; DE3100842A1

Abstract

A socket (2) for prestressing a bolt (14) contains a probe (3) electrically connected to an ultrasonic unit (5) and a computer (6) for comparing the bolt length with that of a standard bolt. In order to avoid the use of cable connections and sliprings and to facilitate production-line prestressing of bolts, the probe (3) is electrically coupled to the ultrasonic unit (5) without contact via a device (7) concentrically surrounding the socket (2). The socket (2) is driven by a drive spindle (12). The device may comprise an annular rotor coil (9) connected to the socket (2) and a stator coil (10) spaced therefrom by bearings (11), pulses being transmitted inductively. Alternatively, the device comprises a spaced rotor electrode (9a) and a stator electrode (10a), pulses being transmitted capacitatively. <IMAGE>

Description

SPECIFICATION Apparatus for measuring bolt prestressing The present invention relates to apparatus for measuring prestressing or preloading of bolts using ultrasonic pulse echoes.

German Patent Specification No. 28 53 988 discloses a method for indirectly measuring the prestressing of bolts where an ultrasonic pulse is used to measure the length of the bolt before and after tightening. With the aid of a computer, the force applied in prestressing the bolt is determined from the change in length measured. To determine the operand required, a bolt of the same type is prestressed in a testing machine and the change in length of the bolt is determined for the load set on the machine. For the tightening of the bolt, the probe is installed in a socket to transmit the ultrasonic pulses reflected by the end of the screw via a lead to an ultrasonic unit connected to a computer. By installing the probe in the socket a device is obtained for continuously measuring the prestressing force applied during tightening without removing the probe.However, this device is unsuitable for applying the prestress in batch and line production. It is only suitable for tightening bolts manually.

An object of the present application is to provide apparatus for measuring the bolt prestress which is suitable for tightening bolts in batch and line production with minor deviation from the desired value of the bolt prestress without interference from a cable lead.

The invention provides apparatus for measuring prestressing or preloading of a bolt using ultrasonic pulse echoes comprising a probe contained in a socket to be placed in use on a bolt head, wherein the probe is electrically coupled to an ultrasonic unit and a computer without contact via a device concentrically surrounding the socket, the end of the socket remote from the bolt head in use being coupled in a torque-locking manner to a drive spindle.

Contactless coupling enables transmission of ultrasonic pulses from the ultrasonic unit into the bolt shaft and returning of the reflected pulses to the ultrasonic unit and the computer to be effected without the signals being affected by variable transition resistances, such as slip rings and without the necessity of a cable lead which is unsuitable for batch and line production.

In a preferred embodiment of the invention the device comprises a rotor coil and a stator coil concentrically surrounding the socket with an air gap therebetween, the rotor coil being mechanically connected to the socket so as to transmit rotation and being electrically coupled with the probe, the stator coil being supported so as to be rotatable, but axially non-slidable on the socket via a bearing and being coupled electrically by a lead to the ultrasonic unit and the computer.

The inductive transmission of the pulses eliminates measuring inaccuracies such as are unavoidable in the case of slip rings by variable transition resistances. Because the use of inductive transmission does not require a cable as a permanent lead, there is full freedom in applying the measuring apparatus. In automated production, the socket can be easily placed on the bolts to remain there until the tightening operation has been completed on the desired prestress having been attained. There is the added advantage that the coupling of the probe to the bolt head is always the same and is not changed as a result of the socket being placed on for measuring and subsequently removed.

In an alternative preferred embodiment of the invention, the device comprises a rotor electrode and a stator electrode concentrically surrounding the socket with an air gap therebetween the rotor electrode being connected for rotation to, but electrically insulated from, the socket, the rotor electrode being electrically connected with the probe, the stator electrode being supported so as to be rotatable but axially non-slidable on the socket via a bearing and being coupled electrically by a lead to the ultrasonic unit and the computer.

As with inductive coupling capacitative coupling also permits a permanent interfering cable lead to be dispensed with. Consequently, unlimited freedom in use for batch and line production is obtained.

Preferably, a spacer ring is placed between the probe and the bolt head, this spacer ring being maintained centrally by a drilled hole relative to the probe.

Placing a circular spacer between the probe and the bolt head enables the gap between the two to be maintained constant and, consequently, also the coupling effect. This applies both to the coupling of a piezo-electric ultrasonic crystal by a coupler and to the application of probes which rely on the electro-dynamic principle via an air gap.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows in part section a socket with an integral probe and inductive coupling placed on the head of a bolt, Figure 2 shows in part section a socket with an integral probe and capacitative coupling, and Figure 3 shows on an enlarged scale a detail of a probe and a spacer.

In Figure 1, a socket 2 is shown in place on a bolthead 1. A probe 3 is fitted directly in the socket 2 in such a manner that the probe 3 is pressed constantly and centrally onto the bolthead 1 by means of a spring 4. A device 7 allows ultrasonic pulses to be transmitted from an ultrasonic unit 5 to the probe 3 and, after reflection by the probe 3, to be returned to a computer 6. In the embodiment illustrated, the device 7 operates by induction and permits a contactless transmission of pulses from the probe 3 via the line 8 to the ultrasonic unit 5 and the computer 6 for evaluation.

The inductively-operating device 7 comprises a rotor coil 9, which is mounted concentrically on the socket 2 and in a manner so as to transmit rotation, and stator coil 10 concentrically surrounding the rotor coil 9 with an air gap therebetween. The stator coil 10 is mounted on a bearing 11 carried by the socket 2 so as to maintain the air gap constant. The socket 2 is driven by a drive spindle 12, which is connected in a torque-locking manner to the upper end of the socket. In order to obtain exactly defined coupling values, a circular spacer ring 1 3 is placed between the probe 3 and the bolthead 1 so as to ensure that a constant coupling gap is maintained between the probe 3 and the bolt head 1.

During actual measuring, while a bolt 14 is being tightened, ultrasonic pulses are conducted from the ultrasonic unit 5 via the line 8 to the stator coil 10 and then transmitted indirectly across the air gap to the rotor coil 9 and to the probe 3 fixedly connected thereto. After passing through the bolt 14, the ultrasonic pulses are reflected at the free end of the bolt and are returned along the same path to the ultrasonic unit 5, when they are delivered to the computer 6 where the elongation of the bolt 14, and thus its prestressing or preloading, is determined.

As an alternative to the inductive coupling of Figure 1, the contactless device 7 of Figure 2 may use capacitance coupling. The rotor and stator coils 9, 10 are merely replaced by rot and stator electrodes 9a, 1 Oa, respectively. In the case of capacitative coupling, care must be taken, by use of special insulation, to prevent a conductive connection between the rotor and stator electrodes 9a, 1 Oa via the bearing 11.

A detail of the probe 3 and the bolthead 1 with a predetermined coupling gap 1 5 therebetween is shown in Figure 3. The coupling gap 15 in the example illustrated is maintained by a casing 1 7 of the probe 3 which tapers into a knife edge 1 6 rather than by the spacer 1 3. The probe 3 contains a piezo-electric crystal or, alternatively, an electrodynamic sensing member.

Claims

1. Apparatus for measuring prestressing or preloading of a bolt using ultrasonic pulse echoes comprising a probe contained in a socket to be placed in use on a bolt head, wherein the probe is electrically coupled to an ultrasonic unit and a computer without contact via a device concentrically surrounding the socket, the end of the socket remote from the bolt head in use being coupled in a torque-locking manner to a drive spindle.

2. Apparatus as claimed in Claim 1, wherein the device comprises a rotor coil and a stator coil concentrically surrounding the socket with an air gap therebetween, the rotor coil being mechanically connected to the socket so as to transmit rotation and being electrically coupled with the probe, the stator coil being supported so as to be rotatable, but axially non-slidable on the socket via a bearing and being coupled electrically by a lead to the ultrasonic unit and the computer.

3. Apparatus as claimed in Claim 1, wherein the device comprises a rotor electrode and a stator electrode concentrically surrounding the socket with an air gap therebetween the rotor electrode being connected for rotation to, but electrically insulated from the socket, the rotor electrode being electrically connected with the probe, the stator electrode being supported so as to be rotatable but axially non-slidable on the socket via a bearing and being coupled electrically by a lead to the ultrasonic unit and the computer.

4. Apparatus as claimed in any one of Claims 1 to 3, wherein the probe carries a spacer for contacting the bolthead in use, the spacer being maintained centrally relative to the probe by a hole.

5. Apparatus as claimed in any one of Claims 1 to 3, wherein the probe has a casing having a tapered knife-edge end for contact in use with a bolthead for maintaining a coupling gap between the probe and bolthead.

6. Apparatus for measuring prestressing or preloading of a bolt using ultrasonic pulse echoes substantially as herein described with reference to any one of the embodiments shown in the accompanying drawings.