Tools – Wrench – screwdriver – or driver therefor – Machine
Reexamination Certificate
1999-12-09
2001-12-25
Smith, James G. (Department: 3723)
Tools
Wrench, screwdriver, or driver therefor
Machine
C081S429000
Reexamination Certificate
active
06332378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process and device for producing screw connections and can be used preferably in assembly processes in industrial production, workplace operation and in monitoring in plant engineering where there are high safety requirements.
2. The Prior Art
It is known from the prior art that in industrial manufacturing electronically controlled screw systems are being used more and more frequently. These comprise a drive unit and a tool spindle with a screw tool which screws in a screw with a predetermined screwing torque in order to press together the components to be joined together with a preset force. This force is designated as prestress force and originates from the change in length of the screw shank, designated hereinafter as elongation. In many applications it is necessary to adjust the prestress force to a predetermined value as precisely as possible. For this, it is required to determine the prestress force. It is known from the prior art to determine the prestress force indirectly by means of measuring the screwing torque during screwing. It is mainly torque or angle of rotation measuring systems integrated into the screw system which are utilised for making a screw connection. When a pre-adjusted torque is attained in screwing in the screw, a signal emitted by the torque measuring system is used for switching off the drive unit, or the screw is turned further about a preset angle of rotation, determined by the angle of rotation measuring system.
The following problems arise with indirect determination of the prestress force by torque measurement: as a screw is being tightened, a small portion only of the applied torque for the elongation of the screw, that is, for the production of prestress force, has an effect. The majority of the torque is absorbed by the friction on the screw thread and on the screw head. If the friction conditions are changed, these changes have a strong effect on the prestress force. Therefore, the abovedescribed devices cannot guarantee that narrow tolerances of prestress force are adhered to.
In order to overcome these difficulties, the prestress force has to be measured directly. Different processes and devices have been put forward for this purpose. A measuring ring in the form of a plain washer is known to be arranged underneath the screw head. This measuring ring is a sensor which gives off an electrical signal with the action of force. If the screw is tightened, the screw head presses on the measuring ring, and the prestress force can be measured directly. This process is very cost-intensive, since the measuring ring remains under the screw head following tightening of the screw connection. This process is accordingly restricted to special applications such as, for example, aeronautics or nuclear energy technology. Ongoing long-term monitoring of the contact force with measuring rings is only partially possible, as the latter comprise an uncontrollable null drift.
DE 4017726 describes a fastening screw having a shank provided at least partially with a thread and an actuating end on which a head, a bolt stop or the like, whereby a first end surface is formed on the actuating end of the fastening screw and a second end surface is formed on the free end, and measuring surfaces for ultrasound measuring are provided on both end surfaces, which extend over only a portion of the end surfaces and are arranged offset axially to the end surfaces in terms of an elevation and/or a depression. In this device the change in length of the screw is measured by ultrasound. By means of methods known to the specialist the prestress force is determined from the characteristic values of the materials and the geometric dimensions of the screw.
DE 19507391 concerns a screwing device for ultrasound-controlled tightening of screw connections.
Still another realization of ultrasonic measurement is shown in DE 4025430 concerning a torque controlled screwing head having an electroacoustic sensor to measure distortion.
Determining the change in length of the screw by using ultrasound does, however, have drawbacks. To detect the change in length with precision, the ultrasound has to be introduced into the bolt or screw in a defined manner. The technical problems to be solved in this process are considerable.
In particular, the bolt or screw to be used has to be provided with a special design of a ultrasound-initialising surface and a reflection surface. These surfaces need to be manufactured with high precision within close tolerances. Such requirements cannot be met by mass-production like that of standard screws. Thus, manufacturing costs for special screws to be used in processes comprising the measurement of prestress forces by ultrasound are considerable.
The use of laser beams for monitoring forces effective in specially designed bolts is disclosed in SU 1687423. This known monitoring does not fit to the production of screw connections under control of prestress forces.
DE 26045510 describes a process for determining the binding power by means of a bore reaching from the screw head axially into the screw shank and by means of a metering gauge inserted into the bore measuring the expansion of the screw shank. The drawback to this process is that contacting mechanical measuring for serial application is error-prone and inaccurate.
A similar process to this is described in EP 0182185, comprising a lock screw with a pressed-in pin which moves as the screw is being tightened and whose movement is measured mechanically.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the prevailing problems of the prior art of producing screw connections by means of screws or bolts under the effect of predetermined initial tension forces, in particular to improve the quality of such screw connections in relation to technological and economical expenditures therefore.
In order to achieve this object, according to the present invention, a new process and a new device for producing a screw connection are provided.
The present invention thus consists in a process for producing a screw connection comprising two parts for instance in form of sheets, pressed together by means a screw having a screw head and a screw shaft or shank which is at least partially threaded, and a longitudinal hole preferably in form of a pocket bore extending from an open end on an outer surface of said screw head to a closed end within the screw shaft, the screw being screwed and further tightened by means of a screw driver producing a rotational movement so far that a predetermined prestress force is effected within the screw shaft which is lengthened under the tension of said prestress force, the process comprising the following process steps:
The screw is set to the parts to be connected by means of the screw driver, and it is positioned by an optical distance measuring device at the open end of said longitudinal hole on the outer surface of the screw head so that the optical distance measuring device can emit a measuring light beam through the longitudinal hole to its closed end. The optical distance measuring device can capture the light beam when reflected on a reflecting surface provided on the closed end, and can process interference of emitted and captured light for producing data of the distance between the optical distance measurement device and said reflecting surface.
The length of said longitudinal hole is measured by measuring the distance between the optical distance measuring device and the closed end of said hole.
The data of the length of the longitudinal hole is recorded first when screwing of the screw by means of the screw driver is started but not yet advanced to a tightening of the screw so far as to produce a significant tension force within the screw shaft.
The process continues to screw in and further tightening of the screw, and to measure the length of said longitudinal hole, and to record data received by continued measurement of the length of the longitudinal hole.
The record
Jacobson & Holman PLLC
Smith James G.
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