impact driver

A pulse is a screwdriver -operated with a pneumatic or electric power tool to screw and loosening of screws or nuts. The drive motor is mechanically connected to the output spindle. The power transmission takes place in a unit pulse through the medium of oil. A pulse unit comprises a cylinder filled with hydraulic oil, and the output spindle. The interior of the cylinder is provided with chambers which are separated from one another by webs. On the output spindle are one or more drive plates, which move radially on the inner cylinder wall. The cylinder is kept at a constant high speed during operation of the motor. Meet now the traction drives the output shaft on the webs, they seal off certain chambers and can accumulate the previously free-flowing oil during a revolution. It developed in the short term, a pressure in the drive direction. This pressure is transmitted to turn on the traction plates which pass it on to the output spindle with which they are firmly connected. Shortly after the pulse, the seal breaks off again and the cylinder continues to rotate. The process of short-term, pulse-like pressure build-up is repeated with each revolution of the motor. These pulses are increasing with tightening time getting stronger and run asymptotically towards the maximum power output.

Principle of operation

The operating principle of the impact screwdriver uses the transmission of a high speed pulse, which the rapidly rotating cylinder together with the generated short-term accumulated oil to a system with a low angular momentum and lesser mass, which is the output spindle. The counter of the pulse, in principle the static output shaft is much smaller than the initial angular momentum. Wherein almost no reaction force is transmitted to the power transmission at the screwing, and the angular momentum is almost entirely in the output shaft and thus to the fitting.

Since the pulse screwdriver has a very low reaction forces quite high torques can be realized without special supports as the impact wrench ( engl. impact wrench ). In addition, the impulse wrenches for power generation requires no reduction gear, which makes him very quickly during the screwing. However, in contrast to the impulse wrenches impact wrenches has no mechanical parts against one another beating. The momentum transfer of the rapidly rotating cylinder on the output spindle via the medium of oil. The oil absorbs the rise of the pulse and makes the impulse wrenches compared to mechanical impact wrenches both low noise and less vibration.

A further advantage resulting from the nature of the power transmission, the better control of the output torque or the power. Since the spikes move asymptotically to a maximum generated power, the output torque does not increase after a certain point. In addition, the oil pressure in the pulse unit can be used even for a torque control, for example in the form of a disconnection of the screwing tool. Here, a spike through a valve causes a mechanism which interrupts the compressed-air supply or of the tool.

Impulse wrenches are available in various designs, which are now the most combinations of drive, construction and performance monitoring on the market.

Use of pulse tools

Impulse wrenches are today primarily used in industrial assembly. Here they have replaced the formerly used impact wrench, who often come only in very high torque of 800 Nm or service areas for use. Generally impulse wrenches are widely used in all kinds of assembly processes, whether. In pre-or final assembly In the automotive production impulse wrenches are traditionally favored by Japanese manufacturers, as the focus here is on a fast production. European and American manufacturers, however, use mostly Screwdrivers, which then, however, often need to be supported due to the reaction force. With the controlled pulse tools recently available but the interest is also higher here again. Last but not least, the new approach for measuring pulse tools (see below) has made this type Wrench again attractive for demanding glands.

History of the impact screwdriver

The invention of the impact screwdriver goes back to the early 1960s. Until then, and even later in the industrial assembly of bolted joints predominantly impact wrench were used. This allowed the rapid assembly of a screw with simultaneously low reaction force to the user. This could even high torques are applied with comparatively small and handy screwdrivers. The disadvantages of the impact wrench are its high noise, strong vibration and very little control of the applied torque.

The invention, which Donald K. Skoog in 1964 filed under the heading " fluid impulse torque tool" for the Ingersoll Rand Co. (No. 3116617 ), should combine the advantages of constant turning wrenches with those of impact wrenches. Torque control, speed and ergonomics are not required in the form of torque support were the main objectives of the new invention.

Shortly after this patent filed in 1965 Thor Power Tool Co., another under the title "Impulse tool" (No. 3.21494 million and 3,214,941 ) that was to go beyond Skoogs patent. The goal was to produce the pulse unit easier and be able to control the torque better. Thor subsequently launched the first commercial pulse screwdriver under the name " Impulsator " on the market. However, this could not prevail on the market at that time and so the invention was forgotten.

It was not until the late 1970s attacked the Japanese manufacturer URYU Seisaku Ltd.. the idea of the impact screwdriver on again. The Japanese carmaker searched for quieter, better ergonomic screwdriving tools. Also the question of torque control is important in order to produce higher-quality products for the world market. With the impact wrench located so far mainly in the use of these requirements could not be met.

In 1978 URYU his first impulse wrenches on the market. These screwdrivers in the U- series had a pulse unit with only one drive plate. So first they followed the development of the American inventions. URYU succeeded in the following years but the problem of tightness, on which depends the power transmission to reliably solve. Thus, the impulse wrenches could take a significant place in the market.

With the market success of the impulse wrenches has been developed. The pulse units in 1984 by two powerful drive plates. Early models were developed with Torque-off in 1982 initially but with a timeout. Until 1988 models have been developed with higher torques up to 800 Nm and therefore just over set the output range of pulse tools today. 1989 allowed the use of a dual chamber air motor smaller designs. In the same year was then, the oil pressure generated in the pulse unit is used for triggering the switch-off. Other manufacturers used for this purpose, the repulsive force of the pulse unit to trigger the shutdown of a mechanism. Especially in Japan in 1985 impulse wrenches were developed soon after the Abschaltmodellen, which had a built-in torque transducer. The shut-off is carried out via a solenoid valve which is actuated by an external control. The time of shutdown are here before the measurement of the transducer. In these controlled pulse tools, the controller can evaluate different ratios and thus detect errors and spend. Also, a tool off at different torque, bringing extended its range of use.

The first electric impulse wrenches came as cordless tools on the market in 1992. As Japanese auto manufacturers using impulse wrenches in large numbers, required more energy efficient, quieter still screwdriver, URYU developed in 2000 impulse wrenches with wired electric drive. These are available in a controlled and non -controlled version.

The most recent stage of development form controlled pulse wrenches, who also built a rotary transducer next to the torque transducer. This can also monitor the angle of rotation in the screw or the screwdriver control afterwards.

Impulse wrenches are now offered by several manufacturers. They have secured in the past decades since its reissue a firm place in the market for screwing and in some areas - especially at higher torques - the preferred installation tool. The problem of how their performance can be measured traceable is solved adequately with the appearance of the VDI / VDE guideline 2649 in 2011. This impulse wrenches are interesting for those users who need to document the performance of their screwing in a traceable measurement chain. This has been the domain of constantly rotating tools such as Screwdrivers.

Measurement of pulse tools to torque

Basic considerations

The measure of the evaluation of the performance of a driving tool in industrial applications is the torque. Requirements for a screw to be made predominantly in the torque, as this is a quantity which can be determined non-destructive and relatively easy outside the Schraubverbandes. It can even be measured during the Verschraubvorgangs. In addition, they can be returned via a measuring chain in each case up to the national standard, which fulfills one of the basic conditions of measurement technology.

The torque is in screw but only an auxiliary variable. Target of a screwing torque, but is not the production of a clamping force or biasing force between the components. But clear conclusions of an applied torque to a clamping force can not be reached because the implementation depends in particular on fluctuating friction coefficients, but also by many other factors.

Each measuring system is calibrated statically. Calibration means, it is compared with the physical variables or normal, which is to measure it. With a torque transducer, this is a combination of length and strength. It is realized mainly through a measurement beam with attached weights, in which the torque sensor is loaded by torsion. During the calibration of the measurement uncertainty of the transducer or the entire measurement chain is determined by being traceable compared with a more accurate by several factors measuring system.

With a screwing torque is brought dynamically to the screw. A Wrench Turns slow and constant, we can assume a quasi- static system. This means a statically calibrated torque transducer, which is upstream of the tool, provides results in which the measurement uncertainty is hardly increased by the dynamics. If, however, the torque generated on the fly, these assumptions are no longer valid. Natural frequencies of the measuring system and mechanical vibration problems in the signal transmission, for example, in rotating measuring waves go here a lot more to the measurement result and distort this. In a quasi- static system, the measurement uncertainty is almost known, in an extremely dynamic anymore. The measuring system is no longer necessarily that to which it was calibrated.

In practice, electrical interference are usually mitigated by special circuits or filters. Although one can also record the dynamic torque curve of a impact screwdriver with current high-resolution measurement, but it comes in the interaction of systems to our borders, which makes a comparison of the results impossible. Disturbance and natural frequencies of the measuring system must be filtered, without thereby erase readings. Since each pulse screwdriver in principle has its own pulse characteristics, the determination of the appropriate filtering is a complex matter and can only be carried out empirically by comparing measurements. This means that different torque - measurement systems in use with impulse tools but not comparable and therefore do not meet an important basic condition of metrology. In practice, it happens that different torque - measurement systems with impulse tools See part widely different results, even though they were originally calibrated all correct ( static).

Comparative measurement of pulse tools according to VDI 2649

The VDI / VDE Directive 2649 sets since 2011, the procedure and the basic conditions for a test set-up for comparative measurements of pulse tools. In this case, this policy builds on the preliminary work on an ISO working group that had but only issued as a further basis for discussion a Technical Specification ( ISO TS 17104 ).

Since the direct power measurement of pulse tools on the torque measurement technology has a large influence on the result, here the biasing force is used as a benchmark. Thus, the basic conditions such as traceability and reproducibility can be met. The measurement bias force releases the judgment largely dynamic influences, which makes the measurement of the torque impulse tools as problematic. In order to receive now clear conclusions on an output torque, involves the measurement system in addition to the biasing force measuring cell and the Schraubsimulation because their characteristics is an essential part of the assessment. Only if the screw has a constant friction conditions, the error in the measured structure can be determined.

The assessment of consistency is done by a comparative measurement. Here in the Testverschraubung torque is introduced via a low-speed, highly accurate screw. For each gland, we measured the biasing force generated. The relationship between torque and preload force is in this case a K-factor, which determines a measurement series may only have a certain deviation. This is the error that generated the screw while measuring the impact screwdriver on biasing force known. With the determined average K factor, the measured values ​​are then converted into torque at the measurement of the impulse wrench in biasing force. The assessment of the impact screwdriver ultimately takes place due calculated torque values ​​. To make sure that have not changed with respect to the screw, is after measuring the impact screwdriver again repeated the first series of measurements and calculated a new average K- factor. This must not exceed the predetermined deviation itself and should vary only within a certain limit of the K- factor of the first series of measurements.

Measurement of pulse tools in practice

The VDI / VDE Directive 2649 specifies and describes a measurement process, such as impulse wrenches evaluated under the same conditions and thus can be compared with each other. This can be distinguished from non- exact exact pulse tools. The calculated torque in the measurements but only in the given measurement setup validity and can not be transferred to any bolting. Therefore, the power output of pulse tools must always be in a current union - will be reviewed and measured against - ie on the workpiece. This is done in most cases by immigration measuring the torque. In the ideal case, in 2649 a correlation be drawn, which facilitates the adjustment and verification between a gland in the production and the screw on the neutral test according to VDI / VDE. On closer inspection, this of course also applies to mechanical screwdriver if they are tested on a reference, not the power screwdriving. But measurements of pulse tools according to VDI / VDE 2649 show the performance and the true repeatability of modern pulse tools and end the confusion which had determined the judgment at the hitherto conventional torque measurements.