Pulse wrench
A pulse wrench is a tool operated by a compressed air or electric motor for screwing in and loosening screws or nuts . The drive motor is not mechanically connected to the output spindle. The power transmission takes place in a pulse unit through the medium oil . A pulse unit consists of a cylinder filled with hydraulic oil and the drive spindle. The interior of the cylinder is provided with chambers which are separated from one another by webs. One or more drive plates are located on the output spindle, which move radially on the inner cylinder wall. The engine keeps the cylinder at a constant high speed during operation. If the drive plates of the output spindle hit the webs during one revolution, they seal off certain chambers and allow the previously free-flowing oil to build up. A pressure develops briefly in the drive direction. This pressure is in turn transferred to the drive plates, which pass it on to the output spindle with which they are firmly connected. Shortly after the impulse , the seal tears off again and the cylinder continues to rotate. The process of brief, impulse-like pressure build-up is repeated with each motor revolution. These impulses become stronger with increasing screwdriving time and run asymptotically towards the maximum power output.
Working principle
The functional principle of the pulse wrench uses the transmission of a high angular momentum , which the rapidly rotating cylinder generates together with the briefly pent-up oil, to a system with a low angular momentum and lower mass, which is the output spindle. The counter-impulse of the essentially static output spindle is much lower than the original angular impulse. As a result, almost no reaction force is transmitted to the screwdriving tool during the power transmission and the angular momentum is almost completely transferred to the output spindle and thus to the screw connection.
Since the impulse wrench has a very low reaction forces may as in the impact wrench (engl. Impact wrench) rather high torques without special supports can be realized. In addition, the impulse wrench does not require a reduction gear to generate force, which makes it very fast during the screwing-in process. In contrast to impact wrenches, the impulse wrench does not have any mechanically striking parts. The impulses from the rapidly rotating cylinder to the output spindle are transmitted via the medium oil. The oil dampens the increase in the impulse and makes the impulse wrench both quieter and less vibration than the mechanical impact wrench.
Another advantage that results from the type of power transmission is the better control of the output torque or the power. Since the generated pulse peaks asymptotically approach a maximum power, the output torque no longer increases from a certain point. In addition, the oil pressure in the pulse unit can even be used for torque control e.g. B. can be used in the form of a shutdown of the screwdriver. A pulse peak triggers a mechanism via a valve, which interrupts the compressed air or power supply of the tool.
Impulse wrenches are offered in a wide variety of designs, with most combinations of drive, design and power control being on the market.
- Driven by a compressed air or electric motor
- Supply: compressed air, mains operation, battery
- Designs: pistol, straight or angle screwdriver
- Non-switching tools with adjustable maximum power output
- Mechanically shut-off tools via adjustable oil pressure, reaction torque or time-out
- Controlled tools with built-in torque transducers . The shutdown takes place via the measured torque value by a control.
- Controlled tools with built-in torque and angle sensors . The switch-off takes place either via the measured torque or angle of rotation value.
Use of pulse wrenches
Impulse wrenches are mainly used today in industrial assembly . Here they have replaced the previously common impact wrenches, which are often only used at very high torques from 800 Nm or in service areas. Impulse wrenches are generally used in all possible assembly processes, whether in pre-assembly or final assembly. In automobile production , pulse wrenches are traditionally preferred primarily by Japanese manufacturers, as the focus here is on fast production. European and American manufacturers, on the other hand, mostly use screwdrivers, which then often have to be supported because of the reaction force. With the most recently available controlled impulse screwdrivers, interest has risen again here too. Last but not least, the new approach to measuring pulse wrenches (see below) has made this type of screwdriving tool attractive again for more demanding screw connections.
History of the impulse screwdriver
The invention of the pulse screwdriver dates back to the early 1960s. Up until then and afterwards, impact wrenches were predominantly used in the industrial assembly of screw connections. These allowed the quick assembly of a screw with a low reaction force for the user. This enabled high torques to be applied with comparatively small and handy screwdrivers. The disadvantages of the impact wrench are its high noise level, strong vibrations and very little control of the torque applied.
The invention, which Donald K. Skoog had submitted in 1964 under the title "Fluid impulse torque tool" for the Ingersoll Rand Co. (No. 3116617), should combine the advantages of constant rotating screwdrivers with those of impact wrenches. Torque control, speed and ergonomics in the form of unnecessary torque support were the main goals of the new invention.
A short time after this patent, Thor Power Tool Co. filed another in 1965 under the title "Impulse tool" (No. 3214940 and 3214941), which should go beyond Skoog's patent. The goal was to manufacture the pulse unit more easily and to be able to control the torque better. Thor subsequently launched the first commercial pulse wrench under the name "Impulsator". However, this could not establish itself on the market at that time and so the invention was forgotten again.
It wasn't until the late 1970s that the Japanese manufacturer URYU Seisaku Ltd. the idea of the impulse screwdriver again. The Japanese automobile manufacturers were looking for quieter, ergonomically better screwdriving tools. The question of torque control also became more important in order to be able to manufacture higher quality products for the world market. These requirements could not be met with the impact wrenches that were mainly in use until then.
In 1978 URYU launched its first impulse wrench. These U-series screwdrivers had an impulse unit with only one drive plate. So they initially followed the structure of American inventions. URYU subsequently succeeded in reliably solving the problem of tightness, on which the power transmission depends. This enabled the impulse wrench to gain an important place in the market.
With the market success, the impulse wrench was further developed. The impulse units became more efficient in 1984 with two drive plates. Models with torque cut-off were developed early on, but initially with a time cut-off in 1982. Models with higher torques of up to just over 800 Nm were developed up to 1988, thus defining the performance range of pulse screwdrivers to this day. In 1989, the use of a double-chamber air motor allowed smaller designs. In the same year, the oil pressure generated in the pulse unit was used to trigger the shutdown. Other manufacturers used the recoil force of the pulse unit to trigger the shutdown via a mechanism. In Japan in particular, shortly after the shutdown models in 1985, impulse wrenches were developed that had a built-in torque transducer . The switch-off takes place here via a solenoid valve that is operated via an external control. The time of switch-off is given by the measured value of the sensor. With these controlled impulse screwdrivers, the control can evaluate various key figures and thus recognize and output errors. A tool can also switch off at different torques, which extends its area of application.
The first electrically operated pulse wrenches came on the market in 1992 as cordless tools. When Japanese automobile manufacturers, who use pulse wrenches in large numbers, demanded more energy-efficient, even quieter screwdrivers, URYU developed pulse wrenches with wired electric drive in 2000. These are available in controlled and non-controlled versions.
The last stage of development for the time being are controlled impulse wrenches, which have built in a torque transducer as well as a rotation angle transducer . This means that the angle of rotation can also be monitored when screwing or the screwdriver can be controlled accordingly.
Impulse wrenches are now offered by various manufacturers. In the last few decades they have secured a firm place in the screwdriving tool market since they were reissued and are the preferred assembly tool in some areas - especially with higher torques. The problem of how their performance can be traceably measured has been adequately resolved with the appearance of the VDI / VDE guideline 2649 in 2011. This makes pulse wrenches interesting for those users who need to document the performance of their screwdriving tools in a traceable measuring chain. Up until now, that was the domain of constantly rotating tools such as screwdrivers .
Measurement of pulse screwdrivers for torque
Basic considerations
The measure for assessing the performance of a screwdriver in industrial use is the torque. Specifications for a screw connection are mainly made in the torque, since this variable can be determined non-destructively and relatively easily outside of the screw connection . It can even be measured during the screwing process. In addition, it can be traced back to the national standard via a measurement chain , which fulfills one of the basic requirements of measurement technology.
The torque is only an auxiliary variable in screw technology. The aim of a screw connection is not to generate torque, but to generate a clamping force or preload force between the components. However, clear conclusions cannot be drawn from an applied torque on a clamping force, since the implementation depends in particular on fluctuating coefficients of friction , but also on many other factors.
Every measuring system is statically calibrated. Calibration means that it is compared with the physical quantities or standards that it is supposed to measure. With a torque transducer , this is a combination of length and force . It is mainly implemented using a measuring beam with attached weights, in which the torque transducer is subjected to torsion . During calibration, the measurement uncertainty of the transducer or of the entire measurement chain is determined by traceably comparing it with a measurement system that is several orders of magnitude more accurate.
During a screwing process, torque is dynamically applied to the screw. If a screwdriver turns slowly and constantly, one can assume a quasi-static system. This means that a statically calibrated torque transducer , which is connected upstream of the tool, delivers results in which the measurement uncertainty is hardly increased by the dynamics. However, if the torque is generated quickly, these assumptions no longer apply. Natural frequencies of the measuring system, mechanical vibrations and problems with signal transmission in z. B. rotating measuring shafts have a much stronger influence on the measurement result and falsify it. With a quasi-static system the measurement uncertainty is almost known, with an extremely dynamic system it is no longer known. The measuring system no longer necessarily shows what it was calibrated to.
In practice, electrical interference is usually reduced by special circuits or filters. It is true that the dynamic torque curve of a pulse wrench can also be recorded with common high-resolution measuring technology, but the interaction of the systems leads to individual limits that make it impossible to compare the results. Interference and natural frequencies of the measuring system must be filtered without deleting measured values. Since every impulse wrench in principle has its own impulse characteristic, the determination of the suitable filtering is a complex matter and can only be carried out empirically by comparative measurements. This means that different torque measuring systems are not comparable when used with pulse wrenches and therefore do not meet an important basic requirement of measuring technology. In practice, it happens that different torque measuring systems for pulse wrenches sometimes show results that differ greatly from one another, even though they were originally all correctly (statically) calibrated.
Comparative measurement of pulse wrenches according to VDI 2649
The VDI / VDE guideline 2649 has been regulating the procedure and the basic conditions for a test setup for comparative measurement of pulse wrenches since 2011 . This guideline builds on the preliminary work of an ISO working group, which, however, only issued a technical specification as a further basis for discussion (ISO TS 17104).
Since the measurement technology has too great an influence on the result in the direct power measurement of pulse wrenches for torque, the preload force is used here as a measured variable. This means that the basic conditions such as traceability and reproducibility can also be met. The measurement of the preload force frees the assessment largely from dynamic influences, which make the torque measurement of pulse wrenches so problematic. In order to obtain clear conclusions about a given torque, the measuring system contains not only the preload force measuring cell but also the screw simulation, since its characteristics are an essential part of the assessment. The error in the measurement setup can only be determined if the screw connection has constant friction conditions.
The constancy is assessed by means of a comparative measurement. Torque is introduced into the test screw connection via a slowly running, very precise screw spindle. The pre-tensioning force generated is measured for each screw connection . The ratio of torque and pre-tensioning force results in a K-factor, which, determined over a series of measurements , may only have a certain deviation . This means that we know the error that the screw connection generates when measuring the preload force with the pulse wrench. With the determined average K-factor, the measured values are converted back into torque when measuring the impulse wrench for preload force. The assessment of the impulse wrench is ultimately based on the calculated torque values. To ensure that the conditions in the screw connection have not changed, the first series of measurements is repeated after the measurement of the impulse screwdriver and a new average K-factor is determined. This itself must not exceed the specified deviation and should only deviate from the K-factor of the first series of measurements within a certain limit.
Measurement of pulse screwdrivers in practice
The VDI / VDE guideline 2649 specifies and describes a measuring process, how impulse wrenches can be assessed under the same framework conditions and thus also compared with one another. This allows you to differentiate between exact and imprecise impulse screwdrivers. However, the torque calculated during the measurements is only valid in the given measurement setup and cannot be transferred to any screwdriving application. Therefore, the power output of pulse screwdrivers must always be checked and counter-measured in a current screw connection - i.e. on the workpiece. In most cases this is done by measuring the torque afterwards. In the ideal case, a correlation can be drawn between a screw connection in production and the screw connection on the neutral test bench in accordance with VDI / VDE 2649, which makes setting and verification easier. On closer inspection, of course, this also applies to screwdrivers if they are tested on a reference, i.e. not on a screw joint. Measurements of impulse wrenches according to the VDI / VDE 2649 guideline show the performance and the true repeatability of modern impulse wrenches and put an end to the confusion that had determined the assessment of torque measurements that were customary up to now.
Individual evidence
- ↑ Patent specification "Fluid impulse torque tool" (PDF file)
- ↑ Patent specification "Impulse tool" (PDF file)
- ↑ a b c VDI guideline: VDI / VDE 2649