Torque wrench
Torque wrench is the general name for a hand-held screwdriving tool with which a defined tightening torque can be exerted on a connecting element such as a screw or nut in order to ensure the necessary clamping force between the components to be connected, even when the maximum operating forces are applied . According to DIN EN ISO 6789, a distinction is made between indicating torque wrenches (measuring wrenches) and releasing types (click or click wrenches).
Torque wrenches are available in different sizes and designs, each covering a specific torque range. The maximum achievable tightening torque is limited, on the one hand, by the force that the user can apply and, on the other hand, by the lever length up to which a manual tool can be used (around 1.5 meters). This means that the theoretical limit is around 1500 Nm . In practice, keys with more than 1300 Nm are rarely found. Higher torques can be prepared by combination of a torque wrench with torque multipliers or hydraulic wrenches (see the hydraulic wrench ) are generated. Colloquially , torque wrenches are also known as nutcrackers , power wrenches or kilo keys .
application areas
The screw connection is a frequently used connection in mechanical engineering . Suitable assembly tools must be used to close the connecting elements. With the standardized and in practice common screwing tools, for example with open-ended, ring and socket wrenches , most screwing cases can be carried out in the manufacture and maintenance of products . The moment transferred when using hand-held tools depends on the physical condition of the user and his or her individual feelings. In addition, with larger wrench sizes, the standardized lever arm was dimensioned in such a way that it is not possible to transfer the necessary torque without tools. If the demands on a screw connection increase and the screw needs to be pretensioned with a defined force , a suitable tool is required. This applies not only to screw connections that are to be loaded with large preload forces, but also for those that may only be preloaded with low torques. The assembly of sensitive circuit boards with semiconductors from the field of mobile radio or microelectronics are an example of this. The consistent lightweight construction in the automotive industry also ensures an increase in the number of connections that have to be pre-tensioned with a precisely defined torque, as the increased use of new construction materials such as magnesium , aluminum and plastic means that the number of sensitive screw connections is steadily increasing. The lower tensile strength of these lightweight materials compared to steel materials leads to damage to the thread and thus to damage if the screw connection is overloaded .
Car owners usually associate the term torque wrench with tightening the wheel bolts of aluminum rims when changing a wheel . The automotive industry specifies the exact tightening torques to be adhered to for every vehicle model - even when using steel rims.
Another field of application is implantation medicine , where dental implants or artificial hip joints are screwed using torque wrenches. But also in the aerospace industry, screwing is done with precise torque. Torque wrenches have a permanent place in modern assembly technology and it can be assumed that the use of screw connections in lightweight components or sensitive mechanical or electronic components will continue to increase in the future.
Various concepts are used to prevent overstretching or overtightening of screw connections. On the one hand there are angle-controlled tightening methods, on the other hand there are torque- controlled methods. In the angle-controlled tightening process, the screw is tightened with a defined angle of rotation, which is done via a mechanical or electronic sensor . In the torque-controlled tightening process, the torque is measured during tightening and an action is triggered when a specified setpoint is reached .
Torque wrench types
Torque wrenches are available in various mechanical or electronic versions with different functional principles. In the group of mechanical torque wrenches, there are the so-called buckling wrenches, which break or break when the desired torque is reached and thus prevent a further introduction of a torque. The mechanical torque wrenches also include the releasing torque wrenches ( click wrenches ) and the slippers .
Cracking keys require expert handling by the user, since the initiation of the torque must be stopped by the user after the trigger signal. In the case of slippers, the release accuracy is independent of the skill of the user, as they slip through when the limit torque is reached and prevent further torque introduction. In the case of electronic torque tools, safe handling by the user is again required, since an optical or acoustic display of the tightening torque signals when the application of a torque must be stopped.
In order to ensure the screwing quality of torque tools, they require a high release accuracy when reaching the set torque with a corresponding reproducibility of the values (repeatability). This requires a clearly recognizable trigger signal and the least possible influence of differences in handling, for example through fast or slow tightening.
Triggering torque wrench
When the torque wrench is triggered, a specific setpoint or target torque is set using a scale or with the aid of a test device. As soon as the torque wrench on the drive part (force axis) reaches the preset torque, this is indicated by an audible, tangible and / or visible signal, usually by a noticeable click / crack or a colored indicator popping out.
The most common designs consist of a steel tube, the length of which is matched to the area to be covered, with the release mechanism housed in it and an adjustment device and scale connected to the handle . A reversible or reversible ratchet (also called a ratchet ) is often integrated as a tool holder . The drive then takes place via a square pin with dimensions in the inch system ( 1 ⁄ 4 , 3 ⁄ 8 , 1 ⁄ 2 , 3 ⁄ 4 , 1 or 1 1 ⁄ 2 ″), onto which various socket wrench inserts ( sockets ) can be attached . Other common tool carriers are, for example, the rectangular holder or the dovetail guide for interchangeable tool heads in different profiles such as open-end, ring, open-ring or hexagon wrenches.
In 1938 the world's first self-releasing torque wrench was registered for a patent and brought onto the market ( Saltus-Werk Max Forst , Solingen).
Indicating torque wrenches
With indicating torque wrenches, the value of the torque that is applied by the tool on the output part is indicated via a mechanical scale, dial gauge or electronic display. Many designs use massive levers with a torsion bar as a drive, the rotation of which can be read off as a measure of the torque in relation to a scale coupled to the lever.
The simplest form of this type consists of a long lever arm between the handle and the wrench head, made of a material that bends elastically when tightened. A second, smaller rod with an indicator is fixed at the head end and runs parallel to the lever arm. This second arm remains unloaded and therefore straight. A calibrated scale is attached to the end of the handle and the bending of the main arm causes movement on the indicator scale. When the desired torque is displayed, the operator stops applying the force. The key is simple, consistently accurate, and inexpensive.
The origins of the flat wrench as it is known today go back to the early 1930s when Walter Percy Chrysler developed a torque wrench for Chrysler Corporation and Micromatic Hone Corporation, which was then manufactured and sold by Cedar Rapids Engineering Company. Their regional sales manager, the young engineer and sports aviator Paul Allen Sturtevant from Elmhurst (IL), developed Chrysler's imperfect idea further, patented his own invention in 1938 and marketed it through his own PA Sturtevant Company, Addison (IL).
Electronic torque wrench
In the case of electronic (digitally indicating) torque wrenches, the measurement is made using a strain gauge on the torsion bar . The measurement signal obtained is by measuring amplifier to be displayed in the power unit (N m, lbf • ft etc.) and converted on the digital display ( display shown). Several different screw connections (screw connection data or limit values) can be stored. These programmed limit values are permanently visualized by means of LEDs or displays while the screw connections are being tightened. At the same time, this type of torque wrench can store all measurements made in an internal memory. This can be read out via an interface (e.g. RS-232 ) or printed out directly via a connected printer. A frequent application of this type of torque wrench is production-accompanying documentation or QA procedures.
Electronic torque angle wrench
The torque measurement takes place as with the electronic torque wrench; in addition, an angle is determined from a joining torque / threshold value. The angle is measured with the help of an angle sensor or electronic gyroscope . The angle measurement can be used to identify connections that have already been screwed. Statistical evaluations can be carried out using the internal measurement memory. Curves can be analyzed using software using the integrated curve memory (force-displacement diagram). This type of torque wrench can also determine the breakaway torque , further torque and final torque of a screwing process. The yield point can be displayed using a special measuring method ( yield point-controlled tightening). This torque wrench is primarily used by automobile manufacturers for documentation of torque-angle-controlled screw connections, since an angle must be applied to the connection ( screw ) to be screwed in addition to the torque specification (N m) . (e.g. 50 N m + 90 °; where 50 N m = joining torque / threshold value and + 90 ° = the angle to be further drawn from the threshold value)
The Saltus-Werk Max Forst GmbH reported in 1995 to the first electronic torque wrench with angle measurement referenzarmloser for international patenting.
Mechatronic torque wrenches
The torque measurement is carried out as with the releasing torque wrench, the torque is also measured digitally (click and final torque) as with the electronic torque wrench. So it is a combination of electronic and mechanical measurement. All measurements are transmitted and documented by radio data transmission.
Others
In addition to the N m scale based on SI units , a second scale is often attached, for example with the unit “ft.lbs” ( pound · foot ). The Anglo-American system of measures and weights is still widely used in some international technical areas (aerospace). 1 Nm corresponds to 0.73756215 lbf ft (pound-force foot), or vice versa 1 lbf ft corresponds to 1.3558179 Nm. The no longer valid unit kp · m ( kilopond meter) or kgf · m (also m · kg) is rarely found. 1 kp m corresponds to 9.80665 N m or vice versa 1 N m corresponds to 0.10197162 kgf m.
Practical orders of magnitude for workshop use start at around 0 to 6 Nm and range up to around 200 to 800 Nm.
A well-known example of its use is tightening wheel nuts or bolts on motor vehicles. It should not be done by hand as far as possible, but rather "gently but firmly" with a torque wrench. If the tightening torque is too low, the wheels can loosen; if the torque is too high, there is a risk of the screws or bolts becoming overstretched and bracing the brake parts , wheel hubs and rims. This is not immediately apparent and is therefore dangerous as the wheel can lock or come loose at high speeds. Typical tightening torques for cars are in the range between 40 and 150 N m (thread between M 10 × 1.25 and M 14 × 1.5).
Much lower torques, which must also be adhered to precisely, are required in dentistry when screwing in implants and are generated using special dental torque wrenches.
A special form of the torque wrench is the torsiometer - a very precise design with which mechanical frictional resistance, for example in steering gears, can be measured.
In addition to so-called production keys, compressed air or power tools (screwdrivers) in which a torque limiter is integrated or attached as an attachment are preferably used for fixed torque values in continuously repeated work steps such as in large-scale production, on cycle lines, in vehicle construction, etc.
Torque tools are subject to the standard EN ISO 6789: 2017-07. This recommends a recalibration interval after a period of use of 12 months or about 5000 load changes . This is necessary because the spring and the limiting mechanism are subject to wear.
