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The torque wrench - structure, function, types

Anyone who changes the tires of their car themselves is familiar with the torque wrench. But this helpful tool can also be used for countless other applications, or is even indispensable. The following information, tips and hints will help you make the right choice when buying a torque wrench.

What exactly is torque and why is it particularly important?

Before considering how a torque wrench works and how you should choose the right tool for your own needs, it is worth taking a quick look at the torque itself. After all, anyone who has understood what torque actually is can easily understand how it works and ultimately also how it works Importance of a torque wrench. [skip]

The torque


Torque is simply the rotational effect of a force on a body.

Just as the physical quantity force for a linear movement indicates the momentum in a defined direction, the torque indicates the energy which, so to speak, “rotates” a body.

This can best be imagined using the example of a wheel. The torque sets the wheel in a rotating motion. If you have a stationary body instead, for example the same wheel with the brake applied, it will not move. If the torque exceeds the resistance of the body, it will fail at its weakest point. Taking the wheel as an example, if the brake were applied, either the brake would yield or another component would fail.

Why the torque makes the difference

The torque, and thus also the torque wrench, is always of interest when a connection is created by an axial rotary movement.

In plain language: Through a screw connection. Interestingly, it is usually not decisive which force the screw itself can absorb. To come back to the example of the wheel already given, this means that the decisive factor is not the force with which the wheel presses against the screws along the bolt axis. In this direction of force, bolts or screws can usually absorb enormous forces, so that the critical area is hardly reached here. Instead, the torque is required to screw the nut into the mating thread with a precisely defined force, or the screw, analogously, with predefined energy.

The goal is quite simple: The screw connection should be so tight that the screw or nut cannot loosen under normal conditions, even due to vibrations or thermal stresses. On the other hand, the screw must not be tightened so tight that the material of the screw itself is weakened by the rotational movement. In the direction of rotation, the resilience of metal is considerably less than its resistance to linear forces. In short, it is about tightening a screw or nut so tightly that it will no longer loosen, but will not yet break.

Since screws, bolts and nuts are subject to strict regulations today, the required force can be defined very precisely. It is also possible to determine the force from which damage is to be feared, i.e. which torque must not be exceeded.

The torque wrench

In order to derive real benefit from the knowledge of the force with which a screw connection can and must be tightened, a suitable tool is now required. The torque wrench is exactly that tool. It enables regulation of the torque that is exerted on the screw connection when it is tightened.

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Layout and function

But what is behind the name torque wrench and how does it work?

The basic structure of a torque wrench is quite simple. Like normal socket wrench sets, the torque wrench has a handle and a receptacle for socket wrenches of various sizes. It can be used with a standard socket wrench set for different screw sizes without any problems.

The core of every torque wrench, on the other hand, lies between these two more common components. Depending on the type and design, either the determination and display of the applied torque or its limitation takes place between the socket wrench and the handle. Details on this can be found in the later section "Types and versions". Basically, however, it can already be stated that torque wrenches generally either display the torque so that the user can adjust the energy used, or they limit the torque on the socket wrench to a set upper value. If this is exceeded, the key signals this point and gives way.

It is not an essential part of the torque-limiting function, but rather an offer of convenient handling of the key in general the most used toggle functionas you know from any simple socket wrench set. It is usually integrated in the head of the key and allows easy switching from clockwise to counterclockwise rotation.

Areas of application

A torque wrench can be used wherever screw connections are made. For the do-it-yourselfer, this is primarily all work to do with your own car. If you take a closer look at the instructions for use of a vehicle, you will notice that the screws to be tightened generally indicate the permissible torque. With this information, the manufacturer ensures that the screw in question is on the one hand tightened enough not to loosen and on the other hand is not damaged by excessive torque. The result is the highest possible level of safety for the use of the individual component, but also for the vehicle as a whole.

But other areas also require the use of a torque wrench. Whether garden shed, play swing or other kit, today countless manufacturers provide torques for every screw or nut, no matter how unimportant, for their own safety and for the safety of the user.

If we leave the DIY sector, there are countless applications for torque wrenches in industry and construction, for example. The higher the load on a screw connection, the more important the correct torque becomes in order to minimize the potential danger.

Types and designs

As with most other technical devices or facilities, various types have also become established for torque wrenches. Since each manufacturer then implements one or the other type again in its own, again individual way, the market offers a multitude of different tools for the same use as a result. Ultimately, however, all common models can be traced back to the few types described below.


A small division:

TriggeringNotification end
  • Buckling wrench
  • Crack key
  • slipper
  • mechanical keys
  • electronic keys
  • mechatronic keys


Various torque wrenches at a glance

In general, it can be stated for all torque wrenches that the decisive properties are a high reproducibility of the results, as well as an exact specification of the applied torque, regardless of handling differences by one or the other user. These requirements are understandable, since the work should of course be carried out with the same accuracy and reliability in every application. Furthermore, the work result is only reliable when the tool reacts as little as possible from a different position or a different approach to the socket wrench.

Triggering torque wrench

The first large group of torque wrenches are the so-called triggering torque wrenches. These are so named because they “trigger” when the desired moment is reached, that is, they consciously prevent further momentary transmission. In this group there are again different versions of keys, which clearly differ in their functionality.

  1. Buckling wrench are tools that simply snap or break off when the maximum torque is reached and thus prevent further power transmission. These keys are very simple and safe to use, because the way they work alone makes it impossible to pass on excessive moments. However, for obvious reasons, this type never caught on. In the DIY sector in particular, a tool that can only be used once is hardly effective.
  2. Crack key on the other hand, the most famous torque wrenches may be. They usually consist of a long tube between the socket wrench and the handle, in which the adjustable mechanism for triggering is housed. The setting is usually made using one or more rotary wheels at the end of the handle. If a screw connection is created with the click wrench, the key is noticeable by a loud crack when the maximum torque is reached. It is important that at this point the power transmission must be stopped immediately by the user, as the key does not automatically prevent this. Thus, the crack key is very comfortable in its handling and setting and therefore widespread, but it requires a certain amount of attention and a careful approach by the user.
  3. Similar to the crack key, solves the so-called slipper when the specified torque is reached. Contrary to the aforementioned type, the slipper is characterized by the fact that it simply slips through from the maximum torque reached, so that any further power transmission is prevented. The handling is therefore significantly safer, since the deliberate termination of the power transmission is not necessary. The key itself triggers in the truest sense of the word and ends the power transmission. This makes it particularly suitable for inexperienced users or for only occasional use.

Indicating torque wrenches

In contrast to the releasing torque wrenches, indicating torque wrenches do not act on their own. They only show the current moment and thus enable the user to adjust the force.

The most common indicating torque wrench, if not the most widely used torque wrench of all, is the mechanical scale wrench. The structure is extremely simple, as it consists of a handle and square holder only of a partially flexible connecting tube and a pointer with a scale attached to it. The higher the force, the more the pipe bends. Since the pointer does not go along with the bend, it deviates from the pipe axis and shows the value of the force associated with the evasive movement on the scale.

Other indicating torque wrenches are mostly electronic or mechatronic types. The modes of operation differ greatly here. As a result, however, will on a digital display ultimately the pending moment is displayed analogous to the mechanical key described above. With these models, too, the user has to work attentively and regulate the force himself. There is no automatic shutdown, cracking or slipping with these types.

Which torque wrench is the right one?

Which torque wrench is the right one depends heavily on the use and the user. For DIY needs, a mechanical indicating key is often sufficient, as the simple construction makes it the cheapest model. Triggering keys are safer if they are only used occasionally. Ultimately, however, you should always work with the necessary care and attention in order to avoid errors.

Tips and information on handling the torque wrench

Once the right torque wrench has been found, it is important to use it correctly. Tip number one is always to take a look at the operating instructions. Even if many people consider this step unnecessary, it helps to avoid operating errors and thus to protect the material from damage and to steer the work result in the desired direction.

The right attitude

The top priority when using a torque wrench is the correct setting of the maximum torque. However, this aspect only comes into play with triggering keys, since the display keys simply do not offer any setting options. The process of setting itself depends on the design of the key. The setting is usually made using two counter-rotating wheels at the end of the handle. How the existing scales can be read and set must be carried out according to the specifications of the respective operating instructions, since even optically almost identical keys can differ in small but essential details.

The right application

In order not to falsify the display of a torque wrench, it depends on the correct application. First of all, this means that the key should be applied cleanly and completely to the screw to prevent it from slipping. However, correct operation is much more important for the result. The force should only be applied to the handle and not additionally done by the second hand, for example on the pipe. This is because, in particular, when the operator's second hand reaches in front of the mechanism, the latter can no longer grasp the force of this hand and, if necessary, limit it.

Why should the torque wrench be relaxed again after work?

For permanently coherent display or triggering results it is imperative to use the torque wrench after work to relax again. The background to this is the fact that a material that is permanently under tension tends to undergo plastic deformation in the long term. As a result, the mechanics already have a changed position in the rest position, which can have a significant effect on the result. One could generally say a key that is permanently energized will sooner or later become "soft" and is no longer suitable for an exact display.

Therefore the torque wrench should be calibrated

Regular calibration of a torque wrench helps to keep the measurement results permanently accurate. As already described on the subject of releasing the key, material changes over time. This happens in the long run even if the key is correctly relaxed. In addition, affect Temperature differences on the material in the way of that the preload in the material itself may differ. This does not make a torque wrench unusable, but the displayed result will be inaccurate. It therefore makes sense to calibrate a torque wrench from time to time and thus guarantee an exact work result over the long term.

Other tips when buying a torque wrench

The described details about different torque wrenches should already be of great help when deciding on one or the other type of torque wrench. Once the desired type has been determined, it is still worthwhile to compare different offers before buying.

With the same functionality, keys from different manufacturers can differ significantly in terms of operation and of course also in price. If you use your torque wrench sporadically, you can do without some comfort in favor of a better price. On the other hand, the choice of a model with a clearer display or setting option can give the occasional user more security in the application, so that even a higher price may make sense here. In short, before the actual purchase you should always take a look at the breadth of the range in order to be able to consider all options and variants when making a purchase decision.

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UNITEC torque wrench with nuts 20809
  • Tool for professional maintenance and repair work on cars, for tightening wheel nuts, cylinder head bolts, etc.
  • Adjustment range from 40 to 210 Nm
  • Drive: 1/2 "(12.7 mm) square
  • including 17mm, 19mm socket and extension
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