Tightening methods in practice: The best technique for your application
A reliable bolted joint is crucial for the safety and durability of products in all industries. But which tightening method is best suited to specific applications, e.g. in the automotive industry, mechanical engineering or electrical engineering? In this article, we show practical examples of different tightening techniques and help you to select the optimum method.
Why the right tightening method makes all the difference
Every bolted joint must withstand certain loads. Settling phenomena, friction fluctuations and changing environmental conditions can influence the preload force. An incorrect choice of tightening method can lead to loose or overloaded connections - with expensive consequences.
Tightening methods and their practical applications
The torque-controlled method is often used in series production as it is easy to implement and widely used. The screw is tightened with a defined torque. In automated screwdriving processes, however, the tightening method is only one part of the overall process. For stable cycle times and reproducible workflows, screw separation must also be reliably adapted to the specific application.
In the angle of rotation method, the screw is first tightened with a starting torque and then turned through a defined angle of rotation. This method ensures a more precise and repeatable preload force.
In yield point-controlled tightening, the bolt is loaded to just before its yield point in order to achieve an optimum preload force.
Adaptive methods such as DEPRAG Clamp Force Control (CFC) ensure a more constant preload force under changing assembly conditions. This minimizes assembly errors and makes processes safer.
1. Torque-controlled tightening: the standard for many applications
Advantages:
- applicable for practically all bolting applications
- easily measurable control variable
- control variable can still be checked after assembly
- relatively low equipment costs (simple tightening tools)
Disadvantages:
- usually large preload force scatter (error when estimating the friction coefficients, scatter of friction coefficients, shut-off accuracy)
- oversizing of the screw necessary (safety)
2. Angle-controlled tightening: makes maximum use of the screw
Advantages:
- highest possible assembly preload force (overelastic)
- relatively low preload force dispersion
- no oversizing of the screw required
- relatively low equipment requirements (simple tightening tools)
- minimizes the influence of friction fluctuations and ensures greater process reliability.
Disadvantages:
- sufficient uniform elongation of the screw required
- limited reusability of the screw (overelastic)
- subsequent control of the assembly result not possible
- incorrect measurements possible, e.g. with screws that are too hard or too soft
- time-consuming experimental determination of the assembly specification
3. Yield point-controlled tightening: maximum material utilization in the elastic range
Advantages:
- highest possible assembly preload force (elastic)
- relatively low preload force dispersion
- no oversizing of the screw required
- reliable assembly even of short screws
- repeat assembly possible
- maximum safety and material efficiency, as the screw is ideally utilized.
Disadvantages:
- subsequent checking of the assembly result not possible
- incorrect measurements possible (e.g. due to washers or soft seals)
- very high measuring effort (controlled screwdriving system required)
4. DEPRAG Clamp Force Control (CFC): the intelligent solution for variable conditions
Advantages:
- maximum process reliability with changing assembly conditions
- compensation of friction fluctuations
- avoidance of assembly errors through real-time monitoring
- repeat assembly possible
- ideal for demanding materials and fastening applications
Disadvantages:
- higher acquisition costs
- more complex determination of assembly parameters
5. Friction torque value procedure: Checking threads and gears
The friction torque value procedure is not primarily used for assembly, but for testing threads or checking the ease of movement of gears. It measures the friction torques that occur and provides information on the quality and functionality of the components.
Advantages:
- ensuring the required thread quality or smooth running of gearboxes
- early detection of manufacturing errors
- improvement of process quality
Disadvantages:
- higher acquisition costs
Which Questions Should Be Answered Before Selecting a Tightening Method?
Selecting a suitable tightening method does not start with the fastening tool, but with the specific fastening application. The key requirements for the joint should already be defined during the design and planning phase.
The following questions can help guide the selection process:
| Question | Significance |
|---|---|
| What preload force must be reliably achieved? | Influences the required level of accuracy and repeatability. |
| Which materials are being joined? | Material properties affect settling behavior and joint load capacity. |
| How critical is the bolted joint to the product's function or safety? | Safety-critical joints require higher levels of process reliability and traceability. |
| Do tightening data need to be documented or stored for traceability? | Documentation and quality requirements are mandatory in many industries. |
| Are varying assembly conditions or significant friction coefficient fluctuations expected? | This may influence the choice of tightening method. |
| What economic requirements exist regarding assembly time and equipment investment? | Not every technically feasible method is economically justified. |
Answering these questions provides a solid foundation for selecting the appropriate tightening method and designing the fastening process.
Process monitoring, documentation, and traceability play an important role, especially for safety-critical or function-critical bolted joints. Requirements for this are described, among others, in VDI/VDE 2862 for safe screwdriving processes.
Which tightening method is the right one?
In order to determine the optimum tightening method for your bolting application, other aspects and key figures - such as the minimum and maximum preload force (Fmin and Fmax) - must also be taken into account. The ratio Fmax / Fmin can be used to calculate the so-called tightening factor, which provides information about the consistency of the preload force and the load variability:
-
methods with a low tightening factor generally offer high repeat accuracy
-
higher tightening factors require more robust (larger) screws
While yield point and angle-controlled tightening achieve a tightening factor of ≈ 1, the factor for torque-controlled tightening with angle monitoring is already ≈ 1.5, and even up to 4.0 for torque-limited tightening (depending on the tightening tool).
In the example below, an M8 screw could be used for the tightening process with yield point control, whereas an M16 screw is required for the same preload force with torque-limited tightening.
The designer should therefore determine during the development process which tightening method will be used to assemble the component later on. This has a direct impact on the size of the screw to be used. Especially in lightweight design, it is advantageous to fully utilize the screw, i.e., to tighten it up to the yield point, so that smaller screws can be used. Smaller screws are lighter and more cost-effective. They also reduce the size of the components to be assembled and therefore make them lighter. Fastening technologies such as friction drilling also show how closely tightening methods, component design, and material utilization are connected.
In an industrial environment, expert advice is essential in most cases. Each fastening case represents an individual challenge and must be assessed specifically. Our experts will be happy to provide you with a professional assessment of your fastening task.
