Table of contents
What is Criss-cross bolt tightening ?
Cross tightening, also known as criss-cross bolt tightening, is a fastening technique employed in the industry to ensure a uniform and balanced tightening of bolts or screws on a structure, mechanical component, or any other assembly. This method is particularly critical in applications where even load distribution or the prevention of distortions is essential.
Criss cross tightening sequence is not a controlled tightening method. Instead, criss-cross tightening technique is not a method that involves strict control or specific torque values but rather focuses on achieving a balanced and uniform cross tightening pattern across bolts or screws in an assembly.
Why perform a criss-cross tightening sequence ?
Criss-cross tightening sequence allows for a progressive approach to joining two components. It is commonly employed to minimize misalignment between the components. This step-by-step tightening method naturally centers the components, ensuring a uniform contact of surfaces. Cross tightening prevents crushing between components, avoiding the risk of elongation or breakage of assembly screws. In other words, this technique prevents, in certain cases, reaching the elastic limit of the screws.
The criss cross pattern bolt tightening process involves successively tightening opposing or adjacent bolts or screws, forming a cross-shaped pattern. The objective is to distribute tension uniformly across the entire assembly surface, thereby minimizing the risks of deformation, uneven stresses, and leaks.
Here is how cross tightening is generally executed:
Identification of Tightening Points: Before commencing the tightening process, the tightening points on the assembly are identified. These points are often symmetrically positioned around the assembly.
Initial Tightening: Begin by lightly tightening each bolt or screw at its respective starting point. This establishes initial contact between the contacting surfaces.
Cross Tightening: Once the initial tightness is achieved, proceed to cross tightening. Alternately tighten the bolts or screws, forming a cross pattern. For instance, if starting by tightening the top-left bolt, move on to the bottom-right, then the top-right, and so forth. Repeat this sequence until all bolts are properly tightened.
Tension Verification: During the tightening process, tension is typically measured using an ultrasonic bolt tension meter to ensure that the load is evenly distributed.
Cross tightening, with its systematic approach, not only aligns components effectively but also ensures a reliable and balanced distribution of forces across the entire assembly, mitigating potential issues related to uneven stresses or distortions.
How does Criss-Cross Bolt Tightening Work?
Here, you will find the proper sequence for criss-cross tightening to progressively tighten the bolts in your assembly. This approach establishes an optimal tightening order to minimize deformations and stresses on the components being fastened, ensuring an even distribution of clamping pressure. Whether it’s criss-cross tightening of a casing, cylinder head, wheel, or more generally, a flange, the following steps should be adhered to.
In this context, the tightening sequence is provided for a maximum of 24 bolts. With this technique, you can effectively tighten assemblies comprising anywhere from 3 to 24 bolts. The objective is to define the order in which they will be tightened based on the numerical series below:
Bolt numbering for tightening: 1 – 9 – 6 – 17 – 21 – 14 – 4 – 12 – 7 – 20 – 24 – 15 – 2 – 10 – 5 – 18 – 22 – 13 – 3 – 11 – 8 – 19 – 23 – 16
By adhering to this sequence, you ensure a systematic tightening process that contributes to a uniform distribution of tension across the entire assembly. This minimizes the risks of uneven deformations and excessive stresses, thereby preserving the structural integrity and optimal performance of the assembly.
Cross Tightening of 6 Bolts
If you are looking to perform the cross tightening of 6 fasteners, follow these systematic steps.
Begin by selecting any bolt as number 1; the specific bolt chosen initially is inconsequential. Next, assign a number to each subsequent bolt along the circle, moving in the counterclockwise direction. The chosen numbers should correspond to the number of fasteners to be tightened. In this case, with 6 fasteners, select numbers from 1 to 6 while maintaining the order given by the series. After 1 and 6, you proceed to 4, 2, 5 and finally 3.
The series: 1 – 9 – 6 – 17 – 21 – 14 – 4 – 12 – 7 – 20 – 24 – 15 – 2 – 10 – 5 – 18 – 22 – 13 – 3 – 11 – 8 – 19 – 23 – 16
You can then tighten the bolts in the order indicated by the numbering, from 1 to 6, as shown by the green arrows.
Following this sequence ensures a methodical cross tightening process that contributes to a uniform distribution of tension across the entire assembly. This minimizes the risks of uneven deformations and excessive stresses, ensuring the integrity and optimal performance of the assembly with 6 bolts.
Criss cross tightening sequence of 8 Bolts
To perform the cross tightening of 8 bolts, follow this systematic procedure.
Start by selecting any bolt as number 1; the specific bolt chosen initially is inconsequential. Next, assign a number to each subsequent bolt, moving in the counterclockwise direction.
The chosen numbers should correspond to the number of fasteners to be tightened. In this case, with 8 fasteners, select numbers from 1 to 8 while maintaining the order given by the series. Therefore, after 1, you proceed to 6, followed by 4, 7, 2, 5, 3, and finally 8.
The series : 1 – 9 – 6 – 17 – 21 – 14 – 4 – 12 – 7 – 20 – 24 – 15 – 2 – 10 – 5 – 18 – 22 – 13 – 3 – 11 – 8 – 19 – 23 – 16
You can then tighten the bolts in the order indicated by the numbering, from 1 to 8, as shown by the green arrows. Following this sequence ensures a methodical cross tightening process, promoting a uniform distribution of tension across the entire assembly, minimizing the risks of uneven deformations and excessive stresses.
Cross tightening pattern of 20 bolts
Following the same principle as before but for 20 bolts:
To tighten 20 bolts in a cross pattern, adhere to the systematic approach outlined below.
Start by selecting any bolt as number 1; the specific bolt chosen initially is inconsequential.
Assign a number to each subsequent bolt, moving in the counterclockwise direction.
The chosen numbers should correspond to the number of fasteners to be tightened. In this case, with 20 fasteners, select numbers from 1 to 20 while maintaining the order given by the series.
Therefore, following the series provided:
Series: 1 – 9 – 6 – 17 – 21 – 14 – 4 – 12 – 7 – 20 – 24 – 15 – 2 – 10 – 5 – 18 – 22 – 13 – 3 – 11 – 8 – 19 – 23 – 16
You can tighten the bolts in the order indicated by the numbering, ensuring a methodical cross tightening process that promotes a uniform distribution of tension across the entire assembly. This minimizes the risks of uneven deformations and excessive stresses for the assembly with 20 bolts.
The Key Advantages of the Criss cross pattern bolt tightening
The benefits of criss cross tightening include a more uniform distribution of the load across the assembly, thereby reducing the risk of deformations or uneven stresses. This contributes to ensuring the stability and integrity of the structure or assembly while guaranteeing optimal performance.
Cross tightening is commonly employed in applications such as engine assembly, automotive construction, aerospace, and other fields where strict tolerances and uniform load distribution are crucial to ensuring the proper functioning and safety of mechanical components.
Various bolt tightening methods
Cross tightening achieves a clean and fair tightening, but it doesn’t address the issue of how to tighten the bolts. Should it be done through torque tightening? Hydraulic bolt tightening? Tension tightening with a bolt tension meter ?
Ultrasonic bolt tension meter
Bolt load measurement involves measuring the elongation of a screw resulting from the tension generated during tightening. When a screw is tightened, it stretches proportionally to the applied force. By measuring this elongation, we can determine the clamping force exerted on the screw, providing an indication of the effort applied in the assembly.
Torque wrench/torque wrench with angle
Torque-based tightening is based on the concept of applying a rotational force to a nut or bolt. This force is generated using a tool such as a torque wrench. The goal is to apply an adequate tightening force to hold the components together while ensuring not to exceed the recommended torque level to avoid any risk of deformation or damage.
Have you chosen to use torque wrench ? Learn how to perform torque tightening. Wondering what the maximum hand tightening torque is?
Hydraulic bolt tightening
The use of an hydraulic bolt tensioner offers several advantages, such as time savings and more reliable tightening. It stands out for its ability to provide a tightening torque that would be impossible to achieve solely with manual force or a torque wrench. By adjusting the output pressure of a pump and using a clamping flange, this solution delivers satisfactory results.
Have you chosen to use an hydraulic bolt stretcher? Learn how to perform hydraulic tightening.
