In the production process of paper machines, various rolls play an indispensable role, from the dewatering of wet paper webs to the setting of dry paper webs. As one of the core technologies in the design of paper machine rolls, “crown” — despite the seemingly slight geometric difference it involves — directly determines the uniformity and stability of paper quality. This article will comprehensively analyze the crown technology of paper machine rolls from the aspects of definition, working principle, classification, key influencing factors in design, and maintenance, revealing its important value in paper production.

1. Definition of Crown: Significant Function in Minor Differences

“Crown” (expressed in English as “Crown”) specifically refers to a special geometric structure of paper machine rolls along the axial direction (lengthwise). The diameter of the middle area of the roll body is slightly larger than that of the end areas, forming a contour similar to a “waist drum”. This diameter difference is usually measured in micrometers (μm), and the crown value of some large press rolls can even reach 0.1-0.5 mm.

The core indicator for measuring crown design is the “crown value”, which is calculated as the difference between the maximum diameter of the roll body (usually at the midpoint of the axial direction) and the diameter of the roll ends. In essence, crown design involves presetting this tiny diameter difference to offset the “middle sag” deformation of the roll caused by factors such as force and temperature changes during actual operation. Ultimately, it achieves the uniform distribution of contact pressure across the entire width of the roll surface and the paper web (or other contact components), laying a solid foundation for paper quality.

2. Core Functions of Crown: Compensating Deformation and Maintaining Uniform Pressure

During the operation of paper machine rolls, deformation is inevitable due to mechanical loads, temperature changes, and other factors. Without crown design, this deformation will lead to uneven contact pressure between the roll surface and the paper web — “higher pressure at both ends and lower pressure in the middle” — directly causing serious quality issues such as uneven basis weight and uneven dewatering of the paper. The core value of crown lies in actively compensating for these deformations, which is specifically reflected in the following aspects:

2.1 Compensating for Roll Bending Deformation

When core rolls of paper machines, such as press rolls and calender rolls, are in operation, they need to apply significant pressure to the paper web. For example, the linear pressure of press rolls can reach 100-500 kN/m. For rolls with a large length-to-diameter ratio (e.g., the length of press rolls in wide-width paper machines can be 8-12 meters), elastic deformation of downward bending in the middle occurs under pressure, similar to a “shoulder pole bending under load”. This deformation causes excessive contact pressure between the roll ends and the paper web, while the pressure in the middle is insufficient. Consequently, the paper web becomes over-dewatered at both ends (resulting in high dryness and low basis weight) and under-dewatered in the middle (resulting in low dryness and high basis weight).

However, the “drum-shaped” structure of crown design ensures that after the roll bends, the entire surface of the roll remains in parallel contact with the paper web, achieving uniform pressure distribution. This effectively addresses the quality risks caused by bending deformation.

2.2 Compensating for Roll Thermal Deformation

Some rolls, such as guide rolls and calender rolls in the drying section, undergo thermal expansion during operation due to contact with high-temperature paper webs and steam heating. Since the middle part of the roll body is more fully heated (the ends are connected to bearings and dissipate heat faster), its thermal expansion is greater than that of the ends, leading to a “middle bulge” of the roll body. In this case, the use of conventional crown design will exacerbate the uneven contact pressure. Therefore, a “negative crown” (where the diameter of the middle part is slightly smaller than that of the ends, also known as “reverse crown”) needs to be designed to offset the additional bulge caused by thermal expansion, ensuring uniform contact pressure on the roll surface.

2.3 Compensating for Uneven Roll Surface Wear

During long-term operation, some rolls (such as press rubber rolls) experience more frequent friction at the edges of the paper web (as the edges of the paper web tend to carry impurities), resulting in faster wear at the ends than in the middle. Without crown design, the roll surface will show a “bulge in the middle and sag at the ends” after wear, which in turn affects pressure distribution. By presetting the crown, the uniformity of the roll surface contour can be maintained in the early stage of wear, extending the service life of the roll and reducing production fluctuations caused by wear.

3. Classification of Crown: Technical Choices Adapted to Different Working Conditions

Based on the type of paper machine (low-speed/high-speed, narrow-width/wide-width), roll function (pressing/calendering/guiding), and process requirements, crown can be divided into various types. Different types of crown differ in design characteristics, adjustment methods, and application scenarios, as detailed in the following table:

4. Key Influencing Factors in Crown Design: Precise Calculation to Adapt to Production Requirements

The crown value is not set arbitrarily; it needs to be comprehensively calculated based on roll parameters and process conditions to ensure its effective functioning. The key factors influencing crown design mainly include the following aspects:

4.1 Roll Dimensions and Material

1. Roll Body Length (L): The longer the roll body, the greater the bending deformation under the same pressure, and thus the larger the required crown value. For example, long rolls in wide-width paper machines require a larger crown value than short rolls in narrow-width paper machines to compensate for deformation.
2. Roll Body Diameter (D): The smaller the roll body diameter, the lower the rigidity, and the more prone the roll is to deformation under pressure. Therefore, a larger crown value is required. Conversely, rolls with a larger diameter have higher rigidity, and the crown value can be appropriately reduced.
3. Material Rigidity: Different materials of roll bodies have different rigidities; for example, steel rolls have much higher rigidity than cast iron rolls. Materials with lower rigidity exhibit more significant deformation under pressure, requiring a larger crown value.

4.2 Operating Pressure (Linear Pressure)

The operating pressure (linear pressure) of rolls such as press rolls and calender rolls is an important factor influencing crown design. The greater the linear pressure, the more significant the bending deformation of the roll body, and the crown value needs to be increased accordingly to offset the deformation. Their relationship can be roughly expressed by the simplified formula: Crown Value H ≈ (P×L³)/(48×E×I), where P is the linear pressure, L is the roll length, E is the elastic modulus of the material, and I is the moment of inertia of the roll cross-section. For example, the linear pressure of press rolls for packaging paper is usually greater than 300 kN/m, so the corresponding crown value needs to be larger than that of press rolls for cultural paper with lower linear pressure.

4.3 Machine Speed and Paper Type

1. Machine Speed: When high-speed paper machines (speed > 1200 m/min) are in operation, the paper web is much more sensitive to pressure uniformity than that in low-speed paper machines. Even minor pressure fluctuations may cause paper quality defects. Therefore, high-speed paper machines usually adopt “controllable crown” to realize real-time compensation for dynamic deformation and ensure stable pressure.
2. Paper Type: Different paper types have different requirements for pressure uniformity. Tissue paper (e.g., toilet paper with a basis weight of 10-20 g/m²) has a low basis weight and is extremely sensitive to pressure fluctuations, requiring high-precision crown design. In contrast, thick paper (e.g., cardboard with a basis weight of 150-400 g/m²) has a stronger ability to withstand pressure fluctuations, so the requirements for crown precision can be appropriately lowered.

5. Common Crown Issues and Maintenance: Timely Inspection to Ensure Stable Production

Unreasonable crown design or improper maintenance will directly affect paper quality and cause a series of production problems. The common crown issues and corresponding countermeasures are as follows:

5.1 Excessively Large Crown Value

An excessively large crown value leads to excessive pressure in the middle of the roll surface, resulting in low basis weight and high dryness of the paper in the middle. In severe cases, it may even cause “crushing” (fiber breakage), affecting the strength and appearance of the paper.

Countermeasures: For fixed crown rolls used in low-speed paper machines, it is necessary to replace the rolls with an appropriate crown value. For controllable crown rolls in high-speed paper machines, the hydraulic or pneumatic pressure can be reduced through the controllable crown system to decrease the crown value until the pressure distribution is uniform.

5.2 Excessively Small Crown Value

An excessively small crown value results in insufficient pressure in the middle of the roll surface, leading to inadequate dewatering of the paper in the middle, low dryness, high basis weight, and quality defects such as “wet spots”. At the same time, it may also affect the efficiency of the subsequent drying process.

Countermeasures: For fixed crown rolls, the roll body needs to be reprocessed to increase the crown value. For controllable crown rolls, the hydraulic or pneumatic pressure can be increased to raise the crown value, ensuring that the pressure in the middle meets the process requirements.

5.3 Uneven Wear of Crown Contour

After long-term operation, the roll surface will experience wear. If the wear is uneven, the crown contour will be deformed, and “uneven spots” will appear on the roll surface. This further causes defects such as “stripes” and “indentations” on the paper, seriously affecting the appearance quality of the paper.

Countermeasures: Regularly inspect the roll surface. When the wear reaches a certain level, timely grind and repair the roll surface (e.g., regrind the crown contour of press rubber rolls) to restore the normal shape and size of the crown and prevent excessive wear from affecting production.

6. Conclusion

As a seemingly subtle but crucial technology, the crown of paper machine rolls is the core for ensuring uniform paper quality. From fixed crown in low-speed paper machines to controllable crown in high-speed, wide-width paper machines, the continuous development of crown technology has always centered on the core goal of “compensating deformation and achieving uniform pressure”, adapting to the needs of different paper-making working conditions. Reasonable crown design not only solves quality problems such as uneven paper basis weight and poor dewatering but also improves the operating efficiency of paper machines (reducing the number of paper breaks) and reduces energy consumption (avoiding over-drying). It is an indispensable key technical support in the development of the paper industry towards “high quality, high efficiency, and low energy consumption”. In future paper production, with the continuous improvement of equipment precision and the continuous optimization of processes, crown technology will become more refined and intelligent, contributing more to the high-quality development of the paper industry.