In the Rolling (metalworking) process, the rolling mill stand is a fundamental structural component that directly affects mill stability, rolling force resistance, and product dimensional accuracy. Understanding the structure and types of mill stands is important for engineers involved in rolling mill design, operation, and maintenance.
This article explains what a rolling mill stand is, the differences between open and closed stands, and common connection methods used in open-type mill stands.
What Is a Rolling Mill Stand?
A rolling mill stand is the structural frame that supports and installs the main working components of a rolling mill. It accommodates parts such as:
- Rolling rolls
- Roll bearings
- Roll adjustment mechanisms
- Rolling Mill Guide Equipment
Because the stand must withstand significant rolling forces while maintaining the required dimensional precision of the rolled product, it must possess sufficient strength and rigidity.
Depending on the mill configuration and production requirements, rolling mill stands are generally classified into two main types:
- Closed mill stands
- Open mill stands
What Is a Closed Mill Stand?
A closed mill stand is constructed as a single integral frame, forming a rigid structural loop. This design provides high strength and stiffness, allowing it to withstand large rolling forces.
Closed stands are typically used in rolling mills where heavy loads are present, such as:
- A primary rolling mill with high rolling force
- Slab rolling mills
- Plate and strip rolling mills
In some cases, they are also applied in section mills that require higher structural rigidity.
In mills equipped with closed stands, rolls are usually inserted or removed along the roll axis through the stand window during roll changes. These mills typically do not require dedicated roll-changing equipment such as:
- Roll-changing cars
- Balance sleeves
- C-hooks
However, when closed stands are used in horizontal arrangement section mills, roll replacement becomes relatively difficult because nearby stands and drive shafts may obstruct axial roll removal.
What Is an Open Mill Stand?
An open mill stand is not an integral frame. Instead, it consists of two main parts:
- The stand body
- A removable top cap
This design is widely used in horizontal section rolling mills, mainly because it greatly simplifies roll changing.
In mills with open stands, the roll can be lifted vertically after removing the top cap, avoiding the axial space limitations that occur with closed stands.
Advantages
- Easier and faster roll replacement
- Better accessibility for maintenance
Disadvantages
- Lower structural rigidity compared with closed stands
- More machined surfaces required
- Higher manufacturing cost in some cases
Connection Methods Between the Top Cap and Stand Body
For open mill stands, the connection method between the top cap and the stand columns significantly influences both roll changing efficiency and stand rigidity. Several commonly used connection structures include the following.
1. Bolt Connection
The top cap is fixed to the stand columns using long bolts.
Characteristics:
- Simple structure
- Large bolt deformation due to bolt length
- Lower overall rigidity
- Roll change is slower because nuts must be removed and reinstalled
2. Vertical Pin and Wedge Connection
In this configuration, vertical pins combined with wedges secure the top cap to the stand body.
Characteristics:
- Faster roll changing than bolt connections
- Moderate structural stiffness
3. Sleeve Ring and Inclined Wedge Connection
This design replaces bolts or cylindrical pins with sleeve rings.
The lower end of the sleeve ring is hinged to the column with a pin, while the upper end uses an inclined wedge to clamp the top cap and column together.
Characteristics:
- Convenient roll replacement
- Larger cross-section than bolts or pins
- Improved mill rigidity
4. Cross Pin and Inclined Wedge Connection
In this structure, the top cap and column are first connected using a horizontal pin, then tightened using an inclined wedge.
Characteristics:
- Simple structure
- Small deformation of connecting elements
However, under wedge tightening force and impact loads, deformation along the pin shear plane can occur, making disassembly difficult and increasing roll change time.
5. Inclined Wedge Connection
Compared with the previous methods, the inclined wedge connection offers several advantages:
- Smaller elastic deflection of the top cap
- Simple and reliable connecting structure
- Reduced lateral deformation of the stand columns
After the wedge is tightened, the upper portion of the column is firmly clamped by the wedge and top cap, significantly limiting lateral deformation. Because of its improved rigidity, this structure is often referred to as a semi-closed mill stand and has been widely adopted in rolling mills.
