In the operation of a Прокатная мельница, the stability of the upper roll is a critical factor that directly affects the quality of the product and the lifespan of the equipment. When the upper roll is adjusted, or during the pause between passes, gravity pulls the upper roll assembly (including the roll, chocks, and bearings) downwards. This creates a gap between the adjusting screw and the nut, as well as between the bearing housing and the screw end.
Without a proper balancing system, when the metal enters the bite, the upper roll is suddenly pushed upward to close this gap. This results in a severe impact load—often called “roll jump”—which can damage the threads of the screwdown mechanism, wear out bearings prematurely, and cause gauge variations in the rolled steel.
To prevent this, balancing devices are installed. Their primary function is to exert a continuous upward force that exceeds the weight of the upper roll assembly, keeping the parts in tight contact even when no steel is in the mill. Ниже, we detail the four main types of balancing devices found in modern and traditional rolling mills, along with their operational parameters and maintenance considerations.
Calculations: How Much Force is Needed?
Before looking at the types, it is important for maintenance engineers to understand the physics. The balancing force isn’t just equal to the weight of the parts; it must be slightly higher to ensure stability.
Standard Formula for Balancing Force (Q)
Q = (1.2 к 1.4) × G
Где G is the total weight of the upper roll, upper roll chocks, and any suspended accessories.
1. Ordinary Spring Balancing
This is the simplest form of balancing used in the industry. It is typically found on section mills or profile rolling mills where the upper roll position is relatively fixed or moves very little.
Mechanism
Springs are mounted on the top of the mill housing (the stand). Long suspension bolts (pull rods) run through the housing and hook onto the upper roll chocks. When the roll is lowered, the springs compress, storing energy. When the roll needs to rise, the springs release this energy to lift the assembly.
Operational Parameters
- • Movement Limit: Best for mills where the roll lift is small, обычно not exceeding 50–100 mm.
- • Reliability: Very high, as there are few moving parts.
- • Drawback: The balancing force fluctuates. As the springs compress, the force increases; as they extend, force decreases. Longer springs are preferred to keep this force relatively constant.
2. Spring Balancing on Counter-Nut
This system is largely obsolete but may still be encountered in older plate mills. It was an engineering attempt to solve the “varying force” problem of ordinary springs.
In this design, the springs are supported on a special nut that moves up and down at the same speed as the upper roll. While theoretically sound, the mechanical complexity made it difficult to maintain. The overall height of the mill stand had to be increased significantly, requiring taller factory buildings. Most mills of this type have been retrofitted with hydraulic systems.
3. Counterweight (Heavy Hammer) Balancing
For heavy-duty mills where the upper roll must move frequently and over large distances—such as blooming mills or slabbing mills—springs are insufficient. This is where the counterweight system shines.
How It Works
This system uses simple physics: levers and weights. A heavy “hammer” or weight block is located in the foundation beneath the mill stand. It is connected via a system of levers and vertical push rods that act directly on the underside of the upper roll chocks.
Pros and Cons
- + Constant Force: Unlike springs, gravity is constant. The balancing force remains exactly the same regardless of the roll’s position.
- + Диапазон: Excellent for large vertical movements.
- – Infrastructure: It requires a deep and complex foundation to house the heavy weights and lever arms, increasing initial civil engineering costs.
4. Hydraulic Balancing
This is the standard for modern strip mills, 4-high plate mills, and most high-precision rolling facilities. It offers the best balance of performance and space efficiency.
The Modern Solution
Hydraulic cylinders are built directly into the mill windows or the chocks. Pressurized fluid holds the upper roll assembly against the screws. When the screws retract, the hydraulic pressure pushes the roll up instantly.
Ключевые особенности
Does not require tall ceilings (like Type 2) or deep foundations (like Type 3).
Easy to change rolls as cylinders can be retracted quickly. Однако, it requires a dedicated hydraulic station.
In freezing environments, standard water-based fluids may freeze. Emulsions or specialized spindle oils must be used.
Comparison of Balancing Systems
For production managers and engineers, choosing or identifying the right system depends on the mill type. Here is a technical comparison:
| Тип | Roll Movement | Complexity | Best Application |
|---|---|---|---|
| Ordinary Spring | Small (< 100мм) | Низкий | Profile/Section Mills |
| Spring on Nut | Середина | Very High | Old Plate Mills (Rare) |
| Counterweight | Большой | Середина (Heavy Base) | Blooming/Slabbing Mills |
| Hydraulic | Variable | Высокий (Requires Pump) | Modern Strip/Plate Mills |
Selection and Maintenance Summary
The choice of balancing device is dictated by the rolling mill’s design purpose. While spring systems are cost-effective for mills with minimal roll adjustment, they fail in high-precision environments where the gap must be eliminated with constant force. The industry has largely moved toward hydraulic balancing for new installations due to its compact nature and ease of automation.
For maintenance teams, regular inspection of these devices is crucial. For hydraulic systems, seal integrity is paramount to prevent pressure drops that could lead to roll slippage or impact damage. For spring systems, checking for metal fatigue and ensuring the pull rods are not elongated is necessary to maintain the required safety factor of 1.2–1.4 times the roll assembly weight.
