Self-locking Screw Jacks Are Used To Maintain Gap Adjustments on Slitters, Levelers, And Pipe Mills in Steel Industry

Self-locking screw jacks are used in heavy-duty machinery like slitters, levelers, and pipe mills to maintain precise, consistent gap adjustments under load. Their inherent ability to hold their position without needing a brake or other external locking mechanism makes them ideal for these applications. They convert the rotational force of a motor or hand wheel into linear movement, allowing for high-precision, synchronized adjustments of heavy roller loads. Example in a pipe mill: In a pipe mill, a screw jack might be used to adjust the position of a forming roll. The precise gap between rolls must be set and maintained to ensure the correct diameter and seam of the pipe. Once the jack moves the roll into position, its self-locking nature keeps it there, resisting the immense forces exerted on the roll by the steel as it is formed.

Worm gear mechanism: The self-locking feature is characteristic of machine screw jacks that use a worm gear system. The low efficiency of the worm gear, combined with the friction and the screw's pitch angle, prevents it from being back-driven by a load.

Rollers gap adjustment: Slitters, levelers, and pipe mills require operators to make fine adjustments to the gaps between rollers or blades to accommodate different material thicknesses and specifications. One or more screw jacks are positioned on the upper rollers. Rotating the jack's screw causes the roller to move up or down, increasing or decreasing the gap, or "roll gap," between it and the bottom roller. This fine-tuning determines the final thickness of the sheet metal, paper, or plastic.

Load support: In these mills, the jacks support very heavy loads and are subjected to tension and compression. The self-locking feature is critical for safety and process integrity, ensuring that the heavy rolls or blades don't creep out of position.

Static positioning: Once the correct gap is set, these self-locking machine screw jacks hold the position indefinitely. This means they can hold heavy loads in position indefinitely without "creeping" or shifting, which provides stability and safety for the material and the equipment. This is essential for maintaining a consistent product quality, especially on long production runs.

Synchronization: In applications with multiple screw jacks, such as large leveling machines, the jacks can be synchronized and driven by a single motor with a control system. The similar gear ratios allow them to move in unison, even when subjected to unevenly distributed loads. This ensures that both sides of the roller are raised and lowered at the exact same speed, keeping the roll perfectly straight and balanced.

Provides precision: Screw jacks offer greater precision than hydraulic or pneumatic systems, which is critical for producing consistent, high-quality material. Some models, called anti-backlash screw jacks, are designed to eliminate play for even greater accuracy in positioning.

Worm gear screw jacks are often preferred over hydraulic systems in rolling machines for several reasons:

Greater precision: The mechanical nature of screw jacks allows for extremely fine and consistent positioning that is difficult to achieve with the constant-pressure variations of hydraulics.

Improved safety: Screw jacks can be self-locking, meaning the load is held securely in the event of a power failure. Hydraulic systems, in contrast, require constant pressure to hold their position and can fail if pressure is lost.

Increased energy efficiency: Screw jacks only use power when they are moving, while hydraulic systems require constant energy to maintain fluid pressure.

Lower maintenance: Screw jacks are a cleaner technology with fewer components than hydraulics, reducing the risk of oil leaks and the need for frequent maintenance.

Types of screw jacks used:

There are several types of screw jacks that can be adapted for use in rolling mill applications, depending on the required precision, speed, and duty cycle.

Machine screw jacks: These use an Acme or trapezoidal thread and are known for handling heavy loads at lower speeds. They are robust, reliable, and cost-effective. A high gear ratio provides the self-locking feature ideal for holding the roll gap under static loads.

Ball screw jacks: These use a ball screw mechanism with recirculating ball bearings to minimize friction. They offer higher efficiency, faster operation, and greater precision than machine screw jacks, making them suitable for high-speed, high-precision applications. However, they are not self-locking and require a brake motor to hold the load.

Bevel gear screw jacks: These use bevel gears to convert the rotary input into linear motion. They provide high load capacity, are very efficient, and can be used with either machine screws or ball screws.