Analysis of Plate Rolling Machine Roller Fracture

1. Roller Friction and Wear Characteristics

1) Wear mechanism

Abrasion is the process by which two objects on a friction surface come into contact with each other and gradually lose material due to the mechanical action of friction. It will cause the object size to decrease gradually in a direction perpendicular to the friction surface. The contact surface is subject to local mechanical damage under high stress, and the damage form is affected by surrounding environmental factors. According to the cause of damage, the damage is divided into adhesion wear, abrasive wear and fatigue wear. Regarding adhesion wear, although the surface of the work roll is heat-treated, the surface film will be compressed and cracked due to the normal load and tangential movement at the same time, causing the new surface to directly contact and cause adhesion (cold welding). If the load and speed are large, the temperature of the friction surface will increase, which will further promote the surface film to break and form adhesion. The cycle of adhesion-peeling-re-adhesion, forms adhesion wear. For abrasive wear, it is also possible and common. Because the plate is hard or has hard spots, impurities, etc., under the tangential movement, the harder contour peaks or abrasive grains on the surface of the roller cut out strip-shaped grooves like cutting, causing material to fall off. For work rolls that make rolling and sliding compound friction, under the action of alternating stress, the surface of the material is fatigued and the material falls off, causing fatigue wear. The number of stress cycles causing fatigue wear decreases as the stress deserves to increase, especially as the proportion of sliding friction increases.

2) Wear analysis

Under certain friction conditions, the wear process is divided into three phases, namely the running-in phase, stable wear phase, and severe wear phase. Run-in is an unstable stage of wear. It has a small percentage during the entire working time and a high wear rate, but it decreases with the increase of working time. The stable wear stage has the longest time, which is characterized by slower wear and a stable wear rate. The severe wear stage is a sharp increase in wear rate. For the roller, it means that the stress concentration is serious, which causes the roller to easily break. For the plate rolling machine, the idle running-in of the machine is of great significance to the efficiency and service life of the machine. Running in can make the machine surface in elastic contact conditions, which can provide a minimum wear rate and stable friction value. The important rule of running-in is that the stable roughness is independent of the original roughness, but depends on the wear conditions. For the work of the machine is not a kind of long-term running-in. The running-in result is affected by factors such as material load, speed, mechanical and physical properties, but as long as the pressure does not exceed the critical value, it is better and better to maintain elastic contact.

2. Causes of Roller Fracture Types

1) Initial fracture of the roller

The initial fracture of the roller is generally caused by the excessive pressure of the upper roller and insufficient strength safety factor of the roller.

2) Intermediate fracture of the roller

The smaller the diameter of the rolled sheet, the greater the contact force and tangential stress. Therefore, the stress on the roller is excessively concentrated due to adhesion and abrasive wear, and the roller may be broken at any time.

3) Late fracture of the roller

If the design and operation are in compliance with the specifications, after the fatigue stress of the roller reaches a certain number of cycles, stress concentration will be formed, resulting in fatigue fracture; and with the increase of overload and tangential stress, this fracture will accelerate.