1. Introduction
Deviation of the conveyor belt from the central axis is one of the most common and critical faults in industrial conveyor systems. This problem results in significant operational losses, including material spillage, increased wear on the belt and conveyor components, unplanned production stoppages, and potential threats to personnel safety. A systematic approach to root cause analysis is mandatory for effective elimination and prevention of such malfunctions.
2. Overview of the Component and System
The conveyor belt is a vital element for transporting bulk materials in most industries. It consists of a drive drum, a tension drum, roller supports (tube-shaped, straight, centering), the tape itself, and a frame. Each of these elements plays a key role in keeping the belt moving in a straight line.
- Drive drum: Transmits motion to the belt. Its surface often has a bulge (crown) to center the tape.
- Roller supports (rollers): Support the belt and load on the working leg and the empty belt on the return leg. Roller alignment is critical for stable motion.
- Conveyor belt: The main element for transporting material. Its condition, tension and the quality of the joints affect the centering.
In modern transportation systems, especially large-sized or high-speed ones, automated belt centering systems can be used. These systems often include hydraulic actuators controlled by proportional valves such as the REXROTH LC40A10D7X. Designed for precise flow control, this hydraulic valve can be integrated into a hydraulic tensioning or active roller alignment system. Its malfunction, for example, due to contamination, wear of seals or incorrect calibration, can lead to inaccurate positioning of the actuators, which directly affects the centering of the tape and leads to its deviation. The working pressure for such valves is usually up to 350 bar, and the positioning accuracy can be within ±0.1 mm when functioning properly.
3. Evidence of Malfunction
Conveyor belt centering malfunctions are manifested by a number of visual, acoustic and measurement signs:
- Visual signs:
- The belt constantly shifts to one side, touching the elements of the conveyor frame.
- Visible traces of abrasion (5-10 mm deep) on the edges of the tape or on the side elements of the structure.
- Spillage of material outside the belt, especially in loading and unloading areas.
- Uneven wear of the tape surface.
- Deformation or damage to the edges of the tape.
- Measurement data:
- Roller alignment: Using a laser leveler, a deviation of more than 3 mm vertically or horizontally on a 1-meter area was detected. According to EN 14973, the maximum permissible deviation for rollers should not exceed 2 mm per meter of length.
- The temperature of the roller bearings: Thermographic analysis (for example, using an IR thermometer) shows a local increase in the temperature of the roller bearings to 80-100°C, which is 40-60°C higher than the ambient temperature (20-30°C). This indicates increased friction or jamming.
- Vibration: Vibration analysis (according to ISO 10816-3) on the roller bearing housings shows an RMS velocity of over 4.5 mm/s, while baseline values for healthy rollers are less than 2.8 mm/s. Peak values can reach 10-15 mm/s.
- Belt tension: Measurement of belt tension (with a dynamometer or strain gauges) shows deviations from the manufacturer's recommended values (for example, a deviation of 10-15% from the optimal tension of 100 N/mm for a specific type of belt).
- Operational indicators: Frequent unplanned stops, reduced productivity, increased electricity consumption (up to 5-10% due to increased friction).
4. Investigation of Root Causes
For the systematic identification of root causes, the "Fishbone" method (Ishikawa diagram) is applied, focused on the key categories affecting the work of the conveyor.
4.1. Equipment (Machine)
- Incorrect crown of drive/tension drum: The crown of the drum, which should be equal to 1/200 of the width of the drum, may be missing, insufficiently defined or worn. This leads to the loss of the natural centering of the tape.
- Misaligned roller supports: Rollers can be installed at an angle, skewed, or not perpendicular to the axis of belt movement. Permissible skew of rollers according to DSTU EN 14973:2018 should not exceed 0.5% of the belt width. Blocked or jammed rollers also create increased drag and strip the tape.
- Roller bearing wear: Bearings with increased backlash or those that are seized prevent the rollers from rotating freely, causing friction and belt displacement. A typical MTBF for quality bearings is 50,000 - 100,000 hours, but can be significantly reduced due to contamination or overloading.
- Deformation of the conveyor frame: The supporting frame can be bent or skewed due to mechanical impacts, improper installation or subsidence of the foundation.
- Misfunction of the automatic centering system: In systems with active elements such as hydraulic steering rollers, a malfunction of the proportional valve (eg REXROTH LC40A10D7X) due to dirt, wear or electronic failure can lead to inaccurate or missing adjustments.
4.2. Material (Tape and Cargo)
- Uneven loading: Material fed to the belt off-center or unevenly distributed across the width creates asymmetric pressure, causing the belt to shift. Optimal loading involves distributing the load over 80% of the belt width.
- Damage or wear to tape edges: Damaged tape edges (eg cuts, fraying) can cause the tape to warp and deviate.
- Tip Joint Defects: Improperly made, damaged or worn tape joints may vary in thickness, stiffness, or not be perpendicular to the tape axis, creating deflection points.
- Incorrect belt tension: Insufficient tension leads to sagging of the belt and its unstable movement. Excessive tension increases the load on the bearings and can deform the belt.
4.3. Method (Maintenance and Operation)
- Insufficient maintenance: Lack of regular checks of roller alignment, bearing condition, belt tension and drum crown.
- Incorrect installation: Errors during the initial assembly of the conveyor or replacement of components, which lead to the initial skew of the frame or rollers.
- Inadequate personnel training: Operators and maintenance personnel lack sufficient knowledge of proper setup, operation, and diagnosis of centering problems.
4.4. Staff
- Lack of Qualification: Technical personnel are insufficiently qualified to perform accurate alignment and diagnostics.
- Technology Violation: Ignoring established procedures during setup or repair.
5. Identified Root Causes
- Misaligned or jammed rollers (probability: high). Confirmed by laser alignment measurements (>3mm per meter) and elevated bearing temperature (>80°C). This is the most frequent reason that requires regular control.
- Uneven loading of the tape (probability: high). A shift of the material to one edge of the tape in the loading area is visually observed. This creates an asymmetric load that results in permanent displacement.
- Incorrect belt tension (probability: average). Measurements show deviations from the recommended values by 10-15%. Insufficient tension allows the tape
