Abstract
Belts are among the critical components in power transmission systems of electric motors, playing a key role in the optimal performance of industrial machinery. Premature belt failure can lead to unexpected production line shutdowns, increased repair costs, and reduced efficiency. Common causes of failure include surface wear and tear, cracking and Belt Breakage due to excessive tension or aging, loosening and slippage, and chemical contamination. Vibration analysis based on frequency spectrum and motor current analysis are two effective methods for identifying this important fault. Therefore, this paper examines the importance of belts, common failure types, and advanced diagnostic methods.
Introduction
The use of belts in motors for power transmission and connecting components is common for several reasons:
- a) Reduced noise and vibration: Belts, due to their flexibility and shock-absorbing properties, reduce vibration and noise, resulting in smoother motor operation.
- b) Slip-free power transmission: Timing belts enable precise, slip-free power transfer, ideal for synchronized component operation.
- c) Lower cost and easier maintenance: Belts are cheaper and easier to replace than chains or direct couplings and do not require constant lubrication.
- d) Flexibility in design: Belts allow changes in the distance between two shafts, enabling more flexible mechanical designs.
However, belts have disadvantages such as shorter lifespan compared to other mechanical coupling methods, susceptibility to Belt Breakage or slippage under poor tension, and limited capability in high-power transmission.
Common Belt Faults
Belts in motors and power transmission systems may suffer from various faults due to time or poor operational conditions, including:
- a) Wear and abrasion: Continuous use leads to surface wear, especially on V-shaped or toothed belts, reducing friction and increasing slippage.
- b) Cracking: Prolonged exposure to heat, oil, chemicals, or aging causes dryness and cracks, which may lead to sudden belt breakage.
- c) Belt Breakage: Excessive tension, sudden shocks, or severe wear can completely result belt breakage.
- d) Stretching and loosening: Older or low-quality belts may stretch and become loose over time, reducing power transmission, increasing slippage, and heat.
Another belt fault is resonance, which occurs when the natural frequency of the belt is close to the motor or drive shaft’s rotational frequency (RPM). When resonance happens, belts experience intense vertical and lateral vibrations. Adjusting belt tension (tightening or loosening) while measuring vibration responses at pulleys or bearings usually helps identify and fix this problem.
A common method to control vertical vibration is installing a damper (metal rod or idler pulley) perpendicular to the belt, close to or lightly touching it. This should be located approximately one-third of the belt’s length from the larger pulley.
Belt fault frequencies are of subharmonic type. Worn, loose, or misaligned belts may generate belt frequency harmonics. Frequencies up to 3x or 4x the belt frequency may appear, with 2x harmonics often dominant. The belt frequency is calculated by:
Where:
- RPM = motor speed (revolutions per minute)
- D = pulley diameter
- L = belt length
For accurate diagnosis in belted motors, a low-frequency range should be selected in spectral analysis, and 2x, 3x, and 4x harmonics are key indicators of belt defects.
Various types of misalignment can also occur in belt drives. These not only cause destructive vibrations but also accelerate wear of both belt and pulleys. Pulley misalignment causes intense vibrations at 1×RPM, mainly visible in the axial direction. In fan pulleys, the axial vibration peak typically appears at the fan’s rotational frequency. When a belt drives an overhung rotor (suffering imbalance), phase analysis is essential for accurate diagnosis.
Fault Diagnosis via Vibration and Current Analysis
Vibration Analysis is a powerful technique for diagnosing belt faults in rotating systems. Faulty belts produce distinctive vibration patterns that can be identified through proper measurement and analysis. Early detection helps prevent severe damage. Key indicators include spectral frequency, harmonics, and overall vibration level.
Vibration symptoms of faulty belts include:
- a) Unbalance-induced vibrations: A stretched or worn belt may cause pulley vibrations, typically at 1× RPM.
- b) Misalignment-induced vibrations: Misaligned pulleys cause lateral belt force, resulting in 2× RPM vibrations. Spectra show strong peaks at 1× RPM and 2× RPM.
- c) Wear-related vibrations: Worn or cracked belts cause abnormal periodic vibrations. Multiple harmonics (e.g., 3× RPM, 5× RPM) may appear in the spectrum.
- d) Looseness-induced vibrations: Loose belts lead to random vibrations and elevated high-frequency noise, with irregular peaks between 100–1000 Hz.
Motor Current Signature Analysis (MCSA) can also diagnose belt faults by observing changes in electrical parameters. Mechanical changes due to belt faults affect the motor load, altering current patterns. These changes include:
- a) Amplitude modulation from torque fluctuations
- b) Frequency modulation from momentary speed changes
- c) Sideband harmonics in the current frequency spectrum
This method is especially useful for sensitive or inaccessible systems, where continuous vibration monitoring is difficult. Proper implementation has shown up to 80% of sudden belt failures can be predicted.
Real-World Scenario
This section presents results of vibration and current analysis for a 400V, 50Hz, 37kW, 4-pole motor driving a pump via belt. The effective vibration speed was measured at 6.74 mm/s, which, per ISO 20816, indicates a warning condition. Figure (1) illustrates the signal sampling process using the MCM1 device on a sample motor.
Figure (3) shows the vibration velocity spectrum. Green lines indicate belt fault frequencies up to the fourth harmonic. The second harmonic shows a significant 0.4 mm/s peak, indicating a belt fault. Visual inspection confirmed that the belt was flaking. This fault increased both the 1× RPM and 2× RPM components in the vibration signal, as well as high-frequency components. Additionally, the noise floor around 15 Hz rose, indicating belt looseness.
Figure (5) presents the current frequency spectrum, where modulation effects related to belt fault vibrations are clearly visible.
Given the critical role of belts in motor performance, using combined diagnostic methods enables early fault detection and helps prevent costly shutdowns. Employing online monitoring systems along with periodic inspections is an effective strategy to improve the reliability of belt-driven systems.
Implementing condition monitoring systems on key production lines, training operators for early fault recognition, using high-quality belts with proper installation, and planning preventive maintenance based on diagnostic data greatly enhance system efficiency and productivity. This study shows that a systematic approach can extend belt life and prevent unexpected failures.
