Squealing and Noise

V-belt squealing is the most recognizable belt problem. It is caused by the belt slipping against the sheave groove walls, creating a high-pitched noise. Squealing is a symptom, not a root cause. The underlying issue is almost always one of these:

• Insufficient tension. The belt does not have enough grip to transmit the required torque without slipping. This is the most common cause. Check and adjust tension per manufacturer specifications.
• Worn sheave grooves. As sheave grooves wear, they become wider and shallower. The belt rides higher in the worn groove, reducing the contact area between the belt sidewalls and the groove walls. Even with correct tension, the belt slips because the grip surface is too small. Replace worn sheaves.
• Oil or grease on belt or sheave. Any lubricant on the contact surfaces reduces friction. Identify the contamination source, clean the sheaves thoroughly, and replace the contaminated belt (oil-soaked belts cannot be cleaned effectively).
• Overload. The driven load exceeds the belt's torque capacity. The belt slips because it physically cannot transmit the required force. Verify the belt is correctly sized for the application. Increase belt width, add belts, or move to a larger section if undersized.
• Startup squeal only. A brief squeal at motor startup is common on drives with high-inertia loads and is not necessarily a problem if it stops within 1 to 2 seconds. Persistent squeal lasting more than a few seconds indicates a real tension or grip issue.

Belt Slipping

Slipping occurs when the belt cannot transmit the required torque without losing grip. It may or may not produce audible squealing. Signs of slipping include heat buildup on the belt, glazed (shiny, hard) belt surfaces, reduced driven equipment speed, and belt dust accumulation on the sheaves.

Causes and fixes:

• Under-tension: Re-tension the belt. Use a tension gauge or the deflection method per manufacturer specifications. V-belts lose tension during the first 24 to 48 hours of operation (run-in stretch), so re-tension after the initial run-in period.
• Worn sheave grooves: Inspect with a sheave gauge. If the belt sits too high in the groove or the groove walls are polished smooth, the sheaves are worn and must be replaced. Worn grooves are the most overlooked cause of chronic slipping.
• Overload: Verify the design HP (motor HP x service factor) does not exceed the belt's rated capacity. See our V-Belt Selection Guide for service factor tables.
• Insufficient wrap angle: If the belt wraps less than 120 degrees around the smaller sheave, there is not enough contact for reliable grip. Increase center distance or add an idler to increase wrap.
• Oil contamination: Clean sheaves and replace belt.

Cracking and Splitting

Cracks appear across the belt's back (outer) surface or along the sidewalls. The belt may feel stiff or brittle compared to a new belt.

• Heat degradation. Chronic exposure to temperatures above the belt's rated range (typically 140 to 185 degrees F for standard neoprene) causes the rubber to harden and crack. Identify and address the heat source. Upgrade to a cogged belt for better heat dissipation, or shield the drive from external heat.
• Sheaves below minimum diameter. Running on sheaves smaller than the belt's minimum causes excessive bending stress each revolution. The outer surface stretches beyond its fatigue limit and cracks. Verify sheave sizes against the minimum for your section. See our V-Belt Size Chart for minimum sheave OD by section.
• Age. All rubber belts degrade over time, even when not in use. Belts stored for more than 3 to 5 years in hot or ozone-rich environments may crack before installation. Store replacement belts in a cool, dry location away from electric motors.
• Over-tension. Excessive tension increases bending stress at each sheave, accelerating fatigue cracking. Reduce tension to manufacturer specifications.
• Ozone exposure. Electric motors and welding equipment produce ozone, which attacks rubber. Drives near these sources may show surface cracking even when temperature and sheave size are correct.

Uneven Edge Wear

One edge of the belt wears faster than the other, or the belt shows abrasion marks on one side only.

• Sheave misalignment. This is the cause in the vast majority of cases. When sheaves are not parallel and coplanar, the belt enters one groove at an angle, causing one sidewall to wear against the groove wall more than the other.
• Bent shaft or worn bearings. A bent shaft or bearing with excessive play allows the sheave to wobble, creating a dynamic misalignment.
• Incorrect belt in groove. Using a belt that is too narrow for the groove (e.g., a slightly undersized cross section) causes the belt to sit unevenly.

Fix: Realign sheaves using a straightedge or laser alignment tool. Both sheaves must be parallel (angular alignment) and in the same plane (offset alignment). Check shaft runout and bearing condition. Verify correct belt section for the sheave groove.

Belt Turnover (Flipping)

The belt rotates (flips) sideways in the sheave groove so that it runs on its edge instead of its sidewalls. This is a rapid failure mode because the edge cannot transmit power or grip the groove.

• Shock loads or pulsation. Sudden torque changes cause the belt to lift and twist. Reciprocating compressors and crushers are common sources.
• Vertical or steep-angle shafts. Gravity pulls one side of the belt into the groove more than the other, creating an unbalanced force that rotates the belt.
• Worn sheave grooves. A worn groove is wider than spec, giving the belt room to rotate.
• Misalignment. Severe misalignment can induce a twisting force on the belt.

Fix: The definitive solution for belt turnover is switching to banded V-belts. The tie band physically prevents any individual belt from rotating. Also inspect and replace worn sheaves, correct alignment, and consider a soft-start motor controller for shock-load applications.

Belt Breakage

The belt snaps completely, separating into two pieces. This is usually a catastrophic failure that stops the equipment immediately.

• Severe overload. A jammed driven component, locked shaft, or sudden extreme torque spike exceeds the tensile cord's ultimate strength.
• Foreign object damage. A bolt, tool, or piece of debris caught between the belt and sheave.
• Incorrect belt section. A belt section that is too small for the load fails under normal operating torque.
• Accumulated fatigue. A belt that has been running with chronic cracking, slipping, or edge wear eventually fails completely.

Fix: Identify the overload source and address it (overload protection, correct belt sizing, debris shields). If the belt section is undersized, select the correct section using our V-Belt Selection Guide. For extreme shock load applications, consider Gates Predator belts with aramid tensile cords.

Glazing and Hardening

The belt's sidewall surfaces become shiny, smooth, and hard instead of the normal matte rubber texture. A glazed belt has reduced friction and slips more easily, creating a self-reinforcing cycle: slipping causes heat, heat causes glazing, glazing causes more slipping.

• Chronic under-tension. The belt has been slipping for an extended period, polishing and hardening the contact surfaces.
• Worn sheave grooves. Even with correct tension, a worn groove provides less grip surface, causing the belt to microslip and glaze.
• Heat exposure. Elevated ambient temperature or proximity to heat sources hardens the rubber compound.

Fix: A glazed belt cannot be restored. Replace the belt. Inspect and replace worn sheaves. Adjust tension to specification. If heat is the cause, upgrade to a cogged belt for better heat dissipation or address the heat source.

Oil and Chemical Contamination

The belt swells, softens, becomes sticky, or shows visible deterioration without mechanical overload or misalignment being present.

• Oil or grease. Leaking bearings, oil mist from nearby equipment, or accidental spills. Oil causes neoprene to swell and lose grip.
• Solvents. Cleaning solvents or chemical splashes from adjacent processes.
• Coolant or cutting fluid. Metalworking fluid dripping onto the drive.

Fix: Identify and stop the contamination source (fix bearing seals, add shielding, redirect drip paths). Clean the sheave grooves thoroughly with a non-petroleum solvent. Replace the contaminated belt. An oil-soaked belt cannot be cleaned or salvaged. If oil contamination is unavoidable, consider upgrading to a belt with better oil resistance (some premium compounds offer improved chemical resistance).