The motor pulley and pump pulley must be in the same plane. If one is offset or angled, the belt runs at an angle and chirps as it enters each pulley. Misalignment also causes uneven belt wear -- one side wears faster than the other.

What you'll see

A persistent chirping or squeaking noise while the compressor runs -- not just at startup. The belt may visually ride to one side of a pulley, or wear more on one edge. If you look at the belt from the side while running (from a safe distance), you may see it entering the pulley at a slight angle rather than straight on. On dual-belt drives, one belt may wear faster than the other if the misalignment causes it to carry more load.
Before you assume this is the problem

If the noise only happens at startup and goes away at running speed, it's more likely a tension issue than alignment. Alignment problems typically cause continuous noise during all operation. See: Belt Too Loose.

See all causes of belt squeal / screaming sound →

How to diagnose

  1. Check alignment with a straight edge

    Place a long straight edge (ruler, level, or even a straight piece of wood) across both pulleys, touching the flat face of each. Both pulleys should contact the straight edge at the same points. If one pulley is further in or out on its shaft, the belt runs at an angle. This is the most common misalignment -- angular offset. Also check that the pulleys are parallel (not tilted relative to each other).

    Result: Straight edge contacts both pulleys evenly = alignment OK. Gap on one side = misaligned.

  2. Check for uneven belt wear

    Look at the belt edges. If one edge is worn more than the other, the belt is running at an angle from misalignment. On a properly aligned system, both edges wear evenly.

    Result: Even edge wear = alignment OK. One edge worn more = misalignment.

How to fix it

  1. Adjust pulley position on the shaft

    If one pulley is offset, loosen its set screw and slide it along the shaft until both pulleys are in the same plane. Tighten the set screw and recheck with the straight edge. On some compressors, the motor position needs adjusting to achieve alignment -- loosen the motor mount and reposition the motor.

  2. Check the motor mount for shifting

    If the motor has shifted on its mounting plate (common after belt tension adjustments), the alignment changes. Make sure the motor is positioned so the pulleys align, then tighten the mounting bolts securely. Some compressors have alignment marks on the motor plate to help with positioning.

Common mistakes

Adjusting belt tension without rechecking alignment afterward. Every time you move the motor for tension, you can change the alignment. Check both tension and alignment together. Also: using pulleys of different widths can make alignment look impossible -- the belt sits at a different depth in each pulley. Make sure both pulleys are the same type and width for the belt being used.

Parts & tools

Straight edge or laser alignment tool. Set screw wrenches (usually Allen keys). Wrenches for motor mounting. Replacement belt if the old one has uneven wear.

Review safety precautions before starting →

Safety

Check alignment with the compressor off. A belt running off a pulley at speed can whip dangerously.

This issue can also cause

  • Unusual Noise / Knocking New or unusual sounds from the compressor: often caused by loose parts, worn bearings, or valve problems.