Key Formulas

Where G is the balance grade (mm/s), ω = 2π×n/60 (rad/s), M is the rotor mass (kg), e_per is the permissible specific unbalance, U_per is the permissible residual unbalance, and F is the resulting centrifugal force.

Balance Grade Selection for Crushers

Crusher Type	Typical Grade	Typical RPM
Horizontal shaft impact (HSI)	G16 – G40	500–800
Vertical shaft impact (VSI)	G6.3 – G16	1000–2000
Hammer mill	G16 – G40	1000–1800
Jaw crusher (flywheel)	G16	200–400
Cone crusher	G6.3 – G16	300–600

Per-Element Mass Tolerance

When replacing hammers or blow bars, individual element mass variation contributes directly to rotor unbalance. Each element sits at a specific radius from the rotation axis. The per-element mass tolerance should be:

Where r_element is the CG radius of the element and N_elements is the number of elements.

Impact on Bearing Life

Unbalance force acts as an additional rotating radial load on bearings. The basic bearing rating life (L10) is highly sensitive to applied load:

• Ball bearings: L10 ∝ (C/P)³
• Roller bearings: L10 ∝ (C/P)^(10/3)

Even moderate unbalance forces can significantly reduce bearing life when combined with the already-high process loads in crushers.

Practical Balancing Procedure for Crushers

• Step 1: Weigh all hammers/blow bars individually and record masses
• Step 2: Sort elements by mass and pair heaviest with lightest
• Step 3: Install paired elements on opposite sides of the rotor
• Step 4: Verify total mass difference between opposing positions is within per-element tolerance
• Step 5: After installation, run the crusher and measure vibration at both bearings
• Step 6: If vibration exceeds limits, perform single-plane field balancing

Centrifugal Force and Bearing Life

The centrifugal force from unbalance adds a rotating radial load to the bearings. The bearing L10 life relationship is:

• Ball bearings: L10 = (C/P)³ × 10⁶ / (60 × n)
• Roller bearings: L10 = (C/P)^(10/3) × 10⁶ / (60 × n)

Where C is the dynamic load rating, P is the equivalent dynamic load (including unbalance force), and n is RPM. Even a small unbalance force can significantly reduce life when added to the already-high process loads in crushers.

Vibration Limits for Crushers

Due to the inherent impact nature of crushers, vibration limits are higher than for smooth-running machines:

• Good: < 10 mm/s velocity RMS on bearing housings
• Acceptable: 10–18 mm/s — typical for operating crushers
• Alert: 18–28 mm/s — investigate, check wear items
• Danger: > 28 mm/s — shut down and inspect

Foundation and Structural Considerations

Crusher foundations must be designed to absorb impact forces. The foundation mass should be 3–5× the crusher mass for adequate vibration isolation. Key checks:

• Anchor bolts: Verify torque at every major maintenance event
• Isolation mounts: Inspect rubber isolators for deterioration and correct deflection
• Concrete condition: Check for cracks, especially around anchor bolt pockets
• Grout integrity: Verify no voids between baseplate and foundation

Crusher Types and Balance Considerations

• Horizontal Shaft Impact (HSI): Blow bars are the primary wear item. Replace as a set and weigh individually. Rotor is typically balanced to G16.
• Vertical Shaft Impact (VSI): Higher speeds demand tighter balance (G6.3–G16). Wear table and anvil ring affect balance indirectly.
• Hammer Mill: Multiple hammers on pivot pins. Reversible hammers must be rotated in matching pairs. G16–G40 depending on speed.
• Jaw Crusher: Flywheel balance is critical. Eccentric shaft unbalance is inherent by design but must be within tolerance.
• Cone Crusher: Mantle and bowl wear affect balance. Head assembly balance is checked during major rebuilds.