Both cast and forged clamps are viable, but they have different strengths:

Cast clamps (ZG35CrMo / ZG42CrMo) — best for most applications:

• Advantages: Can produce complex 3D geometries (curved clamp faces, integrated ribs, non-uniform thickness) that would be impossible or expensive to machine from a forging. Lower cost for large clamps (>100 kg).
• Limitations: Internal casting quality depends on riser design and NDT inspection. Porosity or shrinkage at critical sections can initiate fatigue cracks. ZHILI mitigates this with FEA solidification simulation + 100% UT + 100% MT.

Forged clamps (35CrMo Forged) — best for critical/maximum safety applications:

• Advantages: Forging refines the grain structure, eliminating the internal porosity risk inherent in castings. Higher impact toughness (40-60 vs 30-50 J/cm² for equivalent cast grade). More uniform mechanical properties through-thickness.
• Limitations: Higher material and machining cost (1.3-1.5x vs equivalent casting). Limited to simpler geometries that can be forged and machined. Not practical for clamps over 500 kg.

ZHILI recommendation:

• Use forged 35CrMo for: SAG mill VRM clamps, mills with known vibration issues, replacement clamps where a previous clamp failed, Loesche 56+ or Pfeiffer MPS 5600+ (largest roller sizes with highest clamping loads).
• Use cast ZG42CrMo for: Standard raw meal and cement mills, mills with stable operation and no history of clamp issues. When properly cast and NDT-inspected, ZG42CrMo clamps are completely reliable for 99% of applications.

Bolt torque and tightening procedure are as critical as the clamp material itself. Improper bolt tension is the #1 cause of clamp failure:

Bolt specifications:

• Grade 12.9 only. Never use Grade 8.8 or 10.9 bolts on roller clamps — they do not have sufficient yield strength to maintain preload under VRM vibration.
• Nord-Lock washers or Belleville spring washers are mandatory. Standard flat washers will not prevent bolt loosening as the clamp and roller thermally cycle (ambient to 200 C every startup-shutdown).

Three-step tightening procedure:

• Step 1 (50% torque): Tighten all bolts in a star/crisscross pattern to 50% of final torque. This draws the clamp evenly onto the roller face and prevents warping.
• Step 2 (100% torque): Tighten all bolts to final torque in the same star pattern. Use a calibrated hydraulic torque wrench — never an impact wrench (impact wrenches produce inconsistent tension).
• Step 3 (verification): Go around all bolts a second time at 100% torque. The first bolts tightened will relax slightly as later bolts are tensioned — the second pass ensures uniform preload.

Bolt re-torque schedule: Re-torque all clamp bolts after 24 hours of initial operation (thermal cycling settles the clamp). After that, check bolt torque at every planned shutdown — a single loose bolt means the entire clamp is losing preload. Any bolt found at less than 80% of specified torque requires removing and inspecting the bolt for fretting wear or stretching. Never re-tension a stretched bolt — replace it.

ZHILI bolt kit: Supplied with every clamp — Grade 12.9 bolts, Nord-Lock washers, torque specification card, and step-by-step tightening procedure diagram. Never reuse old bolts with a new clamp.