Pugmill Paddle Arm

Function

The arm's primary mechanical function is to accurately locate the tip at the correct swept radius and helix angle to ensure complete mixing within the pugmill geometry. If an arm bends, cracks or wears to the point that the tip engagement angle changes, mixing action deteriorates — potentially causing segregation of the hot mix or inefficient bitumen coating of coarser aggregate fractions. A cracked or fractured arm is a plant-stopping failure: should an arm fragment enter the pugmill discharge gate, it can cause serious damage to downstream equipment. Inspecting arms at each tip replacement and replacing at first sign of cracking or significant shank wear is established best practice.

Construction & quality

Arms are either drop-forged from 42CrMo4 alloy steel, quenched and tempered to 300–350 HBW, or cast from austempered ductile iron (ADI Grade 900/600) for weight reduction on larger pugmill configurations. The shaft-mounting bore is machined to H7 tolerance on a twin-key or spline coupling. The tip-mounting face and bolt holes are precision-bored and spot-faced to ensure consistent tip geometry across the full shaft set. Arm surfaces are shot-blasted to Sa 2.5 and coated with high-build epoxy primer for corrosion protection between tip change intervals. Mass is balanced within ±50 g per arm to limit shaft vibration at operating speed.

When to replace

• Shank face wear depth exceeding 3 mm from direct aggregate contact — indicating the worn tip has allowed aggregate to reach the arm body.
• Any visible crack in the arm body, particularly at the shaft-mounting radius — treat as an immediate replacement trigger.
• Arm bow or permanent set exceeding 3 mm over the arm length, indicating an overload event.
• Bolt-hole thread damage preventing secure tip retention under operational torque.
• Loose arm on shaft — fretting at the key or spline seat indicating taper wear requiring shaft-fit inspection.