Pugmill Shaft Seal and Bearing

Function

The shaft bearing carries radial loads from the paddle arms — resisting the resistance torque of the aggregate charge — plus any residual axial thrust, while maintaining precise shaft centreline position for consistent tip clearance across all trough positions. The shaft seal excludes hot bituminous mix from the bearing housing. Seal failure allows mix to enter the housing, carbonise the bearing grease, and accelerate bearing wear to seizure within hours. Loss of shaft radial position from advanced bearing wear degrades mixing uniformity and may cause paddle tip contact with the trough liner.

Construction & quality

Bearing: spherical roller (ISO 22200 series) with C3 internal clearance for elevated-temperature service and shaft deflection accommodation. Bore 80–160 mm depending on mixer shaft diameter and batch capacity. Housing: solid or split cast iron or fabricated steel plummer block, with labyrinth inner seal and external multi-lip contact seal. Shaft seal (mix-side): FKM (Viton) V-ring or split labyrinth seal with grease-purge inlet port. Purge grease is delivered at 5–15 g/hour by the plant's centralised automatic lubrication system or by a dedicated manual grease point, maintaining positive inward pressure at the V-ring lip. V-ring temperature rating to 200 °C continuous.

When to replace

• Bituminous mix exuding past the shaft seal toward the bearing housing, visible as a dark residue trail on the end plate
• Bearing housing temperature above 90 °C at normal ambient conditions, indicating lubrication degradation or increased friction
• Carbonised or black grease at the bearing inspection plug, confirming mix ingress into the housing
• Perceptible radial play in the shaft detectable by rocking a paddle arm by hand with the mixer stopped
• Squealing or rumbling noise from the mixer end-plate area at low shaft speed during the post-batch coast-down period