Detailed Breakdown of Sliding Bearing Families

To achieve optimal longevity, mechanical design engineers must align the bearing’s material properties with the specific operational dynamics.

1. Solid Lubricating Bronze Bearings (Maintenance-Free)

Engineered for oscillating movements and high impact loads, these oilless bearings rely on a cast bronze base integrated with solid lubricant plugs (Graphite or PTFE compounds).

• Material Spotlight (CuZn25Al5): This high-strength alloy offers exceptional static load capacity (up to 150 N/mm²). However, engineers must note that due to thermal dissipation limits in boundary lubrication, its maximum dynamic PV limit is ~1.5 N/mm²·m/s.

• Friction Coefficient (μ): 0.08 – 0.25

• Shaft Requirements: The shaft must be a minimum of 100 HB harder than the bearing base, with a surface roughness of Ra 0.2 – 0.8 μm.

2. PTFE Lined Metal-Polymer Bushings

PTFE (Polytetrafluoroethylene) composite bearings consist of a metal backing, a sintered bronze interlayer, and a PTFE sliding surface. They are the go-to solution for maintenance-free, dry sliding applications.

• Maximum PV Limit (Dry): ~1.8 – 3.6 N/mm²·m/s.

• Crucial Design Note: Shaft surface roughness is highly critical here. Engineers must specify a shaft roughness of Ra 0.1 – 0.4 μm. Why? A surface too rough will scratch the polymer layer, while a surface too smooth will prevent the essential transfer film from forming.

• Tolerances: Housing H7 | Bushing Inner H9 | Shaft f7/f8. (Strict interference control is required for the polymer layer).

3. POM Lined Metal-Polymer Bushings

Unlike PTFE, POM (Polyoxymethylene) sliding layers feature lubrication pockets and are designed as a low-maintenance solution. They excel in environments where continuous grease or oil is present, easily handling dirt and abrasive particles.

Critical Mechanical Design Constraints

Selecting the material is only half the battle. Geometric configuration and environmental factors dictate the final performance.

Managing Thermal Expansion

For applications operating at temperatures exceeding 100°C, thermal expansion must be accurately calculated to prevent clearance reduction and subsequent shaft seizure.

Formula for Linear Expansion:

ΔD = α × D₀ × ΔT
(ΔD = Expansion in mm | α = Coefficient /°C | D₀ = Base Diameter | ΔT = Temp difference)

⚠️ Engineering Warning: To accurately calculate radial clearance changes, you must consider the differential expansion between the shaft material (e.g., Soft steel: 1.12 x 10⁻⁵ / °C) and the bearing housing.

Dimensional & Geometric Tolerances

• Length-to-Diameter Ratio (L/D): Keep general applications between 0.5 – 2.0. For systems dealing with high loads or uneven contact, restrict the L/D ratio to 0.8 – 1.0 to prevent edge loading.

• Chamfering: Proper chamfers (e.g., 1.0 x 45° for a 50mm ID bearing) are critical to facilitate freeze-fitting or mandrel press-fitting without damaging the bearing edges.