Oilless Bearings Benefits:

High Load Capacity: The high-strength bronze can support substantial loads without deforming.

Low Maintenance: The self-lubricating properties reduce the need for regular maintenance and re-lubrication.

Long Service Life: The combination of material strength and self-lubrication contributes to a longer service life.

Resilience to Wear: The hardness achieved through heat treatment helps resist wear, even under continuous operation.

Versatility: These bearings can be used in a variety of applications, including industrial machinery, automotive suspension systems, and heavy equipment, where they can operate reliably in challenging conditions.

Selecting Bronze Bearing Materials

Bronze sleeve bearings often outlast the equipment they serve when properly designed, manufactured, and maintained. Bronzes are the most versatile class of bearing materials, offering a wide range of properties from various alloys and compositions. This article aims to help designers select the right bronze for their application.

Bearing Design

Good bearing design involves three fundamental elements:

1. Understanding the service environment
2. Designing for proper lubrication
3. Selecting the excellent bearing material

Accurately assessing expected service conditions is crucial. Creating or identifying the lubrication mode is equally important. The bearing material selected must suit both the service environment and operating mode.

Service Conditions

Major areas of consideration for service conditions include:

• Load, steady and impact
• Speed at design load
• Oscillating motion
• Corrosive environments
• Dirty environments and/or lubricants
• Temperature
• Frequent start-stop operation
• Questionable or interruptible lubricant supply
• Shaft or journal misalignment
• Hardness differential, bearing vs. shaft

Satisfying the often conflicting demands of the particular mix of operating conditions makes almost every bearing design a compromise. Conditions involving extremes of load, speed, or temperature are especially challenging.

Operating Modes

The next step is establishing the in-service lubrication mode – hydrodynamic, boundary, or mixed film. Sometimes this can be done by design, other times it must be accepted by default.

Hydrodynamic Mode

In this mode, the bearing is completely separated from the shaft by a continuous film of oil. The oil is under hydrodynamic pressure created by the relative motion between bearing and journal. Hydrodynamic conditions are established by:

• Velocity greater than 25 sfpm
• Unidirectional motion (no oscillation)
• Proper lubricant viscosity
• Proper lubricant flow

A bearing’s operating mode is described by a bearing characteristic number, A, which determines whether the bearing will operate hydrodynamically. If load, speed, and other factors can be adjusted such that A falls between 0.0005 and 0.50, the bearing should operate in the full-film mode.The designer’s job has been simplified by the availability of tables which combine all the factors germane to full-film operation for common bearing configurations, listing more than 90,000 bearing conditions. Since there is no metal-to-metal contact, frictional coefficients can be as low as 0.001, and bearing life should be indefinite.

Boundary Lubrication Mode

The most demanding condition from the bearing material standpoint is when the bearing and shaft actually touch or are separated only by a microscopically thin film of oil. Boundary-film conditions exist when:

• Bearings are grease-lubricated
• Bearings are only periodically lubricated
• Motion is reciprocating (oscillating)
• Surface velocity is less than 10 sfpm

Bearings operating under boundary conditions demand the greatest care in material selection. The bearing alloy must have low inherent friction, resist welding and seizing, and have high thermal conductivity.

Mixed Film Mode

Between the hydrodynamic and boundary regimes lies the mixed-film mode, where a portion of the bearing’s surface is supported by a hydrodynamic oil film while the rest operates with metal-to-metal contact. This is probably the most common of the three operating modes.

Material Properties

Many millions of bearings operate successfully in the boundary and mixed-film modes for their entire service lives. Even hydrodynamic bearings pass through boundary and mixed-film modes during start-up, shut down, or when faced with transient upset conditions. Material selection is an important design consideration for all sleeve bearings, no matter their operating mode.The general attributes of a good bearing material are:

• Low coefficient of friction versus hard shaft materials
• Good wear behavior against steel journals (scoring resistance)
• Ability to absorb and discard small contaminant particles (embedibility)
• Ability to adapt and adjust to the shaft roughness and misalignment (conformability)
• High compressive strength
• High fatigue strength
• Corrosion resistance
• Low shear strength (at the bearing-to shaft interface)
• Structural uniformity
• Reasonable cost and ready availability

Frictional coefficients for bronze alloys against steel range between 0.08 and 0.14. During wear or with no lubricant present, the coefficient may range from about 0.12 to as high as 0.18 to 0.30. By comparison, the coefficient during wear for aluminum on steel is 0.32 and for steel on steel it is 1.00.A good bearing material should be able to embed small dirt particles in its structure, keeping them away from the steel shaft. Conformability is related to the material’s hardness and compressive yield strength. A high yield strength is also related to good fatigue resistance, largely defining the material’s load-carrying capacity.Corrosion resistance is especially important in aggressive environments or for bearings that stand idle for long periods. Structural uniformity is also crucial, as the alloy’s properties should not change as surface layers wear away.

The Bearing Bronze Families

The bearing bronzes fall into five alloy families with similar compositions and properties:

1. Tin bronzes
2. Leaded tin bronzes
3. High-leaded tin bronzes
4. Aluminum bronzes
5. Manganese bronze

Members of each family have similar properties, with differences allowing selection based on economic considerations or the need to tailor alloys for particular applications

Product Overview

a. Low maintenance frequency. b. More capable of handling heavy loads compared to rolling bearings. c. Usable under harsh temperature conditions of “high heat” and “cold.” d. Compared to rolling bearings, bushings are thinner and save space. e. Suitable for both linear and rotational movements.

Product Features

a. Special brass alloy substrate impregnated with lubricating oil specifications. b. Exhibits excellent wear and sintering resistance under lubricated conditions with oils and greases. c. Ideally suited for high-speed, light-load conditions when lubrication is provided.

Expanded Performance Limits for Common Plain Bearing Materials

This table provides typical maximum working values for various bearing materials under different lubrication conditions. These values serve as a crucial guideline for design engineers to ensure the longevity and reliability of a bearing in a specific application.

Understanding the Key Terms:

• Bearing Pressure (P): This is the specific load on the bearing, calculated by dividing the total force (in Newtons) by the projected area of the bearing (in mm²). It is expressed in N/mm², which is equivalent to Megapascals (MPa). Exceeding the maximum pressure can lead to material deformation or seizure.

• Surface Velocity (V): This measures how fast the shaft’s surface moves relative to the stationary bearing surface, expressed in meters per minute (m/min). High speeds generate more frictional heat.

• PV Factor (P x V): This is the product of pressure and velocity and is the most critical performance indicator. It represents the bearing’s ability to dissipate the frictional heat generated during operation. Exceeding the maximum PV factor for a material will cause its temperature to rise rapidly, leading to lubricant breakdown, excessive wear, and premature failure.

Working Values Chart

*Note on Sintered Bronze:

• The values for Sintered Bronze are for applications without additional external lubrication. This material is porous and is impregnated with oil during manufacturing, making it self-lubricating.

• It excels in high-speed, low-load applications where external lubrication is impractical or undesirable. The internal oil provides a continuous lubricating film as the bearing heats up during operation. Force-feed oil lubrication is generally not applicable as it would flush out the impregnated oil.