REPLACING BRONZE BEARINGS WITH CIP COMPOSITES
CIP Composites are laminated polymer materials made by impregnating textile with thermoset resins. The physical properties of CIP materials make them superior bearing materials. CIP Composites offer design engineers an attractive, cost effective alternative to the traditional material used for bushings, bearings, wear pads, and thrust washers. The following describes the physical characteristics and application factors to replacing bronze bearings with CIP Composites.
Coefficient of Friction
CIP Composites have lower coefficients of friction than bronze in both static and dynamic situations. It is common to achieve 50% lower coefficients of friction when transitioning to composites. Bronze bearings pose a risk for metal to metal contact if grease is non-existent or not sufficient to support the shaft, and may develop high break-away torque requirements after prolonged periods of rest. CIP Composites are self-lubricated, minimizing the concerns with lubrication systems, specifically in high load, slow speed, or oscillating applications. CIP Composites eliminate noisy stick-slip problems, extend operating life and reduces wear, making for better operating efficiency.
There can be concerns with moisture absorption for non-metallic materials. Moisture absorption can create swell and expansion of the bearing material and potentially premature failure. Some common plastics in use today have water absorption rates as high as 5-7%, which can lead to bore closure of the bearing and seizing on the shaft. CIP Composites have a negligible absorption of less than 0.1%, and effectively eliminate this concern when moving away from a metallic solution.
Thermal Expansion Rate & Operating Temperature
One of the dominate differences between bronze and composites is the rate of thermal expansion. CIP Composites provide a higher coefficient of thermal expansion rate that can result in a more efficient freeze-fit installation. Once the bearing is in operation, an increase in temperature (whether from surface friction or the external environment) causes the composite material to expand and can reduce running clearance. This higher expansion rate of CIP Composites is predictable and easily accounted for when establishing the dimensional specifications.
In large bearing and component sizes, the weight savings of going from a bronze or other metallic solution to CIP Composites is substantial. Greatly reducing the material handling issues and installation challenges customers may have experienced with typical greased metallic bearing solutions.
CIP Composite materials are more elastic and will conform non-permanently at a lower pressure than bronze, allowing it to work well in misaligned conditions. At higher operating pressures composite materials conform more to the shaft and the housing than bronze, better distributing the pressure across the bearing contact area which can help eliminate the effects of galling which is a leading cause of bronze bearing failures.
Externally lubricated bronze bearings rely on boundary layer lubrication to prevent damage due to metal to metal contact. This layer of lubrication is susceptible to pressure distribution. In the areas with edge loading or uneven pressures, the lubrication will reside in lower pressure regions; resulting in loss of lubrication in the critical high pressure regions. CIP Composites do not rely on external lubrication to provide this boundary layer; the lubrication is evenly distributed through the bearing material.
load and working pressure
Based on the materials elastic characteristics for a given running clearance, CIP Composite materials will distribute the load to a wider contact surface and more evenly distribute pressure, whereas bronze will distribute the load to a narrower contact surface. The difference in pressure distribution is largely attributed to the increased conformal contact of the composite bearing, in bronze the contact surface is more dependent on the running clearance. Acceptable distribution can be achieved using composites with larger running clearances.
Shaft speed is translated into surface speed based on the shaft diameter, rotational speed and degree of rotation (oscillating or full 360°). Revolution per minute is converted to feet per minute and is correlated to the amount of heat generated at the bearing surface. The higher the surface speed, the more heat generated. CIP Composite materials are thermal insulators and heat generated at the surface does not dissipate well. Bronze materials are thermal conductors so heat from the surface is dissipated more efficiently.
Pressure Velocity (PV)
PV is the product of pressure and surface speed and is critical to evaluating the amount of heat generated from friction. Each bearing material offers different capabilities of PV, however, coefficient of friction and application conditions must be considered. In principle tolerated speeds decrease as the bearing pressure is increased.
Considering both the surface speed and operating frequency allows for a more accurate judgment of the potential heat build-up. Critical to composite materials, the operating frequency determines the available time to dissipate heat. In applications with continuous operation at higher PV’s, heat build-up must be calculated into the running clearance to allow for thermal expansion.
Replacing bronze with CIP Composites offers environmental, performance and maintenance benefits. The experienced staff at CIP can provide custom tailored design/engineering proposals for your application and most custom manufactured orders ship within three weeks, with the ability to expedite orders.
CIP is dedicated to providing the highest quality products with a focus on exceeding requirements. We strive for complete satisfaction through continuous improvement in service and quality.