Authors: Arthur Jones & Aliesha Pocratsky
Sykesville, PA 15856
- Oil impregnation and near-net-shape processes make PM bearings affordable.
- PV ratio, operating temperature, K-value, and oil percentage are key design parameters.
- Frequent testing and process control ensure quality.
“Stator Stakes Claim to Top Powder- Metal Prize,”
MACHINE DESIGN, July 10, 2008 showcases creative uses of PM.
Edited by Jessica Shapiro, [email protected]
By Arthur Jones and Aliesha Pocratsky
They could if they are made from powder metal (PM). Engineers have relied on bearings and bushings made from PM for over 80 years, and recent advances have made PM parts even more attractive. Reliable test data, quality control, and specifications can help designers select and apply these versatile parts.
Designers considering PM bearings for a specific application should look at the required operating temperatures, K-value, oil content, and pressure-velocity (PV) ratio. These four properties are essential pieces of information for engineers who predict how bearings will operate in the real world. At the design phase, the engineer is relying on the strength and oil capacity of the bearing to meet, if not exceed, the design requirements.
Any bearing selection process should first evaluate the maximum pressure (P) in psi exerted on the bearing and the maximum velocity (V) of the shaft in surface feet/minute (sfpm). The relationship between P and V is given as a combined numeric metric in rpm psi, the PV ratio. PM bronze bearings are a good choice when the PV ratio reaches but does not exceed 50,000.
Another metric to consider is the K-value, the radial crush force that can break or flatten a bearing. The Metal Powder Industries Federation (MPIF), an industrial association responsible for powder-metalbearing standards across the industry, requires that selflubricating sintered-bronze bearings have a K-value of at least 26,000 psi of force.
Engineers should also consider the bearing’s operating temperature while in-field. While the bearing material itself can withstand high temperatures, actual bearing performance depends on the temperature range of the lubricant.
In normal operations, bearings can heat up to about 120°F without any performance loss. However, synthetic oils can boost performance if the bearing must operate in harsher settings where the ambient temperature is below 30°F or above 200°F. Adding substances like graphite to the lubricants can further extend performance.
Oil content measures the amount of oil stored in the bearing’s interconnected pores as a percent of the part’s total mass. PM bearing manufacturers strive to create parts that have both high K-value and many capillarylike interconnected pores that let the bearing store the maximum amount of oil. Most oil-impregnated bronze PM bearings have an oil content of 19 to 20%.
The quality of a bearing depends on the raw materials from which it is manufactured and the manufacturing process itself. (See sidebar for details on the PM process.) Designers should choose PM bearings made from high-quality precursors. The bearing supplier should test incoming powders’ particle size, chemical composition, and other quality markers. Long-term relationships with trusted suppliers provide an extra level of certainty about raw material quality.
The manufacturer should also be committed to continuous maintenance and, if needed, replacement of tooling to ensure consistent products. An on-site machine shop can help ensure quick tool turnaround and tooling quality.
Designers should monitor in-process quality continuously. The technicians setting up the presses and monitoring control parameters should be skilled, experienced, and knowledgeable.
Frequent testing of metrics such as K-value, oil content, and density can catch quality problems early. Symmco, for example, tests randomly selected bearings for these properties. Test results are compared both to MPIF standards and to process history to permit continuous improvement. Symmco’s technical team studies how molding, sintering, sizing, and oiling processes affect ultimate bearing performance.
Having a machine on-site that can test bearings under load lets Symmco simulate the demands on the bearing during operation and ensure bearing quality. Ongoing tests also look into how the type of oil impregnated into the bearing affects the steady-state temperature.
The near-net-shape process these metrics monitor is the driver behind the lower cost of PM bearings. Pressing the parts into a custom mold results in inside-diameter and outside-diameter tolerances between 0.001 to 0.002 in., so depending on the application, a finishing step may not be needed. Designers may also have the option of molding in additional features such as keyways or windows instead of machining them with a secondary process.
The self-lubricating properties of oilimpregnated PM bearings also provide design flexibility. As the temperature of the bearing rises in service, the oil releases, creating a cushion between the shaft and the bearing surfaces. When rotation of the shaft or bearing stops and the bearing cools, it again absorbs the oil that was released onto the shaft, although reabsorbtion never reaches 100%.
Self-lubrication means it may not be necessary to machine oil grooves into the faces of the parts. The bearings can also self-lubricate in high-load uses that tend to squeeze out applied lubricants.
The resulting low coefficient of friction saves energy and lowers torque drive requirements. Self-lubrication ultimately brings less downtime and longer life.
In a powder-metal (PM) part, powdered elemental metals or alloys combine with lubricants to produce a homogeneous mix. A mixture of copper and tin powders makes a bronze part.
A die is filled with the mixture and then sees pressures from 10 to 45 tons/in.2 to produce a powdermetal briquette that is 105 to 110% of the final part size.
Next, the briquette heats up in an inert atmosphere to a high temperature (1,500°F for bronze) for sintering. The sintering temperature is below the melting point of the metal but is still sufficient to fuse the individual metal particles into a strong network of interconnected grains surrounding pores.
After sintering, the part is again compressed to the desired size and physical properties.
A second heating in a protective atmosphere, followed by an oil quench improves hardness and strength. A lubricant appropriate to the application can be impregnated into the part’s pores which act like an oil sponge. Designers can specify food-grade, medical-grade, military-spec, and other lubricants depending on the application.