|
Permawick Design Guide for Sintered Bearing Motors
Medium Service 50,000 + hours, Replaces Ball Bearings
Design Inputs
- Life: 50,000 + Hours
- Load: Light to moderate, 35 Pounds Max.
- Speed: 200 Feet/Minute Max.
- Temp: 100C Max Operating temperature
The following figures show a bearing design for FHP motors using a sintered metal, self-aligning bearing. Motors with this type of design and construction are typically found in air moving, appliance, HVAC, small pumps and lightly loaded applications.
 The key design feature in these examples is oil re-circulation. By recycling the oil from the slinger back into the Permawick, where it is re-absorbed and transferred to the bearing, long service life is achieved. Oil re-circulation requires the use of Permawick wicking materials. Felts, gels and greases do not have the re-absorption capabilities of Permawick and felts and gels lack sufficient oil-holding capability to achieve long service life.These designs take advantage of another property of Permawick. They both permit the injection of lubricant and assembly of parts in a single operation. Using either the tab-down design or the light press-fit design, the Permawick injection machine can inject the Permawick and assemble the parts at the same time. This eliminates rivets, welds, etc. and consolidates assembly, while permanently lubricating the endshield in one, simple process.
 Note: Hardened shafts and synthetic lubricants will add only pennies to the overall cost per endshield but push the quality of the bearing assembly well past 50,000 hours - a real competitive advantage over ball bearings.Permawick
The choice of Permawick, whether a mineral oil blend or a synthetic like PSL313NP should be determined by the expected temperature. The NP fiber should always be used for sintered bearings and the HH should be used in conjunction with a felt contactor. NP fiber has an engineered release rate just short of the capillary force exerted by the pores in the sintered bearing. NP fibers provide the right "resistance" to oil release and maximize bearing efficiency. Similarly, HH fibers are engineered to release just short of the Felt pull and keep the bearing/oil system in balance. The capillary force is in sintered metal is stronger than that in felt. Using HH fiber in sintered metal could potentially result in too much oil release to the journal and sometimes lead to oil leakage. NP fibers would tend to resist oil release to a felt and could potentially starve the journal. Each material was designed specifically for its application and proper use will ensure success.
To extend motor life, a re-circulating design should be used. Refer to the oil catcher and oil slinger discussion. The extended oil catcher does 2 things: it increases the reservoir for Permawick, thereby increasing the store oil in the system and it provides an oil pickup place for slung oil. Permawick is injected in the reservoir and the oil catcher as a single, continuous material. Oil picked up by Permawick in the oil catcher will be transferred down to the Permawick surrounding the bushing and effect the re-circulation process.
The method of Permawick injections into the bearing cavity is important to the proper operation of the bearing system. Contact between the sintered bushing and the Permawick is necessary in order for the oil to pass from the Permawick fiber to the bushing and then to the shaft. Contact of the Permawick with the bushing is best when the Permawick contacts the majority of the outer surface, but no less than 20%. Permawick should not contact 100% of the outer surface, because room should be left in the cavity to allow for swelling of the material under elevated temperatures. Never fill the bearing cavity 100% as this could result in leakage.
The tool and method shows how Permawick is injected simultaneous with the press-fit assembly of all the mating parts of the motor end bracket bearing assembly.
The figure above shows the Injection-Assembly tool. The Permawick is highlighted in yellow. Upon injection, a light press-fit can close the tabs in the endshield. securing the cover assembly to the endshield and injecting at one time.
Bearings
Material: Diluted Bronze or Iron-graphite
Bronze - Conforming to ASTM Spec. B202-58T Type 1, Class A
- % Copper 87.95 - 90.5%
- % Iron 1.0 Max
- % Tin 9.5 - 10.5%
- % Carbon 1.75 (Graphite)
- Other .5 Max
- Density 6.4 - 6.8
- Porosity 12 - 18% Max
Iron-Graphite - Conforming to ASTM Spec for sintered iron diluted with 20% bronze
The selection of bronze over iron is sometimes made for marketing purposes. The advantage of bronze over iron is minimal and usually does not justify the added cost. Bronze bushings are less abrasive than IG, have a lower coefficient of friction and are at times, quieter than IG bearings. Where noise is particularly important, bronze is recommended. To further reduce the bearing noise lever, a more porous (less dense) bushing can be specified, with a density range of 6.0 - 6.4. Increasing the density of bronze bushings improves the wear quality to a significant but small degree,. The density increase, however, should not be such as to seriously effect the flow of oil to the shaft. At elevated temperatures, bronze bearings (200-225F) have a greater detrimental effect on mineral oils than do IG bearings. This detrimental effect is due to the free copper in the matrix of the bushing. Since copper is catalytic in the oxidation process, this should be avoided.
The length of the sintered bearing (the L/D ratio) is not constrained by the teaching of the hydrodynamic theory of lubrication, since sintered bearings operate in the area of thin and spotty film at best, but most often in the boundary area. Therefore, an L/D of between 1.0 and 1.5 is recommended. Length beyond 1.5 is subject to end wear and oil starvation due to misalignment and shaft deflection. Sintered bearings present a problem for the bushing manufacturer to produce a bushing of homogenous structure to entire length of the bushing. Wear particles may bring the degradation of the mineral oil to an unacceptable level, where elevated temperatures are encountered and a bronze sintered bushing is required. The election of a synthetic oil must be considered for use as the impregnating oil, as well as in the Permawick lubricant.
Note: the chamfer on the ID of both ends of the bearing. The chamfer helps guide the shaft into the journal without damage and allows oil to move outward onto the face, lubricating axial loads.
Retaining Spring
Material: Spring Steel hardened to 40-45 Rockwell "C", 10-15 pounds force at assembly
The spokes of the retaining spring ring should be oriented so as not to block the pass through the hole in the motor bracket stamping. When the outside oil catcher is not used, the orientation of the tang need not be adhered to, since the pass-through the holes would not be in the motor bracket. The retaining spring should be .005/side larger than the cylindrical portion of the bushing, so as not to interfere with the spherical portion of the bushing to align itself to the conical nest of end bracket. The OD of the tangs of the retained spring should, therefore, be .001 - .004 loose in the shoulder of the motor bracket provided for it. The spring retainer should have an odd number of fingers with a minimum opening of 50% of the surface area. This allow for the part to be placed in the bracket without being readily located and allows for Permawick to be injected into the part with the minimum chance of hitting the fingers. Typical nozzles use 2 holes. An odd number of fingers ensures that at least one inject hole will clear the metal. Note that this design "floats" the retaining spring. It is not trapped between the cover and the endshield. Floating the spring reduces the risk of side loading of the bearing.
Oil Slinger and Shaft
Oil Slinger
Material: Nylatron
The Slinger/Thrust Washer should be tight on the shaft O.D. to ensure rotation with the shaft. If anticipated thrust load on the motor is expected to be less than 15 pounds, then the nylatron surface of the slinger can rub against the porous edge of the bushing wall (minimum wall thickness .100 inches). If higher thrust loads are anticipated, the hard steel (Rockwell 40-45C) should be interfaced between the slinger and the bushing. It is critical that the OD of the slinger DOES NOT come in contact with the Permawick material in the cover. If a rotating surface is in contact with a reservoir of oil, it will act as a pump and quickly drain the system of oil.
Shaft
Bearing service life can be greatly increased by holding two key specs on the shaft - finish and hardness. Shaft finish should be approximately 12-15 RMS. Shaft hardness should be a minimum of 95 Rockwell B. Much longer bearing service life can be obtained by hardening shafts to 35 Rockwell C. In sintered metal bearings, shaft particles are worn off by the abrasive metal. These wear particles (typically iron) act as a catalyst, causing sludge and varnish to form, which lead to bearing failure. Hardened shafts limit the amount of wear and synthetic oils eliminate the catalytic reaction.
A combination of hardened shafts and synthetic oil adds only pennies to the cost of the assembly, but can double bearing service life, reaching 50,000 hours for a fraction of the cost of ball bearings. This is a huge competitive advantage for motor makers.
Internal Cover
Both inner and outer oil catchers (for shaft-down applications) should be plated with a suitable material to prevent rust in many air moving applications. Moisture can condense and remain in the reservoir. Permawick rejects moisture, but prolonged exposure of the thin sheet steel to moisture can lead to corrosion.
Note the burr or turned down lip at the top of the retainer. This lip is useful for holding the Permawick material in place inside the cover. The ID of the cover should be at least 1/4" larger that the OD of the bearing to allow for easy injection of the Permawick. See above tooling drawing. This cover can have a flat OD, as pictured, for tab-down endshields or it can have a burr or no-back OD for press-fit into endshields.
Endshield
Stamped endshields form a nest for the bearing to sit in that allows for self-alignment. Note the grooves pressed into the bearing nest area. These grooves allow for oil that puddles below the bearing (think horizontal) to return along the OD of the bearing, effectively re-circulating off the back side. Note also the turned-in lip at the bottom of the bearing nest. This is a critical feature of a Permawick design, because it allows an area for oil to puddle, feeding the abovementioned grooves. Endshield designs can support tab-downs to secure the internal cover, or press-fit covers.
Note that no gaskets, O-rings or sealers are required. Proper Permawick placement eliminates leaks, simplifying the design.
|
