The following figure shows a bearing design for FHP motors using a sleeve bearing. Motors with this type of design and construction are typically found in appliance, commercial and industrial motors, larger pumps and heavily loaded applications.

The design seeks to take advantage of the inject able properties of Permawick, which permits the injection of lubricant, and assembly of parts in one, simultaneous press-fit operation.

Riveting, spot welding, bent tap and cold heading assembly techniques are eliminated, along with the more expensive assembly processes. A single ST2 Permawick machine can inject Permawick and assemble the end bracket in one operation. 

 

Material: Steel backed Tin Babbitt -SAE 12 (89% Tin, 3.5" Copper, 7.5% Antimony)

Tin Babbitt is Predominantly, the Bushing material used in rigid bearings for Fractional Horsepower Electric Motors. Sold Bronze Bushing have lost favor in FHP motors due to higher costs, machine ability, and the deleterious effects on mineral oils.

Tin Babbitt Advantages:

1.   Has excellent load carrying ability (150 PSI). Caution: Above 200 PSI Babbitt has a tendency to cold flow, resulting in increased bearing clearance, resulting in increased wear and noise level.

2.   Chemical compatibility with mineral oils. The presence of tin in mineral oil acts as a oxidation inhibiter, retarding oil oxidation and degradation of the oil. Caution: Do not use lead Babbitt alloys. The presence of lead in mineral oil acts as a catalyst in the oxidation process and accelerates the degradation of oil.

3.   Excellent Conformability - Allows for slight misalignment of assembly and conforms to shaft distortion under load.

4.   Excellent Imbedability - Allows for the imbedding of hard dirt particles, wear debris, etc.

The length of the bushing should be from .9 to 1.2 times the diameter of the shaft  i.e. A L/D ratio of .9 to 1.2

A L/D of .9 is the optimum L/D taught by the Hydro-Dynamic theory of Lubrication. In the case of wick fed bearing, the amount of lubricant fed to the bearing is considerably less than the amount of lubricant fed from a totally liquid system, as in an Drop Feed Oilier. Where the L/D is .9 the amount of lubricant going out the ends of bearing can be too great for a wick fed oil system to replenish. Conversely an L/D of greater than 1.2 can create a condition of lubricant starvation at the bearing ends. Experience dictates that a L/D of between .9 and 1.2 serves well for bearing designs covered in these recommendations. To avoid feather edges and burrs, chamfering the bushing ends is recommended. However, since chamfering reduces the effective length of the bushing, it should be kept to a minimum i.e. from "break corners" to .005. Caution: A bushing length which yields less then a .9 L/D ratio will increase the unit loading of the bushing unnecessarily and requires too great a flow of lubricant to replace the end losses.

The window area of the bushing should be 10-12% of the developed area of the bushing. (See figure 2)

The general shape of the window should be oval. Ideally the oval should be comprised of two parallel sides connected on the ends by a diameter equal to width of the window. The width should be such that it is compatible to the thickness of the available sheet thickness of the felt supplier. The length of the overkill window should run parallel to the axis of the bushing and extend to within .060 - .080 of the bushing end (E Dimension, Figure 2). Conversely, square and rectangular shaped windows should be avoided, since square corners tend to collect wear, debris and oxidized oil, negatively effecting the flow of lubricant to the shaft. 

The window should be positioned opposite the bushing seam and punched so that the die punch enters the Babbitt first to avoid the steel backing from forming a burr in the window. Caution: Avoid a coined area around the window. The desired condition is that the felt contactor and Babbitt present a continuous and uninterrupted surface to the shaft.  

Oil grooves should be used when the window ends are greater then .060/side from the end of the bushing. The grooves should extend from the bushing window to within .060 from bushing edge (See "A" Dimensions, Figure 2), and extend radially 90 degrees from the center of the bearing window. Caution: Oil grooves should not extend radially beyond 180°.

When bushings are made from the suppliers existing tools, the window length often does not extend sufficiently close to the bushing end. Oil grooves are then used to move the lubricant laterally to avoid lubricant starved ends. Oil grooves that extend radially beyond 180° tend to inhibit the formation of the Hydro-Dynamic oil film in the loaded area of the bushing. 

A surface finish of 16-24 RMS or better is recommended. Caution: A bushing finish of greater then 30 RMS must be avoided.

Smooth bushing surfaces are essential in establishing a Hydro-Dynamic oil film. Surface finish of 24 RMS or smoother is recommended. When bushings are made of bronze or aluminum, the surface finish should be in the range of 10-16 RMS. The Babbitt thicker than .015 is subject to cold flow, increasing bearing clearance and adversely effecting noise level and wear rate.

The effect of bushing finish on bearing load and bearing life is shown below on Chart No. 1.

 

 

Wool felt with percent specific gravity of 34.2 - designated as felt class - 34R2 - or SAE designation F2. Where cost permits, F-1 (White Virgin) is recommended.

F-2 felt density is greater than the density of the surrounding Permawick, and as a result, oil will migrate from the Permawick to the felt contactor, and from there to the rotating shaft.

The "A" dimension shown in Figure 3, should be large enough to extend into the bore of the bushing approximately .005 - .010. The sides of the contactor felt should fit snugly against the inner wall of the bearing hub secured by rails cast in the end bracket hub.

A hole of the convenient diameter (3/16-1/4) located, as shown in Figure 3, should be provided to effect contact of the Permawick with a cross cut area of the felt. If the design does permit a hole, a bias but as indicated by the dotted line shown in Figure 3, can be substituted. Felt contactors are die-cut or stamped from sheets or felt. The wool fibers of the felt are interlocked and lie parallel along the axis of the felt sheet. A cut across the sheet presents a section of fibers cut across the fiber diameters. The rate with which oil will pass from cut section to cut section is greater than the rate that oil will pass from the uncut sides of the felt sheet (B Dimension) (Approximate 3-5 times). It is desirable to have a cut section of the felt contactor in contact with the Permawick to accelerate the flow of oil to the shaft.

Felt sheet designate as SAE-F-1 can be used in place of F-2. The F-1 felt is made of virgin wool, more homogenous fibers and is white in color. The density of F-1 and F-2 are for all practical purposes the same and, therefore, will transfer the oil from the Permawick to the shaft with the same efficiency. F-2 felt is died pink for identification.

Caution: The pressure applied on the shaft by the felt contactor should not be great enough to materially effect the power draw (wattage) of the motor or cause a substantial increase in the coast-down time. Too great a contact pressure causes the surface of the contactor to glaze over on the surface in contact with the shaft and inhibits oil flow.

20-25 Grams Permawick 280 HH (See Spec. Sheet #4015-2)

280 HH (Mineral Oil) is the recommended Permawick to meet the designated bearing capability. When a bearing temperature in the range of 200° to 225° F is anticipated Permawick 313 HH (Synthetic) is recommended. Use of 313 HH should be accompanied with approximately 25% reduction of bearing load. A charge of 20-25 grams of Permawick is recommended to obtain a minimum bearing life of 20,000 hours.

Proper injection of Permawick is very important in achieving the expected bearing capability. Figure 4 shows the bearing hub cavity properly filled with Permawick . The Permawick enters the cavity through two orifices adjacent to and straddles the contactors, contacts the sides of the contactor and enters the die cut hole, and then continues to fill the cavity in the direction indicated by the arrows. This method provides a favorable density gradient with the Permawick density at the contactor, being greater then the Permawick in the other areas of the Permawick cavity, causing the oil to migrate (Figure 4). The main body of Permawick in the cavity to the contacts.

 

The Permawick should not be tightly packed in the cavity, but should be filled as shown in Figure "4", leaving a knit line where the Permawick from each injection orifice meets at the opposite side of the cavity. This is accomplished by injecting a metered amount of Permawick in the cavity. A small void formed by the knit line is desirable. Providing a sump area for the collection of a small amount of oil released during the Permawick injection.

Figure "5" shows the important elements of the injection nozzle press fit fixture.

The Permawick enters the bearing cavity through two orifices. Each orifice of the injection nozzle is connected to a separate chamber in the metering cylinder (see drawing #802) from which a measured amount of Permawick is ejected by the mating ejection piston. Caution: An injection nozzle which takes the Permawick from a common source and then divides the Permawick just prior to entry into the bearing cavity should be used with caution, since the fibrous nature of Permawick prevents it from dividing equally at all times. This in turn causes an unequal amount of Permawick on each side of the contactor, and at worse, causes one orifice to block completely and all the Permawick will enter from one side of the contactor and not reaching the other side.

The lower portion of Figure "5" shows the important elements of a fixture that simultaneously press fits the inner oil catcher (item 5, Fig. "1"), controls and properly forms the Permawick in this cavity. This is accomplished by means of a rubber boot that expands to the proper shape and form upon compression.

The proper filling and method of injection of Permawick is an extremely important element of these design recommendations, and must be followed as vigorously as any other feature of design, such as finish, clearance, etc.

The Equipment required to accomplish the injection of Permawick, as described above, is an ST-1 or ST-2 Permajector machine, equipped with an 802 metering cylinder.

 

 

Cold Roll Steel suitably plated to avoid rust. 

 

The design of the oil catcher shown in Figure 6 and 8 provides a means for press fitting that avoids scratching of the wall of the mating part. The lead radius provided by this design has an ironing effect as it is pressed in, avoiding scratching of the matting wall. Caution: Scratches as produced by the design shown in Figure 7 are in fact capillaries, which will carry free oil from the reservoir to the outer surface of the motor. 

The Hole of the oil catcher (Dimension B and D, Figure 6 and 8), should have its edge turned inward to form a return lip as shown. The "B" dimension should be the shaft diameter plus .010. The "D" dimension should be large enough to clear the outside diameter of the rotor oil slinger.

The return lip feature is important in order to obtain good oil retention in the bearing system. When the motor is running in a horizontal position, oil on the inner face of the oil catcher is prevented from dropping down on to the shaft where, at the clearance between the shaft and oil catcher, a ring of oil can be formed and go out along the shaft. The lip provides a barrier to this source of oil loss. Oil coming down the face of the oil catcher is deflected by the lip and runs down and radially around the lip and to the bottom of the cavity, where it is returned to the main body of Permawick in the bearing hub of the end bracket, and reabsorbed by the Permawick.

When the motor is known to run in a horizontal position, filling of the inner and outer oil catcher with Permawick or a felt ring is not necessary to accomplish oil re-circulation. In this instance oil thrown off by the slinger will flow to the bottom of the cavity where it is returned to the main body of Permawick.

Controlling the amount of Permawick that enters the inner oil catcher is accomplished by reducing the amount of Permawick injected into the cavity (See Figure 5). The rubber boot, however, should be retained in the fixture to control the form of the Permawick to assure that the Permawick in the oil catcher does not touch the slinger.

When the motor is to be used in the shaft up position, then only the inner oil catcher if the upper bracket and the outer oil catcher of the lower bracket needs to be filled with Permawick to accomplish oil re-circulation. Conversely where the motor is to be used with the shaft in a down position, only the upper bracket, inner oil catcher, and the outer oil catcher of the lower bracket need be filled.

When a greater amount of oil in the reservoir is desired to accomplish extended life requirement, then both the inner and outer oil catcher should be filled.

Figure 9 shows an alternate design to effect oil re-circulation without the need to fill the outer oil catcher with Permawick. The extended tang on the felt contactor as shown in Figure 9 is integral with the main body of the felt contactor, and as such reabsorbs the oil collected in the oil catcher and return it to the reservoir.

Spring steel 53 - 55C Rockwell

The hardness of the thrust plate is very important in order to obtain maximum thrust loading (25-35 LBS) when mating with a nylatron thrust washer.

The notch shown in figure Recommendation 1 provides a visual indicator when loading the thrust plate, to insure that the burr in the center hole is faced down. As shown in figure "1", the thrust plate butts up against the bushing hub. This aids in the passage of oil to the thrust area, as well as allowing the bushing hub to act as a heat sink, removing the heat from the rubbing surfaces of thrust plate and washer to the larger mass of the end bracket. When this recommendation is used, control of end bump and end play take-up must be designed in other elements.

Figure Recommendation 2 shows a means to provide end-play take-up, as well as end bump absorption. In this design the thrust plate has extended arms bent to allow the thrust plate to move laterally, and are anchored between a ledge in the bearing hub and the oil catcher. The spring rate of the extending tangs should be adjusted so as not to use the entire thrust capacity of the bearing. Ideally, the accumulated tolerances of motor assembly should not use up the entire .030 provided by lateral movement of the thrust plates. Accumulated assembly tolerances should allow an end play of .005 -.008 to .005 - .010 compression of the thrust plate. The end play allowance should be in relation to the allowable noise level.

 

The Bushing Hub should provide a slot to accept the felt contactor, as well as two rails to support the contactor. Tolerance on "A" dimension should be held to a minimum - commensurate with machining capability. The control of the tolerance on the "A" dimension is very important as it relates to end play of the final assembly and pre-load on the thrust washer. Excessive end play can result in noise generation while excessive pre-load on the thrust area subtracts from the allowable thrust capacity of the motor, as well as increase the wattage.

For life requirements of 25,000 hours, the Permawick reservoir cavity should have sufficient capacity for at least 25 grams of Permawick (16 GM/CU IN). For extended life, additional Permawick can be contained in the thinner and outer oil catcher.

Slinger Nylatron/End Sandwich - Buna Cold Roll Steel (Half Hard)

Figure "13" shows some of the important elements of design when recommended. Thrust plate is used as it relates to thrust washer/slinger, and end play washer sandwich.

The combination thrust washer/slinger detail must be able to move laterally along the shaft to allow its positioning against the thrust plate during assembly, taking up the accumulated assembly tolerance. The fit between the bore of the slinger/washer should be as tight as possible to prevent oil from escaping through the clearance and still allow for lateral movement during assembly without putting undo force between the thrust plate and combination slinger/thrust washer. It is important to limit the amount of oil that can escape through this means.

A chart showing the force necessary to move a nylotron washer along a round shaft with 16 RMS finish, is provided as a guide to the motor designer, as shown in table 3.

It is extremely important that the slinger/thrust washer rotate with the shaft. A roll pin through the U slot in the slinger and into the shaft is just one means to accomplish positive rotation, there are of course, many other possibilities open to the designer. The important point is that the designer provides positive rotation.

The end play take-up sandwich is composed of a Buna Fleximere washer sandwiched between two hard steel washers.

B1112 or Motor designer option

Finish 8-12 RMS

Quality (Out of round) May .000050

The shaft finish and hardness have a very important influence on bearing life and load capacity. Shaft finish having a greater influence of the two. Some literature has shown that finish of less than 8 RMS inhibits hydro-dynamic film formation, is too smooth a surface to provide drag on the oil to bring into the film and area. The cost of a finish better than 8 RMS is not justified in relation to the benefit obtained. A shaft finish of 12 RMS or better has a measurable and real benefit. In may cases an improvement of shaft finish from 20 RMS to 12 RMS allows an increase of bearing load from 75 PSI to 150 PSI, or at the same load a significant increase of bearing life, all other factors being equal. (See Figures 6-8.)

Where a bearing is expected to be subjected to load, in excess of 25 CB/IN and a service life in excess of 20,000 hrs, a shaft hardness of 38-45 Rockwell "C" is recommended.

Shaft surface treatment such as phosphating, and black oxide are beneficial and helpful to good bearing life and capacity.