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load ratings & misalignment capabilitiesDefinitions for Rod End and Spherical Bearing TerminologyRadial Load | Axial Load | Static Load | Dynamic Load | Static Radial Limit Load | Static Radial Ultimate Load Load Ratings: Spherical Bearing | Rod End Bearing | PV Factor | Dynamic Oscillating Radial Load NHBB Testing Capabilities: Mechanical | Polymer | Specifying Misalignment Radial LoadA load applied normal to the bearing bore axis (A). Axial LoadA load applied along the bearing bore axis (B). Static LoadThe load to be supported while the bearing is stationary. Dynamic LoadThe load to be supported while the bearing is moving. Dynamic Load Static Radial Limit LoadThe static load required to produce a specified permanent set in the bearing. It will vary for a given size as a function of configuration. It may also be pin limited, or may be limited as a function of body restraints as in the case of a rod end bearing. Structurally, it is the maximum load which the bearing can see once in its application without impairing its performance. Static Radial Ultimate LoadThe load which can be applied to a bearing without fracturing the ball, race or rod end eye. The ultimate load rating is usually, but not always, 1.5 times the limit load. Plastic deformation may occur. Static Axial Limit LoadThe load which can be applied to a bearing to produce a specified permanent set in the bearing structure. Structurally, it is the maximum load which the bearing can see once in its application without impairing its performance. Static Axial Ultimate LoadThe load which can be applied to a bearing without separating the ball from the race. The ultimate load rating is usually, but not always, 1.5 times the limit load. Axial Proof LoadThe axial load which can be applied to a mounted spherical bearing without impairing the integrity of the bearing mounting or bearing performance. It is always less than the static axial limit load. Bearing movement after proof load is usually .003 or less. RotationIs the relative angular displacement between the ball and race that occurs within the plane perpendicular to the axis of the ball bore. The direction of rotation is about the axis of the ball bore. Rotation MisalignmentIs the relative angular displacement between the ball and race that occurs within any plane that coincides with the axis of the ball bore (see Radial/Axial above). The direction of misalignment is about any axis perpendicular to the ball bore. Oscillating Radial Load or Dynamic LoadThe uni-directional load produces a specified maximum amount of wear when the bearing is oscillated at a specified frequency and amplitude. This rating is usually applied to self-lubricating bearings only. The dynamic capability of metal-to-metal bearings depends upon the degree and frequency of grease lubrication, and that of dry film lubricated bearings upon the characteristics of the specific dry film lubricant applied. Radial PlayRadial play (or radial clearance) is the total movement between the ball and the race in both radial directions less shaft clearance (when applicable). Industry specifications have established the gaging load at ±5.5 lbs., and this is now considered as the industry standard. Unless otherwise specified, the industry wide standard for metal-to-metal spherical bearing and rod end radial clearance is “free-running to .002 max.” Radial play is sometimes referred to as “Diametral clearance.” The two terms are synonymous.
Axial PlayAxial play (or axial clearance) is the total movement between the ball and the race in both axial directions. The gaging load is again ±5.5 lbs. Axial play is a resultant, being a function of radial play, of ball diameter and race width. The ratio between radial and axial play varies with bearing geometry. Axial Test Fixture Fatigue Load of Rod EndsAerospace Standard series rod end bearings AS81935 must be capable of withstanding a minimum of 50,000 cycles of loading when tested as follows: The loading must be tension-tension with the maximum load equal to the fatigue loads listed on the NHBB drawing of the ADNE and ADN series rod end bearings. The minimum load must be equal to 10% of the fatigue loads. Load RatingsThe load rating of a bearing is determined by the dimensions and strength of its weakest component. External factors, such as mounting components, pins, bolts, and housings are not considered part of a bearing when load ratings are investigated but should be considered separately. Spherical Bearing Load RatingsThe weakest part, or load-limiting area, of a spherical bearing is its race. For this reason, formulas have been developed that use the race to calculate static load ratings based on size and material strength. The static load rating formulas for self-lubricating and metal-to-metal spherical bearings are shown below. These formulas will yield approximate ratings, which should be used as ballpark numbers for bearing design.The allowable radial stress values given in the tables were determined from the ultimate tensile strength specifications for various race materials. Allowable axial stress values were derived from material yield strengths.
Allowable Stress Metal-to-Metal Bearings (psi)
AMPCO® is a registered trademark of AMPCO Metal Inc. Rod End Bearing Load RatingRod end bearing load ratings can be generated only after carefully determining the load restrictions that each element of the rod end bearing imposes on the entire unit. In order to generate a frame of reference, consider the rod end bearing as a clock face, with the shank pointing down to the 6 o’clock position. The limiting factors in rating a rod end bearing are as follows:
Most rod ends will fail under tension loading in about the 4 o’clock-8 o’clock portion of the eye or hoop. The Net Tension Area (NTA) can be found as follows:
The shank stress area (SSA) is a function of being either male or female, as follows:
The axial load capability of a rod end is a function of the following:
PV FactorWhile not a type of loading, the PV factor is very useful in comparing and predicting test results on high speed-low load applications such as helicopter conditions. PV is the product of the stress (psi) and the velocity (fpm) applied to a bearing. Caution must be advised when considering extreme values of psi and fpm. The extreme must be considered individually, as well as together. Because the PV factor is derived from the geometry and operating conditions of a bearing, it serves as a common denominator in comparing or predicting test results. For this reason PV values are included in the wear curves for liners in the Self-Lubricating TEFLON® Liner Systems section. The formula for determining the PV value for a spherical bearing is as follows:
Dynamic Oscillating Radial LoadThe dynamic oscillating radial load ratings given in this catalog for ADB, ADW, ADBY, ADB-N, ADW-N, ADBL and ADWL series self-lubricating spherical bearings are based on testing in accordance with AS81820. For conditions other than those specified by AS81820 contact NHBB Applications Engineering. NHBB Testing CapabilitiesMechanical Test EquipmentNHBB has a variety of equipment to test spherical and rod end bearings under diverse conditions. NHBB performance data exceeds military and individual manufacturers’ design requirements. Maximum capabilities of NHBB testing machines are shown below.
Polymer Test EquipmentNHBB has the following thermal analysis (TA) equipment to support and control the quality of composites/polymers through analytical techniques that measure the physical and mechanical properties as a function of temperature and time:
Formula for Determining Misalignment of Rod End & Spherical Bearings
How NHBB Specifies Catalog Bearing and Rod End MisalignmentThe misalignment angle of a rod end or spherical bearing refers to the angle between the ball centerline and the outer member centerline when the ball is misaligned to the extreme position allowed by the clevis or shaft design, as applicable. The illustrations show varying types of bearing misalignment and a formula for calculating each where:
The misalignment angle illustration below shows how misalignment angles for standard ball spherical bearings and rod ends are represented in NHBB catalogs. The misalignment angle is calculated per the Standard Method. Neck ball (high misalignment) bearings and rod ends are represented in the same manner, but are calculated per the High Misalignment Series Method. NHBB prefers not to use rod end clevis misalignment for the following reason. The rod end clevis misalignment formula presupposes a clevis configuration as shown in which the clevis slot and ball faces are of equal width and in direct contact. In aircraft applications the configuration shown is more typical than that shown in the Rod End Clevis Misalignment illustration . As pictured in the Typical installation, the clevis slot is wider than the ball to permit installation of flanged bushings and/or spacers. This results in a higher but more variable misalignment capability, and the angle of misalignment becomes a function of the user’s bushing flange or spacer diameter instead of the fixed rod end head diameter.
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