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Load Ratings & Misalignment Capabilities

Definitions for Rod End and Spherical Bearing Terminology

Radial Load | Axial Load | Static Load | Dynamic Load | Static Radial Limit Load | Static Radial Ultimate Load
Static Axial Limit Load | Static Axial Ultimate Load | Axial Proof Load | Rotation | Misalignment
Oscillating Radial Load or Dynamic Load
| Radial Play | Axial Play | Fatigue Load of Rod Ends

Load Ratings: Spherical Bearing | Rod End Bearing | PV Factor | Dynamic Oscillating Radial Load

NHBB Testing Capabilities: Mechanical | Polymer | Specifying Misalignment


Radial Load

A load applied normal to the bearing bore axis (A).

Axial Load

A load applied along the bearing bore axis (B).



Radial/Axial Load

Static Load

The load to be supported while the bearing is stationary.

Dynamic Load

The load to be supported while the bearing is moving.

Dynamic Load

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Static Radial Limit Load

The 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 Load

The 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 Load

The 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 Load

The 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 Load

The 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.

Rotation

Is 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

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Misalignment

Is 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 Load

The 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 Play

Radial 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.

Radial Test Fixture

Method of Measuring Radial Play


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Axial Play

Axial 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 Ends

Aerospace 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 Ratings

The 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.

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Spherical Bearing Load Ratings

The 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.

 

Static Load Rating Formulas for
Self-Lubricating Spherical Bearings

Allowable Stress PTFE Lined Bearings (psi)

Race Material
Radial
Axial
Ultimate
Limit
Ultimate
Limit
17-4PH, Rc28 MIN
112500
75000
67500
45000
ALUM 2024-T351
60000
40000
36000
24000

Static Load Rating Formulas for
Metal-to-Metal Spherical Bearings



Standard Groove Sizes

Bearing Size
Bore Code
G
Width
3 & 4
.062
5-10
.078
12-16
.094
20 & above
.109

Allowable Stress Metal-to-Metal Bearings (psi)

Race Material
Radial
Axial
Ultimate
Limit
Ultimate
Limit
17-4PH, Rc32-36
150000
100000
125000
83000
4130 Rc32-36
150000
100000
125000
83000
A286 (AMS 5737)
140000
93000
95000
63000
AMPCO® 15 Bronze
75000
50000
45000
30000

AMPCO® is a registered trademark of AMPCO Metal Inc.

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Rod End Bearing Load Rating

Rod 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:

  1. The double shear capability of the bolt passing through the ball bore.
  2. The bearing capability, a function of race material or self-lubricating liner system.
  3. The rod end eye or hoop tension stress in the 3 o’clock-9 o’clock position.
  4. The shank stress area, as a function of male or female rod end configuration.
  5. The stress in the transition area between the threaded shank transition diameter and the rod end eye or hoop.

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:


This simple rod end load rating formula does not take into consideration such variables as special body shapes, thin race sections, hardness variation, lubrication holes, grooves, and hoop tension, which could significantly affect the load rating. Contact NHBB Applications Engineering for assistance in determining the load rating for specially designed Rod Ends and Sphericals.

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:

  1. The retention method used to mount the bearing in the rod end eye (see the Bearing Installation and Retention section).
  2. The axial load capability of the bearing element.
  3. The bending moment, if any, placed on the rod end.

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PV Factor

While 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 PTFE Liner Systems section.

The formula for determining the PV value for a spherical bearing is as follows:

Dynamic Oscillating Radial Load

The 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.

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NHBB Testing Capabilities

Mechanical Test Equipment

NHBB 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.

NHBB Testing Capabilities
 
Force
Material Testing (Universal Testing Machine)
110,000 Lbs.
Static Compression/Tension
200,000 Lbs.

Low Speed Oscillation (up to 50 cpm)

Uni-directional Loading

(1 machine, 2 station) (700°F)

(1 machine, 2 station) (700°F)

20,000 Lbs.

70,000 Lbs.

Moderate To High Speed Oscillation

Uni-directional Load (room temp.)

(1 machine, 2 station) (1000 cpm)

(1 machine, 2 station) (1500 cpm)

(1 machine, 6 station) (200-600 cpm)

1,000 Lbs.

1,000 Lbs.

8,000 Lbs.

Low Speed Oscillation

Reversing or Alternating Load (room temp.)

(1 machine, 2 station) (up to 50 cpm)

40,000 Lbs.

High Speed, Oscillation

Reversing and Alternating Load (room temp.)

(2 machines, 1 station each) (400 cpm)

2,500 Lbs.

Airframe Track Roller

Testing Machine (roller against flat plate)

60,000 Lbs.

 

Polymer Test Equipment

NHBB 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:

  1. Differential Scanning Calorimeter (DSC)
  2. Thermogravimetric Analyzer (TGA)
  3. Dynamic Mechanical Analyzer (DMA)
  4. Thermomechanical Analyzer (TMA)
  5. Thermo-Oxidative Stability Test (TOS)
  6. Acid Digestion System
  7. Fourier Transform Infrared Spectroscopy (FTIR)

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Formula for Determining Misalignment of Rod End & Spherical Bearings

 


Standard Method

Most standard rod end and spherical bearing misalignment angles specified in NHBB catalogs are based on this method.


Design Reference

This method may be used as design reference for installation purposes, but should not be used as a functioning misalignment under load.


High Misalignment Series Method

(Neck balls only)

 

How NHBB Specifies Catalog Bearing and Rod End Misalignment

The 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.
NOTE: Since angle “A” applies equally on both sides of the centerline, it follows that total misalignment of the bearing is double the value obtained for “A”.

The illustrations show varying types of bearing misalignment and a formula for calculating each where:

A = angle of misalignment
D = head diameter (rod end)
S = shoulder diameter (neck ball)
W = width of ball

B = bore of ball
E = ball spherical diameter
T = housing (race) width

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.



Rod End Clevis Misalignment


Misalignment Angle



Typical Rod End/Clevis Installation

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