load ratings & bearing life

Life Modifiers | Lubricant Life | Calculation of Fatigue Life

Static Load Ratings

The static load rating (Co) given in the product listings is the radial load that a non-rotating bearing will support without damage. In evaluating static load conditions, any forces exerted during assembly and test must be considered along with vibration and impact loads sustained during handling, test, shipment and assembly.

Dynamic Load Ratings

Dynamic loading (C) includes built-in preload, weight supported members and the effect of any accelerations due to vibration or motion changes. The dynamic load rating (C) for a radial or angular contact ball bearing is a calculated, constant radial load that a group of apparently identical bearings can theoretically endure for a rating life of one million revolutions. The dynamic load rating is a reference value only; a base value rating life of one million revolutions has been chosen for ease of calculation. The dynamic load rating values (C) given in the product listings (see catalog) include the effects of race-to-ball conformity and are in accordance with ABMA Standard #12.

Rating Life

The rating life (L10) of a group of apparently identical ball bearings is the life in millions of revolutions that 90% of the group will complete or exceed. For a single bearing, L10 also refers to the life associated with 90% reliability.

The magnitude of the rating life, L10, in millions of revolutions for ball bearing application is: L₁₀ = (C/PE)³

The method of computing design life (L) and the nomographs are also in conformance with industry standards, with allowance for the effects of curvature on the equivalent radial load resulting from the application of thrust load. Life calculations can be significantly affected by many factors such as the material or the lubricant. Miniature and instrument ball bearings are normally made of either AISI 440C Stainless Steel or SAE 52100 Chrome Alloy Steel.

Life Modifiers

NHBB recommends that the load rating published for 52100 be reduced by 20% for 440C. This is a conservative approach to ensure that the bearing capacity is not exceeded under the most adverse conditions. This is incorporated in the a2 modifier as shown. The table below provides selected modifiers for calculating failure rates down to 1% (L₁).

Table of Reliability Material Life Modifier a₂

Elastohydrodynamics (EHD Effect)

The presence of a thin film of oil at the mutually contacting ball-to-raceway interface enhances the load capacity of a ball bearing. The lubricant life modifier nomograph includes the effect of the elastohydrodynamic lubricant film and can be used to assist in lubricant selection.

Other Life Adjustments

Seldom are loads ideally applied. The conventional rating life often has to be modified due to application abnormalities, intentional or unknown. The following conditions have the practical effect of modifying the ideal, theoretical rating life (L10).
• Vibration and/or shock-impact loads
• Angular misalignment
• Oscillatory duty
• High-speed effects
• Operation at elevated temperatures
• Fits
• Internal design

While it is difficult to provide the exact effect upon life under any of these conditions, NHBB can provide bearing life estimates based on semi-empirical data to help you forecast bearing life for your application.

Lubricant Life

In many instances a bearing’s effective life is governed by the lubricant’s life. This is usually the case for applications involving very light loads and/or very slow speeds. In such instances, the conventional fatigue life calculated will be unrealistically high. The lubricant’s ability to provide sufficient film strength is affected by:
• Quantity and condition of the lubricant in the bearing
• Environmental conditions (e.g. ambient temperature, area cleanliness)
• The load-speed cycle

Specialized oils and greases are available which exhibit favorable performance characteristics over an extended period.

Calculation of Fatigue Life

While miniature and instrument bearings may fail from causes other than fatigue, the fatigue life should be calculated to ensure against failure. To calculate fatigue life, follow the procedure in the table below.

Fatigue Life

Step	Item/Operation	Symbol	Pounds and Inches
			Obtain from	Example
Given	Radial Load Thrust Load Speed	R T N	Given Given Given	2 lbs. 5 lbs. 15,000 RPM
1 2 3 4 5 6	Obtain lubricant viscosity (consult with factory) Select bearing Read dynamic load rating Read static load rating Read ball size Select radial play range	V   C C0 Db	—   Radial Open Unflanged Radial Open Unflanged   Standard Radial Play Ranges	15 cs. SSR-3 140 lbs. 59 lbs. 3/32“ P25
7	Obtain initial contact angle using Db and table		Contact Angles	9.5°
8	Obtain T1 from T1 nomograph using Co, T and above	T1	T1 Nomograph	9.5 lbs.
9	Obtain R1 from R1 nomograph using above	R1	R1 Nomograph	0.95 lbs.
10	Obtain PE where PE=T1 + R1 or PE=R whichever is greater.	PE	9.5 + 0.95	10.45 lbs.
11	Obtain LB10 from B10 life nomograph (number of revolutions or hours) using C and PE above	LB10	B10 Life Nomograph	2700 hrs.
12	Obtain lubricant life modifier from lubricant effect nomograph using C, T + R, V and N above	a1	Lubricant Effect Nomograph to Obtain a1 Factor	1.5
13	Obtain reliability life modifier (a2) from the table for LB10	a2	Table of Reliability	0.50
14	Obtain design life L where L = a1 a2 Lb10	L	1.5 x 0.5 x 2700	2000 hrs.

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