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'''DN Factor''', also called ''DN Value'', is a number that is used to determine the correct base oil viscosity for the lubrication of various types of [[Bearing (mechanical)|bearings]]. <ref name="Dynaroll.com">{{cite web |title="Speed Limitation: Speed and Lubricant Factors" |url=http://resource.dynaroll.com/speed-limitation.asp |accessdate=Oct 19, 2018}}</ref><ref name="InterflonUSA.com">{{cite web |title="How to Calculate DN Factor" |url=https://interflonusa.com/how-to-calculate-dn-factor/ |accessdate=Oct 19, 2018}}</ref><ref name="McNallyInstitute.com">{{cite web |title="DN Factor" |url=http://www.mcnallyinstitute.com/d-html/d019.htm |accessdate=Oct 19, 2018}}</ref><ref name="MachineryLubrication.com">{{cite web |title="Choosing a High-Speed Grease" |url=https://www.machinerylubrication.com/Read/30210/high-speed-grease |accessdate=Oct 19, 2018}}</ref><ref name="Klueber.com">{{cite news |title="Lubrication of Rolling Bearings: Tips and Advice" |url=https://www.klueber.com/ecomaXL/files/Lubrication_of_rolling_bearings_tips_and_advice.pdf |accessdate=Oct 19, 2018}}</ref>

It can also be used to determine if a bearing is the correct choice for use in a given application.<ref name="McNallyInstitute.com"></ref> It is a product of bearing [[diameter]] (D) and [[speed]] (N).<ref name="Dynaroll.com"></ref><ref name="InterflonUSA.com"></ref>

D = diameter (in millimeters) of the bearing in question. For most types of bearings, there are actually two required measurements: the inner diameter and outer diameter. In such cases, D = (A+B)/2, where A = inner diameter and B = outer diameter. The sum of these two values is then divided by 2 to obtain the median diameter, sometimes also called pitch diameter.<ref name="MachineryLubrication.com"></ref>

N = bearing speed. This is the maximum amount of revolutions per minute (RPM) that the bearing will move.

The DN factor of a bearing is obtained by multiplying the [[median]] diameter (A + B)/2 by RPM, and sometimes by a correction factor.<ref name="InterflonUSA.com"></ref><ref name="OKS Germany">{{cite web |url=https://www.oks-germany.com/en/whats-new/top-feature/rolling-bearing-lubrication/ |website=oks-germany.com |accessdate=Oct 19, 2018 |title="Rolling bearing lubrication"}}</ref> This correction factor may vary from manufacturer to manufacturer. No consensus exists among [[tribologist]]s as to a constant correction factor across manufacturers.

== Example formula==
For a single or double row cylindrical bearing, the following formula would be used to obtain the DN factor. It includes a correction factor of 2:

<math>DN factor = ((A+B)/2) * RPM * 2</math>

Where:
* A and B represent inner and outer diameters, respectively
* A and B are divided by 2 to find the median diameter
* RPM is the maximum speed of the bearing
* 2 is the correction factor for this particular (hypothetical) manufacturer

==Usage==
Once the DN factor of a bearing has been obtained, it can be used to consult grease selection charts in order to determine the correct lubricant. [[Viscosity]] must be matched to the needs of the bearing in order to obtain maximum efficiency, and to avoid lubricant runout due to overheating, which is a consequence of metal-on-metal contact, as well as the failure of grease to extract heat from the bearing system.<ref name="MachineryLubrication.com"></ref><ref name="Klueber.com"></ref>

Viscosity is quantified according to the [[National Lubricating Grease Institute]] (NLGI) consistency number, which is regarded as the standard measure of grease thickness.

Knowing the DN factor of a bearing is critical to preventing lubricant starvation, which is characterized by decreasing lubricant film thickness coupled with increased bearing speed. <ref name="K. Jauhari">{{cite journal |last1=Jauhari |first1=K |title="Oil lubrication on high-speed spindle bearing system: A review" |journal=Proceeding of Tribology Industry |date=2014 |volume=16 |pages=216-231 |url=https://www.researchgate.net/publication/270685952_Oil_lubrication_on_high-speed_spindle_bearing_system_A_review/download |accessdate=Oct 19, 2018}}</ref> Starvation occurs when bearing speed (N) exceeds the ability of the lubricant to flow back into the bearing track. This phenomenon can be the cause of metal-on-metal contact, which causes rapid wear and necessitates early replacement. Jauhari shows that degree of starvation is a function of relative lubricant layer thickness for given operating conditions. He also states that "the rolling fatigue life of [a] bearing depends greatly upon the viscosity and film thickness between the rolling contact-surface [sic]."

== References ==
Online calculators exist to determine DN factor and correct grease viscosity.<ref name="InterflonUSA.com"></ref><ref name="MachineryLubrication.com"></ref>

{{reflist}}

[[Category:Tribology]]
[[Category:Bearings (mechanical)]]
[[Category:Measurement]]

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