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Informative Annex 5

These Informative Annexes are for the use of the Task Force and are not to be included in the API Standards

Bearing Reference Speeds & Allowable Ndm

This Annex in addition to Annex 6 provide background information on factors, specification, and procedures used in calculating the various speed limits (Ndm) of various types of rolling element bearings.

Up until publication of ISO 15312 in 2003 many of the bearing manufacturers listed in their catalogs a speed rating for oil and grease based on the following:

“Generally the operating temperature which can be permitted, with respect to the lubricant being used or to the material of the bearing components, that sets the limit. The speed at which the bearing temperature is reached depends on the frictional heat generated in the bearing (including any externally applied heat) and the amount of heat that can be transported away from the bearing. Bearing type size, internal design, load lubrication and cooling conditions as well as cage design, accuracy and internal clearance all play a part in determining the permissible speed.”

“Speed Ratings

Speed ratings for grease and oil lubrication are quoted in the bearing tables. The speed rating for a given bearing represents the speed at which, under load corresponding to L10h life of 150,000 hrs, there is a balance between the heat which can be removed from the bearing via the shaft and housing and lubricant and the heat generated in the bearing by friction at the given temperature interval above ambient.”

“The speed ratings for grease lubrication are 15-25% lower (depending on bearing type) than the speed ratings for oil lubrication, which apply to oil bath lubrication. These values are lower to take into account the initial temperature peak that occurs when starting up a bearing which has been filled with a grease during mounting or which has been just re-lubricated. The operating temperature will sink to a much lower level once the grease has been distributed in the bearing. The pumping action inherent in certain bearing designs e.g. in angular contact ball bearings and taper roller bearings which becomes more accentuated as speeds increase, or the pronounced working of the grease which occurs in full compliment cylindrical roller bearings also make it necessary for the speed ratings for grease lubrication to be lower than those for oil lubrication.” (1)

(1)SKF General Catalog 4000 1991 pg52 & 127.

The SP requirement of 50,000 Hrs L10 life @ 3600 RPM equates to a C/P loading of 22.1 which is 4.5% of the allowable dynamic load on the bearing. Bearing manufacturers catalogs start to reduce the speed ratings when the loading is above approximately double the 4.5% figure. Therefore the speeds do not have to be corrected for load.

ISO 15312

In 2003 the new ISO 15312 “Rolling bearings – Thermal speed ratings – Calculation and coefficients” was published. ISO 15312 presents procedures that set a “thermal” reference speed. This thermal reference speed is the speed at which there is equilibrium between the heat that is generated by the bearing and that which is dissipated from the bearing to the shaft, housing and lubricant. The reference conditions according to ISO 15312 for obtaining the heat balance are:

  • A temperature rise of 50 K above an ambient temperature of 20 ºC (68 ºF) as measured on the bearing stationary outer ring or housing washer.
  • For radial bearings: a constant radial load of 5% of the basic static load rating C0.
  • For thrust bearings a constant axial load of 2% of the basic static load rating C0.
  • Open bearings with NORMAL clearances.

For Oil lubricated bearings:

  • Mineral oil with out EP additives having a kinematic viscosity at 70 ºC of v=12 mm2 /s (ISO VG 32) for radial bearings.
  • v=24 mm2 /s (ISO VG 68) for radial bearings.
  • Oil bath with the oil reaching up to the middle of the rotating element in the lowest position.

For grease lubricated bearings:

  • Regular lithium soap grease with mineral base oil having a viscosity of 100 – 200 mm2 /s at 40 ºC (ISO VG 150).
  • Approximately 30% of the free space in the bearing.

As of January 2004 some bearing manufacturers have changed their catalogues by doing away with the previous two column speed ratings for oil and grease and replacing it with a calculation procedure based on ISO 15312. These base “thermal speed ratings” are much higher than the previous limits for oil and grease.

However, these base thermal speed ratings obtained by using ISO 15312 require factors to be applied to them that in general, LOWER the allowable base “Thermal speed rating” of the bearing.

A typical comparison of the bearing manufacturers “old” catalog speed ratings and that corresponding to ISO 15312 follows:

FAG 50mm bore, 100 mm OD
Limiting Speed / Reference Speed / Oil / Grease (no seals) / Grease (Seals)
1989 catalogue WL 41510/2ed-Pg / 7000 RPM / 6000 RPM / 4000 RPM
2000 catalogue WL 41520/3 based on ISO 15312 pg 162 / 14000 RPM / 9500 RPM

There is another limiting speed that is determined by the stability or strength of the cage centrifugal and gyrational forces acting on the rolling elements, precision and other speed limiting factors such as seals, and lubricant for sealed bearings. These new limiting speeds are also indicated in the manufacturer’s catalogs. The lower of these two speed limits govern.

ISO 15312 Assumptions

It has been decided to keep the Table 6.9-2 based on the previous bearing manufacturers experience and not modify it based on ISO 15312. In part this is due to the assumptions in ISO 15312. ISO 15312 assumes there is heat dissipated to the shaft, however many applications actually have heat conducted from the hot product to the bearing. ISO 15312 also is based on normal (N) bearing clearances. API requires larger bearing clearances per Group 3. ISO 15312 also calculate the same speed for oil and grease lubricated bearings. It does not account for oil mist or forced lubrication.

Load Affects in 15312

In the new ISO 15312 the allowable equilibrium speed is based on a 5% of the static radial load rating. The previous SKF thermal calc was based on a brg life of 150,000 Hrs. which equates to a C/P of 32 or a 3% dynamic load. i.e:

L10h = [1,000,000/60 (N) ] (C/P)3

(150,000 X 60 X 3600/1,000,000)1/3 = C/P

32=C/P

100/32=3%=P= load is 3% of the allowable dynamic load.

From the SKF catalog for a 50 mm shaft Dia. and bearing. OD = 130 mm: C0 (static) = 52,000 N, and dynamic C = 87,000 N.

5% static = 2600 N, 3% of dynamic = 2610 N. Thus for this one case, the new ISO base case is the same as the previous 150,000 Hr SKF catalog design.

Dynamic vs Static (C0) Bearing Load

Why is the dynamic load higher than the static in the above case ? i.e. Dynamic = 87,000 N and static is 52,000 N?

Static is based on deformation and dynamic is based on fatigue of 1,000,000 revolutions. For example, if 87,000 N is applied to this bearing it should be able to run 1MM revolutions before a crack develops.

Static Load: The basic static load rating is based on ISO 76:1987 which corresponds to a calculated contact stress at the center of the most heavily loaded rolling element/raceway contact of :

4600 Mpa for self-aligning bearings

4200 Mpa for all other bearings

4000 Mpa for all roller bearings

This stress produces a total permanent deformation of the rolling element and raceway which is approximately 0.0001 of the rolling element diameter. The loads are purely radial for radial bearings and centrally acting axial loads for thrust bearings.

Dynamic Load: It expresses the bearing load that will give an ISO 281:1990 basic rating life of 1,000,000 revolutions. Practical experience has shown that seemingly identical bearings operating under identical conditions have different individual endurance lives. To account for this, it is industry practice to present dynamic load ratings in the catalogs based on the probability that 90% of identical bearings can meet or exceed the specified, or expected life i.e. L10h life. This is the total load the bearing “sees” including hydraulic loads, belt drives, vibration, etc.

Manufacturers dm n

To determine a “factor of safety” used in table 6.9-2 the Tables in Annex 6 were calculated. They present Ndm values for 25mm (1 in.), 50 mm (2 in), 75mm (3 in.), and 100 mm (4 in) diameter shafts and the corresponding minimum and maximum bearing OD’s for each of these shaft sizes and for the different bearing types. These Ndm figures are used to determine the maximum allowable Ndmlimits in Table 6.9-2. Annex 6 analysis is summarized in the table at the end of Annex 6.

1/29/2019