Limiting Speed of Bearings
The rotation of the rolling bearings is limited to certain limits. During the operating conditions, the increase in speed of the bearing also led to the increase in temperature because of the friction. The maximum speed at which bearing can be operated continuously without failure and excessive heat generation, such speed can be determined by the formula called limiting the speed of the bearings. The factors which affect the limiting speed of the bearings are, bearing type and size, cage form and material, applied load, lubrication method, and heat dissipation method including the design of the bearing’s surroundings.
The limiting speeds can be given as C/P ≥ 12.5 and Fa /Fr ≤ 0.25 for the bearings of standard design and subjected to normal loads.
Different lubricants are used for different operating speeds. Lubricants used for high speed have different characteristics. When the limiting speeds increase to more than 70 % of the bearing limiting speed special type of oil or lubricants should be used, else contact NIBL.
Correction of Limiting Speed of Bearings
The limiting speed must be corrected to obtain the corrected limiting speed when the load on the bearing P exceeds 8% of the basic load rating of bearing C, or when the axial load on bearing Fa exceeds 25 % of the radial load of bearing Fr,
by using below-mentioned equation –
na = f1 . f2 .n
na = Corrected limited speed (rpm)
f1 = Correction coefficient determined from Fig. 4.1
f2 = Correction coefficient determined from Fig. 4.2
n = Limiting speed under normal load conditions
Fa = Axial load (N)
Fr = Radial load (N)
P = Dynamic equivalent load (N)
C = Basic dynamic load (N)
While selecting the bearing in the case where the operating speed is more than the limiting speed the factors like the desired bearing, then the accuracy grade of the bearing, internal clearance class of the bearing, cage used in the bearing and bearing material, type of lubrication, etc., must be carefully studied. In such a case, forced circulation oil lubrication, jet lubrication, oil mist lubrication, or oil-air lubrication must be used.
Limiting Speed for Rubber Contact Seals for Ball Bearings
The maximum permissible speed for contact rubber sealed bearings (2RS type) is determined mainly by the sliding surface speed of the inner circumference of the seal. These allowable rubbing speeds vary depending upon the seal rubber material.
