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Bearing Life/Travel Life - LM76's pillow block bearing life expectancy can be expressed in terms of linear travel of the bearing on the shaft and is known as "bearing travel life" (LL). The "bearing travel life" is a common term for linear motion applications. However, bearing life calculations are normally expressed in terms of millions of revolutions and are known as the "rating life"(L10). The (L10) or rating life of a particular bearing is defined by the number of revolutions that 90% of the tested bearings will complete before the first evidence of fatigue develops. The following formula gives the relationship of the bearing life (L 10) to a constant bearing load (P) and a Basic Dynamic Load (BDR) which gives a theoretical life of 1 million revolutions.
The recommended operating load for LM76, Jet Rails bearings should be 50% or less of the dynamic rating (BDR). This rating reduction generally compensates for some variables such as shock,vibration, etc. which the bearing may endure. If the actual speed is other than 33.33 R.P.M. the basic dynamic load rating (BDR) must be adjusted by thge speed factor FS. F S = bearing speed factor(FS is valid between 0.7 - 6.0 only.) BDR = basic dynamic load rating BDRS = Speed adjusted basic dynamic load rating BDRs = BDR/FS The speed factor can be calculated from the following formula:
Ns = Bearing speed in feet per minute When the required life L10 (in
millions of revolutions), the actual load P (Lb.) and the bearing speed Ns
(feet per minute) are known the required dasic dynamic load rating (BDR)
can be calculated by using the following formula: BDR = P x F L X FsFL = Bearing Life Factor P = Load per bearing |
Example: Determine the proper size bearing for a carriage which is mounted on 4 bearings and is traveling at 10 feet per minute, carrying 3,000 Lbs., 8 hours a day with actual running of 50%, 5 days a week, and to be in service for 7 years. Solution: First determine the life rating (L 10) in terms of millions of revolutions that will be necessary for this application.L10 = Y (years) x W (weeks) x D (days) x H (hours) x M (minutes) x C (% of operation time) x S (speed in ft/min.) x Nd (speed converter) L10 = 7 x 52 x 5 x 8 x 60 x 0.50 x 10 x 4.5 = 19.66 millions of revolutions; P = The load per bearing = 3,000/4 = 750 lb; BDR = 750/0.5 = 1,500 lb. The nearest bearing with higher BDR tha 1,500 Lb. for PB-20 with 1,660 Lb. Ns = The system speed is 10 ft/min. Determine the effect of the speed on the bearing life by using the speed factor formula:
BDRs = 1,660/1.09 = 1,523
L 10 = 10.60 million of revolutions |
| Pillow Block Size | 8 | 10&12 | 16 | 20 | 24 | 32 | 48 | 64 |
| Nd | 8.25 | 6.82 | 5.72 | 4.50 | 4.36 | 3.82 | 1.91 | 1.53 |
