Loads of Kaydon thin section bearings
Kaydon thin section bearings support a shaft or housing to permit their free motion about an axis of rotation while under applied loads. Kaydon thin section bearings are available in a variety of types to accommodate radial, axial and moment loads.
Radial and axial loads
Load can be applied to bearings in either of two basic directions, where the resultant moment load (M) can be calculated from:
where
| M | moment load [N·m] |
| Fa | axial load [kN] |
| Sa | offset distance from the bearing axis [m] |
| Fr | radial load [kN] |
| Sr | offset distance from the radial plane [m] |
Axial loads (Fa) act parallel to the shaft (bearing's axis of rotation), whereas radial loads (Fr) act at right angles to the axis of rotation. When these loads are offset from either the bearing axis (distance Sa) or radial plane (distance Sr), a resulting moment load (M) will be created.
Load analysis
The use of computer software enables a more complex and accurate method of determining bearing life than previous hand calculations. The actual loads are applied to the bearing and the resultant load on every ball in that bearing is determined. From this calculation, the static safety factor and basic rating life, L10, can be determined. To better understand this, consider primary radial, axial and moment loading.
Primary radial loading
- Larger clearances will have fewer balls carrying the loads, resulting in shorter dynamic lives.
- Larger preloads may overload the bearing before the loads are applied.
Primary axial and moment loading
- Larger clearances will permit a higher contact angle than the ball has with the raceway, and therefore better accommodate the applied loading.
- The ball-to-raceway contact ellipse area may, however, truncate over the edge of the raceway, causing other problems.
- Greater preloads may again overload the bearing before the loads are applied.
Variable load cases
Often a Kaydon bearing system must operate in several modes such as "idle" and "working", and so the loads may vary substantially. It is advantageous to calculate the life of the bearing under the total loading spectrum.
To do this, the individual bearing life under each load case can be calculated alone, then combined to provide the system life for a particular duty cycle.
To perform this calculation, divide the total loading into discrete sections that can have their respective percentage of revolutions represented as part of the total. (Please refer to Calculating bearing life)
Bearing load ratings
Dynamic radial load ratings
Calculation of dynamic radial load ratings using the formulae in ABMA Std. 9 and ISO 281 can be overly optimistic because they assume certain design details that are not valid for Kaydon thin section bearings. These load ratings are included for comparison purposes only.
Kaydon dynamic radial load ratings are calculated using more realistic assumptions based on actual design details and are validated by decades of endurance testing.
Static load ratings
The actual static load that a thin section bearing can withstand depends on the amount of support provided by the shaft and housing.
Bearing life
The basic rating life, L10, is that life which 90% of a representative group of identical bearings can be expected to achieve or exceed before subsurface material fatigue appears. The life that 50% of the bearings may be expected to achieve or exceed is approximately 5 times the L10 life. This is known as the L50 or median life.
Kaydon load ratings enable an estimation of the bearing L10 life for a one-dimensional load case.
The life can be estimated using:
If the speed is constant, it is often preferable to calculate the life expressed in operating hours using:
Where,
| L10 | basic rating life [millions of revolutions] |
| L10h | basic rating life [operating hours] |
| C | Kaydon dynamic load rating (shown as CK in Kaydon tables) |
| P | equivalent dynamic bearing load |
| n | rotational speed [r/min] |
| p | exponent of the life equation = 3 for ball bearings = 10/3 for roller bearings |
Kaydon dynamic load ratings are based on actual basic rating life testing data (1 000 000 revolutions L10 basic rating life). This is the industry standard that was established for ease of calculation.
There is no significant difference between the dynamic load rating for inner ring rotation versus outer ring rotation. This is due to the relatively small ratios of ball diameter to pitch diameter in thin section bearings.
It is not advisable to apply loads equal to the dynamic load ratings in an actual application. Continuous rotation under these conditions would not normally yield an acceptable basic rating life.
The actual value of a basic rating life calculation is only valid for an individual load case and the bearing internal clearance or preload. It is very rare to have a truly radial or axial or moment load, so these are not normally used for a basic rating life calculation.