Axial Thrust
- Definition
Pressures bearing on the pump in a direction in line with the shaft, caused by static mass load and pressure differential between the front and back sides of the impeller under dynamic conditions.
- Four Axial Thrust Modes
Axial thrust loads can be divided into four time modes: Static, Steady State, Cyclic, and Transient.
- Static Axial Thrust
Load presented to the pump in the axial direction, by the mass or weight of the pump, system piping, and fluid contained in the pump and system piping. Total mass at rest is often considered a steady state load but static cyclic loading can occur due to changes in the system while the pump is not operating.
Static loading is a special concern when using tilting pad thrust bearings. Tilting pad thrust bearings typically have a high ability to withstand high static thrust loads. However, these types of bearings require a pressurized film of lubricant to maintain clearance between the tilting pad and the collar. The pressure and flow of that lubricating film is produced by the bearing collar rotation. Therefore, as the pump starts or stops, that pressurized lubricating film is diminished or absent entirely. Applications with high static loads AND which must be started and stopped frequently or slowly, may encounter problems with tilting pad bearings.
- Dynamic Steady State Axial Thrust
Axial thrust on the impeller shaft and bearings, arising from a pressure differential between the front and back sides of the impeller under dynamic conditions. A steady state thrust load at a given flow rate under constant system conditions.
- Dynamic Cyclic Axial Thrust
Cyclic axial thrust is caused by variable intake and discharge pressures as system conditions change. The majority of axial thrust bearing failures may be due to cyclic axial thrust problems.
Suction and Recirculation Cavitation cause cyclical axial thrust loads with either random or symmetrical periodicity, overlaid on the steady state axial thrust load. Suction and Recirculation Cavitation can cause severe cyclic axial thrust loads capable of fatiguing the thrust bearing, shaft, and the impeller attaching bolt. The impeller attaching bolt used on end suction type pumps is particularly vulnerable to cyclic fatigue failure. Rolling element bearings are more vulnerable to cyclic axial thrust than are tilting pad (Kingsbury) thrust bearings.
- Dynamic Transient Axial Thrust
Pressure applied to the impeller shaft and bearings, arising from seemingly random sources, typically caused by the system into which the pump is installed. Transient thrusts can produce extremely high and extremely short bursts of pressure energies. Inertial energy release (water hammer or pressure surge), is a common source of transient axial thrusts. Valve closures, pump shut-downs, and fluid source shut-downs, are all common sources of transient axial thrust loads caused by the release of inertial energies. Transient thrusts exhibit periodicity if the source of the transient is periodical. The source of a transient thrust can be accomplished by identifying a system variable having the same periodicity as the thrust.
Diagnosis of Axial Thrust ProblemsThrust bearing failures obviously indicate that there may be axial thrust problems. However, failure of Non-Thrust dedicated bearings, can also indicate an axial thrust problem. Close examination of the bearing may be able determine if the cause of failure was axial thrust, or other problems such as radial thrust, dirt or sand in the bearing, (especially in product lubricated tilting pad thrust bearings).
Axial thrust bearing failures can be caused by cyclic, static load, steady state, or transient thrusts. Close examination of the bearing may help identify the source. A data logger may be able to identify the source of excessive axial thrust pressures by identifying a system variable capable of creating excessive axial thrust.
