SHAFT DEFLECTION – DIFFERENT TYPES S042
There are a number of forces acting on the shaft to cause it to deflect from the centerline of the bearings. These forces or loads include:
- The forces causing rotation (torque) of the shaft.
- The weight of the parts.
- Radial and axial hydraulic forces.
- Mechanical loading.
The deflection can be along the length of the shaft (axial) or 90° to the length of the shaft (radial). In the following paragraphs we will be looking at both kinds of deflection.
Let’s start with the axial deflection of the rotating shaft.
- Remember that sleeve bearings allow a lot of axial movement. Precision bearings limit axial movement, but can still allow enough radial movement to open lapped mechanical seal faces.
- Up to 65% of its efficiency a centrifugal pump thrusts towards the thrust bearing. Beyond 65% of its efficiency the shaft thrusts towards the volute. At pump start up there is a lot of axial movement of the shaft. This is one of the reasons why we lose so many mechanical seals at pump start up.
- Attaching a mechanical seal to the shaft adds to the axial thrust of the shaft because the stuffing box pressure works on the seal area attached to the shaft or sleeve. The resultant thrust is normally towards the bearings.
- Thermal growth causes shaft axial growth that can be compensated for in the coupling internal clearances. Unfortunately this does not help the mechanical seal.
- Impeller adjustment of open and semi-open impellers can move the shaft towards or away from the volute depending upon the pump design. In the United States the Duriron Pump Company is unique in that impeller adjustment move the shaft in the direction of the bearings. Remember that there is an inital impeller setting and “on going” settings that have to be made for casing and impeller wear.
The main causes of a radial deflection of the rotating shaft from the pump centerline
- Operating off the best efficiency point (BEP) causes the shaft to deflect in a radial direction. The deflection is normally 60° or 240° from the pump cutwater, measured in the direction of shaft rotation if you are using conventional Francis Vane impellers with a specific speed between 1500 and 4000. Other specific speed numbers deflect in the same manner, but in a different axial direction.
- Dynamic unbalance of the rotating assembly is a continual problem. Impeller wear and damage from solids in the pumpage contribute to the unbalance.
- A bent shaft. Many shafts are distorted during the bearing and seal removal process.
- A non-concentric shaft sleeve.
- A non-concentric mechanical seal attached to a sleeve or shaft.
- Misalignment between the pump and its driver. Couplings cannot compensate for this misalignment.
- Pipe strain. Either physical or thermal.
- Thermal growth in a non-centerline pump design.
A combination of radial and axial shaft deflection from the pump centerline
- Bad bearings.
- Poor bearing fit.
- Cavitation. There are five types to consider.
- Water hammer.
- Running at or passing through a critical shaft speed.
- Any kind of severe vibration problems will cause this.
The shaft is not centered in the stuffing box.
- A bolted on stuffing box has slipped.
- The pillow block bearings of a double-ended pump are not on the same centerline as the pump stuffing boxes.
- The seal gland was not centered off the shaft. It was referenced against the inside of the stuffing box or a shoulder on the stuffing box face.
Vibration of the rotating shaft. There are multiple causes of vibration:
- Mechanical causes of vibration
- Unbalanced rotating components. Damaged impellers and non-concentric shaft sleeves are common.
- A bent or warped shaft. This often happens during the bearing and seal removal process.
- Pump and driver misalignment.
- Pipe strain. Either by design or as a result of thermal growth.
- Thermal growth of various components, especially shafts.
- Rubbing parts.
- Worn or loose bearings.
- Loose hold down bolts.
- Loose parts.
- The product is attaching to a rotating component, probably the impeller.
- Damaged parts.
- There is not enough mass or weight in the pedestal. If you weigh the pump and its driver there should be a least five times that mass in the pump pedestal.
- The pedestal is not wide enough. If you drop a vertical line from the center of the motor, two lines radiating out thirty degrees from this centerline should pass through the base, not the sides of the pedestal.
- Hydraulic causes of vibration
- Operating off of the best efficiency point (BEP) of the pump.
- Vaporization cavitation.
- Impeller vane running too close to the pump cutwater.
- Internal recirculation
- Air getting into the system through vortexing etc.
- Turbulence in the system (non-laminar flow).
- Water hammer.
- Other causes of vibration.
- Harmonic vibration from nearby equipment.
- Operating the pump at a critical speed. Watch out for this problem in variable speed and pulley driven pumps.
- Seal “slip stick” at the seal faces.
- The product is vaporizing at the seal faces
- A pump discharge recirculation line aimed at the seal faces. Each pass of an impeller vane gives a pulse to the fluid going to the stuffing box.