Shaft deflection, the different types
SHAFT DEFLECTION, THE DIFFERENT TYPES ST004
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).
- 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.
- The motor rotor moved to its magnetic center at start up. The coupling did not compensate for this axial movement because thermal growth had taken up the clearance in the coupling.
- 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.
- Shaft thrust.
- There is a thrust towards the bearings caused by the combination of the fluid changing direction in the impeller and acting on the shaft and/or impeller surfaces. This thrust is offset by a thrust towards the wet end caused by the impeller shape.
- With centrifugal pumps the resulting axial force can be in either direction, depending upon how close the pump is operating to its best efficiency point. Above 65% of its best efficiency, the thrust is towards the wet end. Below 65% of the best efficiency the thrust is towards the power or bearing end. There is little to no movement at 65% of the pumps best efficiency. This means that at start up the shaft moves in both directions accounting for a higher percentage of seal failure at start up.
- Vertical mixer shafts often lift vertically when solids are mixed with liquid because the solids lift the mixer blade. Mixer people call this “skidding”. 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 most of their 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 back plate, volute and impeller wear.
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.
- The impeller is out of balance.
- Wear, corrosion or cavitation damaged the impeller.
- Product has built up on the vanes or in the balance holes.
- The impeller diameter was reduced and the impeller was not re-balanced
- The impeller never was balanced.
- The shaft is bent.
- Usually occurs during sleeve removal or if the bearing was installed with an arbor press.
- Improper storage with the long shaft supported only on the ends.
- Heating the shaft to remove the sleeve is another common cause
- A non-concentric shaft sleeve.
- A non-concentric mechanical seal attached to a sleeve or shaft.
- Serious misalignment. The misalignment can be the result of pipe strain or 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.
Both radial and axial shaft deflection from the pump centerline
- The bearings are worn excessively.
- Contamination of the lubricant is the biggest cause. Grease or lip seals have a useful life of only 2000 hours (84 days).
- Poor fit or installation.
- Cavitation, there are five types to consider:
- Vaporization caused by too high a product temperature or too low a suction head.
- Air is entering the stuffing box. A common problem with pumps that run in a vacuum or taking a suction from an evaporator or condenser.
- Internal recirculation. Occurs when the Suction Specific Speed is too high, or when either the impeller or wear ring clearance becomes excessive.
- The vane-passing syndrome occurs if the O.D. of the impeller is too close to the pump cutwater. This clearance should be at least 4% of the impeller diameter in the smaller size impellers and at least 6% in the larger diameter impellers (greater than 14 inch or 355 mm.)
- Turbulence. It occurs if there is not laminar flow through the lines.
- Water hammer.
- Running the pump at, or passing through a critical shaft speed.
- Any kind of severe vibration problems will cause both radial and axial deflection of the shaft.
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 diameter of the stuffing box lip, or a shoulder cast and machined 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 of a rotating component.
- The shaft is hitting the wear ring, or a stationary wear ring is contacting a rotating wear ring.
- The shaft is hitting the seal gland or stationary face.
- The shaft is contacting the bushing we normally find in the inboard end of the stuffing box.
- A seal rotating component is hitting the stuffing box inside diameter.
- The shaft or sleeve is contacting the disaster bushing in an API (American Petroleum Institute) gland.
- A recirculation line aimed at the seal faces is causing a pulse each time the impeller vane passes the fitting.
- A gasket or fitting is protruding into the stuffing box.
- 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.
- The impeller vane is 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.
- The seal is vibrating in harmony with some rotating component. The same thing that causes a rear view mirror to vibrate in an automobile. Changing the speed of the equipment or “damping” the vibrating component can stop most harmonic vibration.
- Operating the pump at a critical speed. Watch out for this problem in variable speed and pulley driven pumps.
- Slipstick, caused by:
- Poor lubricating fluids.
- Hot water.
- Some detergents.
- Dry running applications.
- Too high a face load.
- Using unbalanced seals.
- Poor installation.
- Face load has changed because of temperature growth or impeller adjustment.
- You are using a high friction face combination. Often occurs if you use two hard faces.
- Harmonic vibration.
- The product is vaporizing at the seal faces
- Happens frequently with products that contain water, and are operated at elevated temperature.
- Vaporization can occur at the seal face because of high face load or if you use unbalanced seals
- The stationary seal face is not perpendicular to the rotating shaft. This causes the spring loaded rotating face to move back and forth twice per revolution.
- The stuffing box face is not square to the shaft. The stuffing box face is often a rough casting.
- Tightening the gland bolts through a gasket will cock the stationary face.
- Pipe strain.
- Temperature growth.
- A convection tank or some other heavy device is hanging off of the gland.
- Bearing fit or wear.
- Coupling alignment.
- Shaft deflection. You can look this subject up in the alphabetical section
Another contributor to the opening of seal faces and their destruction is excessive heat. The heat can come from several sources:
- The ambient temperature of the product you are sealing may be excessively hot.
- Heat generated within the pump
- The heat generated at the seal faces
- On February 18, 2018