SUBJECT: Shaft displacement and the
original equipment seal (O.E.M.) 11-7
The following picture describes a typical original equipment seal
(O.E.M.) used in the process industry. It should run trouble free
until the carbon face (B) wears away.
To experience long life with this type of
- It must be installed with the proper face load.
- Stationary face "A" must be perpendicular or square to the
- The seal components must be chemically compatible with the
fluid in the stuffing box and any cleaners or solvents that might
be flushed through the lines.
- The seal must be designed to handle the pressures and
temperatures experienced by the product and cleaners it is
- The seal must be able to handle the shaft speed.
- Nothing must interfere with the ability of the seal components
to follow shaft movement while the flat seal faces stay in
contact. Solids, crystallizing fluids and high viscosity fluids
are examples of fluids that will clog sliding components and cause
seal faces to separate with excessive shaft movement.
- Excessive shaft displacement could cause a series of sealing
problems that would include:
- Fretting and shaft damage between the Teflon wedge (2) and
the sleeve (c).
- Excessive movement of the springs that could them to work
harden and fatigue them quickly.
- A high risk of the seal faces opening allowing solids to
penetrate between the faces, imbed into the softer carbon, and
cause face damage.
- A change in the spring load on the faces that could either
open the faces prematurely or cause excessive heating between
the lapped faces.
- The seal case or carbon face could come into contact with
the inside diameter of the stuffing box, either causing damage
to a seal component, or opening the lapped seal faces.
- The rotating shaft or sleeve could come into contact with
and damage the stationary seal face.
Just about all original equipment seals (O.E.M.) leak long before
the carbon faces wear out because this type of seal is very sensitive
to shaft displacement. Here is a list of some of the common causes of
the shaft and seal components to be axially and radially
Causes of axial displacement of the rotating
Remember that sleeve bearings allow a lot of axial movement.
Precision bearings limit axial movement to thousands of an inch
(hundredths of a millimeter), 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.
- Attaching a mechanical seal to the shaft adds to the axial
thrust because of the stuffing box pressure working on the seal
area attached to the shaft or sleeve . This thrust is normally
towards the bearings
- Thermal growth causes shaft axial growth that should be
partially compensated for in the coupling internal
- 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 moves 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
Causes of a radial displacement of the
- 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.
- Dynamic unbalance of the rotating assembly, especially the
- A bent shaft.
- A non concentric shaft sleeve.
- Set screwing a mechanical seal to a shaft or sleeve will cause
the seal to run non-concentric with the rotating shaft or
- Misalignment between the pump and its driver.
- Pipe strain.
- Upward thermal growth in a non-centerline design pump.
Both radial and axial shaft
- Bad bearing.
- Bad bearing fit.
- Cavitation. There are five types to consider.
- Water hammer.
- Running at or passing through a critical shaft speed.
The shaft is not centered in the stuffing
- A bolted on stuffing box has slipped.
- The pillow block bearing of a double ended pump are not on the
same centerline as the pump stuffing boxes.
of the rotating shaft. There are multiple causes of
- Mechanical causes of vibration
- Unbalanced rotating components. Damaged impellers and non
concentric shaft sleeves are common.
- A bent or warped shaft. This often occurs during the sleeve
- Pump and driver misalignment. Remember that these
components must be aligned when the pump and driver are hot and
all expansion has taken place.
- Pipe strain. Either by design or as a result of thermal
- Thermal growth of various components, especially
- Rubbing parts.
- Worn or loose bearings.
- Loose hold down bolts.
- Loose parts.
- Product attaching to a rotating component.
- Damaged parts.
- There is not enough mass 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 center line should pass through the
base, not the sides of the pedestal.
- Hydraulic causes of vibration
- Operating off of the best efficiency point (B.E.P.) of the
- 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. This often happens when you are seal a
- A pump discharge recirculation line aimed at the seal
- Loss of power to the pump
- A parallel pump is closing the check valve on the problem
Along with the seal problems just mentioned, excessive radial
movement of the shaft could cause contact between:
- The impeller and the volute casing or backplate. A clearance
of about 0.015" (0,5 mm) is typical.
- The stationary and rotating wear rings you find in closed
impeller pumps. A clearance of 0.003" per inch (0,03 mm/ 10 mm)
diameter of wear ring is a typical clearance.
- Between the inner and outer portions of the labyrinth seals
find in many bearing seal applications.
The excessive shaft displacement could also:
- Overload the shaft bearings causing excessive heat.
- Put an uneven load on the
grease seals you find in most bearing seal applications.
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