Shaft runout, shaft deflection, and shaft whip are issues that often occur in industrial equipment using rotating shafts. It is important to check your machinery for these problems regularly. If left unattended, they can lead to severe problems with your equipment or, worse, total equipment failure.
Diagnosing and addressing these issues are essential for ensuring the proper operation and longevity of rotating equipment, such as pumps, motors, and turbines. They can lead to excessive vibration, premature wear, and even catastrophic failure.
What Is Shaft Runout?
Shaft runout is the degree to which a shaft deviates from its true circular rotation. Every shaft has a center of rotation, often referred to as its centerline, and any deviation from this centerline is considered shaft runout.
If shaft runout is severe enough, it can cause several problems with industrial equipment, including excessive vibration, seal wear, and bearing damage. Typically, the maximum allowable amount for runout is 0.002" (2 mils or thou). For higher RPM machines (more than 3600 RPM), a tolerance of 0.001" (1 mil or thou) should be used.
To check for runout, use a dial indicator and magnetic base. Zero the dial indicator on the outside diameter of the coupling hub and slowly rotate the shaft. Watch for runout and measure the amount. Next, move the indicator to the shaft adjoining the coupling hub and measure it the same way. If the hub and shaft travel approximately the same amount, a bent shaft likely caused the runout.
What Is Shaft Deflection?
Shaft deflection occurs when a constant radial load in one direction causes your equipment's shaft to deflect or slightly bend downward. Whenever the shaft rotates 180 degrees, it continues to deflect downward in a similar way.
If the pump is not operating at the Best Efficiency Point (BEP), it is subject to recirculation cavitation and high radial loading, which will cause loss of performance, efficiency, and very high vibrations and shaft deflections.
API, an acronym for the American Petroleum Institute, is an organization that develops technical standards for oil and natural gas industries. API 610 is the centrifugal pump standard put forward by API, and for centrifugal pumps, it specifies a maximum of 0.002-inch shaft deflection at the location of the seal faces. Some older pumps and non-API pumps may not meet this specification. In particular, older pumps designed for packing may have excessive shaft deflection. Shaft deflection is reduced by increasing the shaft diameter and/or reducing the bearing span or shaft overhang.
Four situations worsen shaft deflection:
1. Radial force in the impeller
2. The length from the impeller to the radial bearing
3. The shaft's diameter (the smaller the diameter, the more deflection)
4. The shaft material's properties (different materials have different flex)
When you use a deflected shaft, it continues to rotate along its centerline even if the centerline isn't straight. This rotation produces a "reversal of stresses" in the shaft that can cause it to crack and eventually break. You must limit shaft deflection in your equipment as much as possible to prevent internal parts from rubbing together.
If a shaft deflects too much, it will grind its rotating parts against the stationary parts. If your equipment uses stainless steel parts (many industrial pumps do), these parts will begin to gall as soon as they make contact with each other. Galling almost always becomes progressively worse, and if left untreated, it can cause a complete seizure of your equipment's rotating elements.
What Is Shaft Whip?
People often confuse shaft deflection and shaft whip. Radial load causes shaft deflection, whereas shaft whip results from issues inside the pump or other equipment's rotor. Shaft whip occurs when the shaft deviates along a turn of 180 degrees from its centerline and rotates back 180 degrees every rotation, creating a cone shape. Unbalanced impellers typically cause this shift because that's where the highest velocity energy usually occurs.
Use a Dial Indicator to Measure Shaft Runout
A dial indicator mounted on a magnetic base can be used to measure shaft runout as the shaft is being turned slowly. The runout measurement is reported in terms of the total movement of the indicator needle. Runout measurements in actual operating conditions may be impractical to measure and are likely to be far greater than measurements taken while rotating the shaft slowly, without actual operational loads. When space is restricted, a dial test indicator can be used in place of the illustrated dial indicator.
Conclusion
Shaft runout, deflection, and whip can lead to severe problems in your equipment if left unattended, costing you thousands of dollars in repairs and replacements. You must address these issues as soon as possible to maintain the efficiency and longevity of your industrial equipment.
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