I often talk with the pump owners, who believe that the root cause of the shaft fault is only the strength of the data (or lack of strength). I regard this article as an equivalent topic.
In my column in February 2017, I commented on the main reasons for the discontinuation of cantilever pump shaft. Usually the shaft will crack because the pump is required to do some work that it is not planning for. Root cause failure analysis (RCFA) usually alerts fatigue failures due to excessive shaft deflection. Fatigue failure is also known as the failure caused by fatigue due to reverse twists and turns of rotation. Shaft deflection is a preventable result of improper pump operation and/or maintenance selection in the wrong curve area. There are some exceptions to this common fault form, such as metallurgical defects and/or faults in the manufacturing process.
Errors do occur, but for most manufacturers, the number of errors is statistically close to the 6 Sigma level. That is to say, there are 3.4 defects in every 1 million opportunities (DPMO), or it is considered that 99.999966% of all axes have no defects. Reducing shaft deflection will be reflected in longer pump life, lower cost of ownership and higher reliability.
Axis planning
There are many considerations when planning the pump shaft. All reliable pump manufacturers who expect to continue to operate will plan the shaft for normal starting and operating elements, although a small number of pump manufacturers will generally have higher margins in abnormal conditions and safety margins. Economic factors will drive the key parameters that planners need to consider, which at least include torque, speed, and various stresses, such as key seat geometry and location, fillet radius, surface finish, hydraulic load, component components, and bearing location/span.
To compensate, I would like to point out that the shaft diameter depends primarily on the amount of torque expected/required. Because of the various diameters required to install and maintain components, pump shaft planning presents difficult calculations and manufacturing selections. IMOAB pump
In addition to the above elements, please consider the temperature, pH value and the percentage of suspended solids in the pumped liquid before deciding the axis data suitable for your application. See my February 2017 column and/or the resources listed for more details.
data
Common shaft materials include austenitic 304 and 316 stainless steel, martensitic 410 and 416 stainless steel, CD4MCu dual phase steel, carbon steel 1040 and 1045, alloy steel 4140 and precipitation hardening martensitic stainless steel 17-4 PH. Your pump shaft may use different materials, but it may have similar mechanical, thermal, electrical and chemical (alloy composition) characteristics.
In addition, be aware that all information is flexible to some extent. When considering elastomers such as rubber and plastic, it is easy for us to understand this concept, but when talking about metals, we seldom think of elasticity - but all metals are elastic. , IMOUS pump
Young's modulus - what is modulus?
The word module has a Latin root. I sincerely apologize to my Latin teacher, Mrs. Bruniger, and roughly translate it to mean measurement or measurement. In the field of mathematics, it is another way of saying absolute value. In physics, modulus is a constant factor or ratio. In computer science, it means something else. In metallurgy, we think it is the ratio of the slopes of two different curves.
Note: For information science, there are three types of moduli.
Bulk modulus: the ratio of the fraction reduction of stress and main body volume, or the index to measure the compression resistance of data.
Shear modulus: the ratio of the tangential force per unit area to the angular deformation of the object or the resistance of the data to the shear deformation. Shear modulus is the ratio of shear stress to shear strain.
It is closely related to the elastic modulus of this series (also called Young's modulus): Young's modulus is the ratio of tensile stress to tensile strain or the ratio of longitudinal stress to strain.
Generally speaking, we can regard Young's modulus simply as a ratio (technically, the ratio has no unit). More importantly, it tells the pump planner whether the shaft data will bend, deform or crack under a given condition.
Young's modulus is expressed as E. During which E=stress ÷ strain.
See Figure 1.
Young's modulus is a measure of elasticity and should not be mixed with strength, resistance or hardness. Because the most common shaft data have similar Young's modulus values, the decision to change the data due to some harmful problems can rarely solve the root cause of shaft problems. Different shaft data selection will have different corrosion resistance, hardness and tensile strength characteristics. If the shaft does not deflect excessively, specific selection may be more suitable for your application.
Young's modulus is essentially the elastic property of data - that is, how many times can you bend before it cracks? Elasticity is a feature of data, indicating how to recover it to its original shape after deformation. Technically, more importantly, how far can you twist the data in hundreds of thousands of cycles before exceeding the limit of data? This is also known as the "endurance limit". When the shaft did crack, we jokingly called it "accidentally exceeding the" Goodman Line "in the data engineering.
If you investigate the eight most commonly used pump shafts, you will find some differences in hardness, strength and corrosion resistance. You will also notice that the Young's modulus values of all data are in the same range. One exception is titanium, which has a higher value.
Average Young's modulus in English units (psi)
316-SS 29.0 x 106 psi
17-4 PH 28.0 x 106 psi
4140 28.5 x 106 psi
CD4MCu 29.0 x 106 psi
For SI units (metric SI units), the value of psi units will be converted to approximately 190 to 195 Gipascals (GPa). Sometimes the unit is expressed in newtons per square meter (N/m2).
Shaft deflection (zigzag)
The shaft deflection in the running pump is the result of three main factors. Any shaft with runout exceeding 0.0025 inch to 0.003 inch should be replaced.
The second reason is that the rotor is unbalanced. Unbalance is usually not a problem of the shaft, but of the impeller, which is why it is always a smart idea to balance the impeller. At least reach the World Standardization Arrangement (ISO) 1940 6.3 level or better balance. The shaft deflection from the unbalanced impeller is called "whip".
The third reason is that the radial thrust generated by the pump when running away from the allowable/preferred working area (A/POZ) is not weakened. Please note that all three problems can occur together.
A simple step in reducing deflection is to use a solid shaft instead of a sleeve shaft. Of course, not all operators and applications can adapt to this decision. In addition, many pump manufacturers offer the option of a larger diameter shaft to reduce deflection. Reducing deflection can not only prevent the shaft from fatigue failure, but also extend the life of mechanical seal. Most seal manufacturers require that the deflection of the sealing surface be less than 0.002 inch to obtain a successful seal life. For more information, see my February 2016 column. Another advantage of reducing deflection is to extend the bearing life.
Achieve two shaft deflections per revolution of the shaft. Thus, a pump running at 3550 revolutions per minute (rpm) will deflect 7100 times per minute. For details on calculating the radial thrust, see my column devoted to this topic in January 2021.
Shaft deflection coefficient
The last and most important factor of the cantilever pump is the cantilever amount of the shaft and the corresponding deflection, which is referred to as the "L/D ratio" of the shaft (correctly expressed as L 3 ÷ D 4 or L 3/D 4). L is the axial spacing from the impeller centerline to the radial bearing center, and D is the diameter of the shaft. The ratio is derived and simplified from the deflection formula of cantilever beam used in statics. I do not plan to derive the formula in this column, but I will outline this connection and point out that one of the main factors in the deflection formula is the elastic modulus E (Young's modulus). This is a more complex calculation/derivation.
L/D ratio is also called "slenderness ratio" or "shaft stiffness ratio". It indicates how much the shaft will deflect (twist) due to radial hydraulic force when the pump operates far away from the planned point (optimal efficiency point or BEP). The L/D ratio is actually only applicable to cantilever pumps.
I assume that shaft defects can not be prevented simply through data selection, but can be alleviated through geometric decisions.
The most common shaft failure mode is fatigue. Fatigue is due to excessive shaft deflection, which is a function of radial hydraulic load, rotor balance and shaft stiffness. The strength of the shaft has little effect on the stiffness. Stiffness is a function of elastic modulus (Young's modulus) and L/D ratio (geometric dimension).
Common shaft data are attributed to the strict value scale of Young's modulus (29 x 106 psi). Therefore, it is not necessarily a prudent decision to change the shaft data only to avoid/mitigate shaft fracture. If you address the root cause operating factors, you will experience higher pump reliability.
see
Mechanical Engineering Planning (5th Edition), Joseph Shigley and Charles Mischke
Analysis of root cause problems - understanding mechanical problems, Neville Sachs
How do components fail? (ASM publication) Donald Wulpi
Statics (Engineer Mechanics) 3rd Edition, F Beer and ER Johnson
Appendix K of API Standard 610 (11th Edition)
The author of this article, Jim Elsey, is a mechanical engineer with more than 50 years of experience in rotating equipment for global industrial and marine applications. He is an engineering consultant of a famous pump enterprise and also a loser of a pump enterprise. He is an active member of the American Society of Mechanical Engineers, the National Association of Corrosion Engineers and the Navy Submarine Alliance. , IMO screw pump ,US Gear pump
