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Flow Meter Selection Guide All about Rotors...

A "Rotor" is defined as a "rotating member of a machine" by Collins English Dictionary in the Complete & Unabridged 2012 Digital Edition. Meriam-Webster defines a rotor as "a part of a machine that turns around a central point."

When referenced as a component in an Oval Gear Meter, the rotor is an oval shaped gear that in tandem with a 2nd identical gear rotates within a confined space. Each rotation of the gears allows a known volume of fluid to pass through the meter. The flow rate and/or flow totals are then calculated based on the number of times the gears rotate.

Rotor Basics

The oval gear meter's compatibility for any given application is determined by meter's materialsOval Gear Rotors of construction. Specifically, we're speaking of the wetted components, those that come into contact with the liquid.

The way a rotor is made and cut determines its effectiveness to measure fluids of varying viscosity and temperature, while the material is made from determines its compatibility for the application.

In the discussion below, we expand on the features and benefits of PPS, Aluminum and Stainless Steel rotors. In the final section, we explain the basics of the "Sintering" process as it relates to Rotor construction and then finish with a a primer on when and where to consider the use of High Viscosity (Keishi Cut) Rotors.

Choosing the Best Rotor Material for your Application

PPS is an excellent rotor material providing a meter with outstanding accuracy, wear life, serviceability, and very favorable cost of ownership. PPS rotors should be chosen whenever the application conditions allow it; factors to consider are compatibility with the measured liquid, and suitability of the operating temperatures.

PPS vs. Aluminum or Stainless Steel

When your process fluids are compatible all three materials, why choose PPS over Stainless or Aluminum? Consider the following:

  • Reduced weight of PPS compared to Aluminum; leading to a slight reduction in pressure losses, and improved accuracy at start-up as there is less mass to move when the liquid starts to flow. This same benefit is even more pronounced when PPS is compared to SS

  • Reduced manufacturing costs which translates to lower purchase costs. This same benefit is even more pronounced when PPS is compared to SS

  • Reduced manufacturing costs also lead to lower maintenance/repair costs. This same benefit is even more pronounced when PPS is compared to SS

  • PPS is immune to galling (adhesive wear) that will occur between aluminum components; this means that when an aluminum flowmeter with PPS rotors suffers severe damage (from incorrect installation, incorrect use, catastrophic damage of upstream equipment, dirty liquids) in most cases the aluminum measuring chamber will remain undamaged meaning the flowmeter can be repaired with a replacement set of rotors. With meters containing aluminum rotors there will often be damage to the measuring chamber from contact with the aluminum rotors and the subsequent galling; this will often lead to a meter that is not repairable.

  • PPS is immune to many more liquids/chemicals than aluminum

  • PPS bearings are very resistant to wear, due to their high content of glass fibers. This gives a bearing that is significantly less sensitive to lubrication quality than the roller bearings used in aluminum rotors.

  • Noise: PPS rotors produce significantly less operating noise than do metal rotors – often 10-15dB less at maximum speed. This can be very significant for indoor installations, especially when multiple flowmeters are installed in a single building. This same benefit applies when PPS is compared to SS

  • For many applications where a Stainless Steel flowmeter is required PPS rotors can still be used in place of stainless steel rotors for a significant cost saving without any reduction in meter performance or life span.

Click Here to learn more about PPS...

ALUMINUM - Rotors with steel roller bearings provide good performance on all lubricating Aluminum Rotors hydrocarbon liquids. They are a good choice for applications where the liquid is compatible with both aluminum and Carbon Steel.

In general, aluminum Rotors should only be chosen over PPS rotors in applications where large and abrupt temperature variations are present and/or where operating temperatures are expected to be above 248°F (120°C).

STAINLESS STEEL - Consider using stainless steel whenever the application abnormal temperature variations, or where the chemical/corrosion resistance of 316 stainless steel is required and/or preferred. In addition, you should consider the following:

  • Stainless Steel is very stable with increasing temperatures, and in combination with a stainless steel meter body there is negligible effect on the accuracy of the flowmeter with large temperature variations.

  • 316 Stainless Steel has excellent chemical resistance and corrosion resistance, making it suitable for the vast majority of chemical applications.

  • Due to the mechanical strength, stiffness, and temperature stability of this material, Stainless Steel Rotors are considered to be exceptionally accurate in a wide variety of application conditions.

How Rotors are Made

Sintered Rotors - Sintering is the process of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction.

Sintering happens naturally in mineral deposits or as a manufacturing process used with metals, ceramics, plastics, and other materials. The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum.

Stainless steel rotors are manufactured from 316 sintered stainless steel which is a bonded process using stainless steel powder.

Keishi - The "Keishi Cut", which is a Japanese term for High Viscosity rotors. Keishi Cut rotor's have every other gear tooth machined on the flat side of the gear, to provide pressure relief between the gear teeth. In most instances, this special machining can reduce the maximum pressure drop across the meter by approximately 50%. High viscosity (or Keishi Cut) rotors are normally used for fluids with a viscosity above 1000cP.

One other consideration for applications where the normal operating viscosity is below 1000cP; Remember to factor in the minimum operating temperature. For instance, at start up during the winter months, there are many places where minimum temperatures can and do drop well below 0°C, thus increasing the viscosity of the oil or grease significantly to a point well over 1000cP. In these instances, High Viscosity rotors are recommended.

Not sure which rotors are best suited for your application? No problem... Call the Flow Specialists at for expert application assistance! We're available Mon-Fri, 8am-5pm Eastern. Call (866)905-9788

Flow Specialist - Bill Michie Written By: Bill Michie  
Flow Applications Specialist
Cross Company
Instrumentation Group
Phone: (866) 905-9790 (M-F, 8am-5pm Eastern)
Contact the Experts

Special thanks to the Technical Team at GPI Austrialia for preparing, publishing and giving us permission to use the valuable information provided in this post.
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