OBTANIUM

Near-Perfect New Centrifugal Pump Wear Rings and Bushings

New resin and Kevlar bulk molding compound adds strength to pump wear parts.

For years, the pump industry has had problems with centrifugal pumps. The continual wear of rings and bushings has been an issue. Despite many efforts to improve life and quality of these products, none have measured up to expectations. Now new wear rings and bushings can be molded and machined from proprietary bulk molding compounds produced by a molding producer in Salisbury, N.C., that can improve these components.

Centrifugal pumps’ high-speed shafts rotate the impellers. The impellers force liquid from a supply source through the pump to the user. The housing of the pump must remain stationary. The bushing guides and supports the high-speed/high-torque shaft at the interface between it and the stationary housing.

Until recently, the friction at this interface has caused either the shaft or the bushing to wear, and the pump became unbalanced and rapidly lost efficiency. Replacing either the shaft or the bushing because of this wear is the main problem and cost in keeping these pumps in operation.

Centrifugal Pump History

Overcoming wear has vexed centrifugal pump manufacturers for years. In early days both the pump shaft and bushings were made of cast iron. They were replaced with brass or bronze to increase the service life of the pump. However, seawater, boiler chemicals, unfriendly environmental conditions as well as other contaminants attacked these metals and caused pump failure.

Converting the shaft and housing to stainless steel solved the corrosion problem for these components, but the material did not work well as a bushing. Because of the relatively higher cost of the pump shaft and housing versus the cost of the bushing, manufacturers and the industry chose to make the shafts and housings of harder stainless steel and sacrifice the bushings to wear. Therefore, the service life of a centrifugal pump depends on, and is limited by, the life of the bushings that interface with the moving and the stationary parts of the pump.

Copper-based metal bushings, when used in conjunction with stainless steel, are a problem due to the higher relative coefficient of thermal expansion (CTE) of the copper-based materials. As the softer metal expands, the gap between the shaft and bushing widens, inducing vibration. When cool, the gap shrinks onto the shaft, and the wear increases.

Non-metallic bushings, usually made of thermoplastics, have the necessary toughness but not the dimensional stability required for a pump bushing. All thermoplastic-based bushings, by definition, can be re-melted. As a result, as the temperature inside the pump rises, thermoplastics begin to melt and lose their dimensional stability. Adding glass fiber and other hard fillers in an attempt to achieve the needed dimensional stability causes excessive shaft wear because these materials are harder than the stainless steel that makes up the shaft.

Other attempted nonmetallic solutions include:

Impregnated graphite carbon—this material produces good wear characteristics, CTE and chemical resistance, but is extremely delicate and easily damaged during assembly. In addition, any foreign matter in the liquid damages the bushing when in operation.
Glass filled Teflon (TFE) fluorocarbon—this material has excellent chemical resistance and wear resistance, but has 10 times the CTE of stainless. After several heating and cooling cycles the bushings simply “fall out.” The bushings’ stress relieves, and the force of the liquid pushes it out of the pump.
Phenolic impregnated linen—being a thermoset plastic, this material has natural hardness and low CTE. The material meets all the requirements for a pump bushing, except that it is not naturally lubricious and wears quickly. Neither the linen filler nor the resin is sufficiently wear resistant.
Wear-resistant, glass-filled bulk molding compound (BMC)—the glass and other fillers that provide hardness and wear resistance to the bushing will abrade the shaft.

The search for materials was started in 2008 to satisfy a wish list of pump wear components with wide reaching and ambitious targets. The goals were to develop material that can stand up to the centrifugal pump environment in all applications without wearing either the bushing or the shaft.

This component should:

Be machined to a smooth surface so tight tolerances can be obtained around the pump shaft over a wide and changing temperature range to eliminate vibration
Be tough and hard so that foreign material inadvertently carried in the liquid does not damage the bushing
Provide excellent chemical resistance so that it is not attacked by what is purposely or inadvertently pumped with the liquid
Be soft enough not to wear the shaft but tough enough to withstand press fit assembly
Offer good adhesion between the resin and fibers so that the hold is stronger, therefore leaving a smooth and dimensionally accurate machined surface
Have low water absorption so that it does not absorb liquids, which could cause expansion or contraction

Simply stated, the ideal bushing should be molded from a material that is tough, soft, hard, flexible, stiff, thermally-stable and easily-machined. It should not absorb liquids and must be cost effective.

Molded and machined Kevlar-based wear rings

The New Solution

The consulting-producer team had considerable experience with thermoset molding compounds. Thermoset’s natural hardness and chemical resistance appeared desirable in a pump bushing. However, these compounds require fibers, usually glass, to give them strength and toughness. Glass is harder than the metal of the shaft and, as a result, causes shaft wear.

To overcome this problem, they used Kevlar to impart necessary strength to the molded part without the abrasive qualities of glass. Kevlar was not previously used because, in the words of a project engineer, “Nothing sticks to Kevlar.” However, the molding producer had recently developed a resin that stuck to Teflon, so the consultant decided to try it with Kevlar.

Drawbacks of Kevlar

Without adhesion, the fiber reinforcement does not add much to the strength of the part. Without adhesion, when machining parts, the machined surface is fuzzy, therefore eliminating the possibility of holding tolerances.

When using any fiber-filled plastics to mold parts, two separate forces determine the strength of the molded part. The first is the strength of the fiber and the second is the adhesion of the plastic to the fiber.

The ideal condition is for the adhesive force between the plastic and the fiber to be greater than the strength of the fiber, or as great as it can be. When a good adhesive force is achieved, the part can be machined to a smooth surface without protruding (fuzzy) fibers.

The second major drawback of using Kevlar filled BMC as a bushing material is that Kevlar absorbs 7 percent moisture at ambient temperature. This causes problems with Kevlar “bullet proof” vests.

As the Kevlar absorbs moisture, the vest becomes less effective. A molded and machined part used inside a high-pressure pump that absorbs liquid will quickly become ineffective.

New Resin Eliminates the Problems

The new resin, when used to manufacture a Kevlar filled BMC compound, solves both these problems. Molding a near-net shape part and machining to size produces a bushing that meets all the requirements listed in the section above.

In addition, it absorbs water at a rate about the same as a glass-filled epoxy, which is the bench mark for water absorption in a fiber-filled molded part. In other words, water absorption is not a problem.

Parts with the new BMC were molded, machined and assembled into a pump for testing. The molded part had a 5-inch outside diameter (OD) and 0.1-inch wall thickness. When quality control professionals checked the part, they found that the OD was 0.010 inch oversize, or 0.010 inch larger than the hole in the steel part into which it was to be pressed.

Rather than re-machining, the plastic part was positioned above the hole in the stainless steel pump housing, and the two parts were placed into a 20 ton press. Three steel plates were placed on top of the two parts to be assembled. As the press applied pressure, a crack like a gunshot was heard.

“Well, there goes your plastic part.” an engineer commented. After disassembly the stack showed that one of the steel plates had cracked, leaving the molded part perfectly seated. That was a 0.010-inch compressive press fit.

Despite skepticism, pump bushings produced with Kevlar-based thermoset material meet all the requirements for pump wear parts: The near-perfect bushing’s material is tough, soft, hard, flexible, stiff, thermally stable, easily machined, non-absorbent and cost effective.

Pumps & Systems, May 2012

PRLC – LW 1947

Description:
PRLC-LW is a Carbon Fiber, Graphite, and Ceramic filled Bulk Molding Compound manufactured and designed to have outstanding wear and thermal characteristics. Typical applications include wear bushings and bearings, engine components, and more.

Disclaimer:
NOTICE TO USERS: All values represented are based on laboratory tests and does not represent or reflect conditions that exist in production. The data provided should not be used to or intended to substitute for any testing you may need to conduct to determine suitability of a material for a particular use. No warranty or legal responsibility is accepted or implied, and all information is accepted at buyer’s risk. This information may be subject to revision as new knowledge and experience become available.

We have not encountered any signs of wear, although we cannot assure the same outcome for all cases.

Randy Lewis of P.R. Lewis Consulting, LLC, in Lake Wylie, S.C. is an industrial engineer who has spent almost 40 years in every area of the plastics molding and compounding industry. He works with Zeon Technologies to invent new thermoset molding compounds for applications that “Can’t be done,” with Wear resistance, chemical resistance, and high temperature being the focus. Lewis can be reached at [email protected]. This article was also edited by Mike Burnet, Zeon Technologies, Inc.