Near-Perfect New Wear Rings and Bushings
New bulk molding compound (BMC) molds remarkable wear parts
For years, industry has had problems with rotating shafts and fixed bushings. The continual wear of rings and bushings has been an issue. It has always been possible to mold a wear component that would not wear but they wore the shaft. 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 manufacturer in Gastonia, N.C.
The major use thus far has been in centrifugal pumps. The wear bushings surrounding high-speed shafts which 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 cost in keeping these pumps in operation.
Overcoming wear has vexed rotating device manufacturers since Archimedes invented his screw 2200 years ago. The initial approach to solve this problem was to place soft wood rings between the hard wood rotating shaft and the stationary housing. The soft wear rings or bushings were intended to wear sparing the more expensive components. When they reached a critical point the machines were disassembled and the bushings were replaced.
In early days of metal pumps brass or bronze to increase the service life of the pump. However, seawater, chemicals, unfriendly environmental conditions, coefficient of thermal expansion, as well as other contaminants attacked these metals and caused device failure.
Converting the shaft and housing to stainless steel solved the many corrosion problem for these components, but the stainless did not work well as a bushing on a stainless shaft. Because of the relatively higher cost of the 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 rotating device 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 as these materials are harder than the stainless steel that makes up the shaft.
Other attempted nonmetallic solutions include:
- Impregnated graphite carbon—this material produces excellent 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 for pump wear components with a wide reaching and ambitious target. 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. We were given this wish list by Carver Pump in Muscatine, IA.
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
- Have sufficient adhesion between the resin and fibers that the adhesion is stronger than the fibers so they ,cut instead of pulling out of the matrix, 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.
The New Solution
The Titan team has 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 use either Carbon Fiber or 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 for its’ manufacturer, “Nothing sticks to Kevlar.” However, Titan had recently developed a resin that stuck to Teflon, so it was decided to try it with Kevlar.
Difficulties of using Kevlar as a Filler in Plastics
Without adhesion, fiber reinforcement does not add much to the strength of the part. Without adhesion, when machining parts, the machined surface is fuzzy because the fibers pull out instead of cutting; thereby, eliminating the possibility of holding tolerances. When using any fiber-filled plastics in molded parts, two separate forces determine the strength of the molded part. The first is 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. When a required adhesive force is achieved, the part can be machined to a smooth surface without protruding (fuzzy) fibers. The force to cut the fiber must be less than the force required to break the adhesive bond between fiber and matrix.
The second major drawback of using Kevlar as a filler is that Kevlar absorbs 7%t moisture at ambient temperature. Moisture inside the plastic matric causes steam in processing and weakens the molded part and a molded and machined part used inside a high-pressure pump that absorbs liquid will quickly become ineffective. Machined fibers have cut surfaces exposed to moisture. A proprietary manufacturing process prevents the Kevlar from absorbing liquid. As you can imagine this can be important when the product spends its’ life submerged in liquid under pressure.
Molded and machined Kevlar-based wear rings
New BMC Eliminates the Problems
The new BMC, when used to manufacture a Kevlar or carbon fiber 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 Carbon Fiber or 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.