Pumps & Systems, April 2013
Archimedes invented the first centrifugal pump about 200 B.C. in Ancient Greece. Since then, pumps, centrifugal and otherwise, have had the same continuous problem—wear. One part must turn while the other remains stationary. Centrifugal pumps have changed little since then—an impeller turns and liquid is pumped in the desired direction. The first wear bushing was probably soft wood to protect the hard wood. Therefore, wear equals downtime.
New materials, sophisticated impeller angles and propulsion are the only differences in 2,200 years. Wear between the stationary and rotating components is still the main cause of downtime. It has always occurred at the interface between moving and stationary surfaces and has been an accepted, unavoidable drawback. The industry practice for pump manufacturers is to design a cheap sacrificial bushing, just like the soft wood, to abrade away and spare the more expensive shaft. To replace the shaft or the bushings requires downtime, which is the most expensive factor. Pump users need multiple pumps so that one can be taken offline for rebuild of the wear components while another is serviced. Increasing uptime is an advantage of parts molded from the new bulk molding compound.
A new, composite, bulk molding compound (BMC) does not follow the age-old maxim of reducing the coefficient of friction (CoF) between the shaft and the wear bushings as a means of reducing friction/wear. Instead, the surface contact area between the shaft and the bushing is simply reduced. Wear components are molded from the compound and filled with soft, strong fibers and round, ceramic spheres. The perfectly round and smooth ceramic spheres protrude from the surface of the molded part and only a small area of the surface of each sphere actually makes contact with the shaft instead of the entire molded bushing.
The Archimedes Screw 200 BC
The Archimedes screw after 2,200 Years
A thermoset resin is the key to the functionality of this new BMC because of its high glass transition temperature (Tg), chemical resistance and unsurpassed adhesion to fillers. Bushings molded from this compound made with the new resin have a typical Tg of around 250 C (482 F) with continuous use temperature in most applications of around 200 C (392 F). Excursion temperatures to 300 C (572 F) are not uncommon. In one application, a pump was returned from the field for rebuild with the epoxy paint on the outside of the housing scorched from friction heat generated internally by abuse. The rebuild manufacturer reported that the new BMC wear bushing and the shaft under the wear bushing looked like new.
As previously stated, the ceramic beads protrude from the surface of the molded part and are the only contact point between the bushing and the shaft. Just like marbles imbedded in concrete, it does not matter the CoF of concrete because the concrete does not touch the shaft. The round beads only make contact with the shaft and only at the point of intersection or the tip of the ball. These perfectly round spheres do not wear the shaft because of their shape and smoothness. The fiber reinforcement used is softer than the shaft which leaves nothing capable of inducing shaft wear. After more than three years in the field in water pump applications, neither the bushings nor the shafts have shown measurable or visible wear.
The ability of this resin to adhere to fillers is the reason Kevlar and ceramic beads can be used to hold them in place for machining. The fiber reinforcement is similar to rebar in concrete—without it there is little strength. Nothing adheres to Kevlar, according to the manufacturer, but adhesion to Kevlar was achieved with this resin that was equal to epoxy resin to glass. This is why parts molded from this composite BMC are the first long-fiber, Kevlar-filled parts that can be machined cleanly and hold tolerance (see Figure 3).
Dimensional creep with temperature is another reason that other resins do not adhere to their fillers. Adhesion and exceptionally-low creep allow the composite BMC parts to keep their fillers in place.
Kevlar and ceramic-filled wear bushing components machined to (+/-) 0.001 inches (0.254 millimeters)
All thermoplastics, by definition, must move with heat or creep. When creep occurs, adhesion and mechanical lock on the fillers is lost, long before dimensional stability. Most crystalline thermoplastics lose function at their Tg while thermosets retain functional strength and adhesion to fillers for excursions of 50 C to 100 C above their Tg. The Kevlar starts to fail at 180 C but recovers when cooled. As long as the base resin holds it in place, all is well. The carbon fiber is not affected by temperature in the ranges of pump operation and also conducts the heat away from the wear component. This thermal conductivity allows for higher operating temperatures by conducting the heat to the housing and out of the pump.