GRAPHALLOY® FAQs
Contact one of our sales engineers if you have a specific,
design-related question Or visit our blog to get help from our experts
GRAPHALLOY FOR GENERAL APPLICATIONS Q. What is GRAPHALLOY and when should I use it? A. GRAPHALLOY is a solid, uniform combination of graphite and metal. Molten metal is impregnated into a graphite substrate under high temperature and pressure. Used as a bushing or wear surface, GRAPHALLOY survives dusty atmospheres, high temperature, submersion and corrosive environments. GRAPHALLOY does not swell, shrink, or cold flow. Graphalloy is self-lubricating and used in difficult applications when a rolling element bearing would fail. It deposits a graphite layer on the inner race which is not normally removed by high temperatures or submersion in a fluid. It is not normally a competitive replacement for a ball bearing where there are no environmental complications. Q. What are the load ratings for your various GRAPHALLOY bearings? A. Our GRAPHALLOY bushings do not have a load rating based on an L10 life such as a rolling element bearing would have. Since GRAPHALLOY is a sleeve bearing, life is based on both load and speed (PV or pressure (psi) x velocity (ft/min)), as well as the environment in which the bearing is used. All of the application factors are taken into account in selecting a GRAPHALLOY material grade for the bushing. Q. Many of your part numbers have -XXX at the end of the part number. What does this refer to? A. The -XXX at the end of a GRAPHALLOY part number is a place holder for the material grade designation. As we have many different GRAPHALLOY material grades, we endeavor to find the most appropriate material grade for the application that the bushing will be used in. Our Engineering Group will help you select the grade based on our prior experience in similar applications. The material grade selection may also affect the bushing OD and ID dimensions by a slight amount as some applications require more or less press fit or running clearance. Q. Why do GRAPHALLOY bushings have oversize OD and ID dimensions relative to my housing and shaft diameters? A. A GRAPHALLOY bushing needs to be retained in its housing via an interference fit. The coefficient of thermal expansion of GRAPHALLOY material grades is low relative to steel and other housing materials. In order to maintain retention at elevated temperatures, the initial press fit needs to be sufficient to overcome this difference. Graphalloy is also fully elastic and the interference between the bushing OD and the housing ID will close up the bushing ID upon installation in the housing. The bushing also needs to have a clearance with the rotating shaft to allow movement. If the housing and shaft are similar materials and are heated to similar temperatures, then the clearance in the bushing ID will increase slightly through the temperature range (the compression fit becomes relieved and the bushing ID follows the expansion of the housing ID). Q. Can I machine a bushing out of GRAPHALLOY stock? A. The majority of our GRAPHALLOY bushing solutions are specially selected material grades for specific applications. They usually are sized and machined to finished dimensions. GRAPHALLOY material grades machine in a manner similar to cast iron and we do try to stock some
semi-finished bushings in some of the more popular material grades. These are bushings with oversized
OD’s and undersized ID’s in six or eight inch lengths. This is not the most economical way to
provide a GRAPHALLOY solution, as the cost includes material that ultimately ends up being machined away. It
does, however, provide for material at hand for emergencies. GRAPHALLOY FOR PUMP APPLICATIONS Q. Hard carbon materials are available; will they perform better? A. Carbon materials can be made over a wide range of hardnesses - from soft brush materials to nearly ceramic hard mechanical seal face materials. Hard carbons can be much harder than usual shaft materials. These hard carbons can score shafts to the point where they are actually cut. Bushings for pumps are intended to be wear parts, absorbing the shocks, rubbing and abrasion of normal use without damaging the shafting. GRAPHALLOY is chosen for pumps to provide non-galling, self-lubricating bearings. GRAPHALLOY with a moderate hardness is used in most pump applications to insure that expensive shaft materials will be protected.
A. The short answer is no. Bearings and seals perform very difficult functions. Seals require “light band” quality surfaces to do their jobs against equally perfect mating parts and to eliminate gaps that would be detrimental to seal performance. Normally, GRAPHALLOY bushings are not performing a sealing function. A rougher surface is both permissible and desirable as an aid in maintaining a layer of lubricating pumpage. The self-lapping qualities of GRAPHALLOY will coat the shaft surface and polish the ID of the bushing. A high polish finish on a new pump bearing would be counter-productive and lengthen or prevent normal break-in. The ideal surface finish is 8 to 16 microinches. Q. GRAPHALLOY bearings can be pressed in with much greater press fits than hard carbon materials; how does this help? A. Generally, the harder the carbon material, the HIGHER its elastic modulus (stiffness). GRAPHALLOY has a LOWER elastic modulus and therefore tolerates greater press fits. Being more flexible allows GRAPHALLOY to better withstand the shock of shop handling, pump installation and most importantly, pump operation. Hard carbon bushings chip more easily when dropped and are at a disadvantage when encountering similar shocks in use. Since hard carbon materials cannot tolerate heavy fits, they are difficult or impossible to use for high-temperature applications - they can become loose as their housing bore expands in the heat. Q. Is GRAPHALLOY a true “Run-Dry” material? A. GRAPHALLOY is an alloy of carbon graphite and metal. The graphite provides continuous lubrication, without the need for grease oil or other forms of lubrication. GRAPHALLOY deposits a thin film of graphite on the shaft to reduce friction and the metal impregnant help to transfer heat from the rubbing surface to the housing. GRAPHALLOY has much higher temperature limits than many thermoplastics available today. Run-Dry transients will not melt the GRAPHALLOY as it may do to other materials. Normally the pumps can be put back in service immediately after dry running transients while plastics usually require replacement. Q. How do I choose the correct grade of GRAPHALLOY ? A. There are many grades to choose from. The GRAPHALLOY grade selection for pumps is primarily a function of operating conditions of the application. The key questions are temperature, pumpage, load, speed, type of pump and whether run dry or flashing is expected. The best appraoch is to fill out a Request for Quote and have our engineering team make a recommendation. Q. How do thermoplastics compare to GRAPHALLOY? A. There are several advantages in selecting GRAPHALLOY compared to thermoplastics:
Q. What does GRAPHALLOY do when the housing and the
shaft expand due to temperature?
A. Correctly installed, the expansion of GRAPHALLOY will follow the housing it is pressed into. The expansion of the housing and the shaft must be considered when calculating the press fit and running clearance during pump operations. GRAPHALLOY’S unique properties enable the designer to maintain a consistent clearance over a wide temperature range. Q. Does GRAPHALLOY run cooler than thermoplastics? A. GRAPHALLOY material conducts heat better than thermoplastic bearing materials. This provides for cooler
operating temperatures during dry run starts, flashing and transient upsets.
8/23/2017 |