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The bearing gurus at Clevite share insight regarding current and future rod and main bearing development.
Installed bearing sets create a slightly eccentric bore, which promotes oil film capture and enables the rotational movement of the shaft to generate a hydroplane of oil film around the circumference.
Bearing crush is critical to holding the bearing in place. With the cap installed, this exerts radial pressure, forcing the bearing backs outward radially.
When big bucks and series championships are on the line, every nuance is considered. Bearings are no exception.
According to MAHLE Clevite’s Bill McKnight, top NASCAR Cup teams are carefully selecting bearings and measuring each. They will typically order about 500 bearings and check each individual bearing shell for height (in terms of crush factor) and thickness, etc. They’ll carefully categorize each bearing as tight, loose or intermediate, which then allows them to pick and choose bearings depending on the specific engine application.
The trend among the Cup teams is to run tighter clearances, creating higher oil film pressure.
McKnight also noted that some teams are buying Clevite bearings from a proprietary facility in Scotland whose technicians painstakingly measure and package sets guaranteed to be exact matches. “These measured-and-matched bearings are rather expensive, costing around $30 each, so only teams with very healthy budgets are taking advantage of this,” McKnight said. “NASCAR engine developers are also currently testing cryogenic treatment of bearings to determine if this offers any benefit, but the jury is still out with regard to this. All teams are definitely taking advantage of bearing surface coatings, which is no longer considered a questionable approach. The use of anti-friction and oil-embedding coatings have definitely proven their worth.”
McKnight noted that Clevite’s first effort at cryogenically treating race bearings for drag applications was made during the summer of 2007 for the top fuel and funny car teams, the hardest users of bearings we have. Tests continue, improvements look to be small, perhaps 5-7 percent, but when you’re using all we’ve got in a bearing and still asking for more, that may be enough. The treatment consists of freezing finished product at well below -300F for seven to eight hours.
Clevite has made a running change in the TriArmor, coated bearings. Improved coating durability is the result without sacrificing any of the features of the original bearing, except for a darker blue color to the coating. The change was a running production change last spring so most, if not all, of the line is new material when you order parts from your supplier. Clevite also supplies the custom bearing coaters with Clevite race bearings for their production (including HM Elliott, Calico and Polydyne). Coated bearings are very popular in the professional ranks of both circle track and drag racing-friction reduction and resistance to scuffing are two key benefits.
In the NASCAR realm, a few changes have been instituted. Standardizing on a minimum size of 1.850″ for the rod pins and 1.999″ for the mains is now completed. Clevite has numerous choices for these engines, including both H series and V series materials, 2.000″ main sets, main sets allowing teams who ran a 2.017″ main journal last year to use those same blocks with a 2″ crank (.017, .018, .019″-under sets) narrowed rod shells, dowel holes in some sets, extra clearance sets and inventory on the older sizes still used by some teams (1.770″ rods and 2.300″ main, RO7 blocks).
As NASCAR teams continue to narrow the rod bearings in search of less friction and more horsepower, the locating lug on the bearing insert has been placed very close to the edge of the bearing, creating what might be considered a stress point. Clevite is currently in phase-two testing of a new indentless lug insert for those applications. This provides a smooth, uninterrupted surface at the parting line. The company expects to start shifting the production of NASCAR rod shells to that design some time in 2008.
Also for drag racing applications, pro teams, many of whom are using Clevite off-the-shelf race bearings, are starting to take a look at what the NASCAR teams have been doing. Reduced mass, lower friction and coatings are all subjects being talked about at the track. One way to look at it is if your engine bearings look really good at teardown for refreshing the motor, you’re probably leaving something on the table in terms of performance improvements. Now, bear in mind, for the vast majority of racers, there are easier places to get more horsepower than engine bearing modifications but, for those at the very top levels, perhaps not.
BEARING DESIGN AND CONSTRUCTION
Six criteria that must be considered in terms of high-performance bearings include bearing construction and materials; proper housing and shaft geometry; proper bearing geometry; proper surface finishes; sufficient supply of clean oil; and adequate oil viscosity.
In terms of construction, cast bearings provide superior strength as opposed to sintered bearings produced from powdered metal. Sintered bearings lack the continuous copper phase that is needed for strength. A high-performance bearing produced using a steel back, a cast copper-lead primary layer, a nickel dam (laid on top of the copper-lead mass) and a lead/tin/copper overlay provide the best performance for bearings used in high-stress applications.
In short, all high-performance bearings are of the cast copper-lead type.
Main and rod bearing shells feature a slight projection area when installed in the saddle or cap. When the cap is installed and fully tightened to specification, this crush height forces the bearing shells to attempt to expand outward, applying radial pressure concentrically around the bearing housing. This creates the proper geometric shape of the bearing I.D. and locks the bearing in place, preventing bearing movement relative to the bearing housing.
Contrary to popular belief, the small locating tangs/grooves featured adjacent to bearing parting lines are not responsible for locking the bearings in place. These tangs and grooves serve to locate the bearings during assembly only. The installed radial pressure is the primary force responsible for locking the bearings in place. Bearing shells are also slightly larger across the open end as compared to the housing (saddle/cap). When forced into the housing, this slight interference fit also holds the individual bearing shells in place during the assembly process.
Bearing shells are slightly tapered (thinner) approaching and at the parting line.
This creates an eccentric I.D. that promotes a pressurized oil film as the shaft rotates, allowing the oil film to compress and expand as the shaft rotates. This creates a hydroplaning effect, providing an oil film to quickly establish between the journal surface and the bearing. This is the reason that rod bearings are not grooved because you want to create a hydroplaning effect for maximum oil travel between the bearing and journal. The bearing I.D. eccentricity also dictates how an installed bearing I.D. is to be measured. The tightest areas (in terms of minimum clearance between the bearing and journal) will always be at the top and bottom (12 o’clock and 6 o’clock) positions, so bearing I.D. must be measured between these two points.
Performance bearings use a maximum amount of crush. If you over-compress the bearing, you’ll create a thick spot on the bearing wall, possibly minimizing clearance and oil travel. If you have insufficient crush, the bearing will be loose, resulting in bearing surface polishing or fretting (metal transfer).
According to Clevite’s John Havel, if you’re faced with slight geometry problems regarding journal geometry or misalignment, a good choice is Clevite’s V-series bearing, which features a 0.0010″ lead/indium overlay and no nickel dam. This bearing offers a softer surface edge that will be more forgiving if you’re dealing with slight misalignment. The P-series features a 0.0005″ lead/tin/copper overlay. The H-series rod bearings feature a 0.0005″ lead/tin/copper overlay, and the H-series main bearings feature a 0.0010″ lead/tin/copper overlay.
GENERAL CLEARANCE RECOMMENDATIONS
Start with 0.0010″ of clearance per inch of journal diameter. For example: 2.100″ journal diameter X 0.0010 = 0.0021″ clearance. For high-performance applications, add 0.0005″. If, for example, initial clearance is determined to be 0.0021″, add 0.0005″ for a final clearance of 0.0026″. From this point, tighten clearance as your experience dictates in specific applications.
Note: Use of a dial bore gauge is always the recommended method for measuring oil clearance. Instead of measuring journal diameter and then measuring installed bearing diameter, zero the bore gauge at the actual journal diameter. When you measure bearing diameter, you’ll obtain a direct clearance reading without the need to perform math procedures, avoiding potential math mistakes.
Havel emphasizes that if clearance modification is needed, do not increase or decrease clearance by modifying housing size outside of tolerance limits. An undersized housing will over-crush the bearing and an oversized housing will reduce crush and bearing retention.
Currently, Clevite utilizes finite element analysis computer modeling to examine the elastic deflections of all bearing-related areas. EHL, or elasto-hydrodynamic lubrication, allows engineers to more accurately determine the affects of dynamic forces in relation to forces and oil clearances. This understanding of loads, metal deflection and affects on clearance has allowed a more precise view of what the bearings are subjected to, and furthers engineers’ ability to develop bearings that will function properly in high-stress dynamic racing applications.
Tags: BEARING CRUSH, BEARING DESIGN, BEARINGS, BILL MCKNIGHT, CLEVITE, CRYOGENICS, DRAG RACING, NASCAR, OIL CLEARANCE, OIL FILM, TRI ARMOR
