By: Bertie Scott Brown:
Harmonic balancers dampen torsional crankshaft vibrations and they perform their function adequately on common engines.
But if changes are made to the engine, particularly to a racing engine, harmony might be lost. For example, if engine speeds at full throttle are significantly increased, the harmonic balancer could become out of tune. How would you ascertain if threatening torsional vibrations have accompanied the higher revs and how would you measure them? If necessary, how would you eradicate them? This problem is particularly acute on historic and vintage race engines where engine speeds have inevitably increased over time, yet often no provision was made for the harmonic balancer on the original engine.
Earlier this year Virkler & Bartlett, (V&B) the Virginia race engine shop and hard-core problem solvers were invited to analyze potential crankshaft torsional vibrations in a vintage Maserati racing engine. Their calculations anticipated significant fourth order torsional peaks at 7,600rpm and smaller, fifth order peaks at 6,200rpm.
What are fourth and fifth order torsional peaks?
Camshafts and crankshafts vibrate torsionally (in twist) in the running engine. Camshafts are affected by the forces related to the opening and closing of the valves; crankshafts by the combustion events. As a result twisting deflection occurs in both shafts. Two of the many terms used to define torsional vibration are Frequency and Order. Frequency refers to how many things occur in a period of time; Order by the number of events per crankshaft revolution.
If, for example, a rotating shaft is disturbed by two of these vibrations or resonances each revolution, they are defined as second order and they always happen 180 degrees apart. Fourth order torsional vibrations indicate four resonances occurring 90 degrees apart.
Over the years, engine revs had increased on the Maserati race engine from its original 7,000rpm red line to 8000rpm. V&B predicted the torsional vibration by first modeling the rotating assembly in Solidworks, the 3D Computer Aided Design (CAD) software. Analysis of the CAD model predicted fourth order torsional resonances that caused the crank to oscillate or deflect at around 0.5 degrees while rotating at 7,600rpm.
Original Equipment Manufacturers find that oscillatory crankshaft movement should be less than 0.2 degrees or the crankshaft is likely to break due to fatigue failure. V&B validated the 3D CAD predictions by coupling specialist equipment to their dynamometer. They concluded the resulting fourth order torsional vibrations would be particularly worrying on long straights where the engine would slowly climb past the troubling 7,600rpm engine speed in 4th and 5th gears.
“We could not use a conventional damper on the crankshaft nose,” said V&B’s Robert Bartlett, “because the crank doesn’t protrude from the front of the engine, so the damper had to be incorporated with the flywheel-clutch assembly.” Damping can be affected with elastomer, fluid or pendulum dampers. V&B chose a pendulum damper because it is heat resistant and could be tuned to eliminate the 7,600rpm fourth order peak.
Of all the descriptions of pendulum dampers, Kevin Cameron’s is one of the most poignant. In 2016, he commented how impressed he was with the pendulous crankshaft dampers in radial piston aircraft engines of the early 1930s. “Without them, the incoming metal props were failing in fatigue. As dampers dealt with, first the cylinder firing frequency and then the secondary frequency, things smoothed out and 18 inches of prop tip pretty much stopped coming through the fuselages at 800-fps.”
V&B’s pendulum damper for the 1950s Maserati vintage race engine features 12 tubular chambers. Each chamber contains a loose roller. “Imagine the flywheel on its side,” suggests Bartlett, “then touch the roller in its chamber and it will oscillate as a rolling pendulum within a certain period that is a function of roller and chamber diameter. When the engine is running, centripetal acceleration places all the rollers at the outside of the chambers. Now imagine the crankshaft at a resonant point such as 7,600rpm. The period of the pendulum rollers is chosen so they begin to oscillate in time with the 4th order resonance. As the rollers climb the walls of their chambers, they create a strong force that fights or damps the 4th order crankshaft resonance, to about 0.1 degree.”
Tuning the damper
To tune and verify the damper V&B used a Hall effect sensor. This device reads the teeth on a spur gear and feeds its information into advanced equipment that can perform a Fourier analysis on any torsional vibration. “We could have reduced the 5th order 6,200rpm peak by using appropriate rollers in several pairs of chambers, but that would leave more activity at 7,600rpm. It’s a smaller peak and the engine should pass through 6,200rpm quickly, so we can live with it.”
V&B re-create unobtainable parts from ignition systems to wheels and everything mechanical in between. They not only manufacture the original part but also, if required, improve its design and performance. Bob Bartlett claims, “We accomplish tasks other companies are reluctant to attempt.”
For more information contact:
Virkler & Bartlett LLC
1975 Slatesville Rd
Chatham, VA 24531
Telephone (434) 432-4409
Memories from our youth
In the later part of the 1950s, there was much anticipation about the arrival of a new 1600cc twin cam engine design for the desirable MGA sports car. It would replace BMC’s 1500cc B-series OHV (overhead valve or pushrod operated) unit, but the new engine was afflicted by a damaging PR series of problems, the worst of which was melted piston crowns. Company engineers eventually discovered the engine’s 1-3/4in SU type H6 carburetors suffered lean conditions. The pistons were vulnerable at two specific engine resonant frequencies that occurred at 2,500rpm and 5,500rpm. A solution came when they substituted Weber 45 DCOE carburetors, their flexible O-ring joints resolving the dilemma. The vibrations had caused fuel starvation. Frothing fuel leaned the mixture and melted the piston crowns. The fault took a long time to correct, and in the meantime the popular sports car’s image became so notorious it was replaced by the ubiquitous MGB.
Later it was reported that the F1 race engine producer Coventry Climax had identified the cause of the problem but either their message was not transmitted well or ignored.