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Motorcycle Engine Vibration—Friend Or Foe? What, exactly, is carrying the load?

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By Kevin Cameron

Unless we are talking about a possible “manliness coefficient” for the riding of certain bikes (classic British twins, unashamed Sportsters), vibration is usually seen as getting between us and a good time. Kenny Roberts’ almost-successful three-cylinder two-stroke 500cc Grand Prix racer started life with vibes that messed up its carburetion and broke parts. Vibration can put our hands to sleep and, in extreme cases, leads to double vision (as in the “pogo” shaking sometimes reported by US astronauts during launch or from the “tire shake” that can occur in drag racing).

But in certain special circumstances, vibration has actually been useful. Today the readings of lab instruments appear on video screens, but in my student days every meter had an indicating needle attached to a tiny shaft pivoted in jewel bearings like those in Kevin Schwantz’s treasured mechanical watches. We were taught always to tap on the glass faces of such meters before taking a reading to shake the pivots enough to overcome any friction present. Quite often the needle would shift position significantly. To this day, I feel the impulse to tap.

The cockpits of today’s aircraft have large LCD screens on which are displayed the “virtual instruments” necessary for the current flight situation, but in times past a flight engineer sat before a panel which, on a four-engine aircraft, carried 32 or more instruments. Because of the vibration of large aircraft piston engines, no gauge tapping was necessary. Today’s pilots refer to such instruments as “steam gauges.”

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When the gas-turbine era arrived, aircraft engine vibration almost disappeared because there were no longer great big pistons and valves whanging back and forth. To save flight engineers from constantly tapping critical gauges (monitor that turbine inlet temp!), instrument panels had to be equipped with artificial vibration in the form of buzzers.

Harley-Davidson’s big twins have drum-and-forks-shifted multi-speed gearboxes, and shift quality remained good until The Motor Company decided to add engine balance shafts. Suddenly their gearboxes were half-shifting. Why, after decades of reliable shifting, would this problem suddenly appear?

The engineers soon realized that vibration had helped to overcome friction between the rotary shift drum and its bearings, and between drum and shift forks. With substantial engine vibration, the shifting mechanism had “rattled obediently” into the next detent, completing the shift. But without vibration’s help, the drum might stop along the way or even be kicked back into the previous gear.

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In prototype testing, they gave the shift drum low-friction rolling-element bearings and improved certain surface finishes to reduce friction in the shift mechanism. They were eventually rewarded with a return to good shift quality. I had been down this same route myself in trying to improve shifting in race engines of the 1970s.

Near the end of the 19th century, ocean-going ships were propelled by enormous triple-expansion steam engines. These were units of three cylinders—a small high-pressure cylinder, exhausting into a larger intermediate-pressure cylinder, which in turn sent its exhaust steam for further work-extracting expansion in a great big low-pressure cylinder. All that metal in motion led to constant slight cyclic flexure of the ship’s hull and the tremendous shafts that transmitted power to the propellers.

The thrust bearings that transmitted force from the prop to drive the ship forward consisted of a stack of multiple collars fixed to the shaft running in a thrust box containing corresponding stationary plates lubricated by pumped oil.

All was well until Charles Parsons’ 1893 invention of the steam turbine, which hardly vibrated at all. Suddenly conventional thrust boxes, which had worked well for decades, overheated and seized.

The emerging scientific understanding of lubrication revealed why. Something must cause an oil wedge to form between the moving parts. Oil is drawn between the surfaces at the wider, low-pressure end of the wedge, and is pulled into the loaded zone of the bearing by its own viscosity, generating in this way pressures of thousands of pounds per square inch. Oil pumps merely send the oil to where it is needed, but the pressure that supports the load is generated solely by the motion of the parts.

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Seen this way, the problem was clear: With the yanking and thumping of piston steam power, the shaft and thrust collars inside the thrust box were constantly forced into just enough misalignment to generate oil wedges capable of carrying the load. How much load? Eight thousand hp at 15 mph is a thrust of 200,000 pounds.

Innovators on both sides of the Atlantic came up with the same solution: a single thrust collar on the shaft, pressing against a circular array of six or eight segment-shaped tilting pads in a thrust block attached to the ship’s hull. As the shaft turned, the thrust pads tilted just enough to form each its own oil wedge, capable of supporting any desired load. Ships powered by steam turbines were driven by such multipad thrust blocks through two world wars and on into the eventual replacement of steam by today’s more efficient two-stroke marine diesel engines.

Pistons sliding in cylinders tilt ever so slightly to create the oil wedges that support them. Crankshaft journals do the same by being forced just enough off-center by the applied load to form ever-so-slightly crescent-shaped oil wedges. Typical main-bearing clearance in a motorcycle engine is 0.0012 inch, and under load the minimum oil-film thickness is squeezed to as little as 0.00005. That produces a very slightly tapered oil clearance, just enough to work like a charm.

Source: Cycle World


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