Hunting Prevention of High Speed Intermodals

[Overmod]

Not on a three-piece truck without slew. Look at the way the truck is constructed, particularly how the sideframes sit on the bearing shells.

Remember that conventional 'wisdom' has the idea that tightening up the gauge a slight amount (say, 1/4") will result in closer guiding at high speed. (I remember seeing this discussed in Trains many years ago, on the Pennsylvania, with a comment like "We may be longer [than short lines], but we're not as wide.") This would seem to have the effect of reducing the possible amplitude of flange-to-flange oscillations (and to an extent decreasing the arc of swing and hence incident angle between flange taper and gauge corner area) but I believe it will also have the effect of increasing flange contact on lateral excursion or small-period oscillation -- which would produce effects as advertised if there were fundamental oscillating tendencies in the suspension or trucks.

"Constant-contact sidebearers" implies there was not full contact in the original design. It further suggests the problem was either driven by, or could be operationally solved by, changes in the vehicle roll characteristics. Don, can you provide some full details on this?

[Anonymous]

Three axel trucks typicaly have Hyatt bearings on either the two outside axels or on the middle axel. Hyatt bearings allow lateral motion - end play - with a thrust bearing on the inside face of the bearing's cover plate to limit that motion and take lateral load.

Could it be that if a truck entering a curve or some other type of momentary upset to tracking is normally set hunting on a quickly decreasing wave as it corrects itself.

An axel, lateraly shifting, out of phase with this motion would cause the hunting to increase?

[Overmod]

I don't think you'll see hunting develop as much on curve entry as on curve exit -- my opinion is that symmetrical forced oscillation is much more likely than asymmetrical.

I would think that laterally-shifting axles that move out of phase with induced resonant motion would break hunting rather than increasing it. I'm not altogether sure that an axle with side play, but no skew, would induce the kinds of driving motion that lead to hunting (I'd expect to see some lateral jitter of the axle in the truck frame long before I saw physical truck hunting, particularly on long-wheelbase six-wheel trucks)

Part of this might involve whether or not there is progressive loading of the lateral axle thrust bearing. If the assumption is just that a metal face or race on the axle end comes in contact with rollers, plates, etc., and there's no spring or block to provide cushioning, there might be some kinetic shock from flange contact accelerating the wheel and axle assembly laterally. I would still think, though, that it would be rare to get the wheelsets oscillating at a critical frequency to induce hunting in the truck frame per se...

[Anonymous]

QUOTE: Originally posted by oltmannd

jruppert-

It's more a matter of stable vs unstable and at what speed threshold the instability occurs. For a given system, things may be stable at 5 mph meaning there can never be hunting. However at 70 mph, the system may be unstable. This does not mean there WILL be hunting, but there CAN be hunting, if something excites it. Some of the main variables in a railcar related to hunting are length (truck centers and overall), wheel taper, gauge "tightness", polar moment of intertia and speed. Short, empty cars are notorious as are bulkhead flats for hunting.

An example:

Amtrak tested their roadrailer equipment for weeks at speeds up to 110 mph on the NEC only to have it hunt like crazy in NM at 90 mph on the ATSF. The difference turned out to be the ATSF had slightly tighter gauge. The problem was fixed with constant contact sidebearings.

Maybe I should take the list of variables you have given as well enough of an answer. Maybe explaining this stuff is a subject too lengthy for the forum.

One more question though; hunting trucks at opposite ends of a car? A series of hunting cars in a train? I never thought of it.

[Anonymous]

Hey! your reply surprised me!

You're probably right about the out of phase motion, I was thinking not in terms of energy transfer but momentary changes to geometry.

[oltmannd]

QUOTE: Originally posted by Overmod

Not on a three-piece truck without slew. Look at the way the truck is constructed, particularly how the sideframes sit on the bearing shells.

Remember that conventional 'wisdom' has the idea that tightening up the gauge a slight amount (say, 1/4") will result in closer guiding at high speed. (I remember seeing this discussed in Trains many years ago, on the Pennsylvania, with a comment like "We may be longer [than short lines], but we're not as wide.") This would seem to have the effect of reducing the possible amplitude of flange-to-flange oscillations (and to an extent decreasing the arc of swing and hence incident angle between flange taper and gauge corner area) but I believe it will also have the effect of increasing flange contact on lateral excursion or small-period oscillation -- which would produce effects as advertised if there were fundamental oscillating tendencies in the suspension or trucks.

"Constant-contact sidebearers" implies there was not full contact in the original design. It further suggests the problem was either driven by, or could be operationally solved by, changes in the vehicle roll characteristics. Don, can you provide some full details on this?

Constant contact side bearings provide bolster yaw damping and are generally the cheapest way to "solve" hunting problems. They don't do much for carbody roll. The downside to them is that they tend to keep trucks from tracking straight.