Loco "wobbles"

"mindesign" wrote

Could be any number of things, but a non-concentric wheel is a possibility as is binding valve gear or any other potential faults including wheels which are not properly quartered.

John.

Using beer as smoke fluid?? :-) Suggest you get a NRMA back to back gauge and check proper alignment of the wheels, along with the suggestions John gave. I found this cures most problems Rob

Many thanks folks .... will look into the back to back gauge this weekend.

John, I have heard mention now several times of this thing called "quartered wheels" Could you please explain the nature of it and what it means when wheels go out of quarter?

thanks

Steve

A steam locomotive needs to have its cranks positioned so that as one side is going through TDC or BDC the other side is half way through its stroke and is in a position to ensure that the other side doesn't 'lock up'. For two or four cylinder locos this means setting the cranks at 90 deg to each other. On three cylinder locos cranks are 120deg apart. There is one exception -- the LMS Compounds had the outside cranks at 90 deg and the inside one at 135deg to the outside ones.

I doubt very much that Bachmann could possibly produce a loco with the 'quartering' incorrect on all axles as they will be using automatic assembly equipment. If you were building a kit or re-wheeling an RTR loco you would need to do this 'quartering' yourself by eye and trial and error. For some reason this terrifies some folk, but in practice and with some practice, and an orderly way of doing it, you should get to a point where all the wheels rotate smoothly.

I think your wobble maybe an eccentric wheel. Both Bachmann and Hornby seem to produce odd locos like this. Remember these models are being assembled by little Chinese ladies on piecework so they are not going to be the most thorough going at inspection. In fact the price they are selling these products at suggests that the 'inspector' is the final customer. If you can't get your 4MT to run sweetly take it back to the shop and ask for a replacement. You'll have done your job as final inspector and are entitled to the reward.

Alistair Wright '5522' Models

Steve,

In the case of steam locomotives with outside coupling rods, the crankpins on the driving wheels on an axle are set at (normally) 90 degrees to each other - or quarter of a circle. This is to allow the best transfer of torque between the driving axles on the loco. The main criterion is that the angle between crankpins on each axle should be exactly the same. If a wheel slips on an axle and the angle between crankpins alters compared to the other axles on the frame, then the axles are said to be 'out of quarter'.

When axles go out of quarter, in the worst case, the frame binds up completely. If the axles are slightly out of quarter, you get tight spots on each revolution of the axles. This will give uneven running. But if the axles have an amount of play in their bearings, then the axles will also try to rock laterally to avoid the binding action of the out of quarter driving wheels, and the loco can then exhibit a side to side waddling action.

Jim.

Thanks again all .....

Jim, I think you have correctly answered what is wrong with my loco ... I will experiment over the weekend after I get my "rev-up" at the Train Show here in Box Hill, Melbourne, Australia

Cheers all and thanks again

Steve

Phil: A point which seems to have been overlooked in some ways is that there will be inherant wobble if the loco has connecting rods to (dummy) pistons because thesa will impose an unavoidable load at 2 out of 4 'quarters' of a wheel's revolution.

In the real thing, this also would cause a twisting effect on the body of the loco v the wheelset - as the considerable forcefrom the piston is transferrred to (one of) the wheels sets at a time. Hence why 3 or 4 cylinder locos (or engines for that matter) are smoother than 2.

In the model case, the connecting rods and rpresentation of the valve gear are an additional load on the rotation at certain stages.

This is of course made worse when the quartering is wrong, or the additional details are out of alignment.

A simple O-4-0,, or 0-6-0 should not suffer from this wobble, having only connecting rods - UNLESS it has wheel quartering or alignment errors (in which case, as previously commented on, it might even sieze.

A simple way to check, where the motor can be removed (eg old Hornby chassis, but not BACHMANN) is to remove the motor, and to freewheel the chassis along the track.

As also mentioned, Bachmann use split axles which may fail (I bought a 2nd hand 2MT which had part of the axle 'case' split - care being needed when repairing to ensure correct alignment.

"Alistair Wright" wrote

Logic says you're right, but it does seem to happen. It may of course be that in isoltaed cases the bush at the centre of the wheels doesn't grip the wheel and axle tightly enough and the thing moves *out of quarter* at the first attempt at running.

It only takes one wheel to be out of quarter for the thing to run like a dog.

John.

A simple 0-4-0 will have a natural wobble. Likewise a short wheelbased

Because of the interaction between the coned treads and the rail, a pair of wheels actually oscillates along the track. A 4-wheeled bogie or short engine will tend to hunt because of this oscillation - ie zig-zag along the track.

The out of balance forces due to model valve gear is minimal (no piston mass on most electric powered H0 models). I have not noticed any difference between outside cylinder models and inside cylinder model as far as wobble goes.

Binding can also result from excessive axle side play in models.