4-4-0 Compensation Issue

[Fairweather]

Good afternoon all,

 

I've been building a David Andrews County 4-4-0, and have hit a snag. It is compensated with the rear driver fixed, and the front driver connected by beams to the rear ones, allowing the axle to "float". However, while it runs fine initially, after a minute or so a wobble starts in the front left driving wheel, which escalates till that wheel lifts about 0.5-1mm off the rail each revolution, raising as the coupling rod passes 9 o'clock, settling back on the rail with the rod in the 4 o'clock (anti-clockwise). I've checked to see if flash on the wheels could be an issue, but that doesn't seem to be it. Any trouble shooting suggestions would be much appreciated, I've hit a bit of a brick wall!

[asmay2002]

What you've decribed isn't compensation.  If the rear driver is fixed on a 4-4-0 the front drivers would be linked to the front bogie via a compensting beam for a 3-point suspension.  Alternatively the bogie pivot could be one point and the two driving axles would both float via a beam on either side.

[Fairweather]

Ah ok. Well, the way the kit has gone together is as described. Not sure what a more appropriate term would be? Either way, the bogie is sprung, the lead axle floating, the rear one fixed.

 

In addition, pressing down on the beam/bearing to stop the float increases the load on the motor. Possibly the bearing is slightly out of position?

[Grovenor]

after a minute or so a wobble starts in the front left driving wheel, which escalates till that wheel lifts about 0.5-1mm off the rail each revolution, raising as the coupling rod passes 9 o'clock, settling back on the rail with the rod in the 4 o'clock (anti-clockwise).

 

That suggests that the rods are binding, either a slight error in quartering or in the length of the rods. The wheel lifts as that axle is not carrying any load so there is nothing to hold it down.

When you have eliminated the binding it would be a good idea to add wire springs to press that axle down onto the track so that it shares some of the load.

Regards

[Sandy Harper]

The whole point of compensation is that what ever one wheel, or axle, does there will be opposite movement of the corresponding wheel or axle. You can't have compensation beams working on fixed axle and a axle with up and down movement. That isn't compensation. Somewhere along the line I think you may have misinterpreted the build instructions and introduced an element of both fixed chassis and compensation.

 

I would tend to agree with Grovenor. Remove the 'beams' and add wire springs to the front bearings to give independent springing to the front axle. There should only be downward movement on the front axle bearings and at the same time check that the axles are 'square' in all directions. Once you have got that sorted only then, adjust the crank pin holes in the rods if needed.

 

Hope this helps

Regards

Sandy

[t-b-g]

You may not even need to remove the beams. At least they will hold the axle centres at the same spacing as the coupling rod spacers.

 

It might be worth trying just a simple leaf/wire spring bearing down on the leading driving wheels, one spring each side, as near the frames as possible.

 

The loco does have a sort of three point suspension, with the three points being the rear driving wheels and the bogie pivot, so holding the leading drivers down may be all you need to sort the problem out.

 

Tony 

[Grovenor]

 

 

I would tend to agree with Grovenor. Remove the 'beams' and add wire springs to the front bearings to give independent springing to the front axle.

i did not suggest removing the beams, as described they are needed to hold the axle spacing. Just sort out the binding rods and add the springs as Tony says.

Regards

[Sandy Harper]

i did not suggest removing the beams, as described they are needed to hold the axle spacing. Just sort out the binding rods and add the springs as Tony says.

Regards

Sorry Keith

Bad English there. What I should have said was that I agree with your conclusions 'and also suggest removing the beams'.

 

I have never understood why these beams were put in the kit in the first place. If you build the chassis correctly there is no need for them. Suggesting that you use them to maintain axle spacing supposes that you have made a right hash of fitting the front bearings! Correctly fitted bearings/horn blocks should rise and fall vertically. Using a beam, pivoted from the rear axle, will cause the front axle bearings to rise and fall in an arc. I would suggest that the reason for the binding is this, because the when the wheel gets to the extremities of the arc, because of the beam, the rods will still be trying to move vertically. Effectively shortening the distance between the crank pins. The rods, being of fixed length, cannot compensate for this and therefore bind.

 

If you insist on using these beams then the front crankpin holes will need to be reamed out to give a much larger hole than normal to allow the rods to take up the difference. Not an ideal solution.

 

Another reason could be that the front axle hole is well made and will only allow the bearing to move vertically. The pivot beam is then the problem, as it travels along the arc it effectively shortens in relation to the axle spacing, and causes the bearing to ride up the bearing hole causing it to jam at the top.

 

Get rid of the beams!

 

Regards

Sandy

[Izzy]

Your description of the driving wheel rising each revolution suggests that either the quartering is not quite right, that the load at rising point is greater than the load imposed on the axle i.e. the downward force, or a combination of the two. It is a situation often found with sprung/compensated chassis.

 

The answer is to ensure the coupling rod centres match the axle/wheelbase centres, quartering is correct, and sufficient load is applied to the driven axles to overcome the rotational drive forces of the coupling rods.

 

Izzy

[Grovenor]

Sorry Keith

Bad English there. What I should have said was that I agree with your conclusions 'and also suggest removing the beams'.

 

I have never understood why these beams were put in the kit in the first place. If you build the chassis correctly there is no need for them. Suggesting that you use them to maintain axle spacing supposes that you have made a right hash of fitting the front bearings! Correctly fitted bearings/horn blocks should rise and fall vertically. Using a beam, pivoted from the rear axle, will cause the front axle bearings to rise and fall in an arc. I would suggest that the reason for the binding is this, because the when the wheel gets to the extremities of the arc, because of the beam, the rods will still be trying to move vertically. Effectively shortening the distance between the crank pins. The rods, being of fixed length, cannot compensate for this and therefore bind.

 

If you insist on using these beams then the front crankpin holes will need to be reamed out to give a much larger hole than normal to allow the rods to take up the difference. Not an ideal solution.

 

Another reason could be that the front axle hole is well made and will only allow the bearing to move vertically. The pivot beam is then the problem, as it travels along the arc it effectively shortens in relation to the axle spacing, and causes the bearing to ride up the bearing hole causing it to jam at the top.

 

Get rid of the beams!

 

Regards

Sandy

If the bearings are located by the beams and hence move in an arc as you describe this should be better for the rods as the axle centre spacing is fixed so the rods will always be correct, However in this scenario you cannot also have close fitting hornblocks, the hornblocks are an alternative. The effect of hornblocks is that the wheel centre distance increases slightly as the bearing moves up or down in the hornblock and consequently there must be enough play in the connecting rod bearings to allow for that effect. But, as all the compensated and sprung locos around show that is not a problem. However I don't believe that the linking beams are causing the binding, it will be the quartering or the rod lengths, and that needs to be solved before adding springs to put some load on the axle.

Regards

[garethashenden]

Without actually seeing the model, it sounds like there was an option to build it rigid or with twin beam compensation. It sounds to me that the two systems have been mixed accidentally. The beams should pivot in the middle (or at the point that fits the desired COG), rather than at the end. This way the weight is distributed to both axles. As it sits now, the weight is only going to the rear axle.

[DCB]

It sounds like you have made the chassis up with no weight on the leading coupled axle, The cure is probably one or more springs bearing down on the leading axle or a lot of weight bearing directly onto the leading coupled axle, which is exactly what I have been trying to achieve on an M7 0-4-4T.

 

The best system I know for a 4-4-0 is to have a rigid 4 coupled frame with the two front axles of the tender floating so half the weight of the tender is on the back of the loco and the bogie is weighted but not sprung so the weight ends up positioned almost exactly mid way between the coupled axles

 

If sprung chassis with hornblocks work I doubt they work for long as the ones I have seen are simply not robust enough for my habits of operating 20 plus Hornby dublo metal wagons.

[jim.snowdon]

Altogether better would have been either modifying the beams to be centre pivotted, with the weight shared between the two axles, or fitting proper hornguides and springs to both driving axles. Even then, I suspect that something is awry with the axle and coupling rod centres that needs sorting out, and not by opening up the holes in the rods. That may work on a 6-coupled rigid chassis with rigid rods, but doing it on a 4-coupled chassis, rigid or sprung/compensated, is asking for one of the rods to lock up. It is essential  that the rod centres match the axle centres.

 

Since we are working in 7mm scale (David Andrews doesn't do 4mms as far as I am aware), any of the available hornguides and axleboxes are up to it.

 

Jim

[Grovenor]

Since post #4 the advice already given has been repeated several times, it would be nice to get some feedback from Fairweather.

Regards

[Fairweather]

Indeed, many thanks for all of the above. Work has sadly taken me away from the kit, so I won't be able to provide much of an update any time soon. All I had time to try before I left was removing one of the coupling rods, which seemed to solve the problem, though it was always a bit of an intermittent issue. When I get back I intend to go through the suggestions, starting with the possibility of the rods binding, before moving on to additional springing. Again, many thanks to all of you for your help, I'll report back in the hopefully not too distant future on how the problem ebolves

[Fairweather]

Resurrecting this topic to report back. Reaming out the rod improved the situation, but didn't completely solve the problem. Further tests showed that the bolt on the crankpin was tightening with each revolution, eventually impeding the movement of the rods. A bit of glue to hold it in place solved the problem. Many thanks to all, next step, improving the springing.

[jim.snowdon]

Thanks for the update. The fact that opening up the hole improved matters, along with the comment about the crankpin tightening up both point otwards the axle and rod centres having been slightly different. A good clue as to which side is out of kilter is to roll the chassis along until the rods bind, and then feel each rod to see which one is tight. The rod that is out will go tight at the 3 and 9 o'clock positions, ie the dead centre positions, when the rod is in line with the axles.Opening out the holes in the rods is ok up to a point - too much and it is possible to lock the rods or, if the wheels aren't positively located on the axles (more likely in 4mm scale), dragging one wheel out of quarter.

 

Jim