Trial running a chassis

[burgundy]

I have a chassis at the point where the wheels are on (and have been tested without the motor), the motor and gearbox is in (and has been run without being attached to the wheels). I now need to run it all together to see if there are any remaining lumps and bumps to sort out. I therefore need to attach the coupling rods temporarily so that they can be removed again if need be. But equally they need to be sufficiently secure so that the crankpin nuts (Alan Gibson) do not unscrew themselves and disappear into the carpet. How do others manage this stage please?

Grateful for any suggestions.

Thanks

Eric

[buffalo]

I use a short piece of suitable diameter wire insulation screwed on to the ends of the crank pins. I believe this is a fairly widely used method.

 

Nick

[bertiedog]

Just doing them up tight should suffice, but as a temporary measure a spot of thread retainer grade loctite could be used, or a lacquer varnish like nail varnish, which will break up as it is undone afterwards. There may be enough thread to add a second locknut to ensure security.

 

Make sure there are no binds as the rods are put in place, and turn over to find out any binds, and remove high spots etc, bend the pins, or open holes very slightly to get it to turn over, then tighten up the gear etc, if the method allows and test very gently under power.

 

If the chassis requires the gear to be attached to the axle permanently, you must check the rolling chassis first and then fit the gearbox, and hope the lot goes together the same way.

 

This is why the derided grub screw attached gears actually have an real advantage, release them and the chassis can be roll tested.

 

If all has been done as the assembly went ahead, then all should work first time. If there are still binds then re-do as required, but if very slight then run in on rollers till it is smooth in both directions, then clean the gears and put fresh lube on and it is finished for the chassis work. The nuts may then be fixed with the Loctite thread retainer again.

Stephen.

[Kenton]

Tool for the job

 

[Ed] ha, beaten to it while looking for the image.

[Tim V]

One of the older style propelling pencils makes a good tool for putting on the nuts.

[Mallard60022]

I use a short piece of suitable diameter wire insulation screwed on to the ends of the crank pins. I believe this is a fairly widely used method.

Nick

 

Concur - this is the best temporary method. If Mr Wright does it etc.

P

[Jol Wilkinson]

Using a temporary "soft" fixing such as a plastic tube (wire insulation) may be okay with Romford/Markits crankpins where the pin runs directly in the coupling rod, but it doesn't work well with AG or other systems. It appears to have been devised for the Romford/Markits system where the coupling rod retaining bush is a press/solder fit, and so not easily removed.

 

The separate crankpin bush will have some clearance on the threaded pin and also with the rod. Leaving the bush loose on the pin can give an inaccurate setting, which when tightened up could cause a bind. Fix the bush onto the pin (Loctite or paint/varnish) first. The adhesive should also centre the bush through surface tension. When set fix the rods in place with proper nut for a trial. It gives proper running conditions for testing and the nuts are easily removed for stripping down/adjustment of the rod holes. They can be re-fitted with a "temporary" thread locking adhesive at the final build.

[buffalo]

That's a very good point about the loose fitting of the bush on the pin (screw) with Gibson pins. It's probably an indication that, with your far greater experience, you work to much finer tolerances than I do However, I would add that I've seen more binding problems resulting from the lack of concentricity between the inner bore and outer surface of Gibson bushes than from the slop in the bore. In my very limited experience I've found it necessary to select Gibson bushes for concentricity and discard a few that are beyond even my tolerances.

 

Nick

[burgundy]

Thanks to all who have responded to this query.

I am trying out the nail varnish suggestion as it was readily available - I did get a slightly odd look from my wife, but she is sufficiently used to questions like this that she accepted my explanation......... (the same way that I accepted her explanation that she needed gin for her watercolour painting).

I tend to associate Loctite with a pretty permanent solution, especially when the Gibson axle "nuts" are actually round and offer no flats to provide any purchase in the event of the need to remove them again.

My main worry at this point is that, although the wheels rotate smoothly, there is still a bit of friction. I think this is a result of the washers that I have put behind each of the wheels. The frames are 14mm front to front, so that with a back to back of 16.5mm on the wheels, there was too much rattle space. The loco is a Billinton radial tank and drive is on the rear coupled axle (Mashima 1630 plus High Level 108:1). I did consider leaving the centre axle unwashered but that seemed to generate lateral movement that reduced the smoothness. That may, however, have made it rather rigid for even 36" radius curves/turnouts, hence the reluctance to Loctite it all together once and for all.

I will see how it gets on with some test running.

Again, many thanks

Best wishes

Eric

[burgundy]

The nail varnish on the crank pin threads has worked very well and, so far, none of the nuts have disappeared into the carpet.

Unfortunately, my suspicion that I would probably need to disassemble the connecting rods may also have been justified. The initial runs, with the chassis sitting on a Bachrus saddle and power applied directly to the motor terminals, seemed to run very smoothly. Therefore this afternoon I added the pickups to the wheels and wires to the motor so that it could run collecting current through the pick ups and therefore testing the complete chassis. Backwards, it continues to run smoothly, but, forwards, a tight spot has appeared, which caused the motor to start to rotate around the axle and straightened out the two stage gearbox. There is quite a distinct jerk, which seems to correspond with each rotation of the wheels. No damage done, but I shall now need to work through the chassis to find out where it is binding. I checked to make sure that there were no pips on the back of the wheels which might be catching on the pickups. My next guess is that the movement in the compensation is creating a tight spot on one of the crankpins. I shall start at it again tomorrow, having slept on the problem.

The other issue that I now need to think about is how to pin down the motor. I had assumed that the pickup wires would have reduced the scope for movement, given relatively little torque coming back through the gears. However, this particular incident suggests that something more positive may be required. My first thought is to use bathroom sealer (Dow Corning or similar). Does anyone have any other favourite solutions please?

Many thanks

Eric

[Kenton]

The other issue that I now need to think about is how to pin down the motor.

Tie it down - a length of single core pvc sheathed wire twisted to hold the motor on its mounting bracket.

[bertiedog]

All motors and gears develop torque reaction, so the rear of the motor must have a firm grasp on the chassis at some handy point, but still be flexible should springing be in use, so a thin wire is used, handrail or the like, too hold it, and don't forget the reaction is proportional to load, so will be greater in use with a train, and greater with the drag of pickups as well.

Stephen

[bertiedog]

The motor floats on the axle, but rotates against the forces of torque from the worm gears, so it is attached at X only , by the wire, nothing else except the axle, this works with springing as well as rigid, just make sure the pivot is secured to frame as far away as possible with the springing in use, other wise it may bind.

Stephen.

[Guest Max Stafford]

Would you solder this restraining wire to the motor casing then?. I can't think of how else it could be done unless I'm missing something rather important.

 

Dave.

[bertiedog]

In practice there will be a handy attachment point somewhere, the motor case, soldered to the the gearbox frame, epoxied on to the case, or a brass strip around the motor, with the wire trapped in the bolted join, or soldered to the strap, any way will work., it all amounts to the same solution.

 

The wire can go in the opposite direction to a point in front of the motor, (or above or below in extreme cases), same overall effect of a torque restraint....but try to get it as long as possible in the space to ease the radial path of the movement, (if sprung).

 

Stephen.

[John_Hughes]

The nail varnish on the crank pin threads has worked very well and, so far, none of the nuts have disappeared into the carpet.

Unfortunately, my suspicion that I would probably need to disassemble the connecting rods may also have been justified. The initial runs, with the chassis sitting on a Bachrus saddle and power applied directly to the motor terminals, seemed to run very smoothly. Therefore this afternoon I added the pickups to the wheels and wires to the motor so that it could run collecting current through the pick ups and therefore testing the complete chassis. Backwards, it continues to run smoothly, but, forwards, a tight spot has appeared, which caused the motor to start to rotate around the axle and straightened out the two stage gearbox. There is quite a distinct jerk, which seems to correspond with each rotation of the wheels. No damage done, but I shall now need to work through the chassis to find out where it is binding. I checked to make sure that there were no pips on the back of the wheels which might be catching on the pickups. My next guess is that the movement in the compensation is creating a tight spot on one of the crankpins. I shall start at it again tomorrow, having slept on the problem.

The other issue that I now need to think about is how to pin down the motor. I had assumed that the pickup wires would have reduced the scope for movement, given relatively little torque coming back through the gears. However, this particular incident suggests that something more positive may be required. My first thought is to use bathroom sealer (Dow Corning or similar). Does anyone have any other favourite solutions please?

Many thanks

Eric

 

Eric, I wish I had a pound for every time this has happened to me!

 

I suspect that you probably have the tight spot in both directions, but it only makes the motor jump up in one direction simply because it's restrained in the other direction (by the motor mount, presumably).

 

The problem could be in the gear-train; I've had this happen to me a couple of times, and it's a real pig to track down.

 

If just one of the teeth in the gear-train is slightly worn or defective the resultant hitch perfectly mimics a bind in the coupling rods. Check the gears very carefully to see if anything is even slightly wrong. This can show up at any time, but adding a load to the drive-train - by adding the pickups - may have brought it to light. Check this first!

 

If that's all OK then the problem must be somehow related to the pickups which you've just added. Assuming that they're not actually catching on the back of the wheels - which you have checked so that ought to be OK - then I suspect that you do really have a slight bind in the setup which didn't show up when running earlier because the wheels were free to take up a position that minimised or eliminated it; their lateral(?) movement has now been impeded by the pickups, so the bind has appeared. So it's back to taking out the motor and working carefully to see just where the bind is. Take your time, and don't throw it all at the cat!

 

I've used the bathroom sealer dodge on occasion, and it works fine. Heck, I've even used Blutack a couple of times! It isn't exactly a good engineering solution but seems to do the job perfectly!

[Jol Wilkinson]

I usually add an extra frame spacer against which the motor casing can rest and "fix" the motor to this with a double sided foam sticky pad. This gives a bit of flexibility, doesn't transmit so much noise and can be removed (with a little white spirit or similar to dissolve the adhesive) if ever required.

[burgundy]

Thanks to all for your suggestions.

The gearbox is attached to the unsprung axle which makes the challenge slightly less complicated. On the other hand, the two stage gearbox has a joint in the middle, so that the final drive can be set at any point on an arc. Therefore the front of the motor is not actually secured - other than to the top stage of the gearbox.

So, based on the various suggestions, I have put a simple wire harness around the body of the motor and secured it to the spacer immediately below the motor, as Kenton suggested. On its own, the wire harness was not sufficient, as the gearbox was still able to unfold itself and cause the motor to move horizontally and twist, so I have put a pad of sealant under the motor (not real Dow Corning, but a D-I-Y store own brand which I am afraid may not be the same thing at all). I have also put a tiny bit of superglue onto the joint in the gearbox, to stop that unfolding itself when the torque reverses, although this may be ineffective if any lubricant has already penetrated to that area.

I have had a very close look at those gears that are visible, but it is a HighLevel 2 stage box which is folded back on itself, so the lowest level of gears are not easy to see. The smear of lubricating grease does not help either. However, I think I have convinced myself that the point at which the binding occurs is in sync with the rotation of the driving wheel - not in some fraction of that rotation, which might point to one of the stages in the gearing. Therefore the prime suspect at the moment is one of the crankpins, which may be a little bit out of true.

My plan is now to leave the sealant to harden off overnight and have a go at the errant crankpin tomorrow. In the meantime, I can put some paint on the interior of the cab.

Again, thanks for your suggestions

Best wishes

Eric

[John_Hughes]

Just a question about how you positively know the coupling-rod centres are dead on.

 

I find the Perseverance axle jigs to be very useful indeed as a check for this, though they're a bit long in the tooth these days; they still surface on eBay from time to time though...

 

How much slop can you have before problems arise? I'd say about as much as the prototype, that is of the order of a couple of thou or possibly just a little more. So go easy with the reamers!