4mm Smooth running

Discussion in 'Techniques' started by DavidB, 11 September 2021.

  1. DavidB

    DavidB Western Thunderer

    It is years since I made a chassis but have just made an 0-4-0. Unfortunately, with the connecting rods on it is a tad tight and although I can get the wheels to go round, they bind at one point.

    Is there a routine to work through in order to ease the connecting rods and get smooth running? I am worried I could file a bit here and another bit there, elongate holes or make them too big and make things worse.

    Pointers welcome!!
  2. simond

    simond Western Thunderer

    I’m used to 7mm, and pre-quartered wheels, but I’ll offer a few suggestions.

    for a 4-coupled chassis to run sweetly, you need

    Axles to be parallel in plan view (they occasionally may not be in front view if one is equalised)
    Coupling rod centres to be same length as wheelbase, both rods identical, of course.
    Quartering to be identical on both axles.
    Crank throws to be identical on all four wheels (ok, they could be different on each side, but…)
    Sufficient running clearance in axleboxes and crankpin journals.
    Clearance between wheel face (and any protruding bits & pieces) and back of rods

    The running clearances will need to be more generous if your loco has suspension a/o equalisation.

    so, an approach; there may be others, whatever, try to work through it methodically.

    Remove gearbox, remove pickups, remove rods, does the loco run sweetly, and straight, down a gently inclined smooth-ish piece of wood?
    If it doesn’t run smoothly, check bearing clearances, if it doesn’t run straight, check the axles are parallel.

    Put rods on, chassis on track, get a very weak elastic band and a bit of wire, pull it along gently, locate the binding. Is there more than one point in each revolution?

    is something catching? Axle, balance weight or hub on back of rod?

    referring to one side, if it’s binding at 12 & 6 o’clock, I’d guess there’s a mismatch of wheelbase and rod centres on the other side. If it’s binding at 3 & 9 o’clock, it’s the side you’re looking at.

    If the binding is at 45 degrees…. Mmmm, must be a combination of things. Swap the rods, just to check it’s the same.

    I’m not conversant with how crank pin bushes are arranged in 4mm, but I only clean the holes in the rods to ensure they’re smooth, if I need more clearance, I would turn down the bush to get it, because if I make a bollox of it / overdo it, a bush is pennies, the rods are not so easy to replace.

    hope this helps
    GrahameH, michl080 and daifly like this.
  3. DavidB

    DavidB Western Thunderer

    Thank you, Simon. Some steps I have already done but for the rest, it is going to be one of those 'good games, played slowly'!
    simond likes this.
  4. eastsidepilot

    eastsidepilot Western Thunderer

    One of the first things I do when building a chassis, before rods etc are fitted, is to pass a reamer through the axle bearings to make sure the bearings are parallel. It's important to also include the gearbox and it's axle bearings if these are plain brass. In 7mm scale with 3/16" ( 4.76mm) axles I use a 4.8mm reamer. In 4mm I'm assuming 1/8" (3.175mm) ? axles so try a 3.2mm reamer.
    Any problems after this can be put down to crank pin, rod dimensions etc. but as Simon mentions in his post the axles must be running parallel.
    If they are not it will throw everything else out.
    The crank pins must also be square to the face of the wheel bosses.

    Last edited: 12 September 2021
    Ressaldar and simond like this.
  5. DavidB

    DavidB Western Thunderer

    Thank you. I have an 1/8" parallel reamer and did as you suggest so I think the snags are to do with the crankpins. I made a jig and used a drill press to drill the wheels for the crankpins. The wheels were put on with a GW press.
    I also followed Simon's suggestion and have taken a little off the bushes rather than opening the hole in the connecting rod. The wheels now turn continuously with only a hint of hesitancy, so I am almost there.
    michl080 and simond like this.
  6. DavidB

    DavidB Western Thunderer

    I have got the chassis to run smoothly backwards but the motor kept jerking on each revolution when in forward gear. After an incredibly amount of taking apart, reassembling and poring over this and that with a magnifying glass (minus the deerstalker and pipe), I think I have found the problem.

    This lies in the gearbox which is a bespoke High Level. The plastic gears have sharp edges to the teeth and as two of the gears (which don't themselves mesh) rub their sides together, I think the rough edges are catching. I have dropped the gearbox out and can get it to jam so am fairly confident this is the snag (pun intended!). I don't want to take the box apart as the axles have been Araldited in and has wheels and bearings in place, so will attempt to smooth the gear edges in situ.

    In future, I will be checking that the edges of the plastic gear wheels are smooth before I put a box together.
    michl080 likes this.
  7. DavidB

    DavidB Western Thunderer

    Sorted. There was also piece of unwanted plastic in one of the teeth and although small (I needed a glass to see it), it was still enough to disrupt the smooth running.

    Lesson?? Check your gear wheels, minutely, before making up a gearbox.