replacing loco gears

[leon]

I have a 4-6-2 Model Power loco for which I bought a replacement gear set from North West Shortline because the idler gear had stripped. I will replace these gears myself but was wondering if anybody had some advice to share with me.
Thanks;
Leon :?

[doctorwayne]

I'm not familiar with that loco, Leon, but the idler gear is usually the easiest one to replace. If the old gear was stripped of some or all of its teeth, it would be a good idea to go over all of the other gears (I'm guessing there's only the worm and an axle gear) to ensure that there are no remnants of those missing teeth remaining in places where they'll do more damage. This includes areas within the teeth of those gears, but also anywhere else where they might interfere with operation - bearings, electrical pick-up, or lead and trailing trucks.
In most cases, the worm and axle gear are formed from hard material (steel or brass), with the idler of a softer material - brass if the worm and axle gear are steel, or plastic if the other gears are brass (if there's only one gear between the worm and axle, it can be called either an idler gear or a worm gear). The theory behind using dissimilar materials in this manner is that the softer (and easier to replace) gear will be the one which fails if there's a problem.
One of the most common causes of gear damage is a mis-formed worm, especially at the ends of the tooth. You may want to ensure that the centre of the worm's thread is the portion which engages with the idler - this may require the relocation of the worm on its shaft.
Also watch closely to ensure that the tooth of the worm does not bottom-out in those of the idler and also that it's not merely skimming the top of the idler's teeth. This can be adjusted through shims placed strategically under the motor.
One you have everything properly installed, apply some plastic-compatible lubricant, finish re-assembly, and give 'er a test run.

Wayne

[nachoman]

Is this N scale?

[leon]

As always in the past few years, Doctor Wayne, you give good advice for one not to be afraid to follow. What is concerning me is the gear set. It consists of a worm, ideler and an axel gear. If the set has the same number of teeth I would change only the idler gear. But if the teeth are different what problems would I possibly be up against when replacing all three gears?

And in reply to Nachoman: this locomotive is HO guage.

Leon

[doctorwayne]

Well, replacing all three gears wouldn't necessarily entail any problems, but it would, of course, be a little more work.
Not being familiar with that loco, I'll speak in generalities on what you might encounter.
Usually, changing the axle gear is the most involved procedure. You'll need to first disconnect the siderods from that wheelset. If the fasteners holding them in place are slotted to accept a screwdriver, or hex-headed for a wrench, then the unseen part of the fasteners is threaded and needs to be removed accordingly. A plain round head on the fastener usually indicates that it's a press-fit pin, and can usually be grasped with pliers, then pulled out with a twisting motion.
Once the driver set is isolated from the others, remove the screws holding the bottom cover plate in place. If you can support the loco's frame upside down, you should be able to lift the geared driver out without disturbing anything else, but be aware of any current collecting wipers that may bear against the backs of the wheels.
Once you have the driver set removed, the first job is to mark the position of the wheels on the axle ends. While it's preferable to remove the non-insulated driver to gain access to the gear, it's not always the one which comes off most easily - this is somewhat dependent on your chosen wheel removal method. Use a sharp, pointed instrument (X-Acto blade, machinist's scribe, or draughting compass point) to scribe two non-parallel lines across the axle ends and wheel hubs. These alignment marks will assist your quartering efforts when you re-assemble the re-geared axle and the removed-driver.
The best way to remove a driver is with a press such as the ones offered by NorthWest Short Line, but you can get acceptable results using a vise and a piece of heavy sheet metal (an Athearn-type car weight should work) into which you've cut a slot slightly wider than the diameter of the axle. With the jaws of the vise open wide enough to accommodate the axle, place the slotted plate atop the jaws, then slip the wheelset's axle into the slot, with the driver to be removed on top, resting on the slotted plate. The jaws of the vise should be as close together as possible without touching any part of the suspended wheelset or its gear. Place the tip of a centre punch as close as possible to the centre of the exposed axle end, then lightly tap the punch with a hammer. This should dislodged the axle from the wheel, either partially or completely, and you may be able to then remove the wheel by hand.
With that wheel out of the way, re-position the axle so that the gear is resting atop the steel plate, then, with a small block of hardwood or similar material atop the axle's end (it should be solid, but of a material softer than the steel axle) strike the block lightly with the hammer. You may need to repeat several times until the gear can be removed by hand.
The most crucial part of this operation is installing the new gear and re-installing the removed-driver, as both have to go onto the axle perfectly perpendicular to it. Again, the NWSL tool is a good choice for this job, but you can also use your vise as a press. It will put the gear only partway onto the axle though, then you'll need suitable spacers to continue applying pressure to the gear while avoiding contact between the jaw of the vise and the end of the axle. The gear should end up centred on the axle, and you can then use the vise to press the remaining wheel into place. Try to start it with the alignment marks corresponding, and then twist the installed wheel on the axle to re-align it perfectly.

The idler gear replacement procedure should be fairly obvious based on its method of assembly.

To replace the worm, whether it's mounted on the motor shaft or on a separate drive shaft, I've had good success removing the old worm by placing the free end of the shaft vertically on a hard surface, then placing a pair of small pliers, with the jaws not fully-closed, resting atop the upper end of the worm - the jaws shouldn't touch the shaft. Holding the handles of the pliers near their ends, use a small hammer to tap the pliers downward, taking care to not strike the motor if it's part of the assembly. You should be able to inch the worm off its shaft in this manner. The new worm can be pressed onto the shaft using your vise, although if the worm needs to be mounted on the motor shaft, do not, under any circumstances, apply pressure to any part of the motor except the shaft, and in that case, the opposite end of the shaft must be fully supported.

Much of this procedure is easier to do than it is to describe, so you needn't be overawed by the prospect. It may be an even easier task than I've outlined, so if you can post a photo of the boiler-less mechanism, something simpler may become apparent.

Wayne