Brass Technical Tips
May 1998 Brass List
"Thoughts on Gears, Gearboxes and Drive Shafts"
(A topic with some teeth in it)
This issue delves into various aspects of gears, gearboxes and drive shafts that you may encounter in the wonderful world of brass locomotives. It is not uncommon for certain models, from certain builders, to develop gear problems over a period of time, namely cracked synthetic gears on metal axles/shafts. This phenomenon can occur even though the model may never have been run. It is the result of a cast synthetic gear shrinking on a metal shaft that does not shrink; something has to give and it is the weaker gear that ends up cracking and introducing a lovely slip into the gear train if it is a driving gear, or a bind if it is a driven gear. We will also provide a few insights on gearboxes and drive shafts which may not be generally known.
1) Cracked Gears:
One may encounter cracked gears in earlier Samhongsa and KMT gearboxes in diesel models from Oriental Ltd. and Alco Models, for instance. Luckily, most of these occur regularly enough that our good friends at NWSL (NorthWest Short Line) have provided a series of quality brass or Delrin replacement gears, that will cover the most commonly encountered situations. Refer to the NWSL Catalog, Section 4.11 for applications.
If you are calling Caboose Hobbies for these replacements, it is very helpful if the following information about the gear can be provided: a) Importer/Builder/Model, b) where gear is used in gearbox, c) Outside Diameter of gear in mm., d) gear width in mm., e) bore size (O.D. of shaft it is mounted on) and f) number of teeth. Also, specify whether it is a spur gear (teeth 90 degrees to gear face) or a worm gear (teeth at a slight angle to face) and direction of tooth angle(i.e. increases left to right or right to left).
NWSL Gearbox Installation:
Read the instructions enclosed in the NWSL gearbox kits carefully, before you start the installation. They are very thorough in their coverage. I would only re-emphasize the need to chamfer the ends of shafts or axles before mounting gears on them. This will make starting the assembly much easier and assists the shaft in entering the gear straight and square.
Motor Shaft/Gearbox Shaft Couplings:
Flexible Tubing: Many original factory installations use a length of flexible rubber tubing to couple these shafts. This coupling method can work satisfactorily as long as the shafts are fairly close in line without much of an angle between them, and the tubing remains pliable. This last aspect deteriorates over time, and the tubing can become an inflexible solid. Check this even if you are not installing a new gearbox and/or motor. Even if the old tubing still appears flexible and grips the shafts, it can develop a 'set' from sitting in an 'at rest' position, which can induce a vibration in the drive line when running.
The classic replacement, over the years, has been Neoprene fuel line tubing. This usually can do an adequate job, but if it came from a roll, it too can have a built in 'arc set', and induce rotational vibration. A better material is the newer silicone rubber tubing, which is much more resistant to taking a set from being stored on a roll.
In any case, cut the tubing to length with square ends and mount it so the ends are square to the shafts. If there is much more than a 1/4" gap between the shaft ends, it is beneficial to install a gap filler in the tubing to bridge this space and support the flex tubing in this area. A piece of brass or plastic rod works well for this; it should be an easy slip fit into the flex tubing. This approach will prevent any tendency of the tubing to 'whip' in this otherwise unsupported area.
NWSL Drive Shafts and U-Joints:
Flexible tubing couplers are inadequate if there is much of an angle between the shafts to be coupled, or the shafts change angles in operation (as on diesel trucks or the front engine unit of an articulated steam locomotive). Here, a drive shaft with U-Joint couplings is vastly superior, if not a virtual necessity.
If you are changing a flex tube coupling on a steam locomotive model to a drive shaft, you need to consider attaching the motor to the gearbox with what is known as a torque arm. In its simplest form, this is a slim rod connecting the motor or motor bracket to the gearbox; its purpose is to prevent the gearbox from rotating around the driver axle when the motor runs and the worm turns. This can bind the drive shaft when the gearbox rotates toward the motor and can allow the drive shaft to fall out when the gearbox rotates away from the motor. Obviously, this was not a problem with the flex tube coupling, because the two shafts are held together and there is no free intermediate piece, as is our drive shaft.
Why does the gearbox want to rotate around the driver axle, you ask? Think of it this way: The geared driver turns because the gearbox is fixed in position and the motion of the worm and any idler gears imparts movement to the axle gear and hence the driver. But this movement is also, at the same time, trying to turn the worm and idler gears, and hence the gearbox, around the driver axle gear (This is the Law of Physics that states the Action/Reaction Principle of Forces). If the gearbox is not fixed to something, it will try to rotate, as that motion takes less force than turning the driver itself, at least until the gearbox encounters a stop on the frame. Hence we have the need for the torque arm. It does not have to be anything massive, just a 1/8" x .040" piece of brass flat stock is adequate.
A word or two here on drive shaft construction.
The preferred method of assembly, using a NWSL kit, is to mount the slotted U-Joint cups on the motor and gearbox shafts, and the U-Joint eared balls on the drive shaft itself. This keeps the heavier cups on the shafts that have the most support, i.e. from the motor and gearbox shaft bearings. As for the U-Joint balls, they are pressed onto the ends of the drive shaft itself, which is an appropriate length and diameter of steel shaft, also from NWSL. I strongly recommend that these be mounted so that the ears are offset 90 degrees from each other. The ears on the U-Joint balls should NOT be parallel to each other. This may fly in the face of other advice; in fact, the pre-made short drive shaft included in most NWSL U-Joint kits do have parallel ears. This is probably not a big deal on such a short drive shaft, although I suspect that this is done that way mostly as a manufacturing/ casting convenience. The 90 degree offset recommendation is based on a long established automotive drive shaft principle. This states that for constant rotational power transfer, the driving U-Joint should be 90 degrees out of phase with the driven U-Joint. This principle becomes increasingly significant as the angle increases between the driving and driven shafts (the motor and gearbox shafts, respectively). Granted, the rotational forces and drive shaft coupling play are nowhere near what you encounter in automotive applications. BUT, if you are striving for the smoothest operation of your drive line, why not take advantage of this easily applied methodology?!?
A final thought:
It is a good idea to apply a smidgeon of Loctite when installing the U-Joint parts to the shafts. Be careful, though, that the Loctite on the motor and gearbox shafts does not travel into the motor or gearbox bearings. If it does, you will find that this stuff truly lives up to its name, literally, when you apply power to the motor the next day!
Thoughts on packing and shipping brass models, and some answers to questions which have been submitted. This column will most likely run every other month from now on, as space is available. They still expect me to repair models here occasionally.
See you later.