Glossary of Terms

It can be quite difficult at first to get your head around the terminology used when talking about model railway mechanisms. Below is a brief description of the most commonly found components in a Graham Farish diesel locomotive. This list will be enhanced in the future with the addition of pictures for each component and hopefully an expanded diagram showing how they all fit together.

ARMATURE: Another name for the “motor”, for diesels it consists of a central shaft with copper windings, surrounded by iron cladding, and also a commutator on the end, which could be either copper or silver in appearance. There were variations for steam locomotives, DMU’s / Railcars and electric locomotives, with the difference being gears or a flywheel on the end.

COMMUTATOR: This is the “round drum” at the end of the motor which is either copper or silver in appearance. It will have either three or five segments (3-pole vs 5-pole). The carbon brushes sit opposing each other on the commutator, commutating electricity to the motor.

WINDINGS: There are thousands of copper windings on the motor, which, when current is passed through, create the magnetic force field required for the motor to turn. This is possible due to the force field’s opposition to the magnet.

MAGNET: sitting just above the motor windings, the magnet provides the opposing magnetic forcefield required for the motor to turn.

POLE PIECE FOR MAGNET: These are the curved “pointy” metal segments which hang down from the magnet and shroud the motor. Do not get confused with this and the term 3-pole / 5 – pole. Their job is simply to spread the magnetism of the magnet around the windings, increasing the efficiency of the motor.

BRUSHES: Made from carbon and a hardener, these look like short cuttings of pencil lead. Their job is to commute power to the motor, via the commutator. They make contact with the commutator by “brushing” against it.

BRUSH HOLDER: this is the round brass cylinder which looks like a top hat. It is in this that the brush and brush spring sits.

BRUSH CLIP: A square U-shaped copper clip, this clips over the brush holder to hold the brush and spring in place.

SPRINGS: Used for two purposes; 1) Used to apply pressure to the brush so that it pushes up against the commutator, 2) Used in many of the wagon / coach / locomotive couplings to give a dampening effect. The spring used in both is the same.

BEARINGS: Referred to as bushes by some, these are used to hold the drive shafts in place. There are various types of bearing, but the most common on the diesels is a square plate with a shaft hole in the middle. They were made of a white nylon material. Currently they are available in a similar material but are black.

DRIVE DOGS: A small male-to-female fitting, these white plastic fixtures coupled the armature shaft at both ends to the rest of the drive shaft, which in turn had the pinion (worm) gears on the ends. It was, in effect, a device for transmitting the rotation of the armature to the rest of the drive shaft.

DRIVE SPRINGS: These replaced the drive dogs and were a tightly wound coil with a marginally larger diameter than the drive shafts. They fit snugly onto the armature shaft and the shaft with the pinion gear on it, again providing a means to transmit the rotation of the armature to the pinion gear. They are seen as an upgrade to the drive dogs for the following reasons; 1) the gap in the drive dog coupling led to noise when the shaft rotated, 2) the drive dogs would often split on the shafts, leading to slippage, whereby the rotation of the armature would not be transmitted to the pinion gear.

PINION GEAR: Often referred to as the worm gear, it is the brass “corkscrew” drum at each end of the drive shaft in diesels, or next to the motor on steam engines.

GEAR: Technically meaning any type of gear, it is commonly used as a term for the small round “toothed” gears on axles. These came as 12, 16 and 25 toothed gears.

IDLER GEAR: Refers to any “intermediate” gear in the bogie tower, that is a gear which is neither a drive axle gear, nor a gear which meshes with the pinion (worm) gear. In most cases these would be 12 toothed, but could also be 16 toothed.

DRIVE AXLE / DRIVE GEAR: The wheelset (wheels and axle) with a gear fitted to it. These were always 16 tooth gears on diesels.

BOGIE TOWER: Also known as the gear tower, or bogie chassis, this housed the gears and wheels.

BOGIE FRAME: This can be thought of as the “wheel frame”. It clips onto the bogie tower and shrouds the wheels.

STUB AXLE: Used to provide an axle for the gears, these came in two types; 1) knurled end, 2) knurled middle. Later examples of 2) came with a raised middle instead of a knurled middle. The knurled, or raised, section was such so as to offer grip to whatever the axle was supporting.

CHASSIS: This was the carcus of the locomotive. Made from MAZAC alloy, they provided housing for the motor, drive shaft, bearings and other components.

BASE PLATE: This plastic plate is attached to the underside of the diesel with nuts and bolts, or on larger diesels it’s not really a base plate, but rather a top place. You can think of it as the “lid” to hold the motor, bearings and drive shaft in place.

PICK-UP: Copper strips / wipers (steel on very early models), these clip onto the bogie tower and sit just behind the wheel backs, wiping against them. Their job is to conduct electricity from the track, via the metal wheels, up into the chassis.

CERAMIC CAPACITOR: A small orangey-brown “blob”, this component was fitted to suppress RF interference with appliances such as radio and television sets. A by-product of this was less sparking on the commutator. Sparking of the commutator results in a quicker build-up of carbon deposits and pitting. This will lead to poorer running and higher brush wear. The capacitor should have 471K written on it, with 471K = 470pF = 0.47nF.