Meccano Gears 101
The use of Meccano gears in modding steam engines is very common as they are both easy to obtain and reasonably priced.
Meccano gear combinations are designed to mesh on the standard 1/2" spacing yielding ratios of 1:1, 1:2, 1:3, 1:4 and more.
Using a worm gear speed reductions from 19:1 to 133:1 are possible.
Pinions and Spurs
The smaller gear is usually refered to as the pinion, the larger as the spur in simplistict terms.
Common pinion gear widths are 1/4" (no following letter), 1/2" ( part # followed by "a") and 3/4" (part # followed by "b").
1/8" wide gears are also available and have a confusing part numbering system:
Bosses are threaded for 5/32 BSW grub screws.
Shafts are 8 SWG = 0.160" = 4.1 mm in diameter, this is slightly over 5/32".
A #20 drill bit (0.161") works for most meccano holes, to drill pulley bosses predrill first then chase with the #20 bit. This can be somewhat of a trial and error situation depending on how the bit is sharpened, in some cases a #19 may be a better fit.
What fits What?
The two most commonly used pairs of gears which fit the standard 1/2" spacing are a #25 and a #27 for a 3:1 ratio and a #26 and a #27a for a 2:1 ratio, both fit over a 3 hole spacing.
A pair of #26 pinions will also mesh over a 2 hole spacing for a 1:1 ratio.
The chart below gives the combinations available:
Other Work Arounds
Gear ratios that cannot be obtained using the standard 1/2" spacing can sometimes be accomplished with the use of curved parts, in the example above a part #89a is used to mesh a #25 pinion with a #27a spur gear.
Brackets with any spacing required can be made from light metal, drill one hole and insert the first gear mounted on a shaft. Mesh the second gear with a strip of paper between them, when in position use a special center punch ground to fit the hub of the gear to mark the spot to drill the second hole.
Meccano strips make excellent templates to drill the standard 1/2" spaced holes in your own brackets.
Grub screws have a habbit of disappearing so I make them from bolts that have worn out slots. Thread them into a nut, grip the nut in a vice hard enough so that the bolt will not turn. Cut the bolt to length, saw a slot in the end, a jewelers works best for this, then file end smooth. Remove from vise, rotate 1/4 turn and grip again in vise, squeezing gently to free screw, then remove screw from nut.
Sprockets and Chain
Meccano supplies an assortment of sprockets to be run with ladder chain, a word of caution, be sure it is actual Meccano chain, there are other chains out there that are close but do not fit. To check wrap a length of chain around one of the larger sprockets, it should fit in the gulleys all the way around.
The length of chain required is adjusted by adding or removing links, I prefer open the loops by wraping the chain around a large sprocket with the loops up and then open them with a small screwdriver. The wire is very hard and just holding them in your fingers can result in the screwdriver blade slipping and peircing your skin.
Longer runs of chain will require a tightener otherwise they will tend to jump off the sprockets, the tightener should be put on the slack side of the chain. The tighteners work best when held under tension with an expansion spring.
The sprockets range from 3/4" in diameter (14 teeth) to 3" in diameter (56 teeth)
To determine the speed ratio divide or multiply the driver number of teeth by the driven number of teeth, for example a # 96a (14 teeth) driving a # 95a (28 teeth) will result in a 2:1 reduction.