Since the variety of gears available is very limited, finding the right combination may be difficult. N scale applications may be next to impossible. Larger scales may use some gears listed with consideration of teeth stress. Axle and shaft diameters narrow the selection further. Reboring and reaming to enlarge require extreme precision to maintain concentricity. NWSL bushings can be used to reduce bore to shaft size. An ordered procedure will help shorten the selection process. NWSL 72 DP gears and HO drivers etc. will be used for examples.
First determine the maximum driver gear (DG) outside diameter (OD) which must clear the rail top (RT) by at least .022" to avoid interference with uncoupling magnets etc.. Kadee's gluing jig sets height at just under .020. Measure the driver diameter (DV) and subtract .044" to find the maximum gear OD. Using a common HO 50" driver the diameter (DD) is .574 ". The number of teeth is found by:
(DD - .044) * 72 = (.574-..044) * 72 = 40.374 or 38.16 teeth.
SEE GEAR MEASUREMENT
But this is the OD not PD, so subtract 2 teeth for the addendum to get 36 teeth. For simple reductions the axle center (AC) to motor shaft center (MC) distance (CD) is found from the pitch radii by:
CD = PRwg + PRwm + .004" = 36 / 144 + .015 \ 2 + .004 = .2615"
If you know the distance on the old gears, this may lead to some clues on how to tilt or slide (ala Mantua) the motor to obtain the mesh. Many times manipulating the motor on the frame and eyeballing yields hints. An idler gear may be necessary to clear the frame or level the motor. Although obsolescent, the NMRA Gear Data Sheet lists many still used worm gear data with dimensions. See WORM GEAR SELECTION TABLE for an updated list. There are other sources for gears, but most are too expensive for general use.
Double reduction trains add clearance problems, in that the worm wheel must clear the axle. The combination of all 3 gears will effect this. NWSL's package, pitch diameter charts or the GEAR SELECTION TABLE may help as short cuts
Critical components shown in yellow.
Using a desired ratio of 72:1 with the 36 tooth DVG the upper ratio is 2:1. There are several combinations that yield it. Trying 20 and 10, the axle to reduction shaft distance (AC-RS) is:
AC-RS = PRdg + PRrg + .004 = 36/144 + 10/144 + .044 = .323"
If a 1/8" OD bearing (BR) is used .323 - .0625 = .261" may not clear the frame top (FT) with an adjustment mounting plate.
Trying 40 and 20 to gain height:
AC-RS = (36 + 20)/144 + .004 = .393"
The worm wheel may not clear the axle:
Clearance = AC-RS - ORwg - Raxle = .393 - 42/144 -.0625 = .039".
It does clear. A 0 or minus result would indicate a no go. If both extremes fit then 30 and 15 will clear the axle.
The last step is to determine which usable combination of gears will provide a good frame to motor shaft distance for mounting. Keep in mind that in almost all cases, shims will be required to adjust distances. Measure the axle center to frame top distance and motor shaft height to help determine other factors as in examples.
Using a drawing program, such as XCAD PROFESSIONAL 3000, with accuracy better than .0001", to plan, can save much wasted time in metal work.
See EXAMPLES
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