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3. Double Reduction Stage Drives
Typical electric drive motors will have maximum shaft
speeds around the 2500 - 3000 rpm mark. A single stage reduction will
give maximum reductions of roughly 5 or 6:1, given practical constraints
on the sizes of available sprockets and space to fit them in. This
leaves drive wheel speeds of roughly 400 - 500 rpm when the motor
runs at top speed. For many small wheeled slow speed EV's this is too
fast, and the lack of reduction ratio can also mean lack of drive torque
at the wheels (and conversely torque overload at the motor shaft).
At the bottom of Page 1 a solution is described using
electric drive motors with built-in gearheads, however these are not
always available and the solution might be to build your own two stage
speed reduction.
Some examples:
|
Two Stage Roller Chain Drive
 With
this set up a bearing mounted intermediate shaft is driven by
the roller chain stage from the electric motor output shaft. The
intermediate shaft carries a second sprocket which drives a
second roller chain stage to the vehicle's road wheel - here the
final drive sprocket is mounted directly on the wheel hub which
is bearing supported on a fixed shaft.
With, say, 5:1
reductions at each stage this arrangement will give a 25:1
overall speed reduction. This will reduce a motor shaft speed of
2500 rpm to 100 rpm at the road wheel with a proportionate
multiplication of drive torque through the transmission - there
is a lot of scope for adjusting the overall speed ratios by
changing either or both of the stage sprocket sizes. |
|
Roller Chain
+ Geared Drive
Here a combination of roller
chain and straight spur gear stages are used to produce a two
stage transmission. The roller chain element allows flexibility
in the mounting of the electric motor and the spur gear stage
provides a compact high reduction stage.
Both shafts rotate and are
bearing mounted and the vehicle's road wheel is torque fixed to
the final drive shaft. Note how the final shaft diameter is the
larger - it has to deal with both larger drive torques and
bending loads from the road wheel.
The intermediate and final shaft
separation has to be carefully controlled to maintain the
correct centre distance for the gear mesh. |
|
Double stage chain drive as used
in the Voiturette
|
Combined chain & gear drive as
used in the 500W Kart |
....... go to Page 3 - More Double Stage Reduction
Drives
