Roller
chain drives are, off course, a standard means of
mechanical power transmission, however our interest in
roller chains arose principally from the use of low cost
Chinese built drive motors as the main motor type for
our designs. These motors are more usually used in
electric scooter/trike spares and often come with a
chain sprocket already attached to the output shaft. My
initial concerns about noise proved unfounded and we
have found that roller chain drives, even at the high
motor shaft output speeds (3000rpm +), are very quiet.
Generally, for the same reduction ratio, chain linked
shafts have to be a bit further apart than gear linked
shafts, however they are more tolerant of slight shaft mis-positioning and removing the chain breaks the
mechanical link between the shafts which can sometimes
help in assembly or disassembly activities.
Sprockets
need to suit the chain type used (or vice-versa) so pay
attention to the specs. On the scooter motors type #25
chain is most common. This is 1/4" pitch American
specification chain (ISO No. ASA-25) which is different
from UK spec 6 mm pitch chain (04B-1) so don't mix up
the two! Larger pitch chain is stronger so a step up to
3/8" pitch chain in the form of either the US spec #35
(ASA-35) or the UK spec 06B-1 will give more drive
torque capacity. Again, don't mix and match chains or
sprockets from the different specifications. A good
range of larger sizes is available from 1/2" pitch and
upwards if needed.
Sprockets
are similar to gears insomuch as the number of teeth
matters (to get the reduction ratios you require) and
they are available in bossed or un-bossed (plate wheel) forms. For
bonding to shafts the bossed form is best as this offers
the greater bond area for joint strength, however the
un-bossed sprockets are good for bolted attachment to the hubs on drive wheels without taking up
too much space.
If
designing-in a chain drive particular attention needs to
be paid to the shaft centre distance - this needs to be
calculated to suit the chain pitch, number of teeth on
the sprockets and the length of chain to be used. The
equations needed to do this are available in several
engineering design texts however they are most
conveniently obtained from the chain suppliers
themselves,
check here. The common approach is to estimate chain
length to suit the desired approximate shaft spacing,
then to calculate the exact spacing once the precise
chain length is selected. It is wise to allow position
adjustment of one or other of the sprockets to enable the
chain to be tensioned slightly on assembly to remove slack.
As with your steel gears you need to keep your chains
lubricated in operation.
For a good on-line
technical guide for chains take a look at
chain-guide.com.