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Motor RPM

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					Gear Ratios


Calculations
   The most used drive system on a
FIRST robot uses sprockets and
chains. It is vital to the success of a
robot to properly adjust the size of
sprockets in order to obtain the
maximum speed and power.
   Balancing speed versus power can
be daunting but with a few basic
guidelines it can be made easy. Let‘s
examine a couple things to keep in
mind while choosing the right gear
ratio . . .
     Two Broad
      Types of
     Sprockets
a) Driver Sprockets are directly
   driven by a motor
b) Driven Sprockets mesh with
   the driven sprocket
         Gear Ratio Velocity
  First, notice which sprocket has the
  most teeth. This will be the sprocket that
  completes one revolution. Take the
  number of teeth on the driven sprocket
  and divide that number by the amount
  of teeth on the driver sprocket.
Driver Sprocket:   30 Teeth
Driven Sprocket:   10 Teeth
                          30/10   = 3:1 Ratio
      Revolutions per Minute
                 Ex. 1:
To calculate the Motor RPM (60 rpm) x gear ratio (3:1)
  RPM output of                 = 180 rpm
  a given gear
  ratio, take the   Ex. 2:
  velocity ratio    Motor RPM (100 rpm) x gear ratio (1:4)
  and multiply                   = 400 rpm
  that number
  by the RPM
  output of the
  motor source.
RPM with a Compound Gear
• The easiest way to calculate RPM on a
  compound system is to split up the
  problem into two equations or pairs
  depending on the amount of gears
  involved.
• After finding the ratio for each pair,
  multiply them together with the motor
  RPM to get your final RPM.
• see next slide for examples
Ex. 1: This example involves 4 gears. Gear A-
  Driver Sprocket, Gear B and Gear C are
  Fixed, and Gear D. The motor RPM in this
  example will be 100 RPM.
Driver Sprocket-Gear A (30 teeth) / Gear B (10 teeth) = 3:1 Ratio
       Gear C (20 teeth) / Gear D(50 teeth) = 2:5 Ratio

      3:1 Ratio x 2:5 Ratio x 100rpm = 120 rpm

Ex. 2: This example also involves 4 gears. Gear A-
  Driver Sprocket, Gear B and Gear C are Fixed,
  and Gear D. The motor RPM in this example will
  be 60 RPM.
   Driver Sprocket-Gear A (20 teeth) / Gear B (15
   teeth) = 4:3 Ratio
    Gear C (50 teeth) / Gear D (10 teeth) = 5:1 Ratio
       4:3 Ratio x 5:1 Ratio x 60rpm = 400 rpm

				
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posted:4/14/2013
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