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Introduction
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Introduction



You use gears to slow down a motor and give it more strength. Torque is rotating strength. If the input gear is smaller, your larger gear will move slower, but the torque will be stronger.

As the speed increases, the strength of the torque decreases (large input).
As speed decreases, strength increases (small input).

You also use gears when your mechanism that is controlled by a motor is separated from the motor by geometry. The gears allow you to transfer the rotation of the motor across this geometry. When designing, you will often work from mechanism back to motor.

You could use gears with the same number of teeth to move the rotation. These gears are also going in opposite directions—you could create a claw.



Gears are comparable to continuously applied levers; as one tooth is engaging, another is disengaging. The amount of teeth on each gear wheel affects the action on the gear wheel it engages or meshes with. The gear wheel being turned is called the input gear and the one it drives is called the output gear. Gears with unequal numbers of teeth alter the speed between the input and output. This is referred to as the gear ratio.

Gears also alter the direction of rotation. In the above example gear wheel A is rotating clockwise, but as it turns, gear wheel B is moved anti-clockwise.





If you know any two things about a gears—outer diameter and number of teeth—you can use some simple equations to find everything else you need to know, including the correct center distance between them.
Number of Teeth (N)
How many teeth are there in the gear

Pitch Diameter (D):
The circle on which two gears effectively mesh, about halfway through the tooth. The pitch diameters of two gears will be tangent when the centers are spaced correctly.

Diametral Pitch (P):
The number of teeth per inch of the circumference of the pitch diameter. Think of it as the density of teeth—the higher the number, the smaller and more closely spaced the teeth on a gear. Common diametral pitches for hobby-size projects are 24, 32, and 48. The diametral pitch of all meshing gears must be the same.

Circular Pitch (p) = pi / P:
The length of the arc between the center of one tooth and the center of a tooth next to it. This is just pi (ï€ = 3.14) divided by the diametral pitch (P). Although rarely used to identify off the shelf gears, you may need this parameter when modeling gears in 2D and 3D software like we're doing here. As with diametral pitch, the circular pitch of all meshing gears must be the same.

Outside Diameter (Do):
The biggest circle that touches the edges of the gear teeth. You can measure this using a caliper like Sparkfun.com's # TOL-00067.

Note: Gears with an even number of teeth are easiest to measure, since each tooth has another tooth directly across the gear. On a gear with an odd number of teeth, if you draw a line from the center of one tooth straight through the center across the gear, the line will fall between two teeth. So, just be careful using outside diameter in your calculations if you estimated it from a gear with an odd number of teeth.

Center Distance (C):
Half the pitch diameter of the first gear plus half the pitch diameter of the second gear will equal the correct center distance. This spacing is critical for creating smooth running gears.

Pressure Angle:
The angle between the line of action (how the contact point between gear teeth travels as they rotate) and the line tangent to the pitch circle. Standard pressure angles are, for some reason, 14.5&Acir;° and 20°. A pressure angle of 20° is better for small gears, but it doesn't make much difference. It's not important to understand this parameter, just to know that the pressure angle of all meshing gears must be the same.

Addendum:
how much the tooth protudes beyond the pitch radius

Dedendum:
how deep goes the tooth before any fillet take place

Pressure angle:
The angle between the radial direction and the tangent to the tooth at the pitch circle. 20 is a good value.

Some Rules

The value of the base circle is based on pressure angle.
  • The smaller the pressure angle, the smaller the base radius
  • The addendum must be less than the dedendum
  • The dedendum must be more than the difference between the pitch and the base radius.


All of these gear parameters relate to each other with simple equations.
To Get You Have Equation
Diametrical Pitch(P)
Number of teeth per unit length		 Circular Pitch(p)
Number of Teeth(N) & Pitch Diameter(D)
Number of Teeth(N) & Outside Diameter (Do)		 P=π/p
P=N/D
P=(N+2)/Do(approx)
Circular Pitch(p)
Length of the arc from one tooth to the next		 Diametrical Pitch (P) p=π/P
Pitch Diameter(D) Number of Teeth(N) & Diametrical Pitch(P)
Outside Diameter (Do) &Diametrical Pitch(P) 		D=N/P
D=Do-2/P
Number of Teeth(N) Diametrical Pitch(P) & Pitch Diameter(D) N=P*D
Center Distance(CD) Pitch Diameter(D)
Number of Teeth(N) & Diametrical Pitch(P) 		CD=(D1+D2)/2CD=(N1+N2)/2P

Blender Mechanical Gears 0.0.2©2004 Stefano <S68> Selleri







Stepping down has the advantage of producing more power although at a slower rate.







This Stepping up produces a much faster output speed, but mechanically delivers less power




CALCULATING RATIOS

If the input gear (A) has 10 teeth and the output gear (B) 30 teeth, then the ratio is termed 3 to 1 and is written down as 3:1 
Ratio = No. of teeth on the output gear B (30)>
        No. of teeth on the input gear A (10)
		= 3/1 and is written down as 3:1

Simply divide the amount of teeth from the output by the input gear to work out the ratio. In the above example, for every complete revolution of the input gear the output turns 1/3 of the way round. In other words it takes three turns of A to rotate B once. This means you are slowing down the action and is referred to in engineering terms as Stepping Down. If B were the input gear and A the output gear, then the opposite happens and we Step Up. Then with one turn of the input gear the output gear would turn three revolutions, giving a ratio of 1:3.

parallel gears work in a similar way to the In line ones but are much easier to construct. The larger one uses popsicle sticks instead of dowel and is referred to as a Paddle gear.



Designing

When designing and making gear wheels you need to apply a little common sense. For example, the load or pressure put on the gears in automata are usually very small compared to that of a car gear box. This allows you to get away with things that you couldn't in other machines. However, you still have to follow some simple engineering guidelines. You will need to identify what you want to get from the gears. Try running through this simple check list;
  1. Do you want the gears to step up or step down? (speed up or slow down the performance).

  2. Do you want the gears to run parallel, or at an angle of 90° to change the direction of the drive?

  3. What size do you need to make them? (small space means making smaller gears and this can get tricky).





 
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