Figure 1: Visual comparison . Steppers (Left) are dense and simple. Servos (Right) are longer and include a visible feedback encoder housing on the rear. The Million Dollar Question: "Which Motor Do I Need?" If you are designing a CNC machine, a packaging robot, or a conveyor system, you face the same dilemma every time: Stepper or Servo? Make the wrong choice, and you face two disasters: The Stepper Trap: Your machine "loses steps" (positional error) without knowing it, scrapping parts. The Servo Trap: You spend $5,000 on a system that could have been done for $500, blowing your budget. This guide bridges the gap between mechanical requirements and electrical reality. 1. The Stepper Motor: The "Digital Ratchet" Think of a Stepper Motor like a very strong, magnetic ratchet. It divides a full rotation into equal steps (typically 200 steps per revolution, or 1.8°). Pros: Incredible Holding Torque: Ste...
Key Geometric Calculations
In Part 2, we analyzed the loads. Now, we must size the geometry.
Designing a chain drive involves a specific sequence: determining the sprocket size, estimating the center distance, calculating the required chain length in "pitches," and then recalculating the exact center distance.
1. Pitch Diameter
The pitch diameter is the theoretical circle that passes through the centers of the chain pins.
- D1 = Pitch Diameter of Driver Sprocket (Small)
- D2 = Pitch Diameter of Driven Sprocket (Large)
- N1 = Number of Teeth on Driver
- N2 = Number of Teeth on Driven
- p = Chain Pitch
D1 =
p
sin( 180 / N1 )
Calculator Note:
Most calculators default to Degrees mode.
Most calculators default to Degrees mode.
- If using Degrees: Use
180 / N - If using Radians (e.g., Excel): Change 180 to Ï€ →
sin(Ï€ / N)
2. Center Distance Guidelines
The Center Distance (C) is the distance between the shafts. While chains are flexible, sticking to standard design ranges ensures long life.
Figure 1: The geometry of a two-sprocket chain drive, defining the Center Distance (C) and Angle of Contact (θ).
- Minimum: The sprockets must not touch. Ideally, C should be at least 30 to 50 times the pitch (30p - 50p).
- Maximum: Long spans can cause vibration. If C > 80 pitches, guide rails or tensioners are recommended.
3. Chain Length Calculation
This is the most critical step. Unlike a belt, you cannot have a fraction of a chain link. The total length (L) must be an integer number of pitches.
The standard formula to approximate the length in pitches (L) based on a preliminary center distance in pitches (C) is:
L = 2C +
+
N2 + N1
2
(N2 - N1)2
4 π2 C
Design Rule of Thumb:
"It is preferable to have an odd number of teeth on the driving sprocket and an even number of pitches (links) in the chain. This prevents a specific link from constantly engaging the same tooth, distributing wear evenly."
4. Exact Center Distance Recalculation
Once you calculate the chain length, you must round it to the nearest even integer. Because you changed the length, you must now calculate the exact center distance to mount your motor correctly.
This formula calculates the Center Distance in standard units (mm or inches), where p is the pitch:
C =
[
2L - (N2 + N1) +
√[2L - (N2 + N1)]2 - 0.81(N2 - N1)2
]
p
8
5. Angle of Contact
Finally, verify that the chain wraps sufficiently around the small sprocket. The arc of contact (θ) should ideally be greater than 120° to prevent teeth jumping (ratcheting).
θ = 180° -
× 60
2 (D2 - D1)
C
References
- Robert L. Mott, Machine Elements in Mechanical Design
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