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VFD vs Soft Starter for Conveyors: Preventing Gearbox Failure

You selected the right AGMA Class gearbox . You calculated the belt tension perfectly. But the moment you hit "Start," the belt snaps or the gearbox makes a terrifying clunk. The culprit is likely your Starting Method . In conveyor systems, the starting torque profile matters more than steady-state power. Note: We previously discussed VFDs as Energy Savers for pumps and fans. For conveyors, however, the goal is not lowering your electric bill—it is preventing your gearbox from exploding. Table of Contents 1. The Physics of Shock Loads 2. Why Soft Starters Stall Conveyors 3. The VFD Torque Advantage 4. Comparison: Cost vs. Protection 5. Final Verdict Advertisement 1. The Physics of Shock Loads When an AC induction motor starts Direct-On-Line (DOL), it draws 600% to 800% of its rated current (Inrush Current). More importantly, it produces a sudden spike known as Locked-Rotor Torqu...
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Chain Drives Design: Load Analysis & Tension Factors (Part 2)

Roller chain drive system diagram showing tight and slack strands

Figure 1: A typical chain drive system. Note the difference between the "Tight Strand" (transmitting power) and the "Slack Strand."

Understanding the Loads on a Chain

In Part 1, we looked at the types and advantages of chain drives. Now, we must tackle the math and physics behind them.

Designing a chain drive isn't just about picking a chain that fits the sprocket. You must account for the Total Tensile Load. If you only calculate for the torque transmission, your chain will likely fail due to unseen forces like shock, inertia, or vibration.

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1. Nominal Tensile Load

The Nominal Tensile Load is the baseline force required to transmit power. However, this load is rarely static. It fluctuates in a cycle as the chain moves through the system:

  • Tight Strand: As the chain engages the driven sprocket, tension is at its peak (transmitting the torque).
  • Slack Strand: As it leaves the driver sprocket, tension drops significantly.

This constant cycling between high and low tension creates Fatigue Loading. Over millions of cycles, this is the primary cause of chain plate failure.

2. Shock vs. Inertia Loads

It is crucial to distinguish between these two types of dynamic loads:

Shock Load
Shock loads are repetitive spikes in force caused by the nature of the machinery.
  • Example: A rock crusher or a piston pump.
  • Solution: We use a Service Factor (Ks) to oversize the chain. For heavy shock loads, a factor of 1.5 to 2.0 is often applied.
Inertia Load
Inertia loads are occasional, high-intensity events, usually occurring during Startup or Jamming.
  • Example: Starting a fully loaded conveyor belt with a large flywheel.
  • Solution: Ensure peak startup torque does not exceed the Yield Strength of the chain.
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3. High-Speed Effects

As chain speed increases, physics introduces two new enemies:

Centrifugal Tension

Just like a belt, a chain has mass. As it travels around the curve of a sprocket, centrifugal force tries to throw the chain outward. This adds a "phantom load" to the chain that doesn't help transmit power but definitely adds to the stress.

Centrifugal Force Formula

Fc = m v2

Since velocity (v) is squared, doubling the chain speed quadruples this destructive force.

Chordal Action (The Polygon Effect)

As discussed in Part 1, the chain wraps around the sprocket as a polygon, not a circle. This causes the chain strand to rise and fall (vertical vibration) and the speed to surge (velocity variation).

At high speeds, this Chordal Action turns into significant Impact Noise and wear on the sprocket teeth.

4. Vibration and Resonance

Every chain span has a Natural Frequency, much like a guitar string. If the frequency of the shock loads (e.g., from the chordal action or the machine itself) matches the chain's natural frequency, Resonance occurs.

Resonance can amplify the tensile load to destructive levels, causing the chain to "whip" violently. This is a common cause of catastrophic failure in long-span drives.

Next Step: Failure Modes

Now that we understand the loads, how do we predict when a chain will break? We will explore wear, fatigue, and galling in the next part.

Continue to Part 3:
Chain Drives Design: Formulas for Pitch, Length, and Center Distance (Part 3)

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