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...
Figure 1: Rolling contact bearings replace sliding friction with rolling motion to improve efficiency.
Rolling contact bearings (often called anti-friction bearings) use rolling elements—either balls or rollers—to carry applied loads with minimal friction. Unlike journal bearings which rely on a fluid film, these bearings allow machinery to operate with reduced starting torque and minimal wear.
They are the standard solution for everything from electric motors to automotive transmissions, manufactured with hardened raceways and separators (cages) to maintain precision spacing.
Advertisement
1. The Big Three: Bearing Types & Applications
While there are dozens of variations, most anti-friction bearings fall into three primary categories based on the shape of the rolling element.
Figure 2: The anatomy of a standard deep-groove ball bearing.
| Type | Best For... | Limitations |
|---|---|---|
| Deep-Groove Ball | High speeds, moderate radial loads, and light thrust loads. The most common standard motor bearing. | Lower load capacity than rollers. |
| Cylindrical Roller | Very heavy radial loads. The line contact of the roller spreads the force. | Cannot handle thrust (axial) loads well. |
| Needle Roller | Extreme radial loads in tight spaces (low profile). | Cannot handle thrust; requires hardened shaft surface if no inner ring is used. |
2. Standards and Interchangeability
One of the greatest triumphs of mechanical standardization is the AFBMA (Anti-Friction Bearing Manufacturers Association) numbering system.
Interchangeability Rule: A 6204 ball bearing from SKF will have the exact same boundary dimensions (ID, OD, Width) as a 6204 from Timken or NSK. This allows engineers to design shafts without being locked into a single supplier.
3. Accuracy and Manufacturing Quality
Rolling contact bearings are manufactured to incredible precision. Balls and rollers are commonly held to diametral tolerances of 0.0001 in (0.0025 mm) or less.
Advertisement
4. Bearing Life and Fatigue
Unlike plain bushings which wear down over time, rolling bearings fail due to Rolling Contact Fatigue. Repeated stress cycles eventually cause the metal surfaces to spall or flake.
Figure 3: Understanding load direction is critical. Ball bearings handle both; Rollers handle mostly radial.
Engineers calculate the L10 Life, which is the number of hours that 90% of a group of bearings will survive under a specific load.
Basic Life Equation
L10 = (C / P)^n
C = Basic Dynamic Load Rating (from catalog)
P = Equivalent Dynamic Load (actual applied load)
n = 3 for Ball Bearings, 10/3 for Roller Bearings
5. Design Checklist
- Load Direction: Does the application have Thrust (axial) load? If so, a standard roller bearing will fail immediately. Use a Ball or Tapered Roller bearing.
- Shaft Fits: A rotating shaft requires a Press Fit (Interference) on the inner ring to prevent it from spinning on the shaft and destroying the journal.
- Housing Fits: The stationary outer ring usually gets a Slip Fit to allow for thermal expansion.
- Lubrication: Grease is standard for most speeds. Oil bath/mist is required for high-speed heat dissipation.
Reference:
Robert L. Mott, Machine Elements in Mechanical Design
Comments