Productivity Tip: Optimizing Gear Reliability and Life

Gears are one of the most fundamental components of any operation – they literally keep the equipment turning. Gears are found in virtually every industry, from gear mills in mining to open gears in construction applications to gearboxes used in steel, general manufacturing, oil and gas and many other industries.

Keeping gears in good working condition is paramount to success. Any downtime can result in significant productivity loss and costs, depending on the application. A gear mill, for example, can lead to millions of dollars of costs through downtime and part replacement/installation.

To avoid these scenarios, operators need to be able to look for early signs of gear wear so that corrective action can be taken. This two part series will help operators manage this challenge by examining the different types of gears available, common wear and failure modes, gear inspections and the different types of lubricants used.

Gear wear pattern basics

There are eight common gear designs, which all fall into two types of categories:

  • Parallel: These gear designs include spur, helical, herringbone and double helical.
  • Right angle: These gear designs include bevel, spiral bevel, hypoid and worm.

The choice in gear design is governed by application. What all gears have in common is how wear and failure manifests during operation. Wear, for example, is typically caused by repeated stresses, metal-to-metal contact and abrasive influence – regardless of gear type.

Let us first examine contact-related wear. These wear modes include:

  • Normal wear: This is caused by sliding or rolling forces. After initial break-in, the irregularity of the tooth surfaces is kept in good balance.
  • Polishing: The state of the tooth surface becomes smooth like a mirror as the asperity of the surface is removed gradually.
  • Moderate wear: Wear on tooth surface can be identified by checking tooth contact.
  • Abrasive wear: Dirt particles are trapped in the lubricant, and their dimensions compromise the oil film thickness. This results in score marks from the tip of the addendum to the pitch line, and from the pitch line through to the bottom of the dedendum.
  • Adhesive wear: This occurs when the film strength of the lubricant is insufficient to keep the peaks and valley of the opposing surfaces apart under typical operating conditions. This is often described as:
    • Frosting: Slight pitting occurs when only a thin oil film is generated and when heavy loading is applied.
    • Scoring: Surface deterioration is caused by alternate deposition and tearing of tooth surface.
    • Destructive scoring: Visible scratching and the tooth profile is destroyed.

Now let’s look at wear caused by repeated bending stress. These wear modes include:

  • Surface fatigue: Quite simply, this occurs when the gear material gives way due to repeated bending stress, an endurance failure. Sub-surface stresses result in small pits on the surface of the teeth. This stress leads to the three phases of pitting:
    • Initial pitting: A wearing phenomenon that occurs during the beginning of the gear usage. It stops its progression when the tooth surface is broken-in.
    • Progressive pitting: A wearing phenomenon occurs and does not stop its progression after the start-up phase is complete.
    • Spalling: Material fatigue that occurs under the surface, and quite large pieces of steel fall off the gear face. Once you are at the spalling stage, you are well on the way to gear failure.
  • Plastic deformation: Marked and deformed tooth profile, caused by intolerable heavy loads on the material of the tooth.
  • Bending fatigue breakage: Breakage that occurs on the tooth when unexpected heavy loads are applied to the tooth.
  • Overload breakage: This is more prevalent in new gear reducers as manufacturers are reducing physical size and increasing loading.

The following figures show what some of these wear modes look like:

Now that you have a foundational understanding of gear wear modes and how they look, we’ll tackle gear inspections and lubrication basics in our next post in this series.

I hope today’s read was educational, and please “Like” the post if you enjoyed it or ask me any questions you may have in the comments section below.