Gear failure in gearboxes is a common occurrence in industry. There are various factors that affect the rate and severity of gear failure, including tooth geometry, corrosion, lubrication and heat treatment. By choosing a trusted supplier and expert in reducer overhauls, you can avoid costly gear failures. At Rapid Gear, we specialize in the machining of gears, racks and splines, as well as in reverse engineering.
Design Parameters & Heat Treatment
One of the predictors of gearbox failure is surface roughness. Macropitting and micropitting are two similar failure modes that are affected by surface roughness. With the use of cutting-edge gear grinders, this issue can be avoided. Micropitting is a failure mode in which microscopic chunks of metal are removed from the tooth flanks. These pits are so small that they can only be seen using a microscope. Macropitting, on the other hand, is the removal of large chunks which are visible to the naked eye. Often micropitting can be seen around macropitting, because micropits can grow and turn into macropits. Macropitting is caused by several factors, including improper lubrication, misalignment of gears and material properties and defects increasing Hertzian contact stress on the tooth surfaces.
Some common ways to prevent macropitting are:
- Reducing contact stress
- Using carburized or clean steel as they have a high fatigue resistance
- Honing/grinding gear teeth
- Using a lubricant which is cool (this allows maximum viscosity), clean (hard particles can cause denting) and dry (water in lubricants is detrimental)
- Using a highly viscous lubricant (this allows a thicker elastohydrodynamic, or EHL, film to form between the teeth)
The causes of micropitting is Hertzian fatigue and plastic flow which can be exacerbated by high surface roughness, vibrations in the gearbox and oil contamination.
The common ways to prevent micropitting are:
- Honing/grinding gear teeth
- Using a lubricant which is cool, clean, dry
- Using a highly viscous lubricant
- Avoiding shot peening the flanks of the teeth
- Making the pinion harder than the gear because the pinion works harder since it has more cycles
- Coating the teeth with phosphate, copper or silver
- Run-in with a special lubricant and controlled loads (avoid cold starts)
Another way design parameters of a gear can cause gear failure is overload. If a gear receives more load than it was designed to handle, it can permanently damage the gear teeth. Overloading can cause three types of gear failure:
- Plastic deformation of the teeth
- Brittle fracture
- Ductile fracture
Plastic deformation of the teeth occurs when the load is too high and plastically deforms the teeth, causing inaccurate spacing. In turn, this inaccurate spacing causes gouging of the teeth by having them collide in the gears’ next revolution.
The likelihood of brittle and ductile fracture not only depends on design parameters, but also on the heat treatment of the teeth. If a gear tooth is too hard, it is prone to cause brittle fracture. Likewise, if it is too soft, it is more likely to cause ductile fracture. The main difference between ductile and brittle fracture is that brittle fracture happens instantaneously, whereas ductile fracture is a long, slow process. It is important to note that a gear tooth can experience a combination of ductile and brittle fracture. However there are several ways to differentiate between the two:
- Brittle fracture leaves a bright and shiny surface, but ductile fracture is dark and dull
- Brittle fracture leaves a rough, coarse and grainy surface, but ductile fracture is fine, stringy and smooth
- Brittle fracture leaves radial ridges and chevrons, but ductile fracture leaves shear lips
To prevent brittle fracture, you can:
- Use fine-grained, clean, tough and hardenable steel (high nickel, molybdenum and low chrome, manganese, carbon, phosphorus and silicon contents)
- Reduce the load, flaws and tensile stress on the teeth
- Avoid running at temperatures lower than the transition temperature of the steel
To prevent ductile fracture, you can:
- Reduce load
- Make the teeth thicker (this distributes the load)
- Avoid edges and corners since cracks usually start on these
- Choose a material with a high yield strength and no brittle inclusions
Bending fatigue is another common failure mode. Bending fatigue failure occurs when a crack forms, usually at the bottom of a tooth, and propagates. This can be prevented by:
- Using clean, hardenable, carburized, fine-grain steel with shot peened root fillets (not flanks)
- Reduce the concentration of bainite and pearlite in the steel, as these are hard and brittle
- Reduce bending stress and the number of flaws and microcracks in the teeth
Lastly, subcase fatigue can be extremely problematic. This is because it occurs subsurface, at the case-core boundary of the tooth and branches up to the surface. Consequently, all that is visible on the surface is a hairline crack, but it is much more complex than that. In order to avoid this:
- Reduce contact stress
- Use hardenable steel
- Use optimal case/core properties (hardness of around 60 HRC for the case and 35 HRC for the core)
- Ensure the subsurface stress is below the subsurface strength
Corrosion & Lubrication
Corrosion can come in many different forms and can be hard to avoid, but nonetheless, it is crucial to do everything possible to minimize it. It is also important to note that corrosion can cause other types of failure, like bending fatigue cracks due to stress corrosion.
Abrasion is a common form of corrosion in which the oil lubricating the gearbox gets contaminated with small, abrasive particles. This is often due to the oxide layer on the gear teeth getting removed as the gears wear, allowing small oxide particles to contaminate the lubricant. These particles rub against the surface of the gear teeth and remove thin layers of metal, leaving the unoxidized metal exposed. This can be problematic because the oxide layer on metals protects the gear teeth from damage. Abrasion can be detected by the smooth and clean ruts it leaves on the tooth’s surface.
You can prevent abrasion by:
- Using a surface-hardened gear with smooth surfaces
- Using a high viscosity lube and changing it after the first 50 hours of running and using a fine filtration (or changing the lube every 2500 hours of running)
- Using tight seals and filtered breathers
- Having good housekeeping maintenance (such as making sure the inspection part of the machine remains closed)
Another common type of corrosion found in gearboxes is fretting corrosion. Fretting corrosion is a type of corrosion that occurs due to small, repeated movements between two surfaces (usually without the use of a lubricant). This contact causes the protective oxide layer of the metals to be removed and causes corrosion. Fretting corrosion depends on the contact pressure, vibration frequency and duration of contact. It is easily identifiable by the red oxide colour it leaves on the corroded metal.
To prevent fretting corrosion, it is recommended to:
- Use a sufficient amount lubricant
- Change designs to dampen vibrations
- Increase the hardness of at least one of the materials (use a relatively hard metal with a relatively softer metal)
It is also important to highlight the costs associated with gear failure. Whenever gear failure occurs in a gearbox, a new gearbox must replace it. Although this is obvious, what is not as evident is the potential astronomical costs due to said gear failure. A complete gearbox rebuild can take several months, so ensuring your gearbox is up and running again, as fast as possible, is crucial. Detecting gear failure early on will save a company a great deal of time and money, especially if a replacement gearbox can be acquired before the original gearbox’s failure causes it to no longer be functional.
Ultimately, there are numerous failure modes to look out for in gear teeth. In a gearbox, not only can the gears themselves fail, but the bearings and seals can fail as well, so it is important to verify that your gearbox is always in optimal condition. All in all, it is important to pay attention to craters (macropits or micropits), corrosion, lubricant quality and quantity and cracks to avoid gear failure. Cracks and micropits are especially difficult to detect since micropits cannot be seen by the naked eye and cracks can, depending on the nature of the crack, appear much smaller than they truly are or they might not even appear on the surface of the tooth. Lastly, the most effective way to reduce the likelihood of failure is by doing business with a gear specialist, who can guarantee high quality products.
Lucas Foti