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Galling is wear caused by the friction between two metal surfaces that are in sliding contact. Galling is distinguished from other forms of wear between parts by the transfer of microscopic particles from one component to the other (ex. screws, bolts, and threaded fasteners).
Material is pulled from one part and adheres to its mated part creating a bump, or gall. The result is that the parts are no longer able to function together as intended, and unlike other forms of wear that cause costly problems over time, the negative effects of galling occur almost immediately.
How can the negative impacts of galling affect the design application?
- Premature Failure: Galling can cause threads to seize and tear from the fastener or hole. When a bolt locks up to the point where it can only be turned by friction, the fastener can quickly breakdown.
- Fouled Operating Environment: Small particles and debris that result from galling can flake off of the component and enter the operating environment. This is a major concern for applications such as high-pressure valves that can become contaminated and cause blockage.
- Expensive Repairs: Damage to external threads found on screws or bolts is generally an easy fix, but galling can damage internal threads of the mated part, which is much more costly and complicated to repair.
From the design engineer’s perspective, the issue does not become clear until significant cost and time has already been invested in development and/or production. Changes can be made to prevent galling and its costly results.
1. Proper Lubrication
The most common galling prevention strategy is proper lubrication, but this is not the only possible solution. A lesser considered, but often more effective method of eliminating galling is to change the machining process of the threads.
2. Specifying the Threading Method
A design engineer will specify threaded parts but often will not specify the corresponding machining methods. Instead, the machining process is left to the machine shop to decide. This is a missed opportunity for the designer to avoid premature failure, improve tensile strength, and even reduce production costs.
Two common machining methods are cutting and rolling.Thread Cutting involves the use of sharpened tools that physically remove material to form threads. These threads are rough with microscopic chips that can flake off and cause galling.
Thread rolling is a cold-working process in which steel dies are used to force the material outward into the shape of threads. The material is not removed but displaced, and threads are left with smooth finishes.
Rolled threads not only eliminate the galling issue, they provide other benefits as well.
Smooth Finishes
Galling is most likely to occur in cut threads because of the microscopic particles and debris that remain on
the thread. Rolled threads guarantee a smooth surface and reduce friction.
Because rolling does not disturb the natural material grains, these threads are free of imperfections such as burrs, chips, tears and chatter—common starting points for galling. Rolled threads provide chip-less operation and thread uniformity.
Tight Tolerances
Threaded fasteners with rolled threads also have tighter tolerances. Parts with tighter tolerances provide fewer opportunities for sliding and friction. Thread rolling is extremely accurate and yields the same
dimensions and tolerances for the first part as the last.
Superior Strength
When compared to cutting, rolled threads have superior tensile and fatigue strengths. This is mostly due the cold-working, material displacement process.Less Fatigue: Because the natural material grains are left continuous and unbroken, rolled threads are much more resistant to fatigue and stripping.
Increased Tensile: Rolled threads can withstand much higher levels of stress than cut threads. American Standard for Test and Materials International (ASTM International) tensile tests have shown 10% increases in tensile strength over cut threads.
Hardened Roots: The high pressures used to form the threads put strain on the roots, causing hardness to be up to 30% greater than cut threads.
When to Choose Rolled
Specifying the machining method is a potential solution to galling in applications such as nuclear, military, valves, food processing, medical, and aircraft. Some of the application specifics include:
Food Processing: In many food processing and manufacturing equipment, stainless steel, which is particularly susceptible to galling, is used for fasteners. Because the food processing industry prohibits the use of most lubricants, this may not be an option to avoid galling so choosing rolled threads can be an effective solution.
Military, Aerospace, & Nuclear: These applications require fasteners without burrs, in hard-to-machine or exotic materials (ex. stainless steel, titanium, and Inconel). Rolling is an efficient means of achieving a smooth, burr-free surface.
Other High-Precision Applications: Any high-precision applications that require accurate and consistent thread geometry to resist part interference would benefit from rolled threads.
3. Material Selection
Before deciding on rolled threads as a solution, designers should be aware of the properties of the desired material, as some can be difficult to roll and are more prone to galling than others.
Ductility
Ductility, or a material’s ability to deform without breaking, is a major characteristic that determines its ability to be rolled. High ductility in materials can be a precondition for metal particles transferring during motion. However, the high pressure induced during rolling makes the metal surface denser and less likely to form these particles.
Identifying the ductility of the desired material prior to assembly can determine if rolling is a possible galling prevention method.
Galling Susceptibility
Design engineers should also be aware that some commonly-chosen materials are more susceptible to thread galling than others.
Oxide Layers: Some metals, such as stainless steels, aluminum, and titanium, are known to have oxide surfaces for corrosion protection. When fasteners tighten or slide against each other, the oxide layers are scraped away leaving damage that causes build-ups that can stick and lock up.
Hardening Rates: Galling is more likely to occur between metals that harden at the same rate following rolling. If possible, choose two metals that harden at different rates for the fastener and mated part.
Considering these galling prevention strategies can be a proactive step in improving the quality and longevity of metal fasteners and threaded parts, while also reducing the likelihood of expensive and complicated repairs.
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