What is Thread Galling, And How Can It Be Prevented?
What Does Galling Mean?
Thread galling, also known as ‘cold welding’, is a form of adhesive wear which occurs during the movement of components and often leads to stuck Nuts and seized Screws and Bolts. There are several causes of thread galling and ways you can prevent it. Before we look at anti galling tips to stop bolts from seizing, let's take a look at why it's important to avoid stainless steel galling.
Why Is Galling An Issue?
Galling in static fasteners can lead to corroded components which work loose or break. The consequences of galling in actuation can be much more severe because dynamic components within a machine can either stop working as intended, or stop moving altogether.
For example, in a CNC machine, the Lead Screw moves the internals of the machine by highly precise increments for every rotation of the screw. A galled thread can cause a machine to encounter increased resistance,
What Causes Galling?
Engineering components that are made from stainless steel, aluminium and titanium have a self-forming protective oxide film. When this protective oxide layer is rubbed or scraped off, the metals come into direct contact with each other and start to gall. Galling metal is caused by the pressure of movement from tightening the fastener.
What Is Actually Happening When A Fastener Starts Galling?
Friction causes the surface oxides to temporarily break down and the metals come into direct contact with each other. The surface oxides clog the thread and the microscopic high points shear and lock when the surfaces slide past each other, which increases the heat and friction.
As tightening continues, the increased pressure results in more material being sheared off the threads. This continues with even more clogging, shearing and locking until the threads are destroyed and the fastener becomes stuck. It is then almost impossible to remove the fastener without cutting the Nut. Take a look at the list below to see how to prevent screws from sticking.
How To Prevent Stainless Steel Fasteners From Galling
Use Different Alloy Grades Or Material Types.
Engineering components made from austenitic stainless steel have a tendency to gall. Therefore mating them with a material which has a different hardness such as a Duplex Stainless Steel Nut, significantly reduces the chances of galling. Duplex stainless steel is commonly 50% austenite and 50% ferrite, which work hardens at a different rate and has a different crystalline structure and microscopic surface structure.
The same alloy grades are prone to galling. This is caused by the microscopic high points hooking into the other surface, locking together like velcro. For more information about crystalline structures of stainless steels, read our article: The Difference Between Ferritic Austenitic and Martensitic Stainless Steels.
Slow Down The Tightening Speed.
The torque specification must be adhered to, but slowing down the tightening speed, significantly helps to prevent bolts from seizing.
Using Hand Tools For Installation.
Another way to prevent galling bolts is to tighten the fastener by hand, which significantly reduces the heat and friction that would be caused by using an impact wrench or drill.
Ensure That The Threads Are Clean And Undamaged.
Damaged, rough or dirty threads are particularly prone to galling because the friction is increased during installation.
Lubricant reduces friction on the mating threads, but care should be taken because it has an adverse effect on the tightening torque of the fastener. (You will also need to check the end use of the fastener before selecting a lubricant. For example, stainless steel is often used in food related applications where some lubricants are unacceptable.)
Choose Coarse Threads Where Possible
Components with finer threads are more prone to galling because of increased surface contact during installation, which uncreases resistance, and generates additional heat and friction.
Use Cold Rolled Components
Lead screw image Galling metal is more common with lathe cut components because they typically have a rougher surface which creates more friction during installation. Deburring tools can be used to make the surface smoother and reduce friction. Cold rolled components are usually used for actuation applications, especially those that rely on the frequent movement of a Lead Screw. All of Accu’s Lead Screws are carefully cold rolled between dies to create a smooth surface to allow compatible Nuts to easily glide along the thread.
For the best ways to remove a component with a stripped head in addition to removing fasteners that seem like they are jammed in place forever, see our article on ‘How To Remove A Stripped Screw’. The article summarizes removal techniques such as additives that can be used to create more grip.
Purchasing Screws and other fasteners such as Nuts, Washers and Bolts from an ISO 9001 supplier helps to ensure the consistent quality of the threads. Threads which are dirty, damaged or rough add friction which increases stainless steel galling. Careful consideration should be given when choosing the mating threads. For example an austenitic Stainless Steel Bolt could be mated with a Duplex Nut, which makes the threads less likely to gall. Coarse threads or cold rolled threads should be chosen where possible so that there is less friction during installation. Another way to reduce galling is to use hand tools to slow down the tightening torque.