What is Thread Galling, And How Can It Be Prevented?
What Does Thread 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 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 is much more severe because the machinery is dependent on the component’s movement.
For example, in a CNC machine, the Lead Screw moves the turret by a precise increment for every rotation of the screw. Therefore a galled thread means that the parts will no longer be machined to precise length and diameter dimensions.
What Causes Thread 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, resulting in thread seizure.
What Is Actually Happening When A Fastener Starts Galling?
Fastener galling occurs when friction causes the surface treatments, such as 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 for ways to prevent screws from sticking or seizing and suggestions that can help corrosion and galling resistance.
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 hardens at a different rate and has a different crystalline structure and microscopic surface structure.
Applications where the same alloy grades are fastened together can be more prone to galling, than those where different alloy grades or materials are chosen. 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 of the fastener 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, as opposed to using power tools. This significantly reduces the heat and friction that would be caused by using an impact wrench or drill.
- Choose Coarse Threads Where Possible
Components with finer threads are more prone to galling because of increased heat and friction is generated during installation.
Where possible, opting for coarse threads can help to reduce the likelihood of encountering thread galling within your application.
- 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.
Regularly cleaning the fasteners and fixings within an application will significantly reduce the risk of thread damage caused by metal-to-metal galling, whilst also ensuring optimal performance and longevity of the components.
- Use Cold Rolled Components
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. Precision Lead Screws are carefully cold rolled between dies to create a smooth surface to allow compatible Nuts to easily glide along the thread.
For more information, check out our article on the differences between Rolled Threads and Cut Threads.
- Using Lubricant To Protect The Threads
Lubrication for thread protection helps to reduce 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 unsuitable.
How To Remove Seized Fasteners
Removing a seized fastener from an assembly can seem like a difficult task, particularly if the drive of the fastener has also been damaged. This phenomenon is commonly referred to as a ‘stripped screw’.
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, please see our article on ‘How To Remove A Stripped Screw’. This article summarises removal techniques such as additives that can be used to create more grip.
Common Methods For The Prevention And Mitigation Of Thread Galling
Purchasing Stainless Steel Screws and other fasteners such as Washers, Nuts 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.
Our tutorial video, as found below and on the Accu YouTube channel, gives a brief overview of How To Avoid Thread Galling.
Careful consideration should be given when choosing the mating threads for the fasteners within your application. For example an austenitic Stainless Steel Bolt could be mated with a Duplex Nut, which makes the threads less likely to gall.
As mentioned above, coarse threads or cold rolled threads should be chosen where possible so that there is less friction during installation, with another way to reduce galling through the use of hand tools to slow down the tightening torque.