Whenever two poorly lubricated metal surfaces come into contact, you have a strong chance of galling damage. Galling is a kind of cold-welding process in which two metals that are in contact with one another eventually adhere to one another. Galling is especially troublesome for titanium, aluminum, and stainless-steel fasteners.

A protective oxide layer provides some galling prevention, but the tighter the connection, the more likely the oxide layer will become damaged and enable galling to begin. The following is a closer look at common causes of thread galling on stainless fasteners.

Torque and Galling Effects

The coefficient of friction between stainless steel fasteners is higher than most other combinations, requiring more torque to achieve the proper clamp load. If a high clamp load is desired in stainless steel fastened joints, often galling will occur before enough clamp force is produced, which is not always clear to the assembler. Two things can occur, either we have a low pre-load, or the bolts will twist off.

High Temperatures and Galling Effects

Service conditions can cause additional challenges for stainless steel joints, for example, high temperatures and/or exposure to aggressive gasses which are prevalent in applications such as gas turbines or diesel engines.  Thermal expansion can cause permanent distortion while heat and gasses can cause scaling on the surfaces making disassembly difficult during service or repair.

How to Counteract Galling

Lubrication added during assembly is one method of minimizing the galling effect of the threads in stainless steel joints. Molylub, with solid molybdenum disulphide particles or other solid film lubricants containing silver, aluminum, or copper may also be used. Lubricants containing graphite are not advised due to reactions between carbon and chromium at higher temperatures.

Teflon sealing tape may also be used for larger threaded components such as pipes and valves.

Pre-Applied Coatings

To avoid the mess of traditional lubrication, and the possibility of forgetting to apply it, a pre-applied lubricant may be the best option for galling prevention. Wax or similar topcoats may be applied with a dip-spin method or more advanced tribological coatings such as Bossard ecosyn^®-lubric can be used. Tribological coatings are highly engineered to reduce wear and maintain consistent friction which helps to achieve a consistent joint pre-load.

Interested in learning more about galling and how to properly tighten stainless steel fasteners? Email us at ProvenProductivity@bossard.com to get in touch with a Bossard Engineer.

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All design engineers are faced with questions like “What torque is the correct torque?” and “How do I know my assemblers are achieving the correct torque?”. When Bossard engineers hear these questions we typically respond by asking “What clamp load are you trying to achieve?”. When asked, engineers usually reply with confused expressions, or the unveiling of a torque chart from engineering manuals or colleagues.  There is no doubt time and effort went into these torque charts, but how does the testing to create the chart relate to the application the engineer is working on?

Bossard’s biggest piece of advice is to test, test, test! The approach that yields the most success and maximizes the strength of fasteners is to test the application until failure. Until you know the point that the current fastener fails, you are throwing darts at your target clamp load. In some instances, friction varies from lot to lot and can affect the clamp load enough resulting in loose fasteners. Testing until failure helps you achieve a baseline average of total fastener strength. This baseline encompasses friction and is incontestable. The only thing left for the engineer to decide is the percentage of the baseline that is needed for the application. Typically, 75% of the failure torque is used for assembly but there are times in critical applications that fasteners are needed to be pushed further.
When investigating current torque strategies at your company don’t be afraid of torque charts. They can often be used as a great starting point. When a clamp load needs to be met to hold something together, challenge the tightening strategy and test it to prove its validity. You will never regret testing the actual application.

Have more questions about clamp load and proper torque? Reach out to Bossard at ProvenProductivity@bossard.com to receive personalized advice from one of our Application Engineers!

Ben Oostdik
Application Engineer
boostdik@bossard.com

Throughout a typical day, our engineers are asked at least once, “What torque should I use?” for any given size and application. This question is very loaded and requires many details to arrive at a hypothesized torque. The torque is considered hypothesized until testing is performed on an actual application. Until it is tested, it is not confirmed.

The factors that govern any “proper” torque strategy are the strength of the fastener, the surface pressure limits of the mating material, and the friction under the head and in the threads. The ultimate goal of torque is clamp load, so these factors essentially influence total clamp load. Total clamp load, or preload, in most applications is required to keep anything together.

The most common method of finding the proper torque is looking it up on a torque chart. Most companies have some sort of organized torque chart based on size and some refer to the surface finish of the fastener. This is an attempt to consider the friction of common fastener finishes. Our engineers would rate this method very low on an accuracy scale when a specific clamp load is desired. The method we recommend to any design engineer requires a load cell and measuring of the clamp load while tightening. This confirms the design and the engineer’s desired clamp or preload. The torque to get to this clamp load will be the proper torque.

When friction varies and the clamp load needs to be achieved every single time (like a head bolt on an engine) a torque turn method is the proper choice. This method takes a bit more testing to be repeatable but is very accurate. By testing several joints and measuring the clamp load and torque simultaneously, a snug torque can be acquired and then an angle-of-turn added. The snug torque may only be a tenth of the final torque but the angle-of-turn can and will produce accurate and repeatable clamp. For more information on this method, and help determining this angle, contact us any time at ProvenProductivity@bossard.com.

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