necessarily represent the speed at the friction interface, as is evident in couplings, chains, and wire ropes.


Friction in couplings: High speed equipment, low speed contact Couplings are used to transmit torque between shafts while accommodating misalignment and vibration. Grid, gear, and chain couplings typically require lubrication, yet their friction behaviour is often misunderstood.


The primary friction points in valves are: 1. The actuation mechanism (threads, stems, or guides)


2. The sealing interface, typically metal-to-elastomer or metal-to-polymer


Both operate under boundary lubrication with short stroke lengths and high contact stress. In addition, lubricants must be compatible with seal materials, process fluids, and in many cases food or pharmaceutical regulations.


Although couplings are frequently connected to high-speed electric motors, the true friction does not occur at shaft speed. Ideally, both shafts rotate at the same angular velocity, meaning there should be no relative motion within the coupling. In reality, load variations, misalignment, and torsional vibration cause micro-slippage within the coupling elements.


This micro-slippage occurs at very low relative speeds, involves short sliding distances, is highly loaded, and operates in boundary lubrication conditions.


This explains why coupling greases typically use very high base oil viscosities, often exceeding 600 cSt at 40 °C, despite being installed on high-speed equipment. The lubricant is not required to form a hydrodynamic film but to protect against fretting, micro-welding, and wear under boundary conditions.


Additionally, coupling greases must resist centrifugal oil separation. High mechanical stability and oil bleed resistance are essential to prevent lubricant loss during operation.


Correct lubrication also requires proper application during assembly, ensuring grease reaches the internal friction points such as gear teeth, grid elements, or chain rollers.


Friction in valves: Low speed, complex compatibility Valves present one of the most challenging lubrication applications due to hidden friction points, very low operating speed, mixed material contacts and exposure to process fluids.


42 LUBE MAGAZINE NO.191 FEBRUARY 2026


Lubricants for valve applications therefore require excellent seal compatibility, high lubricity under boundary conditions, resistance to washout, chemical compatibility with process fluids and food-grade approval where required.


Perfluorinated oils and silicone-based greases with PTFE solids are commonly used due to their chemical stability and low friction coefficients. However, formulation balance is critical, particularly where silicone oils may affect elastomer integrity or process performance.


Valve lubrication is rarely a “fit-and-forget” application and must be evaluated case by case.


Friction in bolted joints: Where torque is lost Bolted joints are often assumed to be static once assembled, but friction plays a decisive role during both assembly and disassembly.


Only around 10% of applied tightening torque is converted into useful clamping force. The remainder is lost to friction, approximately 40% in the threads, and approximately 50% under the bolt head or nut face.


This means friction control directly affects joint reliability. Variations in surface condition, coatings, and contamination can cause large fluctuations in clamping force for the same applied torque.


Under high loads, microscopic asperity contact can result in micro-welding, leading to seizure and difficult disassembly. Corrosion and thermal cycling further exacerbate the problem.


Anti-seize pastes are therefore widely used in bolted joints. Their high concentration of solid lubricants:


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