A Close Look at Static Friction in Start/Stop Applications
In most systems, the critical factor is a lubricants ability to form a hydrodynamic film to reduce friction and protect against wear. This film forms at specific speeds, as described by the Stribeck curve.
However, many applications do not run continuously but operate with start/stop motions. The critical factors in such contacts are the force and energy required to overcome the static frictional resistance of the contact.
While low break-away force is preferred in most cases, a certain amount of resistance is usually required to prevent unwanted movements, depending on the specific requirements of the application.
A suitable method for determining static friction is to continuously increase force until relative motion is detected between the contacting surfaces. Precise measurements of static friction require that low values of force, speed and deflection be applied and measured with high resolution. Typically, special equipment is needed to make these measurements, and high-precision measuring devices, such as those used in modern rotational rheometers, provide the resolution and accuracy required.
The continuous increase of force and controlled increase in speed after entering the dynamic regime allow both static friction and film forming properties to be characterized, as shown in the graph for two greases. The plot shows that friction coefficient increases steeply during the initial phase of the speed ramp for both greases. Their peak values represent the limiting friction coefficient for each system.
As shown, grease 2 has a higher value of limiting friction, about 0.175, compared to grease 1, with a value of 0.125. This implies that the force required to overcome resistance is higher for grease 2. The curves also show that the friction coefficient for grease 2 is more strongly dependent on rotational speed than for grease 1.
The tests provide accurate, reliable and reproducible characterization of the limiting friction of greases. They can be used to pre-screen greases prior to component or real life testing. In addition, the principles can be applied to other tribological challenges to demonstrate the impact of lubricant, material and operating conditions on lubricant performance.
Frederik Wolf is Product Manager Tribometer at Anton Paar Germany GmbH, Ostfildern, Germany. He can be contacted at Frederik.Wolf@anton-paar.com.
Kartik Pondicherry is Product Specialist Tribometer at Anton Paar GmbH, Graz, Austria. He can be contacted at Kartik.Pondicherry@anton-paar.com.