Stick-Slip Effect

Causes are mutual hooking between the microstruc­tures of the body surfaces on the one hand, on the other hand inter­mol­e­cular forces between the bodies (adhesion). In addition, the stick-​​slip effect also depends on the sequence of motion of the bodies in contact

The stick-​​slip effect propa­gates via structure-​​borne sound – depending on the damping of the material pairing – and can lead to very unwanted noise, for example in vehicles. We know this from creaking and squeaking doors, rattling windshield wipers or creaking noises in vehicle interiors. In addition, stick-​​slip, much more than uniform friction, can lead to increased wear and material fatigue, and can also completely stop the slightest movements in precision machines, threaded spindles and other drives.

However, the stick-​​slip effect also has a bright side. Without stick-​​slip, there would be no string instru­ments that bring us the highest listening pleasures in music.

Just as the physical properties of materials can change due to the influence of temper­ature and humidity, the climate also influ­ences stick-​​slip behavior. This often increases at higher temper­a­tures, as in the case with leather and artificial leather in contact with itself or with hard plastics.

Other polymer materials, such as polyamide (PA), react to high humidity with an increase in the stick-​​slip effect. Compo­nents made of elastomer materials, such as seals in combi­nation with glass or painted surfaces, are also prone to stick-​​slip-​​induced vibration and squeaking when exposed to high humidity.

Factors such as relative velocity also influence noise behavior. Solids with soft surfaces have a higher tendency to stick-​​slip at low speeds, while many hard plastics exhibit the opposite effect. Counter­mea­sures must then be adapted accord­ingly.

The impacts of the stick-​​slip effect with regard to noise gener­ation are always dependent on the natural frequency of the component and thus also on its mass.

In material devel­opment, the avoidance of stick-​​slip is therefore a growing aspect of material require­ments.

Now the question arises, can stick-​​slip be measured?

If so, how?

We developed and launched the first stick-​​slip test rig 20 years ago using a patented test method. In this method, two materials are brought into contact with an adjustable normal force and mutually moved trans­versely at a speed that is also adjustable (see sketch of the operating principle). The static friction deflects a flat spring in the direction of the movement.

At some point, the restoring force of the spring exceeds the static friction of the two materials, and there is a sudden transition from sticking to sliding. Energy is released in that process. We determine this energy with the aid of an accelerometer.

It is not only in the automotive industry where materials get into contact with each other as a result of different vibra­tions and at different temper­a­tures, which can lead to a stick-​​slip effect and the associated anoying­noises.

Annoying noises such as squeaking, creaking or cracking are undesired just as stagnant motion sequences and the resulting malfunc­tions. This statement applies to almost every appli­cation and requirement, and that is why we work together with a wide variety of indus­tries.

In passenger car interiors material pairings of genuine or imitation leather are often found – not only in the luxury segment – which are frequently respon­sible for annoying noises. The resulting customer complaints and the associated high costs for car manufac­turers led the VDA (German Associ­ation of the Automotive Industry) to issue a test standard (VDA 230/​206) in 2005 to address precisely this problem.

Most German and numerous global automotive manufac­turers have adopted this VDA standard 230/​206 in their test speci­fi­ca­tions with the aim of ensuring that their suppliers supply stick-​​slip-​​free materials. Leather, for example, can be optimized by a modified finish. In the case of artificial leather, this is achieved by a modified material formu­lation.

In the case of household appli­ances, the stick-​​slip effect can also be reflected in the form of unpleasant noises, which is partic­u­larly critical in private homes. For example, fridge-​​freezers can make such loud cracking noises that the residents get frightened or even their sleep gets disturbed.

In our laboratory we can perform various tests using specially installed structure-​​borne sound sensors and micro­phones, so that the exact area can be localized as the source of the noise.

In the next step the causal contact points are located in order to find out where the relative movement comes from, which is respon­sible for contact points causing noise at all.

The stick-​​slip effect can also occur in lamps, which is manifested by creaking, squeaking and also cracking noises.

They are caused by temper­ature changes in the lamps, which entail constant material expansion and shrinkage. This causes relative movements due to incom­patible material pairings.

A good example are insulin pens as aids for diabetics to admin­ister a precisely defined amount of insulin easily and accurately.

The rotary knob for dosing has a detent mechanism with acoustic and tactile feedback so that people with visual impair­ments can accurately dose the amount to be admin­is­tered. The sliding phases in the detent mechanism must not be inter­fered by anything in order to avoid operating errors that would have fatal conse­quences.

In our example, incom­patible material combi­na­tions within the insulin pen caused stick-​​slip and impeded acoustic and tactile feedback. We were able to determine the critical material pairings by measurement and success­fully assist our customer in finding alter­native materials.

Lubri­cants are often used to reduce or even eliminate the stick-​​slip effect between two compo­nents or materials.

Unfor­tu­nately, it turned out that under various contact condi­tions, certain lubri­cants either do not help or even make the situation worse due to insuf­fi­cient lubri­cation. The root cause is on one hand, the lubricant does not match the materials of the friction partners, and on the other hand, it does not match the contact situation.

In our physics laboratory we are able to use the H3P test rig to measure the stick-​​slip effect between material pairings under the influence of lubri­cants. Thus we achieve an important contri­bution to avoid annoying noises and optimize abrasion and wear properties.

Stick-​​slip effects can also lead to restric­tions in the customer’s positive product perception on bicycles, for example when pedaling. By using our test rig, respon­sible contact points can be identified, such as the sprocket wheel in contact with the chain. In combi­nation with our experience a suitable coating for the sprocket wheel can be deter­mined that prevents stick-​​slip in a sustainable and long-​​lasting manner and restores the positive product perception of the end customer.

Every day, we encounter situa­tions in which the stick-​​slip effect can occur. In Germany, people sit for an average of 7.5 hours a day. Not only when sitting down, but also when sitting up, standing up or doing a little work out in the office chair, squeaking, creaking and cracking noises can occur. That is caused by the horizontal force induced by the user’s own weight.

During movement, the spring force in the office chair make some materials rub against each other, resulting in noises. The stick-​​slip effect can also occur when the chair surface gets into contact with trousers (e. g. fabric on leather)

In guides, bolted joints and other situa­tions where solids are in contact and induce relative motion, the stick-​​slip effect can result in undesirable effects. These can be annoying noise, vibration, inter­fered precise movement and increased abrasion and wear.

For example, the ship’s shaft bearing of a submarine may be affected, which generates signif­icant vibra­tions due to the stick-​​slip effect and would therefore be easy to locate in a serious case while under water. The tests that we can perform using our test rig on separated samples with the addition of seawater enable us to simulate the situation during cruising and create a solution

• Reduction of static friction by lubri­cation
• The addition of certain additives can influence the sliding properties of plastics
• Decou­pling of the contact by means of sliding-​​optimized films or by felts
• Changing of the contact area and thus of the adhesion
• In case of contacts between hard and soft materials the adhesion can be strengthened by specific modifi­cation of the contact geometry in such a way that relative movement is excluded
• Relative movement, and thus stick-​​slip, can be achieved by a form-​​fit or even material-​​fit connection of the compo­nents