Tribology is the study of friction and wear. When two gears mesh their teeth slide upon each other. The wheels in a watch are designed to roll on each other to minimize friction, but their surfaces can never be perfectly smooth. Friction causes wear, and wear is also influenced by other factors like corrosion from humidity, salt, and chemicals. Mechanical conditions such as speed, load, and frequency of contact between the two parts will also determine the rate of wear. The case of a watch is designed to minimize any potential for for outside influences to impair the lubricants which would accelerate the wear of the movement. Wherever there is friction between two pieces there is never complete contact between their surfaces. There are always rough sections which reduce the actual point of contact to a small fraction of the total surface area of the contact patch. The ratio between to force it takes to slide one piece across the other and the force of the pressure pushing them together while at rest is called the coefficient of friction. When the mechanism is stationary and then slowly set in motion the coefficient of friction is called a static coefficient, and when friction is measured during continuous motion it's called a dynamic coefficient. The energy lost through friction breaks down lubricants. If there is a great deal of energy the temperature can increase depending on how poorly the metal conducts heat. The higher the temperature, the faster a reaction with outside agents can occur that leads to oxidation and wear. 2 extreme examples - a rusted pinion & corroded chronograph pusher: There are different categories of wear: abrasion, erosion, fatigue, chemical, and adhesive wear. In reality, multiple types can happen simultaneously or in succession. Adhesive wear is the most common in micromechanics. When there isn't enough lubricant, two surfaces can bond when they come to rest after being heated by friction. As soon as they return to motion again, the surface of one part may adhere to the other, or break off completely and foul the surrounding environment & lubricants. A good lubricant should have a number of important qualities: - chemically stable - non-corrosive - stable at low temperatures - non-volatile - resist creeping (remain in the same place it was applied) - maintain stability under any degree of humidity Choosing the correct lubricant basically depends on how much stress the pieces are under. A simple rule of thumb is that the more human interaction a part receives, the heavier the lubricant should be. The lightest duty oils are applied to the escapement, where the use of an unsuitable lubricant will impair the timekeeping of the watch. Heavier Olyt grease is used on the barrel walls. The winding system may require a few different types of oil depending on the specific movement. It is critical that the pieces be correctly lubricated - not enough and excess friction will occur; too much and it can contaminate other items in the mechanism. A surface treatment such as Epilame can assist in keeping the lubricant in place on faster moving parts such as pallet jewels and reverser wheels. Synthetic oils were first produced in 1950. They have been vastly improved over the years and now last much longer than their natural counterparts. Greases have undergone changes as well. Instead of adding fibrous soap to oil to produce grease, molybdenum disulphide powder (MoS2) is used to improve the lubricity and stability. If the oil dries out, MoS2 still retains it's low friction properties to relatively high temperatures, even up to 350 °C.