Thread friction

Several publications have described the use of molybdenum disulphide pastes for the lubrication of screw threads. This is an application in which the paste assists assembly and subsequently acts as an anti-seize. The critical factors in screw or bolt assembly are the friction between male and female thread and between bolt-head and workpiece, and their effect on the relationship between assembly torque and bolt tension (clamping force). The stiffness and integrity of a bolted assembly are determined by the bolt tensions, but since it is impracticable to measure bolt tension directly, the required bolt tension is controlled by the torque applied to the bolt or nut. However, the relationship between assembly torque and bolt tension is determined by head friction and thread friction. If the friction is reduced, the required bolt tension will be obtained at a lower assembly torque. 

Dimensional CFD Modeling of Flow Round a Threaded Friction Stir Welding Tool Profile, J. Mater. Process. Technol, Vol 169,2005, p 320-327... 

Problems can arise if thread friction is unknown - for example when the bolt has been cleaned or coated with an aerobic adhesive. In an extreme situation the use of an adhesive with a sufficiently different lubricating effect to machine oil could result in a given torque generating insufficient clamping force. The clamped parts might move under severe vibration even though the fastener itself would not loosen. 

Dry Lubricant. The static and dynamic coefficients of friction for the parylenes are low and virtually the same. This feature is an advantage in the use of a parylene coating as a dry lubricant on the bearing surfaces of miniature stepping motors. Coating a threaded ferrite core significantly reduces the abrasion to coil forms (82). 

When the mold is opened, the part should be easily removable. Cavities are made with a slight taper to reduce frictional drag of the part on the mold. The half of the mold attached to the movable platen is equipped with ejector pins, which push the part out of the cavity while the mold is being opened. When the mold is closed, the pins are flush with the cavity surface. Release agents or lubricants facilitate ejection and shorten the mol ding cycle. Some complex parts require that the mold open in several directions in addition to the direction of the platen movement. For a threaded part, eg, a bottle cap, part of the mold must be rotated to remove the article from the mold. 

Asbestos fibers have also been widely used for the fabrication of papers and felts for flooring and roofing products, pipeline wrapping, electrical insulation, etc. Asbestos textiles, comprising yam, thread, cloth, tape, or tope, also found wide apphcation in thermal and electrical insulation, friction products in brake or clutch pads, etc. In recent years, some of these appHcations have decreased to various extents, although others remain fairly active, typically in friction materials. 

In regard to screw threads, certain observations should be considered. First, the torque values are based on the coefficients of friction of the mating parts and can thus vary significantly. The use of any compatible lubricant that reduces friction will increase the shear and hoop stresses if the torque remains the same. Therefore, with lubricants, reduce the amount of allowable torque. 

Reynolds numbers from 10,000 to 25,000, at which strong evidence exists that under certain conditions, a viscoelastic fluid thread can interact with turbulence eddies and reduce the overall flow friction in the pipe... 

The dry material is readily ignited, bums very rapidly and is moderately sensitive to heat, shock, friction or contact with combustible materials. When heated above its m.p. (103-105°C), instantaneous and explosive decomposition occurs without flame, but the decomposition products are flammable. If under confinement (or in large bulk), decomposition may be violently explosive [1], An explosion which occurred when a screw-capped bottle of the peroxide was opened was attributed to friction initiating a mixture of peroxide and organic dust in the cap threads [2], Waxed paper tubs are recommended to store this and other sensitive solids [3], Crystallisation of the peroxide from hot chloroform solution involves a high risk of explosion. Precipitation from cold chloroform solution by methanol is safer [4], Water- or plasticiser-containing pastes of dibenzoyl peroxide are much safer for industrial use. 

Frictioning involves rubbing or wiping an elastomeric compound into a substrate of textile or metallic cords, which may or may not be held together by pick threads or fill yams, or the substrate may consist of a square woven fabric like hose ducks or belt ducks . 

Transition joints are used to join dissimilar metals where flanged, screwed, or threaded connections are not practical. They are used when fusion welding of two dissimilar metals forms interfaces that are deficient in mechanical strength and the ability to keep the system leak-tight. Transition joints consist of a bimetallic composite, a stainless steel, and a particular kind of aluminum bonded together by some proprietary process. Some of the types in use throughout the cryogenic industry are friction- or inertia-welded bond, roll-bonded joint, explosion-bonded joint, and braze-bonded joint. 

Where high shock conditions are likely to be encountered, some provision must be made to further secure the lead in place by supplementing the side wall friction obtd during consolidation. Scoring of the wall of the lead hole (See Fig l-70b) is the usual practice. It can be accomplished by tapping the hole and then passing a drill thru to remove the crests of the threads, unless very fine threads are used. This smoothes the roughness of... 

---