CONSTRUCTION FIXINGS A solution to a very galling problem
Rawlplug has recently developed the R-HPTII-A4 throughbolt which is specially coated to avoid galling. Below Rawlplug explains how it developed the coating and how it overcame the galling issues.
alling, a phenomenon where mating surfaces seize together is a common problem with stainless steel heavy duty anchors. The issue, although not always apparent, is inherent in stainless steel and other corrosion resistant alloys which produce a self-healing oxide layer. The mechanism of galling is adhesion between the contact surfaces on the atomic scale (when under high pressure) – a process occasionally referred to as cold pressure welding. A potential result of galling is seizure of the nut on the bolt when
applying the installation torque, which can lead to the fixture not being fully clamped to the base material. At this point installers generally over-tighten the nut in a futile attempt to clamp the fixture, as the outcome is usually a sheared bolt. Even if an attempt is made to unscrew the nut, it is usually not possible, resulting in an ineffective anchorage. The problem is not isolated to the bolt/nut thread interface where it is
easy to identify. It can also occur between the expanding collar of a torque controlled anchor and the bolt taper. This situation is potentially more serious as the expander then seizes on the bolt taper, resulting in limited expansion and therefore reduced tensile performance. Engineers at Rawlplug say that they have overcome the first galling
issue on the thread interface by utilising a molybdenum disulphide coating on the hex nut. This is highly effective because the molecules are arranged in layers, with the sulphur bonding very strongly to the base metal and weakly to other sulphur atoms. The combination of the laminar structure and the weak bond, effectively acts as a lubricant, whilst maintaining a crucial separating layer between the stainless steel nut and bolt. In partnership with a coating specialist Rawlplug’s research and development team in Scotland tackled the second issue by
developing a coating for the bolt taper. “We sought a solution which was fast-drying to reduce production costs and opted for a UV curable acrylic-based coating. The
coating has three functions: prevention of galling by creating a separating layer between the expander and bolt taper, control of the coefficient of friction to achieve the optimum expansion and finally, durability. It must withstand the same challenging environmental conditions experienced by the stainless steel during the lifetime of the anchor.”
Introducing the ATS EVO
Friulsider has recently introduced the new ATS EVO, a CE certified Option 1 (ETA 10/0423) anchor for cracked concrete.
barriers, ventilation plants and ceiling heating, suspended walkways, as well as various other applications. Friulsider underlines that the ATS EVO is the result of years of research and tests on various base
materials, coatings and geometric synergies between its components. The coupling between the cone and the anchor body of the ATS EVO ensures an optimal expansion in every application. Also, compared to the previous version, the coating of the cone is white and it has a special
formulation that guarantees higher hardness, higher strength and higher sliding. The clip of the ATS EVO is a single piece with three expanding segments; it has been improved compared to the previous version and the dent has been increased for higher performance. The special nylon bush shape prevents rotation and guarantees a better adhesion. The anchor is made with high tensile steel grade 8.8 to
provide greater shear strength. The ATS EVO pull out strength on cracked concrete is higher than the previous version on non-cracked concrete,
reckons Fruilsider, and it goes head-to-head with the main competitors on the market. Friulsider highlights that the ATS EVO’s points of excellence is installation flexibility, as compared to any other competitor it has minor edge distances (Cmin), minimum spacing distances (Smin) and splitting failure minimum
distances (Ccr,sp and Scr,sp). ATS EVO is high performing even on fire exposure and it has obtained the fire certification F120.
72 Fastener + Fixing Magazine • Issue 68 March 2011
he ATS EVO is a through anchor with high mechanical characteristics that can be used on cracked and non–cracked concrete and solid stone. It comes in three different versions (hex head, threaded bar with nut and countersunk head screw with embedded hexagon) depending on the installation needs, and can be used to install metal or steel structures, industrial signals, noise