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the self-locking and self-sealing precoating precote is a varnish-like coating for the most different types of threads. Its characteristics as a locking and sealing element only become effective by pressure and/or shear stress during assembly. After a brief period the threads are locked vibration-proof and reliably sealed. Screws and thread parts coated with precote are an integral locking element with all of the resulting advantages. The development of precote bases on the modern oT-microencapsulation technique, by means of which the widely proven adhesives for sealing, locking and retaining of threads are enclosed by a thin polymeric wall. precote is formed by incorporation of the microencapsulated single components of modified 2-component acrylate systems into solvent containing bonding agents. After application and drying of the liquid precote results the dry to touch and corrosion preventing precote layer.
Microencapsulated modified acrylates with their variable strength grades offer together with the specific precote bonding agents the possibility of adjustment to the special requirements of practice with regard to strength, coefficient of friction and temperature range.
Using reactive precote material a medium thread-tolerance 6g / 6 H is recommended. In consideration of the fact that the actual thread clearance deviate in a wide range and most time only the pitch diameter d2 is taken with a gage, we recommend to fill the threads > 60 % of the thread depth h with precote. This ensures a proper damage of the capsules and the curing of the coating. A too much coating over the crest may cause an undesired distribution and with small thread sizes a large deviation in torque values. An optimisation of the coating can be only realised with the knowledge of the actual thread combination and in cooperation with the user.
At threads ≤ M6 we recommend a thread filling > 80 % according already the smallest tolerance difference between male- and female thread affect the flange cover negative.
precote combines the advantages of the approved adhesive and the oT-microencapsulation technique. precote represents the modern problem solution in such fields of application, where until now the liquid adhesives have not been applied for safety and handling reasons. Threads coated with precote require no additional mechanical sealing and locking elements - the proper use or handling of which is decisive for efficient work - the assembly, purchase and storage of which cause additional costs - the locking and sealing effect of which is not reliable and disputed. From the physiological and toxicological point of view precote coating is harmless, as the liquid adhesives, partially classified as irritants, are completely isolated by the capsule wall.
Principally the assembly procedure of coated screws and thread parts is the same as with uncoated thread connections; only that adjusting and tightening operations should be completed within 5 minutes, as otherwise structural distortions of the polymer may occur.
The polymerisation of the released adhesive begins on assembly of the coated thread connections. As precote is a fast-curing system, already after 10 to 15 minutes measurable strength values can be achieved. At room temperature final strength is obtained after 3 hours. precote also cures at temperatures around the freezing point, but temperature should not fall below -20°C. However, lower temperatures extend the curing time.
precote can be stored for at least 4 years, if kept at a temperature of +10 to +28°C and a relative air humidity of max. 65%. A short exceeding up to 90% will not influence the product. Taking these facts thread parts coated with precote can be stored under the same conditions as uncoated thread parts.
The locking effect is based on the fact that the breakaway torque (MLB) of the screws locked with precote will be higher than the loosening torque (ML) of unlocked screws. The loosening torque of the unlocked screw corresponds to the tightening torque (MA) minus the double amount of the torque necessary to overcome the pitch of the thread i.e. ML = MA - 2 x Mpitch
a) when unscrewing the pitch must no longer be overcome: from this results 1 x minus Mpitch b) the unscrewing is in a way "downhill" and the Mpitch thus supports the loosening process: from this results again 1 x minus Mpitch. Instead of Mpitch often the term 'useful torque (MN)' is used in technical literature. The breakaway torque (MLB) now is a 'function' of the precote type, of the thread length and of the preload, and may also be as high as the tightening torque (MA) or even higher. The limit of the locking effect is, however, determined by the upper limit of the temperature suitability range. If this is exceeded briefly, the original locking effect is regained after cooling. Disassembly of threads coated with precote is always possible if the thread length does not considerably exceed 2 x d.
The precote coating is always a homogeneous and continuous surface coating. After curing of the adhesive the courses of the thread are hermetically and compulsorily sealed. After curing has been completed precote can withstand pressures up to 350 bar. By this sealing effect also corrosion in the thread connections can be avoided.
We dissuade from a reuse of thread connections coated with precote, because the conditions are quite indefinable. Highly stressed screws should never be used again even without precote layer. If, however, a further use of the thread parts is necessary, we offer the following two possibilities: a) cleaning of the threads and reassembly by use of anaerobic adhesives b) use of "new" unused screws coated with precote as substitutes. If the thread is adjusted somewhat (e.g. because of a subsequently required adjustment) after curing of the precote, the layer will be ruptured. This will naturally result in a loss of the previous locking and sealing effect, but due to the increased coefficient of thread friction value Gew the assembly will be protected against self-loosening.
precote offers a complete system for precoating screws and thread connections for locking and sealing purposes: - mechanical precoating, especially of large quantities by authorized screw manufacturers and coating processors - coating either by hand or by means of suitable devices, for which the experiences gained from the development of our own coating equipments are made available (oT-coating machine CS6). Especially to the manufacturers or users with large quantities of elements of always the same kind, such as screws, thread inserts, thread parts, etc., important advantages are offered by the precote system: - cooperation with the aim of solutions for specific requirements - modern and economic problem solutions in accordance with the latest technical knowledge. If you see potential applications for precote in your specific field, we shall be pleased to examine whether the necessary prerequisites are met. We only ask for an exact description of the planned application with the number of pieces and a sufficient quantity of samples to carry out trial coatings.
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