Systems to ensure the preload SH and TSH
Bolted joints are essential for engineering applications as diverse as wind turbines, oil rigs, combustion engines, automobiles, cranes, aerospace, nuclear, marine, rail, etc., that must endure the most severe working conditions during its effective lifetime.
These conditions vary constantly, the efforts are not uniform, nor are the temperatures, weather conditions, marine or on land, etc., So that bolts and nuts are designed to withstand the maximum loads that may occur in the application , whether continuous or occasional, regular or extreme situations, during the lifetime of the product in which they are mounted.
These joints are calculated with the load conditions in sophisticated software programs. Components are also tested in laboratories or in testing to failure.
The most commonly used screws are hex heads that are preloaded by applying a torque via pneumatic, hydraulic, electric or manual tools.
During this process, the bolt preloads and stretches, causing it to act like a spring that presses the pieces together, so the torque applied is of vital importance. This torque will cause the preload applied to the screw to be equal to that calculated for the binding, otherwise the joint will not work properly and end up deteriorating or breaking.
In the application of torque causes the friction to be involved, this friction between parts of the joint generates dispersions in the transformation of the torque to preload. This dispersion can be minimized by the lubrication of the joint components: bolts, screws, nuts, washers, etc., but still and under normal conditions, the dispersion can be up to 25% of the nominal preload calculated.
We can deduce that if we are able to control the dispersion of the preload, we will increase the security and integrity of the bolted joints.
TSF has patented a solution to ensure the preloads and control the dispersion in the joints obtaining preload variations below 5%. These systems to ensure the preloads are called SH and TSH in its different variants.
Systems to ensure preloads SH and TSH distinguish for their simplicity and ease of use, improving preload application performance by torque or through puller already existing in the market.
In our patented systems to ensure the preload SH and TSH, it is generated in a hydraulic puller designed specifically, defragmented and delivered to the joint by strength transmitters across the parts involved in such joint.
The preload is controlled by the pressure applied to the puller 95%, 90%, 85%, etc., of the elastic limit for each screw and supported by shims or locking bush, according to the variant used of SH and TSH. As the friction is not involved in obtaining the preload, only the relaxation will affect between the components of the joint.
Due to the design of our SH and TSH we can, doing the correspondent protocol, know the absolute elongation of the joint, that is, the total elongation minus the loss due to relaxation of the components of such joint. Through standard metrology systems (caliper, palmer, etc.,) we can obtain the absolute elongation value and therefore, know the preload of each of the bolts, also at the same time; we have unitary traceability of all the bolts constituting the joint. As we have explained, to know the bolt preload is not necessary to use current sophisticated systems such as ultrasound, etc., although, due to the geometrical characteristics of our SH and TSH, those can also be used.
Once assembled the joint and with traditional metrology media, we can in time, check again the elongation to control and verify the sustainability of the preload, allowing us to take maintenance actions if needed, based on precise measurements of the elongation.
When pulling with our systems to ensure the preload SH and TSH, we eliminate the twist in the tying up element and we achieve insensitivity to bending moments, so as pitch and type of thread of the bolt by not intervening friction in tightening the union.
Is noted in the SH and TSH systems to ensure the preload, the ease of assembly and disassembly of the system and the quick achievement of preload and its reset using the same protocol, which at the same time is the same one for all types of bolts in their different metric sizes; this ease of use allows multiple reuses of the parts that constitute the joint. The preload is defined in pressure-preload tables specifically for each metric size and quality of materials.
By not intervening in the SH and TSH systems the friction, we get a potential increase in preload, high stability and low dispersion, which allows us to place fewer tying up elements than in joints assembled by torque control, or we can also use the same number of tying up elements, reducing the metric size of the bolts to use, obtaining the corresponding savings in machining, assembly time, maintenance, etc.
The field maintenance is drastically reduced due to the security of preload achieved, but whenever needed to do it, it is very easy because no special hardware is required for it, a manual hydraulic pump and a puller are enough to perform these tasks.