This (RNZ) is the style that I have sold and used for around 25 years with Matrax runners.
RNZ 2716 is stainless steel 5/16-18 thread
QN 2716 would also work; it is a style with the 4 projections that distribute the load better than RNZ and I see now it is available in stainless as QNZ 2716. http://search.buckeyefasteners.com/catalog3/d/buckeyefasteners/?c=products&cid=aa-weld-nuts&id=QNZ_2716
QNM 080125 is the metric thread version, no four tab metric stainless that I can find here.
How they should be used on Matrax aluminum runners:
The weld nut is installed with the nipple (dimension F) up in the lower track of the aluminum runner. The corresponding bolt or machine screw is attached through a hole in the web of the aluminum runner. If a new hole is drilled it must be done carefully to avoid nicking or gouging the load-bearing flange of the runner.
High strength aircraft alloy aluminum is extremely strong for its weight but is sensitive to cracking, crack propagation, and breaking at stress raisers: discontinuities, sharp corners, cuts and notches. Metal fatigue is an another important cause of failure in high strength aluminum after thousands or hundreds of thousands of loading cycles.
Cracks typically propagate under tension. In some structures a part of the material may be loaded only in tension or in compression, or there may be cycles of only stress or compression, or cycles of stress reversal, that is, alternating tension and compression. If there is only compression in an area or member of a structure that part can better tolerate the existence of notches, deep scratches or similar defects. So, an aluminum runner with a notch in the top/inside forward part of the runner curve or in the tail on top behind the foot board will be unlikely to break. Other low stress or compression-only areas can identified depending on the sled design and the loads applied/experienced. That is the field of stress analysis which in which I worked during my brief career at Boeing on the 747-C project.
*Read about the catastrophic De Havilland Comet aircraft disasters:
“Stress around the window corners was found to be much higher than expected, while stresses on the skin were generally more than previously expected or tested. This was due to stress concentration, a consequence of the windows’ square shape, which caused levels of stress to be two or three times than that across the rest of the fuselage.“
The primary stresses/loads on a well-designed sled runner, wood, aluminum or any other material, are a result of bending when the runner is high centered coming over a bump on the trail, or bridging between two bumps. Under bending loads the inside surface layer or skin of the material being bent is in compression and the outside is in tension. Crashing into a tree or obstacle will also result in bending of the runner but other structure members such as stanchions that are truss elements experience axial compression or tension loads. Many fail at the attachments. A well-built sled with aluminum runners is most likely to have the runners permanently bent without any fracture/breakage.
The web of the runner profile is the unstressed neutral plane in bending up and down as well as sideways; it was designed for this, therefore holes in the web are not a problem. In the web of I-beams and similar profiles there may be large holes that are machined to remove unnecessary material and save weight. They are called lightening holes. The joke in conversation with non-engineers is that they are created to allow lightning to pass through.
The length of the bolt or machine screw is critical so that it is fully engaged in the weld nut but does not project so far through it to interfere with the plastic rail or the plastic runner dovetailed into the track.
It is safer to have/use a plastic strip between the angle or channel or other part attached on top of the runner and the aluminum runner top flanges. This will protect the stressed top flanges from damage from fretting and wear if anything loosens and moves under loads. It is not a good design to clamp a straight length of angle or other stiff component on the top of a curved part of the runner. The only exception is at the front of the runner where there are no large bending stresses.
The nipple (dimension F in the diagrams) should be up and when tightened it will be pulled into the hole (23/64″ or 11/32″) in the web of the aluminum runner. The weld nut is deformed when sufficiently tightened. The weld nut will remain bent/deformed if the bolt/machine screw is removed; the weld nut will stay in place for repairs or replacement of parts attached to it.
It is possible that 8 mm metric thread fasteners can be used with the US thread 5/16-18 weld nuts. 8.8 is a harder stronger metric fastener. It would be ugly if the machine screw snapped off in the weld nut. High grade high strength metric or US thread (grade 5, grade 8) bolts/machine screws must be used that will strip out the weld nut thread (fail-safe mode of failure) before they break.
Although the metric threads are not identical to US there is enough tolerance for the screw to fully enter the weld nut. It will be tight! Be sure that the tightness or torque used to tighten it is more than what is needed to overcome the misfit interference in the threads so it is also sufficient to clamp the parts together and avoid any movement in use/operation that could damage the flanges.
Another thought: If zinc or magnesium strip, tabs or washers could be attached in several places in contact with the aluminum runners that would serve to protect from corrosion especially in salty wet environments. Sacrificial anodes are used for this purpose in many marine applications (ship or boat hulls, outboard motors.) I have not yet found any good source.
See also: https://wordpress.com/post/everythingiknowaboutthatilearnedfrommysleddogs.wordpress.com/1511
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