Online Community of Zenith Builders and Flyers
I wondered if a sheetmetal or a regular screw in a tapped hole would provide greater strength, as we do not like the sheetmetal screws Zenith wants us to use for or CH750's windshield.
We therefore conducted a pull test today, comparing a standard #6 sheetmetal screw with a regular #6 screw. I thought I share our results, as other builders might wonder the same.
The sides of the CH750 cabin frame are made of 0.035" 4130 tube. Since I did not have this material for our destructive test available, I used 0.032" 6061-T6 aluminum. While the absolute load the aluminum can take is significantly less than what the 4130 can withstand, the 'winner' of this test will also show a greater performance in the cabin frame's 4130.
For the test, I tapped the piece of aluminum with a #6 tap and also pre-drilled a hole for a #6 sheetmetal screw. In order to keep the piece from bending, I put it behind a small box wrench and pulled the screw with a big zip tie, which again was attached to a scale on which I pulled until the screw ripped out:
The sheetmetal screw ripped out at 103.2 lbs.:
The regular screw at 123.6 lbs.:
Based on these results, we will tap the holes for the screws that will hold the windshield in place.
I suspect that long term when you take into account vibration and corrosion you will find that the sheet metal screw will provide more holding power (better service) than a tapped screw in the light tubing. Dan.
I can possibly see that a sheetmetal screw might do a little bit better in the case of significant corrosion, but why do you think that vibration would change the picture?
I think that there is significantly less engagement in this application with a tapped screw and therefore is more prone to failure when vibration is present. You said that your hole was tapped with a #6. I assume it would have had been a 6-32 screw which will be the most common size. In a flat piece of material of .035 thickness there will be just over 1 full thread of engagement and the depth of the thread will only be as deep as the root of the thread. In a curved piece of tube there will be less full thread engagement. With a sheet metal screw it will slightly pull the tube to give more uniform engagement and will have slightly more root to crown engagement so in my opinion will be a stronger joint. As a side note to what John said about Zenith calling for locktite is that some plastics are affected by locktite so caution is necessary to not get it on the windshield. Dan.
I agree with Dan about the lack of sufficient threads - the sheet metal screw, however, more-or-less "wedges" itself into the hole and is not thread-dependent, and, the steel tubing will be more resistant to pull-out than the softer aluminum. That, coupled with Loctite, will make for a secure installation. Like I said, in my 6+ years/550+ hrs personal experience and in all the years I've moderated the forum, I can safely say there has never been a reported problem with screws loosening/pulling out on the side frames of the windshield.
Ditto about the Loctite - it crazes plexi "like crazy!" If you drill the plexi holes oversize and sleeve the screw with bushing made from something like soft vinyl tubing, you can put a drop of Loctite on the screw as you insert it into the bushing and that will at least keep it at some distance from the plexiglass as you install it.
Seems to me for your test to be valid, you need to secure the screws in the identical 4130 tubing, not aluminum. Also, it would seem that the windshield probably is pushed against the side frames, rather than being pulled away, but that's just my speculation.
I have an Ed.1 airframe, so the windscreen is different, but the attachment on the sides of the windscreen is similar. I drilled the plexi holes oversize and cushioned the hole with a bushing of soft vinyl tubing so the windscreen could "float" and decrease the likelihood of cracking. The plans called for putting a bit of Loctite on the screws and I put them up "just snug" and quit - no cracks! :>) 6+ years and 550+ hrs and not a single screw has ever moved. So, no problem there nor have I ever heard of any problems with this method.
However, I suspect there are lifting forces across the top of the windscreen, particularly in the later Editions with airfoil-shaped top windows. FWIW, I don't think it is relevant to later Editions (windshield/top window junction is completely different), but my Ed. 1 airframe's windshield joins the so-called "flat" window and both are secured to the spar carry-through. I suspect there could be lifting forces even with this configuration. I did worry that it might be difficult to adjust tension through the windshield plexi, the polycarbonate "flat" window, and the over-lying trim, so I installed riv-nuts in the spar carry-through. This allow rather precise adjustment of the tension and a little Loctite locks-in the adjustment. The riv-nuts allowed for even more threads than simply tapping the spar carry-through and nothing has ever loosened with time and hours.
I would expect that if a regular screw can transfer a higher force in one material than a sheetmetal screw, the same would be the case in a different material. The entire purpose of this test was to make sure that a regular screw provides the same strength as a sheetmetal screw, as I am not a big fan of sheetmetal screws.
is the regular screw secured with a nut on the end ?