I purchased a compact ACME nut from McMaster, but I realized that there was no pressure rating for it on their website so I sent them an email inquiry. Here’s their response…
I spoke with our manufacturer and they do not have a static rating for the 12"-10 bronze Acme nut (95120A111). If you purchase the item and it doesn't work for your application, you can always return it for full credit.
Let me think about that failure mode… If the nut doesn’t hold up to the braking pressure the brake pedal will slide forward and since I’m strapped into a six-point harness I won’t be able to slide forward to even reach the pedal. This would likely be discovered during a panic stop so I might not be around to return it for credit. Yeah, let’s not try that. Nothing against McMaster, the tech person had no idea of my intended application.
Fortunately, McMaster offers a precision ACME nut with a 1,200 pound dynamic and a 4,000 pound static rating. Given that braking is a static activity insofar as the ACME nut is concerned, that’s an ~8x safety factor. The extra weight, size and cost are worth it.
The lead screw bracket was made from 1/4” right angle and welded 1/8” gussets. The flanged ACME nut (bronze) is mounted so that the flange is in compression when braking. In other words, the screws in the flange only come into play when the pedals are being adjusted towards the driver. Even if they fell out, both braking and forward adjustment would continue to work
Completed bracket. A hex nut was welded to the screw to allow manual adjustment (both are 18-1 stainless steel).
I mocked the adjustable pedal box with laser-cut plywood and 3D-printed gibs. Note the bracket and ACME screw in the upper right of the pedal plate.
I discovered that my pedal plate wasn’t wide enough to allow the right gib to clear the throttle pedal. This was the third or fourth time that I’ve tweaked the width of the plate. This is easy to do, but I also need to remember to change the dimensions of the bottom plate. As a software guy, hard coding interdependent values in multiple files just feels wrong. I was also getting tired of doing the math on the gib/plate tolerances every time I made a change. Fortunately, SOLIDWORKS provides an easy way to provide global dimensions, variables and equations that can be used across mulitple parts and assemblies. If I change one of the variables everything is automatically updated. This also makes it easy for someone looking at the design to understand the clearance between the gibs and the plates without needing to open up multiple files and do the math.
I’m still trying to figure out the best place to locate the gibs and the hole in the floor. I complied the list of responses which is shown below. Even for drivers of the same height there will be variances due to differences in inseam, seat recline, seat padding, etc. The 5’-11” driver with the 12” position seems to be an outlier. If anyone else sends me measurements, I’ll compile them.
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