Load Bearing Parts
Greetings Folks,
As the title suggests, I’m working on a project for some load bearing components. Specifically, I’m working on a design for some ‘frame rail guards’ for a heavyweight motorcycle. The bike weighs in the neighborhood of 700 lbs. (318 Kg).
The motorcycle has a pair of shock absorbers mounted longitudinally between the frame rails, forward of the rear wheel. The shocks protrude slightly below the frame rails so some type of spacer must be utilized to prevent damage to the shocks when lifting the bike from beneath the rails.
There will be (4) pieces in total which will ‘snap’ onto the bottom frame rails of the bike to provide a flat, protective surface for a lift to engage in order to raise the bike off the floor for service. Two of the sections will be 182 mm long and two will be 202 mm long.
When fitted to the rails, there will be one long and one short section on each rail for a total length of 384 mm end-to-end. The lift is about 250 mm wide so it will bridge the two sections on opposite sides of the bike when engaged.
The design requires each of the rail guard sections to provide clearance for a cross-beam. This will appear at the end of each section and by printing the parts vertically, will not pose any issues. Two of the rail guard sections additionally require one or more relief cuts to provide clearance for zip-ties and other obstructions.
My printer is a Prusa MK3S+ with a 0.4 mm nozzle. Firmware is 3.11.0-4955. PrusaSlicer is 2.5.0+MacOS-x64. Given all the (recent) thermal anomaly reports, slicer hiccups, etc., I am disinclined to ‘update’ either the FW or PS at the moment, as I have a stable, working setup.
I’ve printed several test parts to insure proper fitment and clearance locations. Initial parts were printed using 0.3 mm draft mode with 2 perimeters and 5% infill. Sections of parts for final fitment were printed using the attached .3mf file with 100% infill, so I am confident in the final design(s) with respect to proper fitment.
Below is the zipped .3mf file of a section with a relief cut and my slicing parameters for review:
Before I begin a long and material intensive set of prints, I have several questions for any of you with experience designing and printing load-bearing parts:
Is PETG appropriate for this use case? I have plenty of it on hand and am very familiar printing with this material. If not, why, and what is recommended instead? I do not have an enclosure but could cobble something together if necessary.
Are 7 perimeters appropriate for this model? Overkill? Underkill? I also sliced a model with 10 perimeters which gave a nice contact patch between the bottom of the curve and the flat bottom of the part, with no infill at that location. Too much?
Is the default 45-degree infill angle acceptable or is there a better solution? Would 90-degrees provide better stability and less chance of deformation under weight?
The section of the model I provided in the attached file includes a relief cut for a zip-tie. Printing this part vertically requires support material in this area. It would be nice if it was ‘reasonably’ easy to remove after the print has cooled. It doesn’t have to leave a pretty face behind, only one that is dimensionally accurate to the design. Any alternate support suggestions?
Is a brim recommended? I think this is an obvious ‘yes’ given the cross section of each part is roughly 28 mm x 48 mm and the print heights will be 182 mm and 202 mm. Probably foolish not to. I only ask because I don’t have any trouble with PETG sticking to the satin build sheet and I rarely use a brim. Given that, I don’t have a lot of experience or knowledge re ‘best practices' for a brim. Any suggestions for brim size and type?
Apologies for the longwinded post. I wanted to try to get all my thoughts down in one place and provide (hopefully) adequate detail. Any and all questions and suggestions are welcomed. Thanks in advance.
Cheers,
Randy