Let torque transfer from the tripod into the aluminium spindle without wearing the spindle.
A stainless steel sleeve inside the outboard tripod housing, called the tripod insert. It is a sacrificial hardness-matched wear surface between the hardened steel tripod rollers and the soft aluminium spindle.
Material was selected on hardness match to the rollers. RCV FSAE tripods are hardened 4340 steel verified at roughly 60 HRC. 304 stainless, used previously, is 70 HRB and reaches 35 HRC maximum after annealing and cold rolling, with 31.2 ksi yield. 4340 steel matches the roller at up to 60 HRC and 125 ksi yield but needs a complex heat treating process. 440C stainless can be hardened up to 57 HRC with 65.3-276 ksi yield and a simple heat treating process, and is the highest-hardness martensitic grade of stainless. 440C was selected: slightly softer than the roller after heat treating, with a simple process.
Heat treatment validated by testing three inserts from the batch with three samples each; average hardness 60.0 HRC.
Heat treatment per Practical Heat Treating, 2nd ed. (Dossett): heat to subcritical annealing temperature and air cool, heat to hardening temperature and oil quench, heat to tempering temperature and air cool, then a second temper and air cool. From the design reports and one-pagers: Machining CV inserts needs at least a set of V-blocks wide enough to take the stainless stock. Depending on stock width, the existing workholding runs four inserts at a time. Clamp the stock very tight in the vise with a parallel under each piece so they all sit at roughly the same height. Use at least a 5-flute end mill; the CR22 report records the longest tool life with these. HSMAdvisor is the recommended source for feeds and speeds. The report is explicit that the manufacturing process is critical to the longevity of this part.
440C stainless steel, heat treated to approximately 60 HRC.
Fault Vault, 2023-07-17, CR22, Manufacturing, Compromised Failure. The right halfshaft was moving and making a strange noise. The right rear inserts were wearing, the stainless steel insert had grooves where the tripods were hitting, and the tripods were binding and shifting in the spindle because the inserts were too small. The trackside approach was to pack them with grease and reinstall the tripods, which only partly relieved it. That entry is the origin of the material change. 304 stainless inserts (70 HRB, 35 HRC maximum) were far softer than the hardened 4340 tripod rollers at roughly 60 HRC. Thickening them eliminated ultimate failures but they still pocketed and wore. CR25 moved to 440C stainless heat treated to a measured average of 60.0 HRC, verified on three inserts from the batch with three samples each.
Material changed from 304 stainless to 440C stainless heat treated to a measured 60.0 HRC, matching the hardness of the 4340 tripod rollers. Insert thickness had already been increased within the existing spindle geometry on the previous car, which stopped ultimate failures but not wear.
Nothing in CrimsonStorage covers these yet.