By Geza-aka-Zombo - Sun Oct 18, 2020 7:24 pm
- Sun Oct 18, 2020 7:24 pm
#26439
At the risk of giving my engineering secrets away, let me speak to ATI damper hub materials. Steel is a good choice as it has good vibration transfer properties (low internal damping), is inexpensive, and is conducive to tapping (for mounting hardware) with little concern for striping threads. Of course corrosion is an issue, so proper finishing is required. Because steel is 3X stiffer than aluminum, less an interference fit between the hub and crank is required, as steel clamps more than aluminum for the same interference. This actually turns out to be a minus as I’ll describe later. Another drawback of steel is that it is a poor conductor of heat (low Kt) and has a low CTE (coefficient of thermal expansion) – these 2 properties make it necessary to excessively heat a steel hub for a long period of time to insure it expands sufficiently to slide on the crank. Cooling the crank (with ice, for example) does not help – putting the crank in the freezer for several hours, or liquid nitrogen is another story, obviously not practical.
Aluminum has a high CTE and Kt, so it will heat more quickly and expand more with a lower temperature increase. Both good properties, especially since the aluminum hub needs to expand more given it requires a larger interference fit to compete with steel’s clamping force. Being corrosion resistant, the 6061-T6 aluminum doesn’t even need a finish; though the anodize I specify brings it up a notch. Tapping directly into the aluminum (without an insert) was a concern I ultimately set aside, as the threads are quite large (especially compared to some of the things screwed into the bare aluminum 928 block casting) and the bolt torque is fairly low. ATI makes hubs using steel and aluminum, so both work fine.
As alluded to above, the specifics of the interference fit is a major consideration and the reason why I selected aluminum. Because the steel requires a smaller interference fit (i.e. 0.0005 to 0.0010”), it is much more difficult to pull off than aluminum, which has a larger interference requirement (i.e. 0.0015 to 0.0025”). Without getting into too much detail, the crank nose diameter has a manufacturing tolerance (assume +/- 0.0002”) – the hub bore also has a similar tolerance. The lower the amount of interference between the 2, the more tightly tolerance the hub bore needs to be to meet the interference fit requirements across all possible combinations of crank nose and hub bore sizes. In a nutshell, the aluminum hub has more leeway because it has more of an interference. Since those designing these hubs don’t have access to Porsche crankshaft machining drawing (at least I don’t), there is too much of an unknown as to the actual crank nose diameter and tolerance – I went the more conservative way – guaranteeing there would always be a sufficient interference, regardless of what the 2 parts measured. ATI, having access to more definitive data on the cranks they design their hubs around, is in a much better position to engineer the lower interference steel design.
Aluminum has a high CTE and Kt, so it will heat more quickly and expand more with a lower temperature increase. Both good properties, especially since the aluminum hub needs to expand more given it requires a larger interference fit to compete with steel’s clamping force. Being corrosion resistant, the 6061-T6 aluminum doesn’t even need a finish; though the anodize I specify brings it up a notch. Tapping directly into the aluminum (without an insert) was a concern I ultimately set aside, as the threads are quite large (especially compared to some of the things screwed into the bare aluminum 928 block casting) and the bolt torque is fairly low. ATI makes hubs using steel and aluminum, so both work fine.
As alluded to above, the specifics of the interference fit is a major consideration and the reason why I selected aluminum. Because the steel requires a smaller interference fit (i.e. 0.0005 to 0.0010”), it is much more difficult to pull off than aluminum, which has a larger interference requirement (i.e. 0.0015 to 0.0025”). Without getting into too much detail, the crank nose diameter has a manufacturing tolerance (assume +/- 0.0002”) – the hub bore also has a similar tolerance. The lower the amount of interference between the 2, the more tightly tolerance the hub bore needs to be to meet the interference fit requirements across all possible combinations of crank nose and hub bore sizes. In a nutshell, the aluminum hub has more leeway because it has more of an interference. Since those designing these hubs don’t have access to Porsche crankshaft machining drawing (at least I don’t), there is too much of an unknown as to the actual crank nose diameter and tolerance – I went the more conservative way – guaranteeing there would always be a sufficient interference, regardless of what the 2 parts measured. ATI, having access to more definitive data on the cranks they design their hubs around, is in a much better position to engineer the lower interference steel design.
Geza
'87 Porsche 928 S4 Auto (3rd owner)
Original owner of:
'97 Porsche 986 5 Speed
'99.5 Audi A4 Avant 2.8 5 Speed
'16 Ford GT350 Track Pack
'87 Porsche 928 S4 Auto (3rd owner)
Original owner of:
'97 Porsche 986 5 Speed
'99.5 Audi A4 Avant 2.8 5 Speed
'16 Ford GT350 Track Pack