Using Threadlocker on Eccentric Bushing Set Screw
Posted: Thu Jun 02, 2011 7:13 pm
(Tried to post this info on my post requesting a torque value, but kept getting an error message)
Problem Summary: How tight to torque the small allen set screw in an older idler shaft eccentric bushing (non-split type). Solution thus far posted: use an L-shaped allen wrench and tighten down using two fingers applied to the short leg of the wrench.
Additional Information:
A neighbor who is a machinist (30+ years) has taken interest in my shopsmith restoring project. He came by when I was fiddling with the gilmer belt tensioning. I spoke to him on my concern regarding this set screw in the eccentric bushing. He studied the arrangement and then got his fancy (i.e., $$$) micro-digital torque wrench and measured/recorded both the set screw's "break" torque (torque to initially loosen the screw) and the "prevail" torque (which appears to be the average torque needed to unscrew the set screw after it breaks loose).
He consulted his machinist guides for this 5/16 -18 x 1/4 size set screw to determine the desired break torque in high vibration equipment (his comment was that the installing torque is set by the manufacturer, but that the "break" torque followed a standard). He then measured the break torque after I fastened it, and stated that my break torque was not high enough due to the loose tolerances of my particular parts and that this break torque was not fully dependent on how tight I had originally installed it. My friend thought that using the two-finger tightening torque should work fine with newer/better parts, but not in my case. His solution was to put some LOW strength threadlocker designed specifically for such applications (i.e., labeled for use with set screws, calibration adjustment screws, and other fastners #2 to 1/4" in size that required periodic adjustment - no special tools required on removal). Product was Permatex Low Strength Thread Locker "Purple" P/N: 222000.
According to him, after using the thread locker and based on my initial installing torque, the calculated thread engagement area, and the condition of my parts - the new break torque should be around 28-30 inch pounds, which he thought would be perfect for this application. He had no real comment on how tight the scre should be fastened in the first place, other than the torque is set by the manufacturer and that our "two-finger" tightening rule with a short allen wrench probably got us about 15 inch pounds +/- and that our procedure looked good to him.
Problem Summary: How tight to torque the small allen set screw in an older idler shaft eccentric bushing (non-split type). Solution thus far posted: use an L-shaped allen wrench and tighten down using two fingers applied to the short leg of the wrench.
Additional Information:
A neighbor who is a machinist (30+ years) has taken interest in my shopsmith restoring project. He came by when I was fiddling with the gilmer belt tensioning. I spoke to him on my concern regarding this set screw in the eccentric bushing. He studied the arrangement and then got his fancy (i.e., $$$) micro-digital torque wrench and measured/recorded both the set screw's "break" torque (torque to initially loosen the screw) and the "prevail" torque (which appears to be the average torque needed to unscrew the set screw after it breaks loose).
He consulted his machinist guides for this 5/16 -18 x 1/4 size set screw to determine the desired break torque in high vibration equipment (his comment was that the installing torque is set by the manufacturer, but that the "break" torque followed a standard). He then measured the break torque after I fastened it, and stated that my break torque was not high enough due to the loose tolerances of my particular parts and that this break torque was not fully dependent on how tight I had originally installed it. My friend thought that using the two-finger tightening torque should work fine with newer/better parts, but not in my case. His solution was to put some LOW strength threadlocker designed specifically for such applications (i.e., labeled for use with set screws, calibration adjustment screws, and other fastners #2 to 1/4" in size that required periodic adjustment - no special tools required on removal). Product was Permatex Low Strength Thread Locker "Purple" P/N: 222000.
According to him, after using the thread locker and based on my initial installing torque, the calculated thread engagement area, and the condition of my parts - the new break torque should be around 28-30 inch pounds, which he thought would be perfect for this application. He had no real comment on how tight the scre should be fastened in the first place, other than the torque is set by the manufacturer and that our "two-finger" tightening rule with a short allen wrench probably got us about 15 inch pounds +/- and that our procedure looked good to him.