Using Threadlocker on Eccentric Bushing Set Screw

[gunner66]

(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.

[JPG]

(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.

The reason for the 'two finger' torque limit is the water pump bearing that the screw is being tightened against. Too much tightening and the bearing will overheat due to being 'squeezed'.

I believe the greenies had the groove in the center and the set screw is pointed so as to fit into the groove and prevent the bearing from sliding in/out. The set screw is not being used in a 'normal' set screw application.

I would not be concerned with the 'break' torque value being low, but the 'prevailing' torque should not be low. I agree with the threadlocker suggestion.

[gunner66]

I agree that the use of a pointed set screw in a groove to secure a bearing is somewhat unique and can cause problems. The use of the LS threadlocker allows you to be able to fasten the screw to about 12-15 in. pds (install torque), but needing around 28 in. pds. (break torque) to loosen it, thus making it more resistant to vibration and thus backing out when you want it to stay put. The "prevail" torque, as I understand it, is then the amount of torque needed to continue backing out the set screw once it is loosened. This old-style bushing is not a great design for this and other reasons, which IMO makes the newer split bushing style a better choice.

[JPG]

I agree that the use of a pointed set screw in a groove to secure a bearing is somewhat unique and can cause problems. The use of the LS threadlocker allows you to be able to fasten the screw to about 12-15 in. pds (install torque), but needing around 28 in. pds. (break torque) to loosen it, thus making it more resistant to vibration and thus backing out when you want it to stay put. The "prevail" torque, as I understand it, is then the amount of torque needed to continue backing out the set screw once it is loosened. This old-style bushing is not a great design for this and other reasons, which IMO makes the newer split bushing style a better choice.

Are you referring to the new two bearing idler shaft? If so I agree if for no other reason than the bearings are replaceable.

[gunner66]

Yes - IMO the newer entire idler shaft assembly using two bearings and with the split bushing does seem better from a maintenance standpoint:

Advantages: replaceable bearings (as you mentioned) and no need for a jamming set screw in the split washer bushing;

However, I do think that the old water-pump style bearing itself is a very robust and reliable bearing that has shown to last quite a long time.

[JPG]

[quote="gunner66"]Yes - IMO the newer entire idler shaft assembly using two bearings and with the split bushing does seem better from a maintenance standpoint:

Advantages: replaceable bearings (as you mentioned) and no need for a jamming set screw in the split washer bushing]

I believe getting a replacement 'water pump bearing' is quite difficult today(I think that is the reason it has been 'redone').

No jam screw, but now an additional screw in the outer end!;)