My understanding is that the unknown factor in torque spec is coefficient of friction and that this is significantly effected (25% to 40%) by lubrication or loctite. Take the TPS TSR rings for example, the torque spec is 65 in lbs for the base nut but no indication of dry or wet in the spec. One could assume dry or wet. Most torque specs are given dry but most ring installs have loctite on them so one could assume wet. Given the huge difference between dry and wet torque values does anyone have any insight or best practices when measuring torque on rings with loctite or lubrication? Should spec be reduced by 25% or more when loctite is applied (dry spec assumption) or increased 25% or more if dry (wet spec assumption)?
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Gunsmithing Ring torque spec; Dry, lubricated, loctite?
- Thread starter AK264
- Start date
I follow manufactures spec on loctite and torque.... Never have I seen different specs for dry vs wet....nor have I thought about it for rings or bases. Interesting....but is it necessary?
Considering that the minimum difference it makes is 25% Id sure like to have a understanding of this. Why bother with a torque wrench when the spec may be off by a factor of +- 25% due to friction coefficient? Its a bit of a bugaboo throughout the fastener industry. Torque spec is often given dry but lubrication is required which makes no sense...
If I remember correctly, badger ordnance rings say not to use thread lock and to just torque....possibly because of what you have brought up.
Which supports a "dry spec". When torqing to a dry spec all of the oil use in manufacturing and oil used to protect the product/fastener in between manufacture and use must be removed with a degreaser and allowed to dry before installation. "clean and dry" To my mind this is not best practice. Loctite aside, best practice in my mind is that anything that is steel has lubricant on it particularily if it is to be outside. Avoidance of seizing and galling also would also mean best practice would be light oiling of fasteners to my mind. Taking the TPS base nuts for example again if their 65 in/lbs torque spec is a dry spec, if the nut has oil on the threads the proper torque would be 49 in/lbs possibly lower. The fastener running through the base of TPS rings is 1/4". Torque specs for 1/4" fasteners run much higher than 65 in lbs even for grade 2 fasteners. This alone would lead me to guess that the 65 in lbs is a "wet spec". The interaction at the base of the rings is not a simple one like clamping two pieces of flat steel together however. There may be limitations of the strength of the clamping piece. 49 in lbs is below minimum torque for all grades of 1/4' fasteners whether wet or dry. This would lead me to believe that 65 in lbs is indeed a "wet spec" and that the dry torque value is in the neighborhood of 80 in lbs, which actually is still low for 1/4" fasteners. Dry torque spec for a grade 5 1/4 20 fastener is 120 in lbs. a grade 8 is 148 in lbs. As I mentioned there are probably interactions in the clamping mechanism that are more complex than simple fastener torque specs. The fact remains that without knowing if the commonly assumed specs given us for rings, 15-20 in lbs for the top and 55-65 for the bottom, are wet or dry we have no way of knowing if we are torqing to manufacturer spec. We have no way of practicing proper torque application. If you are torqing to the wet spec one must ensure the fastener is wet, if torqing to a dry spec one must ensure the fastener is dry. Torqing a wet fastener to a dry spec is a massive overtorque. Torqing a dry fastener to a wet spec is a massive undertorque. Surely the old hands here have come to a conclusion about this quandry?
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Found a interesting calculator.
FUTEK Bolt Torque Calculator | Bolt Torque Calculators
K factor for a lubricated bolt is .15-.18
Be sure to enter correct yield strengths, this is not your fathers aluminum.
FUTEK Bolt Torque Calculator | Bolt Torque Calculators
K factor for a lubricated bolt is .15-.18
Be sure to enter correct yield strengths, this is not your fathers aluminum.
It's about clamping force, generated by application of a certain torque. Naturally, not everyone can afford all that lab equipment required for measuring and control of clamping force, but controlled torque can be applied with use of relatively simple and affordable torque limiting tools. Torque may considered to be a "representation" of the clamping force, with certain limitations taken into account.
While 1/4" screw can, generally speaking, go way above 100 inch-lbs, there is just no need to generate so much clamping force to assemble and keep action and stock together. In regard to "wet" vs "dry" -your estimate of numbers, from my experience is correct, and diference in resulting clamping force may be very substantial. Based on my limited knowledge, if there is no specific "wet" note attached to the torque recommendation, it is assumed that provided torque spec is applicable to the "dry" thread - no lubrication of any kind, be it either oil, wax, grease, dry film, or loctite or even water, which are also, techically speaking, can be treated as lubricants for most threads.
While 1/4" screw can, generally speaking, go way above 100 inch-lbs, there is just no need to generate so much clamping force to assemble and keep action and stock together. In regard to "wet" vs "dry" -your estimate of numbers, from my experience is correct, and diference in resulting clamping force may be very substantial. Based on my limited knowledge, if there is no specific "wet" note attached to the torque recommendation, it is assumed that provided torque spec is applicable to the "dry" thread - no lubrication of any kind, be it either oil, wax, grease, dry film, or loctite or even water, which are also, techically speaking, can be treated as lubricants for most threads.
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Loctite is generally considered to have the same lubricity as a light oil. Sometimes torque spec is given in "as delivered from factory" which assumes a light oil and my belief the above statement "no on-torque adjustments needed" is correct if the spec was "as delivered from factory. Loctite wont gurantee lubricity. Data sheet for 222 does not mention lubricity or k factor. Data sheet for 242 says +- 10% in other words no guarantees. data sheet for 246 which is slightly stronger than 242 lists a k factor of .17 and I believe a k factor of .16 -.18 to be appropriate for 222 and 242 although I can find no official spec to back that up.
TDS
LOCTITE
®
246
,
May
-
2004
Lubricity,
K-Factor
:
3/8
x
16
phosphate
&
oil bolts,
lubricity
steel
nuts
0.17
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My personal opinion is that you are way overthinking this matter (I'm an engineer, so I happen to know a thing or two about overthinking stuff).
I don't know if you're attempting to fix an actual problem that you are currently experiencing, or if you are just trying to turn the matter into an exercise in mental masturbation (which is something that I can appreciate). What I do know is that my experience with quality rings and mounts has led me to simply follow the manufacturer's torque recommendations. I do not lube the threads with oil, and I do not apply any threadlocker unless specifically requested by the instructions.
I've had only two issues with fasteners loosening, both on rifles chambered in .338LM. One happened because my wife interrupted me during the scope install process, and the first few rounds down the tube demonstrated that "finger-tight" does not apply adequate clamping force. The second occurred when I attempted to machine my own ring reducers to adapt a 34mm mount to a 30mm tube and had a bit of difficulty getting the geometry and surface finish worked out (the mating surfaces "took a set" and required re-torquing after approximately 10 rounds).
In every other case, it's been a simple matter of installing and torquing the screws as provided. I save the egghead thinking for stuff that really matters, like rod bolts and head studs. If you really want to get all wrapped around the axle with details, then you have to start determining the effects of variables such as the surface finish of the tapped hole (keeping in mind that the friction of anodized aluminum can be a real bitch to estimate) and changes in the thread fit from the first installation to the nth (keeping in mind that the clamping force provided by a given torque value can vary by up to 2x over ten installation/removal cycles). You'll just drive yourself nuts... er, no pun intended
I don't know if you're attempting to fix an actual problem that you are currently experiencing, or if you are just trying to turn the matter into an exercise in mental masturbation (which is something that I can appreciate). What I do know is that my experience with quality rings and mounts has led me to simply follow the manufacturer's torque recommendations. I do not lube the threads with oil, and I do not apply any threadlocker unless specifically requested by the instructions.
I've had only two issues with fasteners loosening, both on rifles chambered in .338LM. One happened because my wife interrupted me during the scope install process, and the first few rounds down the tube demonstrated that "finger-tight" does not apply adequate clamping force. The second occurred when I attempted to machine my own ring reducers to adapt a 34mm mount to a 30mm tube and had a bit of difficulty getting the geometry and surface finish worked out (the mating surfaces "took a set" and required re-torquing after approximately 10 rounds).
In every other case, it's been a simple matter of installing and torquing the screws as provided. I save the egghead thinking for stuff that really matters, like rod bolts and head studs. If you really want to get all wrapped around the axle with details, then you have to start determining the effects of variables such as the surface finish of the tapped hole (keeping in mind that the friction of anodized aluminum can be a real bitch to estimate) and changes in the thread fit from the first installation to the nth (keeping in mind that the clamping force provided by a given torque value can vary by up to 2x over ten installation/removal cycles). You'll just drive yourself nuts... er, no pun intended
Perhaps I am a egghead because I really enjoy playing with that Futek caculator. Its the first representation Ive found that shows how lubricity affects torque. In the industry I work in every fastener has a torque value, everything has a explicit assembly procedure. The assembly procedures for these beautiful modern rings is vague in comparison and that bugs me. I regard the base and rings to be like a foundation of a house, its got to be right. If things I cant account for happen later I want to know that its not the rings or base. Im method oriented, I dont like what is out there for a method of assembling bases and rings so Im developing one. My cheapo adjustable torque wrench will no longer be used and Im going to go to fixed value Seekonk torque tools, so I have to determine what inch LB values the Seekonks will break at in order to purchase them. I put 222 on every 6-10 fastener I assemble at home. My thought is why wouldnt you? Its cheap insurance. I realize these beautifully engineered modern rings are designed to work without loctite. Ive learned a lot playing with the Futek calculator, that has value to me. I realize there are factors it doesnt account for but it still gives me a feel for the relationship in between the strength of materials, lubricity, and torque spec. I think these beautifully engineered rings and these incredible optics that are available deserve a thorough assembly procedure. If all these things put me in the egghead classification and Im pretty sure they do so be it. Graham is right when he mentions clamping force is a result of the torque applied and is important. Clamping force is not easily measured and exceeding a ring manufacturer specification for torque on the ring fasteners could damage a optic. The other thing correct torque does however is retain the fastener. If one was to find that torque specifications in any application were lower than industry standards for like fasteners in like materials one might conclude a little 222 to be best practice.
Graham is right when he mentions clamping force is a result of the torque applied and is important. Clamping force is not easily measured and exceeding a ring manufacturer specification for torque on the ring fasteners could damage a optic. The other thing correct torque does however is retain the fastener. If one was to find that torque specifications in any application were lower than industry standards for like fasteners in like materials one might conclude a little 222 to be best practice.
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