On Sunday, April 28, 2019 at 6:14:33 AM UTC-7,
clarkm...@gmail.com wrote:
# On Wednesday, February 13, 2019 at 5:48:16 PM UTC-8, unclevito wrote:
# # Should you oil the outside of barrels that are finished in flat black?
#
# Scope base screws are not stressed in shear, but in tension.
# If my 142 gr bullet maximum acceleration is from 500 fps to 2000 fps in 0.4 ms then an 8 pound
# rifle will try to accelerate from 1.27 fps to 5.07 fps in 0.4 ms. This is an acceleration of
# 9500 ft/sec squared.
# If a 2 pound scope were part of that 8 pound rifle during that acceleration the force between
# the rifle and the scope would be
# f = m a = 2 pounds 9500 f/ss = [9500 f/ss] [2 pounds / [Gc = 32 f/ss]]= 594 pounds.
#
# If there were 4 screws in shear that would be 148 pounds force / screw.
# That screw has a 0.12" minor diameter
# A = pi r r = .011 sq in
# Grade 5 bolt [American cap screws] has a shear strength of 72,000 psi
# Each screw would have a shear strength of 814 pounds.
#
# But they are not in shear.
# They are in tension.
# Grade 5 bolts are good for 127,000 psi in tension.
# Each screw is good for 1436 pounds force in tension.
# They are pre loaded to about half that, making a clamping force of 700 pounds.
# The coefficient of static friction between the steel scope base and the receiver is ~ 0.6
# So each screw provides a static state up to 420 pounds recoil force.
# 4 screws 1680 pounds.
# But the hard kicking lightweight rifle and heavy scope only have 594 pounds peak force between them.
# 1680 pounds friction is greater than 420 pounds recoil, therefor the bases stay put.
# If they were to slip, the screws might see shear forces, but that would require operator error
# in tightening the screws.
# The minor diameter of #4 screws is 0.085", so they would work too.
#
# But the coefficient of friction for lubricated steel on steel is 0.16
# The static friction force threshold of slipping =
# [coefficient of friction lubricated steel][clamping force] = [.16][2873 pounds= 459 pounds.
# Because the static friction 459 pounds is less than the recoil reaction on the scope mass
# 594 pounds, the scope should slip and the screws should start shaking loose.
#
# Let us imagine a scope and rifle shooting well for years, and then caught in a rainstorm, it
# is later slathered with oil to prevent rust. Some of the oil creeps under the scope bases. Next
# year the rifle does not shoot so well.
Oh, I doubt it? No shear? Are you kidding me? Shear is what IT IS ALL ABOUT?
When the rifle recoils, the scope screws are NOT in TENSION? THEY ARE IN SHEAR?