Blish Lock Question
Posted 19 June 2004 - 09:18 PM
Posted 20 June 2004 - 12:29 AM
As far as I know, no sound mathematical engineering analysis exists that can prove the Blish lock metal adhesion priciple works in this case, if at all.. I do know that locking lugs must have minimun angles so they don't unscrew under pressure, which seems to be more of a vector analysis exercise, but the angles used in the BL are much larger, and probably don't do squat, lube or not.
Posted 20 June 2004 - 11:17 AM
In any case, the time it takes for moving the lock through the angles before the bolt moves back should delay the action, hence should slow the rate of fire. The British took out the lock piece and put in bolt heads to fill the slots in the bolt for desert (mitigation of sand intrusion) use. I've not seen reports of effect on rate of fire or if the rear of the receiver was cracked from the modification.
Posted 21 June 2004 - 08:12 AM
A felt breech oiler is designed to lubricate the ears on the lock, any oil that gets on the bolt is secondary. If the ears on the lock don’t touch the felt when the bolt goes all the way back, the oiler needs to be replaced.
TSMG’s like to be wet, they just run better with some oil. I don’t see much change in cyclic rate dependent on the amount of oil present- the lock seems to time it up pretty well.
Posted 21 June 2004 - 09:31 AM
Posted 21 June 2004 - 11:40 AM
Posted 21 June 2004 - 09:06 PM
Let me put my 2 cents in on this metal adhesion theory. The coefficient of friction of steel on steel is in the range of .7 to .75 unlubricated (depending on various factors). The coefficient of brass on steel is .3 unlubricated (depending on various factors). Now if you add a lubricant to the brass-steel mechanism the coefficient of friction will decrease. It may range from .09 to .15 to maybe .20. Of course the lubricant used plays a part in what the coefficient of friction will be. The less viscous the lubricant, the higher the coefficient of friction and the more wear (in this case the more wear the Blish lock will experience) in the mechanism.
Lubrication is commonly classified according to the degree with which the lubricant separates the sliding surfaces. The crankshafts in our car engines experiences "hydrodynamic lubrication" where the crankshaft is actually separated from the bearings by the film thickness of the oil. This separation may be only 0.0003" to 0.0008". In "mixed-film" lubrication there is intermittent contact of surface peaks (machined surface finishes for example) and partial hydrodynamic support. With proper design, surface wear can be mild. Coefficients of friciton commonly range from 0.004 to 0.1. "Boundary lubrication" involves continuous and extensive surface contact, but the lubricant is continuously "smeared" over the surfaces and provides a continuously renewed adsorbed surface film which reduces friction and wear. Typical coefficients of friction are 0.05 to 0.20.
Now General Thompson new that under certain conditions the breech on the big guns would unscrew. He noticed that the amount of powder used made a difference. More powder meant more pressure and the tendency for the metals (dis-similar I believe) to stick together. Light powder loads and the breech would unscrew.
So now you have this whacky idea of a hand held trench broom that you want to fire in a fully automatic fashion, yet be able to control it, hit your target and move on. He needs a way to instantaneously hold the breech closed to benefit from the high pressures, but then quickly return the mechanism to the fire mode.
He knows he has to have dis-similar metals, otherwise the friction will be excessive, even when lubricated and in this application you want to control "where" the wear occurs. The bolt has to be steel (bolt: N-2 steel, heat treated) and the lock will be made from aluminum bronze, heat treated) One piece, the lock becomes easily replaced.
It appears to me that Oscar Payne, Eickhoff and maybe Thompson, were thinking of the "boundary lubrication" principle with a little "mixed film" thrown in the mix with the Blish lock. As the cartridge ignites and the pressures build, the lubricated aluminum bronze lock essentially becomes "locked" in place for only a very short period of time, ensuring that a vast majority of the energy of the powder is imparted to the bullet. It might be said that the oil on the ears of the lock is compressed so fast during the "locking mode" that initially is does not have time to flow out of the way and you have somewhat of a "mixed film" lubrication phenomenon. Once the pressure begins to drop, the angle on the lock, gravity and lubrication come into effect. The oiler pads provide the renewed surface film as described in the "boundry lubrication" principle.
You need enough friction to accomplish a locked breech, but not so much that you have excessive wear and or high friction loads. It looks to me like Payne and Eickhoff knew exactly what they needed in order to make the Tommy talk. They had just the right combination of pressure, lock angle, dis-similar metals and lubrication to make the mechanism perform in a very unique fashion. A 1921A Colt-Thompson has a sound all its own, just like a Harley. But let's ask the lucky guys with both Blish and non-Blish Thompsons, "do they sound the same or different?"
My guess is that a Thompson with a Blish lock will have higher muzzle velocity than any other "Thompson" without one, all other things being equal. Anybody got any data that might prove or disprove my 2 cents worth?
I think you would be crazy to run a Blish gun without the lock. The M1 and M1A1 have different buffers and springs and fire at a slower rate. The need for the lock was engineered out of that design.
Just my 2 cents worth and you don't even owe me that!
Posted 22 June 2004 - 01:02 AM
Posted 22 June 2004 - 06:51 AM
Thank you for providing the information regading friction effects.
Several comments. Commander Blish, not General Thompson observed and patented the Blish lock. Auto Ord bought the patent from Blish.
M1/M1A1 Thompsons have a faster ROF than 28s and slower than 21s (see FAQs for respective ranges), approximately equal reciprocating mass as the 28 and higher than the 21, and use same spring as the 28. Thompsons work upside down as well as right side up. These observations lead me to, perhaps mistakenly, believe that the Blish lock, as embodied in Thompson SMGs, provides a mechanical delay as the lock piece is actuated through the compound angles. As such, plenty of lubrication is a good thing to provide it.