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Screw Terminals on B&C breakers...

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steve(at)tomasara.com
Guest

Posted: Fri Feb 06, 2009 7:06 am Post subject: Screw Terminals on B&C breakers...

Bob and the group,



While rewiring my not-yet-flown-by-me Longeze I've been attempting to 

follow the interconnect philosophy (for power wiring) of:



Fast-ons preferred for appropriate (lower) current connections,

Studs w/ ring terminals (with either a self-locking nut or a 2nd (aka 

jam-nut) nut) for higher current,

Screw-terminals, which aren't really lockable, to be avoided.



But I haven't been completely successful yet.  What is recommended for:



 -The screw-terminal B&C breaker used for the alternator field winding?  

Blue locktite?  Or use another breaker?  Or don't worry about it?  Of my 

list, this one bothers me the most as the screws are small enough that 

it's not clear to me I can tighten them enough to get an appropriately 

gas-tight connection without stripping them.  Not to mention the 

(imagined?) possibility of inadvertently loosening them by poking around 

behind the panel and moving the wire (and associated ring terminal) they 

are attempting to clamp.



- The B+ connection on my alternator (ND external regulator type from a 

Toyota) which uses a metric nut and doesn't have room on the stud for a 

jam nut (I haven't looked for a high-temp self locking metric nut but 

that's probably the answer if I can find one),



- The brass screw terminals for the amp-meter leads at the shunt (from 

B&C) (blue locktite?)



On a related note, can anyone point me to the appropriate torque 

standards for brass nuts onto brass studs (as on the B&C shunt and the 

B&C grounding blocks...)



And, lastly, is there any (presumably metallurgical) reason it's a bad 

idea to use a non-brass nut (or jam-nut) on a brass stud?



Thanks in advance,

Steve.

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Feb 07, 2009 6:34 am Post subject: Screw Terminals on B&C breakers...

At 09:01 AM 2/6/2009, you wrote:

 	  Quote: 	
		  



Bob and the group,



While rewiring my not-yet-flown-by-me Longeze I've been attempting 

to follow the interconnect philosophy (for power wiring) of:



Fast-ons preferred for appropriate (lower) current connections,

Studs w/ ring terminals (with either a self-locking nut or a 2nd 

(aka jam-nut) nut) for higher current,

Screw-terminals, which aren't really lockable, to be avoided.


	


    I'm not sure I can beat an enthusiastic drum

    for "avoiding" screw terminals. After all, they've

    been used in civil and military aircraft since day-one

    and I'm aware of no considered movement to reduce

    their numbers. The fasteners we really went after

    were rivets when the molded/bonded composite structures

    were conceived, crafted and qualified.



    As we speak, there are hundreds of thousands of

    airplanes in the word that use breakers like

    and other devices with threaded fasteners not

    unlike . . .



http://aeroelectric.com/Pictures/Wiring_Technique/A36_Firewall_A.jpg

http://aeroelectric.com/Pictures/Wiring_Technique/bus_bars_1.jpg



    What I CAN suggest with enthusiasm is that

    the non-professional assembler of components

    has a higher probability of success (i.e. lower

    risk of future failures) if the processes used

    are "self calibrating". I.e. crimped as opposed to

    soldered, fast-ons as opposed to torque sensitive

    screws, etc. etc. A secondary consideration goes

    to simple-reliability studies where all things

    being equal, the lower parts-count assembly is

    the more reliable.

 	  Quote: 	
		  But I haven't been completely successful yet.  What is recommended for:



-The screw-terminal B&C breaker used for the alternator field winding?

Blue locktite?  Or use another breaker?  Or don't worry about it?


	


    It's not worthy of much worry. First because we're

    building FAILURE TOLERANT aircraft . . . or at least

    that's the goal. As you assemble EVERY bit and piece

    of airplane, study the manner in which it might fail,

    deduce whether or not the failure is pre-flight detectable,

    deduce how you would become aware of the failure in

    flight, deduce criticality of the failure and finally,

    if loss of that component's functionality puts your

    comfortable termination of flight at-risk, then

    craft a Plan-B for dealing with it.

 	  Quote: 	
		  Of my list, this one bothers me the most as the screws are small 

enough that it's not clear to me I can tighten them enough to get an 

appropriately gas-tight connection without stripping them.  Not to 

mention the (imagined?) possibility of inadvertently loosening them 

by poking around behind the panel and moving the wire (and 

associated ring terminal) they are attempting to clamp.


	


    At the current levels this breaker operates at, it's

    not essential that the whole mating surface area under

    the head of the screw be gas-tight. At the microscopic

    level, you will find that when these 6-32 screws are

    torqued "in the ball park", surface irregularities

    (yeah it LOOKS and FEELS smooth . . . but the earth

    is smoother than a billiard ball) will come together

    with sufficient pressure to make the electrical joint.



    That gas-tightness thing for the whole surface area

    has more to do with keeping moisture/oxidants out

    for longevity of the joint . . . that's when we get

    the wrench out and crank that fat wire terminal down

    good. Stuff behind the panel is not subject to much

   splash.



 	  Quote: 	
		  - The B+ connection on my alternator (ND external regulator type 

from a Toyota) which uses a metric nut and doesn't have room on the 

stud for a jam nut (I haven't looked for a high-temp self locking 

metric nut but that's probably the answer if I can find one),


	


    Yeah, and as I recall those threads were pretty

    coarse too. In fact, b-lead terminals are pretty

    high on the list of reasons that alternators are

    returned to B&C for repair. The nut works loose

    and the terminal burns.



 	  Quote: 	
		  - The brass screw terminals for the amp-meter leads at the shunt 

(from B&C) (blue locktite?)



On a related note, can anyone point me to the appropriate torque 

standards for brass nuts onto brass studs (as on the B&C shunt and 

the B&C grounding blocks...)


	


    If you have a torque wrench and your confidence

    goes up by using it, then my all means. I can

    tell you that none of the threaded fasteners

    in the photos above were precision torqued. The

    the band of satisfactory assembly between too

    loose and too tight is really pretty big.



    Most brasses have a tensile strength on the

    order of 50K PSI, so look up the torque values

    for any steel part of the same thread (AC43-13

    has some tables) and adjust the value for brass

    downward in proportion to their relative strengths.



    Just for grins, take a 6-32 steel screw and a

    brass nut (breaker combo) and see how hard it

    is to fail the parts by twisting on them. If you

    DO achieve failure, do it a couple of times. This

    will give you some sense of limits for doing the

    same task in your airplane. I think you'll find

    that you're not inclined to put nearly so much

    force on the parts to achieve satisfactory

    "snugness".



 	  Quote: 	
		  And, lastly, is there any (presumably metallurgical) reason it's a 

bad idea to use a non-brass nut (or jam-nut) on a brass stud?


	


    Not sure I'd worry about it. The fat wires go onto

    studs that are larger and fine threaded. Cranking

    these puppies down without further "insurance"

    is common practice.



    Locktite on the automotive threaded b-lead

    nut is about the only place I'd consider the

    use of a magic elixir useful for fixture.

    It's more important that fat wires on studs

    not impart strong rotational moments under

    vibration. Note the techniques used to

    bundle wires in this photo.



http://aeroelectric.com/Pictures/Wiring_Technique/A36_Firewall_B.jpg



    Bob . . .

 	  Quote: 	
		  Thanks in advance,

Steve.





Checked by AVG - www.avg.com

02/04/09 16:35:00


	




        Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

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