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racerjerry
Joined: 15 Dec 2009
Posts: 202
Location: Deer Park, NY
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 Posted: Mon Apr 25, 2016 2:51 am Post subject: Screw Terminals |
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Screw type terminations scare the hell out of me - except when they are done correctly. I have seen way too many products & screw type terminal strips installed in OBAM aircraft that were designed for static operation that do not have any type of locking mechanism to counter normal vibration. Most quality switches come with terminal screws that have a captive lock-washer as well as a plain thin washer under it so as to provide a locking mechanism for the screw head, but not scar the device being attached.
I am very partial to aircraft quality internal star lock-washers. They are inexpensive, corrosion resistant and do not side load the screw/stud as common split lock-washers are prone to do. They are also thinner than a split washer. If I am installing against a ring terminal; unless it's captive, I normally omit the plain washer and let the stars bite into everything for a solid, secure and vibration resistant connection. The internal star lock-washers should be replaced each time they are removed, but at least check that ears are not completely flattened out. $2 worth will last you a lifetime.
Current prices from Aircraft Spruce:
MS35333-37 (#6) $0.02
MS35333-38 (#8) $0.02
AN960-6L (#6) $0.02 or $1.60 /100
AN960-8L (#8) $0.02 or $1.60 /100
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Jerry King |
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user9253
Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan
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| Posted: Mon Apr 25, 2016 5:41 am Post subject: Re: Screw Terminals |
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It turns out that the split ring lock washers that we have been using for years are not actually lock washers at all. In fact, their spring characteristic can actually cause fasteners to loosen sooner. I have not seen split ring lock wasters used on newer cars. Internal star lock washers are good. Even better are wedge-lock washers. They are used in pairs with the wedges facing each other. When a bolt or nut starts to loosen, the wedges move apart slightly. That causes the fastener to stretch, which makes it difficult for the fastener to loosen anymore. It requires much more force to loosen than to tighten fasteners installed with wedge-lock washers. Matco brakes and Sensenich props use wedge-lock washers without any other locking devices or safety wire.
Youtube movie about wedge-lock washers:
https://www.youtube.com/watch?v=IKwWu2w1gGk
I am not recommending wedge-lock washers for small screws. But I am recommending that split ring lock washers NOT be used anyplace on an aircraft, or for any application with the intent of preventing a fastener from loosening.
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Joe Gores |
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edpav8r(at)yahoo.com
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| Posted: Mon Apr 25, 2016 8:05 am Post subject: Screw Terminals |
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Very interesting product. It appears to work well, but I wonder if the radial impression marks that they create can serve as stress risers in softer materials like aluminum. Also, it looks like it wouldn't take very much over-torque to gall an aluminum surface or scrape off the cladding.
Perhaps this is why aviation still predominantly uses castle nuts on flat washers. Not advocating against Nord-Lock, just thinking out loud...
Eric
| Quote: | On Apr 25, 2016, at 6:41 AM, user9253 <fransew(at)gmail.com> wrote:
[...] Even better are wedge-lock washers. They are used in pairs with the wedges facing each other. When a bolt or nut starts to loosen, the wedges move apart slightly. That causes the fastener to stretch, which makes it difficult for the fastener to loosen anymore. It requires much more force to loosen than to tighten fasteners installed with wedge-lock washers. Matco brakes and Sensenich props use wedge-lock washers without any other locking devices or safety wire.
Youtube movie about wedge-lock washers:
https://www.youtube.com/watch?v=IKwWu2w1gGk
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nuckolls.bob(at)aeroelect
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| Posted: Mon Apr 25, 2016 8:05 am Post subject: Screw Terminals |
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At 05:52 AM 4/25/2016, you wrote:
| Quote: | --> AeroElectric-List message posted by: "racerjerry" <gnking2(at)verizon.net>
Screw type terminations scare the hell out of me - except when they are done correctly. I have seen way too many products & screw type terminal strips installed in OBAM aircraft that were designed for static operation that do not have any type of locking mechanism to counter normal vibration. Most quality switches come with terminal screws that have a captive lock-washer as well as a plain thin washer under it so as to provide a locking mechanism for the screw head, but not scar the device being attached. |
True . . .
All mil-qualified switches will come with internal
tooth lock washers and I've never encountered a
condition precipitated by a loose screw.
Switch and breaker panels are never assembled in
a production airframe. They are put together on
sub-assemblies in good lighting, air conditioned
spaces by people standing upright or sitting
on a chair.
The sub assembly, with a medusa of pendant cables
and wires is assembled to the airframe with a few
screws and 'plugged in' with perhaps a few fat
wires dropping onto studs.
Most breakers and switches with threaded fasteners
have steel screws into brass 'nuts'. Recommended
torque values for fastening are in the 8 in-lb
range . . . and this number is generally 60%
of ultimate. Assembly benches at Beech used
torque limiting screwdrivers to do final tightening
of threaded fasteners. You can often pick up
adjustable, torque limiting screwdrivers off
eBay.
Few OBAM/SE-TC aircraft are assembled like leggos
. . . and none will be easy/convenient/comfortable to
troubleshoot. Further, the assembler is comparatively
Wwaaaayyy down on the learning curve for achieving
consistent tightening torque.
At the same time, we have an opportunity to do
FMEA studies which guarantee that NO single
disconnection for loose screw or any other
reason causes a bad day in the cockpit. Many
variations in desirable switch styles/features
will drive the decision toward threaded fasteners
which are not evil, only different. It's like
the difference between crimped and soldered.
Both are entirely satisfactory when applied
with confidence . . . it's just another skill
to learn.
Bob . . .
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