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Why brass connection washers, not standard steel ones?

cupoftea

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Hi,
When we connect up our 50A, 24V PSU to the load, we use big ring terminals on the end of the load wires. We screw these to the PSU's terminal bolts. The ring terminal gets held in a "sandwich" between two brass washers, and two standard steel bolts...(the brass washers actually 'sandwich' the ring terminal.)
Why are the washers brass? Why not standard plated steel washers?
 

andre_luis

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I'm sure you are able to realize that at first sight only the bottom brass washer would be needed between the terminal ring and the metal surface of the PSU, although not increasing the contact surface of the ring terminal for such a cheap part would be a ridiculous savings when it helps reduce heat at that point of contact. I'm also sure you know that steel has a much lower electrical conductivity than brass, and that metallic ordinary steel washers are primarily proposed to be used as mechanical fasteners, not for electrical performance. But if you really want an explanation, let's say technically, more useful, you should have had the curiosity to measure and determine the part number of the ring terminal and know what current it was specified for, and if it's underated for 50A, it would really make some sense to put some attention on this in order to spread the distribution of electric current at the points of contact.
 

dick_freebird

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Not only the parent material, but how it oxidizes may matter
(over the long haul). Copper oxide is a semiconductor that
will degrade contact resistance some, and takes a long time
to develop any thickness, while iron oxide is a lot worse
electrically and a lot more aggressive.
 

Munyua44

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Hi,
When we connect up our 50A, 24V PSU to the load, we use big ring terminals on the end of the load wires. We screw these to the PSU's terminal bolts. The ring terminal gets held in a "sandwich" between two brass washers, and two standard steel bolts...(the brass washers actually 'sandwich' the ring terminal.)
Why are the washers brass? Why not standard plated steel washers?
the reason behind this is all about corrosion resistance difference between the two material. Iron alloys eg steel get oxidized very quickly hence high rate of corrosion as compared to the copper alloys eg brass material which gets corroded at a slower rate. Therefore the use of the brass washers is economical as it will have more life as compared to iron/steel washers.
 

tonigau

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1 reason to use brass washers is because it is softer than the steel nuts giving rise to an amount of nut lock due to surface deformation. This will also decrease contact resistance to the nut.
A Washer will spread the surface contact force from the nut over a larger area on the lug & this should improve contact resistance even with x2 n surfaces. (I have not tested this)

For connection to ring lugs where there is a bolt, nut & nut, the surface of the lug facing the nut(s) will also conduct well due to the threaded surface area contact to the nut so both should be similar. (unlike some binding posts with a thin brass insert).
That's why its better to thread connections to busbars for high current rather than a drilled hole.

For bolted lug connections, when they are correctly tightened there should be a gas tight seal of the contact surfaces excluding almost all atmospheric pollutants. Have a look at any old bolted connection contact surfaces that's been a long time in harsh environment (eg car alternator out terminal) & the plated steel surfaces look like new.

For my engine driven 130A 3phase welder I have stainless steel terminal bolts & nuts with no washers, (I surface machined the nuts) it spends most time with the nuts loose so if not stainless they would degrade.
 

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