USE OF SPRING WASHERS ON ELECTRICAL CONNECTIONS 4

[xxjohnh]

Manufacturers and field installations seem to differ on the this. When using a a current carrying electrical connection such as bolting a commpression lug to a busbar or busbar to busbar, what is best connection method?

 

[zaza123]

In normal practise when connecting cable lug to the busbar the washer & spring washer are used & then finally Nut is tightened as per torque limit.

 

[jghrist]

The spring washer is only needed if there are different materials involved with different thermal expansion coefficients. For instance, aluminum pads with steel bolts or aluminum to copper pads with any bolt material. Bronze pads with bronze bolts will expand the same and do not need spring washers.

http://www.hubbellpowersystems.com/powertest/catalog_sections/PDF_connect/DJ.pdf

is a useful reference.

 

[ppedUK]

An old sage of a mechanical engineer once told me that a spring washer on top of a plain washer is absolutely useless as a locking device because the plain washer has no grip with the mating surface. It makes sense to me, but I never quizzed him about the thermal expansion scenario.

Maybe our mechanical friends could chip in on this one.

 

[mcgyvr]

http://www.eng-tips.com/faqs.cfm?fid=1257

 

[dpc]

In a previous life, the large consulting firm I worked for always required Belleville spring washers for all aluminum bus connections.

 

[oldfieldguy]

Some of my former clients including a large electrical utility specified belleville spring washers as part of the fastening for connections, without lockwashers.

old field guy

 

[waross]

A lock washer is intended to prevent the nut from turning loose.
A Belville washer is intended to maintain contact pressure and prevent bolt stretching and/or bus-bar cold flow during thermal expansion and compression cycles.
Different issues.
Different solutions.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I've never figured out what the lock washer - the one that looks like a piece cut off a flat coil spring - is for or how they ever became so popular.

A Belleville could work if properly applied. That's a big if though.

 

[xxjohnh]

It seems a lock or spring washer is always required on the threaded end, nothing else. A belleville is a special kind of spring washer and used for special applications. No flat washer is used on the threaded end but could be used as well on the bolt end.

 

[alehman]

It's important to have enough contact pressure in a joint to minimize resistance and reduce corrosion potential. Too much pressure will result in deformation of the conductors which can cause the joint to loosen after thermal cycling.

Bolt torque is poor indicator of contact pressure due to variability in friction.

Spring tension washers, if properly rated and properly installed (two big if's,) help assure the correct initial contact pressure and provide resilience for thermal expansion/contraction. Too often, even in good switchboard shops, there's one big box of belleville's that are used for everything. Also a spring washer squashed flat is useless.

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec7.htm

http://ecmweb.com/mag/electric_belleville_washers_correctly/

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[prc]

Spring washers are split ring circular washers.These are not preferred in electrical applications as in case they break in service,there are chances of them flying off and going to electrically live areas.Their use is limited to lock the nut rotation.

In current carrying joints (say 500A or more) belleville spring washers(disc washers) are used to provide continuous pressure on contact areas to take care of the unequal expansion of current carrying parts and fasteners.The order of the bolting system is bolt head/belliville washer/ plain washer/ current carrying joint (two parts)/ plain washer/belleville/nut/projecting length of bolt.Plain washers are special ones having more OD to transmit the pressure from Belleville.These disc washers can be fitted in series(then their clamping load adds up) or in parallel(deflection adds up with same load)

Current carrying at a joint depends on the surface area(or A /mm2 normally 0.5 A/mm2 in oil and less value for air,surface finish and contact pressure.