Bolt Torque on Bus bars

[JRumsey]

We have recently reviewed our company's bus torque chart and found some of the values are in line with the bolt mfg suggestions (i.e.- 1/4" - 7/16" bolts) but some of the values were much lower than bolt mfg suggestions (i.e.- 1/2" - 3/4" bolts). I started to update our torque chart to match the bolt manufacturers specifications but I have a concern. The torque chart from Fastenal for a 1/2" Grade 8 bolt is 80ft/lbs. If when running main bus in a 480V switchgear I use 1/2" bolts and torque to 80ft/lbs, what happens when the bus bar heats up during use? Theoretically, the bus bar can go as high as 105deg C; have I gone beyond the suggested torque value of the bolt or did the bolt heat up as well and 'give' with the bus?

John

 

[beanland]

Bolting

Bolting

This is a complex issue but some manufacturers have written articles on it. The torque depends on the TPI and whether it is lubricated. No simple answer.

If the bus is copper-copper, then steel or copper bolts with lock washers may be adequate because the coefficients of thermal expansion are similar.

If one or more bus is aluminum, then 1 or 2 Belleville washer(s) are needed to accommodate the expansion. On a 1/2-13 SS bolt, you take a 6000# Belleville to 40 ft-lb, about 1/2 flat. That puts 3000# of pressure per bolt. As the bus heats, the AL expands, the Belleville gives a little then returns as the bus cools. Without the Belleville, the joint can deform and loosen over time.

Old school had aluminum bolts with aluminum bus. Now, more often, SS bolt, bronze nut, SS Belleville and flat washers. BZ nut to prevent galling.

BTW: studies have shown that it is pressure that guarantees a good joint. Joint resistance goes down as pressure goes up.

 

[JRumsey]

Thanks Beanland. These connections are for new build Switchgear, so all are copper to copper with grade 8 zinc plated steel bolts. Agreed the tighter the bolt the lower resistance, i just dont want to over-stress the bolt when the bus heats up. I think this is not probable but i definitely want to avoid popping off bolt heads after a few years of heating and cooling.

John

 

[Strathead]

This is a complex issue but some manufacturers have written articles on it. The torque depends on the TPI and whether it is lubricated. No simple answer.

If the bus is copper-copper, then steel or copper bolts with lock washers may be adequate because the coefficients of thermal expansion are similar.

If one or more bus is aluminum, then 1 or 2 Belleville washer(s) are needed to accommodate the expansion. On a 1/2-13 SS bolt, you take a 6000# Belleville to 40 ft-lb, about 1/2 flat. That puts 3000# of pressure per bolt. As the bus heats, the AL expands, the Belleville gives a little then returns as the bus cools. Without the Belleville, the joint can deform and loosen over time.

Old school had aluminum bolts with aluminum bus. Now, more often, SS bolt, bronze nut, SS Belleville and flat washers. BZ nut to prevent galling.

BTW: studies have shown that it is pressure that guarantees a good joint. Joint resistance goes down as pressure goes up.

And over torquing can cause the Belleville washer to operate improperly. A Belleville washer, by the way is a cupped washer, so basically a spring.

 

[SG-1]

Check the switchgear manufacturers instructions. Over torque can create hot spots in the joint because the excess pressure can deform the copper, resulting in catastrophic failure.

 

[infinity]

The Eaton gear we use has a 50 ft# torque rating for 1/2" hardware and 20 ft# rating for 3/8" hardware. The four bolts connecting the 4000 amp bus links together are 3/8" carriage bolts torqued at 20 foot pounds.

 

[smoothops10]

You can view NFPA 70B online for free. They have charts from multiple mfrs.

1/2" Grade 8 Heat-Treated Steel - 75 lbft
1/2" Coated Silicon Bronze - no lube - 40 lbft
" " lube - 25 lbft
1/2" Al - 25 lbft
1/2" Stainless Steel - 40 lbft

All based on based on national coarse thread pitch.

 

[JRumsey]

And over torquing can cause the Belleville washer to operate improperly. A Belleville washer, by the way is a cupped washer, so basically a spring.

Haha...unfortunately, I am the switchgear manufacturer. I am being questioned on our current torquing process and its values versus the bolt manufacturers standard torquing values. The people who created our current charts are long gone (along with the reasoning behind the values). I am hesitant to just use bolt manufacturers values since the bus bar will heat up and expand; I don't think this is considered in the bolt manufacturers values.

 

[JRumsey]

Check the switchgear manufacturers instructions. Over torque can create hot spots in the joint because the excess pressure can deform the copper, resulting in catastrophic failure.

Infinity, thanks for the information. These values are really close to our original charts. I am just unsure how they were derived.

John

 

[JRumsey]

Smoothops 10, these are the values i am looking to use now. My concern is that if i torque to these values and then the bus bar heats up, i may go over the recommended value.

John

 

[Strathead]

Haha...unfortunately, I am the switchgear manufacturer. I am being questioned on our current torquing process and its values versus the bolt manufacturers standard torquing values. The people who created our current charts are long gone (along with the reasoning behind the values). I am hesitant to just use bolt manufacturers values since the bus bar will heat up and expand; I don't think this is considered in the bolt manufacturers values.

I wouldn't change them without reason. Standard torque values for bolts as I understand them are specific to the bolts and the nuts. It has to do with how much the metal is stretched and stressed vs. the hold value. Take the heads on a care. The torque is very specific to the application. The bolts are specifically made to act a certain way to the required torque pressure on the head in the correct order. If standard torquing were all that was required for a bus connection then why would Belleville washers even be necessary?

 

[infinity]

Here's the table that we are currently using.

 

[Jraef]

Published torque values for the bolts themselves are the MAXIMUMS that should ever be used, but that's not to say they can be used at lower values. The values that you actually use are all application specific. I would go by the industry standards. ANSI C37 has a section on switchgear bolt torque, NETA (National Electrical Testing Assoc.) has a chart for when you don't know the original manufacturer's values.
http://tegg.teggpro.com/teggforms/2001netatables.pdf