Page 1 of 2 12 LastLast
Results 1 to 10 of 13

Thread: Bolt Torque on Bus bars

  1. #1
    Join Date
    May 2015
    Location
    Beaumont, Texas, USA
    Posts
    10

    Bolt Torque on Bus bars

    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

  2. #2
    Join Date
    Sep 2004
    Location
    Vancouver, WA
    Posts
    711

    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.
    e^(i pi) = -1

  3. #3
    Join Date
    May 2015
    Location
    Beaumont, Texas, USA
    Posts
    10
    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

  4. #4
    Join Date
    Dec 2011
    Location
    Ocala, Florida, USA
    Posts
    2,694
    Quote Originally Posted by beanland View Post
    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.


    I know what I don't know, and I know where to go to find it!

  5. #5
    Join Date
    Jul 2009
    Location
    Ware Shoals, South Carolina
    Posts
    1,221
    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.
    Advise is a dangerous gift, even from the wise to the wise.

  6. #6
    Join Date
    Jul 2003
    Location
    New Jersey
    Posts
    25,823
    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.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  7. #7
    Join Date
    Oct 2013
    Location
    MD
    Posts
    69
    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.

  8. #8
    Join Date
    May 2015
    Location
    Beaumont, Texas, USA
    Posts
    10
    Quote Originally Posted by Strathead View Post
    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.

  9. #9
    Join Date
    May 2015
    Location
    Beaumont, Texas, USA
    Posts
    10
    Quote Originally Posted by SG-1 View Post
    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

  10. #10
    Join Date
    May 2015
    Location
    Beaumont, Texas, USA
    Posts
    10
    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

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •