Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Newtons vs Newton meters

  1. Jul 26, 2009 #1
    hello forum members,

    I,m new to this forum and was hoping some one can help me?

    I work with industrial fans, we use a steel built frame with the shaft & bearings located in an housing called "plumber blocks" that are on top of the steel frame. The blocks are bolted down using 16mm bolts that we use 220 Nm on a torque wrench to tighten down, there are two bolts in each block (4 in total, block either end of shaft) so thats 880 Nm in total. The electric motor is bolted on the steel frame on the side with pulleys and rubber belts, the tension of the belts for say a 30kw motor will be approx 60,000 N. The cosine angle = approx 30 degree's so the tension on the blocks should be approx 51961 N.

    I have a friendly arguement with my boss saying that the blocks must be pinned (this means inserting a 6mm pin either side of the block tightly to stop movement), he says no, they have never moved once in the 20 years he has built them.

    My question isn't who is right or wrong, but is there some way of knowing what 880 Nm is compared to the Cosine value of 51961 Newtons?
     
    Last edited: Jul 26, 2009
  2. jcsd
  3. Jul 26, 2009 #2

    rcgldr

    User Avatar
    Homework Helper

    Without the thread pitch, and the friction factors, there's no way to know how much tension the bolts generate with 220nm of torque applied to tighten them, but it's bound to be a large number (well over 100,000 Newtons). 220Nm / 8 mm radius = 27500 N of force at the surface of the bolt in the direction of rotation. If friction is zero, then the threads are similar to a frictionless thin wedge with a side force of 27500N's of force. If friction is zero, and if I've thought this out right, then the tension = 27500N / (sin(θ) cos(θ)) where θ is the angle of the threads. If the pitch was 10 to 1 (about 5.7 degrees), then tension ~= 277000N.
     
    Last edited: Jul 26, 2009
  4. Jul 26, 2009 #3
    Thanks alot jeff,

    Question? was that last calculation based on 1 bolt (220 Nm) or the total 880 Nm ?
     
    Last edited: Jul 26, 2009
  5. Jul 26, 2009 #4

    rcgldr

    User Avatar
    Homework Helper

    one bolt.
     
  6. Jul 26, 2009 #5
    Jeff,

    I'm not trying to be awkward or show disrespect to your calculations, these blocks are slotted and can move, i find 51961 N which is 5302 Kg of tension hard to comprehend against 4 bolts of 220 Nm each? am i thinking wrong?

    Also you mention friction which i understand, would i be right in saying if we oiled the bolts then we would have more Newtons vs Newton meters?

    Also another question? bolts loosen with coefficients of linear/cubical expansion, of which bolts go loose! fact! so the Nm will obviously come down over time.

    So i am right that the blocks need to be pinned, although this is not my original question.
     
    Last edited: Jul 26, 2009
  7. Jul 26, 2009 #6

    Dale

    Staff: Mentor

    Jeff is in the right order of magnitude. I got ~860 kN of tension per screw using the http://www.buyblueprint.com/article/48/" [Broken]. However, as you can see in the equations the force depends very critically on the diameter and pitch of the screw.
     
    Last edited by a moderator: May 4, 2017
  8. Jul 26, 2009 #7

    russ_watters

    User Avatar

    Staff: Mentor

    The torque on the bolt isn't directly relateable to the force holding the parts together - as was shown, it is a function of the torque, firction, and thread pitch.

    ...And you can't just multiply it by 4 - that's a meaningless number.
    If you oiled the bolts, you'd be able to turn them further for the same torque, generating a higher tension in the bolt.
    I'm an HVAC engineer, but I don't understand the configuration you are describing or what you mean by pinning the blocks down - bolts are pins, aren't they? Do you have a picture or diagram you could show us?
     
  9. Jul 26, 2009 #8
    Russ,

    pins are split dowl pins which close up tight when you bang them in, you have to drill the blocks and the steel frame then bang the pins in with an hammer, it stops any movement sideways.
     
  10. Jul 26, 2009 #9

    russ_watters

    User Avatar

    Staff: Mentor

    Doesn't a bolt also stop movement sideways?
     
  11. Jul 27, 2009 #10

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's a solid argument to consider, no ? :smile:
    If you think that the force holding the bolts is way insufficient to keep the blocks in place, then how come that it didn't move in all these years ?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Newtons vs Newton meters
  1. Newton Vs Einstein. (Replies: 25)

Loading...