About Bolts and Nuts - What else in CE? A simpler way to predict bolt preload

Replies

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  heviiguy

  "Predict"? Predict? PREDICT !!!??!!!
  Come on, you've got to be kidding me. Regardless of how complex one makes a calculation, one CANNOT predict an outcome (such as the final bolt preload as a result of a certain torque input) unless one is a fortune teller! It is because of pervasive gross misunderstanding such as this that people continue to wonder why their carefully-designed bolted joints fail.
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  ISHAN TOPRE

  Hello heviiguy,
  
  I think the article in the link mentions above gives some mathematical evidence right? By the word predict perhaps the author means that we can calculate the bolt preloading *almost* perfectly.
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  yadavundertaker mohit

  @heviiguy: nothing is determined accurately but we can find out the outcome very near to the actual result because of very limitations and assumptions so predict is correct according to author
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  heviiguy

  Come on folks: We're Engineers, not Witch-Doctors!
  
  No matter how complex the formula is, whenever a "factor" or "rule of thumb" is included, all bets are off. Anybody who believes that bolt preload can be "predicted" is kidding themselves. It's the same thing as saying that one can predict the temperature of soup within a pot hanging over a campfire by counting the number of logs thrown into the fire. One cannot PREDICT. The only way to know with any certainty, one must MEASURE. Just as one would use a thermometer to measure the soup AFTER it's been heated, one measures the resultant elongation of a fastener AFTER it's been tightened. If this isn't within the acceptable tolerance, additional (or less) "torque" is applied until the elongation is commensurate with the necessary preload.
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  arcsabre

  'Prediction' is possible to a degree of accuracy that depends on the parameters considered for that case. In your case of logs being thrown into the campfire, the factors which would determine the temperature of the soup are the distance of the soup (pot) from fire and the percentage of combustion that takes place. The latter would depend on the arrangement of the logs, which would determine aeration and hence the combustion percentage. This would in turn determine to what extent the soup is heated (and temperature raised). Of course, evaporation of the soup is also a factor but if u want u can neglect it. Considering all these factors, I think it is possible, by some mathematics (calorific value of logs * mass of logs * percentage of combustion)/solid angle of a sphere = heat input to pot/solid angle subtended by the pot from the fire. or another way would be to determine a. the heat at various heights above the campfire for a given set of logs and arrangement, and b. the heat at a particular height if the arrangement of logs or their number is varied, which would affect the heat outflow. Now a relation can be determined between the heat input, the height, number of logs and arrangement (empirically). This can then be used to 'predict' the heat input and temperature raise for any given set of logs at any height.. of course it has to be tested first for a few variations in parameters. Long story short, we can assume that a formula exists to predict such results if we are considering all parameters in the relation correctly and accurately. This can be tested for a few results then extrapolated to the rest.. This is what i think.
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  Ramani Aswath

  I just want to make a mention of 'guesstimates', which are order of magnitude approximations routinely used in research. In most engineering designs a factor of safety is used. This is a measure of the uncertainty in calculations - shall we say 'predictions'?
  Robert Oppenheimer, the father of the atom bomb (Manhattan Project), which ended WW II, was watching the first bomb (called 'The Gadget') tested in New Mexico in 1945. When the blast took place he tore to bits a sheet of paper he had and threw it up in the air. The shock wave from the far away explosion pushed away the paper shreds, which fell a distance behind the physicist. Based on the weight of paper, the distance the mass got pushed and the distance from the centre of explosion, the physicist predicted the power of the bomb, which later measurements showed was quite close.
  
  The post on preload on bolts can well serve for preliminary design.