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Mechanical Engineering

G B Reid MIMechE, SIMarEST

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G B Reid MIMechE, SIMarEST last won the day on September 28 2017

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About G B Reid MIMechE, SIMarEST

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    Benfleet, England
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    Mechanical engineering "generalist", experience in Design (industrial, mechanical, aerospace grade components), Rail, Underground, defence, process flow optimisation, Project Engineering and Mechanical Systems 3D CAD user (various) since 1992. Electrical design installation and verification.

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  1. Hi Danyhil Its a simple moment calculation....the force of the weight of the table the moment (or hinge effect) against the fulcrum (in this case the corners). As the castors are on the corners, any load that will cause the table to topple will be overhanging the edge. 1. find CoG 2. calculate moments to edge of table. (force time distance) 3. Determine CoG of item being calculated and the distance from the fulcrum the force applies - calculate the moment. If both moments apply in the same direction, they are balanced by the legs...if they are in different directions, it is down to whatever is the highest whether it topples or not....but as the legs are on the corners, the CoG would have to overhang which would cause the item ON the table to fall off before the table toppled anyway.
  2. That's a near perfect example of fatigue stress crack propagation. Basically, the repeated impact loading from the jumping has caused a crack to propagate from the thread extend to the point that the material has suffered catastrophic failure. the "tree-ring" appearance on the RHS of the image shows this...but that said, the vertical cracking is a little "odd" and out of place...it's not a simple as it first appears. Whilst it is difficult to be sure with out a full analysis, I would suggest that this is a minor manufacturing defect that has been the propagation source, and that this is unlikely to repeat, but again... Basically, if properly preloaded, the direct load on the axle is minimal, and you should get a cup/cone fracture if any....ie a failure in tension. This is a stress propagation failure, suggesting that one of two possibilities... 1: there was a minor flaw which was the source of a fatigue stress crack and ultimately the failure. 2: the bolt was not suitably preloaded, allowing the stress to act in a non-design way. Thinking through....I think its a combination of both 1 and 2. If preloaded, the "cones" (or the bearings - subject to the design) take the loading by friction on the load-faces and the axle, only tension. The image sort of suggests (and again there is insufficient information to be 100% sure) that the loading has been a flexing one for a significant period of time. The other failure mode would be shear...which this sort of indicates...but again this would be down to a bolt not tight enough as the friction of the load-faces should take all of the loading thus eliminating the shear. I'd suggest just getting hold of a new standard axle and tighten the bolts properly...and if jumping regularly...checking it is tight regularly! All the best
  3. If anyone is interested, it is due to how homogeneous the material is (roughly translated as 'same throughout'). With thinner materials, the final product is generally reasonably homogeneous, but as the plate thickens, it starts to develop more and more intrinsic flaws within the material. Basically, in order to mitigate for this, the quoted strengths have been reduced. Beware of complacency though...these 'reduced' figures are not a "fudge" but verified from tests and can be taken as true for the materials in question. Michal is exactly right, but for those not so technically minded (or those without the materials knowledge) this explanation may help.
  4. I'd use Fd (force of drag) = 1/2 x Cd (coefficient of drag) x rho (air density corrected for humidity...in the UK we use 1.226 in the Eurocode National Annex) x area in m^2 (worst case) x windspeed in ms^-1 (max) squared...due to ground effects it should be okay to use Cd as 1.1...that said surrounding area causing funnelling effects (and thus increasing local windspeed) which could be an issue. It will spit out a force in Newtons - it can be reduced to a force per unit area by dividing by the area....but the equation above gives the figures for your moment and loading calculations. For your windspeed, you need to find the maximum likely in the area. DO NOT just take the figure, it needs to have a statistical likelihood of occurrance 50 years for safety....in the UK generally it is 100mph (45.15 ms^-1), but in the North of Scotland, or offshore, it works out at about 135 (60.35ms^1), but it does vary quite significantly - altitude has a huge effect too! To give a comparison , the "basic winsdspeed" of 22.5ms^-1 spits out ~43ms^-1....the statistical analysis can double the effects! I can't do any more on this as the geographical and environmental factors are unknown....good luck!
  5. Calculate the forces, and the moments. Then use the deflection equations and back calculate. If you need the equations, try 'Roark's formulas <sic> for stress and strain' by Young, Budynas and Sadegh...(imho, it should be formulae, not formulas)
  6. Looks like it may be for redundancy....in case one fails, the circuit can still function. Both 100.1 and 100.2 appear to have the same inlet feeds and the same outlet feeds.
  7. NAFEMS tends to be the authority on modelling, best practice etc. https://www.nafems.org/
  8. Use the forums to build reputation...network at every opportunity and most importantly - Stay Positive no matter what!
  9. Failing that, it is the area integral over the direction applied squared...ie if x direction its the area integral of the y^2 area and if the y direction its the area integral of the x^2 area...
  10. You already know the power...its 15 Watts...one Watt is one Joule per second....just calculate the time element and "Bob's your uncle"!
  11. experiment, experiment, experiment. The strap wrench is a good first step and would most likely be successful. Alternatives can be thought of if the strap wrench doens't work...a combination of rubber-lined v-blocks in a vice for example...?...but the cost increases dramatically.
  12. Your first step here would not be calculation, but geometry! The geometry (as if designing a cam - huge levels of crossover) would give you the critical detail you need for your calculations. The risk of dead spots is purely on being piston driven, but physically impossible in the same due to the conditions...ie can't reach TDC due to pressures and will not stop at BDC due to lowest resistance at this point. On old vintage cars, they would often backfire when stopped as the pressure at TDC would cause the engine to counter rotate slightly as it stopped due to the residual piston pressure.
  13. Doh! Basic error...I didn't read the question properly... ...The room of course! The room cools quickest as that is the only way to break the equilibrium of the temperatures (Room/plates which are both "heated to the same temperature" allowing a differential on the plates forcing them to cool. This may be a semantic answer, but a relevant one.
  14. Assuming both plates are suspended in free air, the vertical unit will cool significantly faster. The convection of the air as a result of heat emanating from the plate will be significantly greater than the horizontal. Effectively, the convection will increase as the warming air travels up the plate faster as it warms more due to the proximity of the sink allowing cold air to replace from the bottom. On the horizontal plate, the air mass above gets 'in the way' of the convective effects due to the effective horizontal area involved...basically, it has to be pushed out of the way by the less dense, warmer air. On the vertically suspended plate, the horizontal equivalent area involved in the convection is significantly lower resulting in significantly reduced air mass effects to slow the convection..this is further aided by the convected air being continually heated as it rises up the plate drawing more energy from the plate.
  15. Hard Water is water with a large amount of calcium carbonate disolved within. When boiled, htis precipitates out to make an unpleasant - but tasteless - grit...it can flake off in large chunks. Soft water is where there is no limescale disolved...although oither salts may be present. Hard water can be converted to soft water for processing by the addition of sodium chloride (common table/rock salt) ot by distillation (or filterring too for that matter). It can also be easily idetified by its loss of surface tension when soap is added (ie it bubbles if its soft!)
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