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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 December 19 2019

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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. No problem! Use hydraulically operated levers and hydraulics themselves... Natural spring or diverted stream from a river via a mechanism operated 'diversions' to control either raise or lower (I.e. in 'lower' position until lowered and then the diversion...well, er...'diverts' to the 'raise' position). It would be an entertaining project to be involved in, but quite achievable. Oh - I watched with the sound off (in office with boss opposite...oops!) - so couldn't hear the wolves....but again could be operated by air displacement from a sealed-sump the reservoir...so again, yes, quite simply!
  2. Okay...I might be misinterpreting this....but a Mechanical one DEFINITELY rather than relying on "wishful thinking"... Ahh...did you mean a 'rotary' or 'static'...? in which case a rotary! That said, it MUST be selected appropriately (there's loads of variations) and installed both properly and carefully!
  3. It depends entirely on the geometry and loading of each pin....they all need to be calculated out as if there is no other present and the material selected as appropriate...they will only shear when they exceed their individual shear loading...take care to ensure double shear is calculated where appropriate.
  4. no matter how willing anyone is to help, it would be useful to know the field....are you building a pet cage, or a bridge, is it an economics project or a hairstyle trend development?
  5. Hi Dr D - I do hope you are well! The glass is an example of easily relatable "brittleness" rather than a "material example" (everyone knows glass breaks when hit...).....that said...the "supercooled fluid" is still a philosophical argument that has never - IMHO - achieved a conclusion one way or the other. Warmest wishes and Season's greetings to all! May 2020 bring success and happiness to all!
  6. It can always be calculated by looking at the section modulus. maximum stress = My/I where M is the Moment at the point being calculated y is half (assuming round) the diameter I is the section modulus (sometimes "Z")...I used to use Z, now almost always use I for some reason...(?!?)...same variable though! for round bar/tube the section modulus (I) is pi(D^4 - d^4)/64 Pi = 3.1415926....etc D = outer diameter d = inner diameter (if present or = 0 if not) As you can see from the formula...for the same weight there would have to be a relatively large increase in diameter...ultimately, the larger the diameter the lower the stress whether it be solid or hollow. If it is hollow, the diameter increases significantly for a much lower ultimate stress in bending...the "^4" has a huge effect! The rigidity is directly proportional to the stress for any given material...the larger the I/Z, the lower the realised stress. Hope this helps
  7. In short, if something is brittle, it will crack (or in some cases shatter) rather than deform. Think of glass - a brittle material....it can be formed in certain circumstances, but it will shatter if impacted...this is due to its brittleness. this is the same for all other materials...some materials are more brittle than others...cast iron for example is very brittle, but pure iron the complete opposite...ie soft and ductile! Materials can also can become so in certain environments - very cold weather on steel for example, or chemical embrittlement where chemical reactions cause a pseudo-brittle effect: hydrogen causes microcracking in steels which leads to brittle behaviour, salt in stainless steels has the same effect (although some grades of stainless steel are relatively safe in salt environments, care needs to be taken in selection.)
  8. Hi Kamal They have given the tensile strength....and the material specific shear strength on p2. The Poissons ration can be back calculated from this. Youngs is a little more problematic...no info. but hopefully this helps. That said it may not be the best solution for carbon fibre...even if it is, make sure you do your surface prep! Have you tried emailing their technical help team...?
  9. Be VERY careful what you are screwing them into....if it is an aluminium carrier, you'll get some serious galvanic effects with stainless, particularly on a pushbike! I think I know the screws you mean and on mine they ARE aluminium! I would have taken a step back and gone for some good-quality graded-steel cap heads. they won't have the same galvanic issues as with stainless and you won't round the drive holes/cross head... Fortunately M4x6LG is a standard size. Good luck!
  10. This is an answer long-after the post - for which I apologise - but I hope of use! It is not really about initial grades, but about ability to interpret systems and gain understanding and apply that understanding. I have known some great engineers who could barely add two plus two, but they understood mechanics and engineering...similarly I have known some fantastic mathematical engineers who I wouldn't trust to tighten a bolt! Maths IS mechanical. If you are struggling with algebra, it is likely something very early that you have missed that has 'bucked' your understanding. I would grab a very basic first introduction book on algebra and read it from the beginning...you may find the "bit" you missed. If that doesn't work, try and understand what other principles are involved where you start to struggle...then address those shortcomings. Something I learned the hard way a good few years back....you never actually LEARN by getting things right....it is only when something becomes a struggle or goes wrong the real learning begins! Maths is very much like a flight of steps, each step resting on the one below. If the lower step is missing, the one on top will fall down. Good luck!
  11. Interesting that you are advised 40kph for 2 days...I used to do "hobby" engine rebuilds (some 30 years ago) for friends (FoC) and recommended 500-800 miles (800 - 1300km) below 50 mph and low revs. It is basically to do with new components requiring time to bed-in. When new they are tighter than in "normal use". By restricting the speed, you restrict the load and minimise the friction-based thermal effects as a result of the initial tightness, allowing the "primary wear" to take place without the components overheating - which would otherwise increase the risk of the engine seizing or worse...broken rings and a need for a rebuild! There are additional considerations too. There are capillary oilways that take time to fill properly and expel all of the air trapped therein. The key thing is really "mechanical sympathy". There is little makes me cringe more than someone revving a cold engine!
  12. 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.
  13. 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
  14. 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.
  15. 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!
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