G B Reid MIMechE, SIMarEST
Members
Content Count
85 
Joined

Last visited

Days Won
15
G B Reid MIMechE, SIMarEST last won the day on September 28 2017
G B Reid MIMechE, SIMarEST had the most liked content!
About G B Reid MIMechE, SIMarEST

Rank
Member
Profile Information

Gender
Male

Location
Benfleet, England

Interests
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.
Recent Profile Visitors

Can I become a mechanical engineer at this point?
G B Reid MIMechE, SIMarEST replied to Kite1439's question in Question & Answers
This is an answer longafter 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! 
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 500800 miles (800  1300km) below 50 mph and low revs. It is basically to do with new components requiring time to bedin. When new they are tighter than in "normal use". By restricting the speed, you restrict the load and minimise the frictionbased 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!

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.

Bicycle axle material
G B Reid MIMechE, SIMarEST replied to MattMatthew's topic in Engineering Materials
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 "treering" 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 nondesign 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 loadfaces 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 loadfaces 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 
Mechanical properties question
G B Reid MIMechE, SIMarEST replied to Roger Casas's topic in Engineering Materials
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. 
Wind Load capacity
G B Reid MIMechE, SIMarEST replied to Mukhtar Assiri's question in Question & Answers
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! 
Torsion of a structure
G B Reid MIMechE, SIMarEST replied to Perilun's question in Question & Answers
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) 
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.

computational mechanics
G B Reid MIMechE, SIMarEST replied to Hamdi Zenkri's topic in Thermal Sciences
NAFEMS tends to be the authority on modelling, best practice etc. https://www.nafems.org/ 
How can i get the moment of inertia?
G B Reid MIMechE, SIMarEST replied to Gerardo Padilla's topic in Engineering Materials
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... 
Holding a cylinder in place strap wrench?
G B Reid MIMechE, SIMarEST replied to Feethurt's topic in Ideas
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 rubberlined vblocks in a vice for example...?...but the cost increases dramatically. 
Crank slider mechanism
G B Reid MIMechE, SIMarEST replied to Perilun's topic in Machine Dynamics & Design Considerations
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. 
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.