More HSC Qs (2 Viewers)

[Husayn]

About time!

Obviously because of the sharp corners which concentrate the stresses. A fix would be to make them curved.

Anything else?

 

[fakingtheday]

Failure is due to stress concentrations forming at the sharp joints. The method to fix it is installing a curved fillet to distrubte stress. This is fun

 

[Husayn]

I think I read about drop-forging once, but I can't remember what it was. Something to do with dropping something on the material to form a shape - fakingtheday shall explain!

 

[Husayn]

Pulled this from the net:

During the drop forging process the heated metal is compressed between two dies. One die is static and the other is dropped with the impact hammer of the press. Almost any desired head style can be created in this manner.

 

[fakingtheday]

Just as you said, the material is in a closed die and dropped on by a ram from above.

 

[Husayn]

OK, this is one that has been bugging me:

I think it is because in Force 1 there is a greater surface area being acted upon.

Care to elaborate?

 

[fakingtheday]

That's a tough one. It's got to do with pressure = force/area so with force one there is less "area" as the rod moves out and therefore a greater force. I think

 

[Husayn]

If we take the pressure as 1 (it says the same pressure is used):

For the Force 1:

1 = Force/Area

Force = Area x 1

So the force is directly proportional.

Let's say Area for Force 1 is 2 whilst for Force 2 its 1

Force 1:

Force = 1 x 2

Force 2:

Force = 1 x 1

So I think it's force 1 is greater because the area is greater.

 

[fakingtheday]

But when you think about it, you are dividing the force by the area, so obviously the smaller the area the greater the force.

 

[Husayn]

Here's an "Engineerning Report" once, I hope peoply are still tuning in

I would say something along the lines of aesthetics and security (more transparent) for societal impacts. Also the use of more weather-resistant materials - concrete cancer/timber decay being a factor - as a result of new preventative measures against corrosion.

 

[Husayn]

That would be with regards to pressure - as its inversly proportional to area. Force is directly proportional...

 

[fakingtheday]

aesthetics is definately one, and i'd say safety, you can see who's around you like in the cities and less likeley to fall down because aluminium or whatever it is doesn't corrode as fast.

 

[Husayn]

OK here is another one.

I would say planes. Wood was once used, but was replaced with steel due to its fatigue resistance and also strength.

Can anyone think of something better?

 

[fakingtheday]

Bicycle: wrought iron and steel is now widely replaced by composites due to their superior strength to weight ratio.

 

[Husayn]

I can mainly think of collaboration between materials and aeronautical engineers with regards to materials with suitable mechanical properties (such as fatigue resistance and thermal insulation).

 

[Husayn]

Bronze or brass alloys because they can be self-lubricating after sintering.

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We got this in our trial and noone got it right, above is my teacher's explanation...

 

[Husayn]

The component may be heated and then rapidly quenched, which causes hardening due to the formation of martensite. Tempering, which involves reheating and then slowly cooling, adding toughness and reducing the brittleness of the material. This is useful as the vehicle axle needs to be fatigue resistant.

 

[fakingtheday]

That last one you've kind of fallen all over yourself in your definition.

Hardening - the steel is heated to the austenite zone and soaked to a single-phase solution and then quenched in water. The resultant grain structure is asicular grains on the surface of the steel (this is the martensite) and larger equiaxed grains in the core of the bearing. Giving a very hard but brittle bearing.

Subsequent tempering involves heating the bearing to well below the recrystalisation temperature. This alleviates inner stresses and toughens/strengthens the overall structure of the bearing. Hence increasing service life and safety.

I don't think this is really a major fatigue thing - more of a wear resistence.

 

[fakingtheday]

Bronze and Brass are copper alloys :-/

I wouldn't pick them, since when do they use bronze or brass in axle shafts? They aren't powderformed anyway, they are precipitation hardened to be formed. Powder forming is for dissimilar materials, eg graphite and copper or something. If he was talking about alloys you'd say copper graphite, because graphite is self lubricating.

Why can't you just say aluminium or medium carbon steel for corrosion and abraison resistence, it's much safer than getting into copper alloys such as bronze and brass.

 

[Husayn]

OK I wasn't too sure about my answer for the axle shaft, thanks for clearing that up.

The marking guidelines for the question state "Many responses to this question displayed a lack of understanding of the purpose of solid bearings, and incorrectly chose steel as the bearing material, rather than a bronze or brass alloy".